Biological membranes Flashcards
1
Q
2.1.5 a)
What is a plasma membrane?
A
- a cell surface membrane
2
Q
2.1.5 a)
Describe the role of membranes within cells
A
- membranes in eukaryotic cells
- separate different areas within a cell (organelles) from each other
- separate organelle contents from cell cytosol - therefore each organelle is a discrete entity and can perform own function
- some organelles divided further by internal
- membranes e.g. mitochondria:
- folded inner membranes = cristae
- give a large SA for some reactions involved in aerobic respiration and localise some enzymes needed for respiration to occur
- inner membranes of chloroplasts:
- called thylakoid membranes
- contain chlorophyll
- on these membranes = some of reactions for photosynthesis occur
3
Q
2.1.5 a)
Describe the role of membranes at the surface of cells
A
- plasma mebrane or cell surface membrane
- separates cells components from external environment (cytosol)
- regulates transport of materials into and out of cell
- may contain enzymes involved in specific metabolic pathways
- digestive enzymes on the plasma membranes of epithelial cells - that line the small intestine, these enzymes catalyse some of the final stages in the breakdown of certain types of sugars
- has antigens, so that organisms immune system can recognise the cell as ‘self’ and not attack it
- may release chemicals that signal to other cells (cell signalling)
- contains receptors for chemical signals and therefore is a site for cell communication/signalling e.g. hormones or drugs may bind to membrane-bound receptors
4
Q
2.1.5 a)
What is compartmentalisation?
A
- formation of separate membrane-bound areas in a cell
- vital to a cell because metabolism includes many reactions that are incompatible
5
Q
2.1.5 a)
What are the advantages of compartmentalisation?
A
- containing reactions in separate parts of the cell allows for different specific conditions required, e.g. chemical gradients
- protects vital cell components
6
Q
2.1.5 a)
Describe what is meant by a partially permeable membrane?
A
- cell membrane forms a barrier, separate cell from its external environment
- need to allow some molecules through
- permeability refers to ability to let substance pass through
- very small molecules (like O2) simply diffuse through the cell membrane - in between structural molecules
what passes through - pearson textbook says small molecules diffuse, kerboodle textbook says that polar molecules diffuse, small non-polar can just pass through, and internet says non-polar freely pass through
- lipid-soluble substances - dissolve in lipid layer and pass through
- other substances pass through special protein channels or carried by carrier proteins
- lipid-soluble substances - dissolve in lipid layer and pass through
7
Q
2.1.5 b)
What is a glycolipid?
A
- lipid/phospholipid with a chain of carbohydrate molecules attached
- these molecules also called cell markers or antigens
- can be recognised by cells of immune system as ‘self’ or ‘non-self’ (for cells not belonging to organism)
8
Q
2.1.5 b)
What is a glycoprotein?
A
- protein with a chain of carbohydrate molecule (of varying size and shape) attached to it
- intrinsic protein (embedded in cell surface membrane)
9
Q
2.1.5 b)
What does a glycoprotein do?
A
- plays a role in cell adhesion (when cells join together to form tight junctions in certain tissues)
- are receptors for chemical signals
- when chemical binds to receptor - it triggers a response from the cell
- could be a direct response or set off a chain of events inside cell
- this is known as cell communication or cell signalling
examples:
- receptors for neurotransmitters (e.g. acetylcholine at nerve cell synapses)
- binding of neurotransmitters triggers or prevents an impusle in next neuron
- receptors for peptid hormones, including insulin and glucagon
- which affect uptake and storage of glucose by cells
- some drugs act by binding to cell receptors e.g. β blockers
- used to reduce the response of the heart to stress
10
Q
2.1.5 b)
What is cholesterol?
