Cell Membrane and Transport - TCE Biology (Tasmania)
“Describe the structure of the cell membrane and explain the mechanisms of membrane transport.”
The cell membrane is a phospholipid bilayer with embedded proteins, described by the fluid mosaic model. It is partially permeable. Small or non-polar molecules cross passively by diffusion, water by osmosis, and larger or charged particles through proteins by facilitated diffusion. Active transport uses energy to move substances against their concentration gradient, and bulk transport moves large amounts in vesicles.
Membrane structure, diffusion, osmosis and active transport are Biology Level 2 content (BIO215123) and are not listed as a topic in TASC's Biology Level 3 course (BIO315124, current for 2026). The part that Level 3 examines is endocytosis and phagocytosis by immune cells: see the innate immune response. Osmosis is also used in osmoregulation and the nephron.
What this dot point is asking
Structure of the membrane
The cell membrane (plasma membrane) is built mainly from a phospholipid bilayer. Each phospholipid has a hydrophilic (water-loving) head and two hydrophobic (water-fearing) tails. In water the molecules arrange themselves with heads facing out toward the watery surroundings and tails tucked inward, forming a double layer.
Embedded in this bilayer are proteins, cholesterol, and carbohydrate chains. The arrangement is described by the fluid mosaic model: fluid because the phospholipids can move sideways, and mosaic because of the varied proteins scattered through it. The proteins act as channels, carriers, receptors, and markers.
A partially (or selectively) permeable membrane allows some substances to pass through freely while restricting or controlling the movement of others, depending on their size, charge, and solubility.
Passive transport: diffusion
Passive transport needs no energy from the cell. The simplest form is diffusion, the net movement of particles from a region of high concentration to a region of low concentration, down the concentration gradient. Small non-polar molecules such as oxygen and carbon dioxide diffuse directly through the bilayer. Diffusion continues until the concentrations are even (equilibrium).
The rate of diffusion increases with a steeper concentration gradient, higher temperature, larger surface area, and shorter distance to travel.
Osmosis
Osmosis is the diffusion of water across a partially permeable membrane, from a region of higher water concentration (more dilute solution) to a region of lower water concentration (more concentrated solution). It is just diffusion of water, but it is named separately because of its importance to cells.
In osmosis, water moves across the membrane toward the side with more dissolved solute (lower water concentration). A cell in a concentrated solution loses water and shrinks, while a cell in pure water gains water and may swell or burst.
A red blood cell placed in pure water (a solution more dilute than its cytoplasm) gains water by osmosis and may burst, because it has no cell wall. The same cell in a strong salt solution loses water and shrivels. In a solution matching its own concentration there is no net water movement, so the cell keeps its shape. This shows why fluids given to patients must match the concentration of body cells.
Facilitated diffusion
Some substances cannot cross the bilayer easily because they are large or charged, for example ions and glucose. They move passively through specific channel or carrier proteins, a process called facilitated diffusion. It still moves substances down their concentration gradient and needs no energy, but it depends on the right transport protein being present.
Active transport
Active transport moves substances against their concentration gradient, from low to high concentration. Because this is the opposite of the natural direction, the cell must supply energy, usually from ATP. Carrier proteins act as pumps, changing shape to move the substance across. An example is the absorption of mineral ions by root cells even when the soil concentration is lower than inside the cell.
Bulk transport
Very large molecules and particles are moved in or out of the cell in membrane-bound sacs called vesicles, using energy:
- Endocytosis brings material into the cell by folding the membrane inward to form a vesicle. Taking in solids is phagocytosis; taking in liquids is pinocytosis.
- Exocytosis releases material from the cell as a vesicle fuses with the membrane, for example secreting enzymes or hormones.
Linking transport to cell needs
A cell constantly exchanges materials with its surroundings: taking in oxygen and nutrients, removing carbon dioxide and waste, and maintaining the right internal conditions. The combination of passive and active mechanisms lets it do this selectively. Active transport is especially important where the cell must move substances against a gradient, which is why cells doing a lot of active transport (such as kidney or root cells) contain many mitochondria to supply ATP.
A common error is to claim all transport uses energy. Diffusion, osmosis, and facilitated diffusion are passive and need no energy from the cell, because substances move down their gradient. Only active transport and bulk transport (endocytosis and exocytosis) require energy, because they move material against the gradient or in large amounts.
Exam-style questions
Questions in the style of TASC 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.
Original7 marksPieces of potato of equal mass are placed in solutions of different salt concentration. After one hour, the piece in distilled water has gained mass, the piece in a salt solution shows no change, and the piece in a salt solution has lost mass. Explain these results in terms of osmosis and water potential, and state what the result tells you about the potato cells.Show worked answer →
A 7 mark answer explains each result by the direction of net water movement.
- Osmosis principle
- Water moves across the partially permeable membrane from a region of higher water potential (lower solute concentration) to lower water potential (higher solute concentration).
- Distilled water (gained mass)
- The external solution has a higher water potential than the cell contents, so water enters the cells by osmosis; the potato gains mass and cells become turgid.
- (lost mass)
- The external solution has a lower water potential than the cells, so water leaves the cells; the potato loses mass and cells become flaccid or plasmolysed.
- (no change)
- No net water movement, so this solution has the same water potential as the cell contents (it is isotonic). This tells you the internal solute concentration of the potato cells is about .
Markers reward correct osmosis direction for all three solutions and the interpretation that the no-change point equals the cell's solute concentration.
Original5 marksCompare facilitated diffusion and active transport across a cell membrane, referring to the direction of movement relative to the concentration gradient and the use of energy.Show worked answer →
A 5 mark answer contrasts both processes on gradient direction and energy.
- Facilitated diffusion
- Movement of substances (for example glucose or ions) down the concentration gradient, from high to low concentration, through specific carrier or channel proteins. It is passive and uses no ATP.
- Active transport
- Movement of substances against the concentration gradient, from low to high concentration, through carrier proteins (pumps). It requires energy in the form of ATP.
- Shared feature
- Both use membrane proteins because the substances cannot cross the lipid bilayer freely.
Markers reward the down-gradient versus against-gradient contrast, the passive versus ATP-requiring contrast, and the use of proteins in both.