(bio) Unit 4 - Biological membranes Flashcards
Membrane Functions?
- COMPARTMENTALIZATION (eukaryotes): create a separate environments for different activities
- provide a SELECTIVELY PERMEABLE membrane: prevent unrestricted exchange of molecules
- TRANSPORT solutes: exchange of molecules across the membrane
- ENERGY transduction: conversion of one form of energy into another
- respond to EXTERNAL SIGNALS: signal transduction
- SCAFFOLD for biochemical activities
What does a membrane phospholipid consist of?
Polar head group, phosphate, glycerol, fatty acid chains
amphipathic
Molecules that are subject to conflicting forces, polar+non polar components in their structure
- solved by the formation of bilayer to become energetically favourable, stable, and spontaneous
Properties of the bilayer lipid
- closed-no free edges
- self sealing for cell fusion, budding, locomotion
How/why do hydrophobic molecules exclude water?
- clusters together to minimize energy cost of organizing water molecules
Describe the movement of phospholipids within membrane
- constantly moving: spinning in place, travelling laterally within leaflet
- phospholipids ‘flipped’ to opposite leaflet during membrane synthesis
Membrane fluidity affected by..
- TEMPERATURE
- changes in LIPID COMPOSITION that affect alignment of phospholipid tails…
How can the alignment of phospholipids tails change the lipid composition in membranes?
- tightly packed tails -> membrane more viscous, less fluid
- freely moving tails -> higher fluidity
How can the membrane fluidity change when temperature and lipid composition interact?
- temp changes while lipid concentration remains constant
- lipid composition changes with constant temperature
What is Transition Temperature (Tm)?
temperature at which membrane transitions between gel phase and fluid phase
- above Tm; the membrane melts thus lipids are more free
-below Tm; hydrophobic tails pack together thus membrane gels (incompatible with life)
What is the transition temperature (Tm) affected by? (3)
- Altering LENGTH of fatty acid chains (long chains->more interactions-> less fluid more packing (hi Tm to melt))
- Altering DEGREE of SATURATION of fatty acids
(via # of cis double bonds; more-> less packing & more fluid (lo Tm to melt))
- unsaturated-> cis double bonds causes kinks - Altering amount of STEROL (eg cholesterol)
(a buffer, inhibits phase transitions when temp changes)
- hi cholesterol at cool temps -> membrane more fluid
- hi cholesterol at warm temps -> membrane less fluid
How do living cells regulate membrane fluidity? (Countering tight packing due to the environment)
- homeo viscous adaptation; potential low temps that could gel membrane, prevented by altering membrane lipid composition
Dealing with low temperatures:
- shorter fatty acid chain length
- increase # double bonds; desaturase enzymes triggered by low temps(plants, bacteria, fish) ; add kinks to chains
Describe the membrane lipids in the three domains of life (all cells, eukaryotic cells, and archaea)
- all cells have membranes consisting of phospholipids
- eukaryotic cells; fatty acid chains ester linked to D-glycerol
- archaea; branched isoprene chains, ether linked to L-glycerol
The types of Membrane Proteins?
- Transporters
- Anchors
- Receptors
- Enzymes
What are the ways proteins can associate with membranes?
Integral; associated with both sides of leaflets
- transmembrane; across entire membrane, proteins dangle on both sides
- monolayer associated; protein embedded in 1 leaflet
- lipid linked protein
Peripheral; loosley associated
What structures do polypeptide chains cross the membrane as? How can Hydrophilic channels be formed?
alpha helixes
R groups have to be hydrophobic and backbone is polar/hydrophilic
can be formed from several alpha helices
What forms when proteins fold into pleated sheets in membranes? Where is it commonly found?
pores, commonly found in endosymbiont-derived organelles and outer membranes of gram/negative bacteria
How can cells resrtict the movement of membrane proteins?
Cytosolic protein (in cell) holds the membrane protein together
Describe the membrane protein distribution in an Epithelium? Why is it organized this way?
proteins localized in the top (apical surface) and bottom (base) , separated by tight junction proteins to prevent free flow of proteins between sides and top/bottom.
Used to create functional domain
What are eukaryotic cells coated with/extracellular surface? Why?
sugars (carbohydrates), “glycocalyx”
for cell-cell recognition + behaviour
How does membrane preserve its asymmetry during transport processes?