A
- eukaryotic cell membranes contain cholesterol
- lipid with hydrophilic and hydrophobic end (like phospholipid)
- regulates fluidity of membranes
- cholesterol molecules positioned between phospholipids in membrane bilayer
- hydrophilic end interacts with heads
- hydrophobic end interacts with tails
- cholesterol pulls phospholipids together
- adds mechanical stability without making them too rigid (regulates membrane fluidity)
- cholesterol prevents phospholipids from grouping too closely, and crystallising (becoming too solid)
- resists effects of temperature changes on the structure of the membrane
11
Q
2.1.5 b)
Describe the arrangement of phospholipids in a membrane
A
- phospholipid bilayer
- hydrophilic phosphate heads of phospholipids form both inner and outer surface of the membrane (therefore in contact with watery exterior and interior)
- hydrophillic heads can interact with water
- hydrophobic fatty acid tail regions are in the centre of the membrane = away from water
12
Q
2.1.5 b)
Who proposed the fluid mosaic model of the cell membrane structure?
A
- Singer and Nicolson
- in 1972
13
Q
2.1.5 b)
Describe the earliest cell membrane theory
A
- membranes seen for the first time soon after invention of electron microscopy
- allowed images to be taken with higher resolution and magnification
- images taken in 1950s showed membrane as two black parallel - supporting earlier theory that membranes were composed of a lipid bilayer
14
Q
2.1.5 b)
What is an intrinsic protein with relation to membranes?
A
- intrinsic protein/integrated proteins
- are transmembrane proteins that are embedded through both layers of a membrane
- have amino acids with hydrophobic R-groups on their external surfaces, which interact with the hydrophobic core of the membrane, which keeps the protein in place
15
Q
2.1.5 b)
What are the two types of proteins in the cell-surface membranes?
A
- intrinsic proteins
- extrinsic proteins
16
Q
2.1.5 b)
What is an extrinsic with relation to membranes?
A
- Extrinsic proteins/peripheral proteins
- present in one side of the bilayer
- normally have hydrophilic R-groups on their outer surfaces and interact with the polar heads of the phospholipids or interact with intrinsic proteins
- can be present in either layer, or some move between layers
17
Q
2.1.5 b)
Describe the role of channel proteins
A
- channel proteins = intrinsic proteins
- involved in the transport across the membrane
- provide a hydrophilic channel
- allows passive movement of polar molecules and ions down a concentration gradient through membranes
- held in position by interactions between the hydrophobic core of the membrane and the hydrophobic R-groups on the outside of the protein
18
Q
2.1.5 b)
Describe the fluid mosaic model
A
- builds upon earlier lipid bilayer model
- model that allows passage of molecules through membrane
- fluid mosaic model (consisting of phospholipid bilayer)
- with phospholipids that are free to move within the layer relative to each other (they are fluid = giving membrane flexibility)
- with proteins embedded in the bilayer (varying size, shape, and position) making up a mosaic pattern
- some proteins can move also = more ‘fluid’
- model structure = explains how cell membranes could be more dynamic and interact with the cells’ environment
19
Q
2.1.5 b)
How may the cell type affect a cell membrane?
A
- contain various proteins and lipids
- type and number will depend on particular cell type
20
Q
2.1.5 c)i)
Why is it a problem if membranes lose their structure?
A
- membranes control the passage of substances into and out of cells (or organelles)
- if membrane loses structure
- loses control of what enters and leaves the cell
- increases permeability of membrane = easier for particles to cross it
- lots of factors (e.g. temperature, presence of solvents) affect membrane structure
- some cells need intact membranes for specific functions
- e.g. transmission of nerve impulses by neurones (nerve cells)
- when neuronal membranes are disrupted, nerve impulses are no longer transmitted as normal
21
Q
2.1.5 c)i)
Many organisms do not generate heat to maintain their body temperature
How would temperature affect the membrane structure?
A
- the organisms temperature would vary with environment temperature
22
Q
2.1.5 c)i)
How are the lipids in a membrane affected with a temperature increase?
