Movement of cells across cell surface membrane

  • Created by: cathymm
  • Created on: 29-10-18 13:40
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  • Movement of cells across cell surface membrane
    • Simple diffusion
      • Water soluble substances unable to pass by simple diffusion because of hydrophobic centre of phospholipid bilayer
      • Diffusion is net movement of water from a region of high concentration to a region of low concentration- against the concentration gradient
      • Diffusion across membrane in biological systems
        • Factors affecting diffusion across membrane:
          • concentration gradient- greater concentration gradient, faster rate of diffusion
          • size of molecule- smaller molecules diffuse faster than large molecules
          • Temperature- diffusion takes place faster at higher temperatures as molecules are given more kinetic energy
          • Thickness of exchange surface- thinner surfaces give more rapid diffusion
          • Surface area of membrane- greater the surface area- faster the diffusion. Microvilli can increase surface area that diffusion takes place
      • passive process- doesn't require use of metabolic energy
    • Facilitated diffusion
      • Supported by proteins
        • carrier proteins-
          • Binding sites for specific molecules
          • take in diffusing molecule, change shape and release molecule on other side of membrane- move molecules across membrane.
        • Ion (channel) proteins
          • channels with a central pore formed by proteins for ions to pass through
            • channels permanently open or gated. Gated ions can open or close controlling ion movement
      • affected by number of carrier or channel proteins in the membrane
      • transport molecules against the concentration gradient- from high to low concentration
      • passive process- doesn't use metabolic energy
    • Active Transport
      • Molecules move from low to high concentration- against concentration gradient
        • metabolic energy required in form of ATP= non passive process
          • cells carrying out a lot of active transport have lots of mitochondria which supply ATP energy required
      • uses protein carriers (pumps)
      • substance to be transported binds against protein carrier. Carrier changes shape releasing substance on other side of membrane
      • Carrier specific to a type of molecule or ion
    • Cytosis
      • Substances transported into and out of cell without passing through membrane
      • Transports large molecules too big for carriers
      • Transports small molecules e.g. water
      • Endocytosis movement of substances into the cell
        • cell surface membrane invaginates around substance to be digested forming a vesicle or membrane bound sac. When vesicles taken into cell, fluid nature of cell surface membrane allows it to reform and fill the gap from cytosis
        • Phagocytosis= transport of solid material into the cell e,g engulfing bacteria by phagocytes
        • Pinocytosis= transport of fluid into the cell
      • Exocytosis- movement of substances out of the cell
        • Secretory vesicles (e.g. from Golgi body) fuse with cell surface membrane, releasing contents outside the cell. Creation of gap reformed in cell surface membrane after exocytosis
        • secretes proteins from cells e.g. digestive enzymes and hormones
    • Osmosis
      • net movement of water from a high water potential to a low water potential across a selectively permeable membrane (high to low water concentration- dilute to more concentrated solution).
        • called net movement a water molecules can move in both directions
      • Water moves through selectively permeable phospholipid membrane by osmosis
      • Water moves through channel proteins called aquaporins
      • Stronger solution (less water) is hypertonic solution to the weaker solution (more water)= hypotonic solution
      • If two solutions are of equal concentration called isotonic solutions
      • Osmosis is movement of water molecules from a less negative water potential to a more negative water potential across a selectively permeable membrane
      • Osmosis in plant cells
        • As plant takes in water by osmosis, its contents become less concentrated so solute potential rises. No pressure potential to restrict intake of water so, solute potential= water potential of cell (membrane and wall not in contact)
        • As pressure potential becomes positive, hinders water entering cellso water potential of cell and solute potential diverges
        • Solute potential still negative at full turgor as there are solutes present in cell.
          • Water potential of water is zero as no further water can enter.
          • At full turgor, maximum water potential between cell membrane and cell wall
        • As cell begins to gain water by osmosis, becoming turgid, the membrane exerts a fore on the cell wall- incipient plasmolysis.
