membranes Flashcards
5 roles of membranes
site for cell signalling
exchange of molecules
compartmentalisation
vesicle formation
site of chemical reactions
role of membranes; site for cell signalling
both within and at surface:
membranes have receptors which are specific to certain molecules, so this allows cell-cell communication (when signalling molecule binds, it triggers a series of chemical reactions within the cell)
role of membranes: exchange of moleucles
at surface: specific transport proteins in the membrane allow active transport to take place
within: ensures certain molecules can enter/leave cells e.g. ATP can move out of mitochondria
role of membranes: compartmentalisation
at surface: allows conditions in cytoplasm to remain relatively constant
within: ensures reaction conditions remain constant inside organelles e.g. pH remains constant inside mitochondria so that respiratory enzymes are not affected
role of membranes: vesicle formation
within cells: vesicles form from membranes of RER/Golgi apparatus
role of membranes: site of chemical reactions
at surface: reactions occur at surface when hormones bind to receptors
within: membranes inside organelles e.g. thylakoid membranes and inner mitochondrial membranes are sites of key reactions
how are mitochondrial membranes adapted for chemical reactions
inner mitochondrial membrane is folded to increase surface area for aerobic respiration reactions. contains respiratory enzymes
how are membranes in chloroplasts adapted for chemical reactions?
membranes are in stacks to increase surface area for photosynthetic reactions.
contain chlorophyll and photosynthetic enzymes
phospholipid bilayer structure
2 layers of phospholipid molecules
each phospholipid has 2 ester bonds and a phosphoester bonds
phosphate heads face outwards
fatty acid tails face inwards
why do the phosphate heads of phospholipids in the bilayer face outwards?
they are polar and hydrophilic so can form H bonds with water
therefore they face outwards towards the aqueous environments
why do the fatty acid tails of phospholipids in the bilayer face inwards?
they are hydrophobic and non-polar so face inwards away from an aqueous environment
why is the phospholipid bilayer called a fluid mosaic model?
fluid: phospholipid molecules are constantly moving relative to each other
mosaic: multiple proteins embedded within the bilayer in a pattern
what in the fatty acid tails affects the fluidity of the bilayer?
whether they are saturated or unsaturated
what are the 2 models of the phospholipid bilayer?
when were they created and by who?
davson-danielli model 1935: proteins form distinct layers (sandwich)
singer-nicholson model 1972:
proteins embedded within the bilayer (fluid mosaic)
similarities and differences between the davson-danielli and singer-nicholson models
similarities:
both have phospholipid bilayer
both have proteins
differences:
DD has protein layers which do not penetrate the bilayer
DD contains no glycolipids or glycoproteins
components of the plasma membrane
phospholipid bilayer
integral (transmembrane) proteins (carrier and channel)
peripheral proteins (extrinsic)
cholesterol
glycoproteins
glycolipids
width of phospholipid bilayer
7-10nm
glycoprotein description
protein with a short carbohydrate chain attached
glycoproteins:
function
site of production
act as receptors for specific molecules (cell signalling)
allows recognition of ‘self’ by immune system
act as antigens
produced in Golgi apparatus
glycolipid description
lipid with a short carbohydrate chain attached
glycolipids function
act as receptors for specific molecules (cell signalling).
allows recognition of ‘self’ by immune system
act as antigens
cholesterol location and function
found interspaced between phospholipids
regulates membrane fluidity
integral proteins location and function
span the width of the bilayer
allow passage of large or hydrophilic or charged particles e.g. glucose
highly specific to certain molecules
carrier protein function
changes shape to allow a molecule to pass (usually active transport or facilitated diffusion)
channel protein function
facilitated diffusion
do not change shape
peripheral proteins location and function
do not span bilayer
involved in chemical reactions, could be an enzyme
how is the structure of a phospholipid different from that of a triglyceride?
one of the fatty acid tails is replaced with a phosphate group, forming a phosphoester bond
difference between a carrier protein and a channel protein?
carrier proteins change shape to allow molecules to pass. channel proteins do not
effect of low temperature of membrane
how can this be overcome?
phospholipids have little kinetic energy so move slowly, meaning the bilayer is not very fluid
this can be overcome y cholesterol between phospholipids and unsaturated fatty acid tails, which increase the fluidity and therefore prevent crystallisation
effect of increasing temperature on membrane
phospholipids gain kinetic energy, so move faster and more
they therefore collide more frequently, increasing the fluidity of the bilayer because there are more gaps between to phospholipids
effect of high temps on membrane
proteins in the bilayer denature and large gaps appear
water floods into cells and bursts them
example of a solvent with properties
ethanol
non-polar
lipid soluble
hydrophobic
solvents effect on membrane
ethanol can diffuse directly across the phospholipid bilayer. can also dissolve lipids
therefore it can insert itself in between phospholipids, which creates large gaps
water floods into cells and bursts them
INCREASES BILAYER FLUIDITY
explain how increasing temperature affects membrane fluidity and permeability with/without cholesterol
general trend: as temp increases, membrane fluidity and permeability increase
with cholesterol: linear increase in fluidity as cholesterol maintains fluidity at high temps
without cholesterol: bilayer is more rigid at lower temps and more fluid at higher temps
besides the effect on phospholipids, why else might membrane function be damaged at high temps?
