HideShow resource information


Animal and plant cells both have a nucleus. Bacterial, animal and plant cells all have a cytoplasm and cell membrane. The nucleus contains DNA in the form of chromosomes. The cytoplasm is a gel like substance where most of the cell's chemical reactions happen. The cell membrane holds the cell together and contains what goes in and out. Plant and bacterial cells have a cell wall which is made of cellulose and supports the cell.

Animal cells:

  • Mitochondria where most of the reactions involving respiration take place. Respioration provides enerygy for cell processes. Cells which need a lot of energy have a lot of mitochondria.
  • Ribosome is where proteins are synthesised.

Plant cells:

  • Chlotoplasts where photosynthesis happens
  • Vacuole is a relatively large structure that contains cell sap, a weak solution of sugar and salts.

Bacterial cells:

  • Don't have a true nucleus but have a singular strand of DNA that floats freely in cytoplasm.
  • Have no chloroplasts or mitochondria.
1 of 19


Chromosomes are long molecules of coiled DNA. DNA is a double helix, each strand is made of small groups called nucleotides. Each nucleotide contains a small molecule called a base and there are four bases; A with T and G with C-these are complementary base pairings. Each base forms cross links with the base on the other strand so they are tightly wound together.

Watson and Crick:

First to build a DNA model in 1953, they used data from other scientists to understand the structure which included; x-ray data showing DNA being a double helix and other data showing bases in pairs. They then created a model to show this. Other scientists repeated their work before it was widely accepted.

Copying DNA:

  • Molecule of DNA unzips
  • Bases on free floating nucleotides pair up with complementary bases on the DNA
  • Cross links form between the bases and old DNA strands, and the nucleotides are joined together to form double strands.
2 of 19

Protein synthesis

DNA controls the production of proteins in a cell. A gene is a section of DNA which codes for a particular protein. Proteins are made up of chains of molecules called amino acids. Every different type of protein has its own number and order of amino acids which gives each protein type a different shape and function. The order of the bases in a gene decides the order of amino acids in a protein. Each amino acid is coded for by a sequence of three bases in the gene. The amino acids are joined together to make proteins, following the order of the bases in the gene. Each gene has a different sequence of bases-which is what allows it to code for a unique protein.

The mRNA:

Proteins are made in the cytoplasm by ribosomes. Ribosomes use the DNA code to make a protein but because the DNA is too big it can't move out of the nucleus so the mRNA takes the code from the DNA to the ribosome by copying the code from the DNA. The mRNA acts as a messenger between DNA and the ribosome.

Proteins in a cell effect its function and some determine cell structure, others control cell reactions. Different types of cells have different functions because they make different proteins. They only make certain proteins because only some of the full set of genes is used in any one cell. Some genes are switched off, which means the proteins thye code for aren't produced. Those genes which are switched on determine the cell function. 

3 of 19

Functions of proteins

Hundreds of proteins all have different functions, these are 4 examples:

  • Enzymes
  • Carrier molecules: used to transports smaller molecules
  • Hormones: used to carry messages around the body
  • Structural proteins: Physically strong

Cells have different chemical reactions like respiration happening inside them but they need to be controlled to get the correct amount to keep it working properly. You can make a reaction happen faster by raising the temperature but this increases the speed of every reaction including bad ones. There is also a limit to the amount of temperature you can raise it to before they get damaged. This is why living things create enzymes which act as biological catalysts. Enzymes therefore reduce the need for high temperatures and they are only there for useful chemical reactions in the body. Every different biological reaction has its own enzyme especially for it coded by a differnt gene and have a unique shape

Chemical reactions involve things either being split or joined. The substrate is the molecule changed in a reaction. Every enzyme has an active site. The enzymes have a high specifity for their substrate because for it to work it needs to fit in its active site. If it doesn't fit the reaction wouldn't be catalysed. A common amnalogy to describe this is the lock and key mechanism.

