B1 Diffusion, Osmosis and Active Transport

What is diffusion?

• The spreading out of particles in a solution or gas from an area of higher concentration to an area of lower concentration.
• The spreading out of particles in a solution or gas from an area of lower concentration to an area of higher concentration.

How is the rate of diffusion affected by higher temperature?

• Increases (particles have more kinetic energy so they are faster)
• Decreases (particles have less kinetic energy so they are slower)

How is the rate of diffusion affected by the size of the concentration gradient?

• Increases as the size of the concentration gradient increases. (the difference in concentration of a substance from another area - the greater the difference the greater the rate)
• Decreases as the size of the concentration gradient increases. (the difference in concentration of a substance from another area - the greater the difference the lower the rate)

How does the surface area of the cell membrane affect the rate of diffusion of substances in and out of the cell?

• Increases as the surface area of the cell membrane increases (because the bigger the surface area of the cell membrane, the more particles can move across it at the same time)
• Decreases as the surface area of the cell membrane increases (because the bigger the surface area of the cell membrane, fewer particles can move across it at the same time)

Give TWO examples of substances transported by diffusion in the body.

• Oxygen and carbon dioxide between blood and the lungs during gas exchange.
• Urea diffuses from cells into the blood plasma (urea is a waste product that is excreted by the kidneys)
• Mineral ions from soil to root hair cells

What is osmosis?

• The diffusion of water molecules from a dilute solution to a higher concentrated solution through a partially permanent membrane.
• The diffusion of water molecules from a concentrated solution to a lower concentrated solution through a partially permanent membrane.

How does osmosis occur?

• A dilute solution contains more water molecules than a concentrated solution. A partially permanent membrane only allows small molecules through such as water but larger molecules e.g. sugar cannot pass through.
• A dilute solution contains fewer water molecules than a concentrated solution. A partially permanent membrane only allows large molecules through such as water but larger molecules e.g. sugar cannot pass through.

What is active transport?

• The movement of substances from a more dilute solution to a more concentrated solution. i.e. against a concentration gradient and energy is required.
• The movement of substances from a more concentrated solution to a less concentrated solution. i.e. against a concentration gradient and energy is required.

Why does active transport require energy?

• It moves substances against a concentration gradient.
• It moves substances down a concentration gradient.

Where does the energy (ATP) required for active transport come from?

• Respiration
• Photosynthesis