AQA GCSE Biology: Unit 2

20 revision cards for the Unit 2 paper, including the following topics:

  • Cells
  • Photosynthesis
  • Genetics
  • Enzymes and Digestion
  • Created by: LOKY
  • Created on: 24-09-16 13:37

Cell Structure

All living things are made of cells. Plant and animal cells have similarities. They both contain the following:

  • Nucleus: Contains the genetic material that controls the cell's activities
  • Cytoplasm: A substance where chemical reactions happen. Contains enzymes which control reactions
  • Cell membrane: Holds the cell together and controls the passage of substances in and out of the cell
  • Mitochondria: Where respiration happens. This releases energy that the cell needs to work
  • Ribosomes: Where protein synthesis happens

Plants have a few extra parts in addition to those listed above:

  • Cell Wall: Made of cellulose, it supports and strengthens the cell
  • Vacuole: Contains cell sap (A solution of sugar and salts)
  • Chloroplasts: Where photosynthesis happens. They contain chlorophyll

Bacteria are single-celled organisms. They have a cell wall and membrane. They have no nucleus so the genetic material floats in the cytoplasm along with ribosomes. Some bacteria with have a flagellum (a tail).

Yeast is a single-celled organism. They also have a cell wall and membrane. Inside is cytoplasm and a nucleus.

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  • Diffusion happens in solutions and gases. It is a natural tendency
  • The bigger the difference in concentration, the faster the diffusion rate
  • Cell membranes use diffusion. They let small molecules (oxygen, glucose, amino acids, water) through but stop bigger molecules (protein, starch) from escaping the cell
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Examples of Specialised Cells in Plants

Palisade Leaf Cells

  • Large number of chloroplasts for photosynthesis. Closer to the part of the cell that is nearest to the surface to take in even more light
  • Tall, thin shape to put lots of them in the top of the leaf. The large surface area lets it absorb more carbon dioxide

Guard Cells

  • Kidney-like shape lets it open and close the stomata (The pores of a leaf)
  • When a plant has lots of water, the cells fill up and go turgid, making the stomata open. If it is short of water, they go flaccid, making the stomata close and reducing water loss
  • Light-sensitive, so they close at night to save water
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Examples of Specialised Cells in Humans

Red Blood Cells

  • Concave shape means larger surface area to absorb more oxygen
  • Packed with haemoglobin, which is what absorbs the oxygen
  • No nucleus, leaving even more room for haemoglobin


  • Contains a huge food reserve to feed the embryo
  • Once fertilised, the membrane changes structure to stop any more sperm adding too much DNA to the egg


  • Long tail and streamlined head to help it swim
  • Mitochondria in its head provides it with enough energy
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From Cell to Organ System

The order is Cells>Tissues>Organs>Organ Systems

Cells carry out a particular function. The process that decides how cells become specialised is called differentiation

Tissues are groups of cells with a similar structure and function. Examples include muscular (contracts to move whatever it's attached to), epithelial (covers parts of the body) and glandular (produces chemicals like enzymes and hormones) tissue

Organs are made up of tissues which work together to perform a certain function.The stomach contains the three examples above:

  • Muscular tissue moves the stomach wall to churn up food
  • Glandular tissue produces digestive juices to digest food
  • Epithelial tissue covers the outside and inside of the stomach

Organs are organised into organ systems which work together to perform a certain function. For example, the stomach is part of the digestve system, which breaks down food. (See Digestive System)

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Digestive System

1. Mouth: Food is chewed by the teeth. Salivary glands produce amylase in the saliva
2. Gullet/Oesophagus: Uses peristalsis to move food to the stomach
3. Stomach: Pummels food with its muscular walls. Produces pepsin and hydrochloric acid. The acid kills bacteria and is the right pH for pepsin to work in (pH 2)
4. Pancreas: Produces protease, amylase and lipase, which is released into the small intestine
5. Liver: Produces bile which neutralised stomach acid and emulsifies fat
6. Gall bladder: Where bile is stored before being released into the small intestine
7. Small intestine: Produces protease, amylase and lipase, which digests food even more. Absorbs soluble molecules into the bloodstream
8. Large intestine: Absorbs excess water from the remains of the food
9. Rectum/Anus: Faeces is stored in the rectum before being excreted

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EQUATION: Carbon Dioxide + Water (with help from light energy) = Glucose + Oxygen

Photosynthesis happens in chloroplasts. Light energy is taken in by chlorophyll. Chlorophyll uses this energy to convert carbon dioxide and water into glucose. Oxygen is also released as a by-product.

