2d Movement of Substances into and out of Cells¶

Part of 2 Structure and Functions in Living Organisms.

Exchange across membranes is one of the big unifying ideas in biology. The key comparisons are between passive movement down gradients and active movement that uses energy.

What You Need to Learn¶

Further detail: Pearson Edexcel International GCSE Biology specification.

On this page you'll learn about diffusion and osmosis, active transport, and factors affecting rate. You'll also cover practical models. The notes bring these ideas together into one clear overview of movement of substances into and out of cells.

Diffusion and Osmosis¶

Diffusion is the spreading out of particles resulting in a net movement from an area of higher concentration to an area of lower concentration. It is a passive process — no energy input is required. Small molecules such as oxygen, glucose, amino acids and water can diffuse across the cell membrane; larger molecules such as starch and proteins cannot.

In biology, diffusion matters in several contexts:

• Single-celled organisms have a relatively large surface area to volume ratio, so diffusion across the cell surface is sufficient to supply their relatively modest metabolic needs.
• In larger, multicellular organisms the surface area to volume ratio is much smaller. Specialised exchange surfaces (alveoli in the lungs, villi in the small intestine, root hair cells in plants) are therefore needed to maintain an adequate rate of exchange.

Explore Surface Area to Volume Ratio¶

Use the interactive below to change cube size and compare how surface area, volume and SA:V change together. The shaded core is a simple way of showing that, in the same time window, diffusion reaches a smaller proportion of a larger structure. That is why larger organisms need specialised exchange surfaces and transport systems. Open full interactive.

Explore Diffusion¶

Use the interactive below to change the concentration on each side of the membrane and the particle temperature, then watch how random motion still produces a net movement from the more concentrated side to the less concentrated side. The tracer particle helps show that diffusion does not move every particle in one direction even when the overall pattern does. Open full interactive.

What this simulation does not show¶

• Real molecules are far smaller and far more numerous than the dots here. A single breath contains about 10^22 molecules, not 200.
• Real molecules collide with each other constantly. Here the particles pass through each other so the random motion is easier to see, but that is a simplification.
• A real cell membrane is not an open gap. It is a phospholipid bilayer, and only some small molecules cross it easily. Here the membrane is shown as fully permeable so the concentration gradient stays as the main focus.

Osmosis is the movement of water molecules through a partially permeable membrane from a dilute solution to a more concentrated solution, or from high water potential to low water potential. The same direction can also be described as movement from high water concentration to low water concentration. It is passive — no energy is needed.

Important concepts in osmosis:

• A dilute solution has a high water potential; a concentrated solution has a low water potential. Water always moves from high to low water potential.
• If an external solution has the same concentration as a cell, it is described as isotonic — no net movement occurs.
• If the external solution is hypertonic (more concentrated than the cell), water moves out of the cell. In animals this causes the cell to shrivel. In plants the cell membrane can pull away from the cell wall — a process called plasmolysis.
• If the external solution is hypotonic (less concentrated than the cell), water moves into the cell. In animals this can cause the cell to burst. In plant cells, water moves into the vacuole and the cell swells against its wall, generating turgor pressure that keeps leaves and stems rigid.

Explore Osmosis¶

Use the interactive below to compare water and solute on either side of a partially permeable membrane. It focuses on the core idea that water can cross the membrane while the larger solute particles cannot, so the net water movement is towards the side with lower water potential. Open full interactive.

What this simulation does not show¶

• Real water and solute molecules are much smaller than shown, and there are vastly more of them. The larger dots here are only used to make the solute visible.
• In a real cell, water building up on one side would increase pressure and gradually reduce the net flow. Here the chambers stay the same size, so water keeps moving until the concentrations balance.
• The membrane here blocks solute completely. Real partially permeable membranes are more selective: some solutes cross slowly, and water often travels through protein channels called aquaporins.

