Biology Additional Edexcel

HideShow resource information

Plant and animal cells

Bacterial Cell Structure

Bacterial cells are much smaller than plant or animal cells. They were first seen under a microscope by Anton van Leeuwenhoek in 1676. As microscopes have improved, scientists have come to understand bacterial cell structure better.

Using electron microscopes we now know that bacteria have a cell wall. This is similar to a plant cell wall but is more flexible. Bacteria do not have a nucleus. They do have two types of DNA – plasmid and chromosomal. The chromosomal DNA carries most of the genetic information. Plasmid DNA forms small loops and carries extra information. Some bacteria have a flagellum – a whip like tail. This helps the bacteria to move itself along. When we talk about these flagellum tails in multiple bacteria, we call them flagella.

1 of 50

Cells and Magnification

We can calculate the length of a magnified object by using the magnification of the lens. Length of object = length of magnified object ÷ magnification  For example, if a specimen appeared 10mm in length under a microscope with a magnification of 1,000 times, the calculation of the actual length would be:   Length of object = 10 ÷ 1000 = 0.01 mm

both cells have a membrane on the outside, and cytoplasm and a nucleus inside. In the plant only are the cell wall, vacuole, and chloroplast.  (

2 of 50


DNA is a long molecule made up of twisted strands of the bases A, T, C and G.

Genes are sections of the DNA and each gene has the code for creating a specific protein. The sequence of bases in the gene controls which amino acids are created and joined to make a specific new protein or enzyme molecule.

Enzymes are large molecules that speed up the chemical reactions inside cells. Each type of enzyme does one specific job. Enzymes are a type of protein, and like all proteins, they are made from long chains of different amino acids.

DNA molecules are very long but are packed into compact structures called chromosomes. Each DNA molecule consists of two twisted strands of bases that form a shape called a double helix.

The two strands are held together by hydrogen bonds between pairs of bases.

When a cell grows and divides into two, it first has to make a duplicate copy of each DNA molecule. This is done by the bonds breaking between the two strands, the strands unwinding, and then new bases joining each old strand to make new strands.

3 of 50

DNA and Enzymes

Aerobic respiration

Most of our energy comes from the process of aerobic respiration, which takes place inside the mitochondria of cells. Mitochondria are oval structures with a folded inner membrane. They occur in large numbers inside muscle cells and liver cells, where a lot of energy is needed. The mitochondria hold a large number of different enzymes which are responsible for different stages of respiration.

Enzyme structure

Enzymes are soluble protein molecules that can speed up chemical reactions in cells. Like all proteins they consist of a string of different amino acid sub units.

The amino acids must occur in the correct sequence for the enzyme to work. This sequence is controlled by the genetic code for the protein. These genetic codes are held in DNA molecules.

4 of 50

Genetic code & DNA

Genetic Code

If A occurs in one strand of a DNA molecule it will form a bond with T in the other strand. C and G will also only bond with each other. In this way it's possible to predict the order of bases on the other strand if the bases on one strand are known.

The sequence of bases in one section (gene) of a DNA molecule controls the sequence of amino acids in one protein molecule. Each amino acid has its own code of three bases. Every time the same sequence of three bases occurs in the DNA molecule, the same amino acid is added to make up the protein.

The discovery of DNA

Throughout the 20th century, many scientists have tried to study deoxyribonucleic acid (DNA). In the early 1950s two scientists, Rosalind Franklin and Maurice Wilkins, studied DNA using x-rays.

Franklin produced an x-ray photograph that allowed two other researchers, James Watson and Francis Crick to work out the 3D structure of DNA. The structure of DNA was found to be a double helix.

In 1962 Crick and Watson, along with Wilkins, received the Nobel Prize in Physiology or Medicine for their discovery. Rosalind Franklin had died four years earlier and her pivotal contribution wasn't acknowledged until much later 

5 of 50

Genetic enginereeing

How it works

Certain enzymes can cut pieces of DNA containing a particular gene from one organism, and join them into a gap in the DNA of another organism. This means that the new organism with the inserted genes has the genetic information for one or more new characteristics. For example, the organism might produce a useful substance, or be able to carry out a new function. We say that the organism has been genetically modified.

The animation shows how the method can be used to produce bacteria that produce insulin. This is a human hormone and valuable to people with diabetes. Bacteria reproduce quickly, so a lot of insulin can be made quickly.