cell signalling: receptors on glycoproteins may change shape
carrier/channel proteins denature so they cannot control what enters and leaves the cell
how can excessive alcohol consumption disrupt membrane function in cells?
alcohol is lipid soluble, so inserts itself between phospholipids and can also dissolve lipids
this increases the fluidity and permeability of the membrane
extra unwanted material can enter and leave the cell e.g. water
cells burst
why must transport into cells across membranes occur?
so that certain molecules can be obtained for chemical reactions e.g. glucose moves into cells in order to be respired
why must transport out of cells across membranes occur?
secreting of large proteins e.g. hormones
excretion (removal) of metabolic waste e.g. CO2, urea
why must transport into/out of cells across membranes occur?
maintains optimum pH for enzyme activity
maintains ionic gradients for neuronal activity
diffusion definition
the passive movement of small molecules (e.g. O2, CO2) or lipid-soluble molecules across a phospholipid bilayer down a concentration gradient
why does diffusion take place?
what does diffusion cause?
because of natural kinetic energy possessed by molecules, which makes them move about randomly.
molecules tend to reach equilibrium, whereby they are evenly spaced
in diffusion:
movement occurs from..
movement doesn’t require…
movement is…
high to low
carrier or channel proteins
passive (no ATP required)
what type of molecules diffuse freely across the membrane?
small, uncharged molecules e.g.O2, CO2
lipid-soluble/hydrophobic/non-polar molecules e.g. ethanol, steroid hormones
both dissolve across the bilayer
how does water move across the membrane and how do these structures aid osmosis?
through aquaporins
increase rate of osmosis by around 1000 times
factors which affect diffusion rate
steepness of concentration gradient
surface area across which diffusion occurs
temperature
nature of molecules/ions e.g. size&polarity
diffusion pathway
carrier/channel protein concentration
how does the steepness of the concentration gradient affect the rate of diffusion?
a greater difference in the number of molecules between 2 areas will increase the rate of diffusion
how does the temperature affect the rate of diffusion?
increased temperature means particles have increased kinetic energy so move faster and more. therefore they spread out faster, increasing the rate of diffusion
how does the surface area across which diffusion occurs affect the rate of diffusion?
increased surface area of membrane means more particles can cross the bilayer at any particular moment in time, increasing the rate of diffusion.
the speed of individual molecules is unaffected
how does the nature of molecules/ions (e.g. size, polarity) affect the rate of diffusion?
larger molecules need more energy to cross a bilayer at the same speed as smaller molecules
non-polar molecules move faster than polar molecules
how does the diffusion pathway affect the rate of diffusion?
shorter diffusion distance means a faster rate of diffusion
how does the concentration of carrier/channel proteins affect the rate of diffusion?
increased concentration of carrier/channel proteins increases the rate of diffusion
what uses facilitated diffusion?
ions (charged particles) and large, polar molecules (e.g. glucose, amino acids)
what does facilitated diffusion occur through?
proteins rather than phospholipids
does facilitated diffusion require energy?
no it is passive and does not use any ATP
concentration gradient for facilitated diffusion?
high to low concentration
through which proteins does facilitated diffusion occur?
mostly channel proteins
sometimes carrier proteins
example of active transport?
sodium potassium pump
ratio for sodium potassium pump
3 Na+ pumped out for every 2 K+ pumped in
what does the sodium potassium pump require?
ATP and hydrolysis reactions
how is the sodium potassium pump carried out?
through plasma membrane
carrier protein=binding site for Na+, K+ and ATP
3Na+ bind to carrier inside cell
carrier protein changes shape due to ATP hydrolysis to allow Na+ ions to exit to extracellular space
2K+ bind to carrier outside of cell
carrier protein changes shape due to phosphate detaching in the ADP phosphate reaction to allow the K+ ions to exit
which type of proteins does active transport use?
only carrier proteins
how does the fluid mosaic model describe the structure of plasma membranes?
phospholipid bilayer has hydrophilic heads facing outwards, hydrophobic tails facing inwards
proteins randomly arranged embedded in bilayer
explain why the graph pigment of beetroot leaking out of cells against temperature accelerates
at low temp, little change in absorbance because the membrane is still intact
at high temp, steep increase in absorbance because the membrane is damaged
explain why surface area to volume ratio of an organism determines whether it needs a circulatory system
large organism has smaller SA:vol ratio
diffusion rate is too slow for sufficient delivery of oxygen, nutrients and removal of waste
what is the role of universal indicator in an experiment where diffusion rate of acid through agar is measured?
to detect presence of acid and measure end point
sodium potassium pump functions
involved in maintaining resting potential of cells
important in neurone and muscle cells
what does ‘PUMP’ mean
ATP is required
examples of bulk transport
exocytosis and endocytosis
why does bulk transport require ATP?
