Transport in cells

Diffusion happens because particles are in constant random motion, and results in particles spreading out evenly.

• •Diffusion is passive — it needs no energy from respiration.
• •Examples: oxygen diffusing into cells and carbon dioxide diffusing out during gas exchange, and urea diffusing from cells into blood plasma.
• •The rate of diffusion increases with a bigger concentration gradient, a higher temperature, and a larger surface area.
• •Single-celled organisms have a large surface area to volume ratio, so diffusion alone supplies enough oxygen and nutrients.
• •Larger, multicellular organisms need specialised exchange surfaces (e.g. alveoli, gills, roots) with a large surface area, thin walls and a good blood supply to overcome their much smaller surface area to volume ratio.

Osmosis is a special case of diffusion that involves only the movement of water.

• •A dilute solution has a high concentration of water (few dissolved solutes); a concentrated solution has a low concentration of water.
• •A partially permeable membrane lets small molecules like water through, but not larger solute molecules.
• •A plant cell in a dilute solution takes in water and becomes turgid — the vacuole swells, pressing the cytoplasm against the rigid cell wall.
• •A plant cell in a concentrated solution loses water and becomes plasmolysed — the cell membrane pulls away from the cell wall.
• •An animal cell has no cell wall, so it can burst (lysis) in a dilute solution, or shrink (crenate) in a concentrated solution.

1. 1Cut equal-sized chips from a potato, blot each one dry, then measure and record its starting mass.
2. 2Place one chip into each of a range of sugar solutions of different concentrations, including distilled water as a control.
3. 3Leave for a set time (e.g. 24 hours), then remove, blot dry, and re-weigh each chip.
4. 4Calculate the percentage change in mass for each concentration: (change in mass ÷ starting mass) × 100.
5. 5Plot percentage change in mass against solution concentration — the concentration where the line crosses zero is the concentration inside the potato cells.

Active transport lets a cell absorb a substance even when there's already more of it inside than outside.

• •Active transport moves particles against the concentration gradient (low to high) — the opposite direction to diffusion.
• •It requires energy from respiration, unlike diffusion and osmosis, which are passive.
• •Root hair cells use active transport to absorb mineral ions (e.g. nitrate) from very dilute solutions in the soil.
• •Cells in the small intestine use active transport to absorb glucose from the gut into the blood, even once the blood concentration is already higher.
• •Cells specialised for active transport contain many mitochondria, to provide the energy needed.