Diffusion, Osmosis & Active Transport (Edexcel IGCSE Biology): Revision Note

Diffusion

• Diffusion can be defined as:

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

• Particles move down a concentration gradient 

• Note that the movement of particles is random, but the result of this random movement is the spreading out of particles until they are at even concentration throughout the available space

• Diffusion is a passive process as it doesn't require energy from the cell, rather the particles diffusing have kinetic energy and they will randomly spread out from a high to a low concentration until they are evenly spread

Diffusion in living organisms

• Gases and small 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 gases and molecules to cross, but not others

  • E.g. small molecules, like oxygen, can diffuse freely across the membrane (so long as there is a concentration gradient) but larger molecules, like starch, cannot

• Examples of diffusion in living organisms include:

  • In a leaf: the concentration of carbon dioxide in the chloroplasts inside of the cells of the leaf will be lower than the atmosphere surrounding it when it's photosynthesising, so carbon dioxide diffuses into the cells by diffusion

  • In the lungs: the concentration of oxygen inside the alveoli (air sacs) is higher than the concentration of oxygen in the blood of the capillaries, so oxygen diffuses into the blood to be transported around the body

  • In the liver: the concentration of urea, a waste product, is higher in the liver cells than the blood flowing through the liver, so urea diffuses out of the liver cells into the blood

Osmosis

• 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

• Scientists often describe osmosis using the term water potential instead of water concentration:

  • Water moves from an area of higher water potential to an area of lower water potential

  • The more solute (eg. sugar or salt) a solution contains, the lower its water potential (the lower its water concentration)

    • And vice versa, the less solute a solution contains, the higher its water potential (the higher the water concentration)

  • Pure water has the highest water potential

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

• 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, requiring energy released by respiration

• 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 pumps 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