Biology- Exchange of Materials

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  • Created by: Lizzi
  • Created on: 01-05-13 15:56

Osmosis and active transport


  • is the diffusion of water.
  • water diffuses from a high to low concentration (along the conc. gradient) through a semi-permiable membrane.
  • water moves in and out of cells by osmosis through the cell membrane.

Active Transport

  • allows cells to take in substances against the conc. gradient.
  • uses energy from respiration.
  • ions from solutions or other substances (eg sugar) can be absorbed by active transport.

Examiner's tips!: remember that osmosis is JUST the movement of water, and that active transport requires energy (unlike osmosis and diffusion). 

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Sports Drinks

Most soft drinks contain the following:

  • water (to replace that lost through sweat)
  • sugar (to replace that used for energy release)
  • mineral ions (to replace those lost in sweat)

For normal amounts of exercise, water is just as effective as energy drinks.

If the concentration of minerals, ions, and fluids in the drink matches that of the lost body fluids (sweat), it is an isotonic drink.

Sport scientists believe that for small amounts of exercise, water is okay. However, they believe that professional athletes benefit more from the isotonic drinks, as they lose more minerals, ions etc.

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The Lungs/ Breathing

Lungs are well suited for gas exchange because they have: a large surface area (due to the alveoli, or air sacs, and the folded walls), alveoli, which have a large SA and thin walls, meaning the lungs have a good blood supply, good ventilation (to increase the diffusion gradient), and a moist surface.

When we breathe in: the intercostal muscles (between ribs & diaphragm) contract, ribcage moves up and out and diaphragm flattens, volume of thorax increases, and pressure in thorax decreases and air is drawn in.

When we breathe out: the intercostal muscles relax, the ribcage moves down and in, and the diaphragm becomes domed, the volume of the thorax decrease, the pressure in the thorax increases and air is forced out.

The movement of air in and out of the lungs is ventilation.

Examiner's tip!: be sure to understand how breathing movements alter the thorax's volume and pressure.

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Exchange in the gut

The gut is covered in small villi which aids exchange:

  • they have a large surface area for diffusion,
  • a rich blood supply (to allow for a steeper conc. gradient),
  • a thin wall (one cell thick) to allow for a shorter diffusion space.
The soluble products of digestion can be absorbed into the villi by diffusion or active transport.

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Exchange in Plants

The Stomata in the leaf allow CO2 to enter, and O2 to dffuse in and out.

Leaves are flat and thin with internal air spaces to increase surface area for gas exchange.

Root hair cells increase the sureface area of roots, so that they can get more water and nutrients.(

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Water is lost from plants through evapouration.

The water vapour escapes the leaf through the stomata when they open to allow CO2.

Wilting can protect the plant from excessive water loss.



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