Biology Triple


Osmosis is the movement of water molecules across a partially permeable membrane from a region of high water concentration to a region of low water concentration.

  • The water molecules pass through both ways through the membrane during osmosis. They move about randomly
  • As there are more water molecules on one side than the other, there's a steady net flow of water into the region with fewer water molecules.
  • This means the strong sugar solution gets more dilute. The water acts as if it is evening it up the concentration either side of the membrane.
  • Osmosis is a type of diffusion.
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Water moves in and out of cells by Osmosis

  • Tissue fluid surrounds the cells in the body
  • The tissue fluid will have a different concentration to the fluid inside the cell.
  • Water will either move into the cell from the tissue fluid, or out of the cell by osmosis
  • If a cell is short of water it will be more concentrated. The solution outside will be more dilute. So water will move into the vell by osmosis.
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Experiment for Osmosis

You cut up an innocent potato into identical cylinders, and get some beakers with different sugar solutions in them. One should be pure water, another should be a very concentrated sugar solution. Different concentrations will be in between.

You meaure the length of the potato cylinders, then leave a few cylinders in each beaker for half an hour. Then you take them out and measure their lengths again. If the potato cylinder has drawn in water by osmosis they will be slighty longer. If water has been drawn out they will be slightly shorter. Plot a graph.

The dependant variable is the chip length.

The independant is the concentration of the sugar solution.

Volume of solution, temperature, time and type of sugar solution should be kept the same to keep it a fair test.

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Gas and Solution Exchange

Substances move by diffusion, osmosis and active transport

Life processes need gases or other dissolved substances before they can happen

E.g. photosynthesis needs carbon dioxide and water need to get into the plant cells. For respiration to take place, glucose and oxygen both have to get inside the cells.

Waste substances also need to mve out of the cells so that the organsim can get rid of them.

In life processes, the gases and dissolved substances have to move through an exchange surface.

Exchange surfaces are adapted : they are thin (short distance to diffuse), large surface area (lots of substances can diffuse at once), animals have lots of blood cells (get stuff into and out of blood quickly), gas exchange surfaces in animals are ventilated (air moves in and out)

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Carbon dioxide diffuses into the air spaces within th eleaf, then it diffuses into the cells where photosynthsis happens. The leaf's structure is adapted so this can happen easily.

  • The underneath of the leaf is the exchange surface. Has little holes called stomata where carbon dioxide can diffuse through.
  • Oxygen and water vapour diffuse out through the stomata.
  • The size of the stomata is controled by guard cells. These close the stomata if the plant is losing water then it is being replaced by the roots.
  • The flattened shape of th eleaf increases the surface area so it is more effective.
  • The air spaces inside the leaf increase the area of the surface so there is more chance for carbon dioxide to get into the cells.

The water vapour evaporates from the cells inside the leaf. Then it escapes by diffusion because there's a lot of it inside the leaf and less of it in the air outside. Evaporation is quickest in hot, dry, windy conditions.

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The Breathing System - Lungs

The Lungs are in the Thorax

  • The thorax is the top part of your body, its separated from the lower part of the body by the diaphragm
  • The lungs are protected by the ribs
  • The air that you breathe in goes through the trachae, this splits into two tubes called bronchi.
  • The bronchi split into smaller tubes called bronchioles.
  • Alveoli are at the end of these, where gas exchange takes place.
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Ventilation is breathing in..

  • Intercostal muscles and diaphragm contract.
  • Thorax volume increases
  • This decreases the pressure
  • This draws air in.

...and Out

  • Intercostal muscles and diaphragm relax
  • Thorax volume decreases
  • This increases pressure
  • So air is forced out
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