There are strong arguments for and against cloning and genetic engineering. It is possible to produce genetically modified animals and plants. Sheep that produce human proteins for treating the symptoms of cystic fibrosis (a disease which causes sufferers to produce abnormally thick and sticky mucus in their lungs, leading to many health problems) have been produced, and even tobacco plants that glow in the dark when they need watering. Some people are excited by the almost limitless possibilities, while some people believe the process is unethical and should be banned. Others are concerned about what might happen in the future.

6 of 50

Genetic modification

GM insulin

Natural insulin can be taken from the pancreas of a pig or cow. It is used to treat diabetes but is limited in supply and doesn't suit all people.

Modern practice is to create insulin synthetically (non naturally or man-made), using genetically modified (GM) bacteria. The gene for insulin secretion is cut from a length of human DNA and inserted into the DNA of a bacterium. The bacterium is then cultivated and soon there are millions of bacteria producing human insulin.

GM insulin has some advantages over insulin taken from pigs or cattle:

It is easier to create high quantities of insulin

It is less likely to cause an adverse reaction

It overcomes ethical concerns from vegetarians and others

7 of 50

Genetic modification

Wild rice

Scientists have added a gene to wild rice that makes it produce beta carotene. This changes the colour of the wild rice to a golden colour. Beta carotene is needed by humans in order to make Vitamin A. The advantage of golden rice is that it can be used in areas where Vitamin A deficiency is common and so can help prevent blindness.

  • Fears that it will crossbreed with and contaminate wild rice
  • Worries that GM organisms might harm people
  • Beta carotene levels aren't high enough to make a difference
  • GM organisms can be expensive

Herbicide resistant crops

Scientists have added genes to crop plants that make them resistant to herbicides. This reduces the quantity of herbicide that needs to be used. Potential disadvantages of this genetic modification include:

  • The potential development of herbicide-resistant weeds
  • Loss of biodiversity as fewer weed species survive as a food and shelter source for animals
8 of 50

Mitosis and meiosis

There are two types of cell division. Mitosis is used for growth and repair and produces diploid cells identical to each other and the parent cell. Meiosis is used for sexual reproduction and produces haploid cells different to each other and the parent cell.

Humans are made of millions of cells. This has a number of benefits:

  • Cells can be specialised to do particular tasks
  • Groups of cells can function as organs making a more efficient but complex organism
  • The organism can grow very large

Cell division

New cells are needed throughout life. These are for growth, to replace damaged cells and repair worn out tissues. Normal human body cells are diploid – they have two of each chromosome. When new cells are made, these 46 chromosomes (in other organisms the number is different) are copied exactly in a process called mitosis.


9 of 50


 Mitosis is the type of cell division used for growth, repair and asexual reproduction. Mitosis occurs wherever new cells are needed. It produces two cells that are identical to each other, and the parent cell.

In mitosis each chromosome is copied exactly. The new chromosomes are moved to opposite sides of the cell, before the cell divides leaving one complete set of 46 chromosomes in each of the two new cells.

Constant cell division ensures that cells never become too large. The larger the cell becomes, the smaller its surface-area-to-volume ratio. Objects with this small ratio find it difficult to maintain exchange of materials with their environment. Large cells could run out of oxygen, and accumulate too much waste, such as carbon dioxide. For this reason it is more efficient for large organisms to be multicellular, rather than single-celled.

10 of 50

The formation of sex cells

In humans all reproduction is sexual. It involves joining together haploid gamete cells from each parent with half the normal number of chromosomes to make a new cell containing both parents genetic material. This is a diploid zygote.

The cells from each parent that combine to form the zygote are called gametes. In humans, the male gamete is called sperm, and the female gamete is called an egg. When the gametes join they form a cell called a zygote. Human sperm and eggs contain 23 chromosomes. Human zygotes contain 46 chromosomes.

The type of cell division that produces gametes with half the normal chromosome number is called meiosis. It is used to produce cells for repair and asexual reproduction.

Gametes contain different genetic information to each other and to the parent cell.

Meiosis is responsible for causing genetic variation.

Useful terms :

  • Gamete - Cell with half the normal number of chromosomes, and only used for sezual reproduction
  • Zygote - Cell formed when two gametes combine
  • Fertilisation - Term to describe the joining of two gametes
  • Haploid - Having half the normal number of chromosomes
  • Diploid - Having the normal number of chromosomes
11 of 50


Meiosis is a different kind of cell division. It is used to produce male and female gametes. A human body cell contains 46 chromosomes arranged in 23 pairs. The gametes are sperm or eggs, and only contain half as many chromosomes (23). This is why meiosis is sometimes called reduction division.