A
- phospholipids aquire more kinetic energy and move around more = increases the membrane fluidity
- permeability increases
- begins to lose its structure
23
Q
2.1.5 c)i)
Give examples of what will be affected if the cell membrane has an increased permeability, due to an increased temperature
A
- will affect the way membrane-embedded proteins are positioned
- may affect infolding of plasma membrane during phagocytosis
- may change ability of cells to signal to other cells by releasing chemicals, often by exocytosis
- to an extent, presence of cholesterol buffers (resists) effects of increasing temperature, as it reduces the increase in membrane fluidity
24
Q
2.5.1 c)i)
How are the lipids in a membrane affected with a temperature decrease?
A
- decreasing temperature = lowers kinetic energy = molecules move more slowly
- saturated fatty acids of phospholipid become compressed
- also many unsaturated fatty acids in the cell membrane, as they become compressed, kink in their tails push adjacent phospholipid molecules away
- maintains membrane fluidity
- therefore proportions of unsaturated and saturated fatty acid within a cell membrane, determines membranes fluidity at cold temperatures
- cholesterol in the membrane also buffers (resists) the effect of lowered temperature to prevent reduction in the membranes fluidity
- does this by preventing phospholipid molecules from packing together too closely
- cholesterol molecules = between groups of phospholipid molecules
- some organisms, e.g. fish, microorganisms and some plants, can change composition of fatty acids in their cell membranes in response to lowered temperatures
25
Q
2.5.1 c)i)
How are proteins affected in a membrane by temperature?
A
- changing temperature affects the movement of phospholipids does not alter integral molecular structure
- but proteins = not as stable as lipids - temperature does affect proteins integral molecular structure
- High temperature (increase in KE) - cause atoms in a large protein molecule to vibrate - this breaks hydrogen bonds and ionic bonds
- bonds that hold their structure together = broken
- protein unfolds - as a results - tertiary structure/shape changes and cannot change back when cooled (protein = denatured)
- carrier + channel proteins in membrane will dentature at high temperatures
- these proteins = involved in transport across membrane, as they denature, permeability is affected
- if membrane-embedded enzymes denature - they will cease to function, if shape of their active site
- under plasma membrane = cytoskeleton threads (made of protein)
- if both embedded proteins + cytoskeleton threads become denatured - plasma membrane will begin to fall apart
- will become more permeable because holes will apear
26
Q
2.1.5 d)i)
What factors affect rate of simple diffusion?
A
- simple diffusion relies only on molecules own kinetic energy
- therefore factors altering this kinetic energy will affect the rate of diffusion
- temperature
- diffusion distance
- surface area
- size of diffusing molecule
- concentration gradient
27
Q
2.1.5 d)i)
How does temperature affect the rate of simple diffusion?
A
- increase in temperature = increase in rate of diffusion
- because molecules have more kinetic energy and move at higher speeds
- decrease in temperature = decrease in rate of diffusion
- because molecules have less kinetic energy and move at slower speeds
28
Q
2.1.5 d)i)
How does the diffusion distance affect the rate of simple diffusion?
A
- particles move at high speeds and are constantly colliding
- this slows down the overall movement
- means that over short distances - rate of diffusion = faster
- as distance increases - rate of diffusion = slower
- because more collisions
- thicker membrane (exchange surface) which molecules have to diffuse across = slower rate of diffusion
29
Q
2.1.5 d)i)
How does surface area affect the rate of simple diffusion?
A
- more diffusion can take place across a larger exchange surface
- cells that are specialised for absorption
- have extensions to their cell surface membranes called microvilli (which increase surface area)
30
Q
2.1.5 d)i)
How does the size of the diffusing molecule affect the rate of simple diffusion?
A
- small molecules/ions diffuse faster than larger molecules/ions
31
Q
2.1.5 d)i)
How does the concentration gradient affect the rate of simple diffusion?
A
- steeper concentration gradient (greater the difference in concentration between two regions)
- = faster rate of diffusion
- to region with fewer molecules (down concentration gradient)
- faster because overall net movement from higher concentration to lower concentration will be larger
32
Q
2.1.5 d)i)
What does a concentration gradient mean?
