Membrane Structure

Question 1

What is a cell membrane?

Answer 1

1. Structure that gives cells it shape.
2. Separates extracellular environment from intercellular environment.
3. Dynamic in shape and keeps changing
4. Allows transport of substances into and out of the cell
5. In eukaryotes each organelle has a membrane
6. Has multiple functions: signal transduction, cell communication, complex reaction sequences, energy transduction

Question 2

What is a cell membrane made of?

Answer 2

1. phopholipid bilayer

2. proteins

Question 3

What is the phospholipid bilayer?

Answer 3

Made up of phospholipids. They had a hydrophillic head with a polar group. Also have a hydrophobic fatty acid tail. The head interacts with the extra/inter cellular environments.
The tails provides structure.
Has two leaflets, the outer and inner.

Question 4

What are Lipids?

Answer 4

Building blocks of membranes.
Non-covalently linked.
Insoluble in water.
Amphipatic - head and tail

Question 5

What are the different types of lipids?

Answer 5

1. Fatty Acids
2. Glycerolipids
3. Glycerophospholipids
4. Sphingolipids
5. Sterol Lipids
6. Prenol Lipids
7. Saccarolipids
8. Polyketides

Question 6

Function of Lipids?

Answer 6

1. Structural - membrane components and protein modification.
2. Metabolic - energy storage
3. Other Functions - cell signalling, hormones, enzyme cofactors, electron carriers, pigments

Question 7

Effects of chain length on lipids?

Answer 7

The longer the chain the higher the melting point.
The longer the chain = more stable = more rigid.
Solubility in water will decrease as chain length increases.

Question 8

Effects of double bonds on lipids?

Answer 8

Lipids contain long chains of carbon, therefore there can sometimes be double bonds.
An increase in the number of double bonds decreases the melting point.
Double bonds usually found in cis configuration. Causes kink in chain and unable to pack in as tightly.

Question 9

Difference between saturate and unsaturated fats?

Answer 9

Saturated fats have no double bonds and are packed in tightly. Have a higher melting point than unsaturated fats.
Unsaturated fats have lipids that contain double bonds. The double bonds cause kinks so the lipids aren’t able to pack in as tightly. This causes a lower melting point than saturated.

Question 10

Structure of Glycerophospholipids

Answer 10

1. Glycerol backbone
2. fatty acid tails often unsaturated
3. Phosphate and alcohol head group - OH on head group is polar

Question 11

Glycerophsopholipid head groups?

Answer 11

Serine
Ethanolamine
Choline
Glycerol
Inositol

Question 12

What is the importance of glycerophospholipids?

Answer 12

Structural component for cell membrane - forms bilayer

Question 13

What are Sphingolipids?

Answer 13

1. Major membrane components

2. Derivatives of the amino alcohol sphingosine

Question 14

Importance of sphingolipids?

Answer 14

1. Forms a mechanically stable and chemical resistant outer leflet of the cell membrane.
2. Linked to cell signalling and cell recognition

Question 15

What are steroids?

Answer 15

Mostly of eukaryotic origin.
Most common in cholesterol (a sterol)
Cholesterol is a mjor component of cell membrane.
Has a small head group able to easily bind/interact with membrane

Question 16

What are the properties of a cell membrane?

Answer 16

1. Flexible
2. self-sealing
3. selectivly permeable
4. two dimensional

Question 17

What is the Fluid Mosaic Model?

Answer 17

1. Every membrane differs in lipid composition. Typically have asymmetrical distribution between leaflets except for cholesterol.
2. Lipids are in constant motion
3. Proteins can move laterally
4. Microdomains - within a leaflet different sections with varying distributions of lipids. Sphingolipids and cholesterol form lipid rafts which are involved in signalling proteins

Question 18

What are the different formations of biomembranes?

Answer 18

1. Micelle - individual lipids are wedged shaped and they form a single layer ball.
2. Bilayer - most common, two layers of cylindrical lipids, flat
3. Vesicle - bilayer but forms a small globular structure

Question 19

Stability of different formations of biomembranes?

Answer 19

Determined by: size of chains, degree of saturation, size of head group and temperature

Question 20

How is the bilayer held together?

Answer 20

Ionic bonds between the head groups.
Head groups form hydrogen bonds with water in surrounding envirnment.
Van der Waals interactions between fatty acid tails.

Question 21

How can lipid bilayers be made in the lab?

Answer 21

1. treat cells with solvent to remove proteins
2. disperse left over phospholipds in water
3. liposome will form

Question 22

What are the different type of lipid motion in the bilayer?

Answer 22

1. Spinning - they can rotate without changing location
2. lateral diffusion - move within the same leaflet. This occurs 10^7 times per second.
3. transverse diffusion - flip flopping between different leaflets this is very rare

Question 23

How can we show lateral diffusion occurs experimentally?

Answer 23

1. Label cell surface with fluroescence
2. Laser beam a patch on the cell surface
3. Wait for lipids to start moving
4. observe blank patch now contains some fluorescent labelled molecules

Question 24

What causes bilayer to go from gel like to fluidlike?

Answer 24

Heat disorders the interactions between fatty acids tails

Question 25

How do lipids effects membrane properties?

Answer 25

1. Fatty acid chains effect fluidity/rigidity by length and conformation 2. Thickness, different type of lipids can alter the thickness of membrane 3. Head type effects curvature. Head and tails similar forms flat membrane (PC). Head small and tail large forms curved membrane (PE)

Question 26

What aspects of membrane function require curvature of membrane?

Answer 26

1. viral budding 2. formation of vesicle 3. stability of curved structure think microvili in small intestine

Question 27

What causes translocation of lipids?

