GCSE level School biology notes: Explaining diffusion, osmosis, transport, active transport

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experiment to investigate osmosis with potato and sugar solution

Movement of substances in living systems - introduction to diffusion, Fick's Law, osmosis, transport and active transport

Doc Brown's biology exam revision study notes

Some of these revision notes on diffusion in and out of cells, explaining osmosis,

 transport of substances like oxygen, glucose, nutrients, carbon dioxide and waste products

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See also Surface exchange of substances in animal organisms

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Sub-index for this section on transport of substances in living organisms

(1) Introduction to diffusion and demonstration experiments, its importance in biology

(2) A particle model and factors affecting the rate of diffusion and Fick's Law of diffusion

(3) The action of partially permeable cell membranes - selective diffusion and examples

(4) Osmosis - examples and explanation

(5) Some details of examples of osmotic action in individual animal or plant cell types

(6) Osmosis experiments - demonstrations of osmotic action

(7) Active transport - explanation and examples

(8) A comparison of diffusion, osmosis and active transport

See also on other pages:

Detailed notes on transport systems in plants

The human circulatory systems

Examples of surfaces for the exchange of substances in animal organisms

Enzymes - section on human digestion, metabolism and synthesis


Typical Learning objectives  Diffusion, Osmosis, Transport and Active Transport

  • Appreciate that we need to understand how biological and environmental systems operate when they are working well in order to be able to intervene when things go wrong.

  • Appreciate that modern developments in biomedical and technological research allow us to do so.

  • Know and understand that the cells, tissues and organs in plants and animals are adapted to take up and get rid of dissolved substances.

  • Know that different conditions can affect the rate of transfer.

  • Sometimes energy is needed for transfer to take place - active transport.

  • You should be able to use your skills, knowledge and understanding to:

    • evaluate the development and use of artificial aids to breathing, including the use of artificial ventilators,

    • evaluate the claims of manufacturers about sports drinks,

    • analyse and evaluate the conditions that affect water loss in plants.

  • Know and understand that differences in the concentrations of the solutions inside and outside a cell cause water to move into or out of the cell by osmosis.

    • The soft cell wall, or outer membrane of an animal cell, acts as a partially permeable membrane.

    • The water surrounding cells, the tissue fluid, contains the dissolved molecules the cell needs to survive eg sugars, amino acids, oxygen, as well as waste carbon dioxide etc.

    • If the cells are short of water ('partially dehydrated'), the concentration of dissolved substances increases, so water diffuses through the cell membrane into the cells to dilute the cell fluids until equilibrium is established. Conversely, if the cell solution is too dilute, then water will diffuse out from osmotic action across the semi-permeable membrane of the cell wall.

  • Know and understand that substances are sometimes absorbed against a concentration gradient.

    • This means transfer occurs in the opposite direction to the natural direction of diffusion and osmosis.

    • Know that this requires the use of energy from respiration and this process is called active transport.

    • Know that active transport enables cells to absorb ions from very dilute solutions.

    • Active transport is required to absorb nutrients like amino acids, sugars like glucose etc. from the gut when the concentration in the gut is lower than their concentrations in the blood supply, and a healthy body requires these nutrients all the time.

    • If the concentrations of nutrients in the gut is higher than that in the blood stream, then the nutrients will naturally diffuse into the blood stream because of the direction of the concentration gradient (more concentrated ==> less concentrated).

    • If the concentration gradient flow is in the direction of blood stream (higher) to gut (lower), then respiration powered active transport must be used to work against the natural diffusion flow.

    • Remember that absorption by diffusion down the concentration gradient through membranes doesn't require energy from respiration


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