A
- concentration difference
- goes from high to low concentration
- Diffusion goes down a concentration gradient
- takes a lot more energy to move substances up/against a concentration gradient
33
Q
2.1.5 d)i)
Describe how diffusion happens
A
- passive transport method
- doesn’t involve metabolic energy (ATP)
- instead utilises energy from the natural notion of particles
- may or may not happen across a membrane or barrier
- Diffusion = net movement of particles (atoms, molecules, ions) from region of higher concentration to region of lower concentration
- Diffusion happens because all molecules have kinetic energy and can move freely + randomly within gas/liquid
- if there is a high concentration of a molecule in a region
- then molecules will collide with each other as they randomly move
- will eventually spread further from each other and move to a region of lower concentration
- this movement is random and will continue until evenly dispersed (concentration equilibrium = reached)
- when the molecules have moved down their concentration gradient - they are still moving randomly but remain evenly dispersed
- = no diffusion (because no net movement)
34
Q
2.1.5 d)i)
What does concentration equilibrium mean?
A
- when there is a balance in concentrations
- = no diffusion (because no net movement)
- Equilibrium doesnt mean particles stop moving but movements are equal in both directions
- remain evenly dispersed - despite random movements
35
Q
2.1.5 d)i)
What is facilitated diffusion?
A
- movement of molecules from a region of high concentration to region of lower concentration
- across a partially permeable membrane
- via protein channels or protein carriers
- does not involve metabolic energy (ATP)
36
Q
2.1.5 d)i)
How does facilitated diffusion happen via channel proteins?
A
- phospholipid bilayer of membranes are barriers to polar molecules + ions
- small molecules that have polarity (e.g. ions)
- are insoluble in lipid
- and therefore cannot interact with the hydrophobic tails of the lipid bilayer
- they diffuse through water-filled protein channels (pores) embedded in the membrane
- around 0.8nm in diameter
- membranes with protein channels = selectively permeable
- as most protein channels are specific to one molecules/ion
- in facilitated diffusion
- movement of molecules = down concentration gradient
- and does not require external energy (ATP)
37
Q
2.1.5 d)i)
How does facilitated diffusion happen via carrier proteins?
A
- facilitated diffusion can involve carrier proteins
- Glucose molecules = too large
- cannot diffuse through the water-filled protein channels in a membrane
- however can bind to a transmembrane carrier protein
- carrier protein changes shape when a specific molecule binds (in this case Glucose)
- then opens to allow molecule (glucose) to pass out on the other side of the membrane
- there are specific carrier proteins for different types of molecules
- in facilitated diffusion
- movement of molecules = down a concentration gradient
- and does not require external energy (ATP)
38
Q
2.1.5 d)i)
What factors affect the rate of facilitated diffusion?
A
- temperature
- concentration gradient
- membrane surface area
- thickness of membrane
- number of channel proteins present
- number of carrier proteins present
- more channel proteins + carrier proteins = higher rate of diffusion overall
39
Q
2.1.5 d)i)
How is the diffusion concentration gradient maintained?
