Cell surface membranes Flashcards
Overton discovery
1895
that fat-soluble substances passed most easily through the membrane, so lipids make up membrane, followed by chemical analysis, showing made up of glycoproteins, proteins and lipids.
gorter and Grendel discovery
enough lipid to form bilayer
structure of a phospholipid
hydrophilic phosphate head
and hydrophobic fatty acid tail
1935 davson and danielli findings
phospholipids form bilayer with hydrophobic tails facing inwards towards non-aqueous centre and phosphate head facing outwards to non-aqueous surroundings.
fatty acid tails form hydrophobic barrier, preventing cell contents from mixing with surroundings.
ALSO: polar proteins form layer each side of lipid bilayer, meaning whole structure turnd out to be 7.5nm.
flaws in davson/danielli model
proteins were found to be globular
turgid
when the surrounding solution has less negative potential and so water flows into the cell down the potential gradient, diluting contents. causing cytoplasm to press hard against the cell wall.
cell wall purpose
prevents cell from bursting, protecting cell
plasmolysed/flaccid cell
when the surrounding solution is more negative and so water has a net flow out of the cell so becomes more concentrated and cytoplasm pulls away from the cell wall.
osmosis
the net movement of water molecules from an area of high water concentration to low water concentration across a partially permeable membrane.
why are animal cells more easily damaged than plant cells by water movement?
because they don’t have a cell wall so will be destroyed if placed in pure water as a large pressure potential will be generated.
what prevents cell damage from pressure potential in the body?
osmoregulation
what prevents aquatic organisms from being damaged by water pressure potential?
often a contractile vacuole aids in the continuous net flow of water out of the cell, preventing the cell from rupturing.
active transport
selective movement of molecules across a membrane, requiring metabolic energy in the form of ATP.
characteristics of active transport
requires energy
highly selective
occurs against concentration gradient.
active transport in cell surface membranes
involve the use of transmembrane proteins known as pumps that react with ATP for specific substances.
different types of protein pumps
transport of a particular molecule/ion
transport of a molecule in the same direction/different directions
sodium potassium pump
useful in neuron axon
pumps 3 sodium out for every 2 potassium in for every ATP molecule used
therefore creates a negative inside to the cell, creating a p.d across the membrane
exocytosis
vesicles of matter across the cell surface membrane
vesicles are often broken off from Golgi apparatus and guided to cell surface membrane via the microtubules of the cytoskeleton to where they merge with the cell surface membrane, invert and then discharge their contents out of the cell.
endocytosis
(phagocytosis) if something touches the cell membrane, the membrane at that point caves in and a vesicle is formed which is drawn into the cell.
mass movement across a cell membrane
what type of transport is it?
exocytosis
endocytosis
ACTIVE TRANSPORT
types of endocytosis
phagocytosis (solid particle transport)
pinocytosis (liquids are carried across)
receptor-mediated endocytosis
Polarity of phospholipid heads?
what property does this introduce?
polar, meaning are hydrophilic as are attracted to other polar water molecules
polarity of phospholipid tails
what property does this introduce?
hydrophobic as are non-polar
polarity
the uneven distribution of charge that occurs in some molecules
how is the phospholipid bilayer visible
via an electron microscope at high magnifications of at least 100,000
model for the cell surface membrane
explanation for name
fluid mosaic model
as both phospholipids and proteins can move about via diffusion and phospholipids amongst the phospholipid heads are scattered as of that of a mosaic
direction of phospholipid movement
frequency
sideways often (free lateral movement) restricted transverse movement
features of phospholipid membrane
bilayer
non-polar hydrophobic interior (tails)
polar hydrophilic exterior facing aqueous surroundings
how do phospholipid saturation affect fluidity of the membrane
more unsaturated, more fluid the membrane
unsaturated phospholipids have double bonds and so kinks in their tails, fitting more loosely.
factors affecting fluidity of membrane
phospholipid tail length
phospholipid saturation
temperature
cholesterol
how does tail length affect fluidity
longer the tail, less fluid the membrane
how does temp affect fluidity
lower temp decreases fluidity
how do organisms who cant regulate their own temperature maintain fluidity?
via increasing the proportion of unsaturated fatty acids
2 types of protein in membrane
integral/intrinsic
peripheral/ extrinsic
transmembrane proteins
proteins which are found spanning the whole membrane
what are transmembrane proteins made out of?
