Cell surface membrane

• What do membranes do

  Create enclosed space separating internal cell environment form external environment .

  control exchange of material cross them as partially permeable
• what is the cell surface membrane model called

  fluid mosaic model
• structure of cell surface membranes

  made from phospholipid bilayer with hydrophobic tails inwards and hydrophilic heads outwards cell

  also contains intrinsic and extrinsic proteins

  cholesterol

  glycolipids

  glycoproteins
• function of proteins in surface membrane

  intrinsic proteins include carrier proteins which allow substances across membrane .

  proteins aid to movement across protein and provide mechanical support
• function of cholesterol 


  makes membrane more rigid and reduce lateral movement . prevents leakage of water and dissolved ions from cell as also hydrophobic.

  regulates fluidity . Sit between bilayer to stop them packing to closely when temp low. prevents bilayer from being to fluid at high temps
• function of glycolipids 


  made of carbohydrate bound to lipids . extend from surface of cell and act as receptors for molecules . allow cells to adhere to one another to form tissues
• function of glycoprotiens 


  carbohydrates attached to extrinsic proteins . act as cell surface receptors and neurotransmitters . allows cell to recognise one another as well to attach and form tissues
• why is it described as fluid

  phospholipids and proteins move around via diffusion

  phospholipids move sideways within own layer

  proteins move about within bilayer
• why is it described as mosaic 


  as when viewed above scattered proteins in phospholipid bilayer look mosaic
• Definition of diffusion 


  The net movement as of molecules from an area of high concentration to an area of low concentration down a concentration gradient. This is simple diffusion and does not require energy as it is passive
• how does concentration gradient affect diffusion

  The steepness affects the rate . A greater difference in concentration means more molecules passion in two directions so faster rate
• how does temperature affect diffusion rate

  Molecules and ions have more kinetic energy at higher temps so move faster resulting in higher rate
• How Does surface area affect diffusion rate

  Higher surface area more space for molecules to diffuse through so rate increases . Surface area of membranes can be increased by folding like microvilli or mitochondria

  as cell increases in size surface area to volume ratio decreases slowing down diffusion as distance greater
• how do properties of molecules and ions affect diffusion rate

  larger molecules diffuse more slowly

  uncharged and non polar molecules diffuse directly across phospholipid bilayer

  non polar molecules diffuse faster than polar ones as solvable in non polar phospholipid bilayer
• what is facilitated diffusion 


  large polar molecules like glucose and amino acids and ions like sodium and chloride can diffuse through phospholipid bilayer . go through bilayer by using channel and Carrier proteins that are highly specific to molecules .
• channel proteins structure 


  water filled pores , allow charged ions to diffuse through membranes . Diffusion does not occur freely . Protein gated as part of protein on inside surface of membrane can move in order to close or open pores . Allows them to control exchange of ions . Passive process
• Carrier proteins function

  protein can switch between two shapes and this causes binding site to open on one side of membrane first and open to other side of membrane when protein switches shape . direction of movement of molecules diffusion across membrane depends on relative concentration on each side of membrane . net diffusion of molecules or ions into or out of a cell will occur down a concentration gradient

  active process requires ATP
• Definition of osmosis 


  diffusion of water molecules from a area of high water potential to a area of low water potential across a partially permeable membrane
• definition of water potential

  tendency of water to move out of a solution
• what is water potential of pure water

  0kpa . therefore any solution that has solutes will have a water potential lower than 0kpa . will have negative value
• osmosis in plant cells placed in pure water or dilute solution 


  water will enter plant cell through partially permeable membrane via osmosis as solution has higher water potential

  as water enters vacuole volume of plant cell increases

  expands protoplasm and pressure increases . cell wall prevents cell from bursting

  pressure created also stops too much water entering and prevents cell from bursting

  when fully inflated plant cell becomes rigid and firms , fully turgid and helps provide support and strength
• plant cell placed in lower water potential

  if plant placed in solution with lower water potential , water will leave plant via osmosis through partially permeable membrane. water leaves vacuole and volume decreases . protoplasm shrinks and no longer exerts pressure on cell wall . process known as plasmolysis plant cell is plasmolysed
• what happens when you place animal cells in Low water potential solution

  animal cells do not have supporting cell wall . water will leave via osmosis through partially permeable membrane and cell will shrink and shrivel . occur when cell in hypertonic environment
• what is hypertonic solution

  when solution outside of cell has a higher solute concentration than inside cell
• what is isotonic solution 


  Water potential equal between both cell and solution
• what is hypotonic solution

  cell has lower water potential than solution so water enters via osmosis
• what happens when you place animal cells in pure water

  pure water/ solution has higher water potential than animal cell . water enters via osmosis through partially permeable membrane .cell continues to gain water cell membrane stretches until It burst as no cell wall to withstand pressure
• why is it important constant water potential must be maintained in cells and body

  so cells do not burst or shrivel
• what does intersect mean in water potential calibration curve

  intersection on graph is the water potential in sample as no osmosis has occurred
• active transport fact sheet 


  movement of molecules and ions through a cell membrane from a area of low concentration to area of high concentration

  requires carrier proteins

  requires energy to make carrier proteins change shape allowing transfer of molecules through membrane

  energy provided by ATP and produced during respiration ATP hydrolysed to release energy
• importance of active transport

  re absorption of useful molecules and ions into blood after kidney filtration in tubules

  absorption of products from digestion

  leading sugars from photosynthesising cells of leaves into phloem for transport

  loading inorganic ions from soil into root hair cells
• what is co transport 


  the coupled movements of substances across a cell membrane via a carrier protein .
• co transport of glucose and sodium ions

  active transport of sodium ions from epithelial cells into blood lowers sodium ions concentration inside cells and creates sodium ion concentration gradient between ileum and epithelial cell

  sodium ions move into cell from ileum by facilitated diffusion carrying glucose via con transport with ATP molecule changing shape of carrier protein

  glucose concentration inside epithelial cell increases and glucose molecules enter blood via facilitated diffusion
• what is another factor that affects rate of facilitated diffusion

  number of channel or carrier proteins available in exchange of surface as one all proteins in use rate can no longer increase
• How are kidney cells adapted for facilitated diffusion

  Have cel membrane that contain a lot of aquaporins that allow facilitated diffusion of water through cell membranes ans allow cells to reabsorb water into body
• how are neurones and muscle celles adapted for facilitated diffusion

  have cell membranes that have a lot of channel proteins for sodium potassium and calcium ions . opening and closing of ions and amount of them play a important role in speed of electrical transmission along membranes of neurones and muscle cells
• how does temperature affect membrane fluidity

  as temp increases phospholipids in membrane move and not tightly as packed increasing permeability

  high temp can melt and breakdown bilayer further increasing permeability

  low temps below 0 can reduce permeability as channel or carrier proteins deform and ice crystals can pierce cell membrane making then highly permeable