Diffusion

Diffusion is the net movement of particles from an area of high concentration to an area of lower concentration, until they are evenly spread out.

The movement is random, and the particles never stop moving around, even when they are evenly distributed.

The bigger the difference in concentration, the faster the rate of diffusion.

Some examples of diffusion in living organisms:

• Oxygen diffuses from alveoli in the lungs into the blood, and from the blood into respiring cells which contain a lower concentration of oxygen
• Carbon dioxide diffuses out of respiring cells into the blood.  In the lungs, the gas diffuses into the air in the alveoli to be breathed out
• Food molecules diffuse from the small intestine into the blood, and from the blood into body cells
• Carbon dioxide enters plant leaves from the air by diffusion, and then diffuses into leaf cells for photosynthesis

Many organs are specialised to exchange materials efficiently by having a large surface area.  For example:

• Alveoli in the lungs
• Villi lining the small intestine
• Root hair cells on plant roots to absorb water and mineral ions
• Leaves have a flattened, broad shape and internal air spaces

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Osmosis

Osmosis is the diffusion of water molecules from a dilute solution, to a more concentrated one, across a partially-permeable membrane.

A partially-permeable membrane allows water molecules to pass through, but not solute molecules.  A cell membrane is partially permeable.

In plant cells, the amount of water that can enter by osmosis is limited by the cell wall, so that they do not burst. 

A cell full of water is said to be turgid.  Turgid cells are important in the support of a plant.

If water leaves a plant cell by osmosis, it becomes soft and flaccid. 

The cytoplasm and membrane may become pulled away from the cell wall.  The cell is said to be plasmolysed. 

If a lot of cells become plasmolysed, the plant will wilt.

Animal cells do not have a cell wall so, eventually, they burst if put into a dilute solution as water enters by osmosis.

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Active Transport

This is the movement of a substance against the concentration gradient. 

This process needs energy from respiration.

Active transport occurs:

• In plant roots to absorb ions from very dilute soil solutions
• To absorb sugar from low concentrations in the small intestine into the blood
• In the kidney to reabsorb sugar into the blood from the tubules
  
  

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