membranes

• COMPONENTS OF CELL MEMBRANES:

  phospholipid bilayer:

  • two layers of phospholipid molecules - hydrophobic head facing outwards in contact with water, Hydrophobic tails facing inwards + away from water, Hydrophilic head facing outwards

  Proteins:

  • globular proteins which carry out the function of membranes - The Fluid Mosaic Model (fluid = flexible layer of lipids and proteins which are free to move around each other, Mosaic = proteins are scattered between the phospholipids at various positions)

  Cholesterol:

  • present between the phospholipids, regulates the membrane stability and fluidity
• Further components of the plasma membrane (only):

  • glycolipids: lipids with carb chain attached
  • glycoproteins: protein with carb chain attached these formc ell surface receptor

  roles of teh glycocalyx: (glycolipids and glycoproteins)

  • cell-cell recognition
  • cell-cell signalling
  • cell-cell adhesion

  the plasma membrane may form microvilli to increase SA or may extend around organlles such as Cilia and Flagella for motility
• types of membrane proteins

  A) channel protein

  B) carrier protein

  C) glycoprotein

  D) 7 nm

  E) transmembrane protein

  F) extrinsic protein

  
• roles of membrane proteins:

  • enzymes: membranes are sites for chemical reactions
  • cell signalling - protein receptors are glycoproteins that have a complementary binding site to a specific cell signalling molecule on the extra-cellular side of the membrane
• hormones, endocrine tissue secreting the hormone and the target tissue containing the receptor:

  • insulin: beta cells of the islets of Langerhans in the Pancreas - target tissue: liver and muscles
  • glucagon: alpha cells of the islets of Langerhans in the Pancreas - target tissue: liver
  • adrenaline: adrenal medulla - target tissue: many cells in the body
  • ADH: posterior pituitary - target tissue: collecting duct in the kidneys
• receptors as drug-binding sites:

  • drug has similar shape to signalling molecule -> can bind to complementary receptor and block OR enhance its action

  antihistamine:

  A) inflammation

  B) receptor

  C) binds to receptor

  
• receptors as drug-binding sites:

  • beta blockers block access of adrenaline to target tissue
• further roles of the plasma membrane:

  3. site of attachment for cytoskeleton: Anchoring points for cytoskeleton - microfilaments attach to move the plasma membrane e.g. formation of pseudopodia, cytokinesis/ cell division

  4. cell-cell recognition: Glycoproteins act as antigens on the surface of the cell

  5. cell-cell adhesion
• 6. transport

  diffusion: Net movement of a substance down its concentration gradient (= from an area of high conc. to an area of low conc.) until equilibrium is reached. The molecules have their own kinetic energy, there is no need for an input of energy/ ATP

  two types of diffusion across cell membranes:

  1. simple diffusion: Diffusion of molecules through the phospholipid bilayer, Includes: Small non-polar molecules, oxygen, carbon dioxide, water, lipid soluble hormones (steroids)
  2. facilitated diffusion: Diffusion of molecules through transmembrane proteins
• 2 types of facilitated diffusion requiring 2 dif. types of proteins:

  • channel proteins/ion channels: contain a hydrophilic channel which allows diffusion of ions across the membrane. channels are gated
  • carrier proteins: allow diffusion of small organic molecules e.g. glucose, amino acids where the conformation (shape) of the protein changes to allow the transport of the molcule across the membrane
• Active transport: Movement of molecules against their concentration gradient using carrier proteins (often called

  pumps) and input of energy ie ATP

  • glucose, amino, ions NEVER water
• the sodium potassium pump - example of active transport

  • all cells contain the Na/K pump or Na/K ATPase ATP is used as a source of energy
  • Na are actively transported out of cells
  • K are actively transported inside of cells
  • this generates concentration gradient for these ions which are used to drive processes such as nervous transmission, muscle contraction, hormone secretion etc.
• Factors which affect the permeability of membranes:

  • permeability = the measure of ease of passage (diffusion) through the membrane
  • membrane permeability: membranes are selectively permeable
  • temperature: when temp is increased further, the phospholipids gain more kinetic energy and move about more. This increases membrane's fluidity and permeability. - membrane proteins become denatured at high temps increasing permeability - permeability increases with increasing temp. so rate of diffusion also increases. - as temp increases so does kinetic energy of molecules => dif. rate increases
• Factors which affect the permeability of membranes: solvents and detergents, pH

