Describe passive and active transport processes that move materials across cell membranes, including diffusion, osmosis (hypertonic, hypotonic, isotonic solutions), facilitated diffusion, protein pumps, endocytosis (phagocytosis and pinocytosis) and exocytosis

What this dot point is asking

QCAA expects you to classify any transport process as passive or active, predict the direction of movement, and name the structures involved. You should be able to apply the tonicity vocabulary (hypertonic, hypotonic, isotonic) to predict the behaviour of plant and animal cells in solutions.

The answer

Transport across the plasma membrane is the bottleneck for every cell. Processes split into passive (no ATP) and active (requires ATP), with the directionality always set by concentration or electrochemical gradients.

Passive transport

Passive processes move substances down their concentration gradient. No metabolic energy is needed; the gradient supplies the driving force.

Simple diffusion. Net movement of particles from high to low concentration until evenly distributed. Across membranes, only small non-polar molecules (O2, CO2, urea, ethanol, steroid hormones) cross the bilayer directly. Rate depends on:

• magnitude of the concentration gradient,
• temperature,
• surface area,
• thickness of the membrane (Fick's law).

Osmosis. Net movement of water across a selectively permeable membrane from a region of higher water potential (more dilute, lower solute) to lower water potential (more concentrated, higher solute). Water passes through the bilayer slowly and through aquaporin channels rapidly.

Tonicity describes a solution relative to the cell:

• Hypotonic. Lower solute concentration outside than inside. Water enters. Animal cells lyse; plant cells become turgid (cell wall prevents bursting).
• Hypertonic. Higher solute concentration outside. Water leaves. Animal cells crenate; plant cells plasmolyse (protoplast pulls away from the cell wall).
• Isotonic. Equal solute concentrations. Net water movement is zero.

Facilitated diffusion. Polar and charged solutes (ions, glucose, amino acids) cannot cross the bilayer directly. They move down their concentration gradient through specific membrane proteins:

• Channel proteins. Pores that open and close (e.g. potassium channels, aquaporins).
• Carrier proteins. Bind the solute, change shape and release it on the other side (e.g. GLUT1 for glucose).

Facilitated diffusion is saturable (channels and carriers have finite capacity) and substrate-specific.

Active transport

Active processes move substances against their concentration gradient and require metabolic energy.

Primary active transport (protein pumps). ATP-driven pumps couple ATP hydrolysis to solute movement.

• Sodium-potassium pump. Moves 3 Na+ out and 2 K+ into the cell per ATP. Maintains the resting potential of neurons and muscle cells.
• Proton pumps. Move H+ against the gradient (stomach parietal cells, plant root cells).
• Calcium pumps. Move Ca2+ out of the cytosol or into the sarcoplasmic reticulum.

Secondary active transport (cotransport). A solute is moved against its gradient by piggybacking on the gradient of another solute (typically Na+). Example: SGLT1 in the small intestine couples Na+ entry to glucose entry against the glucose gradient.

Bulk transport (active)

For large molecules, particles or whole cells, the membrane bends to engulf or release material in vesicles. ATP is required.

Endocytosis. The plasma membrane invaginates and pinches off a vesicle inside the cell.

• Phagocytosis ("cell eating"). Large particles or whole cells. Used by macrophages and neutrophils to engulf pathogens and debris.
• Pinocytosis ("cell drinking"). Small volumes of extracellular fluid and dissolved solutes.
• Receptor-mediated endocytosis. Specific molecules bind surface receptors that cluster into a coated pit before vesicle formation (e.g. cholesterol uptake via LDL receptors).

Exocytosis. Secretory vesicles fuse with the plasma membrane and release their contents to the outside. Used to secrete hormones, neurotransmitters, digestive enzymes and to add new membrane material.

Summary table

Common traps

Saying water moves to "equalise concentration". Be explicit: water moves down the water potential gradient (or equivalently, towards the solution with more solute).

Calling facilitated diffusion active. No ATP. The gradient drives it; the protein only provides a path.

Confusing isotonic with no movement at all. Water still moves in both directions; the rates are equal, so the net change is zero.

Forgetting that plant cells in pure water become turgid, not lysed. The cellulose cell wall prevents bursting; the cell becomes firm under turgor pressure.

Cross-link to Year 12 assessment

Membrane transport returns in Unit 2 osmoregulation and excretion (the nephron is a transport masterpiece), in Unit 3 IA1 data tests where membrane permeability and tonicity often appear as stimulus, and in Unit 3 IA2 student experiments such as classic potato-cylinder osmosis investigations adapted to test understanding of water potential.

In one sentence

Small non-polar molecules diffuse straight through the bilayer; water moves by osmosis from high to low water potential; polar solutes move down their gradients through channels or carriers (facilitated diffusion); against-gradient movement requires ATP-driven pumps or cotransporters; and bulk transport happens by endocytosis (phagocytosis or pinocytosis) and exocytosis.