Membrane Bilayer Flashcards
What are the 5 general functions of biological membranes?
- Continuous, highly selective permeability barrier
- Control of the enclosed chemical environment
- Communication
- Recognition - signalling molecules
i. adhesion proteins,
ii. immune surveillance - Signal generation in response to stimuli
(electrical, chemical)
What is the dry weight membrane composition?
– 40 % lipid
– 60 % protein
– 1-10 % carbohydrate
Membranes are hydrated structures, so what percent of total weight is water?
20%
Why is water important in the membrane?
Water is important to maintain the phospholipid bilayer as it ensures that the hydrophilic head are on the outside and the hydrophobic tails are on the inside.
What would happen to the membranes if the content of water is reduced?
If the water content is reduced, it could lead to slight deformation of the bilayer. It could also lead to changes in protein structure as it would affect the interactions of the protein leading to change in protein function
What does amphipathic mean?
contain both a hydrophilic and a hydrophobic moiety
Describe the structure of a phospholipid
Phospholipids comprise a fatty acid tail coupled via glycerol to a head group that contains phosphate and attached alcohol
What are the dominant phospholipids?
- Phosphatidylcholine 2.phosphatidylserine 3.phosphatidylenthanolamine
- phosphatidylinositol 5.phosphatidylgylcerol
What could the head groups on phospholipids be?
– range of polar head groups
– e.g. choline, amines, amino acids, sugars
What is the general length of fatty acid chain and which are most prevalent?
– Length between C14 and C24
– C16 and C18 most prevalent
What is sphingomyelin?
A related phospholipid in which glycerol has been replaced by a sphingosine
And the phosphocholine moiety has been replaced with a sugar (glycolipid). The alcohol group in sphingomyelin is choline.
What are glycolipids?
Sugar with lipid
What is the difference between cerebrosides and ganglioside Glycolipids?
– Cerebrosides – head group sugar monomer
– Gangliosides – head group oligosaccharide
sugar multimers
What are the 4 phospholipid motions?
- Flexion
- Rotation
- Lateral diffusion
- flip flop
What is the effect of saturated and unsaturated phospholipid fatty acid chains on the membrane fluidity/
Phospholipids with Saturated fatty acid chains pack very close together so the fluidity of the membrane decreases.
Phospholipids with unsaturated fatty acid chains have cis double bonds which introduces kinks which reduces phospholipid packing and therefore increases the fluidity of the membrane
For phospholipids, with unsaturated fatty acid chains how does mobility vary with length?
The longer the chain, the less it moves because more energy is needed to move the heavier chain.
In unsaturated fatty acids, the Cis double bond introduces a kink which deforms the phospholipid structure so reduces packing ability so makes it more fluid so phospholipids move more.
Why is easier for phospholipids to move by lateral diffusion that flip flop?
It is easier for a phospholipid to move by lateral diffusion than to flip flop because more energy is needed to flip flop because more forces need to be broken than moving laterallly
What are the 3 factors that affect membrane fluidity.
- Temperature
- Cholesterol
- Fatty acid chain
How does temperature affect membrane fluidity?
When the temperature is too low, the phospholipids have less energy so move less and pack more closely together in a rigid structure making the membrane less fluid.
When the temperature is too high the phospholipids have lots of energy so move a lot and pack less closely together, making the membrane more fluid.
What is the general function of cholesterol?
To stabilise the membrane
How does cholesterol affect membrane fluidity at low temperatures?
At low Temperatures, the bilayer is more rigid and packed more uniformly, but the cholesterol reduces phospholipid packing by inserting itself in the bilayer and increasing the distance between the phospholipids and thus increases fluidity.
How does cholesterol affect membrane fluidity at high temperature?
At high temperatures, cholesterol increases mass so heat distributed more so less effect of temp on the phospholipids.
At high temperatures, bilayer is more fluid so cholesterol reduces the fluidity and increases the rigidity by forming hydrogen bonds with the phospholipid bilayer and pulling the phospholipids closer together. Thus the membrane molecules are packed more closely. And having the sterol ring which is more rigid also helps to lower the membrane fluidity. Therefore the effect of high temperatures on the bilayer is reduced due to the cholesterol.
Describe the structure of cholesterol
Polar head group
Rigid planar 4 steroid ring structure
Non-polar hydrocarbon tail
How are cholesterols inserted into the phospholipid bilayer?
