Movement of Substances

Question 1

Define diffusion.

Answer 1

Diffusion is the net movement of particles from a region where they are of higher concentration to a region where they are of lower concentration, down a concentration gradient.

Question 2

Describe the role of diffusion in nutrient uptake and gaseous exchange in plants and humans.

Answer 2

Gaseous exchange of CO2 and O2 in the lungs occur via diffusion.
Oxygen, carbon dioxide and other small molecules such as glucose can pass through the cell membrane via diffusion into and out of the cell.

Question 3

What are the factors that affect the rate of diffusion?

Answer 3

For a higher rate of diffusion,

• Steep concentration gradient
• Temperature (higher)
• Smaller particles
• Solid (slowest), liquid (faster), gas (fastest)
• Larger surface area

Question 4

Define osmosis.

Answer 4

Osmosis is the net movement of water molecules from a solution of higher water potential to a solution of lower water potential, through a partially permeable membrane.

Question 5

Describe the effects of osmosis on plant and animal tissues.

Answer 5

Water potential of surroundings affect cell size.

• Higher WP (hypotonic): PC enlarges and becomes turgid [cell wall prevents bursting]. AC expands and bursts or lyses
• Isotonic: No change
• Lower WP (hypertonic): PC undergoes plasmolysis where the membrane and cytoplasm shrinks away from cell wall; cell becomes flaccid. AC shrinks in size and becomes crenated.

Question 6

Which solution (hypertonic, isotonic, hypotonic) is best for plant cells and which is best for animal cells?

Answer 6

PC: Hypotonic
AC: Isotonic

Question 7

Why is turgor important in plants?

Answer 7

• Maintains the shape of soft tissues in plants.
• Enable leaves and young stems to remain erect
• Keeps non-woody plants firm and upright
• Loss of turgidity will cause plant to wilt
• Changes in turgor cause movements of some plant parts e.g. opening and closing of stomata.

Question 8

Describe what hypertonic solutions do to a plant cell.

Answer 8

• The cell sap (in the vacuole) has a lower w.p. than the surrounding solution.
• Water molecules leave the plant cell via osmosis.
• The membrane and cytoplasm shrinks away from the cell wall and the cell becomes plasmolysed.
• The plant cell becomes flaccid.

Question 9

Define active transport.

Answer 9

Active transport is the process in which energy is used to move particles of a substance across a membrane against its concentration gradient, that is, from a region where they are of lower concentration to a region where they are of higher concentration.

Question 10

State an example of a cell that uses active transport and what active transport is used for in the cell.

Answer 10

• Active transport of dissolved mineral salts by root hair

- Active transport of glucose and amino acids by cells in the small intestine of humans.

Question 11

Discuss the importance of active transport.

Answer 11

-Active transport is an energy-consuming process by which substances are transported against a concentration gradient, as in ion uptake by root hair cells and uptake of glucose by cells in the villi.