Understanding Movement of substances into and out of cells: Edexcel IGCSE Human Biology Explained

Every cell in your body depends on the movement of substances across its membrane. The edexcel igcse human biology specification tests three mechanisms, and this section shows you how to distinguish them confidently.

The edexcel igcse human biology movement of substances into and out of cells section covers diffusion, osmosis and active transport, three of the most commonly confused concepts in the entire 4HB1 specification. The good news is that once you understand the logic behind each one, the distinctions are straightforward. These edexcel igcse human biology revision notes walk you through the definitions, the factors that affect each process, and the exam traps that catch students year after year.

Diffusion

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration. It happens because particles are in constant random motion, and over time, they spread out from where they are most concentrated to where they are least concentrated. Diffusion does not require energy from the cell; it is a passive process.

Examples in the human body:

Oxygen diffuses from the alveoli in the lungs into the blood (oxygen concentration is higher in the alveoli than in the blood arriving from the body).
Carbon dioxide diffuses from the blood into the alveoli (CO₂ concentration is higher in the blood than in the alveolar air).
Glucose and amino acids diffuse from the small intestine into the blood after digestion.

Osmosis

Osmosis is the net movement of water molecules across a partially permeable membrane from a region of higher water potential to a region of lower water potential. A partially permeable membrane has tiny pores that allow water molecules through but block larger solute molecules.

The edexcel specification explicitly states that an understanding of water potential is required. Water potential is a measure of the tendency of water molecules to move. Pure water has the highest water potential. Adding solute (such as sugar or salt) lowers the water potential. Water always moves by osmosis from higher water potential (more dilute) to lower water potential (more concentrated).

Common mistake: Many students write that water moves from "low concentration to high concentration." This is ambiguous and often marked wrong. The edexcel mark scheme expects you to say that water moves from a region of higher water potential to a region of lower water potential, or from a more dilute solution to a more concentrated solution. Be precise about what is moving (water molecules) and the direction (towards the lower water potential).

Worked example: osmosis in red blood cells

A red blood cell is placed in distilled water. Predict and explain what happens.

Answer: Distilled water has a higher water potential than the cytoplasm of the red blood cell. Water enters the cell by osmosis through the partially permeable cell membrane. Because animal cells have no cell wall, the cell swells and may burst (lyse). The cell cannot control the influx because osmosis is a passive process driven by the water potential gradient.

Worked example: osmosis in a concentrated salt solution

The same red blood cell is placed in a concentrated salt solution. What happens?

Answer: The salt solution has a lower water potential than the cytoplasm. Water leaves the cell by osmosis. The cell shrinks and becomes crenated (wrinkled). This is why maintaining the correct salt concentration in blood plasma is critical for the health of red blood cells.

Active transport

Active transport is the movement of substances against the concentration gradient, from a region of lower concentration to a region of higher concentration. Unlike diffusion and osmosis, active transport requires energy from respiration (ATP). It also requires carrier proteins in the cell membrane.

Examples in the human body:

Root hair cells in plants absorb mineral ions from dilute soil water against the concentration gradient (this example applies to biology broadly, but the mechanism is identical).
Glucose is absorbed from the small intestine into the blood by active transport when the concentration of glucose in the blood is already higher than in the gut lumen.
Kidney tubule cells reabsorb glucose from the filtrate back into the blood by active transport.

Summary comparison

Feature	Diffusion	Osmosis	Active transport
What moves?	Any particles (molecules, ions)	Water molecules only	Specific molecules or ions
Direction	High to low concentration	High to low water potential	Low to high concentration (against the gradient)
Energy required?	No (passive)	No (passive)	Yes (from respiration)
Membrane required?	Not necessarily	Yes (partially permeable)	Yes (with carrier proteins)

Factors affecting the rate of movement

The specification requires you to understand three factors that affect how quickly substances move into and out of cells:

Surface area to volume ratio: Smaller cells have a larger surface area relative to their volume, which means substances can move in and out more quickly. This is why many cells in the body are small, and why structures adapted for exchange (such as alveoli and villi) have features that increase their surface area.

