Concept of Movement of Substances Across a Plasma Membrane

Passive processes: diffusion and osmosis

Content standard 3.2 requires you to compare the mechanisms substances use to cross the membrane. Diffusion is the net movement of particles (such as gas molecules or dissolved solutes) from a region of higher concentration to a region of lower concentration, down a concentration gradient, until concentration is equal.

Osmosis is a special case of diffusion involving only water molecules moving across a partially permeable membrane, from a region of higher water potential to a region of lower water potential. Both are passive, neither requires the cell to spend energy.

Active transport

Active transport is the movement of substances across a membrane from a region of lower concentration to a region of higher concentration, against the concentration gradient. Because this movement goes against the natural direction of diffusion, it requires energy from ATP (produced by respiration) and specific carrier proteins in the membrane.

Comparing the three processes

Factors affecting rate and bulk transport

The rate of diffusion is affected by the concentration gradient (a steeper gradient speeds diffusion), surface area (a larger area speeds diffusion), temperature (higher temperature speeds particle movement) and the distance the particles must travel. Very large particles, such as whole food droplets or bacteria, are moved across the membrane by bulk transport instead: endocytosis brings material into the cell (phagocytosis for solid particles, pinocytosis for liquid droplets), while exocytosis expels material out of the cell, both by the membrane folding around the material.

How it is examined

Common questions ask you to classify a described movement as diffusion, osmosis or active transport, to explain why a process needs energy while another does not, or to predict how changing temperature, surface area or concentration gradient affects the rate of diffusion. You may also be asked to distinguish endocytosis from exocytosis.

Key terms

• Diffusion, the net movement of particles from a higher to a lower concentration, with no energy needed.
• Osmosis, the net movement of water across a partially permeable membrane, from higher to lower water potential.
• Active transport, the movement of substances against a concentration gradient using carrier proteins and energy from ATP.
• Hypotonic solution, a solution more dilute than the cell, so water moves into the cell.
• Hypertonic solution, a solution more concentrated than the cell, so water moves out of the cell.

Worked exam-style question

Question. A root hair cell absorbs both water and mineral ions from the soil. The soil solution is more dilute than the cell's cytoplasm, but the concentration of mineral ions is higher inside the cell than in the soil.

(a) Name the process by which water enters the root hair cell, and state the direction of movement in terms of water potential. (b) Name the process by which the mineral ions are absorbed, and explain why it needs energy.

(c) State where the energy for part (b) comes from. (d) Explain why mineral ion uptake slows down when the root is deprived of oxygen.

Model answer. (a) Water enters by osmosis, moving from the soil solution (higher water potential) into the cytoplasm (lower water potential) across the partially permeable membrane. (b) The mineral ions are absorbed by active transport; they move against the concentration gradient (from low to high concentration), which does not happen naturally, so it requires energy and carrier proteins.

(c) The energy comes from ATP produced by respiration. (d) Without oxygen, aerobic respiration slows, so less ATP is made; with less energy available, the rate of active transport of mineral ions falls.

Practice question

For each movement, state whether it is diffusion, osmosis or active transport. (a) Oxygen passing from the air in an alveolus into the blood.

(b) Water passing from dilute urine back into the blood at the kidney. (c) Glucose being reabsorbed from the kidney tubule into the blood when the tubule already contains less glucose than the blood.

Exam tip

Source:SRC-DSKP-EN

Frequently asked questions

More for Movement of Substances Across a Plasma Membrane