AQA GCSE Biology B2.2 Organisms in the Environment

revision notes

HideShow resource information

B2.2.1 Photosynthesis Part 1

  • This is the process by which plants and algae make food.
  • Equation:  carbon dioxide + water (+light energy)---> glucose + oxygen.
  • The carbon dioxide is absorbed from the air by the leaves, and water by the roots.
  • Cells of algae and leaves of plants have many chloroplasts, which have a green substance in called chlorophyll which absorb light energy, which is used to carry out photosynthesis.
  • Some of the simple sugar created-glucose-can be used immediately by the cells of the plant. Most of it is converted to insoluble starch and is stored.
  • Iodine solution, a yelow-brown liquid, can be used to test for the presence of starch-it turns dark blue. This can be used to prove that photosynthesis has taken place.
  • Leaf adaptations:
    • Most leaves are broad, maximising surface area for light to fall on.
    • There are air spaces in a spongy layer to allow carbon dioxide to get to the cells, and for oxygen to leave them by diffusion.
    • It has epidermis which is thin and transparent, which allows more light to reach the palisade cells.
    • Palisade cells contain many chloroplasts, and there is a layer of these cells near the top of the leaf in order to absorb more light.
    • There's a thin cuticle made of wax, in order to protect the leaf without blocking light.
1 of 9

B2.2.1 Photosynthesis Part 2

  • The lower epidermis has small gaps called stomata which allow gases to move in and out of the leaf. The gaps are controlled by guard cells on either side of the stoma, and they change shape in order to open/close the gap.

leaf (

2 of 9

B2.2.2 Limiting Factors Part 1

  • Different factors can affect the rate of photosynthesis-limiting factors.
  • Light: For most plants, the brighter the light, the faster the rate of photosynthesis, until a point when the rate of photosynthesis stays the same no matter the brightness of the light. This means that something else is the limiting factor.
    light intensity (
  • Temperature: As the temperature rises, the rate of photosynthesis increases until around 40-50ºC, where enzymes then denature and don't work anymore, and then the rate of photosynthesis falls.

temperature (

3 of 9

B2.2.2 Limiting Factors Part 2

  • Carbon dioxide levels:
    • The atmosphere only has 0.04% carbon dioxide, which means that the level of carbon dioxide is often a limiting factor. Increasing the levels of carbon dioxide increases the rate of photosynthesis.
    • It has the same graph as the light intensity one; the rate of photosynthesis increases as the amount of carbon dioxide does, until a certain point, which means that at that point, carbon dioxide is not a limiting factor anymore.

carbon dioxide ( 

4 of 9

B2.2.3 How Plants use Glucose

  • Glucose may be:
    • Converted to insoluble starch for storage.
    • Used for respiration-it is broken down using oxygen to provide energy for cells.
    • Coverted into fats and oils for storage, sometimes used in cell walls to make them stronger. Also, plants often use these as an energy store for seeds, which provides a lot of energy for a seed when it germinates.
    • Used to produce cellulose(more complex carbohydrates) to strengthen cell walls.
    • Used to make amino acids, by combing sugars with nitrate ions and other mineral ions from the soil. The amino acids are then built up into proteins to be used in cells-this uses energy from respiration.
  • Glucose is soluble in water, which means that if it was stored in cells it could affect the way water moves in and out of cells, and lots of glucose being stored in cells could affect the water balance of the entire plant. Starch is insoluble, which means that it won't affect the water.
  • Energy has to be storeed for when it is dark or light levels are low.
  • Starch can be stored in special storage areas such as tubers or bulbs, to help the plant survive during winter. These starch stores can be taken to eat-such as potatoes and onions, which are full of starch.
5 of 9

