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BIOCH 200 > Biological Membranes > Flashcards

Biological Membranes Flashcards

1
Q

State the rationale used to classify biological molecules as a lipid

A

non polymeric compounds:
- non polar
- hydrophobic
- insoluble in water

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2
Q

List 4 distinct types of molecules that are classified as lipids

A
  1. Fatty acids - AP
  2. Triacylglycerol - HP, contains FA
  3. Membrane lipids - AP, contains Fa
  4. Cholesterol - HP + AP

AP - amphiphilic
HP - hydrophobic

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3
Q

Fatty acids

A
  • long chain hydrocarbon carboxylic acids
  • General formula: CH₃(CH₂)ₙCOO-
  • contains polar and non polar portions (amphipathic)
  • saturated or unsaturated
  • usually cis
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4
Q

identify ⍺, ⍵ and β carbon atoms in a fatty acid

A

⍺ - the carbon connected to the carboxylic group carbon
⍵ - the last carbon in the hydrocarbon chain (highest # carbon)
β - the carbon that is connected to the alpha carbon in the hydrocarbon chain

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5
Q

State why fatty acids are termed amphipathic

A

They contain a long hydrocarbon chain (non polar) and a carboxylic group with a - charge (polar)

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6
Q

Define the terms 1) saturated, 2) monosaturated and 3) polyunsaturated fatty acid

A
  1. hydrocarbon chain contains no double bonds
  2. hydrocarbon chain contains 1 double bond
  3. multiple double bonds
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7
Q

what are the different effects that cis and trans double bonds have on the shape of fatty acids

A

cis - hydrogens are on the same side of the double bond, energetically unfavourable (sterics), but introduce kinks into the structure which lower melting point

trans - hydrogens are on opposite sides of the double bond, elongated/linear aspect, able to stack on top of other fatty acids and create bonds which raise melting point

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8
Q

factors affecting fatty acid melting points

A
  1. Length - longer FA have higher MP, shorter FA have lower MP
  2. Unsaturation - sat. FA have higher MP, unsat. FA have lower MP (greater effect on MP than length)
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9
Q

fatty acid short hand notation

A

(# of carbons) : (# of double bonds) ^ location of double bonds

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10
Q

Describe the structure of a triacylglycerol

A
  • way of storing fatty acids
  • very HP not AP
  • 3 acyl chains (from ester linked fatty acids) attached to glycerol
  • mixed TAGs are most commonly found (made from different FA)
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11
Q

Compare the structure of a triacylglycerol with that of a glycerophospholipid lipid

A

TAG:
- has fatty acyl groups covalently attatched to glycerol
- doesnt have a polar head group

GP:
- also has fatty acyl groups covalently attatched but to glycerol, but glycerol is attatched to a phosphate and polar head group
- presence of a large polar group → amphipathic
- variations exists in both polar head groups and acyl chains → affects size and MP

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12
Q

Why do amphipathic molecules form micelles or bilayers in water?

A
  • eliminate unfavourable contact between water and hydrophobic tails
  • permit solvation of polar head groups
  • water wants to interact with itself: pushes lipid molecules together
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13
Q

compare the behaviour of fatty acids, membrane lipids, and TAGs when they are mixed with water

A

FA: AP, forms micelles to shield the hydrophobic tails, forms bilayers at higher concentrations

ML: AP similar to fatty acids, spontaneously arrange into bilayers

TAG: HP and lack polar groups, do not form bilayers or micelles, insoluble in water, aggregate at surface

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14
Q

what are liposomes?

A
  • lipid bilayers that form spherical vesicles
  • encloses aq solution inside of vesicle seperate from outer aq environment
  • creates a boundary/border between insides and outside

eg micellar water/soap

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15
Q

describe the structure of cholesterol and state how it “fits” into a lipid bilayer

A

structure:
- rigid hydrophobic, non polar hydrocarbon/ring structure
- 27 carbons, 5 rings (four 6 mem)
- weakly AP, one polar aspect (-OH)

bilayer:
- accounts for ~35% of mammilian membranes
- OH associates with polar headgroups of other lipids
- non-polar portion is found in the membrane
- maintains fluidity and rigidity

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16
Q

explain why lipid bilayers are fluid, yet stable, structure

A
  • lipid bilayer is fluid because individual lipid molecules can move laterally within the plane of the bilayer, they have flexibility and mobility within them
  • the noncovalent molecular forces enforce stability: H bonds in acyl chains, electrostatic forces b/w polar head groups and cholesterol enforcing more rigidity
17
Q

state why the exact dimensions of a lipid bilayer are variable

A
  • lipid head groups have different dimensions/sizes
  • the acyl tails vary in length (16-20 C)
  • cholesterol is buried almost entirely in the bilayer (excludes 1 OH group)
18
Q

describe how changes in fatty acid composition affect the fluidity of a biological membrane

A
  • saturation: more unsaturated bonds increase fluidity, more saturated bonds decrease fluidity
  • heat: below TT, acyl chains pack together in van der Waals contact, in a gel-like solid state, above TT lipid molecules move freely and rapidly
  • length: longer = less fluid, shorter = more fluid

membranes must operate above gel temperature (MP) but not be completely disordered

