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Cell Membrane Structure &
Permeability
Plasma Membrane
– The plasma membrane that surrounds the cell is a fluid
mosaic of phospholipids and proteins that separates the
living cell from its nonliving surroundings. The membrane
is selectively permeable and functions to separate the
internal and external environments of the cell
• Phospholipid bilayer
– Hydrophilic on the external and
internal side
– Hydrophobic in the center on the
membrane
• Membrane proteins
– float freely with their hydrophobic
regions in the center of the
molecules and their hydrophilic
regions extending outwards
– some directed or held rigid by
internal cytoskeleton fibers but most
float freely
– The membrane proteins serve as cell
surface markers, some serve as
channel proteins, and some
communicate chemical messages to
the internal environment
Membrane
Structures
Cholesterol
• Cholesterol
– makes the membrane more rigid
– lowers the temperature at which the membrane
solidifies
Cell Transport Mechanisms
• Passive transport (Diffusion)
– Movement of a substance down a
concentration gradient (area where there is
more of a substance to an area where there is
less of a substance) until equilibrium is
reached
– speed depends on the concentration
difference, the size of the substance being
moved, and the thermal energy of the solution
– diffusion of a solute is ALWAYS toward the area
of least solute concentration
– diffusion of a solvent is ALWAYS toward the
area of highest solute concentration
– Diffusion occurs until equilibrium is reached
• Equal solute concentration on both sides
Cell Transport Mechanisms
• Facilitated Transport - Diffusion
aided by a channel protein or a
carrier protein
– necessary for the transport of polar
molecules through the hydrophobic
interior of the plasma membrane
– still follows the rules for passive
transport and osmosis
– most carriers and channels are ion
or molecule specific
– channels greatly speed up the entry
of water and small solutes into the
cell
– ion channels may be gated channels
requiring a stimulus to open
Osmosis
• Osmosis - diffusion of water across a differentially permeable (semipermeable or selectively permeable) membrane
– aided by the presence of aquaporins (channel proteins)
– water diffuses to an area of lower solute concentration to an area of higher
solute concentration
– the side of the membrane with higher solute pressure is said to have greater
osmotic pressure (negative - sucking)
– the side with the lower pressure is said to have greater hydrostatic pressure
(positive - pushing)
– osmoregulation - management of water through the manipulation of tonicity
(solute concentration)
• iostonic - equal solute in 2 adjoining compartments (osmotic & hydrostatic press = 0)
• hypertonic - having a greater solute concentration than the adjoining compartment
(osmotic press > 0)
– a cell in a hypertonic environment loses water and shrivels, turger (firmness) decreases
• hypotonic - having a lower solute concentration than the adjoining compartment
(hydrostatic press > 0)
– a cell in a hypotonic environment gains water and expands, turger (firmness) increases &
plasmolysis (splitting of the plasma membrane) is possible
• Active Transport - diffusion
against a concentration gradient
requiring energy
– movement is toward the area of
lower solute concentration
– requires a carrier protein
– requires the use of ATP (adenosine
triphosphate) for energy
– used to maintain the ion-potential
of the cell membrane
• sodium-potassium pump
– creates an electrochemical gradient
(cytoplasm is negative compared to the
extracellular environment)
Active
Transport
• Exocytosis vs Endocytosis
– Exocytosis - a membrane bound
compartment (from golgi,
lysosome, …) that fuses with the
plasma membrane releasing the
contents the extracellular
environment
– Endocytosis - the infolding of
the plasma membrane around a
molecule releasing a vesicle in
the intracellular environment. 3
types:
• Phagocytosis - cellular eating
• Pinocytosis - Cellular drinking
Bulk Transport