B2: Movement into and out of cells

  1. Diffusion

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

    • move down a concentration gradient as a result of their random movement

  • Some substances move into the cell through the cell membrane through diffusion

  • Diffusion helps living organisms to:

    • obtain many of their requirements

    • get rid of many of their waste products

    • carry out gas exchange for respiration

  • Factors that affect diffusion:

    • Temperature: higher temperature→ more kinetic energy → faster random movement and faster diffusion

    • concentration gradient: large difference → diffusion will happen quickly (fewer particles will move randomly in the opposite direction)

    • surface area: greater surface area → greater diffusion

    • diffusion distance: small thickness → faster diffusion

  1. osmosis

  • Movement of water in and out of cells with a partially permeable membrane through osmosis

  • the diffusion of water molecules from a dilute solution to a more concentrated solution across a partially permeable membrane

    • moves down its concentration gradient

      • high water potential → low water potential

    • partially permeable: allows small molecules through but larger molecules

  • Osmosis in plant tissues

    • water pushes the cell membrane against the cell wall → makes cell rigid and firm

      • creates turgor pressure → cell is turgid

    • provides strength and support for the plant

    • cell wall prevents cell from bursting

    • cells in a dilute solution: water moves into the cell

    • cells in concentrated solution: water will move out of the plant cells

      • cell is plasmolysed

  • osmosis in animal cells

    • concentrated solution: lose water by osmosis → shrivel up

    • distilled water: no cell wall to create turgor pressure → could burst

  1. Active transport

  • Movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration using energy from respiration

  • Vital for the movement of molecules or ions across membranes

    • Uptake of glucose

    • uptake of ions

  • protein carriers

    • active transport uses carrier proteins in cell membranes to pick up specific molecules