GCSE AQA Biology 3

Revision cards summarising in detail each of the topics in Biology 3.

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Active Transport

Active transport is the movement of substances against a concentration gradient and/or across a cell membrane, using enegry from respiration.

  • Active transport takes place against a concentration gradient from an area of low concentration to high concentration and it requires energy from respiration.
  • The rate of active transport depends on the rate of cellular respiration. If a cell is making lots of energy, it can carry out lots of active transport. Cells involved in a lot of active transport usually have lots of mitochondria to provide the energy they need.

How active transport works:

  • The substance being transported attatches to large transport proteins in the cell membrane.
  • Energy released through respiration is used to change the shape of the protein.
  • The protein rotates in the membrane and releases the solute molecule inside the cell.
  • The protein rotates back again, often using energy.

Examples of Active transport:

  • Absorption of mineral ions from the soil by the root hair cells in roots of plants.The mineral ions are found in very dilute solutions in the soil, so active transport is important.
  • Glucose is always moved out of your gut and kidney tubules into your blood, even when it is against a large concentration gradient.
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Exchange of gases in the lungs

Your body needs a constant supply of oxygen for cellular respiration. Breathing brings oxygen into your body and removes waste carbon dioxide produced by your cells.

  • Your lungs are found in the upper part of your body - your chest or thorax, protected by your rib cage. Your lungs are separated from the digestive organs in the lower part of your body (your abdomen) by your diaphragm (a strong sheet of muscle).
  • When you breathe in your ribs move up and out, and your diaphragm flattens from its normal domed shape. This pulls oxygen rich air into your lungs.
  • When you breathe out your ribs move down and in. The diaphragm returns to its domed shape, forcing air out of your lungs again, subsequently removing the poisonous carbon dioxide.

Your lungs are especially adapted to make gas exchange more efficient. They are made up of clusters of alveoli which are tiny air sacs well adapted to gaseous exchange.

  • They have a very large surface area, which is kept moist so that gases can dissolve.
  • They have a rich blood supply. This maintains a steep concentration gradient in both directions, so that oxygen can be removed into the blood, and carbon dioxide delivered to the lungs very rapidly.
  • The layer of cells between the air in the lungs and the blood capillaries is very thin. This lets diffusion take place over the shortest possible distance.
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Exchange in the gut

The food you eat is broken down in your gut to form simple sugars such as glucose, amino acids, fatty acids and glycerol. These food molecules need to be made available to your body cells to provide fuel for respiration.

  • The molecules must move from the inside of your small intestine into your bloodstream. They do this by a combination of diffusion and active transport.
  • During digestion your food is broken down into a soluble form so that are they small enough to pass freely through the walls of the small intestine into the blood vessels.
  • During digestion there is a very high concentration of food molecules in the gut and a much lower concentration in the blood.

The lining of the small intestine is folded into thousands of tiny finger-like projections known as villi. These greatly increase the uptake of digested food by diffusion.

  • Only a certain number of digested food molecules can diffuse over a given surface at any one time. The villi increase the surface area available for diffusion many times.
  • The lining of the small intestine has a rich blood supply, which produces a steep concentration gradient for efficient diffusion.
  • The villi have thin walls (only one cell thick) so there is only a short distance across which diffusion takes place.
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Exchange of materials in other organisms

Gas Exchange in Fish

Fish cannot get oxygen directly from the water they live in because their bodies are covered in tiny protective scales. Fish have evolved a very effective respiratory system which works very well in water.

  • Gills are made up of many thin layers of tissue with a rich blood supply.
  • The gills are thin so there is only a short distance for the gases to diffuse across.
  • The surfaces are always moist as they are surrounded by water!
  • In bony fish the gills are contained in a special gill cavity. Water is pumped over them constantly to maintain a concentration gradient.
  • Some types of shark have to keep swimming all the time to keep water moving over their gills.
  • Gills cannot work in air as they will stick together without water surrounding them. The surface area available for oxygen to diffuse across is reduced and consequently, the fish suffocates.
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