Cell Membrane Structure

Question 1

Learning Outcomes

Answer 1

• Outline the components of cell membrane
• Understand the structures and functions of cell
  membrane
• describe the roles of membrane proteins
  Reading:
• Alberts et al. Molecular Biology of the Cell, Ch10
• Alberts et al. Essential Cell Biology, Ch11

Question 2

Membrane structure

Answer 2

Question 3

Membrane structure

Answer 3

Question 4

Freeze fracture

Answer 4

Question 4

Plasma Membrane – Lipid bilayer

Answer 4

Very thin film of lipid and protein molecules held together mainly by noncovalent
interactions
dynamic, fluid structures, molecules move about in the plane of the membrane
Lipid bilayer – basic fluid structure, impermeable barrier (water-soluble molecules)

Question 5

Building block #1 – Lipids

Answer 5

About 50% of the mass of most animal cell membranes (the remainder – protein)
~ 5 x 106 lipid molecules in a 1µmx 1µm area of lipid bilayer
109 lipid molecules in the small animal cell membrane
All lipid molecules are amphiphilic (amphipathic)
hydrophilic (“water-loving”) or polar end
hydrophobic (“water-fearing”) or nonpolar end

Question 6

Phospholipid

Answer 6

Question 7

4 major phospholipids

Answer 7

Question 8

Cholesterol

Answer 8

Eukaryotic plasma membrane contain especially large amounts of cholesterol
- up to one molecule for every phospholipid molecule
Orientation of cholesterol – hydroxyl group close to the polar head groups of adjacent
phospholipid molecules when forming bilayer

Answer 9

Question 10

Hydrophilic vs. hydrophobic in water

Answer 10

Question 11

Phospholipids spontaneously form bilayers

Answer 11

Shape/amphiphilic nature of phospholipid cause spontaneous formation of lipid bilayer in
aqueous environments
Minimise the exposure of hydrophobic tails from water molecules – self-sealing property

Answer 12

Question 13

Lipid bilayer is a 2D fluid

Answer 13

Question 14

Membranes in cells get more help

Answer 14

Issues with liposome model
- liposomes do not fuse spontaneously (fusion proteins)
- Rare flip-flop (flippases or phospholipid translocators)
Newly synthesised phospholipid (ER) can form both cytosolic and
non-cytosolic monolayers of lipid bilayer

Answer 15

Question 16

Fluidity depends on temperature and composition

Answer 16

Double bonds (kink) – harder to pack
together - more difficult to freeze
Microorganisms with less temperature
controlling capacity may synthesise more
cis-fatty acid with temperature drop

Cholesterol modulates the properties
- tighten the packing (low permeability to water)
while maintaining fluidity

Answer 17

Fluidity depends on its composition

Question 18

Phase separation

Answer 19

Question 20

Raft domain

Answer 20

Question 20

Phase separation

Answer 20

Weak protein-protein, protein-lipid, and lipid-lipid interactions reinforce one another to
partition the interacting components into raft domains.
Functional group – e.g. converting extracellular signals into intracellular ones
Increased thickness

Question 21

Lipid droplets

Answer 21

Cells store an excess of lipids in lipid droplets
- can be used as building blocks for membrane synthesis or as a food source
- Adipocytes (fat cells) are specialised for lipid storage
- Most cells have many smaller lipid droplets
- Stored lipids are exclusively hydrophobic molecules (triacylglycerols and cholesterol esters)
- 3D droplet with monolayer of phospholipids

Question 22

Lipid droplets

Answer 22

Question 23

**Asymmetry is FUCTIONALLY important**

Answer 23

The asymmetrical distribution of phospholipids and glycolipids in the lipid bilayer of human red blood cells (cholesterols are not shown) Phosphatidylcholine Sphingomyelin Phosphatidylethanolamine Phosphatidylserine glycolipid

Answer 23

Converting extracellular signal to intracellular ones - Protein kinase C (PKC) enzyme requires (-) charged phospholipid – concentrated phosphatidylserine in cytosolic face - fragments of phospholipid act as short-lived intracellular mediators (phospholipase C) - apoptosis: flip-flop of phosphatidylserine from cytosolic to extracellular monolayer

Answer 24

Question 25

Glycolipids

Answer 25

Sugar-containing lipid: glycolipids Exclusively found in the monolayer facing away from cytosol Based on sphingosine similar to sphingomyelin Usually form lipid raft (separation) Cell-recognition processes (cell-cell binding) Entry points for certain bacterial toxins or viruses | SEC

Question 26

Glycolipids

Answer 26

Question 27

Building block #2 – membrane Proteins

Answer 27

About 50% of the mass of most animal cell membranes (the remainder – lipid) ~ 50 lipid molecules for each protein molecule (protein molecules are larger) Lipid provides the basic structure of biological membranes, and proteins give its characteristic functional properties. Highly variable per membrane types <25% mass in myelin membrane to serve electrical insulation for nerve cell axons ~75% mass in internal membrane of mitochondria for ATP production

Question 28

Various ways of membrane protein association

Question 28

Lipid anchors

Answer 28

Question 29

a-helical conformation of transmembrane proteins

Answer 29

Question 30

Many membrane proteins are Glycosylated

Answer 30

Question 30

Transmembrane a helices interact with one another

Question 31

Many membrane proteins are Glycosylated

Answer 31

Most plasma membrane proteins are glycosylated Carbohydrates extensively coat the surface of all eukaryotic cells Enormously diverse Usually <15 sugars Functions Mechanical protection Cell-cell recognition and more | MC

Question 31

Some b barrels form large channels

Answer 31

Question 31

Some b barrels form large channels

Answer 31

Mostly found in the outer membranes of bacteria, mitochondria, and chloroplasts Multipass membrane protein – pore forming proteins Most multipass in eukaryotic cells are constructed from a helices

Question 32

Detergents

Answer 32

Detergent molecules are amphiphilic and cone-shape Forming micelles rather than bilayers Irregular shape due to packing constraints

Question 33

Detergents

Answer 33

Question 33

Na+-K+ pump in phospholipid vesicle

Answer 33

Question 34

Membrane protein purification using detergents

Answer 34

Question 35

Protein diffusion in plasma membrane

Answer 35

Question 36

Membrane proteins with various functions

Answer 36

Question 37

FRAP – fluorescence recovery after photobleaching

Answer 37

Question 38

Confine proteins/lipids to specific domains

Answer 38

Question 39

Cell Membrane

Question 40

Membrane bending proteins deform bilayers

Answer 41