4 of 19

More on enzymes

Changing the temperature changes the rate of an enzyme catalysed reaction . Like with any other reaction higher temperatures can increase the rate because heat gets to the enzymes so the substrate particles have more energy which makes them (enzymes and substrates) move about more so they have a higher collision rate. Low temperatures have a slower reaction and a lower collision rate. Bonds holding enzymes break if it's too hot, therefore the shape changes and the active site doesn't fit the substrate so the reaction can't be catalysed and it stops because the enzyme can't function. This is called denaturing and it is permanent. Each enzyme has an optimum temperature where it functions fastest. This is just before the temperature where the enzyme denatures. Human optimum temperature is 37 degrees.

pH levels:

If an enzyme has a pH level which is too high or too low, it interferes with the bonds holding the enzymes together and it denatures. Enzymes also have an optimum pH where the work best and it is often pH 7 (neutral) but some are different e.g. pepsin with a pH of 2 so it is good for acidic situations like the stomach.

The Q10 value for a reaction shows how much the rate changes when the temperature is raised by 10 degrees celcius. You calculate it by dividing the rate at higher temperature by the rate at lower temperature. 

5 of 19


A mutation is a change in the DNA base sequence. If there's a mutation in a gene it can stop protein production of the specific gene or a different protein could be produced instead.

If a mutation occurs in reproductive cells, the offspring may develop abnormally or die at an early stage of development. If a mutation occurs in body cells the mutant cells can multiply in an uncontrolled way and invade other parts of the body which is how cancer happens.

Benefits of mutations:

Sometimes the proteins produced can be beneficial and the new produced protein could be an improvement to the one it is supposed to be. This means the organism has a survival advantage over the population. The muatation would be passed to the offspring and so forth which makes it common in the population. This is how natural selection and evolution happened. Some proteins are neither harmful or beneficial to an organism and have no effect on them. 

They can also happen spontaneously when a chromosome doesn't copy itself correctly but the chances of developing one are increased if you're exposed to ionising radiation(X-rays and ultraviolet light) or certain chemicals which are known to cause mutations (mutagens). If the mutations produce cancer then the chemicals are referred to as carcinogens. Cigarette smoke contains chemical mutagens or carcinogens.

6 of 19

Multiplying cells

Multicellular organisms are bigger which is better because you can travel further, get your nutrients in different ways and fewer things can eat or squash you. It also allows for cell differentiation, instead of having one cell doing everything, multiple cells can do different jobs and can be specially adapted for their job. They are also more complex so they can have specialised organs so they adapt to their environment. Multicellular organisms also need specialised organ systems; one to communicate between different cells:nervous system, one to supply cells with nutrients: circulatory system and a system which controls the exchange of substances with the environment: respiratory system.

Mitosis is when a cell reproduces itself by splitting to form two identical offspring and this happens in the body when you want identical cells

  • Before mitosis, DNA replicates
  • DNA coils in double armed chromosomes, these contain exactly the same DNA.
  • The chromosomes line up at the centre then divide as cell fibres pull them apart and the two arms go to opposite poles of one cell and forms membranes around each set of chromosomes.
  • This means that there are now two new cells which are genetically identical, before it can divide again, the DNA replicates itself to give each chromosome two arms again.
7 of 19

Meiosis, Gametes and fertilisation

Meiosis is another type of cell division which creates gametes. They form in the ovaries and testes. Gametes are sex cells-eggs and sperm. Body cells of mammals are diploid which means that each of it's body cells has two copies of each chromosome in its nucleus- one from the mum and one from the dad. Gametes on the other hand are haploid so they only have one copy of each chromosome. This is so that when the egg and sperm combine, they for a cell with diploid chromosome numbers.

In meiosis DNA replicates and curls to form double armed chromosomes. After, chromosomes arrange into pairs. Humans have 23 pairs of chromosomes. Both chromosomes in a pair contain information about the same features. In the first divison,The pair split-chromosomes move to opposite poles of the cell, no pairs in new cells, just one of each of the 23 different types. Each new cell has a mixture of the parent's chromosomes but only half the usual number of chromosomes. In the second division each chromosome splits in half and one arm ends up in each new cell. There are now four new cells which are genetically different and each gamete only gets half of them at random.