A
- many molecules entering the cell then pass into organelles and are used up for metabolic reactions
- this maintains the concentration gradient and keeps more of the molecules entering the cell
40
Q
2.1.5 d)i)
Give examples of how the concentration gradient across the cell membrane is maintained
A
- O2 diffusing into the cytoplasm of respiring cells then diffuses into mitochondria and used in aerobic respiration
- CO2 diffusing into palisade mesophyll cells of a plant leaf will then diffuse into chloroplasts and be used for photosynthesis
41
Q
2.1.5 d)i)
Explain the effect of having different proportions of transmembrane protein channels and transmembrane carriers in different cell types and give examples
A
- because different cell types have different proportions of transmembrane protein channels and transmembrane protein carriers
- allows cells to control the types of molecules that pass in and out
- neurone plasma membranes have many channels specific to either sodium ions or potassium ions
- diffusion of these ions into and out of the neurone axon is crucial for the conduction of nerve impulses
- at synapses (gaps between neurones)
- there are also calcium ion channels and may be chloride ion channels
- plasma membrane of epithelial cells that line airways have chloride ion channels
- these play a crucial role in regulating composition of mucus to trap particles and pathogens
42
Q
2.1.5 b)
Describe the role of carrier proteins
A
- carrier proteins = intrinsic proteins
- involved in transport across membrane
- play role in both passive (down a concentration gradient) transport
- and active (against a concentration gradient) transport
- these membrane proteins have two regions
- first: a specific site which reversibly binds with specific complementary molecules/ions
- second: region that binds and hydrolyses molecules of ATP to release energy
- for this reason they can be regarded as enzymes
- energy helps carrier protein change its conformation (shape) and in doing so
- it carries the ion from one side of the membrane to the other side to allow molecule to pass out
43
Q
2.1.5 c)i)
What effect do solvents have on a membrane?
A
- Water = polar solvent
- essential in the formation of phospholipid bilayer
- non-polar tails of the phospholipids are orientated away from the water
- form bilayer with a hydrophobic core
- charge (polar) phosphate heads interact with water helping to keep bilayer intact
- organic solvents = less polar than water
- e.g. alcohols
- organic solvents will dissolve membrane, disrupting cells
- alcohols - used in antiseptic wipes
- dissolve the membranes of bacteria in a wound
- killing them + reduces risk of infection
- pure/strong alcohol = toxic, because they destroy cells in the body
- less concentrated = not dissolve membranes but still causes damage
- non-polar solvents
- e.g. benzene
- can enter cell membrane and presence of these molecules between the phospholipids disrupts the membrane
- when membrane = disrupted
- becomes more fluid and more permeable
44
Q
2.1.5 c)i)
Give examples of solvents that amy disrupt a membrane
A
- acetone - dissolve lipids (polar)
- ethanol - dissolve lipids (polar)
- benzene - non polar
- presence of molecules of between phospholipid bilayer disrupts membrane
45
Q
2.1.5 d)i)
What is active transport?
A
- movement of molecules/ions into or out of a cell
- from region of lower concentration to region of higher concentration
- against concentration gradient
- process needs more energy than kinetic energy of the molecules
- also needs carrier proteins to supply molecules energy by the hydrolysis of ATP
46
Q
2.1.5 d)i)
Why might we need active transport?
A
- cells/organelles may need more of a particular molecule/ion
- that they cannot get enough of by simple diffusion or facilitated diffusion
- e.g. root hair cells use active transport to absorb ions from the soil
47
Q
2.1.5 d)i)
What is bulk transport?
A
- another form of active transport
- requires energy from ATP
- used to transport cells and particles that are too large to pass through the plasma membrane or by protein channels and protein carriers
- e.g. enzymes, hormones and whole cells like bacteria
48
Q
2.1.5 d)i)
What are the two types of bulk transport?
A
- endocytosis - bulk transport of material into cells
- exocytosis - bulk transport of material out of cell
49
Q
2.1.5 d)i)
Describe the process of endocytosis
A
- transport of material into cells
- how large particles may be brought into the cell
- do not pass through plasma membrane
- instead a segment of the plasma mebrane invaginates (bends inwards)
- happens when it comes into contact with the material to be transported
- membrane surrounds the particle until it eventually fuses, forming a vesicle
- vesicle pinches off and moves into the cytoplasm to transfer material for further processing within cell
- e.g. vesicle containing bacteria = digested by enzymes
- ATP needed to provide energy for:
- forming vesicles
- changing shape of cells to engulf materials (bend inwards)
- fusion of cell membranes
- moving vesicles
- along cytoskeleton threads using molecular motor proteins
- forming vesicles
50
Q
2.1.5 d)i)
What are the two types of endocytosis?