At fertilisation, the nuclei of the sperm and an egg join to form the zygote. The zygote contains 23 pairs of chromosomes - 23 single chromosomes from the sperm, and 23 single chromosomes from the egg, thereby creating the correct number of 46 chromosomes for all body cells. It also means the zygote contains a complete set of chromosomes from each parent.

12 of 50

The stages of cloning

In 1996 the first large animal was cloned. It was called Dolly, the sheep. Many attempts had been made before the stages of cloning were properly understood. The stages include:

  • Removal of diploid nucleus from a body cell
  • Enucleation, or removal, of egg cell
  • Insertion of diploid nucleus into enucleated egg cell
  • Stimulation of the diploid nucleus to divide by mitosis

In this famous example of cloning, an ordinary cell was used to replace the nucleus of an egg cell, so all of Dolly's cells had identical DNA to the one parent that donated the cell. This form of cloning produced a single, genetically identical, offspring.

Animal cloning raises ethical issues about how far humans should be allowed to interfere in the production of new life. Regulations currently restrict scientific research into human cloning.

Cloning plants is easier than cloning animals. Cloning expensive food crops has been carried out for many years, and causes the public fewer ethical and moral concerns than animal cloning.

13 of 50

How to clone cows using embryo transplants

This technique could be used to make many copies of cows that have a high milk yield. It would produce a herd of cows much faster than if the original cow was used for breeding in the normal way. Sexual reproduction is still involved and the calves are not identical to either parent.

Producing many genetically modified pigs

Pigs may be able to grow replacement organs for use in human transplant surgery. The pigs would first have human genes inserted into their cells so that the organs would not be rejected when transplanted into a human. Inserting human genes into a pig is very difficult. Once it has been done successfully the pig would be cloned so that many copies of the organ could be grown.

14 of 50

Advantages and disadvantages of cloning

Advantage or disadvantage  Cloning situation Advantage

All the new plants are genetically identical – they will all have the desired characteristics.


Organisms that are difficult or slow to breed normally can be reproduced quickly. Some plant varieties do not produce seeds, others have seeds that are dormant for long periods.


If a clone is susceptible to disease or changes in environment, then all the clones will be susceptible.

Disadvantage It will lead to less variation, and less opportunity to create new varieties in the future.

15 of 50


Stem cells

During the development of an embryo most of the cells become specialised. They cannot later change to become a different type of cell.

But embryos contain a special type of cell called stem cells. These can grow into any type of cell found in the body. They are not specialised. Stem cells can be removed from human embryos that are a few days old, for example, from unused embryos left over from fertility treatment.

Here are some of the things stem cells could be used for:

  • Making new brain cells to treat people with Parkinson's disease
  • Rebuilding bones and cartilage
  • Repairing damaged immune systems
  • Making replacement heart valves
16 of 50


Therapeutic cloning

If you were to receive medical treatment with cells grown from stem cells, your body's immune system would recognise the cells as foreign, and they would be rejected and die. But this would not happen if you received cells with the same genes as you.

This could be done by cloning one of your cells to produce an embryo, then taking stem cells from this. This is called therapeutic cloning. Here are the steps involved:

  • Nucleus taken out of a human egg cell
  • Nucleus from a patient's cell put into the egg cell
  • Egg cell stimulated to develop into an embryo
  • Stem cells taken from the embryo
  • Stem cells grown in a container of warm nutrients
  • Stem cells treated to develop into required cell types
17 of 50

Protein synthesis

Genes are sections of the DNA. Each gene has the code for creating a specific protein. The sequence of bases in the gene controls which amino acids are created and joined to make a specific new protein (or enzyme) molecule.

Amino acids and proteins

Each gene acts as a code, or set of instructions, for making a particular protein. Some of these proteins control the cell's internal chemistry. They tell the cell what to do, give the organism its characteristics, and determine the way its body works.

