Lecture 4 Block 2 Flashcards
(26 cards)
What are the bonds between hydrogen and oxygen in H2O?
Polar covalent bonds - this gives water a partial negative charge (has a dipole)
What is the consequence of water’s polarity?
The ability to hydrogen bond - water’s dipole sets up a perfect situation for hydrogen bonding
At any given point in time how many hydrogen bonds does any one water molecule make?
At any point in time each water molecule makes hydrogen bond interactions with 3-4 adjacent water molecules
What are some properties of water that result from this hydrogen bonding?
High surface tension
High heat capacity
High latent heat of evaporation
Solid water takes up more volume than liquid water
What character of water gives it its excellent solvent properties and what does this mean?
The polar character of water is responsible for its exceptional ‘solvent properties’ - its ability to solvate (or hydrate) other polar molecules and therefore hold them in solution.
What is a key contributor to the hydrophobic effect and what is this?
The solvent properties of water (through its polar character) is a key contributor to the hydrophobic effect which is:
The tendency of nonpolar molecules to self-associate in water rather than to dissolve individually, driven by the loss of hydrogen bonding and the higher entropic cost of forming a cavity around nonpolar molecules.
What is the hydrophobic effect a critical contributor to?
The hydrophobic effect is a critical contributor to stabilizing the structure of the biological membranes
What are the two principal chemical constituents of biological membranes?
- Lipid - key contributor to the ‘barrier’ characteristics of membranes
- Protein - key contributor to the ‘functional’ character of membranes
What are phospholipids?
Typically consist of a 3 carbon compound, two long (15+ carbons) hydrocarbon chains that are very hydrophobic alkyl fatty acids, and a negatively charged (and therefore very polar) phosphate residue that also includes a so-called (and very polar also) head group.
What do the head groups of phospholipids include?
They usually include several alcohols including choline (the most common), inositol, glycerol and ethanolmaine; and the amino acid serine.
Why are phospholipids amphipathic?
The presence of both a hydrophobic (water-hating) and hydrophilic (water loving) region within the same molecule makes it an amphiphile
What are two concepts that pertain to the interaction of phospholipids and water?
- The non-polar polar fatty acid “tails” tend to self-associate (driven by the hydrophobic effect) excluding water, whereas the polar head groups support formation of hydrogen bonds and, therefore, attract water.
- The phospholipid bilayer, in addition to being structurally stable, is also very “dynamic” and phospholipids within the bilayer display high rates of “rotational” and “lateral” diffusion.
What types of phospholipid positional movement within the bilayer is possible and which is with energy addition through ATP?
Rotational and lateral diffusion by phospholipids within the bilayer is possible.
Translational diffusion takes to much energy to move the polar head group through the lipophilic interior of the bilayer and requires a LOT of energy input - flippases (specfic ATP-requiring enzymes) are needed to catalyze such movements
What happens when phospholipids are placed in water?
Then the phospholipid concentration in water is high enough that the fatty acid tails can find each other, the physical repulsion (by water) of the hydrophobic PL tails leads to their preferential, spontaneous interaction with each other. The non-polar fatty acid tails, lacking the ability to hydrogen bond, are quite literally squeezed out of solution and form the phosphoplipid bilayers. The polar heads (that are hydrophilic) face outward while the hydrophobic tails face inward. The structure, once formed, takes a LOT of energy to break apart.
What are (in summary) the three steps in bilayer biosynthesis?
- Eznymatic synthesis of the phospholipid
- Insertion of phospholipid into the cytoplasmic face of the smooth ER
- Distribution of phospholipids across the ER membrane by means of an ATP-dependent ‘flippase’
`Why is the middle of a bilayer such an extremely effective barrier to free water diffusion?
At the core of a bilayer is the hydrophobic inner zone that consists only of water-repelling fatty-acid tails. The very low water permeability of the bilayer reduces by ~10,000 fold the free diffusive movement of water between the compartments separated by the membranes.
What is the most important sterol in lipid bilayers in eukaryotes and why?
Cholesterol (CHL) for two reasons:
- influences the thickness of the plasma membrane
- Influences the stiffness (related to strength) of the plasma membrane
What are the two classes of membrane proteins?
Intergral proteins and peripheral proteins.
Integral membrane proteins catalyze a diverse set of cellular processes including?
- Mediating transport of molecules across the membrane
- attaching cells to neighboring cells and surfaces
- interacting with extracellular molecules to initiate a vast array of intracellular events
Are integral membrane proteins typically hydrophobic or hydrophilic?
Neither and both. Integral membrane proteins have an amphipathic structure with regions that readily interact with water and regions that find it more energetically favorable to sit in a lipid environment
Why are integral membrane proteins called integral?
B/C they are in fact an integral part of the structure of the membrane and the only way to remove them is to literally dissolve the membrane with detergent.
What are peripheral membrane proteins basically for?
They include many cytoskeletal proteins that play key roles in organizing the location of selected integral MP’s, and a wide variety of regulatory enzymes whose activity is regulated through interaction with specific integral MP’s.
Are the surfaces of peripheral membrane proteins typically hydrophobic or hydrophilic? Does this make it easier or harder to strip them from the membrane?
The surfaces of peripheral MP’s are typically very hydrophilic and their association with membrane surfaces typically involve weak, but frequently very specific, electrostatic interactions. They are very easy to strip from the membrane by simply increasing the salt concentration, or other gentle methods that leave the membrane intact.
What are “lipid rafts?”
Lipid rafts are small scale collections of closely packed sphingolipids and cholosterol that form moving platforms onto (or int) which specific proteins attach within the membrane bilayer.