When fertilisation happens, male and female gametes combine to make a diploid cell (zygote). The characteristics of this zygote are controlled by the combination of genes on its chromosomes, because it's inherited chromosomes from two parents it shows features of both parents but isn't exactly like either of them (diploid zygote-fertilised egg). A sperm transports male DNA to an egg. They are small with long tails so they can swim to it. They have a lot of mitochondria to provide energy to swim the distance. They also have an acrosome at the front of the head which can release the enzymes they need to digest their way through the membrane of the egg cell. 

8 of 19

Stem cells, differentiation and Growth

Animals grow till a finite size (full growth) and then stop whilst plants grow continuously.In animals, growth happens wiith cell division, in plants, growth happens due to cell enlargement (elongation). Growth by cell division usually only happens in the meristems (tips of the roots and shoots).

Differentiation is when a cell changes to become specialised for its job. Animal cells lose differentiation ability early on whereas plant cells never lose it. Most cells are specialised to do a particular job. Some cells are undifferentiated and can develop into different types of cells, tissues and organs and are called stem cells. They are found in early human embryos and an turn into any kind of cell. Adults also have stem cells in the bone marrow but aren't as versatile but can turn into certain cells but not all of them. 

They are used to cure diseases like blood disorders, these are cured by bone marrow transplants which just replaces faulty bone marrow. Early embryos have a lot of stem cells which scientists can extract to grow them. Eventually growing tissues to treat medical conditions, this is known as stem cell therapy.

People are against stem cell therapy because it could have been potentail life but others say that humans which exist and are suffering are more important. Embryos usually used in research are usually unwanted ones from fertility clinics which would have been destroyed. There are now stocks of stem cells in some countries but it is banned in the US but the UK allows it if they follow strict guidelines.

9 of 19


Growth is an increase in size or mass. Growth in animals in measured by different methods:

  • Length-measures the length or height of an animal or plant which is easy to measure but doesn't tell you about changes in width or diameter.
  • Wet mass- Weight of the plant or animal which is also easy to measure- but this is easily changeable like after an animal has eaten or it has rained on a plant.
  • Dry mass- Dry out the organism and then weigh it and this is good because it doesn't change according to what the organism has eaten or if it has rained but you have to kill the organism to find out its dry mass.

Humans have 5 stages of growth;infancy (0-2 years-rapid growth), childhood (2-puberty-steady growth), Adolescence (puberty-complete development-rapid growth), Adulthood (adolescence-old age-growth stops) and old age(65 years- death). 

Certain parts of the body grow faster than others for example when a baby is in the womb, the brain grows at a greater rate than the rest of the body because a well developed brain gives humans a survival advantage.

10 of 19


Respiration's in every cell in the body, it is the process of releasing energy from glucose. The energy released can't be used directly by other cells so it makes a substance called ATP. This acts as the energy source for many cell processes and transports energy to where it's needed in a cell.  Respiration is controlled by enzymes so the rate of respiration is affected by temperature and pH. Respiration happens aerobically and anaerobiacally. 

Aerobic respiration happens when there is a lot of oxygen available, it is the most efficient way to release energy from glucose. Glucose + oxygen -> carbon dioxide + water + (energy). When respiration rate increases, oxygen consumption and carbon dioxide production increases therefore the rate of oxygen consumption can be used to estimate metabolic rate (amount of energy being used). 

Anaerobic respiration happens when doing a lot of exercise your body can't supply enough oxygen to the muscles so they start respiring anaerobically. It releases much less energy per glucose molecule than aerobic respiration. Glucose is only partially broken down and lactic acid builds up whcih makes your muscles fatigued. When you stop exercising you have oxygen debt  so you need to build up oxygen to break down lactic acid and this is done by panting and then aerobic respiraton can happen again. Your heart rate needs to stay high too because the lactic acid needs to be taken to the liver to be broken down.

