Biological Membranes

Question 1

State the rationale used to classify biological molecules as a lipid

Answer 1

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

Question 2

List 4 distinct types of molecules that are classified as lipids

Answer 2

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

AP - amphiphilic
HP - hydrophobic

Question 3

Fatty acids

Answer 3

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

Question 4

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

Answer 4

⍺ - 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

Question 5

State why fatty acids are termed amphipathic

Answer 5

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

Question 6

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

Answer 6

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

Question 7

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

Answer 7

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

Question 8

factors affecting fatty acid melting points

Answer 8

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)

Question 9

fatty acid short hand notation

Answer 9

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

Question 10

Describe the structure of a triacylglycerol

Answer 10

• 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)

Question 11

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

Answer 11

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

Question 12

Why do amphipathic molecules form micelles or bilayers in water?

Answer 12

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

Question 13

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

Answer 13

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

Question 14

what are liposomes?

Answer 14

• 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

Question 15

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

Answer 15

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

Question 16

explain why lipid bilayers are fluid, yet stable, structure

Answer 16

• 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

Question 17

state why the exact dimensions of a lipid bilayer are variable

Answer 17

• 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)

Question 18

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

Answer 18

• 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

Question 19

state the function of cholesterol in biological membranes

Answer 19

• 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

Question 20

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

Answer 20

• 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

Question 21

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

Answer 21

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

Question 22

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

Answer 22

• Asp
• Glu
• Lys
• Arh

Question 23

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

Answer 23

• alpha helices:
• antiparallel beta sheets:

Question 24

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

Question 25

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

Answer 25

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-)

Question 26

explain why polar substances require proteins to cross a lipid bilayer

Answer 26

- the inside of a bilayer is hydrophobic and repels polar molecules

Question 27

Rate of simple unmediated diffusion depends on:

Answer 27

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

Question 28

define the terms passive and active transport

Answer 28

**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 ## Footnote overall ΔG<0 for transport to occur

Question 29

state how transporter proteins differ from ion channels and porins

Answer 29

**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"*

Question 30

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

Answer 30

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

Question 31

Distinguish between primary and secondary active transport processes

Answer 31

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

Question 32

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

Answer 32

- "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

Question 33

Describe how glucose is transported across the intestinal epithelial cells

Answer 33

- 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