Crossing membranes - Passive processes

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  • Created by: Steff06
  • Created on: 04-04-16 14:40
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  • Passive processes - Diffusion
    • Diffusion:
      • Molecules possess KINETIC ENERGY. Processes such as diffusion only depend kinetic energy are known as PASSIVE processes.
      • The movement of molecules from a region of HIGH concentration to a region of LOW concentration of that molecule DOWN a concentration gradient.
      • In diffusion, molecules are distributed EVENLY. When even, the molecules continue to move around. The state where there is no overall movement is known as EQUILIBRIUM.
      • In living organisms, activities + features ensure equilibrium is NOT reached. E.g CO2 in photosynthesis maintained at conc gradient.
      • Molecules continue to diffuse down their conc gradient without using energy from the cell. RATE of diffusion is affected by a number of processes.
    • Factors affecting diffusion:
      • TEMPERATURE: INCREASING temp increases kinetic energy. Rate of RANDOM movement increases as does rate of diffusion.
        • SIZE of molecule: SMALLER molecules diffuse FASTER than larger ones.
      • CONC GRADIENT: More molecules on 1 side increases con gradient which increases rate of diffusion.
      • STIRRING/MOVING: Stirring a liquid/movement increases movement of molecules/rate of diffusion.
      • SURFACE AREA: GREATER SA = diffusion occurs more rapidly. Cells are adapted to increase SA. E.g. red blood cells are biconcave, alveoli increase SA in lungs.
        • DISTANCE/THICKNESS: Diffusion is SLOWER with THICK membranes as there is a greater distance for molecules to travel.
    • LIPID-BASED molecules:Fat-soluble molecules can pass through the bilayer, diffusing down a conc gradient. Steroid hormones are lip-based and so diffuse through membranes.
      • Very SMALL molecules and ions: CO2 and O2 are small enough to pass through the bilayer. Water molecules are very small so some can pass directly through the membrane even though they are polar (charged).
        • LARGER/CHARGED molecules: Small, charged molecules e.g. sodium/glucose cannot pass through the lipid bilayer. Channel proteins/carrier proteins allow these substances through the membranes. Diffusion of these molecules is known as FACILITATED DIFFUSION.
    • CHANNEL proteins: Form PORES in the membrane shaped to only allow 1 type of ion through. They are GATED so can be opened/closed.
      • CARRIER proteins: Shaped so that a SPECIFIC molecule e..g glucose can fit into them at the membrane surface. When the specific molecule fits, the protein changes shape to allow the molecule through to the other side.
    • SIMPLE diffusion: Gases e.g. CO2/O2, lipid-based like steroid hormones.
      • FACILITATED diffusion using CHANNEL proteins: e.g. sodium and calcium ions.
      • FACILITATED diffusion using CARRIER proteins: e.g. larger molecules like glucose and amino acids.


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