B2.2.4 Making the Most of Photosyntheis

  • People who grow plants have to try to give their plants the best conditions for growing by controlling the limiting factors, which will then give people such as farmers the best profit when the plants grow fast and big.
  • Many have glass or perpex greenhouses. Farmers use a similar idea in polytunnels, used for growing crops like tomatoes, strawberries and potatoes.
  • In a greenhouse, the environment is much easier to control, and the atmosphere inside is a lot warmer, increasing the rate of photosynthesis and causing them to grow faster and produce higher yield.
  • Greenhouses can also be used to grow fruit like oranges which are normally harder to grow in the UK.
  • Rate of photosynthesis in a greenhouse can be increased by using heaters and lamps, except the temperature and light intensity should be controlled , and by adding carbon dioxide to the air. Nitrate ions can also be added to the soil so plants can make amino acids to make proteins.
  • Companies control the limiting factors artificially to help plants grow quicker.
  • Plants can even be grown in water, witha perfect balance of mineral ions instead of soil to maximise rate of growth-hydroponics.
  • It is expensive to control conditions using computer software, but huge profits can be made, and no ploughing or preparing the land is needed, and the plants are clean and unspoilt.
6 of 9

B2.2.5 Organisms in their Environment

  • Many factors affect living organisms.
  • Temperature- In cold climates temperature is a limiting factor which means that plants are generally smaller there and less herbivores are found in that area.
  • Nutrients - The level of mineral ions available strongly impacts the distribution of plants. However, carnivorous plants like the Venus flytraps can survive on low levels of nitrate ions because they can trap, digest and get mineral ions from animal prey.
  • Light levels -Some plants are adapted to living in low light levels-they might have more chlorophyll or bigger leaves, but most plants need higher light levels.
    Breeding cycles of many animals & plants are linked to day length-they only live where day length is right for them
  • Water - As a rule, there are less plants and animals in the desert due to the low availability of water.
    However, when it rains, plants will grow very quickly and then many animals will come to eat them
  • Oxygen and carbon dioxide - Oxygen levels have a big impact. However, some invertebrates can survive in water in low levels of oxygen. Most need a high level of dissolved oxygen. The distribution isn't affected too much by this as oxygen levels vary very little.
       - Carbon dioxide levels can affect the distribution-for example, mosquitoes are attracted to the animals on whose blood they feed by high carbon dioxide levels.
7 of 9

B2.2.6 Measuring the Distribution of Organisms

  • Quadrats
    • This is the simplest way to sample and area (count the number of organisms). It is usually a wooden or metal square frame that you lay on the ground to outline your sample area.
    • A quadrat with 0.5m sides gives a 0.25m2 sample area.
    • You use the same size one each time and sample as many areas as possible to make the data reliable.
    • Sample size is very important, and sample areas must be selected randomly in order to make sure that results reflect the true distributions. To make it random, a person could spin around, walk a certain amount of paces and drop the quadrat. A more scientific way would be to use a random number generator on a scientific calculator.
    • After a number of random samplings, a mean number of organisms per m2 has to be taken. This is quantatative sampling. It can be used to compare distribution of the same organism in different habitats.
    • You could also find the median, mode and range. Median- the middle value of readings when they are in order. Mode-the most common value. Range-the highest value subtracting the smallest value.
  • Transect
    • You could sample along a transect, although it is not random. The most common is a line transect.
    • You could stretch a tape between two points, and then sample along the tape at regular intervals using a quadrat, showing the distribution along it. You can also measure physical factors like light levels and pH that would affect growth of plants along the transect.
8 of 9

B2.2.7 How Valid is the Data?

  • Investigating distribution of organisms can be difficult, so have to be done over a long time and not all variables can be controlled.
  • Reproducible and valid methods must be used to collect data.
  • Reproducible-other people can do the same investigation and get similar or the same results as you.
  • Valid-the investigation must answer the question or prove/disprove your hypothesis.
  • Sample size is very important-the bigger, the more reproducible and valid data you get.
  • Method of sampling should be appropiate-random quadrats for the distribution of plants in an area. To measure change in distribution over a range of habitats, a transect is better.
  • If you are measuring over time, you must be able to replicate your method each time you repeat readings.
  • Changes in distribution is often used to prove environmental change.
  • When carrying out an investigation, as many variables as possible have to be controlled, so others could do the same investigation too, making your results more likely to be reproducible.
  • Outside, it is harder to control variables, but as many as possible still should be controlled.
9 of 9


No comments have yet been made

Similar Biology resources:

See all Biology resources »See all Ecology and Environmental Science resources »