You calculate the respiratory quotient by dividing the amount of co2 produced by the amount of o2 used. If the number is between 0.7 and 1 then they are respiring aerobically and if it's above 1, they are respiring anaerobically.

11 of 19

Functions of the blood


Plasma is a pale yellow liquid which carries everythign that needs transporting around the body; red blood cells, white blood cells and platelets (used in body clotting), water, Digested food products like glucose and amino acids from the gut to all the body cells, carbon dioxide from the body cells to the lungs, urea from the liver to the kidneys, hormones which act like chemical messengers and antibodies which are proteins involved in the body's immune response. 

Red blood cells:

Transport oxygen from the lungs to all the cells in the body. They are small and have a biconcave shape so it has a large surface area to volume ratio for absorbing and releasing oxygen. They contain haemoglobin which makes it red because it has a lots of iron. In the lungs haemoglobin combines with oxygen to become oxyhaemoglobin. The reverse happens in body tissues to release oxygen to the cells. The cells don't have a nucleus so they have more space for haemoglobin so they can carry more oxygen. They are also very flexible so they can get through tiny capillaries. 

12 of 19

Blood vessels

There are three types of blood vessels:

  • Arteries-carry the blood away from the heart
  • Capilleries- involved in the exchange of materials at the tissues
  • Veins- carry blood in to the heart

Arteries have strong and elastic walls to withstand the pressure of the blood. The walls are thick compared to the lumen (the hole down the middle) and contain thick layers of muscle to make them strong.

Arteries branch to capillaries whcih are really tiny. They carry the blood really close to the cells to exchange substances with them. They have permeable walls so substances are able to diffuse in and out. Capillaries supply food and oxygen and take away waste like carbon dioxide. Their walls are only one cell thcik which increases the rate of diffusion by decreasing the distance at which it occurs.

The capillaries join up to the veins which takes blood back to the heart. The blood is at a lower pressure in the veins so the walls don't need to be as thick. They have a bigger lumen to help the blood flow despite the low blood pressure. They also have valves to help the blood flowing in the correct direction. 

13 of 19

The Heart

Double circulatory system:

The first system connects the lungs to the heart. The deoxygenated blood is pumped to the lungs to take in oxygen.The blood then returns to the heart. The second system connects the rest of the body to the heart. The oxygenated blood is pumped to the rest of the body, it gives up its oxygen and then the deoxygenated boood returns to the heart to be pumped out the lungs again. 

Returning the blood to the heart after  it's picked up oxygen at the lungs means it can be pumped around the body at higher pressure which increases rate of blood flow so more oxygen can get to the cells which is necessary because mammals need a lot of oxygen to maintain their body temperature.

Deoxygenated blood enters from the vena cava as the atrium contracts, pushing the blood doown via the tricuspid valve in to the right ventricle. Once the blood has entered the right ventricle, the tricuspid valve closes to prevent any backflow. When the right ventricle contracts, it forces the semi lunar valves to open. Then the blood goes to the pulmonary artery. The deoxygenated blood then leaves the heart for the lungs and the semi lunar valve closes. The cycle then repeats.

Oxygenated blood enters from the pulmonary veins in to the heart, it goes to the left atrium . The left atrium contracts, pushing the blood through the bicuspid valve to pump the blood in the left ventricle and the bicuspid valve closes. The ventricle contracts so the blood then flows to the semi-lunar valves which open to let the blood flow in to the aorta and the blood is pumped around the body. The cycle then repeats again. 

14 of 19

Selective breeding

What is selective breeding?

When humans artificially select the plants or animals that are going to breed and have their genes remain in the population, according to what we want from them like a maximum yield, good health and disease resistance and other qualities like speed, temperament and attractiveness. You chose the one with the best characteristics from two species and breed them together. You then breed the best of the offspring together and this process continues over generations till the desirable trait gets stronger and stronger.

