Diffusion, Osmosis & Active Transport

Diffusion is the net movement of particles (molecules or ions) from a region of high concentration to a region of low concentration — that is, down a concentration gradient.

It is a passive process, requiring no energy from the cell.

Active transport is the movement of particles against a concentration gradient (from a low to a high concentration), using energy released by respiration.

It differs from diffusion because diffusion moves particles down the gradient (high → low) and needs no energy, whereas active transport moves them up the gradient and does require energy.

Water moves into the root hair cell by osmosis.

• The soil water is a dilute solution and so has a higher water potential than the cytoplasm/cell sap of the root hair cell.
• The cell contents are a more concentrated solution (lower water potential).
• Water molecules therefore move from the dilute solution (high water potential) to the more concentrated solution (low water potential).
• This movement of water molecules occurs across the cell's partially permeable membrane, which lets water through but not the dissolved solutes.
• No energy is required — osmosis is a passive process (it is the diffusion of water).

Factor 1 — a steeper concentration gradient.

• If the difference in carbon dioxide concentration between the inside and outside of the cell is greater, diffusion is faster, because there is a bigger overall imbalance driving the net movement of particles.

Factor 2 — a higher temperature.

• At a higher temperature the particles have more kinetic energy and move faster, so they spread (diffuse) more quickly.

(Other acceptable factors: a larger surface area, or a shorter distance / thinner membrane to cross — both speed up diffusion.)

Diffusion

• What moves: any particles (molecules or ions).
• Direction: net movement from a high to a low concentration — down the concentration gradient.
• Energy: none required; it is passive.
• Example: oxygen diffusing from the alveoli into the blood (or carbon dioxide out of a respiring cell).

Osmosis

• What moves: water molecules only, across a partially permeable membrane.
• Direction: from a dilute solution (high water potential) to a more concentrated solution (low water potential) — still down a gradient (of water).
• Energy: none required; it is passive (a special case of diffusion).
• Example: water entering a root hair cell from the soil.

Active transport

• What moves: any particles or ions, via carrier proteins.
• Direction: against the concentration gradient — from a low to a high concentration.
• Energy: yes — energy from respiration (ATP) is needed.
• Example: mineral ions (e.g. nitrate) being absorbed into a root hair cell against the gradient.

Summary: diffusion and osmosis are both passive (no energy) and move substances down a gradient, but osmosis is specifically water across a partially permeable membrane; active transport is the only one that moves substances against the gradient and the only one that uses energy from respiration.

The process responsible is active transport.

• The potassium ions are at a much higher concentration inside the cell (50×) than outside, yet they continue to enter — so they are moving from a low to a high concentration, i.e. against the concentration gradient.
• Diffusion cannot explain this, because diffusion only moves particles down a gradient (high → low); it would move ions out of the cell, not in.
• Moving particles against the gradient requires energy.
• That energy is released by respiration, which requires oxygen (aerobic respiration).
• When the cell is deprived of oxygen, respiration slows or stops, so less energy/ATP is available, and the active transport of ions stops — exactly what the evidence shows.
• This dependence on respiration (and therefore oxygen) is the key sign that the process is active transport, not diffusion or osmosis.