Preserved as membrane moves through endomembrane system
Sugar remains on non cytosolic side (away from cytosol) as it joins plasma membrane (see fig 11, slide 35)
Briefly describe process of secretory pathway (from Unit 0)
Rough ER: protein synthesis that will be exported and inserted into plasma membrane as lysosomes. But they need to be carried as a temporary vesicle first and insert into the golgi body.
Golgi Apparatus: Where the temporary protein carrying vesicles are collected, packaged and distributed
Role of ER in membrane assembly (Secretory pathway)
new phospholipids/fatty acids added to cytosolic side of membrane, scramblases protein transfer random phospholipids to other leaflet so they exist on both sides of the membrane in ER
Membranes with ‘scrambled’ phospholipids (symmetrically distributed) emerge from ER
Role of golgi apparatus in membrane assembly (Secretory pathway)
Membrane from ER arrive to golgi apparatus and its existing membrane. Golgi selectively moves phospholipids with the help of flippases protein to create membrane asymmetry. Moves phospholipids like PS and PE
Membrane asymmetry maintained from this point on
3 ways for substances to enter a cell?
- Directly pass through
- Be transported across bilayer via membrane proteins as carriers
- Be engulfed by the cell to avoid passing through membrane
How do molecules move within a solution
Diffusion - diffused solutes are in constant random motion and will spread out until concentration in all regions are equal
- no NET flux
What can be done if solutes cannot move across a membrane to reach equilibrium?
Water moves down concentration gradient to equalize concentration
; [ ] of water and solutes on both sides are equal as long as water can cross
What is osmosis?
Diffusion of water towards a higher solute concentration through a semi permeable membrane.
Solute - osmotically active particles
What happens when water concentration is equal on both sides?
There is no net movement of water
What is an isotonic Solution?
If concentration of solute is equal inside and outside of cell
What is a Hypotonic solution?
When there is no/little solute on one side of membrane and more on the other side, water will move towards higher [ ]
Thus making cell swell
What is a Hypertonic Solution
When there is more solute outside of the cell than inside, water moves outside towards higher [ ]
Thus making cell shrink
osmotic balance: What is the turgor pressure
The pressure against cell wall in plant and bacterial cells
osmotic balance: What are osmoconformers?
marine animals adjusting their internal salt concentrations to match seawater
“conforms” [ ] to match environment
osmotic balance: What are osmoregulators?
cells have contractile vacuoles that continuously gathers and pumps out water
“regulate” water
ex. terrestrial organisms
What factors need to be considered in order to determine if molecules can pass directly through membranes?
size, polarity and charge
Can small polar molecules pass through membranes?
ex. O2, CO2, N2, Steroid, hormones
Easily cross
Can small uncharged polar molecules cross membranes?
ex. H20, ethanol, glycerol
Not completley freely but can without special mechanisms
Can Larger uncharged polar molecules cross membranes?
ex. amino acids, glucose, nucleosides
Can’t cross to a useful extent
Can ions cross membranes?
No ability
What are the two types of membrane transport proteins
- Carrier protein (shuttle) - change shape to allow passage
- Channel protein (tunnel) - move ions across
What are the similarities between the two membrane transport proteins?
- both allow passageway particular molecules/classes
- most are multi-pass proteins
What are the differences between the two membrane transport proteins?
Channels: anything of a particular size and charge can pass through as long as channel is open
Carriers: one molecules at a time can be transported that has to fit in a specific binding site
Functions of ion channels and some examples
When opened, allow movement of ions DOWN concentration gradient, determined by electrochemical gradient
ex. regulation of cell volume, formation of nerve impulses, secretion of substances into extracellular space, muscle contraction
Features of ion channels
- distinguishes ion charge and size
- highly selective
- faster than carriers
- bidirectional
What is a chemical gradient?
The concentration inside vs outside the cell
What is electrical gradient
Whether ions are being attracted across membrane via oppositely charged molecules or repelled by like charges
What is the electrochemical gradient like for a sodium channel?
Electrochemical gradient where electrical/voltage and chemical gradient work in the same direction
What determines ion selectivity?