Drawbacks of selective breeding:

It reduces the gene pool (the number of different alleles-forms of a gene)in a population because of inbreeding. This can cause health problems due to an increase in genetic disorders due to a limited gene pool. This happens because recessive alleles are more likely to build up in the population because the organisms are more likely to share the same alleles. If a new disease appears and tyhere is not much variation, all the animals could potentially get it.   

15 of 19

Genetic engineering

The idea behind genetic engineering is to move the genes (Sections of the DNA) from one organism to another so it produces useful biological products. Produces organisms with new and useful features very quickly but the inserted gene could have harmful effects like mutations.

The desirable gene is selected and cut from the DNA using enzymes and is then isolated. The useful gene is inserted into the DNA of another organism which replicates to create load of similar organisms which produce the same thing.

Some countries rely on rice for food so tend to have a vitamin A deficiency so scientists used the gene that controls beta carotene from carrot plants and inserted it in to rice plants and humans can change beta carotene into vitamin A. The human insulin gene can be inserted in to bacteria. Some plants have resistance to herbicides, frost damage and disease and now the genes responsible can be cut and put in to useful plats like crops.

People think it's wrong to genetically engineer other organisms for human benefit especially if the organism suffers due to it. People are afraid that in the future people who can afford it will be able to decide what characteristics they want their children to have and those who can't afford it wil be considered genetic underclass. Don't know what will happen in the future and how it could harm the future generations.

16 of 19

Gene Therapy and cloning animals

Gene therapy involves altering someones genes to cure genetic disorders, it isn't working properly yet but they are trying to get it to work for the future. There are two types of gene therapy, in the first one you would change the persons cells, particularly those which are the most affected by the disorder, but this doesn't change the gametes which is an issue because the offspring could still inherit the disorder. The second type of gene therapy changes the genes in the gametes, therefore every cell of any offspring produced from these gametes will be affected by the gene therapy so the offspring won't suffer from any of the diseases but this is currently illegal. There could however be unknown consequences which would be inherited by all future generations. This could lead to parents creating designer babies where they choose their babies genes.


Clones are genetically identical organisms and can occur naturally in both animals and plants. The nucleus of a sheeps egg cell was removed so the egg had no genetic information and another nucleus was inserted in its place called the diploid nucleus from the udder cell of a different sheep and had all of its genetic information. The cell is shcoked and it starts to replicate and mitosis occurs. This dividing clel is now an embryo and can be inserted in to a surrogate mother to be born.

17 of 19

Uses and risks of cloning animals

Cloning lets you mass produce with desirable characteristics and human embryos can be produced by cloning animals adult body cells, embryos could then be used to supply stem cells for stem cell therapy and these cells would have the same genetic information as the patient which would reduce the risk of rejection. Cloned animals might not be as healthy as normal ones and because it is a new science there may be consequences we don't know about yet.

Cloning animals could be a small step away from cloning human embryos to produce human clones. There would also have to be a lot of surrogate pregnancies with high rates of miscarriages and stillbirth. Clones of other mammals have been unhealthy and did early and this would happen with human clones too. If there is a healthy clone it could get psychologically damaged because it is just a clone of another human being.

18 of 19

Cloning plants

Taking cuttings from plants and then planting them to produce genetically identical copies is aslso a type of cloning. It is easier to clone plants because they have the ability to differentiate but animal cells lose this ability early on.

Tissue culture:

You choose the plant you would like to clone based on the desired characteristics and remove several pieces of tissue from the parent plant and these should be taken from the fast growing root and shoot tips and then you grow the tissue in a growth medium with nutrients and growth hormones which is done under sterile conditions to prevent growth of microbes which are harmful to plants. When the tissues produce shoots and roots they can be moved to potting compost to grow.

The plant will be genetically identical so you know what the characteristics of the plant will be so you only grow good ones. You can also mass produce plants which are hard to grow from seeds. If a disease infects the plants however they will all suffer because they have the same genes and there are problems due to genetic variation.

19 of 19


No comments have yet been made

Similar Science resources:

See all Science resources »See all all of B3 resources »