Cells are the basic unit of all forms of life. In this section we explore how structural differences between types of cells enables them to perform specific functions within the organism. These differences in cells are controlled by genes in the nucleus. For an organism to grow, cells must divide by mitosis producing two new identical cells.
If cells are isolated at an early stage of growth before they have become too specialised, they can retain their ability to grow into a range of different types of cells. This phenomenon has led to the development of stem cell technology. This is a new branch of medicine that allows doctors to repair damaged organs by growing new tissue from stem cells.
Cell structure - Eukaryotes and prokaryotes
Plant and animal cells (eukaryotic cells) have a cell membrane, cytoplasm and genetic material enclosed in a nucleus.
Bacterial cells (prokaryotic cells) are much smaller in comparison. They have cytoplasm and a cell membrane surrounded by a cell wall. The genetic material is not enclosed in a nucleus. It is a single DNA loop and there may be one or more small rings of DNA called plasmids.
magnification x real size
(may have to use standard form)
Most animal cells have the following parts:
a nucleus - contains genetic material that controls the activities of the cell
cytoplasm - gel-like substance where chemical reaction occur contains enzymes that control the reactions
a cell membrane - holds the cell together and controls what goes in and out
mitochondria - most reactions for aerobic respiration occur. respiration transfers energy that cell needs to work
ribosomes - where proteins are made
in addition to the parts foung in animal cells, plant cells often have:
permanent vacuole - contains cell sap, a weak solution of sugar and salts
Plant and algal cells also have a cell wall made of cellulose, which strengthens the cell.
Bacteria are prokaryotes
single circular strand of DNA - dont have a 'true' nucleus. the DNA floats freely in the cytoplasm
Plasmids - small rings of DNA (may contain one or more)
bacteria dont have chloroplasts or mitochondria
cells differentiate to become specialised. examples:
Sperm cells are specialised for reproduction - long tail and streamlines head to help it swim. lots of mitrochondria to provide energy. carries enzymes to get through egg cell membrane
nerve cells are specialised for rapid signalling - cells are long to cover more distance. have branched connections at ends so can connect to other nerve cells to form network
muscle cells are specialised for contraction - long so have space to contract. contain lots of mitochondria to gernerate energy needed
root hair cells are specialised for absorbing water and minerals - big surface area for absorbing water and mineral ions from the soil
phloem and xylem cells are specialised for transporting substances - xylem cells are hollow in centre and phloem cells have very few subcellular structures so stuff can flow through them
As an organism develops, cells differentiate to form different types of cells - Most types of animal cell differentiate at an early stage. - Many types of plant cells retain the ability to differentiate throughout life. In mature animals, cell division is mainly restricted to repair and replacement e.g skin or blood As a cell differentiates it acquires different sub-cellular structures to enable it to carry out a certain function. It has become a specialised cell. some cells are undifferentiated cells - they are called stem cells
microscopes let us see things we cant see with naked eye. Microscopy techniques we use have developed over the years as technology and knowledge improved
light microscopes use light and lenses to form an image of a specimen and magnify it. They let us see individual cells and large subcellular structures e.g. nuclei
electron microscopes use electrons to form an image.
An electron microscope has much higher magnification and resolution (gives a sharper image)
This means that it can be used to study smaller cells in much finer detail e.g internal structure of mitochondria and chloroplasts but can also see tinier things such as ribosomes and plasmids
This has enabled biologists to see and understand many more sub-cellular structures.
Bacteria multiply by simple cell division (binary fission) as often as once every 20 minutes if they have enough nutrients and a suitable temperature.
Bacteria can be grown in a nutrient broth solution or as colonies on an agar gel plate.
Uncontaminated cultures of microorganisms are required for investigating the action of disinfectants and antibiotics.
(use area of a circle to calculate sizes of inhibition zones or the area of a colony)
The nucleus of a cell contains chromosomes made of DNA molecules. Each chromosome carries a large number of genes.
In body cells the chromosomes are normally found in pairs.
Mitosis and the cell cycle
Cells divide in a series of stages called the cell cycle. Students should be able to describe the stages of the cell cycle, including mitosis.
During the cell cycle the genetic material is doubled and then divided into two identical cells.
Before a cell can divide it needs to grow and increase the number of sub-cellular structures such as ribosomes and mitochondria. The DNA replicates to form two copies of each chromosome.
In mitosis one set of chromosomes is pulled to each end of the cell and the nucleus divides.
Finally the cytoplasm and cell membranes divide to form two identical cells.
A stem cell is an undifferentiated cell of an organism which is capable of giving rise to many more cells of the same type, and from which certain other cells can arise from differentiation.
Stem cells from human embryos can be cloned and made to differentiate into most different types of human cells.
Stem cells from adult bone marrow can form many types of cells including blood cells.
Meristem tissue in plants can differentiate into any type of plant cell, throughout the life of the plant.
Treatment with stem cells may be able to help conditions such as diabetes and paralysis.
stem cells - 2
In therapeutic cloning an embryo is produced with the same genes as the patient. Stem cells from the embryo are not rejected by the patient’s body so they may be used for medical treatment.
The use of stem cells has potential risks such as transfer of viral infection, and some people have ethical or religious objections.
Stem cells from meristems in plants can be used to produce clones of plants quickly and economically.
- rare species can be cloned to protect from extinction
- crop plants with special features such as disease resistance can be cloned to produce large numbers of identical plants for farmers
transport in cells - diffusion
Substances may move into and out of cells across the cell membranes via diffusion.
Diffusion is the spreading out of the particles of any substance in solution, or particles of a gas, resulting in a net movement from an area of higher concentration to an area of lower concentration.
Some of the substances transported in and out of cells by diffusion are oxygen and carbon dioxide in gas exchange, and of the waste product urea from cells into the blood plasma for excretion in the kidney.
Factors which affect the rate of diffusion are:
- the difference in concentrations (concentration gradient)
- the temperature
- the surface area of the membrane
diffusion - 2
A single-celled organism has a relatively large surface area to volume ratio. This allows sufficient transport of molecules into and out of the cell to meet the needs of the organism.
In multicellular organisms, surfaces and organ systems are specialised for exchanging materials. This is to allow sufficient molecules to
be transported into and out of cells for the organism’s needs. The effectiveness of an exchange surface is increased by:
- having a large surface area
- a membrane that is thin, to provide a short diffusion path
- (in animals) having an efficient blood supply
- (in animals, for gaseous exchange) being ventilated.
Water may move across cell membranes via osmosis. Osmosis is the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.
a partially permeable membrane is just one with very small holes in it so only tiny molecules can pass through e.g. water
water molecules actually pass both ways as water molecules move about randomly all the time
because there are more molecules on one side than the other theres a steady net flow of water into the region with fewer molecules i.e into the srtonger sugar solution
this means strong sugar solution gets more dilute. the water acts like its trying to 'even up' the concentration either side of the membrane
Active transport moves substances from a more dilute solution to a more concentrated solution (against a concentration gradient). This requires energy from respiration.
Active transport allows mineral ions to be absorbed into plant root hairs from very dilute solutions in the soil. Plants require ions for healthy growth.
It also allows sugar molecules to be absorbed from lower concentrations in the gut into the blood which has a higher sugar concentration. Sugar molecules are used for cell respiration.
In this section we will learn about the human digestive system which provides the body with nutrients and the respiratory system that provides it with oxygen and removes carbon dioxide. In each case they provide dissolved materials that need to be moved quickly around the body in the blood by the circulatory system.
Damage to any of these systems can be debilitating if not fatal. Although there has been huge progress in surgical techniques, especially with regard to coronary heart disease, many interventions would not be necessary if individuals reduced their risks through improved diet and lifestyle.
We will also learn how the plant’s transport system is dependent on environmental conditions to ensure that leaf cells are provided with the water and carbon dioxide that they need for photosynthesis.
Principles of organisation
Cells are the basic building blocks of all living organisms.
A tissue is a group of cells with a similar structure and function.
Organs are aggregations of tissues performing specific functions.
Organs are organised into organ systems, which work together to form organisms.
the human digestive system
he digestive system is an example of an organ system in which several organs work together to digest and absorb food.
carbohydrates, proteins, lipids (or fats) are major nutrients that we need in large quantities.
We get these by eating them. They are broken down first and then reassembled into our own carbohydrates, proteins and lipids. This is because:
- most of the molecules in food are too large to pass through the absorbing surface of the gut wall
- the carbohydrates, proteins and lipids are reassembled in the form required, rather than other animal or plant versions
Enzymes catalyse specific reactions in living organisms due to the shape of their active site.
Digestive enzymes convert food into small soluble molecules that can be absorbed into the bloodstream.
Carbohydrases break down carbohydrates to simple sugars. Amylase is a carbohydrase which breaks down starch.
Proteases break down proteins to amino acids.
Lipases break down lipids (fats) to glycerol and fatty acids.
The products of digestion are used to build new carbohydrates, lipids and proteins. Some glucose is used in respiration.
