Unit 5 Flashcards
Biological Membranes
- all cells -> plasma membrane
encloses contents of entire cell - eukaryotic cells -> membrane-bound organelles
-nuclear ‘envelope’ - double membranes of mitochondria and chloroplasts
endoplasmic reticulum
Golgi apparatus
lysosomes / vacuoles
temporary transport vesicles
et al.
Membrane Functions pt 1
- compartmentalization (eukaryotes)
- create separate environments for different activities
- provide a selectively permeable barrier
- prevent unrestricted exchange of molecules
- transport solutes
- exchange of molecules across the membrane
Membrane Functions pt 2
- energy transduction - conversion of one form of energy into another
- respond to external signals - signal transduction
- signals travelling from a distance or from nearby cells
- scaffold for biochemical activities
Membrane Phospholipids
phosphatidyl choline (PC)
phosphatidyl serine (PS)
phosphatidyl ethanolamine (PE)
phosphatidyl inositol (PI)
Movement of Phospholipids within Membrane
- phospholipids are constantly moving
- spinning in place; travelling laterally within ‘leaflet’
- phospholipids are occasionally ‘flipped’ to the opposite leaflet during membrane synthesis but they rarely ‘flop’ back
Lipid bilayers form spontaneously.
- hydrophobic molecules would exclude water, clustering together to minimize energy cost of organizing water molecules
= > energetically favourable - form large droplets or surface film
- are closed – no free edges
- self-sealing
- important feature for cell fusion, budding, locomotion
Membrane Fluidity
how easily lipid molecules move …
- rotationally
- laterally within a membrane leaflet
Membrane fluidity affected by
- temperature
- changes in lipid composition that affect alignment of phospholipid tails
- tightly packed tails -> membrane more viscous, less fluid
- freely moving tails = higher fluidity
- temp changes while lipid composition stays constant
lipid composition changes with constant temp
Transition Temperature (Tm)
- temperature at which a membrane transitions between the fluid phase and gel phase
- above Tm -> membrane ‘melts’ -> lipids free
- below Tm -> hydrophobic tails pack together -> membrane gels -> incompatible with life
Transition Temperature (Tm) and Membrane Fluidity
- cells must maintain fluidity within a relatively narrow range even in the face of changes in environmental temperature
Tm (fluid/gel transition temp) affected by: pt 1
- altering length of fatty acid chains
- longer chains -> more interactions between fatty acid tails -> tighter packing -> less fluid at a given temp
- higher Tm, higher temp to ‘melt’
- range 14-24 carbons in membrane fatty acids
Tm (fluid/gel transition temp) affected by: pt 2
- altering degree of saturation of fatty acids -> # double bonds
more double bonds -> less packing > more fluid at a given temp
- lower Tm, lower temp to ‘melt’
- membrane phospholipids typically have one saturated fatty acid and one with one or more double bonds
trans un sat
double bonds of H, one on each side
cis unsat
double bonds of H, both on same side
saturated
single bonds, 2 H atoms on each side
Tm (fluid/gel transition temp) in eukaryotic cells also affected by:
pt 3
- altering amount of sterol (eg cholesterol)
especially animal cells - can be up to 50% of membrane lipid
- cholesterol acts as a ‘buffer’, inhibiting phase transitions when temp changes
- higher cholesterol at cool temps -> membrane more fluid
- higher cholesterol at warm temps -> membrane less fluid
Regulation of Membrane Fluidity in Living Cells pt 1
homeo viscous adaptation
- maintaining membrane fluidity at temps potentially low enough to cause membrane to enter gel phase by altering membrane lipid composition
Regulation of Membrane Fluidity in Living Cells pt 2
dealing with low temperatures:
- shorter fatty acid chain length
- eg. enzymes that cut C18 C16
- many bacteria
- pond fish dealing with dramatic day / night temp shifts
Regulation of Membrane Fluidity in Living Cells pt 3
- increase # double bonds (= decrease saturation)
- eg. desaturase enzymes triggered by low temps
- bacteria, cold-hardy plants (winter wheat)
- coldwater fish species
Membrane Lipids in the 3 Domains of Life
- membranes of all cells consist of phospholipids.
- glycerol-phosphate plus two hydrocarbon chains
- membrane phospholipids of both eubacterial and eukaryotic cells have fatty acid chains ester-linked to D-glycerol
- archaea have branched isoprene chains instead of fatty acids
- L-glycerol instead of D-glycerol
- ether linkages instead of ester linkages
Polypeptide chains usually cross as
α-helices.
