Lecture 4: Membranes and Signalling Flashcards
Structure of membranes: fluid mosaic model
- fluid lipid bilayer in which proteins are embedded and float freely
- membranes aren’t rigid (dynamic structures)
function of fluid mosaic model
1) protection
2) allow selective exchange of molecules between cell and environment: need the exchange of energy and matter
what do peripheral proteins do for communication
- link microtubule to membrane, they sit there to help with communication or transportation
purpose of integral protein
- also called transmembrane protein
- interact with lipid bilayer/aq environment
- ex. collagen is an integral component of EC matrix
purpose of cholesterol
maintains fluidity (low temp) and rigidity (high temp) of mosaic model
HIGH TEMP
- F.A. chains move freely, cholesterol fits in between to restrict movement
LOW TEMP
- F.A. chains pack tightly, making the membrane rigid
membrane asymmetry
- membranes are asymmetrical
- membrane proteins of one half of the bilayer are structurally and functionally distinct from the other half
important because: specialization (inner and outer), cell signalling, stability
How can we support the idea that membranes are fluid
- membrane proteins start out segregated
- proteins move around in the membrane proving that they are fluid
what does freezing do to your cell
- rigidity in the cell
- slows down processes
how does membrane asymmetry support the fluid mosaic model
- maintains fluid part of the model through the uneven distribution of lipids and proteins
the lipid fabric of a membrane
1) phospholipids are the dominant lipids in membranes
2) membrane fluidity
3) organisms can adjust fatty acid composition
How will water behave in a phospholipid bilayer
phospholipid bilayers will rearrange to form a hydrophilic outer layer facing the water and a hydrophobic inner layer facing each other
maintaining proper fluidity
- fluidity of lipid bilayer dependent on how densely individual lipid molecules can pack together
BASED ON:
1) temperature
2) composition of lipid molecules
- these will help indicate the fluidity of our membrane
COMPOSITION OF LIPID MOLECULES
viscosity and fluidity
- we want to maintain balance, over fluidity can cause leakage
A) VISCOUS
saturated hydrocarbon tails, made by linear fatty acid chains
B) FLUID
unsaturated hydrocarbon tails with kinks, unsaturated fatty acid are:
- non-linear increases fluid
- can’t fit as many
- less dense, more fluid
TEMPERATURE
relative to fluidity
TEMPERATURE DROPS
- phospholipid molecules become closely packed and membrane forms highly viscous semisolid gel
- fluidity of membrane related to degree to which membrane lipids are unsaturated
alternate: unsaturated fatty acid + saturated fatty acid chains, helps maintain proper fluidity
Adjusting fatty acid composition
1) proper fluidity is maintained by adjustment of fatty acid composition
2) DESATURASES: enzymes that produce unsaturated fatty acids during fatty acid synthesis
3) Regulation of desaturates— membrane fluidity
All phospholipids are first
made linear
Temperature affects
the ratio of unsaturated fatty acids to saturated fatty acids which determine fluidity
DESATURASES
- Desaturases are enzymes that introduce double bonds into fatty acid chains, converting saturated fatty acids into unsaturated fatty acids, which affects membrane fluidity and lipid composition.
- LINEAR: INCREASE DENSITY, DECREASE FLUIDITY
- BENT: DECREASE DENSITY, INCREASE FLUIDITY
Increase temperature, decreases…
Desaturases
- as temperature increases, the relative amount of desaturate % likely means there’s too much packing which decreases density and increases fluidity
- but it will drop to counteract over fluidity, and produce saturated lipids to promote rigidity
Sterols
ex. cholesterol
- 4 C ring
- type of lipid, buffers for fluidity
- at high temperature: decreases fluidity
- at low temperatures: increases fluidity
cholesterol
- helps decrease density and increase fluidity
membrane buffer
ex. cholesterol
- stabilize membrane fluidity
- maintain structural integrity
- prevents changes in fluidity with temperature variations
Four key functions of membrane proteins
1) TRANSPORT:
membrane proteins = transport proteins
2) ENZYMATIC
membrane proteins=enzymes
3) SIGNAL TRANSDUCTION
membrane proteins= send signals
4) ATTACHMENT/RECOGNITION
membrane proteins= adhere to cytoskeleton in the cell and ECM outside of cell
(can be used as recognition proteins)
What are the two structural categories of membrane proteins
1) intergral membrane proteins
2) peripheral membrane proteins
1) Integral membrane proteins
- proteins embedded in phospholipid bilayers (interacts with hydrophobic core of the bilayer)
-composed of non polar a.a. coiled into alpha-helices
- transmembrane proteins (sub category): most integral proteins
what are most integral membrane proteins
transmembrane proteins
- Channels, transporters, or receptors
2) Peripheral membrane proteins
- on the surface of the membrane
- don’t interact with hydrophobic core
- held together by hydrogen and ionic bonds via interaction with the lipids or exposed portions of integral membrane proteins
- most on cytoplasmic side of membrane, they form part of the cytoskeleton helping with anchorage
- made up of mixture of polar and non polar a.a.
Passive membrane transport
- Based on diffusion
- SPONTANEOUS
- Two types: SIMPLE and FACILITATED
(simple=small molecules can squeeze in)
(facilitated=use electorchemical gradient) - Two groups of transport proteins carry out facilitated diffusion
what is osmosis
the passive diffusion of water
passive membrane transport
- hydrophobic nature of membrane restricts free movement of molecules
- PASSIVE TRANSPORT: movement across a membrane without ATP
- driven by diffusion
Diffusion
- net movement of substance from region of high to low concentration
- rate of diffusion depends on the
1) concentration difference
2) concentration gradient (on how different the two concentration are)