Download Biology 12 Membrane Notes File

Biology 12 – Membrane Notes
Fluid-Mosaic Model
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Cell membranes are sometime described as a fluid mosaic
! Why Mosaic?... the proteins and carbohydrate chains in/on the phospholipid bilayer form a
pattern (mosaic) within the bilayer
! Why Fluid?... the proteins and carbohydrate chains in/on the phospholipid bilayer can drift
around in the bilayer (fluid)
Selectively Permeable
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Cell membranes are selectively permeable (not just semi-permeable)
! What’s the difference?
o Semi-permeable = a membrane (e.g. dialysis tubing) that materials can pass through
simply based on size and concentration gradient
o Selectively permeable = a living membrane that can use energy to select molecules (even
if they are too big or the concentration gradient is going in the opposite direction)
Six Ways In/Out of Cells
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Molecules enter/leave cells based on size, shape of molecule, charge of the molecule,
concentration gradient, and cell “need”
" some molecules can pass right through the bilayer (①Diffusion and ②Osmosis)
" some molecules travel through protein channels/carriers (③Facilitated Transport and ④Active
Transport)
" some molecules enter/leave using vesicles or vacuoles (⑤Endocytosis and ⑥Exocytosis)
① Diffusion
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natural physical process where molecules move from an area of high concentration to an area
of low concentration
o passive process (doesn’t need extra energy)
o occurs within cells (cytoplasm), outside cells (extracellular fluid) and through the
bilayer
! *molecules must be small and neutral (eg. O2, CO2, alcohol)
o rate of diffusion affected by:
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temperature
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size of the molecule
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concentration gradient
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fluid viscosity
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fluid currents
② Osmosis
= the diffusion of water across a semi-permeable (or selectively permeable) membrane
o water moves from an area of high [water] " low [water] OR water will move to dilute
a solute
• Important Definitions
o Osmotic Pressure = the force that results from the osmotic flow of water
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can “pop” an animal cell or keep a plant cell turgid
o Tonicity = the amount of solute in a solution (in mol/L or %)
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Isotonic = equal solute concentration
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Hypertonic = greater solute concentration
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Animal cells (e.g. Red blood cells) placed in a hypertonic solution will
lose water and shrink/shrivel (“crenation” for RBCs)
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Plant cells in a hypertonic solution will wilt (“plasmolysis”)
Hypotonic = lower solute concentration
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Animal cells placed in a hypotonic solution will gain water and burst
(“lysis” or “hemolysis” in RBCs)
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Plant cells in a hypotonic solution will get tight (“deplasmolysis” or
“turgor”)