Topic 3: Movement into and out of cells

Cells are constantly exchanging substances with their surroundings — taking in oxygen and nutrients while removing waste. But how do these molecules actually move across the cell membrane? In this topic you will learn the three key transport processes: diffusion, osmosis and active transport. Understanding how and why substances move is essential for explaining gas exchange, water uptake in plants and digestion.

Diffusion

Diffusion is the net movement of particles from a region of their higher concentration to a region of their lower concentration (down a concentration gradient), as a result of their random movement. It is a passive process, meaning it needs no energy from respiration.

Diffusion supplies cells with substances they need, such as oxygen for respiration entering a cell, and removes waste like carbon dioxide. The rate of diffusion increases when there is a steeper concentration gradient, a higher temperature (particles move faster), a larger surface area, and a shorter distance to travel.

Osmosis

Osmosis is the net movement of water molecules from a region of higher water potential (a dilute solution) to a region of lower water potential (a concentrated solution), through a partially permeable membrane.

The membrane lets water through but not larger solute molecules. In animal cells, if too much water enters, the cell swells and may burst (lysis); if too much leaves, it shrinks (crenation). Plant cells are protected by their cell wall: when water enters, the cell becomes firm and turgid, which supports the plant; when water leaves, the cell becomes flaccid and the plant wilts. If a plant cell loses so much water that the membrane pulls away from the wall, it is plasmolysed.

Active transport

Active transport is the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration — that is, against a concentration gradient — using energy from respiration.

Because it works against the gradient, active transport requires both energy and special carrier proteins in the membrane. It is important when a cell needs to take up substances that are already at a higher concentration inside, for example root hair cells absorbing mineral ions from the soil, and the uptake of glucose in the small intestine.

Comparing the three processes

It helps to compare the three transport methods directly:

• Diffusion – any particle, down the gradient, no energy needed.
• Osmosis – water only, down its water potential gradient through a partially permeable membrane, no energy needed.
• Active transport – particles moved up the gradient, requires energy from respiration and carrier proteins.

The key distinction in the exam is energy: diffusion and osmosis are passive, while active transport is the only process that uses energy from respiration.