Bile is made in the liver and stored in the gall bladder. It is alkaline to neutralise hydrochloric acid from the stomach. It also emulsifies fat to form small droplets which increases the surface area. The alkaline conditions and large surface area increase the rate of fat breakdown by lipase.
carbohydrases convert carbohydrates into simple sugars
enzyme & amylase (made in salivary glands, pancreas, small intestine)
STARCH ---------------------> MALTOSE(and other sugars eg dextrins)
proteases convert proteins into amino acids
enzyme & protease ( made in stomach(pepsin), pancreas, small intestine)
PROTEINS --------------------> AMINO ACIDS
lipases convert lipids (fats and oils) into glycerol and fatty acids
enzymes & lipase ( made in pancreas & small intestine)
LIPID ----------------------> GLYCEROL & FATTY ACIDS
more on digestion
salivary glands - produce amylase enzyme in saliva
liver - where bile is produced. Bile neutralises stomach acid and emulsifies fats
Stomach - pummels food with muscular walls. produces protein enzyme, pepsin. produces hydrochloric acid to kill bacteria and give right pH for the protease enzyme to work (pH 2 - acidic)
Gall bladder - bile is stored before released into small intestine
Pancreas - produces protease, amylase & lipase enzymes. releases into small intestine
Large intestine - excess water is absorbed from food
small intestine - produces protease, amylase & lipase enzymes to complete digestion. food absorbed out of digestive system into blood
rectum - where faeces temporarily stored
in the thorax ( top part of body)
lungs are like pink sponges and protected by ribcage. surrounded by pleural membranes
air you breathe in goes through trachea. this splits into two tubes called bronchi, one going to each lung
bronchi splits off into smaller tubes called bronchioles
bronchioles end at small bags called alveoli where gas exchange takes place
lungs contain millions of little air sacs called alveoli surrounded by network of blood capillaries. where gas exchange happens
blood passing next to alveoli has just returned to lungs from rest of body so contains lots of carbon dioxide and very little oxygen
oxygen diffuses out of alveolus (high concentration) into blood (low concentration). Carbon dioxide diffuses out of blood (high conc.) into the alveolus (low conc.) to be breathed out
when blood reaches body cells oxygen is released from red blood cells (where theres high conc) and diffuses into body cells (low conc)
at same time carbon dioxide diffuses out of body cells (high conc) into the blood (low conc). its then carried back to lungs
calculate breathing rate in breaths per minute
breaths per minute = number of breaths/number of minutes
double circulatory system
made up of heart, blood vessels, and blood
double circulatory system is two circuits joined together:
1 - in the first one, the right ventricle pumps deoxygenated blood to the lungs to take in oxygen. the blood then returns to the heart
2 - in the second one, the left ventricle pumps oxygenated blood around all the other organs of the body. the blood gives up its oxygen at the body cells and the deoxygenated blood returns to the heart to be pumped out to the lungs again
the heart contracts to pump blood around the body. the walls of the heart are mostly made of muscle tissue
the heart has valves to make sure the blood flows in the right direction - stops it flowing backwards
blood flows into the the two atria from the vena cava and the pulmonary vein
the atria contract, pushing the blood into the ventricles
the ventricles contract, forcing the blood into the pulmonary artery and the aorta, and out of the heart
the blood then flows to the organs through arteries, and returns through veins
the atria fill again and the cycle starts over
the heart needs its own supply of oxygenated blood. conorary arteries branch off the aorta and surround the heart to give it the oxygenated blood it needs
The natural resting heart rate is controlled by a group of cells located in the right atrium that act as a pacemaker. Artificial pacemakers are electrical devices used to correct irregularities in the heart rate. The body contains three different types of blood vessel:
arteries - carry blood away from heart. heart pumps blood at high pressure so artery walls are strong and elastic. thick walls compared to hole in middle (lumen). layers of muscle to make them strong and elastic fibres so they can stretch and spring back
veins - lower pressure blood so thinner walls. bigger lumen to help blood flow. valves keep blood flowing in right direction
capillaries - carry blood really close to every cell in body to exchange substances. have permeable walls so substances diffuse in/out. supply food and oxygen and take away waste eg CO2. walls only one cell thick which increaes rate of diffusion by decreasing distance over which it occurs
Blood is a tissue consisting of plasma, in which the red blood cells, white blood cells and platelets are suspended.
rate of blood flow = volume of blood/number of minutes
red blood cells
carry oxygen from the lungs to all cells in the body
their shape is a biconcave disc which gives it a large surface area for absorbing oxygen
they dont have a nucleus - more room to carry oxygen
contain red pigment called haemoglobin
in the lungs, haemoglobin binds to oxygen to become oxyhaemoglobin
in body tissues, the reverse happens - oxyhaemoglobin splits up into haemoglobin and oxygen to release oxygen to the cells
white blood cells
white blood cells defend against infection
some can change shape to gobble up unwelcome microorganisms, in a process called phagocytosis
others produce antibodies to fight microorganisms as well as antitoxins to neutralise any toxins produced by the microorganisms
unlike red blood cells they do have a nucleus
help blood clot
platelets are small fragments of cells. they have no nuclues
they help the blood to clot at a wound - to stop all your blood pouring out and to stop microorganisms getting in
lack of platelets can cause excessive bleeding and bruising
the liquid that carry everything in blood:
- red and white blood cells and platelets
- nutrients like glucose and amino acids (these are the soluble prducts of digestion which are absorbed from the gut and taken to the cells of the body)
- carbon dioxide from the organs and the lungs
- urea from the liver to the kidneys
- anibodies and antitoxins produced by white blood cells
Coronary heart disease: a non-communicable disease
In coronary heart disease layers of fatty material build up inside the coronary arteries, narrowing them. This reduces the flow of blood through the coronary arteries, resulting in a lack of oxygen for the heart muscle. Stents are used to keep the coronary arteries open. Statins are widely used to reduce blood cholesterol levels which slows down the rate of fatty material deposit.
In some people heart valves may become faulty, preventing the valve from opening fully, or the heart valve might develop a leak. Students should understand the consequences of faulty valves. Faulty heart valves can be replaced using biological or mechanical valves.
In the case of heart failure a donor heart, or heart and lungs can be transplanted. Artificial hearts are occasionally used to keep patients alive whilst waiting for a heart transplant, or to allow the heart to rest as an aid to recovery.
cardiovascular disease - stents
stents are tubes that are inserted inside arteries. they keep them open, making sure blood can pass through to the heart muscles. this keeps the persons heart beating
stents are a way of lowering the risk of a heart attack in people with conorary heart disease. they are effective for a long time and the recovery time from surgery is relatively quick
there is a risk of complications during the operation (eg heart attack) and a risk of infection from surgery. there is also a risk of patient developing a blood clot near the stent - called thrombosis
cardiovascular disease - statins
statins reduce cholesterol in the blood
cholesterol is an essensial lipid that your body produces and needs to function properly. having too much bad cholesterol in the bloodstream can cause fatty deposits inside arteries which can lead to conorary heart disease. statins are drugs that reduce this cholesterol - slows down fatty deposits
advantages - reducing cholesterol can reduce the risk of strokes, conorary heart disease and heart attacks. can increase good cholesterol (HDL)which can remove bad cholesterol
diadvantages - long-term drug that must be taken regularly - could forget to take them. can cause negative side effects which can be serious. effect isnt instant.
Health is the state of physical and mental well-being.
Diseases, both communicable and non-communicable, are major causes of ill health. Other factors including diet, stress and life situations may have a profound effect on both physical and mental health.
Different types of disease may interact.
Defects in the immune system mean that an individual is more likely to suffer from infectious diseases.
Viruses living in cells can be the trigger for cancers.
Immune reactions initially caused by a pathogen can trigger allergies such as skin rashes and asthma.
Severe physical ill health can lead to depression and other mental illness.
The effect of lifestyle on some non-communicable d
Risk factors are linked to an increased rate of a disease. They can be:
- aspects of a person’s lifestyle
- substances in the person’s body or environment.
A causal mechanism has been proven for some risk factors, but not in others.
- The effects of diet, smoking and exercise on cardiovascular disease.
- Obesity as a risk factor for Type 2 diabetes.
- The effect of alcohol on the liver and brain function.
- The effect of smoking on lung disease and lung cancer.
- The effects of smoking and alcohol on unborn babies.
- Carcinogens, including ionising radiation, as risk factors in cancer.
Many diseases are caused by the interaction of a number of factors.
cnacer is caused by uncontrolled cell growth and divison which is a result of changes that occur to the cells and results in the formation of a tumour (a mass of cells). not all tumours are cancerous
Benign tumours are growths of abnormal cells which are contained in one area, usually within a membrane. They do not invade other parts of the body.
Malignant tumour cells are cancers. They invade neighbouring tissues and spread to different parts of the body in the blood where they form secondary tumours.
cancer - risk factors
risk factors increase the chance the chance of some cancers
smoking - linked to lung cancer but has also been linked to mouth, bowel, stomach and cervical cancer
obesity - linked to many types including bowel, liver and kidney cancer. second biggest preventable cause after smoking
UV exposure - UV radiation can cause skin cancer. people who live in sunny climates or spend a lot of time outside are at higher risk.
Viral infection - infection with some viruses increases the chances of developing certain types of cancer. for example infection with hepatitis B and hepatitis C increase the risk of liver cancer.
There are also genetic risk factors for some cancers.
