the characteristics of the plasma membrane as a semi-permeable boundary between the internal and external environments of a cell and the movement of hydrophilic and hydrophobic substances across it, including water (osmosis), simple diffusion, facilitated diffusion, active transport, endocytosis and exocytosis

What this dot point is asking

VCAA wants the fluid mosaic structure of the plasma membrane, why it is semi-permeable, and the six transport mechanisms by which substances cross it: simple diffusion, facilitated diffusion, osmosis, active transport, endocytosis and exocytosis.

The answer

Structure: the fluid mosaic model

The plasma membrane is a phospholipid bilayer with embedded proteins, cholesterol and carbohydrates. It is "fluid" because the components move laterally; it is a "mosaic" because the embedded molecules are scattered through the bilayer.

Phospholipids

Each phospholipid has a hydrophilic phosphate head (water-loving) and two hydrophobic fatty-acid tails (water-fearing). In water, they self-assemble into a bilayer with heads on the outside and tails packed in the middle. This is the structural basis of the membrane.

Proteins

Integral (transmembrane) proteins span the bilayer; many act as channels, carriers, receptors or enzymes. Peripheral proteins sit on one face, often anchored to integral proteins, and act in signalling and structural roles.

Cholesterol

Found in animal cell membranes (not plant). Sits between phospholipids and regulates fluidity: at high temperatures it stiffens the membrane; at low temperatures it prevents the fatty acid tails from packing too tightly.

Carbohydrates

Attached to outer-face proteins (glycoproteins) and lipids (glycolipids). Form a "glycocalyx" that is the basis for cell recognition, immune signalling and cell adhesion.

Semi-permeability

The membrane lets some substances through but not others.

• Small non-polar molecules (O2, CO2, N2, steroid hormones, small lipids) cross directly through the hydrophobic core.
• Water crosses slowly through the bilayer but much faster through dedicated channels called aquaporins.
• Small polar molecules and ions (glucose, amino acids, Na+, K+, Cl-, H+) cannot cross the hydrophobic core. They need protein channels or carriers.
• Large molecules and particles (whole proteins, pathogens) cannot fit through any channel and must enter or leave by vesicles (endocytosis or exocytosis).

Passive transport (no ATP)

Driven by concentration, pressure or electrochemical gradients. The cell does not spend ATP.

Simple diffusion

Movement of small non-polar molecules directly across the bilayer, down their concentration gradient. Example: oxygen entering a respiring cell.

Facilitated diffusion

Movement of polar molecules or ions through a channel protein (always open or gated) or carrier protein (changes shape), down their concentration gradient. Example: glucose entering a red blood cell through the GLUT1 carrier; Na+ entering a nerve cell through a sodium channel.

Osmosis

Movement of water across a semi-permeable membrane from a region of higher water concentration (lower solute concentration) to lower water concentration (higher solute concentration). Aquaporins greatly accelerate this. In an animal cell:

• Hypotonic surroundings (less solute outside): water enters, the cell swells and may burst (lysis).
• Hypertonic surroundings (more solute outside): water leaves, the cell shrinks (crenation).
• Isotonic surroundings: no net water movement.

In a plant cell, the cell wall prevents bursting; the vacuole loses water in hypertonic surroundings, the cell becomes flaccid and may plasmolyse (membrane pulls away from wall).

Active transport (requires ATP)

Moves substances against their concentration gradient, from low to high concentration, through a carrier protein (pump) that uses ATP to change shape.

Example: the sodium-potassium pump in animal cells pumps 3 Na+ out and 2 K+ in per ATP, maintaining the resting potential of nerves and muscles.

Active transport explains why cells can concentrate nutrients (such as glucose in intestinal epithelium) or excrete ions even when external concentrations are higher.

Bulk transport (vesicles, ATP-dependent)

For substances too large for any protein channel.

Endocytosis brings material into the cell. The plasma membrane folds inward around the material and pinches off as a vesicle.

• Phagocytosis ("cell eating"): engulfing large solids, such as a macrophage eating a bacterium.
• Pinocytosis ("cell drinking"): engulfing extracellular fluid.
• Receptor-mediated endocytosis: specific molecules bind receptors that cluster and trigger vesicle formation, such as cholesterol uptake via LDL receptors.

Exocytosis moves material out of the cell. A vesicle fuses with the plasma membrane and releases its contents to the outside. Examples: insulin secretion from pancreatic beta cells; neurotransmitter release at a synapse.

Summary table

Examples in context

Example 1. Cystic fibrosis and CFTR at Royal Children's Hospital Melbourne. The CFTR protein is a chloride channel that uses ATP to actively transport chloride ions across the epithelial plasma membrane. In cystic fibrosis, a mutation (commonly delta-F508) misfolds CFTR so chloride cannot cross, water follows by osmosis, and airway mucus becomes thick. Royal Children's Hospital Melbourne now treats eligible children with the drug combination elexacaftor-tezacaftor-ivacaftor (Trikafta), which rescues mutant CFTR folding so the channel reaches the membrane. Lung function (FEV1) typically improves by 10 to 14 percentage points within months. The case shows how a single membrane transporter underpins osmosis, ion balance and clinical outcome.

Example 2. Plant water transport at Royal Botanic Gardens Cranbourne. Tube-shaped root hair cells at Royal Botanic Gardens Cranbourne absorb water by osmosis. The soil solution has higher water potential than the root cytoplasm, so water moves passively across the partially permeable plasma membrane. Aquaporin channels in the membrane (a form of facilitated diffusion for water) increase the rate roughly tenfold over simple diffusion. During drought, the plants close aquaporins via hormone signalling, slowing water loss. VCE students can see osmosis directly by placing potato cubes in 0.0 to 1.0 mol/L sucrose solutions and weighing them after 30 minutes: cubes in hypotonic solution gain mass, cubes in hypertonic solution lose mass.

Try this

Q1. Distinguish between simple diffusion, facilitated diffusion and active transport with respect to the use of membrane proteins and ATP. [3 marks]

• Cue. Simple diffusion: no protein, no ATP. Facilitated: channel or carrier protein, no ATP. Active: carrier protein and ATP, against gradient.

Q2. Potato cubes were weighed before and after 30 minutes in three sucrose solutions: 0.0 mol/L (gained 8 percent mass), 0.3 mol/L (gained 1 percent), 0.8 mol/L (lost 7 percent). Explain the trend using the term osmosis, and estimate the sucrose concentration that matches the cytoplasm's water potential. [3 marks]

• Cue. Water moves to the lower water potential. Crossover near 0.35 mol/L sucrose, where mass change is zero.

Q3. Refer to the cystic fibrosis CFTR protein. (a) Classify CFTR as a passive or active transporter and justify. (b) Explain why blocked chloride transport leads to thick mucus in airways. (c) Suggest one consequence of CFTR malfunction for sweat-gland osmotic balance. [2+2+2 marks]

• Cue. (a) Active; uses ATP to move chloride. (b) Water follows chloride osmotically; without chloride efflux, mucus dehydrates. (c) Sweat is salty because sodium chloride cannot be reabsorbed across sweat-duct epithelium.