1
Q
how is the structure of the p.m. commonly described in terms of?
A
- the fluid mosaic model, which states that biological membranes are 2D, fluid structures within which lipid and protein molecules diffuse freely
2
Q
what 2 main structural factors affect the stability of the p.m.?
A
- lipid rafts, which are held together by large amounts of cholesterol and contain relatively high concenteations of sphingomyelins. they can diffuse within the lipid bilayer and their main functions include contributing to the fluidity of the memrane and helping to regulate signaling processes
3
Q
how does cholesterol help modualte the fluiditiy of the p.m. at a range of temperatures?
A
- at high temperatures, including physiological temp. cholesterol decreases membrane fluidity by impeding the diffusion of phospholipids within the bilayer
- at low temperatures, cholesterol increases membrane fluidity by preventing phospholipid tails from clustering together
4
Q
unsaturated fatty acud tails promote fluidity by preventing the tails from stacking, as can occur with?
A
- saturated fatty acid tails
5
Q
phospholipids are mobile in what direction?
A
- horizontal direction, as they can move around relatively freely within a single layer of the bilayer membrane
6
Q
how do phospholipids shift from one side of the membrane to another?
A
- catalyzed using flippases which is energetically costly
7
Q
what are the 3 main classes of lipids present in the p.m.?
A
- phospholipids
- sterols
- glycolipids- cell signalling and cell adhesion (glyceroglycolipid or sphingolipid)
- phospholipids and glycolipids usually contain an even number of C atoms
- unsaturated fatty acids in the bilayer membrane are nearly always found in the cis orientation
8
Q
what are the 3 main classes of membrane proteins?
A
- transmembrane (integral)
- peripheral
- lipid anchored
9
Q
what is the function of integral proteins?
A
- includes membrane-spanning proteins with a hydrophilic cytosolic domain that interacts with the interior of the cell and a hydrophobic membrane-spanning domain that anchors into the cell membrane and a hydrophilic extracellular domain that interacts with the extracellular environment
- common and important class with examples like proton pumps, ion channels, and GPCR
10
Q
what is the function of peripheral proteins?
A
- these proteins are transiently attached to integral membrane proteins or are associated with peripheral regions of the lipid bilayer
- they tend to interact with the biological membrane only transiently before resuming their function within the cytoplasm. this category includes some enzymes and hormones
11
Q
what is the function of lipid-anchored proteins?
A
- they are covalently bound to single or multiple lipid molecules that anchor the protein within the membrane without the protein contacting the membrane
- ex. G proteins which are intracellular membrane bound structures that help coordinate the signaling cascade initiated by GPCR
12
Q
for proteins destined for insertion or association with the p.m. what happens?
A
- an N terminus signal sequence directs the newly synthesized proteins to the ER where they are inserted into the lipid bilayer.
- once inserted, the proteins are then transported to their final destination in vessicles, which eventually fuse with the target membrane
13
Q
proteins can be glycosylated which refers to?
A
- the addition of oligosaccharide chains to a peptide chain
- most common example are O-glycosylation, in which a glycosidic bond is formed to the O atom present in serine and threonine side chains or N-glycosylation in which the oligosaccharide binds to a N on the side chain of asparagine
- ABO blood type system so membrane glycoproteins often involved in cell recognition and communication processes
- most common example are O-glycosylation, in which a glycosidic bond is formed to the O atom present in serine and threonine side chains or N-glycosylation in which the oligosaccharide binds to a N on the side chain of asparagine
14
Q
what is a liposome?
A
- a lipid bilayers enclosing a spherical space used in labs for research
15
Q
what is a micelle?
A
- an aggregate composed of a single layer of lipids in an aqueous solution, where the hydrophilic head region is in contact with the solvent, while the hydrophobic tail region is sequestered in the center of the micelle
16
Q
what are the 4 main methids in which substances cross the lipid membrane?
A
- osmosis
- passive transport (including simple diffusion and facilitated diffusion)
- active transport (primary or secondary)
- exocytosis/endocytosis
17
Q
what is simple diffusion
A
- simple diffusion is a form of diffusion that does not require the assistance of membrane proteins. In essence, the particle or substance moves from higher to lower concentration. However, its movement does not need a membrane protein that will help substances to move downhill.
18
Q
what is facilitated diffusion?
A
- Facilitated diffusion is the diffusion of solutes through transport proteins in the plasma membrane. Facilitated diffusion is a type of passive transport. Even though facilitated diffusion involves transport proteins, it is still passive transport because the solute is moving down the concentration gradient.
