AQA Biology 2a

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Human / Animal Cell

1) Nucleus -- contains genetic material that controls the activities of the cell.

2) Cytoplasm - gel-like substance where most of the chemical reactions happen. It contains enzymes that control these chemical reactions.

3) Cell membrane - holds the cell together and controls what goes in and out.

4) Mitochondria - these are where most of the reactions for respiration take place. Respiration releases energy that the cell needs to work.

5) Ribosomes - these are where proteins are made in the cell.


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Plant Cell

Plant cells usually have all the bits that animal cells have, plus a few extra things that animal cells don't have:

1) Rigid cell wall - made of cellulose. It supports the cell and strengthens it.

2) Permanent vacuole - contains cell sap, a weak solution of sugar and salts.

3) Chloroplasts - these are were photosynthesis occurs, which makes food for the plant. They contain a green substance called chlorophyll.


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Yeast is a single-celled organism.

Yeast is a microorganism. A yeast cell has a nucleus, cytoplasm, and a cell membrane surrounded by a cell wall.


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Bacterial cells have no nucleus.

Bacteria are also single-celled microorganisms.

1) A bacterial cell has cytoplasm and a cell membrane surrounded by a cell wall.

2) The genetic material floats in the cytoplasm because bacterial cells don't have a nucleus.


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1) "Diffusion" is simple. It's just the gradual movement of particles from places where there are lots of them to places where there are fewer of them.

2) That's all it is - just the natural tendancy for stuff to spread out.

Diffusion is the spreading out of particles from an area of high concentration to an area of low concentration.

3) Diffusion happens in both solutions and gases - that's because the particles in these substances are free to move about randomally.

4) The simplest type is when different gases diffuse through each other. This is what's happening when the smell of perfume diffuses through the air in a room:

5) The bigger the difference in concentration, the faster the diffusion rate.

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Cell Membranes

1) They're clever because they hold the cell together BUT they let stuff in and out as well.

2) Dissolved substances can move in and out of cells by diffusion.

3) Only very small molecules can diffuse through cell membranes though - things like oxygen (needed for respiration), glucose, amino acids and water.

4) Big molecules like starch and proteins can't fit through the membrane:

5) Just like with diffusion in air, particles flow through the cell membrane from where there's a high concentration (a lot of them) to where there's a low concentration (not such a lot of them).

6) They're only moving about randomally of course, so they go both ways - but if there are a lot more particles on one side of the membrane, thete's a net (overall) movement from that side.

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Palisade Leaf Cells

1) Packed with chloroplasts for photosynthesis. More of them are crammed at the top of the cell - so they're nearer the light.

2) Tall shape means a lot of surface area exposed down the side for absorbing CO2 from the air in the leaf.

3) This shape means that you can pack loads of them in at the top of a leaf. 

Palisade leaf cells are grouped together at the top of the leaf where most of the photosynthesis happens.

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Guard Cells

1) Special kidney shape which opens and closes the stomata (pores) in a leaf.

2) When the plant has lots of water the guard cells fill with it and go plump and turgid. This makes the stomata open so gases can be exchanged for photosynthesis.

3) When the plant is short of water, the guard cells lose water and become flaccid, making the stomata close. This helps stop too much water vapour escaping.

4) Thin outer walls and thickened inner walls make the opening and closing work.

5) They're also sensitive to light and close at night to save water without losing out on photosythesis.

Guard cells are therefore adapted to their fuction of allowing gas exchange and controlling water loss within a leaf.

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Red Blood Cells

1) Concave shape gives a big surface area for absorbing oxygen. It also helps them pass smoothly through capillaries to reach body cells.

2) They're packed with haemoglobin - the pigment that absorbs the oxygen.

3) They have no nucleus, to leave even more room for haemoglobin.

Red blood cells are an important part of the blood.

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Sperm and Egg Cells

1) The main functions of an egg cell are to carry the female DNA and to nourish the developing embryo in the early stages. The egg cell contains huge food reserves to feed the embryo.

2) When a sperm fuses with an egg, the egg's membrane instantly changes its structure to sstop any more sperm getting in. This makes sure the offspring end up with the right amount of DNA.

3) The function of a sperm cell is basically to get the male DNA to the female DNA. It has a long tale and a streamlined head to help it swim to the egg. There are a lot of mitrochondria in the cell to provide the energy needed.

4) Sperm also carry enzymes in their heands to digest through the egg cell membrane.

Sperm and eggs are very important cells in reproduction.

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Large Multicellular Organisms

1) Specialised cells carry out a particular fuction.

2) The process by which cells become specialised for a particular job is called differentiation.

3) Differentiation occurs during the development of a multicellular organism.

4) These specialised cells form tissues, which form organs, which form organ systems.

5) Large multicellular organisms (e.g. squirrels) have different systems inside them for exchanging and transporting materials.


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Similar Cells

A tissue is a group of similar cells that work together to carry out a particular function. It can include more than one type of cell. In mammals (like humans), examples of tissues include:

1) Muscular tissue, which contracts (shortens) to move whatever it's attached to.

