AQA Triple Biology | Exchanging Materials

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  • Created by: Sam Nunn
  • Created on: 03-03-13 15:15


- A partially permeable membrane will only allow certain particles through them, mainly water.

- Diffusion is the net movement of particles from a high concentration to a low concentration.

- Osmosis is diffusion in water.

- The concentration inside your cells needs to stay the same for them to work , however this does not need to happen outside of them, so osmosis can occour.

- If too much water enters the cell, it can swell and burst. This is known as Lysis.

- On the other hand if too much water leaves the cell through osmosis and will shrink and shrivel. This is known a crenation.

- Animals use osmosis to keep the balance of water in their cells. This keeps the internal conditions stable and safe for the animal.

- Plants rely on osmosis to support their stems and leaves. When the water enters the plant cells by osmosis, it pushes the cytoplasm against the plant cell walls. This keeps the shape of the plant stable as well as building pressure until no more water can physically fit into the cell, so it will never become hard and rigid.

- Differences in concentration inside and outside of the cell cause water to move into or out of the cell by osmosis.

Active Transport

- If substances need to move against the concentration gradient, or across a partially permeable membrane , a process called active transport is needed.

- Active transport allows cells to move substances from an area of low concentration to an area of high concentration. This is opposite to diffusion, as they move against the concentration gradient.

- This allows cells to absorb ions from very dilute solutions and to move substances like sugars and ions etc. from one place to another.

- It takes energy for the active transport system to carry a moelcule across the membrane and back. This energy comes from cellular respiration. The rate of active transport and cellular respiration are closely linked because of this. This basically means that if a cell is making a lot of energy, it can carry out lots of active transport.

- Cells involved with active transport usually have a lot of mitochondria to provide all of this energy.

- Active transport is very important for some cells, particularly plant cells which have to absorb ions and minerals from dilute solutions in the soil.

The Sports Drink Dilemma

- Most soft drinks contain water, sugar and mineral ions, however sports drinks contain sugars to replace the sugar used in energy release during activity. They alos contain water and ions to repleace ions lost in sweating.

- When you excercise you release energy by respiration to make your muscles contract, moving your body and using up sugar. You also sweat out water and mineral ions to help control your body temperature. If you sweat too much, water will leave your cells by osmosis and become dehydrated.

- If these lost substances are not replaced, your…


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