Form 4 · Common mistakes

Movement of Substances Across a Plasma Membrane, common mistakes

The mistakes SPM students make on Movement of Substances Across a Plasma Membrane, why each one loses marks, and the correct version.

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The mistakes, why they lose marks, and the fix

Common mistake	Why it loses marks	What earns the mark
Saying osmosis is the movement of water from high to low concentration.	This description ignores that osmosis is specifically about water potential and a partially permeable membrane, and using 'concentration' loosely can make the direction ambiguous when a solute, not water, is being discussed.	State that osmosis moves WATER from a less concentrated (dilute) solution to a more concentrated solution, i.e. down the water potential gradient, across a partially permeable membrane.
Forgetting that active transport needs energy.	Leaving out the energy requirement loses a mark on its own, since examiners specifically credit the mention of ATP or respiration as a separate marking point from the direction of movement.	State that active transport moves substances against the concentration gradient and requires energy (ATP) released from respiration, using a specific carrier protein.
Using 'turgid' and 'plasmolysed' to describe an animal cell.	These terms describe a change relative to a rigid cell wall, which animal cells do not have, so using them for an animal cell is factually wrong even if the water movement described is correct.	Use turgid and plasmolysed only for plant cells; describe an animal cell instead as undergoing haemolysis (bursting) or crenation (shrinking).
Writing that diffusion needs a partially permeable membrane.	Confusing diffusion with osmosis in this way often causes a student to also give the wrong direction of movement, since diffusion applies to any particle, not only water.	State that diffusion happens in any medium, gas, liquid or across a membrane, and does not require a partially permeable membrane, unlike osmosis, which specifically needs one.
Believing a cell in an isotonic solution has stopped exchanging water with its surroundings.	This misses that water still moves both ways across the membrane, only with no net change in size, which examiners often test with a follow-up question on rate versus net movement.	Explain that water still crosses the membrane in both directions in an isotonic solution; there is simply no net movement because water enters and leaves at the same rate, so cell size stays constant.
Calling facilitated diffusion a form of active transport because it uses a protein.	The presence of a protein does not by itself indicate energy use; the direction of movement relative to the concentration gradient is the deciding factor examiners look for.	State that facilitated diffusion uses a channel or carrier protein but still moves particles down the concentration gradient without using energy, so it remains a passive process, unlike active transport.
Assuming a larger cell or organ always absorbs substances faster than a smaller one.	This ignores that absorption rate depends on the surface area to volume ratio, not overall size, so a larger structure with a lower ratio can absorb relatively more slowly.	State that a smaller structure such as a root hair or a villus usually has a higher surface area to volume ratio, so it absorbs substances relatively faster despite its smaller size.
Writing that a higher temperature always speeds up every membrane process without limit.	This overlooks that active transport depends on carrier proteins, and any statement that ignores denaturation at high temperature will not gain full marks on a question that specifically tests this limit.	Explain that diffusion and osmosis speed up as temperature rises because particles gain kinetic energy, but active transport slows above a certain temperature because the carrier proteins involved denature.
Thinking active transport only happens in plant roots.	This narrow view causes students to miss valid examples in animal physiology, such as the kidney tubule or the nerve cell, when a question asks for an example outside plants.	Recognise that active transport occurs wherever a substance must move against its own concentration gradient, in root hair cells absorbing mineral ions, kidney tubules reabsorbing glucose, and nerve cells running the sodium-potassium pump.
Describing the phospholipid bilayer as having both hydrophilic heads and hydrophobic tails facing outward.	Getting the orientation backwards changes the entire explanation of why the membrane is a barrier to water-soluble substances, and diagram-labelling questions specifically check this detail.	State that the hydrophilic heads face outward toward the watery surroundings on both sides of the membrane, while the hydrophobic tails face inward, away from water, meeting in the middle of the bilayer.
Saying channel proteins and carrier proteins work the same way.	Conflating the two loses marks on structure questions that ask candidates to distinguish the fixed pore of a channel from the shape-changing action of a carrier.	State that a channel protein forms a fixed, water-filled pore for specific ions or small molecules to diffuse through, while a carrier protein binds the particle, changes shape and releases it on the other side, and can work passively or, using ATP, actively.

How to avoid these mistakes

Write the exact SPM definitions of diffusion, osmosis and active transport.
Practise predicting cell changes in hypotonic, isotonic and hypertonic solutions.
Learn the visking tubing and potato-strip experiments and their expected results.
Draw and label the fluid mosaic model, marking the phospholipid bilayer, channel proteins, carrier proteins and glycoproteins, and state what each part does.
List the factors affecting the rate of diffusion, osmosis and active transport, and practise explaining why each one speeds up or slows down movement.
Revise real-life examples such as kidney dialysis, food preservation by salting and intravenous drips, and link each example to the correct transport process.
Practise past-style structured questions that combine a diagram of a cell or membrane with a data table, since this chapter is usually tested by linking a visual to a written explanation.
Note that whether osmosis causes a cell to gain or lose water always depends on comparing the cell's own concentration with its surroundings, not on memorising a fixed direction.

Frequently asked questions

What is osmosis in simple terms?

Osmosis is the net movement of water molecules across a partially permeable membrane, from a solution with more water (less concentrated) to one with less water (more concentrated). No energy is needed, so it is a form of passive transport.

How is active transport different from diffusion?

Diffusion moves particles down the concentration gradient with no energy. Active transport moves them against the gradient and needs energy from respiration plus carrier proteins, which is how roots absorb mineral ions even when the soil has a lower concentration.

Why does a plant wilt when the soil is dry or too salty?

When the surrounding solution is more concentrated than the cell sap, water leaves the cells by osmosis. The cells lose turgor and become flaccid, and if it continues the cell membrane pulls away from the wall (plasmolysis), so the plant wilts.