GCSE Biology Unit 3 - Exchange of Materials

Notes on the first section of GCSE Biology Unit 3 - Exchange of materials

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Active Transport

What is Active Transport?

Active Transport is the movement of particles from a low concentration to a high concentration - Movement against the concentration gradient.

This movement needs energy which comes from cellular respiration. So, cells involved in lots of active transport need lots of Mitochondria.

What situations is it common?

Minerals in the soil are more dilute (low concentration of minerals) than the solution within the plant cells (high concentration of minerals). Active transport means the plants can absorb these minerals.

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Exchange of gases in the lungs 1

What happens when we breathe in?

- Diaphragm contracts and flattens

- Intercostal muscles contract

- Ribs move up and out

This causes the volume to go up and the pressure to go down causing air to flow into the lungs.

What happens when we breathe out?

- Diaphragm and intercostal muscles relax

- Ribs move down and in

This causes the volume to go down and the pressure to go up causing air to flow into the lungs.

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Exchange of gases in the lungs 2

Where does the air go?

When you breathe in air travels through the mouth and nasal passages, it then travels down the trachea, and passes through one of the two bronchi. This then splits into many bronchioles and the air reached the alveoli. This is where the diffusion of gases takes place.

How is the alveoli adapted to diffusion of gases?

- Rich blood supply

- Large surface area

- Moist surface

- Thin walls

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Exchange in the gut

The molecules in your gut need to be able to pass into your bloodstream through both diffusion and active transport.

This is why your food is broken down into a soluble form during digestion.

The food molecules first pass from the gut to the blood vessels by diffusion. This is made easier by the villi.

How is the villi adapted to diffusion?

- Rich blood supply

- Large surface area

- Moist surface

- Thin walls

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Exchange of materials in other organisms 1

Gas exchange in fish

Gills are made up of many thin layers in stacks with a rich blood supply. They are always moist as they work in water.

When a fish is out of water, the gill stacks collapse meaning there is not enough surface area for the fish to get the oxygen it needs.

Gas exchange in tadpoles and frogs

Tadpoles have frilly external gills with a large surface area and rich blood supply. When they turn into a frog, the external gills are reabsorbed - this is know as metamorphosis.

Gas exchange in frogs takes place in:

- The skin (very moist, large surface area, rich blood supply)

- The mouth (large and thin skinned) (Only when on land)

- The lungs (Increase surface area available) (Only when very active on land)

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Exchange of materials in other organisms 2

Gas exchange in insects

No gas exchange can take place through their tough outer covering - so they have an internal respiratory system.

They have tiny holes in them called spiracles these only open when they need oxygen to stop water loss.

These holes lead to a trachea which then leads to many tracheoles. These are freely permeable to gases, have a large surface area and are moist - so most gas exchange happens here.

They have no blood, but the tracheoles come in close contact with the cells.


Whatever the organism, Gas exchange depends on:

Large surface area, moist surfaces, short diffusion distances and a large concentration gradient.

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Exchange in plants 1

Plants diffuse carbon dioxide through their leaves. Leaves are effective for this:

- They are thin (keeping diffusion distance short)

- They are flat (keeping surface area large)

- The have air spaces (so carbon dioxide can come in contact with lots of cells)

How do plants let in carbon dioxide, but keep in water?

- They have a waxy layer (waterproof and gas-proof)

- They have small holes in their surface known as stomata. These holes can be closed when the plant does not need carbon dioxide. This opening and closing is controlled by the guard cells.

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Exchange in plants 2

How do plants take in water?

They take them in through the roots. These are thin, divided tubes with a large surface area.

The root hair cells also increase the surface area, as they have tiny projections which push out between the soil particles.

The membranes of the root hair cells have microvilli which further increase the surface area.

This also means the water only has a short distance to travel up the roots to the xylem.

What about minerals?

Minerals are taken in using active transport, so the plant roots have lots of mitochondria to provide energy for the process.

The roots also provide a large surface area and short travel distance (see above).

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The process of plants losing water by evaporation when their stomata are open is called transpiration.

As water evaporates from the leaves, more water is being brought up the xylem to replace it. This is known as the transpiration stream.

What increases the rate of transpiration?

- Warm, sunny conditions - Hot, dry conditions - Windy conditions

How are plants adapted to stop transpiration?

- Waxy, waterproof layer

- Wilting (This is a defence mechanism against water loss, when the stomata close - preventing water loss - and the leaves collapse - reducing surface area).

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