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Strand 2: Structures & Processes

Movement Through Cell Membranes

Diffusion, osmosis and active transport across cell membranes.

1. Diffusion

In brief:Diffusion is the passive movement of a substance from an area of high concentration to an area of low concentration.

Diffusion is the passive movement of a substance (solute) from an area of high concentration to an area of low concentration. No energy is required - it is a passive process.

Examples: CO₂ diffusing out of a fizzy drink; perfume spreading through a room; oxygen diffusing into blood in the lungs.

Factors Affecting Rate of Diffusion

  1. Concentration gradient - the greater the difference, the faster diffusion occurs
  2. Temperature - higher temperature = more energy = faster diffusion
  3. Surface area - greater surface area of the membrane = faster diffusion
  4. Distance - greater distance = slower diffusion (limits cell size)

Only small molecules (O₂, CO₂) move by diffusion. Large molecules like proteins require energy.

Cell membrane

Fluid-mosaic model of the plasma membrane

Wikimedia Commons (CC)

Osmosis

Osmosis across a semi-permeable membrane

Wikimedia Commons (CC)

Key Points

  • 1Diffusion is passive movement from high to low concentration
  • 2No energy required - moves 'down the concentration gradient'
  • 3Rate increases with: larger gradient, higher temperature, greater surface area
  • 4Greater distance = slower diffusion - this limits cell size
  • 5Only small molecules diffuse across membranes

Learning Outcomes

  • Distinguish between diffusion, osmosis and active transport; examine the role of osmosis in food preservation and plant health

2. Osmosis & Active Transport

In brief:Osmosis is the passive movement of water across a semi-permeable membrane. Active transport requires energy to move substances against the concentration gradient.

Osmosis

Osmosis is the passive movement of water from an area of high water concentration to an area of low water concentration across a semi-permeable membrane.

Effect of Osmosis on Cells

Solution TypeAnimal CellPlant Cell
Isotonic (equal concentration)NormalFlaccid
Hypotonic (more water outside)Swells, may burst (lyse)Turgid (healthy)
Hypertonic (less water outside)ShrivelsPlasmolysis (cytoplasm shrinks from wall)

Turgor = outward pressure of cell contents against cell wall. Plants need turgid cells to stand upright. Plasmolysis = cytoplasm pulls away from cell wall due to water loss.

Osmosis in Food Preservation

Salting meat and making jam places food in hypertonic solutions. Microbes lose water by osmosis and die.

Active Transport

Active transport moves substances from low to high concentration (against the gradient). It requires energy (ATP) and carrier proteins in the membrane. Example: mineral ion uptake by plant roots.

Key Points

  • 1Osmosis: passive water movement across a semi-permeable membrane, high to low water concentration
  • 2Isotonic = equal concentration; Hypotonic = more water outside; Hypertonic = less water outside
  • 3Plant cells are healthiest when turgid (hypotonic solution); animal cells prefer isotonic
  • 4Plasmolysis: cytoplasm shrinks from cell wall in hypertonic solution
  • 5Active transport moves substances against the gradient using ATP and carrier proteins
  • 6Food preservation (salting, jam) uses hypertonic solutions to kill microbes by osmosis

Learning Outcomes

  • Distinguish between diffusion, osmosis and active transport; examine the role of osmosis in food preservation and plant health
  • Investigate factors affecting rates of osmosis across semi-permeable membranes, use primary data to support conclusions