To enable genes to code for proteins, the bases A, T, G and C get together not in pairs but in triplets. This is how it works:

  1. Each protein is made up of large numbers of amino acid molecules.
  2. Each triplet of bases codes for one particular amino acid.
  3. Amino acids are made in the number and order dictated by the number and order of base triplets.
  4. Finally, the amino acid molecules join together in a long chain to make a protein molecule. The number and sequence of amino acids determines which protein results.
18 of 50

How protein is produced

How a protein is produced (

19 of 50


Causes and effects

Mutations are changes that can occur in genes. These changes are random and can be caused by background radiation and chemicals that we come into contact with, eg the chemicals in cigarette smoke. The change causes an alteration to the base pair sequence in the genetic code.Sometimes these changes can be so severe that the cell dies, sometimes the cell can divide uncontrollably and become cancerous, and sometimes the changes are small and the cell survives. Very rarely, the changes may even be beneficial to us and produce new and useful characteristics.

Passing on mutations

If these changes occur in normal body cells, the changes are lost when we die. But if the changes occur in our sex cells such as sperm and ova, there is the possibility that the changes in the gene will be passed onto the next generation.

It is when these changes are passed on to the next generation that natural selection can either ensure that they are selected if they are useful, or disappear from the gene pool if they are not.

New species

The combined effect of these mutations, environmental changes, and natural selection, can sometimes produce changes in the organism that are so great that a new species is produced. This does not happen very often and only occurs when the mutated organism can no longer breed with the original species and is capable of producing fertile offspring.

20 of 50


Transcription is the first part of the process of making a protein. It takes place inside the cell nucleus. Transcription involves copying the DNA and the stages are:

  1. The DNA in a gene unzips so that both strands are separate – one strand is used as a template
  2. Complementary bases attach to the strand being copied – C joins to G and so on
  3. Thymine (Base T) is not present and a different base, U, joins with A in the way that T would have done
  4. This forms a strand of messenger RNA (mRNA)

DNA helix unzipping and being copied (

DNA helix unzipping and being copied

Messenger RNA is small enough to move out of the nucleus and so it travels to the ribosomes in the cytoplasm.

21 of 50


Translation takes place in the ribosomes that are found in the cytoplasm. This is where the messenger RNA is 'interpreted' and the new protein formed. The stages are:

  1. The mRNA attaches to a ribosome. The ribosome "reads" the mRNA.
  2. The ribosome decodes the mRNA in groups of three – base triplets or codons – which are complementary to bases in transfer RNA (tRNA).
  3. The tRNA is specific to an amino acid that it collects and returns to the mRNA.
  4. The amino acids are now lined up in order of the instructions on the mRNA.
  5. Bonds form between the amino acids and a polypeptide chain is formed.
  6. The polypeptide chain folds and becomes a specific shape forming a protein.
22 of 50


Enzymes are large molecules that speed up the chemical reactions inside cells. Each type of enzyme does on specific job. Enzymes are a type of protein, and like all proteins, they are made from long chains of different amino acids.

Biological catalysts

Enzymes are soluble protein molecules that can speed up chemical reactions in cells. These reactions include respiration, photosynthesis and making new proteins. For this reason enzymes are sometimes called biological catalysts. Enzymes speed up (catalyse) chemical reactions occurring inside and outside of living cells. This includes:

  • DNA replication
  • Protein synthesis
  • Digestion

Each enzyme will only speed up one reaction as the shape of the enzyme molecule needs to match the shape of the molecule it reacts with (the substrate molecule). The part of the enzyme molecule that matches the substrate is called the active site.

23 of 50


Enzymes and temperature

At low temperatures, enzyme reactions are slow. They speed up as the temperature rises until an optimum temperature is reached. After this point the reaction will slow down and eventually stop.

Enzymes and pH

Most enzymes work fastest in neutral conditions. Making the solution more acidic or alkaline will slow the reaction down. At extremes of pH the reaction will stop altogether. Some enzymes, such as those used in digestion, are adapted to work faster in unusual pH conditions and may have an optimum pH of 2 (very acidic) if they act in the stomach.

Enzymes and substrate concentration

Enzymes will work best if there is plenty of substrate available. As the concentration of the substrate increases, so does the enzyme activity. However, the enzyme activity does not increase without end. This is because the enzyme can't work any faster even though there is plenty of substrate available.

24 of 50

The lock and key mechanism

Enzymes are proteins that speed up chemical reactions in our cells.

Enzymes work best at their optimum temperature. This is why homeostasis is important - to keep our body temperature at a constant 37°C.