Molecules of cholesterol are immobilized against adjacent phospholipids through the formation of a hydrogen bond between the hydroxyl group of cholesterol and the carboxyl group of the phospholipid.
Describe the aspects of cholesterol’s paradoxical effect in phospholipid bilayer
Cholesterol reduces phospholipid packing by disrupting the uniform phospholipid bilayer structure so increases fluidity but it also reduces phospholipid chain motion which reduces fluidity. Therefore cholesterol maintains the fluidity of the membrane
What effect does cholesterol have on the permeability of membranes to water soluble molecules?
Cholesterol binds to the carboxyl group on the head of the phospholipid so makes the bilayer more rigid so makes it less fluid so mobility decreases so makes the bilayer more uniform and less permeable
What are the three lipids present in the membranes?
- phospholipids
- glycolipids
- Cholesterol
What is the evidence that proteins are present in the membranes?
Functional : The fact that all these are present in membranes indicates the presence of proteins: – Facilitated diffusion – Ion gradients – Specificity of cell responses
Biochemical
– Membrane fractionation + gel electrophoresis
– Freeze fracture
Describe how the proteins in erythrocyte membrane can be identified
- Eyrythrocytes Lysed and spun in a centrifuge to get a white pellet called the erythrocyte ghosts.
- The erythrocyte ghosts are then denatured and run through a polyacrylomide gel to perform electrophoresis
- The membrane is first put in a detergent which coats all the proteins in a negative charge which is attracted towards the positive electrode.
- Small proteins move faster and further, whereas the larger molecules move slowly and travel less
Give 6 proteins that are present in erythrocyte membranes
Spectrin a Spectrin b Band 3 Glycophorin Band 4.1 Actin
Describe how freeze fracture allows us to see membrane proteins
- Freeze cell or membrane so that everything is locked in position.
- A knife is used to press down on the ice crystal until it fractures at the points of weakness - between the two bilayer.
- Therefore fracture off the outside and inside lamella.
- Then we can do low angle shadowing using electron dense metal like osmium onto topography on membrane
- If protein sticking out, there will be a build p of osmium
- Then the preparation can be looked at in an electron microscope
What are the three modes of motion of proteins in bilayers?
- Conformational change
- Rotational
- Lateral
Which mode of motion is not permitted in proteins and why?
Flip flop because it is not thermodynamically possible as a lot of energy is required for the protein to flip flop and could result in breaking the membrane.
What restricts membrane protein mobility?
- Aggregates - membrane protein associations
- Tethering - association with extra-membranous proteins
(peripheral proteins), - both intracellular/extracellular proteins e.g. cytoskeleton or extracellular matrix - Interaction with other cells
- lipid mediated effects - proteins tend to separate out into the fluid phase or cholesterol poor regions
Describe peripheral membrane proteins
– Bound to surface
– Electrostatic and hydrogen bond interactions
– Removed by changes in pH or in ionic strength
Describe integral proteins
– Interact extensively with hydrophobic domains of the lipid bilayer
– Cannot be removed by manipulation of pH and ionic strength
– Are removed by agents that compete for non-polar interactions
E.g detergents and organic solvent
Describe the secreted protein biosynthesis
- Signal sequence on nascent polypeptide is recognised by the SRP
- SRP binds to the polypeptide and ribosome
- Translation halted by binding of SRP
- SRP recognised by SRP receptor/docking protein on ER membrane
- In making the interaction with the docking protein, the SRP is released from the signal sequence
- This removes the inhibitory constraint on translation so translation continues
- The signal sequence then interacts with a signal sequence receptor (SSR) within a protein translocator complex
- The ribosome becomes anchored to this pore complex, through which the growing polypeptide chain is extruded.
- The signal sequence is cut by the enzyme signal peptidase
- Once translation is finished, the ribosome is detached and goes back into the cytoplasm to find more mRNA
Describe the membrane protein biosynthesis
- Signal sequence on nascent polypeptide is recognised by the SRP
- SRP binds to the polypeptide and ribosome
- Translation halted by binding of SRP
- SRP recognised by SRP receptor/docking protein on ER membrane
- In making the interaction with the docking protein, the SRP is released from the signal sequence
- This removes the inhibitory constraint on translation so translation continues
- The signal sequence then interacts with a signal sequence receptor (SSR) within a protein translocator complex
- The ribosome becomes anchored to this pore complex, through which the growing polypeptide chain is extruded.
