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Transport of Substances through Cell Membrane

Transport of Substances through Cell Membrane

 

 

 Type of moleculeExamplesThrough which part of the membraneActive or passiveWith or against the concentration gradient
DiffusionSmall, uncharged moleculesCarbon dioxide and oxygenPhospholipid bilayerPassiveWith (high to low)
Facilitated DiffusionHydrophilic moleculesglucoseThrough channel proteins/via carrier proteinspassiveWith (high to low)
OsmosisWater molecules·Phospholipid bilayerpassiveWith (high to low)
Active TransportSubstancesIons into root hairs, muscles and nerve cellsThrough carrier proteins that change shapeactiveAgainst (low to high)
ExocytosisVery large molecules/particlesProteins (insulin/

polysaccharides)

Not through membraneactiveeither
EndocytosisVery large molecules/particlescholesterolNot through membraneactiveeither

 

Diffusion:

  • Net movement of molecules/ions from an area of high concentration to an area of lower concentration
  • Continues until equilibrium
  • Small uncharged molecules
  • Pass between the lipid molecules
  • No energy needed

 

Facilitated Diffusion:

  • Hydrophilic molecules
  • Insoluble in lipids
  • Cross the membrane by either diffusing through water-filled pores within channel proteins that than span the membrane
  • Different channel proteins for transporting different molecules (each has a specific shape for a specific ion/molecule)
  • Some may be opened or closed depending on a signal (hormone or change in voltage across the membrane) these are GATED CHANNELS
  • No energy needed

 

  • Carrier proteins; ion/molecule binds onto a specific site on the protein which then changes shape
  • Movement can occur in either direction; net movement dependent on the concentration across the membrane
  • Molecules move from high to low
  • No energy needed

 

Osmosis:

  • Net movement of water molecules from a solution with lower concentration to a solution with a higher concentration of solute.
  • Partially permeable membrane

 

Active Transport:

  • Transports substances against a concentration gradient (low to high)
  • Energy required; comes from respiration and the energy transfer molecule ATP
  • Specific carrier proteins are also needed; substance binds to carrier proteins
  • Energy from ATP changes the shape of the carrier protein, causing the substance to be released on the other side of the membrane
  • Occurs in every cell
  • Transports ions across epithelial cells

 

Exocytosis:

  • Very large molecules or particles transported across cell surface membranes
  • Relies on fluid nature of the membrane
  • Release of substances from the cell as vesicles fuse with the cell membrane (usually proteins or polysaccharides)

 

Endocytosis:

  • Substances taken into the cell by the formation of vesicles
  • Part of cell surface membrane engulfs the material to be transported
  • Substance can also be absorbed by attaching to a receptor in the membrane then endocytosis