2) Glandular tissue, which makes and secretes chemicals like enzymes and hormones.

3) Epithelial tissue, which covers some parts of the body, e.g. the inside of the gut.

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An organ is a group of different tissues that work together to perform a certain function. For example, the stomach is an organ made of these tissues:

1) Muscular tissue, which moves the stomach wall to churn up food.

2) Glandular tissue, which makes digestive juices to digest food.

3) Epithelial tissue, which covers the outside and inside of the stomach.

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An organ system is a group of organs working together to perform a particular function. Forexample, the digestive system (found in humans and mammals) breaks down food and is made up of these organs:

1) Glands (e.g. the pancreas and salivary glands), which produce digestive jucies.

2) The stomach and small intestine, which digest food.

3) The liver, which produces bile.

4) The small intestine, which absorbs soluble food molecules.

5) The large intestine, which absorbs water from undigested food, leaving faeces.

The digestive system exchanges materials with the environment by taking in nutrients and releasing substances such as bile.

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Plant Cells, Tissues & Organs

Plants are made of orgas like stems, roots and leaves. These organs are made of tissues. For example, leaves are made of:

1) Mesophyll tissue - this is where most of the photosynthesis in a plant occurs.

2) Xylem and phloem - they transport things like water, mineral ions and sucrose around the plant.

3) Epidermal tissue - this covers the whole plant.

Photosynthesis Equation:

carbon dioxide + water ---sunlight---chlorophyll---> glucose + oxygen

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1) Photosynthesis is the process that produces 'food' in plants and algae. The 'food' it produces is glucose.

2) Photosyntheis happens inside the chloroplasts.

3) Chloroplasts contain a green substance called chloroplyll, wich absorbs sunlight and uses its energy to convert carbon-dioxide (from the air) and water (from the soil) into glucose. Oxygen is also produced as a by-product.

4) Photosynthesis happens in the leaves of all green plants - this is largely what the leaves are for. 

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The Rate of Photosynthesis

The rate of photosynthesis is affected by the intensity of light, the volume of CO2, and the temperature.

Plants also need water for photosythesis, but when a plant is so short of water that it becomes the limiting factor in photosynthesis, it's already in a lot more trouble.

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The Limiting Factor

1) Any of these three factors can become the limiting factor. This just means that it's stopping photosynthesis from happening any faster.

2) Which factor is limiting at a particular time depends on the environmental conditions:

  • at night light is the limiting factor
  • in winter it's often the temperature
  • if it's warm enough and bright enough, the amount of CO2 is usually limiting

You can do experiments to work out the ideal conditions for photosynthesis in a particular plant. The essentail type to use is a water plant like Cnadian pondweed - you can easily measure the amount of oxygen produced in a given time to show how fast photosynthesis is happening.

You could either count the bubbles given off, or if you want to be a bit more accurate you could collect the oxygen in a gas syringe.

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Not Enough Light

  • 1) Light provides the energy needed for photosynthesis.
  • 2) As the light level is raised, the rate of photosynthesis increases steadily - but only up to a certain point.
  • 3) Beyond that, it won't make any difference because then it'll be either the temperature or the CO2 level which is the limiting factor.
  • 4) In the lab you can change the light intensity by moving a lamp closer to or further away from your plant.
  • 5) But if you just plot the rate of photosynthesis against "distance of lamp from the beaker", you get a weird shaped graph. To get a graph like thisone you either need to measure the light intensity at the beaker using a light meter or do a bit of nifty maths with your results.


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Too Little Carbon Dioxide

  • 1) CO2 is one of the raw materials needed for photosynthesis.
  • 2) As with light intensity the amount of CO2 will only increase the rate of photosynthesis up to a point. After this the graphy flatterns out showing that CO2 is no longer the limiting factor.
  • 3) As long as light and CO2 are in plentiful supply then the factor limiting photosynthesis must be the temperature.
  • 4) There are loads of different ways to control the amount of CO2. One way is to dissolve different amounts of sodium hydrogencarbonate in the water, which gives of CO2.


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Temperate has to be Right

  • 1) Usually, if the temperature is the limiting factor it's because it's too low - the enzymes needed for photosynthesis work more slowly at low temperatures.
  • 2) But if the plant gets too hot, the enzymes it needs for photosynthesis and its other reactions will be damaged.
  • 3) This happens at about 45 dgrees C (which is pretty hot for outdoors, although greenhouses can get that hot if you're not careful).
  • 4) Experimentally, the best way to control the temperature pf the flask is to put it in a water bath.


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In all of these experiments, you hav to try and keep all the variables constant apart from the one you're investigating, so it's a fair test:

  • use a bench lamp to control the intensity of the light (careful not to block thelight with anything)
  • keep the flask in a water bath to help keep the temperature constant
  • you can't really do anything about the CO2 levels - you just have to use a larg flask, and do the experiments as quickly as you can, so that the plant doesn't use up too much of the CO2 in the flask. If you're using sodium hydrogencarbonate make sure it's changed each time.
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Artificial Ideal Conditions

1) The most common way to artificially create the ideal environment for plants is to grow them in a greenhouse.