  • amphilic molecules - contain both hydrophylic and hydrophobic parts
  • proteins are denatured by organic solvents
  • organic solvents such as alcohol or acetone dissolve lipds and can therefore break membranes. permeability increases and the contents of the cell leaks out
  • detergents such as SDS emulsify cell membranes
• Factors which affect the rate of diffusion:

  • rate of diffusion = how fast diffusion happens
  • concentration gradient:
  • as conc. grad. increases rate of dif. increases. 2 process are needed to maintain a high conc. grad.:
  • the molecules crossing membrane must be taken away
  • constant supply of molecule
• Factors which affect the rate of diffusion:

  distance over which diffusion occurs:

  • membrane are very thin: 7nm
  • exchange surfaces are only one cell thick

  surface area:

  • as S.A. increases, rate of dif. increases
  • s.a. of many membranes in the body is increases e.g. small intestine, kidnet proximal tubules, capillaries, lungs
  • Fick's Law: rate of dif. = conc. grad. X s.a./ distance
• Factors which affect the rate of diffusion:

  size and type of molecule:

  • as solubility of molecule in the membrane increaes, rate of dif. increases
  • larger the molecule, lower the rate of diffusion
  • diffusion of hydrophobic molecules > dif. of hydrophilic molecules
  • gases (oxygen and CO2) diffuse through phospholipid bilayer
  • ions need ion proteins to cross the membrane
• Osmosis : the net movement of water from a higher water potential to a lower water potential gradient across a partially permeable membrane (until equilibrium is reached)

  • water potential = the ability/tendency of a solution/cell/system to donate water to another sol./cell/system - measured in kilopascals (kPa)

  
• • the water potential of pure water is 0kPa => the water potential of all other solutions must be negative
  • pure water: 0kPa, highest
  • hypotonic solution: -100kPa, higehr (less negative)
  • hypertonic solution: -1000kPa, lowest (more negative)
  • the values are negative because they can be seen to act as a 'pulling' pressure as opposed to a pushing pressure
• The rate of somosis across the membrane can be increased by:

  • the use of protein channels called Aquaporins whihc have a hydrophilic pore. they are inserted into the membrane by the exocytosis by vesciles.
  • active transport of ions to lower water potential and create a water potential graident across the membrane
• water potential in animal cells and plant cells

  wp = tendency of a solution to donate water to another solution

  • animal cells: wp is solely as a result of the solutes dissolved in solution:
  • water potential in cell = solute potential (value of water potential as a consequece of solute molecules only)
  • plant cells - another term is needed to describe the positive pressure exerted by cell wall which stops cell from expanding:
  • pressure potential = pressure exerted by cell wall against the expanding cell (=the hydrostatic pressure exerted by teh expanding cell against the cell wall)
• water potential in animal cells and plant cells (part 2)

  • solute potential - contribution to the value of the wp made by the conc. of the solutes in the cells
  • for plant cells: water potential of cell = solute potential + pressure potential
  • solute potential is always negative
  • pressure potential is always positive in cells
  • water potential = 0 when flaccod -> plasmolysed
  • wp cell = 0 i.e when pressure potential = negative solute potential, when plant call is fully turgid
• Bulk transport: cell membrane transport of macromolecules and particles:

  • only small molecules enter or leave cells through the membrane
  • larger molecules use vesicles:

  exocytosis:

  • intracellular vesicles are made by the Golgi Apparatus
  • the vesicles are moved by the action of the motor proteins which move then along microtubule tracks towards the plasma membrane
  • when vesicle reaches the plasma membrane, it fuses with it and empties its content into the extracellular fluid
  • requires energy, process also known as secretion
• Bulk transport: cell membrane transport of macromolecules and particles:

  endocytosis:

  • the plasma membrane invaginates/forms a dip or tuck and pinches off to carry the cellular fluid into the cell
  • substance to be carried into cell is progressively surrounded by small portions of the plasma membrane
  • plasma membrane pinches off to form an intracellular vescile or vacuole
  • process requires energy
  • reverse of exocytosis
• there are 2 types of endocytosis:

  pinocytosis:

  • cell drinking
  • intake of fluids and solutes by the formation of small vesicles from the plasma membrane
  • the plasma membrane invaginates/forms a dip or a tuck and pinches off to carry the extracellular fluid into the cell

  phagocytosis:

  • intake of large particles e.g. cell debris or bacteria by the formation of vacuoles
  • plasma membrane extends outwards to form pseudopia
  • carried out by specialised cells called phagocytes or by unicellular organisms such as amoeba