Diffusion, facilitated diffusion, osmosis, active transport, endocytosis and exocytosis, and the factors that affect the rate of movement across membranes.

What this dot point is asking

CCEA wants you to describe and distinguish diffusion, facilitated diffusion, osmosis, active transport, endocytosis and exocytosis, to use the idea of water potential for osmosis, and to explain the factors that affect the rate of movement across membranes.

Passive transport

Small, non-polar molecules such as oxygen and carbon dioxide diffuse straight through the phospholipid bilayer. Channel proteins form water-filled pores for specific ions, while carrier proteins bind a specific molecule, change shape, and release it on the other side, all without using ATP. Osmosis is the movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane. Pure water has the highest water potential (zero); adding solute makes the water potential more negative (for example $-200\ \text{kPa}$).

Active transport and bulk transport

A classic active transport pump is the sodium-potassium pump, which uses ATP to pump 3 sodium ions out and 2 potassium ions in against their gradients, important in nerve cells and in absorption in the gut and kidney. Cells that carry out a lot of active transport, such as those lining the small intestine, have many mitochondria to supply ATP and a folded membrane (microvilli) to increase surface area.

Factors affecting the rate

The rate of diffusion increases with a steeper concentration gradient, a larger surface area, a shorter diffusion distance (thinner membrane), and a higher temperature (more kinetic energy). Fick's law summarises these: $\text{rate} \propto \dfrac{\text{surface area} \times \text{concentration difference}}{\text{diffusion distance}}$. For facilitated diffusion and active transport, the number of transport proteins also limits the rate, so the rate plateaus once all carriers are working at maximum (saturation).

Examples in context

Example 1. Glucose absorption in the small intestine. Glucose is absorbed from the gut against its concentration gradient by co-transport: sodium ions are actively pumped out of the epithelial cell, creating a sodium gradient, and glucose is then carried in alongside sodium as it diffuses back. This couples active transport (the pump) with facilitated diffusion (the co-transporter), and explains why oral rehydration salts contain both glucose and sodium, which together drive water uptake.

Example 2. Red blood cells in different solutions. A red blood cell placed in pure water (high water potential) gains water by osmosis and bursts (haemolysis) because it has no cell wall to resist. The same cell in a concentrated salt solution (low water potential) loses water and shrinks (crenation). This is why intravenous drips must be isotonic with blood plasma, otherwise osmosis would damage the patient's blood cells.

Try this

Q1. State two ways active transport differs from facilitated diffusion. [2 marks]

• Cue. Active transport moves substances against the gradient and requires ATP; facilitated diffusion is passive and down the gradient.

Q2. A cell is placed in a solution with a lower water potential than the cell. State and explain what happens. [2 marks]

• Cue. Water leaves the cell by osmosis (down the water potential gradient), so the cell shrinks or plasmolyses.

Q3. A cell at $-450\ \text{kPa}$ is placed in a solution at $-450\ \text{kPa}$. State what happens to the cell and explain why. [2 marks]

• Cue. No net movement of water because the water potentials are equal, so there is no gradient; the cell mass stays the same.