A) Identify the neurons labeled A and B. b) What is the gap labeled C and why is it important? c) Name one chemical substance that transmits nerve impulses. d) Describe the aqueous humour and its adap

4. a)

• A: Motor neuron (or efferent neuron)
• B: Sensory neuron (or afferent neuron)

4. b)

• Identity: The gap labeled C is a synapse.
• Importance: It allows for the unidirectional transmission of nerve impulses from one neuron to another, ensuring that signals are processed and directed correctly within the nervous system.

4. c)

• One example of a chemical substance is acetylcholine (or any other neurotransmitter like dopamine, serotonin).

4. d) i)
The aqueous humour is a clear, watery fluid that fills the space between the cornea and the lens. It is adapted to its function by:

• Maintaining the shape of the front part of the eyeball.
• Providing nutrients (like oxygen and glucose) to the avascular cornea and lens.
• Removing waste products from these structures.

4. d) ii)
One advantage of binocular vision is depth perception (or stereoscopic vision), which allows for accurate judgment of distances and three-dimensional viewing of objects.

5. a)
The paw paw cylinders decreased in length from $50 \text{ mm}$ to $44 \text{ mm}$ and became flexible because they were placed in a hypertonic solution. Water moved out of the cells of the paw paw cylinders by osmosis into the surrounding solution, causing the cells to lose turgor, become flaccid, and shrink.

5. b)
A suitable control set-up would be to place similar paw paw cylinders in distilled water (or an isotonic solution).

5. c)
A diffusion gradient is the difference in concentration of a substance between two regions.

5. d)

• A hypotonic solution has a lower solute concentration (and thus a higher water potential) than the cell's cytoplasm, causing water to move into the cell.
• A hypertonic solution has a higher solute concentration (and thus a lower water potential) than the cell's cytoplasm, causing water to move out of the cell.