transport in cells

Diffusion

Movement of particles from an area of high concentration to an area of low concentration, down a concentration gradient, until equilibrium is reached

Examples of diffusion in cells

Oxygen diffusing into cells from the blood, carbon dioxide diffusing out of cells during respiration

Factors affecting rate of diffusion

Concentration gradient (steeper = faster), temperature (higher = faster), surface area of membrane (larger = faster)

Adaptations of cells for diffusion

Thin membranes for short diffusion distance, large surface area, efficient blood supply to maintain concentration gradient

Osmosis

Diffusion of water molecules from a dilute solution (high water potential) to a concentrated solution (low water potential) through a selectively permeable membrane

Active transport

Movement of substances from a low concentration to a high concentration, against a concentration gradient, using energy from respiration

Examples of active transport in organisms

Root hair cells absorbing mineral ions from soil; glucose absorption in the small intestine

Importance of active transport in plants

Uptake of essential mineral ions (like nitrate and magnesium) even when soil concentrations are lower than inside the plant

Importance of active transport in animals

Absorption of glucose in the gut to ensure no valuable nutrients are lost in faeces

Surface area to volume ratio

As organisms get larger, their surface area:volume ratio decreases, making diffusion alone insufficient for exchanging materials

Adaptations of exchange surfaces

Large surface area, thin membrane for short diffusion path, rich blood supply, ventilation (in animals)

Examples of specialised exchange surfaces

Alveoli in lungs (gas exchange), villi in small intestine (nutrient absorption), gills in fish (gas exchange)

Why multicellular organisms need transport systems

They have a smaller surface area:volume ratio, so simple diffusion cannot meet the needs of all cell