As the temperature increases, so does the rate of chemical reaction. This is because heat energy causes more collisions, with more energy, between the enzyme molecules and other molecules. However, if the temperature gets too high, the enzyme is denatured and stops working.

A common error in exams is to write that enzymes are killed at high temperatures. Since enzymes are not living things, they cannot be killed.

One enzyme - one job

Enzymes are specific. Only molecules with the correct shape can fit into the enzyme. Just like only one key can open a lock, only one type of enzyme can speed up a specific reaction. This is called the lock and key model.

25 of 50

Denaturing of enzymes

The important part of an enzyme is called the active site. This is where specific molecules bind to the enzyme and the reaction occurs.

Anything that changes the shape of the active site stops the enzyme from working. This is similar to a key that opens a door lock. It does not matter what a key handle looks like, but if you change the shape of the ‘teeth’ the key no longer works.

The shape of the active site is affected by pH. This is why enzymes will only work at a specific pH, as well as a specific temperature. Change the pH and the enzyme stops working.

Increasing the temperature to 60°C will cause a permanent change to the shape of the active site. This is why enzymes stop working when they are heated. We say they have become denatured.

26 of 50

Aerobic respiration

Respiration is not the same thing as breathing. That is more properly called ventilation. Instead, respiration is a chemical process in which energy is released from food substances, such as glucose - a sugar.

Aerobic respiration needs oxygen to work. Most of the chemical reactions involved in the process happen in tiny objects inside the cell cytoplasm, called mitochondria.

This is the equation for aerobic respiration:

glucose + oxygencarbon dioxide + water (+ energy)

The energy released by respiration is used to make large molecules from smaller ones. In plants, for example, sugars, nitrates and other nutrients are converted into amino acids. Amino acids can then join together to make proteins. The energy is also used:

  • To allow muscles to contract in animals
  • To maintain a constant body temperature in birds and mammals
27 of 50

The circulatory system

The circulatory system consists of:

  • The heart, which is the muscular pump that keeps the blood moving
  • The arteries, which carry blood away from the heart
  • The veins, which return blood to the heart
  • The capillaries, which are tiny blood vessels that are close to the body's cells

A process called diffusion takes place in the capillaries. Diffusion is where particles of a high concentration move to an area of low concentration. Glucose and oxygen diffuse into the cells from the capillaries. Carbon dioxide diffuses out of the cells into the blood in the capillaries.

During exercise, the muscle cells respire more than they do at rest. This means:

  • Oxygen and glucose must be delivered to them more quickly
  • Waste carbon dioxide must be removed more quickly

This is achieved by increasing the breathing rate and heart rate. The increase in heart rate can be detected by measuring the pulse rate. The stroke volume also increases – this is the volume of blood pumped each beat. The total cardiac output can be calculated using the equation:   Cardiac output = stroke volume x heart rate

During hard exercise, the oxygen supply may not be enough for the needs of the muscle cells. When this happens, anaerobic respiration takes place, as well as aerobic respiration.

28 of 50

The after effect of exercise

During hard exercise when anaerobic respiration occurs with aerobic respiration, an oxygen debt builds up. This is now known as Excess Post-exercise Oxygen Debt or EPOC. This is because glucose is not broken down completely to form carbon dioxide and water. Some of it is broken down to form lactic acid. Panting after exercise provides oxygen to break down lactic acid. The increased heart rate also allows lactic acid to be carried away by the blood to the liver, where it is broken down.

Blood pressure

Arteries carry blood away from the heart.

The blood in the arteries is under pressure because of the contractions of the heart muscles. This allows the blood to reach all parts of the body.

  • Systolic pressure - the higher measurement when the heart beats, pushing blood through the arteries.
  • Diastolic pressure - the lower measurement when the heart rests between beats.
29 of 50


Leaves enable photosynthesis to occur. Photosynthesis is the process by which leaves absorb light and carbon dioxide to produce carbohydrate (food) for plants to grow. Leaves are adapted to perform their function, eg they have a large surface area to absorb sunlight.

Plants have two different types of 'transport' tissue, xylem and phloem, that move substances in and around the plant. When water evaporates from the leaves, resulting in more water being drawn up from the roots, it is called transpiration.

Functions of leaves

The function of a leaf is photosynthesis – to absorb light and carbon dioxide to produce carbohydrates. The equation for photosynthesis is:

Carbon dioxide and water → glucose and oxygen

Did you know:

  • Leaves are the source of all of food on the planet
  • Leaves recycle all of the world's carbon dioxide in the air
  • Leaves contain the world's most abundant enzyme
30 of 50

Factors affecting photosynthesis

Light intensity

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.

