Cell Membranes & Transport



  • important components of cell surface membranes
  • form bilayers 
  • hydrophilic heads point out of bilayer, hydrophobic tails point inwards
  • basis of membrane structure
  • lipid solubles to freely enter and leave the cell, water solubles cannot
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  • arranged randomly


  • surface of bilayer / partially embedded
  • provide structural support
  • recognition sites for identification


  • extend both layers
  • carriers transport water solubles 
  • channels allow active transport of ions
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Fluid Mosaic

  • individual phospholipid molecues move relative to one another
  • proteins embedded in bilayer vary in shape size and pattern
  • cholesterol fits between phospholipid molecules increasing stability
  • glycolipids found on outer layer of membrane are involved in cell to cell recognition
  • glycoproteins also stick out of membranes
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Membrane As A Barrier

  • lipid solubles move through the phospholipids
  • water solubles pass through water filled channels
  • membrane is selectively permeable to water and some solutes
  • lipid solubles move through the cell easier
  • uncharged molecules pass freely through the membranes 
  • hydrophobic core stops ions and polar molecules
  • charged and large molecules are assisted into the cell by intrinsic proteins
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  • passive transport
  •  the movement of molecules or ions from a region of high concentration to a region of lower concentration
  • net movement until equilibrium 
  • affected by
    • concentration gradient- greater the difference greater the rate
    • distance- shorter the distance greater the rate
    • surface area of membrane- larger the area quicker the rate
    • thickness of exchange surface- thinner the membrane greater the rate
    • temperature- increase in temperature, increase in kinetic energy, increase in rate

diffusion = surface area X difference in concentration

                length of diffusion path

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Facilitated Diffusion

  • charged particles or ions and large molecules cannot pass through cell membrane as insoluble in lipid
  • facilitated diffusion is a special form of diffusion for faster movement of these molecules 
  • passive and occurs down a concentration gradient
  • occurs at specific points where there are protein molecules

channel proteins

  • pores lined with polar groups allow charged particles and water solubles to pass
  • hydrophilic
  • each channel is specific to one type of ion
  • open and close to fulfill need of cell

carrier proteins 

  • diffusion of larger polar molecules such as sugars/ amino acids
  • molecule attaches to carrier at binding site 
  • carrier protein changes shape to accomidate and pushes molecule through membrane
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Active Transport

  • energy requiring process for ions and molecules to move against the concentration gradient
  • ions and molecules move in opposite direction of diffusion
  • energy provided by ATP, therefore anything that effects respiration effects active transport
  • process uses carrier proteins
  • e.g protein synthesis, muscle contraction, nerve impulse transmission
    • molecule/ion combines with specific carrier protein
    • ATP transfers phosphate group to carrier protein
    • carrier protein changes shape, carrying molecule/ion inside membrane
    • molecule/ion released, carrier protein returns to original shape
  • active transport cannot take place in the presence of a respiratory inhibitor such as cyanide
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Osmosis & Osmosis & Animal Cells

  • special case of diffusion moving water only
  • the passage of water from a region of high water potential to a region of lower water potential through a partially permeable membrane 
  • water potential is the pressure created by water molecules, units kPa
  • pure water's water potential= 0kPa
  • adding a solute lowers the water potential, giving it a negative value
  • hgh concentration of water molecules= greater potential energy
  • more concentrated solution= more negative WP

osmosis & animal cells

  • animal cells have no cell walls 
  • when placed in different solutions they are effected differently to plant cells:
    • hypotonic (dilute)- cell lysises/bursts
    • isotonic (same)- no net movement
    • hypertonic (concentrated)- cell shrinks
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Osmosis & Plant Cells

  • water enters a plant causing the vacuole to expand pushing the cytoplasm against the wall
  • wall can only expand so far, pressure builds up to resist entry of more water, cell is turgid
    • hypertonic=if WP of external solution is lower than solution inside cell water moves out
    • hypotonic=WP of external solution is higher than internal, water flows in
    • isotonic= WP are equal, no net movement
  • if cell is placed in hypertonic solution it loses water, causing plasmolysis=vacuole shrinks, cytoplasm draws away from cell wall, cell becomes flaccid
  • when the membrane is just moving away from cell wall it is at incipient plasmolysis
  • if cell is placed in hypotonic solution it gains water, causing turgor= takes in water until prevented by opposing wall pressure, cell becomes turgid
  • turgor is important for keeping young seedlings in shape and form

water potential= solute potential + pressure potential

  • hydrostatic pressure pushes out on cell wall met by pressure potential of cell wall
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Bulk Transport

  • moving bigger materials or materials in bulk use different processes


  • engulfing of material by plasma membrane to bring it inside the cell in a vesicle
    • phagocytosis
      • obtaining solid materials by having a lysosome fuse with the vesicle formed
      • digestive enzymes digest the solid material for products to be absorbed into cytoplasm
      • white blood cells do this
    • pinocytosis
      • entry of liquid via same way as a solid, producing smaller vesicles


  • substances leaving the cell after being transported through the cytoplasm in a vesicle
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