Transport in Cells¶

Part of Cell Biology.

Cells must exchange materials with their surroundings. Transport in cells compares three mechanisms for moving substances and explains why surface area matters so much for small organisms and specialised surfaces.

What You Need to Learn¶

Cells must exchange materials with their surroundings. Learn why concentration differences drive diffusion and osmosis, when transport needs energy, and how cell size and exchange-surface design affect supply. Use measurements, ratios and graphs to explain these processes rather than relying only on definitions.

Diffusion¶

• Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration.
• It is a passive process: it requires no energy because particles have kinetic energy and move randomly. More particles move away from the concentrated area than towards it until concentrations equalise.
• Diffusion works in liquids and gases, including in the cytoplasm, and happens across cell membranes and in tissues like the lungs (oxygen into blood), the liver (urea out), and the kidneys.
• Diffusion works faster when: the concentration gradient is steeper, the temperature is higher (particles move faster), the diffusion distance is short and the surface area is large.
• Facilitated diffusion is a special type of passive transport that uses membrane proteins to help substances cross, but still relies on concentration gradients (not against them) and requires no ATP energy.

Watch Diffusion

Set different starting concentrations, then follow individual particles and the net movement across the membrane. The concentrations are relative model values; particles keep moving even after the concentrations become similar. Open full interactive.

Apply it: Oxygen concentration is higher outside a cell than inside. What happens to net oxygen movement if respiration inside the cell slows?

Osmosis¶

• Osmosis is specifically the movement of water through a partially permeable membrane from a region of higher water potential (dilute solution) to lower water potential (concentrated solution).
• Water potential describes water’s tendency to move. Adding dissolved substances lowers it; for the dilute solutions compared here, water moves towards the more concentrated solution.
• In plant cells, water entering by osmosis makes the cell turgid (swollen and firm), with turgor pressure pressing the cell contents against the cell wall. This provides support (especially in leaves and herbaceous plants).
• If plant cells lose water by osmosis (placed in concentrated solution), they become flaccid (wilted and limp). Severe water loss causes plasmolysis, where the cell membrane detaches from the cell wall.
• In animal cells, which lack a cell wall, osmosis can cause problems: too much water entry bursts the cell; too much water loss shrivels it. They depend on surrounding tissue fluid maintaining the right water potential.
• Osmosis is investigated experimentally by measuring changes in mass or length of tissues placed in solutions of different concentrations.

For an osmosis experiment, percentage mass change = (final mass − initial mass) ÷ initial mass × 100. A potato sample changing from 2.0 g to 2.2 g gains 10%; a negative value indicates loss. Rate of water uptake can be estimated as mass gained ÷ time, with units such as g/min. Plot solution concentration on the horizontal axis and percentage mass change on the vertical axis; where the line crosses zero, there is no net water movement.

Follow Water Across a Membrane

Compare a dilute solution with a more concentrated solution. Water can cross the membrane while the solute shown cannot, so the net water movement is towards the more concentrated side. Use equal starting concentrations to check the difference between no net movement and no movement at all. Open full interactive.

Apply it: A bag containing concentrated sugar solution is placed in pure water. Predict its mass change if the membrane lets water through but blocks sugar.

See Osmosis Change a Plant Cell

Change the surrounding solution and observe the vacuole, cell membrane and wall. Water entering a cell increases pressure against its wall; substantial water loss can pull the membrane away from the wall. Open full interactive.

Apply it: A wilted stem becomes firm after standing in water. Explain this using the cell wall and water movement.

Active Transport¶

• Active transport moves substances from a dilute region to a concentrated region, against the concentration gradient, using energy from respiration (ATP).
• Unlike diffusion and osmosis, active transport is an active process requiring the cell to do work to move substances uphill against the gradient.
• Root hair cells use active transport to absorb mineral ions (like nitrate and potassium) from soil, even when ion concentrations are low.
• Cells in the small intestine use active transport to absorb some nutrients when concentrations are low, and to absorb glucose even when already well-fed.
• Active transport is essential for plants to take up nutrients and for intestines to maximise nutrient absorption.

Surface Area to Volume Ratios¶

• Cells exchange materials through their surface area (the cell membrane), so cells with a large surface area relative to their volume can exchange materials faster.
• Small organisms and single-celled organisms have high surface area to volume ratios, allowing efficient exchange without needing specialised surfaces.
• Larger organisms need specialised exchange surfaces (like root hair cells, alveoli, or villi) to increase surface area and maintain efficient exchange relative to their body size.

For a cube of side a, surface area = 6a² and volume = a³. A 1 cm cube has a ratio of 6:1, while a 2 cm cube has 24:8 = 3:1. Increasing size reduces exchange area per unit volume and lengthens internal transport distances.

Alveoli and intestinal villi provide large areas, thin barriers and a good blood supply; ventilation maintains oxygen and carbon dioxide gradients at the lungs. Fish gills combine many thin filaments with flowing water and blood. Root hairs enlarge the absorbing surface of roots, while thin leaves, air spaces and stomata provide short routes for gas exchange. Transport systems then carry substances between these surfaces and distant cells.

Compare Size and Exchange Area

Change the cube size and compare its surface area with its volume. The cube is a geometric model: its surface represents exchange area and its volume represents the material that must be supplied. Open full interactive.

Apply it: Two cubes have sides of 1 cm and 3 cm. Which has more exchange area per unit volume, and why does that matter for a living organism?

Common Confusions¶

• Diffusion vs. osmosis: Diffusion is the movement of any particles from high to low concentration. Osmosis is specifically water movement across a partially permeable membrane. Not all diffusion is osmosis.
• Water potential vs. concentration: Water potential is not another name for water concentration. In these solution comparisons, dissolving more solute lowers water potential, so water moves towards the more concentrated solution.
• Passive vs. active transport: Diffusion and osmosis are passive (no energy needed); active transport requires ATP energy and moves substances against the concentration gradient. This energy requirement makes it "active".
• Turgid vs. plasmolysed: Turgid is when a plant cell is swollen with water (healthy state). Plasmolysed is when the cell membrane has pulled away from the cell wall due to water loss (damage).

Key Terms¶

• Diffusion: the net movement of particles from high concentration to low concentration.
• Osmosis: the movement of water through a partially permeable membrane from dilute to concentrated solution.
• Partially permeable membrane: a membrane that lets some substances pass through but not others.
• Concentration gradient: the difference in concentration between two regions.
• Water potential: a measure of the tendency of water to move from one region to another.
• Active transport: movement against a concentration gradient using energy from respiration.
• ATP: a molecule that transfers energy between reactions in cells.
• Turgid: firm state of a plant cell after water enters by osmosis.
• Flaccid: wilted and limp state of a plant cell after water loss by osmosis.
• Plasmolysis: the separation of the cell membrane from the cell wall due to water loss.
• Turgor pressure: the pressure of water inside a plant cell pushing on the cell wall.
• Facilitated diffusion: passive transport using membrane proteins, following the concentration gradient.

Connected Pages¶

• Cell Structure
• Cell Division
• Principles of Organisation
• Biology Key Terms Glossary
• Required Practical Reference