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Cells & Molecules > Plasma Membrane
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Fluid-mosaic model

  • Plasma membrane consists of a phospholipid bilayer studded with proteins, polysaccharides, lipids
  • The lipid bilayer is semipermeable
    • Regulates passage of substances into and out of the cell
    • H2O and some small, uncharged, molecules (O2, CO2) can pass through
  • Phospholipids have two parts
    • "Head": hydrophilic → attracts and mixes with H2O
    • Two "fatty acid tails": hydrophobic

Function of proteins

  • Carrier (change shape for different molecules) for water-soluble molecules such as glucose
  • Channels for ions (sodium and chloride ions)
  • Pumps use energy to move water-soluble molecules and ions
  • Adhesion molecules for holding cells to extracellular matrix
  • Receptors enable hormones and nerve transmitters to bind to specific cells
  • Recognition sites, which identify a cell as being of a particular type
  • Enzymes, which speed up chemical reactions at the edge of the membrane
  • Adhesion sites, which help some cells to stick together
  • E.g. glycoprotein acts as a receptor and recognition site

Passive transport

  • Uses energy from moving particles (Kinetic Energy)


  • Substances move down their conc. gradient until the conc. are in equilibrium
  • Microvilli are extensions of the plasma membrane
    • They increase the surface area of the membrane, therefore
    • They accelerate the rate of diffusion
  • Fick's lawrate of diffusion across an exchange surfaces (e.g. membrane, epithelium) depends on
    • surface area across within diffusion occurs (larger)
    • thickness of surface (thinner)
    • difference in concentration gradient (larger)
    • Fick’s law = (surface area x difference in conc gradient) / thickness of surface
  • Temperature increases rate of diffusion due to increasing K.E. (kinetic energy)

Facilitate diffusion

  • Transmembrane proteins form a water-filled ion channel
    • Allows the passage of ions (Ca2+, Na+, Cl-) down their conc. gradient //passive - no ATP required
    • Some channels use a gate to regulate the flow of ions
    • Selective permeability - Not all molecules can pass through selective channels
  • How do molecules move across the membrane?
    • Substrate (molecule to move across the membrane) binds to carrier protein
    • Molecule changes shape
    • Release of the molecule (product) at the other side of the membrane
  • Example
    • If you want to move a muscle a nerve impulse is sent to this muscle
    • The nerve impulse triggers the release of a neurotransmitter
    • Binding of the neurotransmitter to specific transmembrane proteins
    • Opens channels that allow the passage of Na+ across the membrane
    • In this specific case, the result is muscle contraction
    • These Na+ channels can also be opened by a change in voltage


  • Special term used for the diffusion of water through a differentially permeable cell membrane
  • Water is polar and able to pass through the lipid bilayer
  • Transmembrane proteins that form hydrophilic channels accelerate osmosis, but water is still able to get through membrane without them
  • Osmosis generates pressure called osmotic pressure
    • Water moves down its concentration gradient
    • When pressure is equal on both sites net flow ceases (equilibrium)
    • The pressure is said to be hydrostatic (water-stopping)

Water potential

  • Measurement of the ability or tendency of water molecules to move
  • Water potential of distilled water is 0, other solutions have a negative water potential
  • Measured in kPa - pressure
  • Hypotonic
    • Solution is more dilute / has a lower conc. of solute / gains water by osmosis
    • Cells placed in a hypotonic solution will increase in size as water moves in
    • For example, red blood cells would swell and burst
    • Plant cells are unable to burst as they have a strong cellulose cell wall
  • Hypertonic
    • Solution with a higher conc. of solutes / loses water by osmosis
    • Cells will shrink in hypertonic solutions
  • Isotonic
    • Solutions being compared have equal conc. of solutes
    • Cells which are in an isotonic solution will not change their shape
    • The extracellular fluid of the body is isotonic
  • Molecules collide with membrane / creates pressure, water potential
  • More free water molecules, greater water potential, less negative
  • Solute molecules attract water molecules which form a "shell" around them
    • water molecules can no longer move freely
    • less "free water" which lowers water potential, more negative

Active Transport

  • Movement of solute against the conc. gradient, from low to high conc.
  • Involves materials which will not move directly through the bilayer
  • Molecules bind to specific carrier proteins / intrinsic proteins
  • Involves ATP by cells (mitochondria) / respiration
    • Direct Active Transport - transporters use hydrolysis to drive active transport
    • Indirect Active Transport - transporters use energy already stored in gradient of a directly-pumped ion
  • Bilayer protein transports a solute molecule by undergoing a change in shape (induced fit)
  • Occurs in ion uptake by a plant root; glucose uptake by gut cells

Endocytosis and Exocytosis

  • Substances are transported across plasma membrane in bulk via small vesicles
  • Endocytosis
    • Part of the plasma membrane sinks into the cell
    • Forms a vesicle with substances from outside
    • Seals back onto the plasma membrane again
    • Phagocytosis: endocytosis brings solid material into the cell
    • Pinocytosis: endocytosis brings fluid materials into the cell
  • Exocytosis
    • Vesicle is formed in the cytoplasm //May form from an edge of the Golgi apparatus
    • Moves towards plasma membrane and fuses with plasma membrane
    • Contents are pushed outside cell
    • Insulin is secreted from cells in this way