Plant tissues include
- epidermal tissues - covers the whole plant
- palisade mesophyll - part of leaf where most photosynthesis happens
- spongy mesophyll - in leaf and contains big air spaces to allow gases to diffuse in and out of cells
- xylem and phloem - transport things like water, mineral ions and food around the plant (through roots, stems and leaves)
- meristem tissue - found at the growing tips of shoots and roots and is able to differentiate into lots of different types of plant cell, allowing the plant to grow
The leaf is a plant organ
plant organ cells
epidermal tissues - covered with waxy cuticle to reduce water loss by evaporation
upper eperdermis - transparent so that light can pass through it to the palisade layer
palisade layer - lots of chloroplasts (little structures where photosynthesis occurs) near top of leaf so can get most light
xylem and phloem - form network of vascular bundles which deliver water and other nutrients to entire leaf and take away glucose produced by photosynthesis. also help support structure
tissues of leaves - adapted for efficient gas exchange e.g lower epidermis is full of little holes called stomata, which let CO2 diffuse directly into leaf. opening and closing of stomata controlled by guard cells in response to environmental conditions. air spaces in spongy mesophyll tissue increase the rate of diffusion of gases
transpiration and translocation
The roots, stem and leaves form a plant organ system for transport of substances around the plant.
Root hair cells are adapted for the efficient uptake of water by osmosis, and mineral ions by active transport.
Xylem tissue transports water and mineral ions from the roots to the stems and leaves. It is composed of hollow tubes strengthened by lignin adapted for the transport of water in the transpiration stream.
The role of stomata and guard cells are to control gas exchange and water loss.
Phloem tissue transports dissolved sugars from the leaves to the rest of the plant for immediate use or storage. The movement of food molecules through phloem tissue is called translocation.
Phloem is composed of tubes of elongated cells. Cell sap can move from one phloem cell to the next through pores in the end walls.
infection and response
Pathogens are microorganisms such as viruses and bacteria that cause infectious diseases in animals and plants. They depend on their host to provide the conditions and nutrients that they need to grow and reproduce. They frequently produce toxins that damage tissues and make us feel ill.
This section will explore how we can avoid diseases by reducing contact with them, as well as how the body uses barriers against pathogens. Once inside the body our immune system is triggered which is usually strong enough to destroy the pathogen and prevent disease.
When at risk from unusual or dangerous diseases our body’s natural system can be enhanced by the use of vaccination. Since the 1940s a range of antibiotics have been developed which have proved successful against a number of lethal diseases caused by bacteria. Unfortunately many groups of bacteria have now become resistant to these antibiotics. The race is now on to develop a new set of antibiotics.
Communicable (infectious) diseases
Pathogens are microorganisms that cause infectious disease. Pathogens may be viruses, bacteria, protists or fungi. They may infect plants or animals and can be spread by direct contact, by water or by air.
Bacteria may produce poisons (toxins) that damage tissues and make us feel ill.
Viruses live and reproduce inside cells, causing cell damage.
Bacteria and viruses may reproduce rapidly inside the body.
protists are single-celled eukaryotes. some protists are parasites which live on or inside other organisms and can damage them
some fungi are single-celled. others have a body which is made up of hyphae (thread like structure) which can grow and penetrate human skin and the surface of plants, causing diseases. they can produce spores which can be spread to other plants and animals
Measles is a viral disease showing symptoms of fever and a red skin rash. Measles is a serious illness that can be fatal if complications arise. For this reason most young children are vaccinated against measles. The measles virus is spread by inhalation of droplets from sneezes and coughs.
HIV initially causes a flu-like illness. Unless successfully controlled with antiretroviral drugs the virus attacks the body’s immune cells. Late stage HIV infection, or AIDS, occurs when the body’s immune system becomes so badly damaged it can no longer deal with other infections or cancers. HIV is spread by sexual contact or exchange of body fluids such as blood which occurs when drug users share needles.
Tobacco mosaic virus (TMV) is a widespread plant pathogen affecting many species of plants including tomatoes. It gives a distinctive ‘mosaic’ pattern of discolouration on the leaves which affects the growth of the plant due to lack of photosynthesis.
Salmonella food poisoning is spread by bacteria ingested in food,
or on food prepared in unhygienic conditions. In the UK, poultry are vaccinated against Salmonella to control the spread. Fever, abdominal cramps, vomiting and diarrhoea are caused by the bacteria and the toxins they secrete.
Gonorrhoea is a sexually transmitted disease (STD) with symptoms of
a thick yellow or green discharge from the vagina or penis and pain on urinating. It is caused by a bacterium and was easily treated with the antibiotic penicillin until many resistant strains appeared. Gonorrhoea is spread by sexual contact. The spread can be controlled by treatment with antibiotics or the use of a barrier method of contraception such as a condom.
Rose black spot is a fungal disease where purple or black spots develop on leaves, which often turn yellow and drop early. It affects the growth of the plant as photosynthesis is reduced. It is spread in the environment by water or wind. Rose black spot can be treated by using fungicides and/or removing and destroying the affected leaves.
The pathogens that cause malaria are protists.
The malarial protist has a life cycle that includes the mosquito. Malaria causes recurrent episodes of fever and can be fatal. The spread
of malaria is controlled by preventing the vectors, mosquitos, from breeding and by using mosquito nets to avoid being bitten.
human defence systems
skin - acts as barrier to pathogens. it also secretes antimicrobial substances which kill pathogens
nose - hairs and mucus trap paricles that could contain pathogens
trachea and bronchi - secrete mucus to trap pathogens. lined with cilia (hair-like structures which waft mucus up to the back of the throat where it can be swallowed)
stomach - produces hydrochloric acid which kills pathogens that make it that far from the mouth
the immune system can attack pathogens
if pathogens do make it into your body your immune system kicks in to destroy them
the most important part is the white blood cells. they travel around in your blood and crawl into every part of you, constantly patrolling for microbes. when they come across an invading microbe they have three lines of attack.
White blood cells help to defend against pathogens by:
- phagocytosis - engulf foreign cells and digest them
- antibody production - lock onto invading cells so they can be found and destroyed by other white blood cells. antibodies produced are specific to that type of antigen. antibodies produce rapidly. if person infected again antibodies produced quicker/easier
- antitoxin production - counteract toxins produced by invading bacteria
People can be immunised against a pathogen through vaccination. Different vaccines are needed for different pathogens.
Vaccination involves introducing small quantities of dead or inactive forms of a pathogen into the body to stimulate the white blood cells to produce antibodies. If the same pathogen re-enters the body the white blood cells respond quickly to produce the correct antibodies, preventing infection.
Vaccines can contain:
- live pathogens treated to make them harmless
- harmless fragments of the pathogen
- toxins produced by pathogens
- dead pathogens
Antibiotics and painkillers
Antibiotics, such as penicillin, are medicines that help to cure bacterial disease by killing infective bacteria inside the body. It is important that specific bacteria should be treated by specific antibiotics.
The use of antibiotics has greatly reduced deaths from infectious bacterial diseases. However, the emergence of strains resistant to antibiotics is of great concern.
Antibiotics cannot kill viral pathogens.
Painkillers and other medicines are used to treat the symptoms of disease but do not kill pathogens.
It is difficult to develop drugs that kill viruses without also damaging the body’s tissues.
Discovery and development of drugs
Traditionally drugs were extracted from plants and microorganisms.
- The heart drug digitalis originates from foxgloves.
- The painkiller aspirin originates from willow.
- Penicillin was discovered by Alexander Fleming from the Penicillium mould.
Most new drugs are synthesised by chemists in the pharmaceutical industry. However, the starting point may still be a chemical extracted from a plant. New medical drugs have to be tested and trialled before being used to check that they are safe and effective. New drugs are extensively tested for toxicity, efficacy and dose. Preclinical testing is done in a laboratory using cells, tissues and live animals.
Clinical trials use healthy volunteers and patients.
- Very low doses of the drug are given at the start of the clinical trial.
- If the drug is found to be safe, further clinical trials are carried out to find the optimum dose for the drug.
- In double blind trials, some patients are given a placebo.
Producing monoclonal antibodies
Monoclonal antibodies are produced from a single clone of cells. The antibodies are specific to one binding site on one protein antigen and so are able to target a specific chemical or specific cells in the body.
They are produced by stimulating mouse lymphocytes to make a particular antibody. The lymphocytes are combined with a particular kind of tumour cell to make a cell called a hybridoma cell.
The hybridoma cell can both divide and make the antibody. Single hybridoma cells are cloned to produce many identical cells that all produce the same antibody. A large amount of the antibody can be collected and purified.
Uses of monoclonal antibodies
Some examples include:
- for diagnosis such as in pregnancy tests
- in laboratories to measure the levels of hormones and other chemicals in blood, or to detect pathogens
- in research to locate or identify specific molecules in a cell or tissue by binding to them with a fluorescent dye
- to treat some diseases: for cancer the monoclonal antibody can be bound to a radioactive substance, a toxic drug or a chemical which stops cells growing and dividing. It delivers the substance to the cancer cells without harming other cells in the body.
Monoclonal antibodies create more side effects than expected. They are not yet as widely used as everyone hoped when they were first developed.
a hormone called HCG is found in urine when pregnant. pregnancy tests detect this
1 - the bit of the stick you wee on has some antibodies to the hormone, with blue beads attached
2 - the test s t r i p has some more antibodies to the hormone stuck onto it (so they cant move)
3 - if youre pregnant: the hormone binds to the antibodies on the blue beads. the urine moves up the stick, carrying the hormone and the beads. the beads and hormone bind to the antibodies on the s t r i p. so the blue beads get stuck on the s t r i p which turn it blue
4 - if youre not pregnant the urine still moves up the stick, carrying the blue beads but theres nothing to stick the blue beads onto the test s t r i p so it doesnt go blue
Cancer diagnosis and treatment
Cancerous cells have antigens. Monoclonal antibodies can be designed to bind specifically with these antigens. When injected into a person's body, the monoclonal antibodies will bind with these cancer cells and clump them together. This makes it easier to identify a cancerous tumour, which can then be treated or removed.
