Test your understanding of Lessons 6–10: cell membranes, transport in plants and animals, gas exchange, and photosynthesis.
⏱ 35 min10 MC + 4 SA22 marksLessons 06–10
Lessons Covered
06
Cell Membranes
07
Transport in Plants
08
Transport in Animals
09
Gas Exchange
10
Photosynthesis
Track Your Score
/22
Multiple Choice — 10 marks
1. The fluid mosaic model describes the cell membrane as:
A A rigid lattice of phospholipids with fixed, immobile proteins
B A single layer of phospholipids embedded with enzymes
C A flexible phospholipid bilayer with proteins that can move laterally within the membrane
D A solid protein sheet coated on both sides with phospholipid molecules
2. Which membrane proteins form water-filled pores that allow specific ions to pass rapidly down their concentration gradient without using ATP?
A Glycoproteins
B Carrier proteins
C Channel proteins
D Pump proteins
3. Water moves from the soil into root hair cells primarily because:
A Root hair cells actively pump water inward using ATP
B The soil solution has a higher water potential than the cytoplasm of root hair cells
C Transpiration creates a positive pressure in the root that forces water in
D High CO₂ levels in root cells draw water across the membrane
4. Which statement best explains how water reaches the top of a tall tree via the xylem?
A Living xylem cells use ATP to pump water upward from root to leaf
B Root pressure alone is sufficient to push water to the top of large trees
C Transpiration from leaves creates tension that pulls a continuous water column upward through cohesion between water molecules
D Xylem vessels actively contract and expand to push water toward the leaves
5. In the human double circulatory system, the pulmonary circuit:
A Carries oxygenated blood from the left ventricle to body tissues
B Carries deoxygenated blood from the right ventricle to the lungs, then returns oxygenated blood to the heart
C Transports absorbed nutrients from the intestines to the liver
D Distributes oxygenated blood to all organs except the lungs
6. Which blood vessels allow direct exchange of nutrients and gases between blood and surrounding tissues?
A Arteries, because they have thick muscular walls to withstand high pressure
B Veins, because they have valves that prevent backflow
C Capillaries, because their walls are only one cell thick
D Arterioles, because they regulate blood flow into capillary beds
7. According to Fick's Law, the rate of diffusion across a membrane is:
A Proportional to membrane thickness and inversely proportional to surface area
B Proportional to surface area and concentration gradient, and inversely proportional to membrane thickness
C Equal to surface area multiplied by membrane thickness
D Proportional to the molecular weight of the diffusing substance only
8. Which combination of features makes the alveoli highly effective for gas exchange?
A Large diameter, thick walls, and low total surface area
B Small total surface area, thin walls, and sparse capillaries
C Very large total surface area, walls one cell thick, moist lining, and a dense capillary network
D Rigid walls, dry lining, and large individual volume
9. During the light-dependent reactions of photosynthesis:
A CO₂ is fixed into organic molecules in the stroma using glucose
B Water is split (photolysis), O₂ is released, and ATP and NADPH are produced on the thylakoid membranes
C CO₂ is fixed directly using light energy without any electron carriers
D Glucose is broken down in the cytoplasm to release energy for ATP synthesis
10. A plant is placed in complete darkness for 24 hours. What happens to photosynthesis?
A Photosynthesis continues because the plant uses stored light energy in the chloroplasts
B Only the Calvin cycle stops; the light-dependent reactions continue using O₂ from the air
C The light-dependent reactions stop; ATP and NADPH are depleted; the Calvin cycle then stops because it lacks the energy and reducing power needed
D Photosynthesis is unaffected because CO₂ is still available from cellular respiration
Short Answer — 12 marks
1. Explain how the fluid mosaic model describes the structure of the cell membrane, and explain how this structure makes the membrane selectively permeable. (3 marks)
1 mark: phospholipid bilayer — hydrophilic heads outward, hydrophobic tails inward, proteins embedded and able to move laterally; 1 mark: bilayer blocks ions and large polar molecules; small nonpolar molecules diffuse freely; 1 mark: membrane proteins (channels, carriers, pumps) provide specific, regulated pathways for particular substances
The fluid mosaic model describes the cell membrane as a phospholipid bilayer in which molecules can move laterally. Phospholipids arrange with their hydrophilic heads facing the aqueous environments inside and outside the cell, and their hydrophobic tails facing inward. Proteins are embedded throughout and can drift laterally within the membrane. The phospholipid bilayer creates the basis of selective permeability: the hydrophobic core acts as a barrier to ions and large polar molecules, which cannot dissolve through the nonpolar interior. Small nonpolar molecules such as O₂ and CO₂ can dissolve directly through the bilayer. Selective permeability is further achieved through membrane proteins: channel proteins form specific water-filled pores allowing particular ions across by facilitated diffusion; carrier proteins bind and transport specific molecules; and pump proteins use ATP to actively transport substances against their concentration gradient. Together, the bilayer and protein complement ensure only appropriate substances can enter or leave the cell.