Hydrophilic channels can be formed from
several α-helices
- hydrophilic side chains form an aqueous pore
- α helix amphipathic
- hydrophobic side chains interact w phospholipid tails
Proteins folded into pleated sheets can form
pores.
- common in outer membranes of gram-negative bacteria and endosymbiont-derived organelles
eg sucrose-specific bacterial porin (S. typhimurium)
Cells can restrict the movement of membrane proteins
Membrane Anchoring, Membrane Domains and Compartments, Membrane Protein-Protein Interactions, Membrane Protein-Lipid Interactions
Membrane Protein Distribution in an Epithelium
- protein a (apical surface) helps protein b stays in the basolateral side
Eukaryotic cells are coated with
- sugars.
‘glyco calyx’
Membrane asymmetry is preserved during transport processes.
Sugar is on non cytosolic side — even when it joins the plasma membrane
Secretory Pathway: outward to plasma membrane
- Rough ER: synthesis of proteins for export (secretion), insertion into membranes, lysosomes
- Golgi apparatus: collection, packaging, and distribution
Membrane Assembly: Role of ER
- free fatty acids in cytosol (catalyzed by enzymes bound to cytosolic side of ER)
- New phospholipids added to cytosolic side (scramblases transfer phospholipids to other leaflet)
- Membranes with scrambled phospholipids emerge from ER — random distribution
Membrane Assembly: Role of Golgi Apparatus
- membranes with evenly distributed phospholipids arrive from ER (due to action of flippases phospholipids no longer symmetrically distributed)
- Membrane asymmetry maintained from this point on
Asymmetrical Composition of Membrane Leaflets
- out: phosphatidyl choline (PC), sphingomyelin
- in: phosphatidyl inositol (PI) and phosphatidyl serine (PS), phosphatidyl ethanolamine (PE)
Asymmetry of Lipid Composition
the appearance of PS in outer leaflet of membrane usually indicates that cell is going to die
cytosol is enriched for
PS and PE
Human RBCs as ‘Model Organisms’ for Plasma Membrane
- best understood plasma membrane
- Cells are inexpensive and available in large numbers
- Already present in single cell suspension
- Simple - no nucleus, no ER, no mitochondria, no lysosomes, very pure preps of plasma membranes
Membrane Transport: Overview
- need to allow passage of certain substances in/out of cell — gases, ions, nutrients/waste products
- Lipid bilayers tend to block passage of polar (water-soluble) molecules
- Substances can enter a cell by — passing directly through lipid bilayer, being transported across bilayer by membrane proteins acting as carriers or channels, being engulfed by cell, avoiding passing through the membrane
How do molecules move? -> diffusion
- dissolved solutes (molecules/iosn in solution) are in constant, random motion
- Solutes will spontaneously ‘spread out’ (increasing entropy) until concentrations in all regions are equal — no net flux
- Across semi-permeable membrane
Water also moves down its concentration gradient
Starts with more solute on right side of membrane — solute cannot cross
* Water moves across membrane to attempt to equalize its concentration
Outcomes:
* concentration of water equal on both sides
* concentration of total solute equal on both sides as long as water is allowed to cross
- Water can cross but large solute cannot; next flux of water will be into upper chamber
Osmosis
- diffusion of water across semi-permeable membrane down its concentration gradient (toward a higher solute concentration)
- Once ‘water concentration’ equal on both sides, no net movement of water
- Water concentration depends on total concentration of osmotically active particles (solutes)
- All ions, molecules dissolved in fluid
Osmosis and Cells
- water is constantly moving through cell membrane in both directions
- If osmotic tone (concentration of osmotically active particles) is equal inside and outside cell — intracellular and extracellular fluid are isotonic
- Net movement of water will be toward fluid with higher concentration of solutes
Osmotic effects on cells with walls
- hypotonic solution — lysed
- Isotonic solution — normal
- Hypertonic solution — shriveled
Tonicity (Mammalian Red Blood Cell)
hypotonic solution - cell swells
isotonic solution, no net loss or gain
hypertonic, cell shrinks
Strategies for Maintaining ‘Osmotic Balance’
osmoconformers
osmoregulators
turgor
osmoconformers
most marine organisms adjust their internal salt concentrations to match seawater
osmoregulators
some single-celled eukaryotes have contractile vacuoles that periodically pump out water
- terrestrial organisms carefully regulate the osmolarity of a fluid they circulate through their bodies such that it is iso-osmotic with their cytoplasm
turgor
- most plants are hyper-osmotic to their environment
- water pulled into cell, presses membrane out to cell wall
What molecules can pass directly through membranes?