2) Greenhouses help to trap the sun's heat, and make sure that the temperature doesn't become limting. In winter a farmer or gardener might use a heater as well to keep the temperature at an ideal level. In summer it could get too hot, so they might use shades and ventilation to cool things down.

3) Light is always needed for photosynthesis, so commercial farmers often supply artificial light after thesun goes down to give their plants more quality photosynthesis time.

4) Farmers and gardeners can also increase the level of carbon dioxide in the greenhouse. A fairly common way is to use a paraffin heater to heat the greenhouse. As the paraffin burns, it makes carbon-dioxide as a by-product.

5) Keeping plants enclosed in a greenhouse also makes it easier to keep them free from pests and diseases. The farmer can add fertilisers to the soil as well, to provide all the minerals needed for healthy growth.

6) Sorting all this out costs money - but if the farmer can keep the conditions just right for photosynthesis, the plants will grow much faster and a decent crop can be harvested much more often, which can then be sold. It's important that a farmer supplies just the right amount of heat, light, etc. - enough to make the plant grow well, but not more than the plants need, as this would just be wasting money.

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How Plants Use Glucose


1) Plants manufacture glucose in their leaves.

2) They then use some of the glucose for respiration.

3) This releases energy which enables them to convert the rest of the glucose into various other useful substances, which they can use to build new cells and grow.

4) To produce some of these substances they also need to gather a few minerals from the soil.

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How Plants Use Glucose


Glucose is converted into cellulose for making strong cell walls, especially in a rapidly growing plant.

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How Plants Use Glucose


Glucose is combined with nitrate ions (absorbed from the soil) to make amino acids, which are then made into proteins.

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How Plants Use Glucose


Glucose is turned into lipids (fats and oils) for storing in seeds. 

Sunflower seeds, for example, contain a lot of oil - we get cooking oil and margarine from them.

Seeds also store starch.

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How Plants Use Glucose


Glucose is turned into starch and stored in roots, stems and leaves, ready for use when photosynthesis isn't happening, like in the winter.

Starch is insoluble which makes it much better for storing than glucose - a cell with lots of glucose in would draw in loads of water and swell up.

Potato and parsnip plants store a lot of starch underground over the winder so a new plant can grow from it the following spring. We eat the swollen storage organs.

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Distribution of Organisms

1) A habitat is the place where an organism lives, e.g. a playing field.

2) The distribution of organism is where an organism is found, e.g. in a part of the playing field.

3) Where an organism is found is affected by environmental factors such as:

temperature, availability of water, availability of oxygen and carbon dioxide, availability of nutrients, amount of light

4) An organism might be more common in one area than another due to differences in environmental factors between the two areas. For example, in a field, you might find that daisies are more common in the open, than under trees, because there's more light available in the open.

5) There are a couple of ways to study the distribution of an organism. You can:

  • measure how common an organism is in two sample areas (e.g. using quadrats) and compare them
  • study how the distribution changes across an area e.g. by placing quadrats along a transect
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Using Quadrats

A quadrat is a square frame enclosing a known area, e.g. 1 m square. To compare how common an organism is in two sample areas, just following these simple steps:

1) Place a 1 m square quadrat on the ground at a random point within the first sample area. E.g. divide the area into a grid and use a random generator to pick coordinates.

2) Count all the organisms within the quadrat.

3) Repeat steps 1 and 2 as many times as you can.

4) Work out the mean number of organsisms per quadrat within the first sample area.

5) Repeat steps 1 to 4 in the second sample area.

6) Finally compare the two means. E.g. you might find two daisies per meter square in the shade, and 22 daisies per meter square (lots more) in the open field.

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Using Transects

You can use lines called transects to help find out how organisms (like plants) are distributed across an area - e.g. if an organism becomes more or less common as you move from a hedge towards the middle of a field. Here's what to do:

1) Mark out a line in the area you want to study using a tape measure.

2) Then collect data along the line.

3) You can do this by just counting all the organisms you're interested in that touch the line.

4) Or, you can collect the data by using quadrats. These can be placed next to each other along the line or at intervals, for example, every 2 m.

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1) Quadrats and transects are pretty good tools for finding out how an organism is distributed.

2) But, you have to work hard to make sure your results are reliable - which means making sure they are repeatable and reproducible.

3) To make your results more reliable you need to take a large sample size, e.g. use as many quadrats and transects as possible in your sample area. Bigger samples are more representive of the whole population.

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1) For your results to be valid they must be reliable and anwer the original question.

2) To answer the original question, you need to control all the variables.

3) The question you want to answer is whether a difference in distribution between two sample areas is due to a difference in one environmental factor.

4) If you've controlled all the other variables that could be affecting distribution, you'll know whether a difference in distribution is caused by the environmental factor or not.

5) If you don't control the other variables you won't know whether any correlation you've found is because of chance, because of the environmental factor you're looking at or because of a different variable - the study won't give valid data.

6) Use random samples, e.g. randomly put down or mark out you quadrat or transect. If all your samples are in one spot, and everywhere else is different, the results you get won't be valid.

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