Photosynthesis can be limited by three things:

  • Availability of light (measured by light intensity)
  • Availability of carbon dioxide (measured by COconcentration)
  • Temperature (obviously measured by temperature)

Light and carbon dioxide are limiting factors. This means that, after a certain point, producing more light or COwont make photosynthesis happen any faster. However, too high a temperature means that enzymes needed for photosynthesis will be damaged.

A greenhouse can create ideal conditions for growing plants. They can trap heat to keep the plants in warm conditions. To give more light to plants, gardeners can buy lamps to supply more light. To supply more CO2, gardeners can buy paraffin burners. Paraffin produces COas a by-product when burned.

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Plant organs and how they use glucose

Plants are made up of stems, roots and leaves. These organs are made of tissues. An example is a leaf:

  • Mesophyll tissue: Where photosynthesis happens
  • Xylem: Transports water through the leaf
  •  Phloem: Transports thing such as mineral ions and sucrose around the leaf
  • Epidermal tissue: Covers the leaf

Once plants have made glucose from photosynthesis, they use it in 5 ways:

  • Respiration
  • Converting to cellulose: This makes a strong cell wall
  • Making proteins: Combined with nitrate ions, glucose makes amino acids
  • Storage in seeds: Glucose is turned into fats and oils (lipids) for storing in seeds
  • Storage as starch: Glucose can also be turned into starch and be stored in roots, stems and leaves as an energy reserve when photosynthesis isn’t happening
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Distribution of Organisms (Quadrats & Transects)

There are 5 factors that can affect the distribution of an organism:

  • Temperature
  • Availability of nutrients 
  • Amount of light
  • Availability of water
  • Availability of oxygen and carbon dioxide

There are two ways of studying these patterns:

  • Quadrats: A square (usually 1m2) that encloses an area. It is placed randomly on the habitat before you count the different types of organism and how many there are. This is done multiple times
  • Transects: A line that measures how organisms are distributed across an area (eg. From the edge of a forest to the middle of a field). You collect data at certain intervals on the line by either counting the organisms that touch the line or using quadrats
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Proteins & Enzymes

Enzymes are catalysts. A catalyst is a substance that increases the rate of a reaction without being changed or used up in the reaction.

Proteins make up structural components of tissues (muscles), hormones, antibodies and enzymes. Proteins are made of amino acids, which are folded into unique shapes. These shapes are vital for the enzyme’s function. Different enzymes work at different pH values. If it’s too high or too low, the pH interferes with the bonds holding the enzyme together and it changes shape. This is known as denaturing.

Chemical reactions usually split things up or join them together. Enzymes can catalyse reactions so that they aren’t used up. Every enzyme has a unique shape to fit to the substance it was made to break up. They only work on one type of substance. The substance fits in the enzyme’s active site, like a key in a lock. It is then split up and released. If the substance doesn’t fit in the active site, then it can’t be broken up.

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Enzymes in the Digestive System

Molecules in food are too big to pass into the bloodstream. They need to be broken down into molecules that can pass through the gut wall. There are three types of enzymes, all of which are produced in the pancreas and small intestine:

  • Amylase: Converts starch into maltose (a sugar). Also produced in the salivary glands (carbohydrase)
  • Protease: Converts proteins into amino acids. Also produced in the stomach (pepsin)
  • Lipase: Converts lipids into glycerol and fatty acids

Bile (produced in the liver and stored in the gall bladder) neutralises the hydrochloric acid (bile is alkaline) from the stomach as it passes into the small intestine. This stops the enzymes there from denaturing. It also emulsifies fat. This means that it breaks the fat into tiny droplets, giving it a larger surface area and increasing the rate of reaction.

