Cell Membrane

Cell Membrane

Membranes
1. All cells surrounded by membranes
  1. In eukaryotics surrounds some organelles too
2. Acts as a barrier to control what enters or leaves
  1. Partially permeable - allows some molecules through
    1. Can move through by diffusion, osmosis or active transport
3. Fluid mosaic model
  1. Composed of lipids, proteins, carbohydrates
    1. Phospholipids form a continuous bilayer
      1. Bilayer is fluid as it moves continuously
        Cholesterol are present in the membrane
        Proteins scattered throughtout like a mosaic
        Polysaccharides can attach to proteins and lipids
        Glycolipid, Glycoprotein
  2. Different components play different roles
    1. Phospholipids form a barrier to dissolved substances
      1. As hydrophobic centre of bilayer repels water
    2. Cholesterol provides stability
      1. Cholesterol is present in all cell membranes except bacteria
      2. Does this by binding between phospholipids to the hydrophobic tails
        Causes them to pack closer together, restricting movement making it more ridgid
        Important for animal cells with no cell wall
        Important for RBC that arent supported by other cells
    3. Proteins in membranes have many functions
      1. Include channel proteins, carrier proteins, and receptor proteins
4. Permeability test
  1. Changing temperature changes the membrane permeability
    1. Below 0
      1. Phospholipids pack close together and become rigid
        Channel and carrier proteins deform, increasing permeablility
        Any ice that forms can pierce the membrane causing very high permeablility
    2. 0-45 celsius
      1. Phospholipids move around and the higher the temp the more permeable it is
    3. Above 45
      1. Bilayer starts to melt
        Water expands, puts pressure on membrane
        Proteins deform increasing permeability of cell membrane
Simple Diffusion
1. Net movement of particles, molecules, or ions from an area of high to low conc down its conc gradient
  1. Can diffuse across membranes as long as they can move freely through the membrane
    1. When diffusion directly across a membrane its called simple diffusion
      1. Higher conc gradient the faster the rate of diffusion
        Microvilli increase surface area for faster diffusion
2. Passive process - no energy used
Osmosis
1. Net Diffusion of water molecules across a partially permeable membrane
  1. From an area of high water potential to low water potential
    1. Water potential (WP) is likelihood of water molecules to diffuse in or out of a solution
      1. Equal WP of solution said to be isotonic
        Rate depends on factors
        WP gradient, Higher gradient, faster conc
        As solutions undergo osmosis, difference in WP decreases
        Rate of osmosis levels off over time
        Thickness of exchanged surface
        Thinner the osmosis path the faster rate of osmosis
        Surface area of exchange surface
        Larger S.A. the higher rate of osmosis
      2. Investigating WP Using serial Dilutions
        Make five serial dilutions of sucrose solution by a known factor
        Add identical potato cylinders in solutions
        After 20 mins, dry chips and weight each chip
        Calculate the % change in mass
      3. Net movement of water from a less negative WP to a more negative WP
Active Transport (AT)
1. Active process - uses energy in form of ATP
2. Usually movement of molecules and ions against a conc gradient
  1. Carrier proteins used to move molecules from low to high conc
    1. Molecule attaches to protein, causing it to change shape, moving molecule across membrane
      1. Co-transporters are a type of carrier protein used in AT
        Bind two molecules at a time
        High to Low conc gradient used for one of the molecules used to move the other one against its conc gradient
Facilitated Diffusion
1. Passive
2. Large molecules, ions, and polar molecules diffuse slowly across membranes
  1. Number of Channel and carrier proteins increase rate of diffusion of these molecules/ions
    1. Higher conc gradient the faster the rate of diffusion

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	Criado por Dominic Weston quase 8 anos atrás