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)
Simple diffusion
- passive transport of substance across the lipid portion of membranes with concentration gradients
- small uncharged molecules move rapidly
- large/charged molecules may be impeded from crossing membranes
- spontaneous
- exergonic
- Gibbs
Factors that influence diffusion
Size and charge (because some molecules are surrounded by a hydration shell, increase the energy barrier for the ion to pass) of molecules
(lipid solubility) for simple
Facilitated diffusion
- passive transport of substances at rates higher than predicted from their lipid solubility
Depends on:
- membrane proteins
- follows concentration gradients
- specific for certain substances
- becomes saturated at high concentrations of transported substance
Where are transport proteins found
- on membrane, can help substances get across still no ATP
Characteristics of transport mechanisms
SIMPLE
- lipids
- no binding of transported substance
- concentration gradient=energy source/transport
- nonspecific molecules get across
- no saturation of high concentrations of transported molecules
FACILITATED
- proteins
- binding of transported substance
- energy source/transport= concentration gradients
- specific molecules get across
- saturated of high concentrations
What do transport proteins carry out
which integral membrane protein
- subset of integral membrane proteins (Must span across membrane)
1) channel proteins: water and ions (ex. K+/Na+)
2) carrier proteins: specific single solutes (sugars, a.a)
Channel proteins
- aquaporin = channel protein
- the concentration gradient follows the direction of transport=diffusion
- gated channel or non gated
Carrier proteins
- protein and binding site will be open where increased concentration site
- protein changing 3D Shape= conformational change
Movement of molecules through carrier proteins
1) conformation so binding site is exposed towards higher concentration
2) solute molecules binds to carrier protein
3) in response to binding, carrier protein changes conformation, binding site is exposed to lower concentration (diffusion=no energy)
4) transported solute is released and carrier protein returns to conformation in step 1
Graph of simple vs facilitated diffusion
facilitated transport: helps things moves faster, but you will also reach a saturation point
- F.T: approaches maximum rate when all transporters are occupied
Osmosis occurs
- across a selectively permeable membrane
- in response to differences in concentration of solute molecules
- selectively permeable membrane must allow water molecules to pass but NOT solute molecules (solute itself cannot cross membrane)
tonicity
water moves:
- from hypotonic solution (lower concentration of solute molecules) to hypertonic (higher concentrations of solute molecules)
- when solutions on each side are isotonic: no NET osmotic movement of water in either direction
Tonicity and osmotic water movement
hypotonic= movement of water into cell increases, can burst
hypertonic=water rushes out of the cell, will kill the cell
isotonic= concentrations are the same, helps maintain integrity
- movement at same rate, net movement=0
what are we moving in Primary vs Secondary active transport
primary- moves positively charged ions (must occur b4 secondary)
secondary- moves both ions and organic molecules
active membrane transport
a) active transport requires a direct/indirect input of energy derived from ATP hydrolysis
b) moves substances against concentrations gradients requiring energy (not spontaneous, energy acts as external aid)
c) depends on membrane transport proteins
d) specific certain substance
e) can be saturated
What are the differences between primary and secondary active transport
a) primary active transport= same protein transport substance also hydrolyzes ATP to power transport directly
b) secondary active transport= transport indirectly driven by ATP hydrolysis (from primary)
- transport proteins don’t break down ATP
- instead use a favourable c.g. of ions as the energy source
what are an example of carrier transport proteins
- pumps
- these pumps allow the charged ion to get through the hydrophobic core (can’t do it themselves)
direct hydrolysis of ATP
primary active transport, c.g. (Down) and direction of transport (Up) in opposite directions
Na+/K+ pump
- REASON FOR AMOUNT: we want a difference in voltage across membranes, creates an electrochemical gradient
- ATP is split into and phosphate, where 3 Na+ go out and 2 K+ come in
PRIMARY ACTIVE TRANSPORT
How do ion pumps maintain membrane potential
through membrane potential is the voltage difference across a membrane
electrochemical gradient- concentration difference and an electrical charge difference
This holds potential energy that we can harness for secondary AT to drive endergonic, non-spontaneous reaction
Symport vs Antiport
both are secondary active transport
a) symport
- cotransport solute moves via channel in same direction
- glucose and a.a.
