Cell Membrane Structure Flashcards
Learning Outcomes
- 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
Membrane structure
Membrane structure
Freeze fracture
Plasma Membrane – Lipid bilayer
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)
Building block #1 – Lipids
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
Phospholipid
4 major phospholipids
Cholesterol
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
Hydrophilic vs. hydrophobic in water
Phospholipids spontaneously form bilayers
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
Lipid bilayer is a 2D fluid
Membranes in cells get more help
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
Fluidity depends on temperature and composition
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
Fluidity depends on its composition
Phase separation
Raft domain
Phase separation
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
Lipid droplets
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
Lipid droplets
Asymmetry is FUCTIONALLY important
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
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
Glycolipids
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
Glycolipids
Building block #2 – membrane Proteins
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
Various ways of membrane protein association
Lipid anchors
a-helical conformation of transmembrane proteins
Many membrane proteins are Glycosylated
Transmembrane a helices interact with one another
Many membrane proteins are Glycosylated
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
Some b barrels form large channels
Some b barrels form large channels
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
Detergents
Detergent molecules are
amphiphilic and cone-shape
Forming micelles rather than
bilayers
Irregular shape due to packing
constraints
Detergents
Na+-K+ pump in phospholipid vesicle
Membrane protein purification using detergents
Protein diffusion in plasma membrane
Membrane proteins with various functions
FRAP – fluorescence recovery after photobleaching
Confine proteins/lipids to specific domains
Cell Membrane
Membrane bending proteins deform bilayers