Selectivity filter (1) and gate of ion channels (2)
What is the electrochemical gradient like for a potassium channel?
Electrochemical gradient where electrical/voltage and concentration gradient work in opposite directions
What determines whether channels are open or closed? (4 options)
- Voltage gated
- Extracellular ligand
- Intracellular ligand
- Mechanically gated
Do membrane transporter proteins only exist on plasma membrane?
No, it can exist on organelles inside the cell
Function of Carrier proteins
Binding of a solute to specific site of carrier protein changes shape of carrier protein.
Solute moves down concentration gradient, therefore carrier proteins facilitates PASSIVE DIFFUSION.
Bidirectional (many)
Features of Carrier proteins
- high specificity (ex. GLUT1 will move glucose but no fructose)
- passive process (facilitates diffusion)
- saturable; a limited amount of space therefore they can be easily saturated with solute
- can be inhibited/ blocked by substances resembling normal substrate
Describe Passive (what are the 3 ways) vs Active transport (what are the 2 ways) ?
Passive transport involves transported molecules moving down its concentration gradient through the cell membrane.
- simple diffusion, channel mediated , carrier mediated
Active transport involves the movement of molecules against the concentration gradient, requires energy + coupled with energy release
- coupled transport and ATP driven pump
How can Active transport be coupled with energy release?
hydrolysis of ATP, absorption of light, movement of electrons
Sodium-Potassium ATPase; where was it found? How does it work?
found in animal cells, movement of 3 Na+ out for every 2 K+ in which is coupled by hydrolysis of ATP because the addition of Phosphate changes the shape of sodium-potassium ATPase.
Addition of phosphate to the pump trigger the change of shape which allows Na+ to be ejected. Once K+ binds, the pump dephosphorylates itself thus returns to original conformation and K+ is ejected into cytosol.
Contributes to slightly negative charge inside the cell.
(electrogenic -> uneven movement of substances)
Significance of Na/K-ATPase
- membrane protein and an enzyme
- running pump consumes a third of NRG produced by animal cells
- contributor to basal metabolic rate
- maintains Na+ graident (HI outside, LO inside)
- to co-transport other molecules
Purpose of ion pumps in general
Allows cells to contribute to certain substances or set up gradients that can be used to drive other processes.
Coupled transport, what is it?; what are the two directions/ways they could exist in?
Coupled transport is an active transport, moving an ion/small molecules of interest against it’s concentration gradient. But it involves the movement of a co-transporting ion that moves DOWN its gradient.
Symport and antiport
- symport; moving solute of interest and co-transporting ion partner in the same direction (ex. glucose and Na+)
- antiport; going in opposite directions
What is uniport carrier protein?
moving a single molecule through a carrier protein, mediating facilitated diffusion.
ex. glut1 transporter
Are coupled transport (antiporters) also considered ion channels and pumps?
Even though they involve ions, they are not ion channels. They are not pumps either bc they do not involve hydrolyzing ATP
Antiporters aka “exchangers” when ions are moving from either side of the protein.
What is coupled-mediated transport also known as?
indirect active transport, secondary active transport
What are membrane transporters made of?
proteins, encoded by genes
Transporters in a membrane depends on…
- genes present in organisms
- whether or not they are expressed, at that specifc time
In a fluid membrane there is..
free lateral movement of proteins, maintaining their orientations
Compared to the cystolic (inner) leaflet , the non cystolic leaflet on the membrane has..
- lower proportions of PE and PS
- similar cholesterol content
- higher carbohydrate content
A plant living in extremely cold environments would want to..
have a higher proportion of unsaturated fatty acids
Transmembrane proteins stay embedded in membrane because..
the locations of their polar and non polar amino acids
What features distinguish membrane carrier proteins from ion channels?
Membrane carrier proteins can become saturated if the concentration of their cargo molecule is high, whereas channels allow passage of ions at very high rates as long as there is a diffusion gradient.
Transport of glucose across the apical surface of intestinal epithelial cells can be referred to as secondary active transport because…
it is co-transported with Na+ via a symport, such that it indirectly depends on the Na+ gradient maintained by the Na+/K+ pump.
Why don’t bacterial cells burst in a hypotonic solution?
Bacteria have a cell wall that prevents lysis.
