Membrane Structure, Cell Comp, Volume Flashcards
Describe the molecular components of a membrane.
Cell membranes consist of lipid bilayers with embedded and associated proteins• Lipid bilayers are ~5 nm thick (50Å) and serve as a barrier to most water-soluble molecules• Proteins spanning the lipid bilayer mediate most of the functions of the membrane (~30% of the all proteins encoded in the genome are membrane proteins)• Lipid bilayers are dynamic and fluid structures; membrane fluidity depends on composition and temperatureComposed of lipids, cholesterol, and proteins.
Identify the parts of a phospholipid sphingolipid and cholesterol.
Phosphoglycerides: 2 fatty acyl chains attached to a glycerol, phosphate, and polar head group at the other end. What type of head group determines which phosphatidylglycerol it is (ethanolamine, inositol, etc). Sphingolipids: have a ‘sphingosine’ (long acyl unit) with either of two polar head groups (which one it is distinguishes which type of sphingolipid it is) and an attached fatty acid chain. Cholesterol: characteristic hydroxylated ring structure surrounded by a fatty acid chain and polar head group on opposing ends.
List the different ways proteins associate with membranes.
Integral proteins (fully or partially embedded in membrane) can go multiple times back and forth through the membrane.. It is also possible to have integral proteins that are only bound to one side of the bilayer; it is also possible to have proteins that are only attached at their ends to the membranes. Peripheral membrane proteins: covalently interacting with proteins bound in the membrane, but not themselves embedded in the membrane.
Describe the asymmetry of membrane bilayers.
Bilayers are asymmetric. Notice that particular phospholipids don’t flip from one side of the membrane to another without enzyme activity (flipase), but lots of lateral diffusion all the time.
Describe the concept of membrane fluidity.
Membrane fluidity: the degree to which lateral motion is possible among adjacent phospholipids on a given side of the bilayer. Some lipids are anchored, some are free-floating within any given membrane. Note that different physical compartments of a membrane can have different degrees of fluidity. Having acyl chains unsaturated makes the membrane more fluid (less tightly packed). Cholesterol, when intercalated in membranes, stiffens the membrane and makes it less fluid.
Explain how cholesterol synthesis is regulated.
When cholesterol concentration goes down, and you need to synthesize more, you want HLH (a transcription factor) to get to the nucleus. At low cholesterol concentrations, Insig unbinds from the targeting domain of SCAP, and SCAP and SREBP + HLH move to the Golgi. Now SREBP gets cleaved by SCP 1+2 and HLH can move to the nucleus. It is a complex control mechanism, but it is also very sensitive to cholesterol concentration.
What is the typical volume of plasma?Extracellular fluid?Intracellular fluid?
3L15L + 5L for the 3rd space27L
Know the two most important functional properties of membranes one conveyed by lipids the other by channels and transporters.
Lipids are also ‘strong’, in an electric sense. That is, they can keep opposite electric charges separated, without collapsing. The excess anions inside a cell create an electrical potential difference with the outside of the cell, and this membrane potential, which governs some vital cell processes, is wholly dependent on the integrity of the plasma membrane. The ability of the membrane to withstand the imposed electric force (which has a whopping strength of about 100,000 volts/cm) is due to its lipid composition. It is impermeable to charged substances! This includes dipoles. The second physiologically important point about membranes is that many charged and polar molecules can cross membranes, no thanks to the lipids. Thank you channels + transporters.
Understand the routes by which a given substance can traverse a membrane.
Channels, diffusion, transporters, tunnels, proteins, etc ← LOL!
Identify physical forces that can determine the gating properties of ion channels.
Some depend on the electric field (membrane potential) across the membrane. The gates in many such voltage-gated channels swing open when the membrane is depolarized. Gates in other channels require mechanical stimulation (stretching of the membrane) to open (e.g., hair cells in the cochlea; touch receptors in the skin). Still others require the binding of a particular chemical. Others open or close depending on temperature (cutaneous thermal receptors). Some gates in channels depend on more than one force, making for complicated behavior. Some channels contain no gates (e.g. aquaporins) , while others contain two gates, and both have to be open to allow ions to pass!
What are the main differences between channels and transporters?
Transporters can move large molecules like glucose and can move substances across their gradient using energy.Channels simply let specifically sized/charged molecules float through.
Be able to determine which direction an uncharged substance will move across a membrane.
According to its concentration gradient.
Determine under a given set of conditions whether a cell will swell or shrink.
Look at the osmotic pressure inside and outside of the cell.
List the three mechanisms that different cells have evolved to keep from swelling and bursting.
A simple solution to the problem would be to make the cell membrane impermeable to water, as well as to the internal solute molecules. However, this would create many new problems for the cell; growing cells, for example, must have a way of accumulating water as they grow (remember, cells are mostly water by volume), and therefore they must have surface membranes permeable to water. Another solution is to build a strong wall around the cell, and so keep the cell from swelling by brute force (that is, apply a hydrostatic force to counter the osmotic force drawing water into the cell). Plant cells, fungal cells, and bacteria do just this, by building tough cell walls outside their plasma membranes. But it’s an expensive way to go, in the sense that the osmotic force is not trivial so that cell walls require considerable metabolic resources (building materials, energy for synthesis, etc.), and greatly limit cell shape. The solution in animal cells to the problem of volume control is to fight fire with fire, and balance the osmotic force osmotically, by having solute molecules in the ECF, in order to balance those in the ICF. Thus, the concentration (or, more accurately, the activity) of water is exactly the same in the ICF and ECF. How simple physiology can be.
Know which direction water will move across a semi-permeable membrane given the solute compositions of the fluids on either side of the membrane.
Do the practice problems on the worksheet :)