- The signal sequence is cut by the enzyme signal peptidase
- Protein synthesis is arrested by the stop transfer signal. The stop transfer signal is a hydrophobic sequence which forms the trans- membranous region of the protein. A lateral gating mechanism releases the membrane protein from the protein translocator into the lipid bilayer.
- The ribosome detaches from the ER and protein biosynthesis continues in the cytoplasm. The result is a transmembrane protein with its hydrophobic N-terminal directed in to the lumen and it’s hydrophilic C-terminal to the cytoplasm.
What percents of total body weight is water?
60%
What proportion of water in the body is in the ECF?
1/3
What proportion of water in the body is in the ICF?
2/3
What is ECF?
Fluid found outside cell membranes - fluid between cells
What is the ICF
Fluid inside cells bounded by the cell membrane
What compartments does the ECF divide into?
Interstitial fluid
Plasma
Lymph
Transcellular fluid
What proportion of ECF is interstitial fluid and plasma?
Interstitial fluid - 75%
Plasma - 25%
Describe the ion composition of the ECF and ICF
- The ECF has more Na+ than ICF
- The ICF has more K+ than ECF
- The ECF has more Cl- than ICF
- The ICF has more proteins than ECF
Which cation is the biggest proportion of the ECF?
Na+
Is the osmolality different in the ECF and ICF and why?
Not different because osmosis ensures equilibrium
Does changing ion conc by a little change osmolality?
No
Which substances can pass through the phospholipid bilayer?
- hydrophobic molecules
- small uncharged molecules - water, urea - not too well though
Which substances cannot pass through the phospholipid bilayer?
- Large uncharged polar molecules.
- ions
- charges polar molecules
Why can Na+ and K+ pass through the capillary wall but not the cell membrane ?
Na+ and K+ pass through the fenestrations in the capillary wall, not through the endothelium.
No fenestrations in the cell membrane so cannot pass through
Describe passive transport
- No energy needed
* Movement down concentration gradient
Describe active transport
- movement against concentration gradient
- requires ATP
Give examples of passive transport
- diffusion
- facilitated diffusion
- osmotic(and oncotic pressures) hydrostatic pressure
What are the examples of vesicular transport(active)?
- pinocytosis
- phagocytosis
What is diffusion
movement from
high ->low concentration
until equilibrium reached
What is flux?
Describes how fast a solute moves (i.e the number of moles crossing a unit area
of membrane per unit time (moles / cm2/ s)
What is flicks first law of diffusion?
The rate of flow of an uncharged solute due to diffusion is directly proportional to
the rate of change of concentration with distance in direction of flow.
How does membrane thickness affect flux?
If membrane is thinner, flux is faster
If membrane is thicker, flux is slower
How does surface area and thickness affect diffusion?
Diffusion is proportional to the surface area of he barrier and inversely proportional to its thickness
What is the driving force for net diffusion?
Concentration gradient
What happens when there is diffusion of 2 solutes?
Each substance diffuses down its own concentration gradient, independent of
concentration gradients of other substances
What is facilitated diffusion
Move from HIGH to LOW concentration through a protein channel
What are gated channels and give examples?
Proteins that open only in presence of stimulus (signal)
• stimulus usually different from transported molecule
• ex: ion-gated channels
• ex: voltage-gated channels
How do large molecules cross the membrane?
Exocytosis (out of cells)
• Moving large molecules into & out of cell requires ATP
• through vesicles & vacuoles
Endocytosis (into cells)
• phagocytosis = “cellular eating”
• pinocytosis = “cellular drinking”
• receptor-mediated endocytosis
How does water move through the membranes?
By osmosis Diffusion of water from high hypotonic solution to hypertonic solution • across a semi-permeable membrane
What happens if a cell is in a hypotonic solution?
Higher solute concentration inside than outside so water enters cell causing cell to swell and possibly burst
What happens if cell is in a hypertonic solution?
Higher solute concentration outside cell than inside so water moves out of cell causing it to shrink.
What is osmole?
The unit of osmolarity
What is the difference between osmolarity and osmolality
Osmolarity refers to the number of solute particles per 1 L of solvent
osmolality is the number of solute particles in 1 kg of solvent.
In a biological system, osmolarity and osmolality is the same but when measuring, it is always osmolality
In clinical practise what can b done to estimate serum osmolality
Serum osmolality can be estimated by doubling the serum sodium because osmolality is mainly determined by Na+ and Cl- in ECF