Answer 27

Enzymes catalyse movement of lipids. Flippase: moves lipid from outside to inside Floppase: moves lipid inside to outside Scramblase: moves lipids in both directions

Question 28

Basics of membrane proteins

Answer 28

- Located in/on membrane bilayer - different membranes have different compositions, lipid:protein ratio - < 1/3 genes code for membrane proteins

Question 29

Functions of membrane proteins?

Answer 29

1. transporters 2. receptors 3. adhesion molecules 4. lipid synthesis 5. energy transduction

Question 30

What are the three types of membrane proteins?

Answer 30

1. Integral 2. Lipid-anchored 3. Peripheral

Question 31

Features of integral protein

Answer 31

1. Firmly associated with membrane - require harsh detergents and organic solvents to remove them. 2. Span the membrane - inside and outside of bilayer. Hydrophobic interactions spanning the membrane and hydrophilic surfaces in the extramembrane domains

Question 32

Features of lipid-anchored proteins

Answer 32

1. Protein covalently linked to a lipid 2. Lipid is embedded in the membrane. 3. the protein does not enter the bilayer

Question 33

Features of a peripheral protein?

Answer 33

1. Released with mild treatment 2. does not contact core of bilayer 3. hydrophilic interactions with membrane surface or other proteins

Question 34

Structures of intergral proteins?

Answer 34

1. transmembrane alpha-helices | 2. transmembrane beta-sheet (beta-barrel)

Question 35

Structure of alpha-helix

Answer 35

Backbone held together by hydrogen bonds. Side chains of amino acids. Embedded in the hydrophobic core. Interactions between hydrophobic amino acids side chains and fatty acid tails. Ionic interaction between head groups. Perpendicular to the plane of the bilayers.

Question 36

Structure of Beta-barrel proteins

Answer 36

- Anti parallel to bilayer. - Form pores (porins) - Some side chains face the membrane and are hydrophobic. - Other side chains face internally creating the pore which is hydrophilic - allows water though. - Not stable - makes a polar section of the membrane

Question 37

Can proteins move in bilayer?

Answer 37

Proteins can move laterally however not as frequently/easily as lipids. Proteins unable to flip-flop.

Question 38

Importance of transmembrane transport

Answer 38

Some molecules can't diffuse through the bilayer. | Cells require some of these molecules that can't get through to function.

Question 39

What is the membrane permeable to?

Answer 39

1. gasses | 2. small unchagred polar molecules e.g.

Question 40

What is the membrane not permeable to?

Answer 40

1. ions 2. large, uncharged molecules 3. charged polar molecules

Question 41

Order of permeability easiest to hardest

Answer 41

1. gasses (CO2, N2, O2) 2. small uncharged polar molecules (Ethanol, water partially) 3. large uncharged polar molecules (Glucose, fructose) 4. Ions (K+, Ca2+, CL-) 5. Charged polar molecules (AMino acids, ATP, glucose, proteins)

Question 42

What are the different transport types?

Answer 42

1. Diffusion 2. Facilitated diffusion 3. primary active transport 4. secondary active transport 5. Ion channels

Question 43

Types of transporter proteins

Answer 43

Uniporter: allows transport in one direction Symporter: allows two different molecules into/out of the cell at the same time (one direction). Antiporter: allows one molecule in and one molecule out at the same time

Question 44

What is passive transport?

Answer 44

Simple diffusion across a cell membrane. Concentration wants to be the same on both sides therefore solutes will move to reach and maintain equilibrium (Chemical gradient). Electrically charged molecules will move to maintain electrochemical gradient.

Question 45

What are the two classifications of transporters?

Answer 45

1. Carriers. Have a high specificity but are slow. | 2. Channel. Have a low specificity but are fast.

Question 46

What is active transport?

Answer 46

Transport of a molecule across the cell membrane that require energy typically as it is acting against the concentration gradient. Can get energy from chemical reaction such as ATP hydrolysis or through coupled transport

Question 47

Classes of ATP-Pumps

Answer 47

1. P-Class 2. V-Class 3. F-Class 4. ABC Superfamily

Question 48

What is a P-Class pump?

Answer 48

Proton Pumps, can be used in ATP synthesis. Found in plants and fungi (H+ pump), mammalian stomach (Na+/K+ pump). Cation transporters. Phosphorylated on Asp as part of cycle. Have a Transmembrane domain with three other domains (A, P and N) branching off in the inter cellular environment.

Question 49

What are F and V Class pumps?

Answer 49

Driven by ATP hydrolysis. | Has a rotating component withing the membrane (F0/V0). Other compenent is external from the membrane (F1/V1).

Question 50

What are ABC transporters?

Answer 50

Pump amino acids, peptides, protein, metal ions, lipids, and compounds (drugs). Uses ATP. Has a transmembrane component and component within the cytosol (NBDs).

Question 51

What is the H+/lactose co-transporter?

Answer 51

Secondary transport. | Lactose goes into cell.

Question 52

What are Aquaporins?

Answer 52

Transport water across membrane. Changes in osmotic pressure causes cells to shrink/swell. Only allows single water molecules through not whole chains.

Question 53

What are Ion Channels?

Answer 53

Differ from transporters allows unrestricted diffusion. Usually have a ligand gate to stop movement. Only open for a ms - very fast. Proton clamp can measure a single ion channel.

Question 54

What is the role of voltage and ligand gated ion channels in neurons?

Answer 54

Important for muscle contraction. AP in neuron opens Ca2+ channel. This opens nicotinic acetylcholine receptor. Depolarisation causes voltage-gated Na+ channel to open - generates AP.

Question 55

What are some consequences of defective ion channels?

Answer 55

Musce stiffness/paralysis. Cystic fibrosis Toxins target ion channels.