Diffusion, Osmosis & Active Transport (Edexcel IGCSE Biology: Double Science)

• Diffusion can be defined as:

The movement of molecules from a region of higher concentration to a region of lower concentration

• Molecules move down a concentration gradient 
• Note that the movement of molecules is random, but the result of this random movement is the spreading out of molecules until they are at even concentration throughout the available space

Diffusion in living organisms

• Molecules move into or out of living cells by diffusion when they cross the cell membrane
  • The cell membrane is partially permeable, meaning that it allows some molecules to cross, but not others
  • E.g. smaller molecules can diffuse across the membrane but larger molecules cannot
• Diffusion allows living organisms to, e.g.:
  • gain nutrients in the digestive system
  • gain oxygen in the lungs
  • remove waste products in the lungs and kidneys

• Osmosis can be defined as:

The movement of water molecules from a region of higher water concentration (a dilute solution) to a region of lower water concentration (a concentrated solution), through a partially permeable membrane

• Osmosis is the diffusion of water, as the water is moving down its concentration gradient
• Partially permeable membranes prevent the movement of larger molecules, e.g. sugars, but allow the movement of small water molecules

Osmosis is the diffusion of water molecules across a partially permeable membrane

Osmosis in animal cells

• Without a cell wall, osmosis can have severe effects on animal cells:
  • In a strong sugar solution (lower water concentration), the cell loses water, becoming crenated (shrivelled)
  • In distilled water (higher water concentration), the cell gains water, eventually bursting as it lacks a cell wall to maintain structure

Effect of osmosis on animal cells

Osmosis in plant cells

• Due to the cell wall, plant cells are protected from bursting:
  • In a strong sugar solution (lower water concentration), the cell loses water, the vacuole shrinks, and the cell membrane pulls away from the wall, making the cell flaccid or plasmolysed
  • In distilled water (higher water concentration), the cell gains water, the vacuole expands, and the membrane pushes against the cell wall, making the cell turgid
    • Turgid cells provide structural support and prevent wilting in plants

The effect of osmosis on plant cells

• Active transport can be defined as:

The movement of particles across a cell membrane from a region of lower concentration to a region of higher concentration

• Energy is needed for active transport because particles are being moved against a concentration gradient
  • Energy is released during cellular respiration
• Active transport across the cell membrane involves protein carrier molecules that are embedded in the cell membrane
• Examples of active transport in cells include:
  • absorption of the products of digestion into the bloodstream from the lumen of the small intestine
  • absorption of mineral ions from the soil into the root hair cells of plants

Active transport involves movement against a concentration gradient, and so requires energy