2. Describe how water moves from soil to leaf in a plant. In your answer, refer to osmosis, water potential, and the transpiration-cohesion-tension mechanism. (3 marks)
1 mark: water enters root hair cells by osmosis — soil has higher water potential than root cytoplasm; 1 mark: transpiration from leaf creates tension (negative pressure) in xylem; 1 mark: cohesion (H-bonds between water molecules) maintains a continuous column pulled upward by tension
Water enters root hair cells by osmosis. Root hair cells contain dissolved solutes giving their cytoplasm a lower water potential than the surrounding soil water. Water moves passively from high water potential (soil) to low water potential (root hair cell cytoplasm) across the selectively permeable plasma membrane — no ATP required. Water then moves through the root cortex toward the xylem by a continuing water potential gradient. In the xylem, the transpiration-cohesion-tension (TCT) mechanism operates: water evaporates from leaf mesophyll cells through open stomata (transpiration), reducing water potential in leaf cells and creating tension (negative hydrostatic pressure) in xylem vessels. Water molecules are cohesive — they form hydrogen bonds with each other — maintaining a continuous, unbroken column from root to leaf. The tension generated at the leaf is transmitted down through the column, pulling water upward throughout the plant. Xylem vessels are dead at maturity with no end walls, forming continuous hollow tubes that minimise resistance to flow.
3. Describe three structural adaptations of the alveoli that maximise the rate of gas exchange. Link each adaptation to Fick's Law. (3 marks)
1 mark per adaptation correctly linked to Fick's Law (3 required): large total surface area (∝ SA); walls one cell thick (∝ 1/distance); moist lining + dense capillary network (maintains concentration gradient)
Adaptation 1 — Very large total surface area (~70 m²): Fick's Law states diffusion rate is proportional to surface area. Hundreds of millions of alveoli create an enormous internal surface, dramatically increasing the rate of O₂ and CO₂ exchange.
Adaptation 2 — Walls only one cell thick (~0.5 μm): Fick's Law states diffusion rate is inversely proportional to membrane thickness. The extremely thin alveolar wall (plus the adjacent capillary wall) minimises the total distance O₂ must diffuse from air to blood, maximising diffusion rate.
Adaptation 3 — Moist lining and dense capillary network: Fick's Law states diffusion rate is proportional to the concentration gradient. The moist lining allows gases to dissolve for efficient diffusion. The dense capillary network continuously removes O₂ (binding it to haemoglobin) and delivers CO₂, maintaining steep concentration gradients on both sides. Ventilation renews air on the other side, further sustaining the gradient.
4. Compare the light-dependent and light-independent (Calvin cycle) stages of photosynthesis. Describe: (i) where each stage occurs in the chloroplast, and (ii) the main inputs and outputs of each stage. (3 marks)
(i) Location: The light-dependent reactions occur on the thylakoid membranes — the internal membrane system arranged in stacks (grana) inside the chloroplast. The Calvin cycle (light-independent reactions) occurs in the stroma — the fluid-filled space surrounding the thylakoids, where the enzyme RuBisCO is dissolved.
(ii) Inputs and outputs: Light-dependent stage — Inputs: water (H₂O) and light energy. Outputs: oxygen (O₂, released when water is split by photolysis), ATP (from photophosphorylation), and NADPH (produced when NADP⁺ accepts electrons). These pass to the Calvin cycle. Calvin cycle — Inputs: CO₂ (fixed by RuBisCO), ATP, and NADPH (from the light reactions). Outputs: glucose (as the 3-carbon molecule G3P), ADP and NADP⁺ (recycled back to the light reactions to be re-energised).
Answers
Q1 — C: The fluid mosaic model describes a flexible phospholipid bilayer with proteins that move laterally. "Fluid" = lateral mobility of lipids and proteins; "mosaic" = varied protein composition. A (rigid lattice) and D (solid protein sheet) are incorrect — the membrane is not rigid. B is incorrect — proteins, not enzymes, are embedded in a phospholipid layer.
Q2 — C: Channel proteins form water-filled pores allowing specific ions down their concentration gradient by facilitated diffusion — no ATP required. Carrier proteins (B) bind and change shape to transport molecules. Pump proteins (D) use ATP to work against the gradient. Glycoproteins (A) are involved in cell recognition.
