WHAT ARE MEMBRANES?
selectively permeable barriers
maintains constant internal environment
enclose cell contents
internal organelles often have different environments to the cytosol
very thin
FLUID MOSAIC MODEL
fluid: = the different components are free to move around
mosaic = the proteins are embedded into the membrane
composed of:
lipid bilayer= 2 layers; head on the exterior; tails on the interior
phospholipids
carbohydrates
cholesterol
PHOSPHOLIPIDS
ampiphilic - both hydrophilic and hydrophobic regions
polar head - -vely charged, phosphate group
non-polar tail - no charge
phospholipids self assemble to create the bilayer
tails can be either saturated or unsaturated:
SATURATED
single bonds between the carbon atoms on the hydrocarbon chain; many saturated fatty acids means neatly packed together so fewer gaps
UNSATURATED
a double bond between the carbon atoms on the hydrocarbon chain; forms a kink due to the presence of the double bond; many unsaturated fatty acids means larger gaps in between
higher conc. of unsaturated acids promote
membrane FLUIDITY
higher conc. of saturated fatty acids REDUCE
membrane FLUIDITY
CARBOHYDRATES
2 main types:
glycolipids
glycoproteins
maintains membrane stability and cell protection
aid cell-cell adhesion (cell adhesion molecules)
facilitate cell recognition
CHOLESTEROL
a lipid (20% of the membrane composed of cholesterol)
has polar and non-polar regions
wedges itself between phospholipid tails
can migrate and flip between membrane layers
PROTEINS
serve different functions: enzymes, carrier proteins, channel proteins, receptors, recognition. cell adhesion
two main types:
integral
peripheral
INTEGRAL PROTEINS
large protein inserted all the way through the lipid bilayer
Firmly inserted into the membrane
Span the bilayer of the membrane
Transmembrane portion
hydrophobic
Extracellular and cytosolic portions are hydrophilic
Carrier proteins and channels are integral proteins
PERIPHERAL PROTEINS
proteins found on the outer or inner surface, attached to other components of the cell membrane
loosely attached to membrane
may be removed easily form the membrane with minimal disruption
Membrane Fluidity Affected by...
Extreme temperatures (hot or cold)
Concentration of unsaturated fatty acids
Cholesterol levels
MEMBRANE FLUIDITY
Low temp
Less kinetic energy
Phospholipids pack together
Membrane less fluid
High temp
More kinetic energy
Phospholipids tend to move further away
Membrane more fluid
Cholesterol
Maintains fluidity by preventing lipids from getting too close or too far apart from each other
are the properties of the individual components the same in membranes of different cell types?
Red Blood cell – equal amounts of lipid and protein and a small amount of carbohydrate
Nerve cells – higher amounts of lipid in cell membranes (80% lipid)
transport of small molecules across the membrane:
passive:
down a conc. (electrochemical) gradient
3 TYPES OF TRANSPORT ACROSS A MEMBRANE
Simple diffusion
Facilitated diffusion
Osmosis Uses inherent kinetic energy
ACTIVE TRANSPORT
Moves against a concentration (electrochemical) gradient
Has directionality
Requires a specific carrier protein
5 types:
Primary
Secondary
Symport
Antiport
Needs an external energy source
SIMPLE DIFFUSION
Small, uncharged, non polar molecules
O2, CO2, NO, urea
No metabolic energy required
Uses kinetic energy (natural motion)
No specificity
Rate of diffusion proportional to concentration gradient
FACILITATED DIFFUSION
Small, polar molecules
H2O, Glucose, Na+
Uses specific integral proteins (carrier proteins)
Rate of diffusion proportional to concentration gradient BUT also on one other factor...
Simple and facilitated diffusion kinetics
The rate of diffusion reaches a plateau (transport maximum; Tm) in facilitated diffusion as carrier proteins become fully occupied.
Osmosis
Net movement of water down a concentration gradient
Water highly polar, but small amounts can ‘sneak’ through the membrane unaided
Water moves freely and reversibly through specific protein channels called AQUAPORINS
PRIMARY ACTIVE TRANSPORT
movement of sodium and potassium ions uses the Na+/K+ ATPase pump
maintains ion conc. differences inside and outside the cells
hydrolysis of ATP results in the phosphyrylation of the pump
3Na+'s out and 2K+'s into the cell (against the gradient)
SECONDARY ACTIVE TRANSPORT (CO-TRANSPORT)
movemnt of a substance (against the conc. gradient) is coupled to ion movement (down the conc. gradient) i.e. the Na+/glucose movement chain
the energy used for the secondary active transport comes from the electrochemical gradient across the membrane
SYMPORT:
transported substances move in the same direction
Na+/glucose transporter
ANTIPORT
transported in the opposite direction
vesicular transport of large molecules
ENDOCYTOSIS
moves large molecules into the cells (ingestion/uptake)
Three types:
Phagocytosis
Pinocytosis
Receptor-mediated
EXOCYTOSIS:
Moves large molecules out of the cell (excretion / secretion)
Used for:
Hormone secretion
Neurotransmitter release
Mucous secretion
VESICLES
Bubble-like, membranous sacs
Made of a phospholipid bilayer containing fluid
Transport bubble
Protects substances being transported
ENDOCYTOSIS:
cell eating’
Ingestion of large particles by specialised cells (phagocytes; in the immune system):
Macrophages
Certain white blood cells
Used for removing bacteria and debris
When a particle (e.g. bacteria) binds to a phagocyte, the cell membrane wraps itself around the particle
Resulting in a phagosome containing the particle
PINOCYTOSIS
cell drinking
process used to take in extracellular fluid with dissolved substances
droplet enters the cell and fuses with an endosome (sorting vesicle)
ENDOCYTOSIS - receptor mediated
main mechanism for specific uptake of macromolecules
very selective method
Uptakes substances (hormones, cholesterol) found in small amounts
Receptors for this are specific membrane proteins
Some viruses can hijack this route and enter our cells
EXOCYTOSIS
Substances to be moved out of the cell
Hormones
Waste products
Neurotransmitters
Process is stimulated by a cell-surface signal that results in a cascade of events:
Substance is enclosed in a vesicle
Transported to plasma membrane
Vesicle attaches to ‘docking’ sites on the membrane
Vesicle
Transported to plasma membrane
Fuses with membrane and ejects substance
a balance in cell membrane content
while exocytosis adds membrane material to the cell membrane, endocytosis removes it from the cell membrane
how do oral rehydration therapies work?
mixture of sodium chloride and glucose
exploits the physiology of ion and water transport to treat dehydration (from diarrhoea, extreme sports etc)
normal conditions:
glucose is transported across the enterocyte (intestinal cell), whilst the Na/K ATPase maintains a concentration gradient
water is also reabsorbed from the intestine following the water potential created by movement of Na and glucose.