Temperature: Higher temperatures give particles more kinetic energy, so they move faster and diffuse more rapidly. This applies to both diffusion and osmosis. For active transport, higher temperatures increase the rate of respiration (up to the optimum), supplying more ATP for the carrier proteins.

Concentration gradient: A steeper concentration gradient (a bigger difference in concentration between two regions) increases the rate of diffusion. The body maintains steep gradients at exchange surfaces through adaptations. For example, the blood flowing past the alveoli constantly carries away oxygen, keeping the oxygen concentration in the blood lower than in the alveolar air and maintaining a steep diffusion gradient.

Linking to the rest of the specification: The edexcel igcse human biology exam expects you to apply these three processes across multiple topics. Osmosis explains how the kidneys regulate water content. Diffusion explains gas exchange in the alveoli. Active transport explains nutrient absorption in the small intestine. When you see a question about any of these systems, think about which transport mechanism is operating and why.

Worked example: explaining gas exchange in the lungs using diffusion

Explain how oxygen passes from the air in the alveoli into the blood.

Answer: The concentration of oxygen in the alveolar air is higher than in the blood arriving at the lungs (deoxygenated blood). Oxygen moves by diffusion down its concentration gradient, from the alveolar air through the thin walls of the alveolus and the capillary (which are both one cell thick) into the blood. The large surface area of the alveoli, the thin walls, and the rich blood supply (which carries oxygen away and maintains the concentration gradient) all increase the rate of diffusion.

Worked example: surface area to volume ratio

A cube-shaped cell has sides of 1 cm. Calculate its surface area to volume ratio.

Step 1: Surface area = 6 x (1 x 1) = 6 cm²

Step 2: Volume = 1 x 1 x 1 = 1 cm³

Step 3: SA:V ratio = 6 : 1

Now consider a cell with sides of 2 cm:

Surface area = 6 x (2 x 2) = 24 cm². Volume = 2 x 2 x 2 = 8 cm³. SA:V = 24 : 8 = 3 : 1.

The smaller cell has a higher SA:V ratio (6:1 vs 3:1), so substances can move in and out more efficiently relative to its volume.

Self-check questions

Write your answer before looking at the guidance below.

Define diffusion.
Why does osmosis require a partially permeable membrane but diffusion does not?
A plant cell is placed in a concentrated sugar solution. Describe and explain what happens to the cell.
Name the energy source for active transport.
State three factors that affect the rate of diffusion.
Explain why a red blood cell bursts in distilled water but a plant cell does not.
Give one example of active transport in the human body.
A student says "osmosis is the diffusion of water." Is this statement accurate enough for the edexcel exam? Explain.

Answers: (1) The net movement of particles from a region of higher concentration to a region of lower concentration. (2) Osmosis involves the movement of water molecules only, through a membrane that allows water through but blocks solute molecules; diffusion can occur in any medium, with or without a membrane. (3) The sugar solution has a lower water potential than the cell's cytoplasm. Water leaves the cell by osmosis. The cell membrane pulls away from the cell wall (plasmolysis) and the cell becomes flaccid. (4) ATP, produced by respiration. (5) Temperature, concentration gradient, and surface area (to volume ratio). (6) The red blood cell has no cell wall, so it swells until it bursts (lyses). The plant cell has a rigid cell wall that resists further expansion once the cell is turgid, preventing it from bursting. (7) Glucose reabsorption in the kidney tubules, or glucose absorption in the small intestine against the concentration gradient. (8) No. The mark scheme requires mention of a partially permeable membrane and movement from higher to lower water potential. Simply saying "diffusion of water" omits these essential details.

These edexcel igcse human biology notes on the movement of substances into and out of cells cover every point the specification requires. On the Green Bridge CBT platform, you will find movement of substances into and out of cells edexcel igcse practice questions that test these concepts under exam conditions. For igcse 4hb1 movement of substances into and out of cells questions mapped to the specification, use the platform's topic-by-topic mode. You can also explore the full edexcel igcse human biology explained series and edexcel igcse human biology practice questions for deeper revision across every section.