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Cell Membrane
The cell membrane regulates what enters and leaves the cell and
also provides protection and support.
 If a substance can diffuse across a membrane the
membrane is said to be permeable to that substance.
 If a substance cannot pass across a membrane the
membrane is said to be impermeable to that substance.
 Most membranes are selectively permeable – they allow
some substances to cross, but not others.
Concentration – the mass of solute in a given volume of solution.
 The cytoplasm of a cell is at a certain concentration.
 The fluid surrounding the cell is at another concentration.
Diffusion – movement of particles from an area of greater
concentration to an area of lesser concentration.
 When the particles in the two areas have moved to where
both areas are at the same concentration, the environment
is said to be at equilibrium.
 Depends on random particle movements.
 Done without requiring energy.
Osmosis – the diffusion of water through a selectively permeable
membrane.
 Isotonic solutions – occurs when two solutions have
reached equilibrium or a solution with the same
concentration as water.
 Hypertonic solution – a solution with a higher
concentration of solute than water.
 Hypotonic solution – a solution with a lower
concentration than water.
These solutions drive osmosis because osmosis does not require
energy.
Osmotic Pressure – the pressure exerted on a cell membrane due
to a concentration gradient (different concentrations on either side
of the cell membrane.)
Facilitated Diffusion – larger or highly charged molecules that
would normally not pass through the lipid bilayer on their own
move through protein channels and gain access to the inside of the
cell.
 The protein channels are very specific to the molecule
they allow through.
Active Transport – the movement of molecules against a
concentration gradient.
 Requires energy.
 Small molecules and ions are carried across by protein
“pumps.” Example: Na+-K+ pump.
 Larger molecules and solid clumps of material are
transported across the membrane by movements of the cell
membrane.
o Endocytosis – the process of taking material into the
cell by means of infoldings or pockets of the cell
membrane. The pocket then breaks loose from the
membrane and forms a vacuole within the cytoplasm.
o Phagocytosis – “cell eating”, extensions of cytoplasm
surround a particle and package it within a food
vacuole. Then the cell engulfs it.
o Pinocytosis – the process of taking liquid into the cell
by means of a tiny pocket formed along the cell
membrane, filled with liquid and pinched off to form
vacuoles within the cell.
o Exocytosis – the release of large amounts of material
from the cell by means of the membrane of a vacuole
containing the material fusing with the cell membrane
and forcing the contents out of the cell.