What drives diffusion?

Movement of particles from a region of high concentration to low concentration

Movement of particles from a region of low concentration to high concentration

Active pumping by membrane proteins

Which of the following best defines osmosis?

Movement of water molecules through a partially permeable membrane from a dilute solution to a more concentrated solution

Movement of any solute from high to low concentration

Movement of water molecules against a concentration gradient using ATP

Movement of glucose through the cell membrane

What happens to an animal cell placed in a very dilute (hypotonic) solution?

It shrinks and becomes crenated

Nothing — animal cells are not affected by osmosis

It becomes turgid but does not burst

Which statements correctly describe active transport?

It moves substances against a concentration gradient

It uses carrier proteins in the membrane

It is a form of passive transport

Osmosis and Diffusion

⏱ 8 min✏️ Quiz at the end

Introduction

Every living cell is surrounded by a partially permeable membrane that acts as a selective barrier. Understanding how substances cross this barrier is fundamental to biology — it explains how your gut absorbs nutrients, how your lungs exchange gases, and how plant roots draw up water from soil. The three key processes are diffusion, osmosis, and active transport.

Diffusion

Diffusion is the net movement of particles from a region of high concentration to a region of low concentration, continuing until the particles are evenly spread out (equilibrium). It is a passive process — no energy is needed.

The driving force is the concentration gradient: the steeper the gradient (the bigger the difference in concentration), the faster diffusion occurs. Several other factors also affect the rate:

Temperature — higher temperatures give particles more kinetic energy, increasing the rate.
Surface area — a larger surface area allows more particles to cross at once.
Distance — the shorter the distance particles must travel, the faster diffusion proceeds.
Particle size — smaller molecules diffuse more quickly than larger ones.

Diffusion in living organisms

Diffusion is constantly at work in the body. Oxygen diffuses from the air sacs (alveoli) in the lungs into the blood, where oxygen concentration is lower. Carbon dioxide diffuses in the opposite direction. In body tissues, oxygen diffuses out of capillaries into cells, and carbon dioxide diffuses from cells into the blood. These exchanges are so efficient because the alveoli and capillary walls are extremely thin (just one cell thick) and have enormous combined surface areas.

Osmosis

Osmosis is a special case of diffusion that applies specifically to water molecules. It is defined as the movement of water through a partially permeable membrane from a region of lower solute concentration (more dilute — higher water potential) to a region of higher solute concentration (more concentrated — lower water potential).

A partially permeable membrane allows small water molecules to pass freely but blocks larger dissolved molecules such as glucose or proteins.

Osmosis in plant cells

Plant cells have a rigid cell wall outside their membrane. When a plant cell is placed in a dilute solution, water enters by osmosis, the vacuole expands, and the cell becomes turgid — firm and swollen. The cell wall prevents it from bursting. Turgidity is vital; it keeps leaves and non-woody stems upright.

If the surrounding solution is more concentrated than the cell's contents, water leaves the cell by osmosis. The vacuole shrinks, the membrane pulls away from the cell wall, and the cell becomes plasmolysed. Severely plasmolysed cells cannot function and the plant wilts.

Osmosis in animal cells

Animal cells lack a cell wall, so the results are more dramatic. In a hypotonic solution (more dilute than the cell), water rushes in and the cell swells and may burst (lysis). In a hypertonic solution (more concentrated than the cell), water leaves and the cell shrinks and crenates (becomes spiky). In an isotonic solution the concentration inside and outside is equal, so there is no net movement of water and the cell maintains its normal shape.

This is why intravenous (IV) fluids given to patients in hospitals are carefully formulated to be isotonic with blood cells — otherwise red blood cells would burst or shrink.

Active Transport

Both diffusion and osmosis are passive — they move substances along a concentration gradient without any energy input from the cell. Active transport is different: it moves substances against the concentration gradient (from low concentration to high), and it requires ATP energy and specific carrier proteins embedded in the membrane.

A key example is the absorption of glucose and mineral ions in the small intestine. After most glucose has been absorbed passively, there is still useful glucose in the gut at concentrations lower than in the blood. Intestinal cells use active transport to pump this remaining glucose across the membrane into the bloodstream.

Plant root hair cells similarly use active transport to absorb nitrate ions from the soil even when the concentration of nitrates in the soil is lower than inside the root cell. This is why plants need energy from respiration to grow well — without ATP, active uptake of minerals would stop.

Comparing the Three Processes

Feature	Diffusion	Osmosis	Active Transport
Substance moved	Any dissolved particle or gas	Water only	Specific ions or molecules
Direction	High → low concentration	High water potential → low	Low → high concentration
Energy needed	No	No	Yes (ATP)
Membrane needed	Not always	Yes (partially permeable)	Yes (carrier proteins)

Why It Matters

These three processes together explain how every cell in your body is supplied with what it needs and cleared of waste. Glucose enters cells by diffusion and active transport. Water balance is maintained by osmosis. Kidney cells use active transport to reabsorb useful molecules from the filtrate back into the blood. Plant roots draw up water by osmosis and minerals by active transport. Without these mechanisms, life as we know it would be impossible.