Movement across membranes

Diffusion is the net movement of particles from an area of high concentration to one of low concentration (down the concentration gradient), requiring no energy.

The rate of diffusion increases with:

• a steeper concentration gradient
• a shorter diffusion distance
• smaller particle size
• a larger surface area.

Examples: oxygen diffusing into cells during respiration; CO₂ diffusing out; glucose absorbed from the small intestine into blood.

Osmosis is the net movement of water molecules across a partially permeable membrane from a region of higher water potential to one of lower water potential. A dilute solution has a high water potential; a concentrated solution has a low water potential. No energy is needed.

In plant cells:

• Dilute surrounding solution → water enters → cell becomes turgid (cell wall resists further swelling, preventing bursting).
• Concentrated surrounding solution → water leaves → cell becomes flaccid → extreme loss causes plasmolysis (membrane peels away from cell wall).

In animal cells:

• Dilute surrounding solution → water enters → cell swells and may burst (red blood cells: haemolysis).
• Concentrated surrounding solution → water leaves → cell crenates (shrivels).

Examples: roots absorbing water from soil; plants wilting when water-deficient; water reabsorbed from the kidney tubules into the blood.

Active transport moves substances against the concentration gradient (from low to high concentration). It requires energy (ATP) from respiration and carrier proteins in the cell membrane.

Because ATP is needed, active transport:

• depends on mitochondria (which generate ATP by respiration)
• is inhibited by toxins that block respiration, or by deprivation of oxygen (no aerobic respiration → no ATP).

Examples:

• Root hair cells absorb mineral ions (e.g. nitrates) from the soil, where they are at lower concentration than inside the root.
• Cells lining the small intestine absorb glucose from the gut into the blood against the concentration gradient.

• Describing osmosis as moving "solute" (it is the movement of water).
• Forgetting active transport needs energy (and so respiration).
• Saying osmosis needs energy (it is a passive process).