Carbon Dioxide

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.

If you plot the rate of photosynthesis against the levels of these three limiting factors, you get graphs like the ones above.

In practice, any one of these factors could limit the rate of photosynthesis.

31 of 50


Transpiration explains how water moves up the plant against gravity in tubes made of dead xylem cells without the use of a pump.

Water on the surface of spongy and palisade cells (inside the leaf) evaporates and then diffuses out of the leaf. This is called transpiration. More water is drawn out of the xylem cells inside the leaf to replace what's lost. As the xylem cells make a continuous tube from the leaf, down the stem to the roots, this acts like a drinking straw, producing a flow of water and dissolved minerals from roots to leaves.

Factors that speed up transpiration will also increase the rate of water uptake from the soil. When water is scarce, or the roots are damaged, it increases a plant's chance of survival if the transpiration rate can be slowed down. Plants can do this themselves by wilting, or it can be done artificially, like removing some of the leaves from cuttings before they have chance to grow new roots.

32 of 50


Factors that affect transpiration rate



Explanation Light In bright light transpiration increases The stomata (openings in the leaf) open wider to allow more carbon dioxide into the leaf for photosynthesis Temperature Transpiration is faster in higher temperatures Evaporation and diffusion are faster at higher temperatures Wind Transpiration is faster in windy conditions Water vapour is removed quickly by air movement, speeding up diffusion of more water vapour out of the leaf Humidity Transpiration is slower in humid conditions Diffusion of water vapour out of the leaf slows down if the leaf is already surrounded by moist air

33 of 50


No heart, no blood and no circulation, but plants do need a transport system to move food, water and minerals around. They use two different systems – xylem moves water and solutes from the roots to the leaves – phloem moves food substances from leaves to the rest of the plant. Both of these systems are rows of cells that make continuous tubes running the full length of the plant.


Xylem cells have extra reinforcement in their cell walls, and this helps to support the weight of the plant. For this reason, the transport systems are arranged differently in root and stem – in the root it has to resist forces that could pull the plant out of the ground. In the stem it has to resist compression and bending forces caused by the weight of the plant and the wind.

TissueProcessWhat is movedStructure Xylem Transpiration    Moves water and minerals from roots to leaves Columns of hollow, dead reinforced cells Phloem Translocation    Moves food substances from leaves to rest of    plant Columns of living cells

34 of 50



Plants absorb water from the soil by osmosis. Root hair cells are adapted for this by having a large surface area to speed up osmosis.

The absorbed water is transported through the roots to the rest of the plant where it is used for different purposes:

  • It is a reactant used in photosynthesis
  • It supports leaves and shoots by keeping the cells rigid
  • It cools the leaves by evaporation
  • It transports dissolved minerals around the plant

Root hair cell (

35 of 50

Reducing water loss


Plants growing in drier conditions tend to have small numbers of tiny stomata and only on their lower leaf surface, to save water loss. Most plants regulate the size of stomata with guard cells. Each stoma is surrounded by a pair of sausage-shaped guard cells. In low light the guard cells lose water and become flaccid, causing the stomata to close. They would normally only close in the dark when no carbon dioxide is needed for photosynthesis.


Most plant cells are turgid at all times. This supports the weight of the plant, which is especially important where there is no woody tissue, such as leaves, shoot and root tip. If the plant loses water faster than it can be absorbed the cells lose turgor pressure and become flaccid. This causes the plant to wilt.


Osmosis is the movement of water molecules from an area of high concentration of water to an area of lower concentration of water through a partially permeable membrane. This can be the cell membrane. An example is the flooding of plants by sea water. Sea water contains many chemicals in solution, such as salt. Osmosis will move water across the plant cell membrane, from the weaker to the stronger solution.

36 of 50

Sampling and techniques

Ecologists study an environment in terms of its biodiversity – the variety of different species in an area. They will want to know certain information about the species present:

  • Where an organism is found (distribution)
  • The number of that organism present (population)

Whenever a scientist studies an area it is usually not possible to look at the entire environment in detail. Therefore, the scientist samples a section or small portion. Sampling several small sections is representative of the whole area. The sampling technique used depends on the habitat and type of organisms present.

