Plasma membranes

• functions of plasma membranes

  1. separating cell contents from the external environment

  2. compartmentalisation organelles

  3. cell recognition and signalling

  4. maintaining chemical gradients for cellular reactions

  5. controlling and regulating transport in and out of the cell
• plasma membrane structure


  phospholipid bilayer
• plasma membrane model


  fluid mosaic
• phospholipid molecules are


  polar
• phospholipid heads are


  polar, hydrophilic (attracted to water)
• phospholipid tails are


  non-polar, hydrophobic (repelled by water)
• other molecules present in plasma membranes (excluding phospholipids)

  intrinsic and extrinsic proteins, glycoproteins, glycolipids, cholesterol
• intrinsic protein structure


  transmembrane proteins (span both layers of the membrane) and have amino acids with hydrophobic R groups on their external surfaces
• how do intrinsic proteins stay in place

  they interact with the hydrophobic core of the bilayer so they don't move
• intrinsic protein function


  transportation of molecules in and out of the cell
• two types of intrinsic protein

  channel and carrier
• channel proteins


  provide a hydrophilic channel allowing the passive movement of polar molecules and ions through the membrane down the concentration gradient
• carrier proteins


  they often change their shape to complete the active or passive transport of molecules through the membrane
• extrinsic protein structure


  present in only one side of the bilayer (those present in the extracellular side tend to be glycolipids and those present in the cytosolic side tend to be glycoproteins), many have amino acids with hydrophilic R groups on their external surfaces
• glycoprotein structure


  type of intrinsic protein with a carbohydrate chain attached
• glycoprotein function


  1. carbohydrate chain form hydrogen bonding with water, helping stabilise the membrane

  2. play important role in adhesion

  3. act as receptors for chemical signals
• glycoproteins as receptors


  a chemical binds to the receptor eliciting a response from the cell
• glycoprotein receptor examples


  1. receptors for neurotransmitters and action potentials

  2. receptors for peptide hormones such as insulin

  3. some drugs such as beta blockers bind to receptors for a specific response
• glycolipid structure


  an extrinsic lipid molecule with a carbohydrate chain attached (only found on the extracellular side of the bilayer)
• glycolipid function


  form cell surface antigens
• cholesterol structure


  polar molecules in the phospholipid bilayer
• cholesterol function


  regulates the fluidity of the membrane by interacting with the phospholipid molecules pulling them together reducing fluidity (also prevents the membrane from crystallising or becoming too rigid)
• cholesterol and phospholipid binding

  cholesterol molecules have hydrophobic and hydrophilic ends and so bind respectively to the phospholipid head or tail
• plasma membranes in cell signalling

  to receive signals, the plasma membranes have extracellular receptors to detect changes in environment and allow communication between cells. this is known as cell signalling
• cell signalling mechanism


  1. glycoproteins, acting as receptors, are specific to a single molecule and therefore have a complementary receptor. these are known as target cells

  2. when the specific molecule binds to the receptor, the target cell causes one of the following to happen:

  a release of a secondary intracellular message, opening/closing of a protein channel for transport or activating an enzyme
• target cells in medicine


  many medicinal drugs (such as antihistamines) target specific receptors on the plasma membrane in order to activate the appropriate immune response
• factors affecting membrane permeability

  temperature, organic solvents
• temperature affecting membrane permeability

  an increase in temperature increases the kinetic energy between the phospholipid molecules in turn increasing fluidity of the membrane and damaging the structure

  proteins in the membrane will denature at 40°C and the membrane becomes disrupted and freely permeable
• organic solvents affecting membrane permeability

  as water polarity is essential for the formation of the phospholipid bilayer, membranes will dissolve in organic solvents, disrupting the cells

  the concentration of organic solvent determines the extent of damage
• types of movement across membranes

  active and passive
• types of passive movement across membranes

  diffusion, facilitated diffusion and osmosis
• passive movement across membranes

  requires a concentration gradient, no metabolic energy input
• there is only type of active movement across membranes

  active transport
• active movement across membranes

  requires a metabolic energy input to move against the concentration gradient
• diffusion definition


  the net movement of molecules from an area of high concentration to low concentration, across a partially permeable membrane

  (passive movement, down a concentration gradient so no metabolic energy required)
• factors affecting the rate of diffusion

  temperature, distance, surface area, size of molecule, concentration gradient
• temperature affecting the rate of diffusion

  as temperature increases the kinetic energy of the molecules increases, making them move faster so diffusion rate increases
• distance affecting the rate of diffusion

  if the distance that the molecules have to travel during diffusion increases, the rate of diffusion will decrease
• surface area affecting the rate of diffusion

  the larger the surface area, the more molecules can cross the exchange surface at once increasing the rate of diffusion
• size of molecule affecting the rate of diffusion

  the larger the molecule the more energy is required for them to move and so diffuse slower, decreasing the rate of diffusion