TT (transition temperature): temp of transition from an ordered crystalline to more fluid state

19
Q

state the function of cholesterol in biological membranes

A
  • because it is rigid and planar: limits rotational movement of neighbouring acyl tails, ↑ VDW interactions
  • low temp: prevents close packing between acyl chains
  • high temp: decreases motion/disorder, increases rigidity
20
Q

state why the lipids and proteins in a membrane bilayer typically move only laterally

A
  • a significant energy barrier is associated ith desolvating a polar head group to move it through a hydrophobic bilayer
  • Flipases: increase the rate of transverse diffusion and allow changes in lipid composition in the layers

Transverse diffusion: flip-flip of lipids in a bilayer

21
Q

List the 3 major types of membrane protien and state how each is associated with the membrane

A
  1. Integral membrane protein: fully embedded into the bilayer, potion in contact with acyl tails of the bilayer must have HP AASC on surface, portions of protein facing in- and outside of the bilayer contain polar AASC
  2. Peripheral membrane protein: exist on one side of the lipid bilayer, mainly contains polar AASC on it’s surface, can easily remove
  3. Lipid-linked protein: exists on one side of the bilayer but contains a small HP aspect lipid anchor covalently attached to the internal NP lipid bilayer

AASC - amino acid side chains

22
Q

what side chains are you likely to find on a periperal membrane protein?

A
  • Asp
  • Glu
  • Lys
  • Arh
23
Q

**describe the two most common structures seen in proteins which cross a lipid bilayer

A
  • alpha helices:
  • antiparallel beta sheets:
24
Q

identify the most likely locations for hydrophobic and polar amino acid side chains, given the structure of an integral membrane protein

A
25
Q

Identify substances that do and do not require proteins to cross a lipid bilayer

A

Do:

  1. Gases (CO₂, O₂)
  2. Hydrophobic molecules (benzene)
  3. Small polar molecules (H₂O, ethanol)

Do not:

  1. Large polar molecules (glucose)
  2. Charged molecule (amino acids, H+, Na+, Cl-)
26
Q

explain why polar substances require proteins to cross a lipid bilayer

A
  • the inside of a bilayer is hydrophobic and repels polar molecules
27
Q

Rate of simple unmediated diffusion depends on:

A
  1. size of the molecules
  2. concentration gradient
  3. lipid solublity
28
Q

define the terms passive and active transport

A

passive: The thermodynamically spontaneous protein mediated transmembrane movement of a substance from high to low concentration.
ΔG = negative

active: The transmembrane movement of a substance from low to high concentrations by a protein that couples this endergonic transport to an exergonic process such as ATP hydrolysis

overall ΔG<0 for transport to occur

29
Q

state how transporter proteins differ from ion channels and porins

A

ion channels:
- PT
- typically formed between subunits (tetramer)
- highly selective and sometimes gated (eg Na+), selectivity depends on the size of the pore and the properties of the side chains/FG found there

porins:
- PT
- non-specific
- contains a water filled pore in the center of a antiparallel beta sheet barrel (trimer)
- non polar exterior and a polar interior
- only restrictve to size

transporter (carrier) proteins:
- can be AT or PT
- do not have membrane-spanning pores
- conformational change alternates openings from one side of the membrane to the other
- selective for substrate transported
“revolving door”

30
Q

define the terms 1) uniport, 2) symport, and 3) antiport

A
  1. transports a single type of solute
  2. transports 2 different types of solutes in parallel
  3. transports 2 different types of solutes anti parallely
31
Q

Distinguish between primary and secondary active transport processes

A

primary:
- uses ATP (or other reaction as a source of free energy)
(ΔGt solute + ΔG rxn) < 0

secondary:
- uses an ion gradient as a source of free energy (eg Na)

(ΔGT solute + ΔGt ion) < 0

sum of free energies has to be 0 for transport to occur

32
Q

describe the mechanism of the Na+ K+ ATPase and state how it uses the energy in ATP to drive the transport (7 steps)

A
  • “electrogenic antiport” meaning it creates a voltage across the membrane and moves solutes in opposite directions

per 1 cycle:
- 3 Na ions exported from cell
- 2 K ions imported
- ATP + water → ADP + Pi + H+ (ATP hydrolysis)

  1. 3 intracellular sodiums bind to the protein
  2. ATP binds
  3. A phosphoryl group is transferred from ATP to an Asp side chain of the pump → releases ADP
  4. protein conformation changes, exposing the Na+ binding sites to the cell exterior, Na+ ions dissociate
  5. Two K+ ions bind
  6. The aspartyl phosphate group is hydrolyzed → Pi is released
  7. The protein conformation changes, exposing K+ to the interior of the cell, K+ dissociates
33
Q

Describe how glucose is transported across the intestinal epithelial cells

A
  • it is a uniport (Na-Glucose transporter)
  • [Na+]in < [Na+]out →ΔG < 0 (favourable)
  • Na+ transport provides the energy for glucose import
  • a glucose transporter on the other side of the membrane allows glucose to move down it’s concentration gradient
  • Na+/K+ maintains low concentration of sodium inside the cell

BIOCH 200 (7 decks)

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