CELL MEMBRANE: structure and function
The structure of cell membranes is fluid mosaic of lipids and
proteins. (held together by hydrophobic interactions)
Most lipids are phospholipids which are amphipathic: both
hydrophobic and hydrophilic.
Therefore forms phospholipid bilayer in water.
Most membrane proteins also amphipathic and stay in
phospholipid bilayer with hydrophilic regions protruding to allow
more contact with water in the cytosol and extracellular fluid as
well as protecting hydrophobic part from aqueous environment.
Lipids move (membranes fluid) by lateral movement SIDEWAYS–
every 10^7 seconds. (also flip-flop but this occurs once per month)
What affects membrane fluidity?
Unsaturated fatty acids have kinks in the tails due
to double bonds and so are more loosely packed and
therefore more fluid.
Saturated fatty acids are linear and packed more
tightly and are less fluid.
Therefore if temperature decreases in a cell
membrane, membrane becomes more solid so in
order to make more fluid, amount of unsaturated fatty
acids increase.
, Temp increases: membrane more fluid = make it less fluid = increase saturated fatty acids
Temp decreases: membrane more solid = make more fluid = increase unsaturated fatty acids.
temp at which fluid becomes solid varies based on lipid type.
Cholesterol (steroid): fluidity buffer. Resists changes to membrane fluidity. makes membrane
less fluid by restricting movement of phospholipids but also stops tight packing of phospholipids
so lowers temp needed to solidify.
Membrane proteins: embedded in lipid bilayer = functions (integral proteins (in bilayer))
FUNCTIONS:
-Transport
Protein spans membrane and creates hydrophilic channel across membrane that is selective for a
particular solute. Other transport proteins pump substances across membrane and change their
shape – hydrolyze ATP for energy.
-Enzymatic activity
Protein is an enzyme where active site exposed to substances. (metabolic pathway)
-Signal transduction
Membrane protein has receptor (binding site in shape of receiving chemical message/hormone)
Proteins relays message to inside of cell by binding to a cytoplasmic protein (its shape changed
by the chemical messenger.)
-Cell – cell regulation
Glycogen proteins can be recognized by other membrane proteins in other cells.
Immune system rejects foreign cells (bacteria) – no receptor proteins
-Intracellular joining
Membranes of DIFFERENT adjacent cells hook together
-Attachment to cytoskeleton and extracellular matrix.
stronger framework for animal cell
MEMBRANES: selectively permeable (emergent property)
How? Lipid bilayer and transport proteins.
Hydrophobic molecules – NONPOLAR: pass through lipid bilayer quickly
Eg) oxygen, carbon dioxide, hydrocarbons
Hydrophilic molecules – POLAR + IONS: do not cross membrane easily.
E.g) sugars(carbs), water, Sodium, potassium, calcium, chlorine (Na, K, Ca, Cl)
Transport Proteins:
Hydrophilic substances avoid contact with lipid bilayer by transporting through transport
proteins.
TYPES of transport proteins:
Channel proteins: hydrophobic channel for certain molecules/ions to use as a tunnel
Example: Aquaporins – facilitate passage of water.
Carrier proteins: bind to molecules and change shape to move across membrane.
The structure of cell membranes is fluid mosaic of lipids and
proteins. (held together by hydrophobic interactions)
Most lipids are phospholipids which are amphipathic: both
hydrophobic and hydrophilic.
Therefore forms phospholipid bilayer in water.
Most membrane proteins also amphipathic and stay in
phospholipid bilayer with hydrophilic regions protruding to allow
more contact with water in the cytosol and extracellular fluid as
well as protecting hydrophobic part from aqueous environment.
Lipids move (membranes fluid) by lateral movement SIDEWAYS–
every 10^7 seconds. (also flip-flop but this occurs once per month)
What affects membrane fluidity?
Unsaturated fatty acids have kinks in the tails due
to double bonds and so are more loosely packed and
therefore more fluid.
Saturated fatty acids are linear and packed more
tightly and are less fluid.
Therefore if temperature decreases in a cell
membrane, membrane becomes more solid so in
order to make more fluid, amount of unsaturated fatty
acids increase.
, Temp increases: membrane more fluid = make it less fluid = increase saturated fatty acids
Temp decreases: membrane more solid = make more fluid = increase unsaturated fatty acids.
temp at which fluid becomes solid varies based on lipid type.
Cholesterol (steroid): fluidity buffer. Resists changes to membrane fluidity. makes membrane
less fluid by restricting movement of phospholipids but also stops tight packing of phospholipids
so lowers temp needed to solidify.
Membrane proteins: embedded in lipid bilayer = functions (integral proteins (in bilayer))
FUNCTIONS:
-Transport
Protein spans membrane and creates hydrophilic channel across membrane that is selective for a
particular solute. Other transport proteins pump substances across membrane and change their
shape – hydrolyze ATP for energy.
-Enzymatic activity
Protein is an enzyme where active site exposed to substances. (metabolic pathway)
-Signal transduction
Membrane protein has receptor (binding site in shape of receiving chemical message/hormone)
Proteins relays message to inside of cell by binding to a cytoplasmic protein (its shape changed
by the chemical messenger.)
-Cell – cell regulation
Glycogen proteins can be recognized by other membrane proteins in other cells.
Immune system rejects foreign cells (bacteria) – no receptor proteins
-Intracellular joining
Membranes of DIFFERENT adjacent cells hook together
-Attachment to cytoskeleton and extracellular matrix.
stronger framework for animal cell
MEMBRANES: selectively permeable (emergent property)
How? Lipid bilayer and transport proteins.
Hydrophobic molecules – NONPOLAR: pass through lipid bilayer quickly
Eg) oxygen, carbon dioxide, hydrocarbons
Hydrophilic molecules – POLAR + IONS: do not cross membrane easily.
E.g) sugars(carbs), water, Sodium, potassium, calcium, chlorine (Na, K, Ca, Cl)
Transport Proteins:
Hydrophilic substances avoid contact with lipid bilayer by transporting through transport
proteins.
TYPES of transport proteins:
Channel proteins: hydrophobic channel for certain molecules/ions to use as a tunnel
Example: Aquaporins – facilitate passage of water.
Carrier proteins: bind to molecules and change shape to move across membrane.