- ex. aquaporins/water channels (move large amounts of water)
- ion channels
19
Q
what is Osmosis?
A
- special form of simple diffusion that only applies to solvents (water)
- osmosis takes place when you have a physical setup in which water can diffuse through a barrier but solutes cannot
20
Q
what is Van’t Hoff’s law?
A
- At constant temperature, the osmotic pressure (π) of a dilute solution is directly proportional to its molar concentration (C) or inversely proportional to volume (V) of the solution
- Π= iMRT
- colligiative property
- M is the total concentration of solute particles
- R is the ideal gas constant
- T is temperature in K
21
Q
what is passive transport
A
- which include the subcategories of simple diffusion, osmosis and facilitated siffusion does not require the input of chemical energy.
- passive transport is a spontaneous process that is “powered” by the increase in entropy associated with transport (thermodynamically favourable)
- simple/osmosis are regulated by the concentration gradient down which particles are flowing
- facilitated depends on the enzyme activity of the carrier protein/channel os it can become saturated
22
Q
even though facilitated diffusion is a form of passive transport so does not need energy input, it is subject to regulation unlike osmosis and simple diffusion. 2 examples are:
A
- ligand-gated channels are ion channels that can open in response to the binding of a ligand.
- voltage-gate channels are membrane channels that respond to changes in membrane potential.
23
Q
what is active transport?
A
- requires input of energy to move a solute against a concentration or electrochemical gradient
24
Q
what is primary active transport?
A
- Primary active transport directly uses a source of chemical energy (e.g., ATP) to move molecules across a membrane against their gradient.
25
what is secondary active transport?
* Secondary active transport (cotransport), on the other hand, uses an electrochemical gradient – generated by active transport – as an energy source to move molecules against their gradient, and thus does not directly require a chemical source of energy such as ATP.
26
most enxymes that perform primary active transport are members of the ?
* transmembrane ATPase family
* transmembrane proteins that catalyze the hydrolysis of ATP to release energy
* more specifically, they couple the movement of solutes to ATP hydrolysis
* ex. sodium/potassium pump
27
what is the function of the sodium/potassium pump?
* this enzyme helps to maintain the cell potential across membranes by exchanging Na+ for K+ across a membrane, moving both ions against the transmembrane concentration gradients established by the pump
* for every ATP hydrolyzed, 3 Na+ ions are transported out of the cell and 2 K+ ions are transported in. this helps maintain the charge imbalance between the exterior and interior of the cell
28
primary active transporters can also be powered by?
* redox reactions or by energy harnessed from the photons of incident light
* ex. the enzymes of the mitochondrial ETC that use the energy released from redox reactions to translocate protons across the inner mitochondrial membrane against their concentration gradient
29
what are antiporters and symporters?
* Symporters and antiporters are involved in active transport.
* Antiporters transport molecules in opposite directions
* symporters transport molecules in the same direction.
30
what is endocytosis?
* an energy-using process by which cells absorb molecules by engulfing them
* pinocytosis- cells engluf liquid substances
* phagocytosis- cells engulf solid particles (debris, microorganisms, other apoptotic cells)
* clathrin-mediated endocytisis which involves small vesicles that have a coat composed mainly of cytosolic protein clathrin. clathrin-coated cesicles are found in nearly all cells and form domains in the p.ml termed clathrin-coated pits concentrate large extracellular molecules that are receptor-specific for the endocytosis of their ligands (uptake of LDL, growth factors, antibodies, etc.)
31
what is the endocytic pathway?
* early endosomes which pass the molecule of interest onto late endosomes and recycle material back to the p.m. and lysosomes which are the final component of the endocytic pathway
* lysosomes are the principal hydrolytic compartment of the cell, function to break down cellular waste products/macromolecules and are highly acidic with a pH of 4.8
32
what is exocytosis?
* is a form of bulk transport in which materials are transported from the inside to the outside of the cell in membrane-bound vesicles that fuse with the plasma membrane.
* these membrane-bound vesicles contain soluble proteins that will be secreted into the extracellular environment, as well as membrane proteins and lipids that become components of the cell membrane
33
what are the 2 main types of exocytosis?
* consitutive exocytosis is performed regularly by all cells to release material to the extracellular matrix or to deliver membrane proteins to the membrane
* non-constitutive exocytosis is mediated by Ca2+ signaling and is used to release vesilces with specific content into the extracellular space (or in the case of neurons, into the synaptic cleft)
MCAT biochem
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