Monoclonal antibodies have also been designed to treat cancer by:
- carrying drugs that have been attached to them, to the tumour
- encouraging your immune system to attack the cancer cells directly
Other diagnostic uses Monoclonal antibodies are also used in a similar way to identify and diagnose infections, such as HIV and AIDS, herpes and chlamydia. Some monoclonal antibodies have been attached to dyes that will glow fluorescentunder UV light. This can make disease identification much easier.
in the lab
Monoclonal antibodies can be used to:
1 - bind hormones and chemicals in blood to measure their levels
2 - test blood samples in laboratories for certain pathogens
3 - locate specific molecules on a cell or in a tissue:
- first monoclonal antibodies are made that will bind to the specific molecules you are looking for
- the antibodies are then bound to a flourescent dye
- if the molecules are present in the sample youre analysing, the monoclonal antibodies will attach to them and they can be detected using the dye
Detection and identification of plant diseases
Plant diseases can be detected by:
- stunted growth
- spots on leaves
- areas of decay (rot)
- malformed stems or leaves
- the presence of pests
Identification can be made by:
- reference to a gardening manual or website
- taking infected plants to a laboratory to identify the pathogen
- using testing kits that contain monoclonal antibodies.
Plants can be infected by a range of viral, bacterial and fungal pathogens as well as by insects.
Plants can be damaged by a range of ion deficiency conditions:
- stunted growth caused by nitrate deficiency - nitrates are needed to make proteins
- chlorosis caused by magnesium deficiency - magnesium makes chlorophyll which is needed for photosynthesis. plants effected by chlorosis have yellow leaves
Plant defence responses
Physical defence responses to resist invasion of microorganisms.
Cellulose cell walls.
Tough waxy cuticle on leaves.
Layers of dead cells around stems (bark on trees) which fall off.
Chemical plant defence responses.
Poisons to deter herbivores.
Thorns and hairs deter animals.
Leaves which droop or curl when touched.
Mimicry to trick animals.
In this section we will explore how plants harness the Sun’s energy in photosynthesis in order to make food. This process liberates oxygen which has built up over millions of years in the Earth’s atmosphere. Both animals and plants use this oxygen to oxidise food in a process called aerobic respiration which transfers the energy that the organism needs to perform its functions. Conversely, anaerobic respiration does not require oxygen to transfer energy. During vigorous exercise the human body is unable to supply the cells with sufficient oxygen and it switches to anaerobic respiration. This process will supply energy but also causes the build-up of lactic acid in muscles which causes fatigue.
Photosynthesis is represented by the equation:
carbon dioxide + water----> light glucose + oxygen
6CO2 + 6H2O --------> C6H12O6 + 6O2
photosynthesis is an endothermic reaction in which energy is transferred from the environment to the chloroplasts by light
rate of photosynthesis
Three factors can limit the speed of photosynthesis - light intensity, carbon dioxide concentration and temperature.
These factors interact and any one of them may be the factor that limits photosynthesis. Limiting factors are important in the economics of enhancing the conditions in greenhouses to gain the maximum rate of photosynthesis while still maintaining profit.
Without enough light, a plant cannot photosynthesise very quickly, even if there is plenty of water and carbon dioxide. Increasing the light intensity will boost the speed of photosynthesis. inverse square law - light intensity ∝ 1/distance²
Sometimes photosynthesis is limited by the concentration of carbon dioxide in the air. Even if there is plenty of light, a plant cannot photosynthesise if there is insufficient carbon dioxide.
If it gets too cold, the rate of photosynthesis will decrease. Plants cannot photosynthesise if it gets too hot.
can also be a limiting factor
the amount of chlorophyll in a plant can be affected by disease (eg infection with the tabacco mosaic virus) or environmental stress, such as lack of nutrients. these factors can cause chloroplasts to become damaged or to not make enough chlorophyll. this means the rate of photosynthesis is reduced because they cant absorb as much light
Uses of glucose from photosynthesis
The glucose produced in photosynthesis may be:
- used for respiration
- converted into insoluble starch for storage
- used to produce fat or oil for storage
- used to produce cellulose, which strengthens the cell wall
- used to produce amino acids for protein synthesis.
To produce proteins, plants also use nitrate ions that are absorbed from the soil.
Aerobic and anaerobic respiration
cellular respiration is an exothermic reaction which is continuously occurring in living cells.
The energy transferred supplies all the energy needed for living processes.
Respiration in cells can take place aerobically (using oxygen) or anaerobically (without oxygen), to transfer energy.
Organisms need energy for:
- chemical reactions to build larger molecules
- keeping warm
aerobic respiration is respiration using oxygen
it is the most efficient way to transfer energy from glucose
goes on all the time in plants and animals
most reactions in aerobic respiration happen inside the mitochondria
word equation - glucose + oxygen --------> carbon dioxide + water
symbol equation - C6H12O6 + 6O2 ------> 6CO2 + 6H2O
used if there is not enough oxygen
when you do vigorous exercise and your body cant supply enough oxygen to your muscles
anaerobic respiration is the incomplete breakdown of glucose, making lactic acid
glucose -----> lactic acid
As the oxidation of glucose is incomplete in anaerobic respiration much less energy is transferred than in aerobic respiration.
anaerobic respiration in plants and yeast cells
plants and yeast cells can respire without oxygen but they produce ethanol (alcohol) and carbon dioxide
glucose --------> ethanol + carbon dioxide
Anaerobic respiration in yeast cells is called fermentation and has economic importance in the manufacture of bread and alcoholic drinks.
Response to exercise
During exercise the human body reacts to the increased demand for energy.
The heart rate, breathing rate and breath volume increase during exercise to supply the muscles with more oxygenated blood.
If insufficient oxygen is supplied anaerobic respiration takes place in muscles. The incomplete oxidation of glucose causes a build up of lactic acid and creates an oxygen debt. During long periods of vigorous activity muscles become fatigued and stop contracting efficiently.
Blood flowing through the muscles transports the lactic acid to the liver where it is converted back into glucose. Oxygen debt is the amount of extra oxygen the body needs after exercise to react with the accumulated lactic acid and remove it from the cells.
in a cell there are lots of chemical reactions happening all the time whihc are controlled by enzymes many of these are linked together to form bigger reactions
enzyme enzyme enzyme
reactant ---------> product ---------> product ---------> product
in some of these reactions larger molecules are made from smaller ones: lots of small glucose molecules are joined together in reactions to form starch, glycogen and cellulose
in other reactions learger molecules are broken down into smaller ones: glucose is broken down in respiration. respiration transfers energy to power all the ractions in the body that make molecules
Metabolism is the sum of all the reactions in a cell or the body.
The energy transferred by respiration in cells is used by the organism for the continual enzyme controlled processes of metabolism that synthesise new molecules.
- conversion of glucose to starch, glycogen and cellulose
- the formation of lipid molecules from a molecule of glycerol and three molecules of fatty acids
- the use of glucose and nitrate ions to form amino acids which in turn are used to synthesise proteins
- breakdown of excess proteins to form urea for excretion.
Homeostasis and response
Cells in the body can only survive within narrow physical and chemical limits. They require a constant temperature and pH as well as a constant supply of dissolved food and water. In order to do this the body requires control systems that constantly monitor and adjust the composition of the blood and tissues. These control systems include receptors which sense changes and effectors that bring about changes.
In this section we will explore the structure and function of the nervous system and how it can bring about fast responses. We will also explore the hormonal system which usually brings about much slower changes. Hormonal coordination is particularly important in reproduction since it controls the menstrual cycle. An understanding of the role of hormones in reproduction has allowed scientists to develop not only contraceptive drugs but also drugs which can increase fertility.
homeostasis and response
homeostasis is the regulation of the internal conditions of a cell or organism to maintain optimum conditions for function in response to internal and external changes. Homeostasis maintains optimal conditions for enzyme action and all cell functions. In the human body, these include control of: - blood glucose concentration - body temperature - water levels.
These automatic control systems may involve nervous responses or chemical responses. All control systems include:
- cells called receptors, which detect stimuli (changes in the environment)
- coordination centres (such as the brain, spinal cord and pancreas) that receive and process information from receptors
- effectors, muscles or glands, which bring about responses which
The human nervous system
The nervous system enables humans to react to their surroundings and to coordinate their behaviour.