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Uses of Enzymes outside the body

Some microorganisms produce enzymes which pass out of their bodies. This means that the enzymes can be used to help us. At home, biological detergents contain proteases and lipases, which break down the stains on clothes. They work best at low temperatures, which means that the washing machine uses less energy. In industry, enzymes can change foods. There are three examples:

  • Proteins in baby foods are pre-digested using proteases
  • Carbohydrases turn starch syrup into a sweeter sugar syrup
  • Isomerases turn glucose syrup into fructose for use in slimming foods and drinks. It is sweeter so you use less


  • They only catalyse the reaction you want
  • Using low temperatures means less energy, therefore it costs less
  • Because they aren’t used up in the reaction, they last for a long time


  • They can be denatured by the smallest increase in temperature. This means the conditions they’re kept in must be under strict control
  • Can be expensive to produce, even if they last for a long time
  • Contamination with nearly anything can affect the enzyme
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Respiration is the process of releasing energy from glucose which happens in every cell. The equation is: Glucose + Oxygen = Carbon Dioxide + Water + Energy (C6H12O6 + 6O2 = 6CO2 + 6H2O)

Aerobic respiration happens in both plants and animals. Most of the reactions to produce this happen in mitochondria. The energy provided can be used for four things:

  • Build larger molecules from small ones (eg. Amino acids to proteins)
  • Allow muscles to contract in animals
  • Maintain a steady body temperature for warm-blooded animals
  • Build sugars, nitrates and other nutrients into amino acids, which are built into proteins, in plants

During exercise, three things increase: Heart rate, rate of breathing and depth of breathing.
These changes cause your blood to flow faster to increase the supply of sugar and oxygen to your cells and remove carbon dioxide from your cells quicker. Some muscles also use up glycogen stores, which are converted back into glucose to provide more energy. Glycogen is how muscles store glucose.

If cells don’t have enough oxygen, they start doing anaerobic respiration, which is the incomplete breakdown of glucose. The equation is Glucose = Lactic acid + energy (C6H12O6 = 2C3H6O3). Lactic acid builds up in muscles, which is painful. It also results in oxygen debt. This means you have to repay the oxygen that wasn’t used in respiration to remove the acid. Oxygen oxidises it to carbon dioxide and water (C3H6O3 + 3O2 = 3CO2 + 3H2O). This means you still breathe hard after exercise.

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Genetic diagrams

Alleles are different variations of the same gene (see DNA). A genotype is what alleles an organism has. In genetic diagrams, these alleles are represented by uppercase and lowercase letters. If the two alleles are the same (both uppercase or lowercase), then the organism is homozygous. If they are different (one uppercase and one lowercase), then the organism is heterozygous. If heterozygous, only one allele can decide what the characteristic is, or phenotype. The characteristic that is present is the dominant allele, represented by the uppercase letter. The lowercase letter is the recessive allele. For an organism to have a recessive trait, both the letters have to be lowercase.

An Austrian monk called Gregor Mendel was the first to discover this. He experimented with tall and small pea plants. He published his results in 1866, concluding that characteristics were passed on to the next generation of plants when ‘adults’ crossed. However, they weren’t accepted because no one knew what he was talking about. No one knew about genes or DNA, which you would have to know about to understand the significance of Mendel’s discovery. His work was overlooked until after his death when genes and DNA were discovered.

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Genetic disorders

Disorders can be passed on as well as characteristics. There are two examples:

  • Cystic Fibrosis: Cause by a recessive allele, this disorder produces thick, sticky mucus in the cell membranes of cells in the air passages and pancreas. The parents don’t have to be a sufferer to carry a recessive allele of the disorder. If they are heterozygous, then they are carriers
  • Polydactyly: Caused by a dominant allele, this disorder means a baby is born with extra fingers or toes. As long as a person has a dominant allele of the disorder, they will be a sufferer.

Embryos can be screened for disorders if the embryo is being fertilised through IVF (In Vitro Fertilisation). Before it is implanted, scientists can remove a cell and study its genes. There is a big debate on whether this should be allowed or not:

Advantages are that the woman won’t give birth to a child that has to suffer a disorder. Treating disorders costs the government and the taxpayer a lot of money and there are laws to stop embryonic screening from going too far (eg. selecting the sex of the embryo). However, it isn't long before people will start demanding their ‘ideal’ child and picking their traits. This also implies that people with disorders are undesirable. This can lead to prejudice in communities. Rejected embryos which are destroyed all have the potential for human life and the actual screening can be expensive.