Pooters - A pooter is used to catch small insects. The user breathes in through the mouthpiece which has a piece of net covering the end. The insects are sucked into the holding chamber via the inlet tube.

Sweep nets -  Sweep nets are used in areas of long grass to catch organisms. They can also be used in ponds.

Pitfall traps are used to catch small, crawling insects. They can be set up and left overnight to catch nocturnal species. All organisms caught should be released unharmed.

Quadrats - Quadrats are square frames of a known size eg 1 m2. They are used to sample all the plant species in a particular area.

37 of 50

Fossil records

Fossil remains have been found in rocks of all ages. Fossils of the simplest organisms are found in the oldest rocks, and fossils of more complex organisms in the newest rocks. This supports the theory of evolution, which states that simple life forms gradually evolved into more complex ones.

Certain environmental conditions drastically slow down the decaying process, helping to preserve the tissues. Examples of this are:

  • Insufficient oxygen, eg when an organism becomes trapped in amber
  • Low temperatures, eg when an organism becomes frozen in a glacier
  • High soil acidity, eg when an organism falls into a peat bog

If these conditions are not present, the remains will not be fossilised. This makes tracing the story of evolution of any one species challenging. In most cases there are big gaps in fossil records, making it like a jigsaw puzzle with half the pieces missing. Problems also arise as soft tissues decay resulting in scientists having to estimate what the organism was like. Finally, there are also lots of fossils that we haven't yet found.

38 of 50


Humans are made of millions of cells. This has a number of benefits:

  • Cells can be specialised to do particular tasks.
  • Groups of cells can function as organs, making a more efficient but complex organism.
  • The organism can grow very large

Growth can be defined as an increase in size, length and mass.

Cell division

New cells are needed throughout life. These are for growth, to replace damaged cells and repair worn out tissues. Normal human body cells are diploid – they have two of each chromosome. When new cells are made, these 46 chromosomes (in other organisms the number is different) are copied exactly in a process called mitosis.

39 of 50

The blood

Blood is a liquid tissue consisting of:

  • Plasma
  • Red blood cells
  • White blood cells
  • Platelets

One of the functions of blood is to transport materials around the body. White blood cells and platelets are part of the body's immune system, but plasma and red blood cells are involved in transport.


Plasma is a straw-coloured liquid. It transports dissolved substances around the body, including:

  • Hormones
  • Nutrients, such as water, glucose, amino acids, minerals and vitamins
  • Waste substances, such as carbon dioxide and urea

Red blood cells

Red blood cells contain a protein called haemoglobin. This sticks to oxygen, allowing it to be carried round the circulatory system.

40 of 50

Cells tissues and organs


Animal cells and plant cells can form tissues, like muscle tissue. A living tissue is made from a group of cells with a similar structure and function, which all work together to do a particular job. Here are some examples of tissues:

  • Muscle
  • The lining of the intestine
  • The lining of the lungs
  • Phloem (tubes that carry dissolved sugar around a plant)
  • Root hair tissue (for plants to take up water and minerals from the soil)


An organ is made from a group of different tissues, which all work together to do a particular job. Here are some examples of organs:

  • Heart
  • Lung
  • Stomach
  • Brain
  • Leaf
41 of 50

Cells tissues and organs

Organ systems

An organ system is made from a group of different organs, which all work together to do a particular job. Here are some examples of organ systems:

  • Circulatory system
  • Respiratory system
  • Digestive system
  • Nervous system
  • Reproductive system
  • Leaf canopy
42 of 50

The heart


The heart is a pump that sends some blood to the lungs and some blood to the rest of the body each time it beats. The blood on the left side is kept separate from the blood on the right side. This is called double circulation and is a more efficient way of delivering oxygen to the tissues than single circulation.

Blood enters the heart through a vein and collects in an atrium. The atrium is emptied into a ventricle which contracts to put the blood under pressure. The blood is forced out through an artery as a valve prevents it flowing back to the atrium. The artery also contains a valve to stop blood flowing back to the ventricle when the ventricle relaxes.

43 of 50

The heart

Deoxygenated blood

Deoxygenated blood passes through these blood vessels, valves and parts of the heart:

Vena Cava, Right atrium , Tricuspid , Right ventricle , Semilunar , Pulmonary artery , Lungs

Oxygenated blood

Oxygenated blood passes through these blood vessels, valves and parts of the heart:

Pulmonary vein , Left atrium , Bicuspid , Left ventricle , Semilunar , Aorta , Body

The left ventricle exerts more pressure than the right ventricle, and so it has a thicker more muscular wall. The atria (plural of atrium) exert less pressure than the ventricles so they have a thinner muscular wall.