b) antiport
- provide active transport via channel of another molecule in one direction providing energy for another molecule to go the opposite direction
- also called exchange diffusion
- RBC for coupled movement of Cl-
Lap-Chee-Tsui
- channel protein that allows Cl- to pass also lets water pass: important for digestion and respiration
- genetic mutation: three nucleotide deletion, loss of phenylalanine
- improper folding and structure improper function, not bringing Cl- and water leads to mucus buildup
- CYSTIC FIBROSIS
exocytosis and endocytosis
Form of transport
are active transport
- transport large molceules
- both require energy
How do we get functional proteins at the surface
- through exocytosis, where any proteins on the secretory vesicle membrane doesn’t become part of the plasma membrane
2 main forms of endocytosis
1) bulk phase (pinocytosis)
2) receptor mediated endocytosis: only specific molecules that bind directly to receptor on surface
signal transduction pathway
- a signal on a cells surface is converted into a specific cellular response
- communication is so important because it could mean life/death
Clathirin
protein that plays a key role in the formation of coated vesicles for endocytosis
- forms pocket
familial hypercholesterolemia
- this is when receptor-mediated endocytosis goes wrong:
- on the lining of blood cells LDL receptors bring them into the cell where its cleaned up
- inherited disease: mutation of LDL receptor, the receptor is built improperly so the function is affected, LDL will collect instead forming plaque and clots
result: high cholesterol in blood
- arteriosclerosis (can lead to heart attack)
phagocytosis uses
pseudopods
controlling vs target cell
controlling: Makes special molecules that send signals to change what a target cell does.
target: processes signal in 3 steps
1) reception: uses receptor protein on cell surface to read
2) transduction: processing of information (relay)
3) response: change in cell
what are surface receptors
- integral membrane proteins
- recognize and bind signal
binding of signal molecule
- induces molecular change in the receptor that activates cytoplasmic end
transmits signal
response of surface receptor
1) inactive
2) active
- when signal molecule is bound a conformational change occurs which causes a change in function
- the receptor activates when the signal molecule enters the binding site
signal transduction pathways
- binding of signal molecule to surface receptor triggers cellular response WITHOUT entering cell
- relayed through protein kinases
protein kinases
- enzymes that transfer phosphate from ATP to target proteins= phosphorylation
- stimulates/inhibits activates of target protein producing cellular response
how do we balance cellular response pathways
1) protein phosphatases
- reverse response by pulling off PO4
- turn off signal transduction pathway
How do we move from
- active protein to an inactive protein
the active protein hydrolyzes ATP in each step for a new PO4 to be added on to an inactive protein to turn it active
- each step hydrolyzes ATP
amplification
- increase in magnitude of each step as signal transduction pathway proceeds
-each enzyme activates hundreds of thousands of proteins that enter the next step in pathway, allowing for FULL response with few signal molecules binding to receptors
Ex. signal enzyme activates 10 of 1st molecules in pathway, which then activate 100 in the 2nd pathway, and then 100 000 in the 3rd…..etc etc
what does the mosaic aspect of the fluid mosaic model refer to
the assortment of proteins
what was the frye-edidin experiment
when human and mouse cells were grown in tissue culture, and the cells were fused where they intermixed eahcothers proteins
- proves the fluidity of the fluid mosaic model, proteins move around
freeze fracture technique
- we would freeze cells
- hit them with a knife to expose the inner/out leaflets of the cell where we observe differences in size, number, shape of cells in both sides
- proves asymmetry
- hormones will need to go to the outside, cytoskeleton needs the inside
what is a property all phospholipids have
amphipathic, allowing them to form micelles, a liposome, or a bilayer in water because of the hydrophobic effect where polar molecules exclude hydrophobic molecules
- acts as a barrier to isolate np from (aq)
-
where are sterols found
only in membranes of animal cells, not in plants or prokaryotes
- restrain the movement of lipid molecules
the larger the concentration gradient
the faster the rate of diffusion
gated channels
- type of channel protein found in all eukrayotes
- switch between open, closed, and intermediate states
- opened based on voltage change across the membrane or signalling molecules that bind
carrier-mediated movement of a solute is also called
uniport transport
how does an aquapOrin work
using a succession of hydrogen bonding sites on the channel in the protein
transport pumps
- positively charged ions are moved
- ex. proton pumps
- temporarily bind a PO4 from atp as it pumps
