Transport across cell membranes

• Compartmentalisation is the isolation of organelles from the cytoplasm and other organelles allowing for cellular processes to happen separately
• Functions of internal membranes

  • compartmentalisation
  • site of biochemical reactions
  • control movement of substances in and out of organelles
• Function of cell plasma membrane

  • define cell boundary
  • control movement of substances in and out of cell
  • cell recognition in immune system
  • receptor site for hormones and neurotransmitters
  • help cells stick together to form tissues
  • allow cells to bend shape and be flexible
• Water can pass through phospholipids slowly because the molecule is small and polar
• The hydrophobic fatty acids face inwards in the bi layer away from watery environments where as the hydrophilic phosphate groups face outwards towards watery environments inside the cell and outside the cell
• Functions of phospholipids

  • allow lipid soluble substances to enter and leave the cell
  • prevent water soluble substances from entering and exiting the cell
  • makes cell membrane flexible and self sealing
• Intrinsic proteins go through the phospholipid bilayer
• Extrinsic proteins are found at the surface of the bilayer and don’t go all the way through
• Protein channels are hollow tubes that allow water soluble molecules to diffuse across the plasma membrane
• Carrier proteins move molecules that can’t cross the membrane by changing shape and allowing the molecules through which can be by active transport or passive diffusion
• The function of extrinsic proteins is provide adhesion between cells. Act as receptors for hormones. Cell recognition
• The function of intrinsic proteins is to form protein channels and carrier proteins
• Intrinsic proteins move water soluble, ionic and polar molecules across the plasma membrane
• Cholesterols is a lipid that strengthens the membrane and is hydrophobic so prevents water loss
• Functions of cholesterol

  • reduces sideways movement of phospholipids in bilayer
  • makes membrane less fluid when at high temperatures
  • prevents leakage of water and dissolved ions from cell
• A glycolipid is a carbohydrate attached to a lipid
• Glucolipids act as recognition sites. Maintain membrane stability. Help attach cells together to form tissues
• Glycoproteins are a carbohydrate attached to a protein in the bilayer
• Glycoproteins act as recognition sites for the immune system. Help cells attach to form tissues
• Cell surface membrane cannot be seen with optical microscope because wave length is too long so has a low resolution
• Diffusion is the net movement of molecules or ions from a region of high concentration to a region of lower concentration until an equilibrium of concentration is formed
• Fluid mosaic model

  Fluid = individual phospholipids can move sideways and switch place with each other

  Mosaic = when seen from above membrane looks like a mosaic of phospholipids, proteins and lipids
• Net movement of particles =

  the number of molecules moving in the direction of the forces - the number of molecules moving in the opposite direction of the force

  moving in - moving out
• Simple diffusion, osmosis and facilitated diffusion is passive
• Active transport and co transport is active
• Dynamic equilibrium is an equilibrium that changes but stays relatively equal
• As temperature increases so does diffusion because more kinetic energy
• Steeper the concentration gradient = quicker the diffusion
• Stirring increases diffusion
• Increasing the surface area increases diffusion
• Decreasing diffusion distance increase diffusion
• Smaller the molecule quicker the diffusion
• Simple diffusion happens through phospholipid bilayer
• Ficks law states that rate of diffusion is dependant on the surface area, difference of concentration of particles and thickness of diffusion surface
• ROD = SAD/TOES

  RATE OF DIFFUSION = surface area x distance/ thickness of surface
• Osmosis is the passage of water molecules from a solution of high water potential (dilute) to a solution of lower water potential (concentrated) through a selectively permeable membrane
• Hypertonic = concentrated with low water potential
• Hypotonic = dilute with high water potential
• Isotonic = equal concentration to water potential
• Protoplast is the living components of a cell excluding cell plasma membrane