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

Worked example

A nerve cell at rest has more Na+ outside than inside, and more K+ inside than outside. Na+ leaks in by facilitated diffusion through channels; K+ leaks out the same way. To maintain the resting gradient, the sodium-potassium pump uses ATP to actively pump 3 Na+ out and 2 K+ in. When the cell fires an action potential, voltage-gated channels open and ions rush down their gradients (facilitated diffusion). After firing, the pump (active transport) restores the resting concentrations.

Common traps

Saying the membrane is "made of phospholipids". It is a phospholipid bilayer, with embedded proteins, cholesterol and carbohydrates. Markers want fluid mosaic.

Confusing osmosis and diffusion. Osmosis is specifically the movement of water across a semi-permeable membrane. Diffusion can refer to any substance.

Saying water cannot cross the membrane. It can, slowly through the bilayer and quickly through aquaporins.

Calling facilitated diffusion "active". It uses proteins but does not use ATP. The gradient supplies the energy.

Saying hypotonic means "low water". Hypotonic means low solute and therefore high water. The opposite (hypertonic) means high solute, low water.

In one sentence

The plasma membrane is a fluid mosaic of phospholipids, proteins, cholesterol and carbohydrates that is selectively permeable, allowing small non-polar molecules to cross by simple diffusion and water by osmosis, polar molecules and ions through protein channels (facilitated diffusion) or pumps (active transport, against the gradient, requiring ATP), and large particles by vesicle transport (endocytosis or exocytosis).