          • At point where cell membrane begins to lose contact with cell wall =incipient plsmolysis
        • Plants e.g. herbaceous (non-woody)need turgor for support
          • Plant cell wall's strength limits expansion of cell membrane as water enters plant cell by osmosis.
            • Opposing forces of cell wall and cell membrane create the turgor support
        • Plant tissues with a shortage of water are no longer turgid and are flaccid
          • If a large number of cells are flaccid, wilting occurs.
        • If a plant cell loses too much water by osmosis, the cell membrane (protoplast) can pull way from the cell wall (except at points where adjacent points are joined by plasmodesmata)- called plasmolysis and cell is plasmolysed
          • Protoplast= cytoplasm (and vacuole and surrounding cell membrane
          • A plasmolysed cell is unlikely to survive.
            • When plasmolysis occurs
              • Plants growing in a field with too muhch fertiliser
                • A seed from a woodland tree carried to a salt marsh and starting to germinate in its environment
      • Osmosis in animal cells
        • Animal cells don't have a cell wall , so nothing to stop cell membrane until it bursts (lysis)
          • Lysis occurs when animal cell is placed in hypotonic solution
        • if animal cells placed in hypertonic solution, lose water, shrink and shrivel up (crenation)
        • In healthy animals, blood and tissue fluid kept at correct water potential so lysis and crenation don't occur
    • Water potential
      • Tendency to take in pure water by osmosis from pure water across a selectively permeable membrane
        • Affected by space available within cell e.g. turgid cell has potential to take in water but no space
      • Measured in kilopascals (kPa)
        • Pure water has water potential of O kPa- unable to take in any more water by osmosis
          • Pure water has water potential of 0 kPa as all water molecules are free- not forming associations with other molecules
      • indication of free energy of water molecules
        • In solutions, not all water molecules are free as they are forming hydration shells around solutes.
          • Solutes reduce ability of water molecules to diffuse throughout the solution
          • Always have a negative water potential- always have some water in hydration shells.
          • More concentrated a solution is, more negative the water potential as less water molecules are free- in hydration shells.
            • More likely to take in water by osmosis
      • Solute potential
        • Potential of a solution to take in water
          • potential= solute concentration only
      • Pressure potential
        • effect of pressure on a solution
          • pressure influences cells ability to take in or lose water by osmosis
        • positive greater than or equal to 0kPa
      • written using Greek letter psi
      • Water potential of cell= solute potential + pressure potenial
  • Osmosis in plant cells
    • As plant takes in water by osmosis, its contents become less concentrated so solute potential rises. No pressure potential to restrict intake of water so, solute potential= water potential of cell (membrane and wall not in contact)
    • As pressure potential becomes positive, hinders water entering cellso water potential of cell and solute potential diverges
    • Solute potential still negative at full turgor as there are solutes present in cell.
      • Water potential of water is zero as no further water can enter.
      • At full turgor, maximum water potential between cell membrane and cell wall
    • As cell begins to gain water by osmosis, becoming turgid, the membrane exerts a fore on the cell wall- incipient plasmolysis.
      • At point where cell membrane begins to lose contact with cell wall =incipient plsmolysis
    • Plants e.g. herbaceous (non-woody)need turgor for support
      • Plant cell wall's strength limits expansion of cell membrane as water enters plant cell by osmosis.
        • Opposing forces of cell wall and cell membrane create the turgor support
    • Plant tissues with a shortage of water are no longer turgid and are flaccid
      • If a large number of cells are flaccid, wilting occurs.
    • If a plant cell loses too much water by osmosis, the cell membrane (protoplast) can pull way from the cell wall (except at points where adjacent points are joined by plasmodesmata)- called plasmolysis and cell is plasmolysed
      • Protoplast= cytoplasm (and vacuole and surrounding cell membrane
      • A plasmolysed cell is unlikely to survive.
        • When plasmolysis occurs
          • Plants growing in a field with too muhch fertiliser
            • A seed from a woodland tree carried to a salt marsh and starting to germinate in its environment

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