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DNA = DeoxyriboNucleic Acid

DNA contains the instructions to put an organism together and make it work. It’s found in the nucleus of animal and plant cells in long molecules called chromosomes. Each cell has 23 pairs of chromosomes.

A gene is a section of DNA that contains the instructions to creating a specific protein. They tell cells in what order to place amino acids in a chain to create the specific one.

Almost everyone has unique DNA, with the exceptions of identical twins and clones. DNA, or genetic, fingerprinting cuts up a person’s DNA into sections before separating them. This forms a unique pattern (unless they have an identical twin or a clone of themselves) which can tell people apart. This is used in forensic science and paternity testing.

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Cell Division

There are two ways that cells divide: Mitosis and meiosis.

Mitosis is when a cell reproduces itself by splitting to form two identical daughter cells. It is used when the organism wants to grow or to replace damaged cells. If a cell needs to divide, it duplicates its DNA so there is one copy for each cell. They form X-shaped chromosomes, with each arm being an exact copy of the other. The chromosomes then line up before cell fibres pull them apart. Each arm goes to opposite ends of the cell. Membranes form around each of the two sets of chromosomes to form the nuclei. The cytoplasm divided to complete the division. Asexual reproduction (used in strawberry plant runners) also uses mitosis. The offspring have the same genetic material so there’s no variation.

Gametes are the sex cells of organisms. In humans, the females have eggs and the males have sperm. Gametes have 23 chromosomes. This is so that when the two combine, the fertilised egg has the right number of chromosomes. The new organism will have traits from both parents, which is how sexual reproduction produces variation. Meiosis has the same structure as mitosis. However, meiosis has a second division, which produces 4 daughter cells.

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Stem Cells and Genetic Variation

Stem cells are undifferentiated cells. They can develop into different types of cells depending on the instructions they’re given. They are found in two places: early human embryos and adult bone marrow. Bone marrow cells can only turn into certain types of cells but embryonic stem cells can turn into any cell. These can be used to treat diseases in people by replacing the faulty cells with the new ones. However, a lot more research is still needed because to produce certain cells, the stem cells need to have a certain environment to change. Some people are against stem cell research because each embryo is a potential human life. A counter-argument is that existing patients are more important that cells that haven’t yet developed a living baby.

Genetic variation is caused by the mix of chromosomes in a fertilised egg, which takes traits from its mother and father. An example is that one of the pairs of chromosomes decides the sex of the baby. In women, this is an ** chromosome. In men, this is an XY chromosome. The Y chromosome causes male traits. In a genetic table, the mix of the two means that the possible combinations are a 1:1 ratio of ** and XY, which is why the human population is close to a 50:50 mix of males and females.

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Fossils are the remains of organisms from many years ago, found in rocks. They form in three ways:

  • Gradual replacement by minerals: The most common outcome. Things that don’t decay easily (eg. Bones and teeth) can last a long time when buried. They’re eventually replaced by minerals when they decay, forming a rock-like substance shaped like the bones or teeth. This is also surrounded by rock, but the fossil remains distinct
  • Casts and impressions: Some organisms are buried in soft material (eg. clay). The clay hardens around it and the organism decays, leaving a cast
  • Preservation: This happens where no decay is possible. In amber and tar pits, there’s no oxygen or moisture so decay microbes can’t survive. In glaciers, it’s too cold and in peat bogs it’s too acidic

No one knows for certain how life began though. Scientists think that this is because the first ever organisms were soft bodied and that their remains completely decayed.

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Extinction and speciation

The extinction of a species is caused by six things:

  • The environment changes too quickly to react to
  • A new predator wipes out the population
  • A new disease wipes out the population
  • A new competitor for food arises. The original species can’t compete
  • A catastrophic event such as a volcanic eruption or an asteroid strike
  • Through the natural cycle of speciation

Speciation is the development of a new species. It occurs when two populations of the same species:

  • Become so different that they cannot produce fertile offspring together
  • Become separated by physical barriers
  • Evolve to have a characteristic that gives them an advantage
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