AS biology

fluid mosiac model, transport across cell membrane

  • Created by: grainne
  • Created on: 31-05-10 16:33

Fluid-Mosaic Model

Fluid:protein molecules able to move laterally

Mosaic: patchwork effect of phospholipid bilayer studded with proteins, polysaccharides, lipid.

The phospholipid bilayer prevents the movement of any water solouble molecule into or out of cell, but allows fat solouble substances to diffuse across.


  • hydrophillic phosphate heads outermost
  • hydrophobic hydrocarbon chains innermost
  • intrinsic protein
  • extrinsic protein:confined to one side of membrane ie external
  • carbohydrates-attatch to protein to form glycoprotein
  • cholesterol: membrane too fluis witout cholessterol at high temperature-lible to burst.
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Function of Protein in Plasma Membrane

  • Carriers for water soluble molecules
  • channels for ions
  • pumps, which use energy to move water soluble molecules and ions
  • receptors which enable hormones an nerve transmitters to bind specific cells
  • recognition sites which help identify a cell
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Cell Physiology

Diffusion: is the movement of particles from an area of higher concentraton to an area of lower concentration, particles diffuse down concentration gradient.

  • passice process- no energy required, but at higher temp, more K.E
  • occurs until equilibrium is achieved
  • depends on several factors:conc gradient, length of diffusion pathway, surface area

Facillitated Diffusion:

  • takes place via protein carriers for larger molecules eg amino acids, glucose.
  • these proteins are specific- allowing only one type of molecule to pass through
  • Protein channels for polar molecules diffuse through hydrophillic channels
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Active transport

involves the expenditure/usage of ATP

achieved by special transport protien carriers

each specific for particular molecule

movement can take place against concentration gradient

ATP used to make the carrier protein change Three DImensional Shape transferring molecule across membrane in the process

cells which are involved n active transport have abundant mitiochondria

factors which limit respiration will accelerate active transport eg. lack of oxygen

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  • engulfing of material bu the cell membrane to form a small vacule
  • eg:phagocytosis, solid material taken up, WBC. and Pinocytosis:liquid or molecule suspension taken up


  • secretory vesicles carry the material to the cell membrane
  • Vesicles coalease with the cell membrane
  • allowing the contents to pass out of the cell eg, digestive enzymes/hormones.
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Osmosis is the diffusion of water molecules across a partially permeable membrane from an area of high water potential (higher concentration of water molecules) to an area of lower water potential (lower concentration of water molecules.)

water potential: Potential of Water molecules to diffuse out of solution. (high w.p -high concentration of water)

Pure Water has the highest Water potential. all substrates have a lower water potential than pure water.

  • HYPOTONIC: lower solute concentration than cell concentration (higher water potential) thus water moves into cell.
  • ISOTONIC: solute concetation same as cell concentarion
  • HYPERTONIC: solute concentation will be higher than water potential, water moves out of cell.
  • SOLUTE CONCENTRATION OPPOSITE OF WATER CONCENTRATION eg:solute concentration -100, water concentration +100
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Osmosis in Plant and Animal cells

Animal Cells

  • in hypertonic solution water moves out of cell
  • cell shrivels
  • crenation, cell crinkles
  • in hypotonic:
  • cell swells>cell ruptures
  • Bursting of RBC is called heamolysis

In Plant Cells

  • in hypertonic
  • vacuole swells, membrane pulls away, plasmolysis (Flaccid)
  • In hypotonic, TURGID- provides sturcture and shape for plants.
  • putsing force of cell wall
  • protoplast has expanded
  • cell expands -cell size has not expanded due to cell walls. cell wall provides the inward pressure (Yp)
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Calculating the osmotic reationship

Yp + Ys=Ycell

  • Yp= inward pressure, usually by cell wall, usually value positve. hyfrostaticpressure is the pressure liquids exert in a enclosed space. usually calculated in Pa
  • kPa>Pa= x103
  • mPa>pa=x106

Ys=water potential of the solution in the vacuole.( abilty of cell to take water) usually value is negative

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