L15 - L19: Cellular Processes Flashcards
cell membrane function (extra)
movement in and out of cell governed by physical forces which allows:
- maintenance of conc. grad.
- spatial organisation of chemical/physical processes within cell
- development of membrane potential
- controlled uptake of nutrients, discharge of waste products and secretion of molecules
membrane structure
thin, 8nm, flexible, sturdy barrier surrounding cell cytoplasm
- 50% lipid, 50% protein held together by hydrogen bonds
membrane lipids structure
two back to back layers of 3 types of lipid molecules:
- phospholipid
- cholesterol
- glycolipids
membrane lipids function
barrier to entry/exit of POLAR substances
phospholipids
- 2 parallel layers of molecules
- each amphipathic: charged polar and uncharged nonpolar region
- membrane thickness determined by tail length
phospholipid abundance
75%
cholesterol
steroid with attached -OH
cholesterol abundance
~20%
glycolipids
lipids with attached carbohydrate group
glycolipid abundance
~5%
glycolipid and cholesterol location
scattered among phospholipid bilayer
membrane protein function
‘gate keepers’ - regulate traffic
- different cells => diff. proteins => diff. properties
peripheral protein features
easily removed
- only bond attaching protein needs to be broken e.g by changing salt conc., washing with other molecules
peripheral protein function example
may attach integral protein to cytoskeleton to bend and give cell shape
integral protein features
not easily removed
- bilayer needs to be broken up e.g by using detergent
transmembrane proteins are
amphipathic
- hydrophobic regions with nonpolar amino acids coiled into helices spanning hydrophobic core of lipid bilayer
- hydrophilic ends interacting with aqueous soln.
amphoteric
- react as both acid and base
membrane protein transport function
- ion channels
- transporter proteins
membrane properties
- fluidity
- selective permeability
- gradients
fluidity
lipids/proteins move within plane but rarely between leaflets => lipid composition of leaflets can be asymmetric
factors affecting fluidity
chemical structure of looped molecules:
- lipid tail length: longer => less fluid
- no. double bonds: more => more packing is upset => more fluid
- amount of cholesterol: more => less fluid
permeability
ability of particular molecules to cross cell membrane
- governed by laws of diffusion
permeability depends on
- size
- charge
- lipid solubility
lipid bilayer is permeable to
- nonpolar, uncharged molecules
- lipid soluble molecules
- small uncharged polar molecules
lipid bilayer is impermeable to
- large uncharged polar molecules
- small ions (due to charge)
transport mediated by membrane proteins
membrane gradients
- conc. grad
- electrical grad
conc grad
- uncharged molecules diffuse down conc grad
- established by selective permeability
electrical grad
- ions influences by membrane potential in addition to conc grad => net ion movement influences by electrochemical grad (effect of both conc and electrical grad)
effect of membrane potential
- maintains difference in charged ions between inside/outside membrane
- membranes mimic capacitors (separate and store charge/energy) => energy generated when ions allowed to flow
energy required to maintain gradients
~30% of resting energy used
- gradient represents stored energy
hyperpolarisation
cell becomes more negative
depolarisation
cell becomes less negative
typical extracellular ion conc
positive
typical cytoplasmic ion conc
negative
Na+ conc
high outside - low inside cell
K+ conc
high inside - low outside cell
- equilibrium at -80mV membrane potential (if less, K+ moves out)
- moves down conc grad until elect grad stops it as leaving -ve charge every time K+ leaves make it more difficult for next K+ to leave
Cl- conc
high outside - low inside cell
- at equilibrium no net movement
- depolarised => Cl- into cell, conc grad takes over
- hyperpolarised => Cl- out, elect grad takes over
passive and active transport classification
- use of energy (passive only uses their kinetic energy)
- direction in relation to grad. (down or against grad)
vesicular transport
moves materials across membranes in small vesicles by exocytosis or endocytosis
non-mediated transport
doesn’t directly use a transport protein (diffuse through bilayer)
diffusion
- random mixing of particles in a solution as a result of particles’ kinetic energy
- more molecules move down conc. grad (net diffusion)
- eventually reach equilibrium - evenly distributed - no net movement
factors affecting diffusion
- steepness/magnitude of diffusion/conc grad: greater difference => faster
- temperature: higher => faster
- mass/size of diffusing substance: larger => slower
- surface area: larger => faster (e.g invaginations in membrane)
- diffusion distance: small => faster
a. cell size: rate sets limit on size of cells at ~20µm
b. membrane thickness: thicker => slower
diffusion function
- nutrient absorption
- waste excretion
- gas exchange
diffusion transports
nonpolar, hydrophobic molecules
osmosis
only occurs if membrane is permeable to water but not to certain solutes
semi-permeable
more permeable to water than solute
osmosis net movement
only in one direction and if an osmotic gradient exists as water moves to eliminate it
ion movement and water movement
closely linked
ion movement => create osmotic difference => stimulate water movement
Pw =
Pd + Pf
Pd
through lipid bilayer
- small
- insensitive to mercury (more to cholesterol)
- temp. dependent (more fluid => more water goes through this way)
Pf
through water channels
- large
- mercury sensitive
- temp. independent (not blocked by lipids in bilayer)
aquaporins
water channels that mediate Pf
- 9 different isoforms each with different permeabilities to water => varying Pw between cells
aquaporins location
- sweat glands have many
- some cells have none:
- very low water flow (Pw) as it only relies on small (Pd) even with osmotic difference
- ion movement without water movement (decoupling)
osmotic pressure
Hydrostatic pressure (acting against) applied to a soln. To prevent inward flow of water across semi-permeable membrane
- opposing force
- colligative property
- proportional to conc. of solute particles that can’t cross membrane
colligative property
depends only on number of solute not types/nature of particles in soln.
osmolarity
measure of solute (molecules/ions) in soln
= øic
- physically measured by osmometers
comparing osmolarity to reference soln
- hyposmotic: lower os
- isosmotic: same os
- hyperosmotic: higher os
water movement based on osmolarity
high to low osmolarity
osmolarity of body fluids
- 280 mOsmol
- maintained as isosmotic => no net water movement => no change in cell volume
- if solute leaks (water leaks) pump throws out again: if leak > pump = cell swelling
change in cell volume is generally
unfavourable esp. in some cells (e.g brain cells)
- body systems are thus involved in controlling osmolarity
tonicity
measure of soln’s ability to change cell volume by altering water content
- depends on membrane permeability of solute
osmolarity and tonicity
not always same thing
- osmosis when solutes impermeable but if they pass, different effect
- e.g urea: isosmotic, hypotonic
tonicity of soln
- isotonic: no change in cell vol
- hypotonic: cause cell swelling, eventually lysis (haemolysis in RBC)
- hypertonic: cause cell shrinkage/crenation
mediated transport
moves materials with help of transport proteins (ion channels, transporters)
ion channel structure
- water-filled pore
- lined by hydrophilic amino acids (ion charges) that shield ions from hydrophobic core
- hydrophobic amino acids facing lipid core of bilayer
- specific amino acids lining pore determine selectivity of channel to specific ions