Cell membrane structure and transport

• Cell membrane:

  • made up almost entirely of proteins and phospholipids
  • controls what enters and leaves the cell
  • receptor (communication)
• Phospholipids:

  • can form bilayers
  • inner layer has its hydrophilic heads pointing in, towards the cell, and interacts with water in the cytoplasm
  • outer layer has its hydrophilic heads, pointing outwards, interacting with water surrounding the cell
  • hydrophobic tails of the two phospholipid layers point towards each other
  • phospholipid component allows lipid-soluble molecules across but not water-soluble molecules
• Proteins:

  • extrinsic proteins - on either surface of the bilayer
  • it provides structural support, form recognition sites (by identifying cell), receptor sites for hormone attachment
  • intrinsic proteins - extend across both layers of the bilayer
  • Used for structural support, some are carriers transporting water-soluble substances across, others allow active transport of ions across by firming channels
• Fluid-mosaic model:

  • individual phospholipid molecules can move within a layer relative to one another (fluid)
  • proteins embedded in the bilayer vary in shape, size and pattern (mosaic)
• Structure of biological membrane:

  • extrinsic protein
  • intrinsic protein
  • phospholipid
  • cholesterol (animal cell) - makes membrane more rigid and stable
  • glycoalyx (glycoprotein and glycolipid) - carbohydrate layer around membrane, some have roles as hormone receptors or in cell to cell recognition
• High temperatures


  • Increase kinetic energy of phospholipid molecules
  • Phospholipid molecules move faster
  • Increase permeability
  • Easier for particles to diffuse through
  • Enzyme may denature at extreme temperatures
• Lipid-soluble substances and small molecules

  • They dissolve in the phospholipid and diffuse across the membrane
• Organic solvent

  • Disrupts membrane
  • Increases permeability of membrane
  • Alters structure
• Water-soluble substances, polar molecules and ions


  • They cannot readily diffuse through the phospholipid and must pass through intrinsic protein molecules
• Phospholipid layer

  • Hydrophobic, so it is easier for lipid-soluble substances and small molecules to pass through than water-soluble substances
• Permeability of membrane

  The ease with which substances can pass through a membrane
• Cell membrane

  • Selectively permeable to water and some solutes
• Organic solvent


  Non-polar solutes dissolve in non-polar solvent
• Smaller the size of diffusing molecule


  Faster the diffusion
• Greater the difference in the concentration of molecules


  More molecules diffuse in a given time
• Larger the surface area


  More molecules can diffuse across in a given time
• Lipid-soluble molecules


  Diffuse faster than water-soluble molecules
• Increased temperature

  Increases the rate of diffusion as molecules have more kinetic energy
• Thinner the membrane


  • Shorter the diffusion pathway
  • More molecules can diffuse across in a given time
• Factors affecting the rate of diffusion

  • Concentration gradient
  • Thickness of exchange surface
  • Surface area of membrane
  • Size of diffusing molecule
  • Nature of molecule
  • Temperature
• diffusion = passive movement of small non-polar lipid-soluble molecules from an area of high to low concentration
• Facilitated diffusion:

  • always uses intrinsic proteins
  • channel proteins open to let small ions pass through and close according to cells needs, channels are hydrophilic so water-soluble molecules can pass through
  • carrier proteins allow diffusion of larger polar molecules across membrane, molecules attaches to binding site -> carrier protein changes shape -> releases molecule on the other side of membrane
  • passive movement
• Active transport:

  • active process, requires energy from ATP to go against concentration gradient
  • molecule/ion combine with a specific carrier protein on the outside of membrane -> ATP transfer phosphate group to carrier protein in the inside of membrane (becomes ADP) -> carrier protein changes shape and carries the molecule/ion across membrane, released into the cytoplasm
  • phosphate ion released from carrier protein back into cytoplasm and combines with ADP to form ATP
  • carrier protein returns to original shape
• Co-transport:

  • two substances simultaneously transported across a membrane by one protein
  • carrier protein changes shape and deposits molecules inside the cell to separately diffuse
  • Uses ATP as source of energy, from low to high concentration
• Bulk transport:

  • endocytosis
  • exocytosis
• Endocytosis = material is engulfed by extensions of plasma membrane and cytoplasm, surrounding it, making it a vesicle

  • Phagocytosis - uptake of only solid material that is too large to be taken in by diffusion
  • granulocytes engulf bacteria, lysosome fuses with vesicle formed and enzymes digest cells, products are absorbed into the cytoplasm
  • Pinocytosis - same mechanism, only smaller vesicles are produced, but is the uptake of liquid
• Exocytosis = process by which substances may leave the cell, being transported through the cytoplasm in vesicle, which fuses with cell membrane e.g. digestive enzymes