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Work through the notes, try the practice questions, then take the quiz. The report tells you exactly what to revise next. (2026)
Question
A model cell is a cube with sides 5 mm. Calculate (a) its surface area, (b) its volume, (c) its SA:V ratio.
Solution
SA = 6 × side² = 6 × 25 = 150 mm².
V = side³ = 125 mm³.
Ratio = SA/V = 150/125 = 1.2; ratio 1.2:1.
Answer
SA = 150 mm²; V = 125 mm³; SA:V = 1.2:1.
Question
Explain how alveoli in the lungs are adapted for efficient gas exchange. (4 marks)
Solution
Each adaptation gets a mark when linked to rate.
Answer
Alveoli are very numerous, giving a large total surface area (1). They have thin walls (one cell thick), giving a short diffusion distance (1). They are surrounded by capillaries with a good blood supply, which carries away diffused oxygen and maintains a steep concentration gradient (1). Ventilation (breathing) keeps a fresh supply of oxygen-rich air at the alveoli (1).
Examiner note
Top-band: each feature linked explicitly to the EFFECT (large SA → more area; thin walls → shorter distance, etc).
Question
A potato chip's mass changed from 4.0 g to 4.6 g after 30 min in sugar solution. Calculate the percentage change.
Solution
Change in mass = 4.6 - 4.0 = 0.6 g.
Divide by initial mass and multiply by 100.
Answer
+15% (mass increased).
Question
A potato chip is placed in 1.0 mol/dm³ sugar solution. Will water move INTO or OUT of the chip? Explain.
Solution
Inside cell: dilute cell sap (high water concentration). Outside: concentrated sugar solution (low water concentration).
Water moves from high to low water concentration → OUT of the chip.
Answer
Water moves OUT of the chip. The cell sap is more dilute than the surrounding sugar solution, so water passes through the partially permeable cell membrane down its concentration gradient.
Question
Explain why the small intestine cannot use only diffusion to absorb all the glucose from food. (3 marks)
Solution
At the end of digestion, glucose in the gut runs low.
Gut glucose concentration drops below blood concentration.
Diffusion would move glucose BACK to the gut. Active transport reverses this.
Answer
As digestion finishes, glucose in the gut becomes lower than in the blood (1). Diffusion would move glucose back into the gut, since diffusion is down a gradient (1). Active transport pumps glucose into the blood against the gradient, using energy from respiration, so none is wasted (1).
Question
Give TWO differences between active transport and diffusion.
Solution
Direction of movement.
Energy requirement.
Answer
Percentage change in mass
When to use
Use to compare osmosis results across chips of different starting masses. Always multiply by 100 at the end.
Net movement of particles from a region of higher concentration to a region of lower concentration (down a concentration gradient).
A difference in concentration of a substance between two regions. The steeper the gradient, the faster diffusion occurs.
Ratio of an object's surface area to its volume. Decreases as size increases. Higher ratio → faster diffusion-driven exchange.
The net movement of water from a region of higher water concentration to a region of lower water concentration across a partially permeable membrane.
The external concentration at which there is no net movement of water in or out of a cell. Mass change is zero.
A membrane that allows small molecules (e.g. water) to pass but blocks larger solutes (e.g. sugar). The cell membrane is naturally partially permeable.
A plant cell that has gained water by osmosis and is firm (vacuole pushes membrane against cell wall). Provides support to the plant.
Movement of a substance across a cell membrane from a region of lower concentration to a region of higher concentration, requiring energy from respiration.
ATP molecules released during respiration in mitochondria; used to power active transport, muscle contraction, and many other cell processes.
Mistake
Saying diffusion uses energy.
Why it happens
Confusing it with active transport.
How to avoid it
Diffusion is PASSIVE — no energy from respiration. Active transport needs energy.
Mistake
Saying diffusion stops when particles reach equilibrium.
Why it happens
Looks static at equilibrium.
How to avoid it
Particles keep moving in both directions; NET movement is zero. Concentration is even, but exchange continues.
Mistake
Calculating cube SA without multiplying by 6 (all faces).
Why it happens
Students compute side² and stop.
How to avoid it
A cube has 6 faces. SA = 6 × side². Always.
Mistake
Saying 'sugar moves across the membrane' in osmosis.
Why it happens
Confusion with diffusion.
How to avoid it
Osmosis = WATER only. Sugar is the solute — it stays put because the membrane is partially permeable.
Mistake
Dividing change by FINAL mass instead of INITIAL.
Why it happens
Misremembering the formula.
How to avoid it
Always divide by the INITIAL mass; % change tells us how much the starting value changed.
Mistake
Forgetting to multiply by 100 to get a percentage.
Why it happens
Rush.
How to avoid it
Final step of % calculation = × 100. Check your answer — % changes are usually in the range of single digits to tens, not decimals.
Mistake
Saying active transport just 'needs energy' without saying it comes from respiration.
Why it happens
Vague answers.
How to avoid it
Always quote 'energy from respiration' or 'ATP from mitochondria'.
Mistake
Saying water moves by active transport.
Why it happens
Mix-up with osmosis.
How to avoid it
Water = osmosis (passive). Ions and glucose against gradient = active transport.