Cell Membranes & Transport Flashcards
What is the function of the cell surface membrane?
- separates internal cell envi. from external envi
- compartmentalises diff. structures (organelles) within cell
- controls exchange of material across them [partially permeable]
- acts as an interface for communication
What is the structure of the cell surface membrane?
- phospholipid bilayer that contains proteins (intrinsic/integral or extrinsic/peripheral)
- intrinsic = embedded in membrane
- extrinsic = found on outer or inner surface of membrane
What is the structure of a phospholipid?
- 1 hydrophilicphosphate head (polar so soluble in water)
- 2 hydrophobic fatty acid tails (non-polar so insoluble in water)
What happens when phospholipids are shaken w water?
- forms a micelle: sphere w hydrophilic heads facing out towards water + hydrophobic tails facing in towards each other
Why are cell membranes described as fluid mosaic?
- fluid = phospholipids + proteins can move around via diffusion
- mosaic = proteins within phospholipid bilayer produce a scattered pattern
What are the diff. types of molecules cell membranes contain?
- lipids = phospholipids, cholesterol, glycolipids [also contains carbs]
- proteins = glycoproteins [also contain carbs], transport proteins, enzymes
What is the structure of cholesterol?
- hydrophobic tail + hydrophilic head
- fit between phospholipid molecules + orientated same way
- absent in prokaryotes membranes
What is the structure of glycolipids?
- lipids w carbohydrate chains attached
- carb chains project out into fluid surrounding cell
What is the structure of glycoproteins?
- proteins w carbohydrate chains attached
- carb chains project out into fluid surrounding cell
What are transmembrane proteins?
- proteins that span the entire membrane
- e.g. transport proteins
What is the function of phospholipids in cell surface membranes?
- acts as a barrier to most water-soluble substances
- so water-soluble molecules (e.g. sugars, AA + proteins) can’t leak out of cell + unwanted water-soluble molecules can’t get in
What is the function of cholesterol in the cell surface membrane?
- regulates fluidity of membrane
- sits between phospholipids: prevents them being compact when T°Cs are low = stops membrane freezing + fracturing
- cholesterol + phospholipid tail interactions: stabilise cell membrane at higher T°Cs by stopping membrane becoming too fluid
- contributes to impermeability of membrane to ions + inc. mechanical strength + stability of membranes
What is the function of glycolipids + glycoproteins in the cell surface membrane?
- acts as receptor molecules due to carb chains on surface
- allowing them to bind w substances at cell’s surface
- some act as cell markers or antigens, for cell-to-cell recognition
What are the 3 main receptor types created from glycolipids + glycoproteins on the cell surface membrane?
- signalling receptors for hormones + neurotransmitters
- receptors involved in endocytosis
- receptors involved in cell adhesion + stabilisation
What is the function of transport proteins in the cell surface membrane?
- 2 types: channel + carrier proteins
- create hydrophilic channels: allows ions + polar molecules to travel through membrane
- each protein is specific to a particular ion or molecule
- allows cell to control what substances enter or leave
What increases a membranes fluidity?
- an inc. in unsaturated fatty acid chains: bc they are bent so are less tightly packed together so there’s less intermolecular forces
- higher T°Cs: molecules have more energy so move more freely, inc. membrane fluidity
What is the definition of simple diffusion?
- net movement of molecules or ions, down a conc. gradient, from an area of high conc. to an area of lower conc. until evenly distributed
What are the factors affecting the rate at which a substance diffuses across a membrane?
- ‘steepness’ of conc. gradient
- T°C
- SA
- properties of molecules or ions
How does the concentration gradient affect the rate of diffusion?
- greater diff. in conc. = greater diff. in number of molecules passing in the 2 directions = faster rate
How does T°C affect the rate of diffusion?
- higher T°Cs = particles have more KE = move faster so higher rate
How does surface area affect the rate of diffusion?
- greater SA = greater number of particles crossing at any moment = faster rate
How does the properties of molecules or ions affect the rate of diffusion?
- large molecules = slower rate bc require more energy
- non-polar molecules = faster rate bc are soluble in the non-polar phospholipid bilayer
What is the definition of facilitated diffusion?
- passive net movement of particles, down a conc. gradient, from an area of higher conc. to an area of lower conc. w the help of a transport protein
What are e.g.s of substances that can’t diffuse through the phospholipid bilayer of cell membranes?
- large polar molecules (e.g. glucose + AA)
- ions (e.g. sodium ions [Na+] + chloride ions [Cl-])
What are the 2 types of proteins that enable facilitated diffusion?
- channel proteins
- carrier proteins
What are channel proteins?
- water-filled pores
- allow charged substances (e.g. ions) to diffuse through cell membrane
- controls exchange of ions
- fixed shape
What are carrier proteins?
-
switches between 2 shapes
- binding site open on one side of membrane, then opens on other side once shape switches
What is the definition of osmosis?
- movement of water molecules from an area of higher water potential to an area of lower water potential across a partially permeable membrane
What is the water potential of pure water at atmospheric pressure?
- 0kPa
What happens if a plant cell is placed in pure water?
- water enters plant cell through its partially permeable cell membrane by osmosis
- bc pure water has a higher water potential than the plant cell
What does a plant cell being described as turgid mean?
- plant cell is fully inflated w water + has become rigid + firm
What happens when a plant cell is placed in a solution w a lower water potential?
- water leaves plant cell through its partially permeable cell membrane by osmosis
- water leaves vacuole = protoplast shrinks + doesn’t exert pressure on cell wall
- plant cell is plasmolysed
What is the process plasmolysis?
- when protoplast shrinks + begins to pull away from cell wall
- only occurs in plant cells
What happens when an animal cell is placed in a hypertonic solution [solution w a lower water potential]?