Information from receptors passes along cells (neurones) as electrical impulses to the central nervous system (CNS). The CNS is the brain and spinal cord. The CNS coordinates the response of effectors which may be muscles contracting or glands secreting hormones.
stimulus----> receptor-----> coordinator-----> effector-----> response
receptors detect stimuli - there are many different receptors eg taste receptors in tongue and sound receptors in ears
receptors can form part of larger complex organs eg retina of eye is covered in light receptor cells
Structure and function
central nervous system (CNS) - In vertebrates (animals with backbones) this consists of the brain and spinal cord only. in mammals, the CNS is connected to the body by sensory neurones and motor neurones
Sensory neurones - carry information as electrical impulses from the receptors to the CNS
motor neurones - carry electrical impulses from CNS to effectors
effectors - all your muscles and glands which respond to nervous impulses
effectors respond to nervous impulses and bring about a change
muscles and glands are known as effectors - they respond in diffrent ways. muscles contract in response to a nervous impulse, whereas glands secrete hormones
synapses and reflexes
synapse - the connection between two neurones. the nerve signal is transferred by chemicals which diffuse (move) across the gap. these chemicals then set off a new electrical signal in the next neurone
reflecx actions are automatic and rapid; they do not involve the conscious part of the brain.
the passage of information in a reflex (from receptor to effector) is called a reflex arc
1- the neurones in reflex arcs go through spinal cord or through an unconcious part of the brain
2- when a stimulus is detected by receptors, impulses are sent along a sensory neurone to a relay neurone in the CNS
3- when the impulses reach a synapse between sensory neurone and relay neurone they trigger chemicals to be released. these cause impulses to be sent along the relay neurone
4- when impulses reach a synapse between relay and motor neurone the same thing happens. chemicals released and impulses sent along motor neurone
5- impulses travel along motor neurone to the effector
6- muscle contracts
7- because you dont have to think about response it is quicker
The brain controls complex behaviour. It is made of billions of interconnected neurones and has different regions that carry out different functions.
cerebral cortex - outer wrinkly bit. responsible for conciousness, intelligence, memory and language
medulla - controlls unconscious activities like breathing and heartbeat (infront of spinal cord)
cerebellum - responsible for muscle contraction (small bit behind spinal cord)
investigating brain damage
Neuroscientists have been able to map the regions of the brain to particular functions by:
studying patients with brain damage-if a small part of brain has been damaged the effect it has can tell you a lot about what that part does
electrically stimulating-brain can be stimulated by pushing a tiny electrode into the tissue and giving it a small zap of electricity. by observing what stimulating different parts of brain does you get an idea of what they do
MRI scans-a magnetic resonance imaging (MRI) scanner can produce a very detailed image of the brains structures. scientists use it to find out what areas of the brain are active when people are doing different things
difficulties of investigating brain function
knowledge of how the brain works has led to the development of treatments for disorders of the nervous system
however the complexity and delicacy of the brain makes investigating and treating brain disorders very difficult. it also carries risks such as physical damage to the brain or increased problems with brain function
The eye is a sense organ containing receptors sensitive to light intensity and colour.
sclera - is the tough, supporting wall of the eye
cornea - transparent outer layer found at front of eye. it refracts (bends) light into the eye
iris - contains muscles that allow it to control the diameter of the pupil and therefore how much light enters the eye
lens - focuses the light onto the retina (which contains receptor cells sensitive to light intensity and colour)
ciliary muscles and suspensory ligaments - contol shape of lens
optic nerve - carries impulses from the receptors on the retina to the brain
focusing on near and distant objects
to look at near objects:
1-ciliary muscles contract, which slackens suspensory ligaments
2- the lens becomes fat (more curved)
3- this increases the amount by which it refracts light
to look at distant objects:
1- the ciliary muscles relax, which allows suspensory ligaments to pull tight
2- this makes the lens go thin (less curved)
3- so it refracts light by a smaller amount
if lens doesnt refract light at right amount person will be short or long sighted
unable to focus on near objects:
this occurs when the lens is the wrong shape and doesnt refract light enough or the eyeball is too short
the images of near objects are brought into focus behind the retina
you can use glasses with convex lenses (a lens which curves outwards) to correct it
the lens refracts light rays so they focus on the retina
the medical term for long-sightedness is hyperopia
unable to focus on distant objects:
this occurs when the lens is the wrong shape and refracts light too much or the eyeball is too long
the images of distant objects are brought into focus in front of the retina
you can use glasses with concave lenses (a lens which curves outwards) to correct it so light rays focus on the retina
the medical term for short-sightedness is myopia
New technologies now include hard and soft contact lenses, laser surgery to change the shape of the cornea and a replacement lens in the eye.
adaptation to light
very bright light can damage the retina - so you have a relfex to protect it
when light receptors in the eye detect very bright light, a reflex is triggered that makes the pupil smaller. the circular muscles in the iris contract and the radial muscles relax. this reduces the amount of light that can enter the eye
the opposite happens in dim light. the radial muscles contract and the circular muscles relax which makes the pupil wider to let more light into the eye
Control of body temperature
Body temperature is monitored and controlled by the thermoregulatory centre in the brain. The thermoregulatory centre contains receptors sensitive to the temperature of the blood. The skin contains temperature receptors and sends nervous impulses to the thermoregulatory centre.
If the body temperature is too high, blood vessels dilate (vasodilation) and sweat is produced from the sweat glands. Both these mechanisms cause a transfer of energy from the skin to the environment.
If the body temperature is too low, blood vessels constrict (vasoconstriction), sweating stops and skeletal muscles contract (shiver).
human endocrine system
The endocrine system is composed of glands which secrete chemicals called hormones directly into the bloodstream. The blood carries the hormone to a target organ where it produces an effect. Compared to the nervous system the effects are slower but act for longer.
The pituitary gland in the brain is a ‘master gland’ which secretes several hormones into the blood in response to body conditions. These hormones in turn act on other glands to stimulate other hormones to be released to bring about effects.
human endocrine diagram
need to know where pituitary gland, pancreas, thyroid, adrenal gland, ovary, testes are
Control of blood glucose concentration
Blood glucose concentration is monitored and controlled by the pancreas.
If the blood glucose concentration is too high, the pancreas produces the hormone insulin that causes glucose to move from the blood into the cells. In liver and muscle cells excess glucose is converted to glycogen for storage.
Type 1 diabetes is a disorder in which the pancreas fails to produce sufficient insulin. It is characterised by uncontrolled high blood glucose levels and is normally treated with insulin injections.
In Type 2 diabetes the body cells no longer respond to insulin produced by the pancreas. A carbohydrate controlled diet and an exercise regime are common treatments. Obesity is a risk factor for Type 2 diabetes.
If the blood glucose concentration is too low, the pancreas produces the hormone glucagon that causes glycogen to be converted into glucose and released into the blood.
Maintaining water and nitrogen balance in the body
Water leaves the body via the lungs during exhalation.
Water, ions and urea are lost from the skin in sweat.
There is no control over water, ion or urea loss by the lungs or skin.
Excess water, ions and urea are removed via the kidneys in the urine.
If body cells lose or gain too much water by osmosis they do not function efficiently.
The digestion of proteins from the diet results in excess amino acids which need to be excreted safely. In the liver these amino acids are deaminated to form ammonia. Ammonia is toxic and so it is immediately converted to urea for safe excretion.
The kidneys are part of the urinary system, together with the ureter, urethra and bladder The kidneys produce urine by filtration of the blood and selective reabsorption of useful substances such as glucose, some ions and water.
The renal arteries take blood with waste products to the kidneys to be filtered. Renal veins then return the filtered blood to be circulated around the body. Blood vessels take the blood though the kidneys where the waste products are removed into convoluted tubules. These tubules join together to form the ureter, which transports urine to the bladder where it is stored. Urine is then passed from the bladder to the urethra to be released.
The water level in the body is controlled by the hormone ADH which acts on the kidney tubules. ADH is released by the pituitary gland when the blood is too concentrated and it causes more water
to be reabsorbed back into the blood from the kidney tubules. This is controlled by negative feedback.
People who suffer from kidney failure may be treated by organ transplant or by using kidney dialysis. Students should know the basic principles of dialysis.
Hormones in human reproduction
During puberty reproductive hormones cause secondary sex characteristics to develop.
Oestrogen is the main female reproductive hormone produced in the ovary. At puberty eggs begin to mature and one is released approximately every 28 days. This is called ovulation.
Testosterone is the main male reproductive hormone produced by the testes and it stimulates sperm production.
Several hormones are involved in the menstrual cycle of a woman:
- Follicle stimulating hormone (FSH) causes maturation of an egg in the ovary.
- Luteinising hormone (LH) stimulates the release of the egg.
- Oestrogen and progesterone are involved in maintaining the uterus lining.
Fertility can be controlled by a variety of hormonal and non-hormonal methods of contraception.
- oral contraceptives that contain hormones to inhibit FSH production so that no eggs mature
- injection, implant or skin patch of slow release progesterone to inhibit the maturation and release of eggs for a number of months or years
- barrier methods such as condoms and diaphragms which prevent the sperm reaching an egg
- intrauterine devices which prevent the implantation of an embryo or release a hormone
- spermicidal agents which kill or disable sperm
- abstaining from intercourse when an egg may be in the oviduct
- surgical methods of male and female sterilisation
The use of hormones to treat infertility
This includes giving FSH and LH in a ‘fertility drug’ to a woman. She may then become pregnant in the normal way.
In Vitro Fertilisation (IVF) treatment.
- IVF involves giving a mother FSH and LH to stimulate the maturation of several eggs.
- The eggs are collected from the mother and fertilised by sperm from the father in the laboratory.
- The fertilised eggs develop into embryos.
- At the stage when they are tiny balls of cells, one or two embryos are inserted into the mother’s uterus (womb)
Although fertility treatment gives a woman the chance to have a baby of her own: - it is very emotionally and physically stressful - the success rates are not high - it can lead to multiple births which are a risk to both the babies and the mother
Adrenaline is produced by the adrenal glands in times of fear or stress. It increases the heart rate and boosts the delivery of oxygen and glucose to the brain and muscles, preparing the body for ‘flight or fight’.
Thyroxine from the thyroid gland stimulates the basal metabolic rate. It plays an important role in growth and development.
Thyroxine levels are controlled by negative feedback.
Plant hormones - Control and coordination
Plants produce hormones to coordinate and control growth and responses to light (phototropism) and gravity (gravitropism or geotropism). Unequal distributions of auxin cause unequal growth rates in plant roots and shoots.