A
- phagocytosis - for solids
- pino(endo)cytosis - cells ingesting liquids
51
Q
2.1.5 d)i)
Describe the process of exocytosis
A
- Bulk transport of materials out of cells
- how large particles may be exported out of cells
- do not pass through plasma membrane
- instead vesicles formed by Golgi apparatus is moved towards and then fuses with the plasma membrane
- contents then released outside of the cell
- ATP needed to provide energy for:
- moving vesicles along cytoskeleton threads using molecular motor proteins
- and for fusing vesicles to the plasma membrane
52
Q
2.1.5 d)i)
What is a solute?
A
- substances dissolved in a solvent
- e.g. water
- forming a solution
53
Q
2.1.5 d)i)
What is a solvent?
A
- liquid in which a solute is dissolved to form a solution
54
Q
2.1.5 d)i)
What is a solution?
A
- a liquid mixture in which the minor component (the solute) is uniformly distributed within the major component (the solvent)
55
Q
2.1.5 d)i)
What is water potential?
A
- measure of the relative tendency of water molecules to diffuse from one region to another
- measured in kilopascals (kPa)
- symbol for water potential is ψ
- Pure water has the highest possible water potential
- given the value of 0kPa
- at standard temperature and atmospheric pressure (25oC and 100kPa)
- all solutions have a negative water potential
- the more concentrated the solution (the more solute)
- the more negative the water potential (lower water potential = more negative)
56
Q
2.1.5 d)i)
What is osmosis?
A
- particular type of diffusion
- doesn’t involve metabolic energy (ATP)
- instead utilises energy from natural notion of water molecules
- Osmosis = net diffusion of water molecules down their water potential gradient (more negative to less negative)
- or from a region where there are relatively more water molecules to a region where there are fewer water molecules
- across a partially permeable membrane
- osmosis happens because:
- all water molecules have kinetic energy and move randomly
- and because water molecules can pass directly through the phospholipid bilayer but solutes cannot
- some membranes have protein channels (aquaporins) which can allow water molecules to cross more rapidly (therefore reach an equilibrium faster)
- if there is a high concentration of water inside a cell
- molecules will collide with each other as they randomly move and eventually spread further from each other
- the molecules will move out of the cell where there is a relatively lower water
- this will continue until equilibrium is reached
- while molecules will still be moving there will be no net movement
57
Q
2.1.5 d)i)
What is hydrostatic pressure?
A
- diffusion of water into a solution = increase in volume of solution
- if solution is in a closed system e.g. cell
- results in an increase in pressure
- this pressure is called hydrostatic pressure
- has the same units as water potential (kPa)
58
Q
2.1.5 d)i)
What is cytolsis?
A
- if an animal cell is placed in a solution with a higher water potential (less negative) than its cytoplasm
- water will move into the cell by osmosis increasing the hydrostatic pressure inside the cell
- all cells habe thin cell-surface membranes (around 7nm) and no cell walls
- the cell-surface membrane cannot stretch much and cannot withstand the increased pressure
- the cell will swell and burst - when this happens it is called cytolsis
59
Q
2.1.5 d)i)
What is crenation?
A
- if an animal cell is placed in a solution with a lower water potential (more negative) than its cytoplasm
- water will leave the cell to the solution by osmosis across a partially permeable pasma membrane down the water potential gradient
- this will cause a reduction in the volume of the cell and the cell to shrivel
- this is described as crenation and the cell is crenated
60
Q
2.1.5 d)i)
How is cytolsis and crenation prevented?
A
- multicellular animals usually have control mechanisms to make sure their cells are continuously surrounded by aqueous solution with an equal water potential (isotonic)
- in blood the aqueous solution = blood plasma
61
Q
2.1.5 d)i)
What happens to an animal cell when it is put in solutions of different water potentials?
A
62
Q
2.1.5 d)i)
What happens to a plant cell when it is put in solutions of different water potentials?
A