44 of 50

Heart problems

Narrow rigid arteries need more pressure to pump blood through them. Increased blood pressure can lead to heart problems over time. Cholesterol contributes to the narrowing of arteries, so a change in diet can lead to a healthier heart.

Leaking heart valves make blood circulation inefficient. They can be replaced in open heart surgery by transplanted valves from a donor, or mechanical valves.

A faulty pacemaker causes irregular beating of the heart which in turn causes blood circulation to be inefficient. Artificial pacemakers powered by a battery can be fitted without needing open heart surgery. The problems of fitting mechanical or electrical heart components include:

  • Rejection by the immune system
  • Finding a way of reducing the size of the components to fit inside the body
  • Providing a power supply for pacemakers

In some cases a heart transplant may be needed. It's difficult to find suitable donors with healthy hearts that match the tissue type of the patient that needs them. People with heart transplants need to take drugs to stop their immune system from rejecting the heart for the rest of their lives. This can lead to greater risk from infections.

45 of 50

The digestive system

Digestion is the breakdown of carbohydrates, proteins and fats into small, soluble substances that can be absorbed into the blood.

Lipases and proteases are used in biological detergents, and enzymes are used in the manufacture of food and drink.

Digestion is the breakdown of large molecules into smaller, soluble molecules that can be absorbed into the body. Digestion happens inside the gut, and relies on enzymes.


Food is moved the digestive system by a process known as peristalsis. This is the contractions of two sets of muscles in the walls of the gut. One set runs along the gut, while the other set circles it. Their wave-like contractions create a squeezing action, moving down the gut.

46 of 50

Enzymes and digestion

Enzymes and their reactions catalysed

EnzymeReaction catalysed Amylase                    Starch → sugars Protease Proteins → amino acids Lipase Lipids → fatty acids + glycerol

Amylase is an example of a carbohydrase. Lipids are fats and oils.

Different parts of the gut

Different parts of the gut produce different enzymes.

Where enzymes are produced

Enzyme Where produced Amylase         Salivary glands, pancreas, small intestine Protease         Stomach, pancreas, small intestine Lipase         Pancreas, small intestine

47 of 50


After the stomach, food travels to the small intestine. The enzymes in the small intestine work best in alkaline conditions, but the food is acidic after being in the stomach. Bile is an alkaline substance produced by the liver and stored in the gall bladder. It is secreted into the small intestine, where it emulsifies fats. This is important, because it provides a larger surface area in which the lipases can work.

digestive enzymes are produced in the pancreas, bile stored in the gall bladder, bile production in liver (

48 of 50

The intestines

Digested food molecules are absorbed in the small intestine. This means that they pass through the wall of the small intestine and into our bloodstream. Once in the bloodstream, the digested food molecules are carried around the body to where they are needed. Only small, soluble substances can pass across the wall of the small intestine. Large insoluble substances cannot pass through.

Absorption into bloodstream

The inside wall of the small intestine is thin, with a large surface area. This allows absorption to happen quickly and efficiently. To get a big surface area, the inside wall of the small intestine is lined with tiny villi. These stick out and give a big surface area. They also contain blood capillaries to carry away the absorbed food molecules.

Diagram of villli, showing the walls which are just 1 cell thick, and the network of capillaries, and the blood vessels.

The villi have a rich blood supply. The blood supply has a lower concentration of food molecules and so diffusion occurs quickly.

49 of 50

Probiotics and Prebiotics


Probiotics contains beneficial bacteria. These are usually either lactobacillus or bifidobacterium that are designed to produce lactic acid in the gut. The manufacturers claim these can make you healthier by:

  • Improving the digestive system
  • Helping the body defend against disease
  • Reducing allergies


The body cannot breakdown prebiotics. They are added as a source of nutrients for the probiotic bacteria. A common form of prebiotics are oligosaccharides.

In your exam you will be expected to consider the evidence for and against the use of probiotics and prebiotics. This may include the use of graphs and data from scientific experiments.

50 of 50


No comments have yet been made

Similar Biology resources:

See all Biology resources »See all Plant and animal cells resources »