ATP is required to move vesicles and also so that vesicles can fuse with the plasma membrane
exocytosis process
vesicle fuses with plasma membrane of a cell and releases its contents
vesicle becomes part of the existing plasma membrane
examples of endocytosis
phagocytosis (eat)
pinocytosis (drink)
endocytosis process withe example
large molecules taken into a cell
e.g. phagocyte engulfs a bacterium
how does a phagocyte engulf a bacterium?
encloses bacterium in a vesicle called a PHAGOSOME
lysosomes (containing hydrolytic enzymes) fuse with the phagosome forming a PHAGOLYSOSOME (lysosomes releasing enzymes)
endocytosis use
useful products of digestion reabsorbed
non-useful parts removed
experimental evidence for active transport
AT stops in atmosphere of nitrogen gas (no O2 available for aerobic respiration, so no ATP produced, so AT cannot take place)
AT stops when metallic inhibitors added (respiration inhibited so no ATP produced so AT cannot occur)
AT stops at high temps (carrier proteins become denatured so AT stops)
what is the most appropriate term to describe the release of neurotransmitters from a neuronal cell?
exocytosis
what does water potential determine and what is it measured in?
determines the direction in which water diffuses across a membrane
measured in kiloPascal (kPa)
as more solute dissolves in a solution, the water potential….
decreases (becomes more negative)
osmosis definition
the net movement of water across a partially permeable membrane down a water potential gradient
water potential in:
pure water
dilute glucose solution
concentrated glucose solution
0kPa
around -10kPa
around -50 to -100kPa
what is solute potential?
how much solute is dissolved in a solution
always negative
what is pressure potential?
how much turgor pressure is exerted on the cellulose cell wall
always positive
calculation for water potential
water potential= solute potential(-) + pressure potential (+)
what is an isotonic solution?
water potential of solution = water potential of cell cytoplasm
what is a hypotonic solution?
water potential outside cell is greater than the water potential inside the cell
what is a hypertonic solution?
water potential inside cell is greater then the water potential outside of the cell
effect of osmosis on animal cell in hypotonic solution?
water moves in by osmosis
plasma membrane bursts
CYTOLYSIS (haemolysis in RBC)
effect of osmosis on plant cell in hypotonic solution?
water moves in by osmosis into cytoplasm and vacuole
turgor pressure exerted on cellulose cell wall
cell becomes turgid
why do plant cells not burst when placed in pure water?
as cell takes in water by osmosis, contents push against plasma membrane, which in turn pushes against cell wall
however, the cellulose cell wall is strong and prevents the cell form bursting
effect of osmosis on animal cell in hypertonic solution?
water moves out by osmosis
cell shrivels
CRENATION
effect of osmosis on plant cell in hypertonic solution?
water moves out by osmosis
cell becomes FLACCID
IF cell continues to lose water, the plasma membrane detaches from the cellulose cell wall (PLASMOLYSIS)
nerve cells located where have opioid receptors?
brain
spinal cord
alimentary canal
what are opioid receptors stimulated by?
what are these?
example of a substance which mimics the effect of these?
enkephalins
cell signalling compounds
morphine
why are receptors for morphine on the plasma membrane rather than inside the cytoplasm
morphine= large and polar so cannot freely diffuse across phospholipid bilayer
nervous transmission myst be fast so morphine binds to a receptor outside
why does morphine interact with opioid receptors but aspirin doesn’t ?
aspirin is the incorrect shape to bind to an opioid receptor
morphine is complementary to an opioid receptor
what drug, when given just before or after morphine, limits morphine’s effects
what is this overcome by?
naloxone
giving more morphine
in what way does naloxone act to inhibit the effect of morphine
naloxone binds to the opioid receptor (complementary)
acts as a competitive antagonist (does not stimulate receptor)
has higher affinity than morphine
why are morphine and naloxone not effective against cells in the heart and lungs
morphine cannot bind to other receptors present on the cells of the heart and lungs
cells of the heart do not have Mu opioid receptors
cell signalling molecules examples
cytokines
hormones
bulk transport definition
the movement of large amounts of molecules across a membrane.
limitations of PAG: effect of temp on membrane permeability of beetroot
SA:vol ratio of beetroot cores not controlled
cores not all taken from same beetroot
not all water baths thermostatically controlled
damage to membrane using mounted needle
effect of limitation of ‘SA:vol ratio of beetroot cores not controlled’
how to improve
affects rate at which pigment leaks out of beetroot
use callipers to measure dimensions of cylinder and calculate SA:vol ratio. use cork borer
effect of limitation of ‘cores not all taken from same beetroot”
how to improve
beetroots could have been different ages, which would affect the state of the membrane and the concentration of betalain
use same beetroot for all cores
effect of limitation of ‘not all water baths thermostatically controlled’
how to improve
temperature dropped and this leads to an underestimate of membrane permeability
use a thermostatically controlled WB for each core
effect of limitation of ‘damage to membrane using mounted needle’
how to improve
overestimate of permeability
use forceps to extract core
how to modify experiment if a limitation is that some strips of specimen have skin e.g. potato
replace strip or use different strip
do not cut off skin, as this will decrease size
