Diffusion, Osmosis and Active Transport

Question 1

Diffusion

Answer 1

The net movement of particles from an area of higher concentration to an area of lower concentration (down a concentration gradient).
The larger the difference in concentration, the faster the rate of diffusion.

Question 2

Osmosis

Answer 2

The net movement of water molecules, across a partially permeable membrane, from an area of higher concentration to an area of lower concentration.

Question 3

Active Transport

Answer 3

Active transport is the movement of particles into or out of cells against a concentration gradient. Unlike diffusion or osmosis, it requires energy.

Question 4

Which three things speed up diffusion?

Answer 4

• A larger concentration gradient
• A higher temperature (= molecules having more kinetic energy)
• A greater surface area

Question 5

How do cells change structure in different solutions?

Answer 5

• In pure water or a dilute solution, a cell will gain water via osmosis - becoming turgid. The cell’s contents will push against the cell wall.
• In a concentrated solution, a cell will lose water via osmosis - becoming flaccid. The cell will become soft and floppy, and the cell membrane will start to pull away from the cell wall.
• In a very concentrated solution, a cell will lose a lot of water via osmosis - becoming plasmolysed. The cell will shrink, and the cytoplasm and organelles will move away from the cell wall.

Question 6

Investigating osmosis in a living system

Answer 6

• Cut up a potato into two identical cylinders and measure their lengths.
• Place each of them in a different beaker (one: pure water; two: concentrated sugar solution).
• Re-measure the lengths of the cylinders: if they have drawn in water via osmosis, they should be longer; if they have lost water via osmosis, they should be shorter.

Question 7

Investigating osmosis in a non-living system

Answer 7

• Place Visking tubing over the end of a thistle funnel connected to a glass tube containing sugar solution, and place the funnel in a beaker of pure water.
• Measure the level of the sugar solution in the glass tube.
• Leave overnight. In the morning, water should have been drawn into the tube via osmosis - forcing the solution to move up the tube.