Biology B3

Plants and the environment:

Photosynthesis, osmosis, transport in plants, minerals in plants, decays - bacteria and fungi, the carbon and nitrogen cycles, food chains, farming

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Leaves enable photosynthesis to happen.

Photosynthesis is the process by which leaves absorb light and carbon dioxide to produce carbohydrate (food) for plants to grow.

Large surface area: To absorb more light

Thin: Short distance for carbon dioxide to diffuse into leaf cells

Chlorophyll: Absorbs sunlight to transfer energy into chemicals

Network of veins: To support the leaf and transport water and carbohydrates

Stomata: Allow carbon dioxide to diffuse into the leaf

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Photosynthesis 2

During the day a plant is both respiring and photosynthesising. So there is a two-way traffic of oxygen and carbon dioxide both into and out of the plant.

During the night when the plant is respiring but not photosynthesising, oxygen is being taken in but not given out - and carbon dioxide is being given out but not taken in.

Plants use up more carbon dioxide in photosynthesis than they produce in respiration. And they produce more oxygen by photosynthesis than they use in respiration. This means the overall result is that plants put oxygen into the atmosphere. This makes it possible for animals to breathe.

Epidermis is thin and transparent To allow more light to reach the palisade cells Thin cuticle made of wax To protect the leaf without blocking out light Palisade cell layer at top of leaf To absorb more light Spongy layer Air spaces allow carbon dioxide to diffuse through the leaf, and increase the surface area Palisade cells contain many chloroplasts To absorb all the available light

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Osmosis takes place in all living cells.

Plants absorb water from the soil by osmosis through their root hair cells. Plants use water for several vital processes including photosynthesis and transporting minerals.

Osmosis explains how water moves from high water concentration to low water concentration through a partially permeable membrane.

You need to remember:

  • the direction of movement of water
  • the effect of water movement on the volume and therefore the pressure of the different solutions
  • that particles move in both directions through the membrane. Changing the pressure or the concentration on one side of the membrane will change the movement of the particles until equilibrium is reached.
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Osmosis in red blood cells.

If a red blood cell is placed in water, water enters the cell by osmosis.

Because the membrane is quite weak the cell will burst as the volume and therefore the pressure in the cell increases.

Red blood cells shrink when placed in concentrated solutions of sugar as water moves out of them by osmosis.

This makes the cells appear wrinkled when viewed through a microscope.

This does not happen inside the body because the kidneys make sure the concentration of the blood stays about the same as the concentration of the solution inside the red blood cell.

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Osmosis in plant cells

Plant cells have a strong rigid cell wall on the outside of the cell membrane. This stops the cell bursting when it absorbs water by osmosis.

The increase in pressure makes the cell rigid. This is useful as plants do not have a skeleton. Instead the leaves and shoots can be supported by the pressure of water in their cells.

If plant cells lose too much water by osmosis they become less rigid and eventually the cell membrane shrinks away from the cell wall.

  • Lysisbursting an animal cell by osmosis
  • Crenationshrinking an animal cell by osmosis
  • Turgida plant cell fully inflated with water
  • Plasmolyseda plant cell that has lost water causing the cell membrane to be pulled away from the inside of the cell wall
  • Flaccida plant cell that is limp through a reduction of pressure inside the cell
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High temperatures prevent decay, low temperatures slow decay

High temperatures destroy enzymes and proteins, killing the organisms responsible for decay. Low temperatures slow the rate of reaction of enzymes and will prevent growth and reproduction.

Oxygen Lack of oxygen will slow or prevent most forms of decay

Oxygen is needed for respiration by organisms. Some bacteria can survive without oxygen, such as those used in biogas generators.

Water Lack of water will slow or prevent decay Water is needed for transport and to support reactions inside organisms. Decay organisms need water to digest their food.

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Food preservation



The sealed can is heated to kill bacteria. When it cools, no more bacteria can enter.

Cooling or chilling

Ready made meals

The food is kept at a low temperature (4°C) so bacteria cannot reproduce (they're not killed).


Vegetables and meat products

Bacteria cannot reproduce at low temperatures and the water in the food is frozen.


Fruit, flour, soups, pasta

Removing water prevents the microorganisms from growing, or digesting the food.

Adding salt or adding sugar

Jam, crisps, meat

Removes water by osmosis

Adding vinegar

Pickles Changes the pH to prevent the enzymes in the microorganisms from working.

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Carbon Cycle

1. Carbon enters the atmosphere as carbon dioxide from respiration and combustion.

2. Carbon dioxide is absorbed by producers to make carbohydrates in photosynthesis.

3. Animals feed on the plant passing the carbon compounds along the food chain. Most of the carbon they consume is exhaled as carbon dioxide formed during respiration. The animals and plants eventually die.

4. The dead organisms are eaten by decomposers and the carbon in their bodies is returned to the atmosphere as carbon dioxide. In some conditions decomposition is blocked. The plant and animal material may then be available as fossil fuel in the future for combustion.

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Nitrogen cycle

1.Nitrogen gas is converted to nitrate compounds by nitrogen-fixing bacteria in soil or root nodules. Lightning also converts nitrogen gas to nitrate compounds. The Haber process converts nitrogen gas into ammonia used in fertilizers. Ammonia is converted to nitrates by nitrifying bacteria in the soil.

2.Plants absorb nitrates from the soil and use these to build up proteins. The plant may be eaten by an animal, and its biomass used to produce animal protein.

3.Urea and egested material is broken down by decomposers. This results in nitrogen being returned to the soil as ammonia.

4.Decomposers also break down the bodies of dead organisms resulting in nitrogen being returned to the soil as ammonia.

5.Higher only: In some conditions denitrifying bacteria in the soil break down nitrates and return nitrogen to the air. This is usually in waterlogged soil. Improving drainage reduces this effect, making the soil more fertile.

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