The cell membrane and transport (SACE Stage 2 Biology)
“Explain the fluid mosaic model and the mechanisms of passive and active transport across the membrane”
The cell membrane is a fluid mosaic of a phospholipid bilayer with embedded proteins, and it is selectively permeable. Substances cross by passive transport, which needs no energy and moves substances down a concentration gradient (diffusion, osmosis, facilitated diffusion), or by active transport, which uses energy (ATP) to move substances against their gradient.
Jump to a section
What this dot point is asking
You need to describe the structure of the membrane using the fluid mosaic model and explain each mechanism of transport, including whether it requires energy and the direction relative to the concentration gradient.
The fluid mosaic model
The cell membrane is described by the fluid mosaic model:
- It is built from a phospholipid bilayer. Each phospholipid has a hydrophilic (water-loving) phosphate head facing outward and two hydrophobic (water-fearing) fatty acid tails facing inward.
- It is fluid because the phospholipids can move sideways within the layer.
- It is a mosaic because various proteins are scattered through the bilayer, including channel and carrier proteins, plus cholesterol and carbohydrate markers.
The membrane is selectively (partially) permeable: small non-polar molecules cross freely, but ions and large or polar molecules need protein channels or carriers.
Passive transport (no energy)
Passive transport moves substances down their concentration gradient (high to low), so it needs no cellular energy.
Diffusion
Diffusion is the net movement of particles from a region of high concentration to a region of low concentration until evenly spread. Small non-polar molecules such as oxygen and carbon dioxide diffuse directly through the bilayer.
Osmosis
Osmosis is the diffusion of water across a selectively permeable membrane, from a region of higher water concentration (lower solute concentration) to a region of lower water concentration (higher solute concentration).
Facilitated diffusion
Facilitated diffusion is passive movement of substances that cannot cross the bilayer alone (such as glucose and ions) through channel or carrier proteins. It is still down the gradient, so it needs no energy.
A membrane that allows some substances to pass while restricting others, which is what lets a cell control its internal environment.
Active transport (requires energy)
Active transport moves substances against their concentration gradient (low to high). This requires energy from ATP and uses specific carrier proteins (pumps). An example is the sodium-potassium pump in nerve cells.
Bulk transport also requires energy: endocytosis brings large materials into the cell in vesicles, and exocytosis releases them.
What determines the rate of transport
SACE often probes the factors that change transport rate, so be ready to reason about each:
- Concentration gradient. A steeper gradient speeds diffusion and facilitated diffusion. Once the gradient reaches zero (equilibrium), there is no net movement even though particles keep moving.
- Temperature. Higher temperature gives particles more kinetic energy, speeding diffusion; it also speeds active transport up to the point where the carrier proteins denature.
- Surface area. A larger membrane area (microvilli, folds) allows more transport per unit time - the same surface-area-for-rate logic as the rest of Topic 2.
- Number of carrier or channel proteins. Facilitated diffusion and active transport saturate once every carrier is working at full capacity, so adding more substrate beyond that point does not increase the rate. This produces the characteristic levelling-off seen on rate graphs.
Why active transport needs a continuous ATP supply
Because active transport moves substances against their gradient, it cannot run on the random motion of particles; it needs the carrier protein to change shape using energy from ATP hydrolysis. This is why cells that do a lot of active transport - such as kidney tubule cells reabsorbing glucose or nerve cells running the sodium-potassium pump - are packed with mitochondria. If a poison blocks respiration and ATP production, active transport stops while passive transport continues, a contrast examiners like to test.
Osmosis in different solutions
A plant cell with cell sap of moderate solute concentration is placed in pure water, then in a strong salt solution.
- In pure water (higher water concentration outside): water moves into the cell by osmosis. The cell swells and becomes turgid, but the cell wall prevents bursting.
- In strong salt solution (lower water concentration outside): water moves out of the cell by osmosis. The cell loses turgor and the membrane pulls away from the wall - the cell becomes plasmolysed.
Always compare water concentration (or solute concentration) inside and out, then state the net direction of water movement.
- Saying facilitated diffusion needs energy
- It uses proteins but still moves down the gradient, so it is passive - no ATP.
- Getting osmosis direction backwards
- Water moves toward the higher solute (lower water) concentration.
- Confusing active transport direction
- Active transport moves substances against the gradient and requires ATP.
- Describing the membrane as a fixed sandwich
- It is fluid: phospholipids and proteins move within the bilayer.
Exam-style questions
Questions in the style of SACE Board exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
SACE 2018-style1 markWhich one of the following correctly identifies a property of simple diffusion and a property of facilitated diffusion: simple does not require energy and facilitated requires energy; simple requires membrane proteins and facilitated does not; simple moves with the gradient and facilitated against it; or both do not require energy?Show worked answer →
The correct combination is that both do not require energy. Simple diffusion and facilitated diffusion are both passive transport, moving substances down (with) their concentration gradient without using ATP. The differences are that simple diffusion does not need membrane proteins while facilitated diffusion uses channel or carrier proteins, and both still move substances with the gradient (not against it).
SACE 2018-style3 marksInsulin is a protein packaged in the Golgi body. Describe how insulin is secreted from a cell.Show worked answer →
Three points describe exocytosis.
Insulin is packaged into a membrane-bound vesicle, which buds off the Golgi body.
The vesicle moves to and fuses with the cell (plasma) membrane.
The vesicle membrane joins the plasma membrane and the insulin is released to the outside of the cell (into the blood). This process is exocytosis and requires energy (ATP).
Practise this
Sources & how we know this
- Stage 2 Biology subject outline (2025 onwards) — SACE Board
- Stage 2 Biology: external assessment — SACE Board