- water leaves cell through its partially permeable cell membrane by osmosis + cell shrinks + shrivels up
What happens when an animal cell is placed in a hypotonic solution (e.g. pure water) [solution w a higher water potential]?
- water enters cell through its partially permeable cell membrane by osmosis + cell stretches until it bursts [cytolysis]
What happens when an animal cell is placed in an isotonic environment?
- no change to cell
- bc solution inside + outside cell has the same solute conc.
Describe a method used to investigate the water potential of a plant tissue?
- use a cork borer to cut 5 potato cylinders (same length/mass) + blot them dry to remove excess moisture
- measure + record initial mass then place into 10cm3 sucrose solutions of diff. water potentials
- using a stopwatch leave cylinders in solution for 30 mins
- then remove + dry them to remove excess liquid
- finally measure + record final length + mass of each potato cylinder
- calculate % change in mass for each potato cylinder
How do you find the percentage change in mass of the potato cylinders?
- (final mass - initial mass / initial mass) x100
What do the results of the % change in mass of the potato cylinders indicate?
- pos. % change = potato gained water by osmosis bc solution had higher water potential than potato
- potato cells turgid
- neg. % change = potato lost water by osmosis bc solution had lower water potential than potato
- potato cells flaccid + may be plasmolysed
- no % change = no overall net movement of water into or out of potato cells bc solution had same water potential as potato
How do you find out the conc.of sucrose in the potato cylinders using a graph?
- plot a graph for % change in mass (y) against conc. of sucrose (x)
- where line of best fit crosses x axis = conc. of sucrose
What are the diff. types of data you can have?
- qualitative = non-numerical (e.g. blood group)
- discrete = numerical that can only take certain values in a range (e.g. shoe size)
- continuous = numerical that can take any value in a range (e.g. height or weight)
What is the definition of active transport?
- movement of molecules or ions into or out of a cell from an area of lower conc. to an area of higher conc. using ATP + carrier proteins
Why does active transport require ATP?
- ATP produced during respiration is hydrolysed to release energy
- energy is used to make carrier protein change shape = to transfer particles across cell membrane
What is active transport important in?
- reabsorption of useful molecules + ions into blood after filtration into kidney tubules
- absorption of some products of digestion from digestive tract
- loading sugar from photosynthesising cells of leaves into phloem tissue for transport around plant
- loading inorganic ions from soil into root hairs
What is the definition of co-transport?
- the coupled movement of substances across a cell membrane via a carrier protein
- movement of 1 molecule is dependent on movement of the other
What is an example of co-transport?
- absorption of glucose + sodium ions from epithelial cells lining the mammalian ileum into the blood
Describe the process of co-transport of glucose + sodium ions in the ileum?
- Na+ ions actively transported out of epithelial cell into blood = dec. Na+ conc. in epithelial cell
- = Na+ conc. gradient between ileum + epithelial cell
- Na+ ions move into cell from ileum by facilitated diffusion, carrying glucose molecules (against their conc. gradient) w them via a co-transport protein
- glucose conc. inc. inside epithelial cell, so glucose molecules enter blood via facilitated diffusion
What are the factors that the rate of diffusion depends on?
- T°C
- SA
- conc. gradient
- thickness of exchange surface
What are the 3 main factors the rate of simple diffusion depends on?
- SA
- conc. gradient
- thickness of exchange surface
How does surface area affect the rate of simple diffusion?
- larger SA = higher rate
How does the conc. gradient affect the rate of simple diffusion?
- greater diff. in conc. on either side of exchange surface = higher rate
How does the thickness of the exchange surface affect the rate of simple diffusion?
- thinner = higher rate
What are the 2 main factors the rate of facilitated diffusion depends on?
- conc. gradient
- number of channel or carrier proteins
How does the conc. gradient affect the rate facilitated diffusion?
- greater diff. in conc. on either side of exchange surface = higher rate (until all channel/carrier proteins are being used)
How does the number of channel or carrier proteins affect the rate of facilitated diffusion?
- greater number of channel or carrier proteins available = higher rate (once all proteins in use = rate can no longer inc.)
What are examples of cells adapted for the rapid transport of molecules across their membranes?
- root hair cells
- epithelial cells of the small intestine
How are root hair cells adapted for the rapid transport of molecules across their membrane?
- adapted for absorption of water + mineral ions from soil
- specialised shape to inc. cell’s SA = inc. rate of water uptake by osmosis
- thinner walls so water can move through easily
- have permanent vacuole w cell sap (more conc. than soil water) = maintains high water potential gradient
How are epithelial cells of the small intestine adapted for the rapid transport of molecules across their membrane?
- have microvilli = inc. cell’s SA so inc rate of diffusion of products of digestion
- each villus has a constant blood supply = continually transports products of digestion away from epithelial cells
- maintains high conc. gradient
What are examples of specialised cell for facilitated diffusion?
- neurones
- muscle cells
- some kidney cells
What factors or conditions affects the permeability of cell membranes?
- T°C
- solvent conc.
Describe a method to show how T°C affects the permeability of cell membranes.
- using a scalpel cut 5 cubes of beetroot (same size)
- rinse beetroot pieces to remove any pigment released during cutting
- add beetroot to diff. test tubes containing 5cm3 of water + place each test tube in a water bath at a diff T°C for 30mins
- remove beetroot pieces, leaving coloured liquid
- use colourimeter to measure light absorbance of 5 samples of coloured liquid
Describe the results of how T°C affects the permeability of the cell membrane of beetroot.
- as T°C inc., membrane permeability inc. so absorbance inc. bc more pigment is released
- bc phospholipids in membrane move more bc have more energy = not as tightly packer together inc. permeability
