▶️ Answer/Explanation
a.
Red blood cells will absorb water by osmosis, since distilled water is hypotonic compared to the cell’s internal environment. As water enters, the cells will swell and may eventually burst (lyse) due to the excessive intake of water.
b.
Leukemia is commonly treated using stem cells.
c.
- Depolarization occurs when Na⁺ ions enter the neuron, causing a change in membrane potential.
- This change triggers local currents, leading to the depolarization of adjacent sections of the axon (if the threshold is reached).
- In myelinated neurons, the impulse moves rapidly by saltatory conduction, jumping from one node of Ranvier to the next.
Markscheme
a. Red Blood Cells in Distilled Water:
Cells absorb water by osmosis and swell/burst (lyse) due to the hypotonic environment.
b. Other Conditions Treated with Stem Cells:
Leukemia (or other hematopoietic diseases) / Skin burns.
c. Nerve Impulse Propagation:
1. Depolarization of one axon segment triggers the next via local currents (Na+ diffusion).
2. Threshold potential (−50 mV) opens Na+ channels, causing depolarization.
3. In myelinated axons, impulses “jump” between nodes of Ranvier (saltatory conduction).
Note: Annotated diagrams are also acceptable.
▶️ Answer/Explanation
a.
- Simple diffusion – Passive movement of small, non-polar molecules (e.g., oxygen, CO₂) from high to low concentration. No energy required.
- Facilitated diffusion – Passive transport of larger or polar molecules (e.g., glucose, ions) through channel or carrier proteins. No energy required.
- Active transport – Movement of substances against the concentration gradient using ATP (e.g., Na⁺/K⁺ pump). Requires energy.
- Osmosis – Passive movement of water molecules from a low solute concentration to a high solute concentration across a semi-permeable membrane. No energy required.
b.
- Two amino acids joined by a peptide bond (CO–NH).
- Water (H₂O) is removed during the condensation reaction.
- R₁ and R₂ are variable side chains of the amino acids.
c.
- ADH is made in the hypothalamus and released by the posterior pituitary when the blood is too concentrated (high osmolarity).
- It acts on the kidneys, especially the collecting ducts, making them more permeable by inserting aquaporins (water channels).
- This causes more water to be reabsorbed into the blood by osmosis, making the urine more concentrated.
- When hydration is restored, less ADH is released, so more water stays in the urine (negative feedback).
Markscheme
a. Types of Membrane Transport:
1. Simple Diffusion: Passive movement of molecules along a concentration gradient (no energy).
2. Facilitated Diffusion: Passive transport through protein channels (no energy).
3. Active Transport: Movement against gradient using ATP (e.g., Na+/K+ pump).
4. Osmosis: Passive water movement across a membrane (hypo → hypertonic).
5. Vesicular Transport: Endocytosis/exocytosis (energy-dependent).
b. Dipeptide Structure:
1. Two amino acids linked by a peptide bond (C=O to N–H).
2. Free NH3+ (N-terminus) and COO− (C-terminus) ends.
3. Chiral carbons with R-groups.
c. ADH Action:
1. Osmoregulation: Hypothalamus detects high blood osmolarity → triggers ADH release from pituitary.
2. Kidney Action: ADH binds to collecting duct/distal tubule cells, inserting aquaporins.
3. Water Reabsorption: Water moves via osmosis into blood, concentrating urine.
4. Negative Feedback: Low osmolarity inhibits ADH → dilute urine.