Explain the fluid mosaic model and the mechanisms of passive and active transport across the membrane

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.

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.