Gibberellins are important in initiating seed germination.
Ethene controls cell division and ripening of fruits.
plant hormone that controls growth near the tips of shoots and roots. it controls the growth of a plant in response to light(phototropism)and gravity (gravitropism/geotropism)
shoots grow towards light: 1- when a shoot tip is exposed to light, more auxin accumulates on the side in the shade than the side in the light 2- this makes the cells grow (elongate) faster on the shaded side, so the shoot bends towards the light
shoots grow away from gravity and roots grow towards gravity 1- when a shoot is going sideways, gravity produces an unequal distribution of auxin in the tip, with more auxin on the lower side 2- this causes the lower side to grow faster, bending the shoot upwards 3- a root growing sideways will also have more auxin on the lower side 4- but in a root the extra auxin inhibits growth. this means the cells on top elongate faster, and the root bends downwards
uses of plant hormones
Plant growth hormones are used in agriculture and horticulture.
Auxins are used:
- as weed killers
- as rooting powders
- for promoting growth in tissue culture.
Ethene is used in the food industry to control ripening of fruit during storage and transport.
Gibberellins can be used to:
- end seed dormancy
- promote flowering
- increase fruit size.
Inheritance, variation and evolution
In this section we will discover how the number of chromosomes are halved during meiosis and then combined with new genes from the sexual partner to produce unique offspring. Gene mutations occur continuously and on rare occasions can affect the functioning of the animal or plant. These mutations may be damaging and lead to a number of genetic disorders or death. Very rarely a new mutation can be beneficial and consequently, lead to increased fitness in the individual. Variation generated by mutations and sexual reproduction is the basis for natural selection; this is how species evolve.
An understanding of these processes has allowed scientists to intervene through selective breeding to produce livestock with favoured characteristics. Once new varieties of plants or animals have been produced it is possible to clone individuals to produce larger numbers of identical individuals all carrying the favourable characteristic.
Scientists have discovered how to take genes from one species and introduce them into the genome of another by a process called genetic engineering. In spite of lots of potential benefits that th3 technology can offer, genetic modification is still controversial.
Sexual and asexual reproduction
meiosis leads to non-identical cells being formed while mitosis leads to identical cells being formed. Sexual reproduction involves the joining (fusion) of male and female gametes: - sperm and egg cells in animals - pollen and egg cells in flowering plants.
In sexual reproduction there is mixing of genetic information which leads to variety in the offspring. The formation of gametes involves meiosis. Asexual reproduction involves only one parent and no fusion of gametes. There is no mixing of genetic information. This leads to genetically identical offspring (clones). Only mitosis is involved
Some organisms reproduce by both methods depending on the circumstances. - Malarial parasites reproduce asexually in the human host, but sexually in the mosquito. - Many fungi reproduce asexually by spores but also reproduce sexually to give variation. - Many plants produce seeds sexually, but also reproduce asexually by runners such as strawberry plants, or bulb division eg daffodils.
meiosis halves the number of chromosomes in gametes and fertilisation restores the full number of chromosomes.
Cells in reproductive organs divide by meiosis to form gametes.
When a cell divides to form gametes:
- copies of the genetic information are made
- the cell divides twice to form four gametes, each with a single set of chromosomes
- all gametes are genetically different from each other.
Gametes join at fertilisation to restore the normal number of chromosomes. The new cell divides by mitosis. The number of cells increases. As the embryo develops cells differentiate.
Advantages/disadvantages of sexual/asexual reprodu
Advantages of sexual reproduction:
- produces variation in the offspring
- if the environment changes variation gives a survival advantage by natural selection
- natural selection can be speeded up by humans in selective breeding to increase food production.
Advantages of asexual reproduction:
- only one parent needed
- more time and energy efficient as do not need to find a mate
- faster than sexual reproduction
- many identical offspring can be produced when conditions are favourable
DNA and the genome
The genetic material in the nucleus of a cell is composed of a chemical called DNA. DNA is a polymer made up of two strands forming a double helix. The DNA is contained in structures called chromosomes.
A gene is a small section of DNA on a chromosome. Each gene codes for a particular sequence of amino acids, to make a specific protein.
The genome of an organism is the entire genetic material of that organism. The whole human genome has now been studied and this will have great importance for medicine in the future.
importance of understanding the human genome
1 - it allows scientists to indentify genes in the genome that are linked to the diffrent type of diseases
2 - knowing which genes are linked to inherited diseases could help us to understand them better and develop effective treatments for them
3 - scientists can look at genomes to trace the migration of certain populations of people around the world.
all modern humans are descended from a common ancestor who lived in africa but humans are now all over the planet. the human genome is mostly identical in all individuals but as different populations of people migrated away from africa they grdually developed tiny diffrences in their genomes
by investigating these diffrences scientists can work out when new populations split off in a different direction and what route they took
DNA is a polymer made from four different nucleotides. Each nucleotide consists of a common sugar and phosphate group with one of four different bases attached to the sugar.
DNA contains four bases, A, C, G and T.
A sequence of three bases is the code for a particular amino acid. The order of bases controls the order in which amino acids are assembled to produce a particular protein.
The long strands of DNA consist of alternating sugar and phosphate sections. Attached to each sugar is one of the four bases.
The DNA polymer is made up of repeating nucleotide units.
The DNA code for the protein remains in the nucleus, but a copy, called mRNA, moves from the nucleus to the ribosomes where proteins are synthesised in the cytoplasm. The protein produced depends on the template used, and if this sequence changes a different protein will be made.
Carrier molecules bring specific amino acids to add to the growing protein in the correct order. There are only about 20 different naturally-occurring amino acids.
DNA structure determines the protein synthesised. If this changes a different protein will be made. A copy of the DNA is made, but is now mRNA. The copy moves to the ribosome into to the cytoplasm. Amino acids are connected together in a specific order at the ribosome to create a specific protein molecule.
Each protein molecule has hundreds, or even thousands, of amino acids joined together in a unique sequence. It is then folded into the correct unique shape. This is very important, as it allows the protein to do their jobs eg enzymes or hormones and it can form structures within the body like collagen.
Effect of genetic variants on phenotype
The structure of DNA is important in synthesising specific proteins needed in biological processes.
Not all parts of the DNA code for proteins, there is a coding and non-coding part of DNA, which can switch genes on and off, so variations in these areas may affect gene expression, and if the correct protein is synthesised or not.
In different cells around the body, genes will be switched on and others will be switched off. This will vary depending on which cells you examine.
Mutations occur continuously. Most do not alter the protein, or only alter it slightly so that its appearance or function is not changed.
A few mutations code for an altered protein with a different shape. An enzyme may no longer fit the substrate binding site or a structural protein may lose its strength.
Not all parts of DNA code for proteins. Non-coding parts of DNA can switch genes on and off, so variations in these areas of DNA may affect how genes are expressed.
Genetic inheritance key words
gamete - a sex cell
chromosome - long molecules of DNA
gene - short section of DNA that codes for a protein
allele - different variations of the same gene
dominant - the allele expressed if present
recessive - the allele only expressed if both alleles are the same homozygous - when the two alleles present are the same heterozygous - when the two alleles present are different genotype - the alleles present on the two chromosomes for a particular characteristic
phenotype - the appearance of a particular characteristic, how it is expressed
Some characteristics are controlled by a single gene, such as: fur colour in mice; and red-green colour blindness in humans. Each gene may have different forms called alleles.
The alleles present, or genotype, operate at a molecular level to develop characteristics that can be expressed as a phenotype.
A dominant allele is always expressed, even if only one copy is present. A recessive allele is only expressed if two copies are present (therefore no dominant allele present).
If the two alleles present are the same the organism is homozygous for that trait, but if the alleles are different they are heterozygous.
Most characteristics are a result of multiple genes interacting, rather than a single gene.
Some disorders are inherited. These disorders are caused by the inheritance of certain alleles.
- Polydactyly (having extra fingers or toes) is caused by a dominant allele.
- Cystic fibrosis (a disorder of cell membranes) is caused by a recessive allele.
Ordinary human body cells contain 23 pairs of chromosomes.
22 pairs control characteristics only, but one of the pairs carries the genes that determine sex.
- In females the sex chromosomes are the same (**)
- In males the chromosomes are different (XY)
1. all plants and animals have charcteristics that are similar to their parents
2. this is because an organisms characteristics are determined by the genes inherited from their parents.
3. these genes are passed on in gametes from which the offspring develop
4. most animals (and many plants) get some genes from mother and some from father
5. this combining of genes from two parents causes genetic variation
6. some charcteristics are determined only by genes. in animals these include: eye colour, blood group and inherited disorders
causes of variation
Differences in the characteristics of individuals in a population is called variation and may be due to differences in:
- the genes they have inherited (genetic causes)
- the conditions in which they have developed (environmental causes)
- a combination of genes and the environment.
there is usually extensive genetic variation within a population of a species
all variants arise from mutations and that: most have no effect on the phenotype; some influence phenotype; very few determine phenotype. Mutations occur continuously. Very rarely a mutation will lead to a new phenotype. If the new phenotype is suited to an environmental change it can lead to a relatively rapid change in the species.
evolution is a change in the inherited characteristics of a population over time through a process of natural selection which may result in the formation of a new species.
The theory of evolution by natural selection states that all species of living things have evolved from simple life forms that first developed more than three billion years ago.
If two populations of one species become so different in phenotype that they can no longer interbreed to produce fertile offspring they have formed two new species.