Explore Plant Cell Osmosis¶

Use the interactive below to move between hypertonic, isotonic and hypotonic surroundings and watch how the vacuole, membrane and turgor change in a plant cell. It gives a visual model for flaccid, normal, turgid and plasmolysed states. Open full interactive.

Active Transport¶

Active transport is the movement of particles from an area of lower concentration to an area of higher concentration — against the concentration gradient. Diffusion moves particles down a concentration gradient; active transport moves them against it. Because this requires working against the concentration gradient, energy from respiration must be supplied. This is why cells carrying out active transport usually have many mitochondria.

Examples in biology:

• Root hair cells absorb mineral ions such as nitrates from the soil by active transport. Mineral ions are usually in higher concentration inside the root cells than in the surrounding soil, so diffusion alone cannot bring them in.
• Gut epithelium: glucose and amino acids from digested food must sometimes be absorbed from the gut into the bloodstream even when the gut concentration is lower than the blood concentration. Active transport moves these nutrients against the gradient.

Factors Affecting Rate¶

These factors explain why single-celled organisms exchange substances easily while large organisms need specialised exchange surfaces and transport systems.

Practical Models¶

Investigating osmosis in potato tissue:

1. Place potato strips into sucrose solutions of different concentrations (including 0% as a control).
2. Dry the strips on paper towel and record their mass before placing them in the solutions.
3. After 20 minutes, remove, dry and reweigh each strip.
4. A gain in mass indicates water entered the cell (hypotonic solution); a loss in mass indicates water left (hypertonic solution). Repeat with potato strips of similar starting mass to improve reliability.

Investigating diffusion in a non-living system: Agar cubes containing sodium hydroxide and phenolphthalein indicator can be placed in hydrochloric acid. Acid diffusing into the cube causes the indicator to become colourless. Measuring the depth of colour change shows how far the acid diffused in a given time, and how surface area to volume ratio affects the rate.

Common Confusions¶

• Osmosis vs diffusion: Osmosis is specifically about water moving through a partially permeable membrane. Diffusion applies to any type of small particle and does not require such a membrane.
• Osmosis terminology: When you describe the solutions, water moves from a dilute solution to a more concentrated solution. When you describe the water itself, it moves from high water potential or high water concentration to low water potential or low water concentration.
• Active transport vs diffusion: Active transport requires energy from respiration and moves substances against the concentration gradient. Diffusion requires no energy and always moves particles down the gradient.
• Gradient language: Use direction words such as down or against a concentration gradient. Avoid vague phrases such as along or across a gradient.
• Turgor vs turgidity: Both refer to the pressure-driven firmness of plant cells when swollen with water. Plant cells become turgid and rigid when water moves in; they become flaccid when water is lost.

Key Terms¶

• Diffusion: the net movement of particles from a region of higher concentration to a region of lower concentration.
• Osmosis: the net movement of water through a partially permeable membrane from a dilute solution to a more concentrated solution, or from high water potential to low water potential.
• Active transport: movement of substances against a concentration gradient using energy from respiration.
• Concentration gradient: the difference in concentration between two regions.
• Partially permeable membrane: a membrane that allows some substances to pass through more readily than others.
• Surface area to volume ratio: the ratio of an organism's or cell's exchange surface to its internal volume.
• Isotonic: describing a solution with the same concentration as the cell, so there is no net water movement.
• Hypertonic: a solution more concentrated than the cell.
• Hypotonic: a solution less concentrated than the cell.
• Turgor: the pressure generated in a plant cell when water enters the vacuole and pushes against the cell wall, helping the plant remain rigid.
• Plasmolysis: the shrinkage of the cell membrane away from the cell wall in a plant cell that has lost water by osmosis.
• Water potential: a measure of the tendency of water to move from one region to another.

Connected Pages¶

• 2b Cell Structure
• 2c Biological Molecules
• 2e Nutrition
• 2h Transport
• IGCSE Biology Key Terms Glossary
• Practical Investigation Reference