2.1.5

• Plasma (cell-surface) membrane


  Partially permeable barrier between the cell and its environment, keeps the contents of the cell separate from its environment, limits what molecules can enter and leave the cell, site for certain chemical reactions, enables cell communication through cell signalling
• Internal membranes (organelle membranes)


  Separate organelles from the cytoplasm, compartmentalise the cell, separate processes so each can occur in a specialised area, enable concentration gradients, act as sites of specific chemical reactions
• Fluid mosaic model of membrane structure

  • Bilayer of phospholipid molecules
  • Cholesterol regulating fluidity and stability
  • Glycolipids and glycoproteins for cell signalling and attachment
  • Protein molecules floating in the phospholipid bilayer, some partially held on the surface (extrinsic), others embedded (intrinsic), some floating freely, others bound to other components
• Phospholipids


  Form a barrier that limits movement of some substances into and out of the cell or organelles, making the membrane partially permeable
• Cholesterol


  Fits between phospholipid tails, inhibits their movement, reduces membrane fluidity, provides mechanical stability, makes membrane less permeable to water and ions, can be converted to steroid hormones, used for waterproofing skin, making vitamin D, and bile salts
• Glycolipids and glycoproteins


  Carbohydrate groups used to recognise the cell as 'self' or 'foreign', act as binding sites for drugs, hormones, and signalling molecules, enable cell attachment
• Proteins


  • Have enzymatic activity and cell signalling functions, form pores allowing movement of molecules, act as carrier molecules for facilitated diffusion, and as active pumps
• As temperature increases


  Membrane permeability increases, phospholipid bilayer may lose mechanical stability and melt, proteins may denature, making membrane completely permeable
• As solvent (e.g. alcohol) concentration increases


  Membrane is more likely to dissolve
• Passive transport


  Movement of molecules that does not need metabolic energy (ATP), uses kinetic energy, occurs down concentration gradient, can be diffusion, facilitated diffusion, or osmosis
• Diffusion


  • Net movement of molecules away from a concentrated source, may occur across a membrane if molecules are fat-soluble or small enough to fit between phospholipids
• Facilitated diffusion


  • Diffusion across a membrane helped by a transport protein, could be a pore protein or a carrier protein
• Osmosis


  • Net movement of water molecules across a partially permeable membrane, down the water potential gradient
• Factors affecting rate of diffusion

  • Temperature (higher increases rate)
  • Concentration gradient (larger increases rate)
  • Molecule size (smaller increases rate)
  • Membrane thickness (thinner increases rate)
  • Surface area (larger increases rate)
• Active transport


  Movement of molecules using metabolic energy (ATP), can move molecules against their concentration gradient, uses membrane-bound proteins that change shape
• Bulk transport


  Movement of molecules through a membrane by the action of vesicles, endocytosis moves molecules into the cell, exocytosis moves molecules out of the cell, uses metabolic energy
• Water potential


  Pure water has a water potential of zero, adding solutes lowers the water potential, water molecules move from higher to lower (more negative) water potential
• Osmosis


  Movement of water molecules across a partially permeable membrane, down the water potential gradient