- Individuals in a species show a wide range of variation and this variation is because of differences in their genes.
- Individuals with characteristics most suited to their environment are more likely to survive and reproduce. This is commonly known as 'survival of the fittest'. The genes that allow these individuals to be successful within their environment are passed on to their offspring, which results in these specific genes becoming more common.
- Those that are poorly adapted to their environment are less likely to survive and reproduce. Their genes are less likely to be passed on to the next generation.
- Over a period of time, a species will gradually evolve.
- Both genes and the environment can cause variation, but only genetic variation can be passed on to the next generation.
- If two populations of one species become increasingly different in phenotype that they can no longer interbreed to form fertile offspring, this can result in the formation of two species.
Selective breeding (artificial selection) is the process by which humans breed plants and animals for particular genetic characteristics. Humans have been doing this for thousands of years since they first bred food crops from wild plants and domesticated animals.
Selective breeding involves choosing parents with the desired characteristic from a mixed population. They are bred together. From the offspring those with the desired characteristic are bred together. This continues over many generations until all the offspring show the desired characteristic.
The characteristic can be chosen for usefulness or appearance:
- Disease resistance in food crops.
- Animals which produce more meat or milk.
- Domestic dogs with a gentle nature.
- Large or unusual flowers.
- Selective breeding can lead to ‘inbreeding’ where some breeds are particularly prone to disease or inherited defects.
genetic engineering is a process which involves modifying the genome of an organism by introducing a gene from another organism to give a desired characteristic.
Plant crops have been genetically engineered to be resistant to diseases or to produce bigger better fruits.
Bacterial cells have been genetically engineered to produce useful substances such as human insulin to treat diabetes.
In genetic engineering, genes from the chromosomes of humans and other organisms can be ‘cut out’ and transferred to cells of other organisms.
genetic engineering - risks/benefits
Crops that have had their genes modified in this way are called genetically modified (GM) crops. GM crops include ones that are resistant to insect attack or to herbicides. GM crops generally show increased yields.
Concerns about GM crops include the effect on populations of wild flowers and insects. Some people feel the effects of eating GM crops on human health have not been fully explored.
Modern medical research is exploring the possibility of genetic modification to overcome some inherited disorders.
main steps in genetic engineering.
In genetic engineering:
- enzymes are used to isolate the required gene; this gene is inserted into a vector, usually a bacterial plasmid or a virus
- the vector is used to insert the gene into the required cells
- genes are transferred to the cells of animals, plants or microorganisms at an early stage in their development so that they develop with desired characteristics
Tissue culture: using small groups of cells from part of a plant to grow identical new plants. This is important for preserving rare plant species or commercially in nurseries.
Cuttings: an older, but simple, method used by gardeners to produce many identical new plants from a parent plant.
Embryo transplants: splitting apart cells from a developing animal embryo before they become specialised, then transplanting the identical embryos into host mothers.
Adult cell cloning: - The nucleus is removed from an unfertilised egg cell. - The nucleus from an adult body cell, such as a skin cell, is inserted into the egg cell. - An electric shock stimulates the egg cell to divide to form an embryo. - These embryo cells contain the same genetic information as the adult skin cell. - When the embryo has developed into a ball of cells, it is inserted into the womb of an adult female to continue its development
Darwins theory of evolution
Charles Darwin, as a result of observations on a round the world expedition, backed by years of experimentation and discussion and linked to developing knowledge of geology and fossils, proposed the theory of evolution by natural selection.
- Individual organisms within a particular species show a wide range of variation for a characteristic.
- Individuals with characteristics most suited to the environment are more likely to survive to breed successfully.
- The characteristics that have enabled these individuals to survive are then passed on to the next generation.
explanation for giraffes long neck:
- giraffe with long neck can reach food higher up
- giraffe more likely to get enough food to survive to reproduce
- giraffes offspring inherit long neck
why Darwin's theory wasn't accepted
Darwin published his ideas in On the Origin of Species (1859). There was much controversy surrounding these revolutionary new ideas.
The theory of evolution by natural selection was only gradually accepted because:
- the theory challenged the idea that God made all the animals and plants that live on Earth
- there was insufficient evidence at the time the theory was published to convince many scientists
- the mechanism of inheritance and variation was not known until 50 years after the theory was published.
other theories - Lamarck
Other theories, including that of Jean-Baptiste Lamarck, are based mainly on the idea that changes that occur in an organism during its lifetime can be inherited. We now know that in the vast majority of cases this type of inheritance cannot occur.
Lamarck's theroy said that a characteristic that is used more and more by an organism becomes bigger and stronger. one that isnt disappears eventually. any characteristic of an organism that is improved through use is passed on to offspring
explanation for long necks in giraffes
1. a giraffe stretches its neck to reach food high up
2. giraffes neck gets longer because its used a lot
3. giraffes offspring inherits its long neck
Alfred Russel Wallace
Alfred Russel Wallace independently proposed the theory of evolution by natural selection. He published joint writings with Darwin in 1858 which prompted Darwin to publish On the Origin of Species (1859) the following year.
Wallace worked worldwide gathering evidence for evolutionary theory. He is best known for his work on warning colouration in animals and his theory of speciation.
Alfred Wallace did much pioneering work on speciation but more evidence over time has led to our current understanding of the theory of speciation.
speciation and isolation
speciation - as long as a population has the opportunity to interbreed and exchange genes, they remain one species. A population of one species can only evolve into more than one species if groups within the population become isolated from each other by barriers that prevent exchange of genes
three types of isolating barrier:
GEOGRAOHICAL - features such as rivers or mountain ranges isolate groups. movement of land-masses by continental drift led to geographica changes millions of years ago
ECOLOGICAL - occupying different habitats or breeding areas, pH, salinity
REPRODUCTIVE - breeding between groups within a population may not be possible because of differences in courtship behaviour, physical differences which prevent mating, or failure of gametes to fuse
The understanding of genetics
In the mid-19th century Gregor Mendel carried out breeding experiments on plants. One of his observations was that the inheritance of each characteristic is determined by ‘units’ that are passed on to descendants unchanged. each characteristic is determined by ‘units’ that are passed on to descendants unchanged.
In the late 19th century behaviour of chromosomes during cell division was observed.
In the early 20th century it was observed that chromosomes and Mendel’s ‘units’ behaved in similar ways. This led to the idea that the ‘units’, now called genes, were located on chromosomes.
In the mid-20th century the structure of DNA was determined and the mechanism of gene function worked out.
This scientific work by many scientists led to the gene theory being developed.
Evidence for evolution
The theory of evolution by natural selection is now widely accepted.
Evidence for Darwin’s theory is now available as it has been shown that characteristics are passed on to offspring in genes.
There is further evidence in the fossil record and the knowledge of how resistance to antibiotics evolves in bacteria.
Fossils are the ‘remains’ of organisms from millions of years ago, which are found in rocks.
Fossils may be formed:
- from parts of organisms that have not decayed because one or more of the conditions needed for decay are absent
- when parts of the organism are replaced by minerals as they decay
- as preserved traces of organisms, such as footprints, burrows and rootlet traces.
Many early forms of life were soft-bodied, which means that theyhave left few traces behind. What traces there were have been mainly destroyed by geological activity. This is why scientists cannot be certain about how life began on Earth.
We can learn from fossils how much or how little different organisms have changed as life developed on Earth.
Extinctions occur when there are no remaining individuals of a species still alive. reasons species become extinct:
- environmental changes
- new predator
- new disease
- cant compete with another specie for food
- catastrophic event eg volcano
Bacteria can evolve rapidly because they reproduce at a fast rate.
Mutations of bacterial pathogens produce new strains. Some strains might be resistant to antibiotics, and so are not killed. They survive and reproduce, so the population of the resistant strain rises. The resistant strain will then spread because people are not immune to it and there is no effective treatment.
MRSA is resistant to antibiotics.
To reduce the rate of development of antibiotic resistant strains:
- doctors should not prescribe antibiotics inappropriately, such as treating non-serious or viral infections
- patients should complete their course of antibiotics so all bacteria are killed and none survive to mutate and form resistant strains
- the agricultural use of antibiotics should be restricted.
The development of new antibiotics is costly and slow. It is unlikely to keep up with the emergence of new resistant strains.
Classification of living organisms
Traditionally living things have been classified into groups depending on their structure and characteristics in a system developed by Carl Linnaeus.
Linnaeus classified living things into kingdom, phylum, class, order, family, genus and species. Organisms are named by the binomial system of genus and species.
The five kingdoms are:
- animals (all multicellular animals)
- plants (all green plants)
- fungi (moulds, mushrooms, yeast)
- protists (Amoeba, Chlorella and Plasmodium)
- prokaryotes (bacteria, blue-green algae)
phylum, class, order
Phylum follows Kingdoms and has many different organisms, including three examples below:
- Chordata, which have backbones
- Arthropod, which have jointed legs and an exoskeleton
- Annelids, which are segmented worms
Class is an additional sub-division, which for example, results in the Chordata phylum being divided into: Mammals, Birds, Amphibians, Fish, Reptiles
Order follows class and as an example, mammals can be further sub-divide into a variety of different groups such as: - Carnivores, primates
Orders are broken down into families. Here are a few examples of which carnivores can be divided into:
Canidae - dogs
Felidae - cats
Genus, the Felidae family can be further sub-divided into four genus examples:
- Acinonyx - cheetah
- Panthera - lion and tiger
- Neofelis - clouded leopard
- Felis - domestic cats
Species is the final classification stage, and the genus Panthera can be divided into:
- Panthera leo (lion)
- Panthera tigris (tiger)
impact of developments in biology on classificatio
As evidence of internal structures became more developed due to improvements in microscopes, and the understanding of biochemical processes progressed, new models of classification were proposed.
