transport

• plasma membranes:

  surrounds cells

  allows condition to be different inside and outside cells

  controls substances in and out of cell
• Phospholipids


  Molecules with hydrophilic heads facing outward attracting water and hydrophobic tails pointing inwards repelling water, allowing lipid-soluble substances to move through the membrane, functions include allowing lipid-soluble substances to enter and leave cell, preventing water-soluble substances from entering and leaving cell, making the membrane flexible and self-sealing
• Proteins embedded in phospholipid bilayer

  Some on the surface of the bilayer giving mechanical support and acting as cell receptors for molecules like hormones, some spanning the phospholipid bilayer forming protein channels for water-soluble ions to diffuse across the membrane, other carrier proteins bind to ions or molecules like glucose and amino acids and can change shape to move molecules across the membrane
• Proteins

  • Provide structural support, act as channels to transport water-soluble substances across the membrane, allow active transport through carrier proteins, form cell-surface receptors for identifying cells, help adhere cells together, act as receptors for hormones
• Cholesterol

  Within the phospholipid bilayer of the cell-surface membrane, adds strength, is hydrophobic important for reducing water loss and dissolved ions from the cell, pulls together fatty acid tails of phospholipid molecules limiting movement but not too rigid, functions include reducing lateral movement of other molecules including phospholipids, making the membrane less fluid at high temperatures, preventing leakage of water and dissolved ions from cells
• Glycolipids


  Carbohydrates covalently bonded with lipids, extend from the phospholipid bilayer to the watery environment outside the cell and act as cell-surface receptors for specific chemicals, functions include acting as recognition sites, maintaining stability of the membrane, helping attach to one another and form tissues
• Glycoproteins


  Carbohydrate chains attached to extrinsic proteins on the outer cell surface membrane, act as cell surface receptors for hormones and neurotransmitters, functions include acting as recognition sites, helping cells to attach to one another and form tissues, allowing cells to recognize each other e.g. lymphocytes
• Molecules forming cell surface membrane

  • Phospholipids
  • cholesterol
  • proteins: channel proteins, carrier proteins, receptor proteins
  • glycolipids
  • glycoproteins
• permeability of cell surface membrane:

  • cell-surface membrane control movement of substances into and out of cell
  • many do not freely diffuse across since:
  • not soluble in lipids and therefore cannot pass through phospholipid layer
  • too large to pass through channels in membrane
  • same charge on protein channel and so are repelled
  • electrically charged (polar) and have difficulty passing through non-polar hydrophobic tails in pospholpid bilayer
• fluid mosaic model

  • fluid: individual phospholipid molecules can move relative to one another, give membrane flexible structure and is constantly changing shape
  • mosaic: because proteins that are embedded in phospholipid vary in shape, size and pattern
• diffusion


  net movement, as result of random motion of its molecules or ions, of substance from region of its higher concentration to region of its lower concentration

  move down concentration gradient

  random movement caused by natural kinetic energy of molecules or ions
• facilitated diffusion 


  passive process, relies on inbuilt motion (ke) of diffusing

  no need for ATP

  diffuse large molecules (glucose, amino acids, ions- Na+ and Cl-)

  down concentration gradient, at specific points on plasma membrane

  use protein such as protein channels and carrier proteins (highly specific)
• channel proteins


  water filled pores

  allow charged substances to diffuse through (ions)

  gated, channel protein on inside surface of membrane, can move in order to close or open pore

  allows channel protein to control exchange of ions
• carrier proteins


  can switch shapes, so can cause binding site of carrier protein to open to one side of membrane, then open other side when carrier protein switches shape

  direction of movement depends on relative concentration on each side of membrane

  net diffusion of molecules/ ions out of cell wall occur down concentration gradient
• simple diffusion depends on

  • concentration gradient: high concentration gradient means faster rate of diffusion
  • how thick exchange surface is: thinner surface means faster rate of diffusion
  • surface area: larger surface area means faster rate of diffusion

  such as microvilli: increase surface area for diffusion
• facilitated diffusion depend on

  • concentration gradient: higher concentration gradient, faster rate of facilitated diffusion
  • number of channel/ carrier proteins: once all in use, facillitated diffusion can not happen quicker
• factors of diffusion

  • steep conc gradient:
  • more on one side of membrane, more will move from that side to other side
  • greater difference means more molecules passing
  • temperature:
  • more kinetic energy at higher temps
  • move faster, higher rate of diffusion
  • surface area:
  • greater surface area = more molecule/ ions so faster rate of diffusion
  • surface area can be increased e.g. by folding (microvilli in intestine)
• osmosis

  diffusion if H2O molecules across partially permeable membrane, area of high water potential to area of lower water potential

  water potential= likelihood water molecules will diffuse in or out of solution

  pure water has highest water potential

  solution of same water potential are isotonic
• rate of osmosis factors

  • water potential gradient: higher water potential= faster rate of osmosis
  • thickness of exchange surface: thinner surface= faster rate of osmosis
  • surface area of exchange surface: larger SA, faster rate of osmosis
• investigate water potential

  use serial dilutions
• osmosis of plant cells


  water enter cell vacuole, volume of plant cell increase

  protoplast increase, push against cell wall and pressure builds

  • turgid: plant cell inflated with water and become rigid and firm- important for support and strength

  water leaves vacuole, volume of plant cell decrease, protoplast shrinks, pull away from cell wall

  • plasmolysed- plasmolysis
• osmosis in animal cells

  hypertonic solution:

  • RBC have higher H2O potential than solution
  • net movement of water out
  • cell shrivels

  isotonic solution

  • water potential and RBC equal
  • no net movement of H2O
  • normal cells

  hypotonic solution

  • RBC lower water potential than solution
  • net movement of water in
  • cell swell, may lyse(burst)
• active transport

  • region of low to high concentration
  • energy from respiration
  • provided by ATP
  • requires carrier proteins

  
• Co-transport


  Coupled movement of a substance across a cell membrane via a carrier protein, involving the facilitated diffusion or active transport of two molecules at a time
• Co-transport


  • Specific co-transport involves the absorption of glucose and Na+ ions
• Active transport

  1. Na+ from epithelial cell into cell generates sodium ion concentration gradient between ileum and epithelial cell

  2. Na+ moves into cell from ileum by facilitated diffusion, carrying glucose molecule, via co-transport

  3. Glucose concentration inside epithelial cell increases

  4. Glucose molecule enters blood via facilitated diffusion