Form 4 · Paper 2 essay guide

Movement of Substances Across a Plasma Membrane, Paper 2 essay guide

How Movement of Substances Across a Plasma Membrane appears in Paper 2 essays: themes, a planning grid, a model answer structure and the keyword list.

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Essay themes this chapter supports

Compare diffusion, osmosis and active transport: Define each process precisely, including direction and energy use → State the type of membrane or medium each requires → Give one example of each in a living organism → Explain how the three might act together in one organ, e.g. the small intestine or kidney tubule → Conclude by linking the choice of process to whether movement is with or against the concentration gradient
Explain the structure of the plasma membrane and relate it to selective permeability: Describe the fluid mosaic model: phospholipid bilayer, integral and peripheral proteins, glycoproteins and glycolipids → Explain why small non-polar molecules cross directly while larger or charged particles need a channel or carrier protein → Link membrane structure to at least one named transport process → Give a real example, such as gas exchange at the alveolus or ion transport at the root hair → Conclude by stating why the membrane is described as partially permeable
Discuss how movement of substances across membranes applies in daily life and technology: Introduce the principle of diffusion/osmosis/active transport that underlies the application → Explain kidney dialysis as an application of diffusion across a partially permeable membrane → Explain food preservation by salting or sugaring as an application of osmosis → Explain why IV drip fluids must be isotonic to blood plasma → Conclude by linking each technology back to the correct transport process and the reason it is chosen
Explain how a plant and an animal cell respond differently to being placed in solutions of different concentration: Define hypotonic, isotonic and hypertonic in terms of water potential → Describe what happens to an animal cell in each solution: haemolysis, no change, crenation → Describe what happens to a plant cell in each solution: turgid, no change, plasmolysed → Explain why the presence of a cell wall causes plant and animal cells to respond differently → Conclude with a real-life link, such as wilting in dry soil or IV fluid concentration

How to structure your essay

Open with one or two sentences that define the topic and set the scope.
Devote one paragraph to each key concept, in a logical order.
Use precise biological terms and, where relevant, a labelled diagram.
Give an example or state where the process happens in the body or plant.
Close by linking the ideas back to the question.

Planning grid

Question	Answer
Introduction	Define the three transport processes and state that the essay will explain how they differ and where each applies.
Point 1	Diffusion, no energy, down the concentration gradient, any particle, e.g. gas exchange at the alveolus.
Point 2	Osmosis, no energy, down the water potential gradient, water only, across a partially permeable membrane, e.g. water uptake at the root hair.
Point 3	Active transport, needs ATP from respiration and a carrier protein, against the concentration gradient, e.g. mineral-ion absorption at the root hair or glucose reabsorption in the kidney tubule.
Point 4	Link to membrane structure, the phospholipid bilayer, channel proteins and carrier proteins explain why some particles need help crossing while others diffuse freely.
Conclusion	Summarise that the correct process depends on what is moving, the direction relative to the gradient, and whether energy is available, then link back to a real example.

Key terms to include

diffusion
osmosis
active transport
concentration gradient
water potential
partially permeable membrane
carrier protein
channel protein
fluid mosaic model
turgid
plasmolysis
haemolysis
crenation
ATP
surface area to volume ratio

A model essay outline

Fluid mosaic model, A phospholipid bilayer with proteins scattered through it; it is partially (selectively) permeable.
Diffusion, Net movement of particles from high to low concentration, down a concentration gradient, without energy.
Osmosis, Net movement of water molecules from a less concentrated (dilute) to a more concentrated solution across a partially permeable membrane.
Active transport, Movement of substances against the concentration gradient, using energy from respiration and carrier proteins.
Effects on cells, In hypotonic solution animal cells burst (haemolysis) and plant cells become turgid; in hypertonic solution animal cells shrink (crenation) and plant cells plasmolyse.

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.