Due to evidence available from chemical analysis there is now a ‘three- domain system’ developed by Carl Woese. In this system organisms are divided into:
- archaea (primitive bacteria usually living in extreme environments)
- bacteria (true bacteria)
- eukaryota (which includes protists, fungi, plants and animals)
Evolutionary trees are a method used by scientists to show how they believe organisms are related. They use current classification data for living organisms and fossil data for extinct organisms.
The Sun is a source of energy that passes through ecosystems. Materials including carbon and water are continually recycled by the living world, being released through respiration of animals, plants and decomposing microorganisms and taken up by plants in photosynthesis.
All species live in ecosystems composed of complex communities of animals and plants dependent on each other and that are adapted to particular conditions, both abiotic and biotic. These ecosystems provide essential services that support human life and continued development.
In order to continue to benefit from these services humans need to engage with the environment in a sustainable way. In this section we will explore how humans are threatening biodiversity as well as the natural systems that support it. We will also consider some actions we need to take to ensure our future health, prosperity and well-being.
Adaptations, interdependence, competition -Communi
An ecosystem is the interaction of a community of living organisms (biotic) with the non-living (abiotic) parts of their environment.
To survive and reproduce, organisms require a supply of materials from their surroundings and from the other living organisms there.
Plants in a community or habitat often compete with each other for light and space, and for water and mineral ions from the soil. Animals often compete with each other for food, mates and territory.
Within a community each species depends on other species for food, shelter, pollination, seed dispersal etc. If one species is removed it can affect the whole community. This is called interdependence. A stable community is one where all the species and environmental factors are in balance so that population sizes remain fairly constant.
Abiotic (non-living) factors which can affect a community are:
- light intensity
- moisture levels
- soil pH and mineral content
- wind intensity and direction
- carbon dioxide levels for plants
- oxygen levels for aquatic animals
Biotic (living) factors which can affect a community are:
- availability of food
- new predators arriving
- new pathogens
- one species outcompeting another so the numbers are no longer sufficient to breed.
Organisms have features (adaptations) that enable them to survive in the conditions in which they normally live. These adaptations may be structural, behavioural or functional.
Some organisms live in environments that are very extreme, such as at high temperature, pressure, or salt concentration. These organisms are called extremophiles. Bacteria living in deep sea vents are extremophiles.
Levels of organisation
Feeding relationships within a community can be represented by
food chains. All food chains begin with a producer which synthesises molecules. This is usually a green plant or alga which makes glucose by photosynthesis.
A range of experimental methods using transects and quadrats are used by ecologists to determine the distribution and abundance of species in an ecosystem.
Producers are eaten by primary consumers, which in turn may be eaten by secondary consumers and then tertiary consumers.
Consumers that kill and eat other animals are predators, and those eaten are prey. In a stable community the numbers of predators and prey rise and fall in cycles.
How materials are cycled
many different materials cycle through the abiotic and biotic components of an ecosystem
All materials in the living world are recycled to provide the building blocks for future organisms.
The carbon cycle returns carbon from organisms to the atmosphere as carbon dioxide to be used by plants in photosynthesis.
The water cycle provides fresh water for plants and animals on land before draining into the seas. Water is continuously evaporated and precipitated.
materials decay because they are broken down (digested) by microorganisms. this happens faster in warm, moist, aerobic conditions because this is when microorganisms are more active
Gardeners and farmers try to provide optimum conditions for rapid decay of waste biological material. The compost produced is used as a natural fertiliser for growing garden plants or crops.
Anaerobic decay produces methane gas. Biogas generators can be used to produce methane gas as a fuel.
rate of decay
the rate of decay is affected by:
temperature - warmer temperatures make things decompose quicker because they increase the rate that the enzymes involved in decomposition work at. if its too hot decomposition slows down or stops because enzymes are destroyed and the organisms die. really cold temperatures slow the rate of decomposition
oxygen availability - many organisms need oxygen to respire which they need to do to survive. the microorganisms involved in anaerobic decay dont need oxygen though
water availability - decay takes place faster in moist environments because the organisms involved in decay need water to carry out biological processes
number of decay organisms - the more microorganisms and detritus feeders there are the faster decomposition happens
Impact of environmental change
Environmental changes affect the distribution of species in an ecosystem. These changes include:
- temperature - eg the distribution of bird species in germany is changing because of a rise in average temperature
- availability of water - eg the distribution of some animal and plant species in the tropics changes between the wet and dry seasons
- composition of atmospheric gases - eg the distribution of some species changes in areas where there is more air pollution
The changes may be seasonal, geographic or caused by human interaction.
Biodiversity is the variety of all the different species of organisms on earth, or within an ecosystem.
A great biodiversity ensures the stability of ecosystems by reducing the dependence of one species on another for food, shelter and the maintenance of the physical environment.
The future of the human species on Earth relies on us maintaining
a good level of biodiversity. Many human activities are reducing biodiversity and only recently have measures been taken to try to stop this reduction.
Rapid growth in the human population and an increase in the standard of living mean that increasingly more resources are used and more waste is produced. Unless waste and chemical materials are properly handled, more pollution will be caused.
Pollution can occur:
- in water, from sewage, fertiliser or toxic chemicals
- in air, from smoke and acidic gases
- on land, from landfill and from toxic chemicals.
Pollution kills plants and animals which can reduce biodiversity.
Humans reduce the amount of land available for other animals and plants by building, quarrying, farming and dumping waste.
The destruction of peat bogs, and other areas of peat to produce garden compost, reduces the area of this habitat and thus the variety of different plant, animal and microorganism species that live there (biodiversity)
The decay or burning of the peat releases carbon dioxide into the atmosphere.
Deforestation and Global warming
Large-scale deforestation in tropical areas has occurred to:
- provide land for cattle and rice fields
- grow crops for biofuels.
Levels of carbon dioxide and methane in the atmosphere are increasing, and contribute to ‘global warming’ consequences of global warming:
- higher temperatures cause seawater to expand and ice to melt, causing sea levels to rise
- the distribution of many wild animal and plant species may change as temperatures increase and the amount of rainfall changes in different areas
- changes in migration patterns - birds will migrate further north
- biodiversity - could be reduced if some species are unable to survive a change in the climate, so become extinct
Scientists and concerned citizens have put in place programmes to reduce the negative effects of humans on ecosystems and biodiversity.
- breeding programmes for endangered species
- protection and regeneration of rare habitats
- reintroduction of field margins and hedgerows in agricultural areas where farmers grow only one type of crop
- reduction of deforestation and carbon dioxide emissions by some governments
- recycling resources rather than dumping waste in landfill
Trophic levels in an ecosystem
Trophic levels can be represented by numbers, starting at level 1 with plants and algae. Further trophic levels are numbered subsequently according to how far the organism is along the food chain.
Level 1: Plants and algae make their own food and are called producers.
Level 2: Herbivores eat plants/algae and are called primary consumers.
Level 3: Carnivores that eat herbivores are called secondary consumers.
Level 4: Carnivores that eat other carnivores are called tertiary consumers. Apex predators are carnivores with no predators.
Decomposers break down dead plant and animal matter by secreting enzymes into the environment. Small soluble food molecules then diffuse into the microorganism.
Pyramids of biomass
Pyramids of biomass can be constructed to represent the relative amount of biomass in each level of a food chain. Trophic level 1 is at the bottom of the pyramid.
Transfer of biomass
Producers are mostly plants and algae which transfer about 1 % of the incident energy from light for photosynthesis.
Only approximately 10 % of the biomass from each trophic level is transferred to the level above it.
Losses of biomass are due to:
- not all the ingested material is absorbed, some is egested as faeces
- some absorbed material is lost as waste, such as carbon dioxide and water in respiration and water and urea in urine.
Large amounts of glucose are used in respiration efficiency = biomass transferred to the next level --------------------------------------- x100 biomass available at the previous level
Factors affecting food security
Food security is having enough food to feed a population.
Biological factors which are threatening food security include:
- the increasing birth rate has threatened food security in some countries
- changing diets in developed countries means scarce food resources are transported around the world
- new pests and pathogens that affect farming
- environmental changes that affect food production, such as widespread famine occurring in some countries if rains fail
- the cost of agricultural inputs
- conflicts that have arisen in some parts of the world which affect the availability of water or food.
Sustainable methods must be found to feed all people on Earth.
The efficiency of food production can be improved by restricting energy transfer from food animals to the environment. This can be done by limiting their movement and by controlling the temperature of their surroundings.
Some animals are fed high protein foods to increase growth.
Fish stocks in the oceans are declining. It is important to maintain fish stocks at a level where breeding continues or certain species may disappear altogether in some areas.
Control of net size and the introduction of fishing quotas play important roles in conservation of fish stocks at a sustainable level.
Role of biotechnology
Modern biotechnology techniques enable large quantities of microorganisms to be cultured for food.
The fungus Fusarium is useful for producing mycoprotein, a protein-rich food suitable for vegetarians. The fungus is grown on glucose syrup, in aerobic conditions, and the biomass is harvested and purified.
A genetically modified bacterium produces human insulin. When harvested and purified this is used to treat people with diabetes.
GM crops could provide more food or food with an improved nutritional value such as golden rice.