biological membranes

• roles of the cell surface membrane

  • separates cell contents
  • controls movement of substances in/out by being selectively permeable
  • microvilli increase sa in small intestine
  • cell recognition - cell surface antigens
  • cell to cell attachment
  • cell signalling
  • enzymes attach to membrane
• roles of membranes inside cells

  • compartmentalisation - organelles isolated
  • isolate enzymes - lysosomes
  • control entry/exit into organelles
  • attachment of ribosomes
  • isolate dna
  • provide internal transport system
  • cristae in mitochondria folded for sa
• glycocalyx

  • glycolipid - attached to phospholipid
  • glycoprotein - attached to protein
• plasma membrane contains

  • glycoproteins
  • glycolipids
  • cholesterol
  • extrinsic and intrinsic proteins
  • channel proteins
  • phospholipids
• phospholipids

  • contain hydrophobic tail and hydrophilic head
  • functions - give the membrane fluidity, barrier to large water soluble molecules, unsaturated fatty acids have kinds which prevents close packing of phospholipids
• proteins in membranes

  • can be: alpha helix, globular, intrinsic, extrinsic, channel and carrier
  • structural support
  • transport
  • help cells adhere together
  • receptors for hormones
• cholesterol

  • fits between phospholipids
  • regulates fluidity of membranes
  • reduces lateral movement of phospholipids, improving stability
• glycocalyx

  • receptor sites for hormones
  • cell markers e.g ABO blood system
  • act as antigens, allowing cells to recognise as self
  • helps cells adhere to one another
  • forms hydrogen bonds with water to stabilise the membrane
• membranes and temperature

  • increase in temperature increases the kinetic energy, increase in movement increases the permeability of the membrane
  • phospholipids vibrate causing them to move apart slightly
  • bigger gaps allow for larger molecules to pass through
  • at very high temps, phospholipids separate completely causing membrane to break down
  • proteins will also denature
• how would the membrane structure vary in organisms living in high temps?

  more cholesterol and more saturated fatty acids - form uniform shapes
• how would the structure of the membrane vary in organisms in low temps?

  less cholesterol, more unsaturated fatty acids - have more kinks and therefore larger gaps
• effect of organic solvents on membrane permeability

  • organic solvents can dissolve lipids
  • ethanol dissolves the lipids, loosing structure of bilayer
  • increasing solvent concentration increases permeability
• diffusion - the net movement of partilles from a region of their higher concentration to a region of lower concentration
• factors affecting rate of diffusion

  • steepness of concentration gradient
  • temperature
  • surface area
  • length of diffusion pathway
  • moving
  • type of molecules
• simple diffusion - moment directly through the phospholipid bilayer

  • eg carbon dioxide, oxygen, lipid soluble vitamins
• facilitated diffusion - passive movement of molecules through a channel or carrier protein
• Chanel proteins

  • specific for certain ions
  • have a hydrophilic lining
  • can be gated
• carrier proteins

  • for large polar molecules
  • when specific molecules bind to the protein, changes shape and allows crossing of membrane
  • does not require atp
• osmosis - net movement of water from a region of high water potential to a lower water potential across a partially permeable membrane
• solute - the substance that dissolves in a liquid to form a solution
• solvent - the liquid in which a solute dissolves
• solution - the mixture formed when a solute has dissolved
• water potential - the tendency of water to leave a solution

  • pure water = 0
  • adding solutes lowers the water potential
• animal cells in low water potential

  • water moves from a higher water potential inside to a lower water potential outside
  • reduction in cell volume
  • cell becomes crenated
  • cells in a tissue pull away from other cells
  • cytoplasm becomes concentrated
  • enzymes are disrupted
• animal cells in high water potential - water moves from a higher water potential outside the cell to a lower water potential inside cell

  • pressure inside cell increases
  • plasma membrane is fragile and cannot withstand pressure
  • no cell wall to prevent bursting
  • lysis
  • haemolytic - red blood cell bursting
• plant cells in low water potential - water moves from higher water potential inside cell to lower water potential outside cell

  • volume of cell decreases
  • contents of cell doesn't push out on cell wall
  • cell becomes flaccid
  • plasma membrane pulls from cell wall
  • cell is plasmalysed
• plant cells in high water potential - water moves from a higher water potential outside cell to lower water potential inside

  • volume of cell increases
  • cell contents expands
  • contents of cell wall push out on cell wall
  • turgid cell
• active transport - the movement of molecules through carrier proteins across membranes against concentration gradient

  1. molecule binds to receptors on carrier proteins
  2. atp binds to carrier protein, hydrolysed into add
  3. carrier protein changes shape and molecule released on other side
• example of active transport

  • root - magnesium ions taken in for chlorophyll
  • intestine - removal of glucose from gut into blood
  • kidney - useful molecules are reabsorbed
  • phloem - sucrose loaded
  • nerve impulses - sodium potassium pump
  • muscles - calcium ions into sarcoplasmic reticulum
• bulk transport - large molecules that cannot fit through carrier proteins or channel protein enter or leave the cell via vesicles, requires atp
• endocytosis

  1. substance is ousted the cell, binds to receptor site
  2. cell surface membrane folds around the substance to engulf it
  3. membrane pinches off to form a vesicle
  4. vesicle can be moved into cytoplasm along cytoskeleton
• exocytosis

  1. vesicles formed by Golgi apparatus move toward the cell surface membrane with the cytoskeleton
  2. vesicle fuses with the membrane
  3. contents of vesicle released outside cell
• cell signalling - communication between cells using chemicals triggering a response inside cells

  1. reception - first messenger binds to receptor eg non steroid hormones
  2. transduction - second messenger activates enzymes eg adenyl cyclase
  3. response