Q3 — B: Root hair cells contain dissolved solutes, giving them lower water potential than soil water. Water moves by osmosis from higher (soil) to lower (root hair cell) water potential across the selectively permeable membrane — no ATP needed. Options A, C, and D are incorrect; the mechanism is passive osmosis.
Q4 — C: The transpiration-cohesion-tension mechanism: transpiration creates tension in xylem; water molecule cohesion (H-bonds) maintains a continuous column pulled upward. Xylem cells are dead at maturity — no ATP used. Root pressure alone is insufficient for tall trees. Options A and D incorrectly suggest active transport.
Q5 — B: Pulmonary circuit: right ventricle → pulmonary artery → lungs (gas exchange) → pulmonary vein → left atrium. It carries deoxygenated blood to the lungs and oxygenated blood back. Option A describes the systemic circuit. Option C describes the hepatic portal system. Option D is incorrect.
Q6 — C: Capillaries have walls only one cell thick — minimising diffusion distance. Their small diameter and slow blood flow maximise exchange time. Arteries (A) are built for pressure. Veins (B) return blood to the heart. Arterioles (D) regulate flow but are not exchange vessels.
Q7 — B: Fick's Law: Rate ∝ (surface area × concentration gradient) / membrane thickness. Option A inverts the SA/thickness relationship. Option C is not a valid expression of Fick's Law. Option D (molecular weight) is not part of Fick's Law for biological membranes.
Q8 — C: Alveoli: very large total SA (~70 m²) → proportional to diffusion rate; walls one cell thick → minimises diffusion distance; moist lining → gases dissolve; dense capillaries → steep concentration gradient. Options A, B, D all describe features that would reduce exchange efficiency.
Q9 — B: Light-dependent reactions (on thylakoid membranes): water split by photolysis → O₂ released; light drives electron transport chain → ATP produced; NADP⁺ reduced to NADPH. Option A describes the Calvin cycle. Option C incorrectly omits electron carriers. Option D describes respiration.
Q10 — C: Without light, the light-dependent reactions stop — no photolysis, no ATP, no NADPH. The Calvin cycle depends on continuous ATP and NADPH from the light reactions — once depleted, CO₂ fixation stops too. The plant continues respiring but cannot photosynthesise. Option A is false; plants cannot store light energy for later use.
SA1: The fluid mosaic model describes the membrane as a phospholipid bilayer — two layers with hydrophilic heads facing the aqueous environments on each side and hydrophobic tails facing inward. Proteins are embedded throughout and can move laterally. Selective permeability arises from two mechanisms: (1) the hydrophobic core blocks ions and large polar molecules while allowing small nonpolar molecules (O₂, CO₂) through freely; (2) membrane proteins — channels allow specific ions by facilitated diffusion; carriers transport specific molecules; pumps use ATP for active transport against gradients — provide regulated, specific pathways for other substances.
SA2: Water enters root hair cells by osmosis because soil water has higher water potential than root cytoplasm (which contains dissolved solutes). Water moves passively from high to low water potential across the selectively permeable membrane. Water then moves through the root cortex into xylem vessels by osmosis. In the xylem, transpiration-cohesion-tension operates: evaporation of water from leaf mesophyll through stomata (transpiration) reduces water potential in leaf cells, creating tension (negative pressure) in xylem. Water molecules are cohesive — hydrogen bonds maintain a continuous unbroken column from root to leaf — allowing the tension at the leaf to pull the entire water column upward.
SA3: (1) Large total surface area (~70 m²) — Fick's Law: rate ∝ SA. Millions of alveoli maximise area available for O₂/CO₂ exchange. (2) Walls one cell thick — Fick's Law: rate ∝ 1/thickness. Thin wall minimises diffusion distance, maximising rate. (3) Moist lining + dense capillary network — Fick's Law: rate ∝ concentration gradient. Moist lining dissolves gases; capillaries continuously remove O₂ and deliver CO₂, maintaining steep gradients for both gases.
SA4: (i) Light-dependent reactions occur on the thylakoid membranes (grana). Calvin cycle occurs in the stroma. (ii) Light-dependent: inputs — H₂O and light; outputs — O₂ (from photolysis), ATP, NADPH. Calvin cycle: inputs — CO₂, ATP and NADPH (from light reactions); outputs — glucose (G3P), ADP and NADP⁺ (returned to light reactions).
Flag for Review
Note any concepts you found difficult. Return to the relevant lessons before attempting Checkpoint 3.