hydrophobic regionsa re made out of one or more alpha helical chains
regions of intrinsic proteins
hydrophilic and hydrophobic
meaning the hydrophobic regions (containing hydrophobic amino acids) are located next to hydrophobic fatty acid tails, repelled by aqueous surroundings
hydrophilic regions are repelled by hydrophobic interior and so face outwards
total thickness of membrane
roughly 7nm on average
3 types of lipid
phospholipid
glycolipid
cholesterol
phospholipids role
form bilayer
non-polar tails and polar heads, therefore making it difficult for polar molecules/ions to pass through, acting as a barrier to water-soluble substances
how can phospholipids be modified chemically
can be modified chemically to act as signalling molecules, activating other molecules, or can be hydrolysed to release small, water-soluble, glycerol related molecules which diffuse through cytoplasm to bind to specific receptors
cholesterol role
hydrophilic heads and hydrophobic tails, fitting between hydrophilic molecules
regulates fluidity, preventing crystallisation
regulates mechanical stability, without it cell membranes will burst open
hydrophobic region prevents polar molecules passing through
frequency of cholesterol in animal vs plant cell surface membranes
plant cell surface membranes have much less cholesterol than animal membranes
absent from prokaryotes
effects of cholesterol on fluidity
at low temps, increases fluidity, preventing close packing, so cells can survive at lower temperatures
also aids in stabilisation of mebrane at higher temps when becomes too fluid
glycolipids and glycoproteins location
periphery of cell membrane, carbohydrate chains project into aqueous surroundings forming hydrogen bonds with the molecules, stabilising structure
glycocalyx
sugary coating on outside of carb chains
animal cell glycocalyx composition
glycoproteins
plant cell glycocalyx composition
glycolipids
glycoproteins and glycolipids function
acts as receptor molecules, binding to particular substances on cell surface
3 groups of receptor
signalling receptors
signalling receptors
coordinat activities of the cell
recognise messenger molecules (eg, hormones) via binding, triggering a series of chemical reactions inside the cell
endocytosis receptors
bind to structures needing to be engulfed
cell markers
(antigens)
allow cell-cell recognition
transport proteins
provide hydrophilic channels/pathways for ions and polar molecules
2 types of transport proteins
channel proteins and carrier proteins
cytoskeleton
system of protein filaments inside cell, determining/maintaining the shape of the cell
signalling pathway
stimuli/signal reaches a receptor which then transmits the signal to a target/effector which brings about a response
process of signalling between cells
typical water-soluble signalling molecule arrives at receptor, bringing about a shape change in the receptor, passing the message to the inside of the cell, allowing it to interact w next component of the pathway
G protein
acts as a switch to bring about rerlease of a second messenger, diffusing through cell. relaying the message / amplifying the message, typically activating an enzyme which in turn activates more enzymes in an activation cascade
3 basic ways in which a receptor can alter the activity of a cell
opening an ion channel (changing membrane potential)
acting as a merman bound enzyme
acting as an intracellular receptor
5 basic methods of transport across the cell membrane
diffusion facilitated diffusion active transport osmosis bulk transport
factors affecting diffusion
steepness of concentration gradient
temp
surface area
diffusion pathway distance
proteins involved in facilitated diffusion
channel/carrier proteins
channel proteins
water-filled pores, allowing charged substances to pass through.
usually gated, allowing control of ion exchange
fixed shape
carrier proteins
change shape, alternatively open to one side of the membrane depending on concentration each side
facilitated diffusion
diffusion of a substance through transport proteins in a cell membrane, providing hydrophilic areas allowing ions to pass through the membrane (otherwise less permeable)
water potential
tendency of water to move out of a solution
factors affecting water potential
water proportion in solution
pressure applied
which direction does water move from?
from high water potential to low water potential
down a water potential gradient until water potential is the same throughout the solution
water potential of pure water at atmospheric pressure
0
solute potential
contribution of the solution to water potential
whats the effect of adding solute on the water potential of a solution
decreases water potential
pressure potential
the contribution of pressure to the water potential of a solution
protoplast
the living part of the cell inside the cell wall
water potential in plant cells
the combination of solute potential and pressure potential
plasmolysis
the process by which the protoplast pulls away from the cell wall
incipient plasmolysis
the point at which the pressure potential has reached zero and plasmolysis is about to occur
why do non-polar molecules travel quicker across the membrane than polar molecules?
as non-polar molecules can dissolve into lipid bilayer and diffuse into the cell whereas polar molecules require transport proteins.
therefore non-polar molecules travel more efficiently, using less energy
technique used by singer and nicholson
freeze fracture
freeze fracturing method
freesing of a cell and then fracturing it so the inner surface membrane can be seen with electron microscopy, splitting apart weak intermolecular bonds holding 2 layers of phospholipid together
steps of freeze fracturing
immersion of cell in chemicals that alter strength of plasma membrane, immobilising macromolecules
cells are passed through a series of glycerol solutions w increasing concentration, protecting from being burst
mounted on gold supports and frozen w liquid propane
fractured in helium-vented vacuum at -150 degrees and colder razor blade fractures
causes evaporation of upper cell surface, creating 3d effect
replica made in gold/platinum w carbon to provide contrast and stability
then raised to room temp and put in distilled water/digestive enzymes, separating sample and replica, rinsing and ready for viewing
How does freeze fracturing prove membrane structure
imprints left on one side of the replica provide evidence for integral proteins, showing proteins are transmembrane,.
cytosis
type of active transport in which plasma membrane folds around substance to transport across a membrane.
