B3.1 Exchange of materials

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Diffusion is the net movement of particles of a gas or a solute from an area of high concentration to an area of low concentration along a ocncentration gradient through a partially permeable membrane.

Partially permeable - allowing only certain substances to pass through .

Partially permeable membrane - a membrane which only allows certain substances to pass through.

Osmosis - the net movemetn of water particles from an area of high concentration (of water NOT solute) to an area of low concentration (of water NOT solute) along a concentration gradient through a partially permeable membrane.

In diffusion, the partially permeable membrane only allows gas particles or particles of a solute to go through.

In osmosis, the partially permeable membrane only allows water particles to go through.

The concentration outside your cells need to stay the same for them to work properly. Changes in the concentration of solutions outside the cells may cause water to move into or out of the cells by osmosis.

Osmosis in animals

If a cell uses up all its water in a chemical reaction, more water immediately moves in via osmosis.

If a cell becomes too dilute due to the amount of water made in a chemical reaction, more water immediately moves out via osmosis.

Osmosis restores balance (Homeostasis)

Osmosis in animals: big problems

If the solution outside the cell is dilute, water would move into the cell. The cell would swell up and may burst.

If the solution outside the cell is more concentrated, water would move out of the cell. The cell would shrink and shrivel up.

Osmosis in plants

Water moves into plant cells through osmosis. The vacuole swells up and presses the cytoplasm against the cell wall. The pressure builds up until no more water can physically move into the cell. The cell becomes hard and rigid.

Osmosis in plants: problems

Plants always need the solution outside their cells to be dilute so that water moves into the cells and makes the cell rigid and firm.

If the solution outside the cells is more concentrated than the solution inside the cells, water moves out of the cells. The cells would not remain hard and firm as there would be no water pressure inside the cell. So, the cell would not support the plant.

Active transport is the movement of substances against the concentration gradient across a partially permeable membrane using energy. Substances move from an area of low concentration to an area of high concentration.

Active transport allows cells to absorb ions from very dilute solutions. It also enables them to move substances, such as sugars and ions, from one place to another through the cell membranes.

The energy required for active transport comes from cellular respiration which takes place in the mitochondria.

Importance of Active Transport

Plants: mineral ions in the soil are usually found in very dilute solutions which are absorbed by the plants through active transport.

Humans: sugars such as glucose are often absorbed from the gut and kidney tubules into

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