Chapter 2 Movement of Substances

Question 1

Define diffusion [2] (2011/B9)

Answer 1

The net movement of substances down the concentration gradient, from a region of higher concentration to a region of lower concentration.

Question 2

Define osmosis [3] (2011/B9)

Answer 2

The net movement of water molecules from a region of higher water potential to a region of lower water potential across a partially permeable membrane.

Question 3

Suggest one disadvantage of giving medicines using a patch [1] (2011/B9)

Answer 3

• Not all of the medicine in the patch might have been delivered.
• The patch may fall off if not attached securely leading to a lower than required dosage being administered.

Question 4

There are three ways in which substances move into and out of cells in living organisms. Compare these three types of movement of substances [3] (2018/B9)

Answer 4

• Any 3 points:
• Both osmosis and diffusion are passive processes where the expenditure of energy is not needed, while active transport requires energy.
• Osmosis involves the movement of water molecules only while both diffusion and active transport involve the movement of other particles, molecules or ions.
• Osmosis is the movement of water molecules down a water potential gradient, diffusion is the movement of particles down a concentration gradient and active transport is the movement of particles against a concentration gradient.
• A partially permeable membrane must be present for both osmosis and active transport to take place but not for diffusion.

Question 5

Explain changes in cells B and C (B is turgid while C is flaccid) [2] (2018/B9)

Answer 5

1. Cell B:
   - Expands and swells as water molecules have entered the cell by osmosis. The surrounding solution has a higher water potential than that in the cell sap.
   - The central vacuole increases in size and pushes the cell contents against the cell wall.
2. Cell C:
   - Shrinks as water molecules leave the cell by osmosis. The surrounding solution has a lower water potential than that in the cell sap.
   - The vacuole decreases in size and the cell membrane shrinks away from the cell wall.

Question 6

Root hair cells have high ion concentrations. Explain how this is maintained and its importance [3] (2018/B9)

Answer 6

• How high ion concentration is maintained: Root hair cells are able to absorb ions against the concentration gradient by active transport. The ions do not move out of the root hair cells owing to the presence of a partially permeable tonoplast and cell membrane.
• Its importance: Due to the high ion concentration, there is a lower water potential in the cell sap as compared to the soil solution. As such, water molecules can easily enter the root hair cells by osmosis down a water potential gradient and be transported up the roots to other parts of the plant via the xylem.

Question 7

Define active transport (self)

Answer 7

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