Biological membranes

Question 1

2.1.5 a)

What is a plasma membrane?

Answer 1

• a cell surface membrane

Question 2

2.1.5 a)

Describe the role of membranes within cells

Answer 2

• 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

Question 3

2.1.5 a)

Describe the role of membranes at the surface of cells

Answer 3

• 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

Question 4

2.1.5 a)

What is compartmentalisation?

Answer 4

• formation of separate membrane-bound areas in a cell
• vital to a cell because metabolism includes many reactions that are incompatible

Question 5

2.1.5 a)

What are the advantages of compartmentalisation?

Answer 5

• containing reactions in separate parts of the cell allows for different specific conditions required, e.g. chemical gradients
• protects vital cell components

Question 6

2.1.5 a)

Describe what is meant by a partially permeable membrane?

Answer 6

• 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

Question 7

2.1.5 b)

What is a glycolipid?

Answer 7

• 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)

Question 8

2.1.5 b)

What is a glycoprotein?

Answer 8

• protein with a chain of carbohydrate molecule (of varying size and shape) attached to it
• intrinsic protein (embedded in cell surface membrane)

Question 9

2.1.5 b)

What does a glycoprotein do?

Answer 9

• 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

Question 10

2.1.5 b)

What is cholesterol?

Answer 10

• 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

Question 11

2.1.5 b)

Describe the arrangement of phospholipids in a membrane

Answer 11

• 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

Question 12

2.1.5 b)

Who proposed the fluid mosaic model of the cell membrane structure?

Answer 12

• Singer and Nicolson
• in 1972

Question 13

2.1.5 b)

Describe the earliest cell membrane theory

Answer 13

• 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

Question 14

2.1.5 b)

What is an intrinsic protein with relation to membranes?

Answer 14

• 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

Question 15

2.1.5 b)

What are the two types of proteins in the cell-surface membranes?

Answer 15

• intrinsic proteins
• extrinsic proteins

Question 16

2.1.5 b)

What is an extrinsic with relation to membranes?

Answer 16

• 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

Question 17

2.1.5 b)

Describe the role of channel proteins

Answer 17

• 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

Question 18

2.1.5 b)

Describe the fluid mosaic model

Answer 18

• 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

Question 19

2.1.5 b)

How may the cell type affect a cell membrane?

Answer 19

• contain various proteins and lipids
• type and number will depend on particular cell type

Question 20

2.1.5 c)i)

Why is it a problem if membranes lose their structure?

Answer 20

• 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

Question 21

2.1.5 c)i)

Many organisms do not generate heat to maintain their body temperature

How would temperature affect the membrane structure?

Answer 21

• the organisms temperature would vary with environment temperature

Question 22

2.1.5 c)i)

How are the lipids in a membrane affected with a temperature increase?

Answer 22

• phospholipids aquire more kinetic energy and move around more = increases the membrane fluidity
• permeability increases
• begins to lose its structure

Question 23

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

Answer 23

• 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

Question 24

2.5.1 c)i)

How are the lipids in a membrane affected with a temperature decrease?

Answer 24

• 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

Question 25

2.5.1 c)i)

How are proteins affected in a membrane by temperature?

Answer 25

• 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

Question 26

2.1.5 d)i)

What factors affect rate of simple diffusion?

Answer 26

• 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

Question 27

2.1.5 d)i)

How does temperature affect the rate of simple diffusion?

Answer 27

• 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

Question 28

2.1.5 d)i)

How does the diffusion distance affect the rate of simple diffusion?

Answer 28

• 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

Question 29

2.1.5 d)i)

How does surface area affect the rate of simple diffusion?

Answer 29

• 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)

Question 30

2.1.5 d)i)

How does the size of the diffusing molecule affect the rate of simple diffusion?

Answer 30

• small molecules/ions diffuse faster than larger molecules/ions

Question 31

2.1.5 d)i)

How does the concentration gradient affect the rate of simple diffusion?

Answer 31

• 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

Question 32

2.1.5 d)i)

What does a concentration gradient mean?

Answer 32

• 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

Question 33

2.1.5 d)i)

Describe how diffusion happens

Answer 33

• 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)

Question 34

2.1.5 d)i)

What does concentration equilibrium mean?

Answer 34

• 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

Question 35

2.1.5 d)i)

What is facilitated diffusion?

Answer 35

• 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)

Question 36

2.1.5 d)i)

How does facilitated diffusion happen via channel proteins?

Answer 36

• 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)

Question 37

2.1.5 d)i)

How does facilitated diffusion happen via carrier proteins?

Answer 37

• 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)

Question 38

2.1.5 d)i)

What factors affect the rate of facilitated diffusion?

Answer 38

• 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

Question 39

2.1.5 d)i)

How is the diffusion concentration gradient maintained?

Answer 39

• 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

Question 40

2.1.5 d)i)

Give examples of how the concentration gradient across the cell membrane is maintained

Answer 40

• 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

Question 41

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

Answer 41

• 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

Question 42

2.1.5 b)

Describe the role of carrier proteins

Answer 42

• 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

Question 43

2.1.5 c)i)

What effect do solvents have on a membrane?

Answer 43

• 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

Question 44

2.1.5 c)i)

Give examples of solvents that amy disrupt a membrane

Answer 44

• acetone - dissolve lipids (polar)
• ethanol - dissolve lipids (polar)
• benzene - non polar
  
  • presence of molecules of between phospholipid bilayer disrupts membrane

Question 45

2.1.5 d)i)

What is active transport?

Answer 45

• 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

Question 46

2.1.5 d)i)

Why might we need active transport?

Answer 46

• 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

Question 47

2.1.5 d)i)

What is bulk transport?

Answer 47

• 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

Question 48

2.1.5 d)i)

What are the two types of bulk transport?

Answer 48

• endocytosis - bulk transport of material into cells
• exocytosis - bulk transport of material out of cell

Question 49

2.1.5 d)i)

Describe the process of endocytosis

Answer 49

• 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

Question 50

2.1.5 d)i)

What are the two types of endocytosis?

Answer 50

• phagocytosis - for solids
• pino(endo)cytosis - cells ingesting liquids

Question 51

2.1.5 d)i)

Describe the process of exocytosis

Answer 51

• 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

Question 52

2.1.5 d)i)

What is a solute?

Answer 52

• substances dissolved in a solvent
  
  • e.g. water
• forming a solution

Question 53

2.1.5 d)i)

What is a solvent?

Answer 53

• liquid in which a solute is dissolved to form a solution

Question 54

2.1.5 d)i)

What is a solution?

Answer 54

• a liquid mixture in which the minor component (the solute) is uniformly distributed within the major component (the solvent)

Question 55

2.1.5 d)i)

What is water potential?

Answer 55

• 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 (25^oC 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)

Question 56

2.1.5 d)i)

What is osmosis?

Answer 56

• 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

Question 57

2.1.5 d)i)

What is hydrostatic pressure?

Answer 57

• 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)

Question 58

2.1.5 d)i)

What is cytolsis?

Answer 58

• 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

Question 59

2.1.5 d)i)

What is crenation?

Answer 59

• 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

Question 60

2.1.5 d)i)

How is cytolsis and crenation prevented?

Answer 60

• 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

Question 61

2.1.5 d)i)

What happens to an animal cell when it is put in solutions of different water potentials?

Answer 61

Question 62

2.1.5 d)i)

What happens to a plant cell when it is put in solutions of different water potentials?

Answer 62