depends on size, polarity, charge
small non polar molecules
easily can cross
small uncharged polar molecules
still able to cross without special mechanisms
larger uncharged polar molecules
somewhat of an ability to cross - needs help
ions
tiny, no ability to cross lipid bilayer
Membrane Transporter Proteins
- carrier protein (shuttle) — require molecules that fit a particular binding site, one molecule at a time can be transported
- Channel protein (tunnel) — ions across membrane, channels mostly detect size and charge, as long as channel open, it can pass
Ion Channels
- when open, allow movement na, k, ca, cl, down their gradients
- Critical in many cell activities — regulation of cell volume, formation and propagation of nerve impulses, secretion of substances into extracellular space, muscle contraction
- Ion flux determined by both electrical and concentration gradients — “electrochemical” gradient
features of ion channels
- Discriminate on both charge and size
- Usually highly selective
- Much faster than carriers (1000x)
- Bidirectional
Electrochemical gradients drive ion movements.
- ‘chemical’ gradient: concentration inside versus outside
- ‘electrical’ gradient: whether it is being attracted across membrane (by oppositely charged molecules) or repelled (by ‘like’ charges)
What determines ion selectivity?
depends on — gate open — ion has to fit
What determines whether channels are open or closed?
- channels are open or closed depending on the slight negative charge on the inside of the membrane
Carrier proteins mediate facilitated diffusion.
- binding of solute at specific site temporarily changes shape of carrier protein
- Solute is moves down its concentration gradient so carrier protein facilitates passive diffusion — many carriers work in both directions
GLUT1 (glucose transporter on mammalian cells)
- will move glucose, not fructose
- D-glucose but not L-glucose
Features of Membrane Carrier Proteins
- specificity
- Passive (facilitates diffusion)
- Saturable
- Can be inhibited/blocked by substances resembling normal cargo (‘substrate’)
Transporting against a Gradient
Active transport:
* using energy (directly or indirectly) to move ions against their gradient
* Transport closely coupled to energy release — hydrolysis of ATP, absorption of light, movement of electrons
Animal Cells: Sodium-Potassium ATPase
- only present in animal cells
- Moves Na out, K in
- Coupled to hydrolysis of ATP — Change in shape caused by addition of phosphate group — moves 3 Na out for every 2 K in — electrogenic
Significance of Na/K-ATPase
- both a membrane protein and an enzyme
- Present in all animal cells — running this pump consumes — one third of the energy produced by animal cells
- Major contributor to basal metabolic rate — target of many drugs
- Helps maintain a Na+ gradient (high outside cell, low inside)
- Animal cells use this gradient move (co-transport) other molecules — glucose, amino acids
Muscle Cells in Animals:
Ca2+-ATPase = pump
other Membrane Pumps
- H+/K+-ATPase (proton pump) in parietal cells lining stomach
- Pump K+ into cell (against gradient) in exchange for H+
Ion pumps in general: - allow cells to concentrate certain substances or set up gradients that can be used to drive other processes
Carrier Proteins and Coupled Transport
- uniport, symport, antiport — symport and antiport coupled transport
- e.g. Na+ is often co-transported with glucose or amino acids
- E.g. antiporters: H+ exchanged for Na+ or K+
Glucose-Na+ Symport in Intestinal Epithelium
- Na-glucose transporter
- glut 2 facilitated diff.
General Features of Coupled Transport
- membrane carrier protein uses driving force of an ion moving DOWN its gradient to move a solute (small molecule, ion) across the membrane even AGAINST that solutes gradient
- symport — ion and other solute move in same direction
- antiport — opposite directions
- Gradient for ion is created by active transport
- Coupled-mediated transport is thus also known as … indirect active transport, secondary active transport
Pumps in Animal vs Plant Cells
- Animal cell — Na+-K+ ATPase, Na+ driven symport
- Plant cell — H+ ATPase, H+ driven symport
Membrane Transporters and Gene Expression
- each membrane has its own characteristic set of channels and carriers — plasma membrane, lysosomal membranes, mitochondrial membranes (inner, outer),..
- Transporters are proteins, encoded by genes
- The array of transporters present in a given membrane depends on — genes present in that organism, whether or not they are expressed … in that cell, in that patch of membrane, at that time
- A mutation in a widely-expressed membrane transporter can have devastating consequences
e.g. channelopathies (Ion channel diseases)
