1.3 cell membranes and transport Flashcards
what are the factors affecting the rate of diffusion?
- temperature
- concentration gradient
- stirring/moving
- surface area
- distance/thickness
- size of molecule
osmosis definition
diffusion of water molecules that are FREE TO MOVE from an area of higher WATER POTENTIAL to a lower water potential through a selectively permeable membrane
what is water potential measured in?
kilopascals (kPa)
what is the water potential of pure water?
0 kPa
what is water potential defined as?
the tendency of water to leave a system by osmosis (due to the free-to-move water molecules)
(is the pressure exerted by water molecules that are free to move in a system)
is the cell cytoplasm’s water potential more positive or negative? why?
cell cytoplasm contains dissolved sugars, salts etc so water potential is more negative
describe a solution with a high water potential
it has a large number of water molecules that are free to move
hypotonic definition
one of two solutions with MORE free water and LESS solute
hypertonic definition
one of two solutions with LESS free water and MORE solute
what happens to an animal cell in a hypotonic solution?
- water will move down the water potential gradient into the cell
- will eventually burst (bc no cell wall)
- HAEMOLYSED
what happens to an animal cell in a hypertonic solution?
- water will move out of the cell
- the cell content shrinks and the membrane wrinkles
- FLACCID
what happens to a plant cell in a hypotonic solution?
- as water moves into the cell, the cytoplasm and vacuole will push against the cell wall, which will stop the cell getting any larger
- TURGID
what happens to a plant cell in a hypertonic solution?
- as water leaves the cell, the cytoplasm and vacuole shrinks and the plant cell membrane pulls away from the cell wall
- PLASMOLYSIS
what symbol is used to represent water potential and what is it called?
psi ψ
if the concentration of a solution is increased, what would happen to the water potential?
would decrease/become more negative
if the water potential on each side of a cell membrane is equal, what will be reached?
equilibrium
what term refers to a solution with the same water potential as cell cytoplasm?
isotonic
what is the cell membrane made up of?
almost entirely of proteins and phospholipids
what can phospholipida form? (that make them an important component of cell membranes)
bilayers, with one sheet of phospholipid molecules opposite another
in which direction does the inner layer of phospholipids have its hydrophillic heads pointing? (in the cell membrane)
what water does it interact with?
the inner layer of phospholipids has its hydrophilic heads pointing in, towards the cell, and interacts with the water in the cytoplasm
in which direction does the outer layer of phospholipids have its hydrophillic heads pointing? (in the cell membrane)
what water does it interact with?
the outer layer of phospholipid layers point outwards, interacting with the water surrounding the cell
where do the hydrophobic taila of the two phospholipid layers point? (in the cell membrane)
the hydrophobic tails of the two phospholipid layers point towards each other, to the centre of the membrane
what does the phospholipid component of a membrane allow across?
lipid-soluble molecules across, but not water-soluble molecules
what are the two ways in which proteins are embedded in the phospholipid bilayer of the membrane?
- extrinsic proteins
- intrinsic proteins
where are extrinsic proteins found in the membrane?
on either surface of the bilayer
what do extrinsic proteins provide?
structural support and from recognition sites, by identifying cells, and receptor sites for hormone attachment
where are intrinsic proteins found in the membrane?
extended across both layers of the phospholipid bilayer
(they span the membrane from one side to the other)
what do intrinsic proteins do in the membrane?
some are carriers, transporting water-soluble substances across and other allow active transport of ions across, by forming channels
what is the fluid-mosaic model?
a diagram that shows the way in which the phospholipids and proteins are arranged in the membrane
what does the phospholipid bilayer contain?
- extrinsic proteins
- intrinsic proteins
- glycolipids
- glycoproteins
- carrier proteins
- channel proteins
- cholesterol
why is the model of membrane structure called ‘fluid-mosaic’?
- lipid layer fluid/can move (fluid)
- the proteins embedded in the bilayer vary in shape, size and pattern (mosaic of protein molecules irregularly or randomly arranged) (mosaic)
why is cholesterol in the membrane?
- it makes the membrane more rigid anf stable
where are glycolipids and glycoproteins found in the membrane?
in the outer layer of the membrane
what is the carbohydrate layer around the membrane called?
the glycocalyx
chemically, what do phospholipids consist of?
glycerol bonded to two fatty acids and a phosphate group
why are cell membranes described as selectively permeable?
because the hydrophobic layer in the middle of thr bilayer is impermeable to polar and charged particles
(non-polar molecules like oxygen, co2 and fat-soluble vitamind dissolve in the hydrophobic layer and can cross the membrane by simple diffusion
are channel proteins hydrophobic or hydrophillic? therefore, what kind of particles can go through them?
they have hydrophilic R groups lining the space down the middle of the protein
these channels are specific to particular charged or polar particles and allow them to diffuse across the membrane in facilitated diffusion
what is the role of some proteins?
some proteins are specific receptors e.g for hormones or neurotransmitters
some are enzymes e.g ATP synthetase is an intrinsic protein embedded into the cristae of mitochondria
is simple diffusion a passive process?
yes
what is simple diffusion?
the movement of molecules or ions down a concentration gradient, until they are equally distributed
what molecules can diffuse by simple diffusion?
- small molecules
- lipid soluble molecules
- non-polar molecules
what are the two types of proteins used in facilitated diffusion?
- channel proteins
- carrier proteins
are carrier proteins specific to the molecule or ion that they transport?
yes
what is a glycoprotein?
when a protein has a carbohydrate attached, it is called a glycoprotein
(attached to protein)
what are glycolipids?
phospholipids attachrd to carbohydrates
(carbohydrate attached to lipid/phospholipid)
what is the glycocalyx mainly involved in?
cell to cell recognition
(all of the carbohydrates projecting outside of the cell are termed the glycocalyx)
why is the glycocalyx only found on the side of the bilayer that faces out of the cell?
it’s mainly involved in cell to cell recognition
where is cholesterol found in the membrane?
between fatty acid tails
what happens if there is not enough cholesterol in the membrane?
what happens if there is too much?
if there’s not enough cholesterol, the molecule becomes too fluid
if there’s too much, the membrane becomes too rigid
what are some examples of molecules that cross membranes by simple diffusion?
- oxygen
- carbon dioxide
- fat-soluble vitamins
is facilitated diffusion a passive process?
yes
what particles/molecules are transported across the membrane by facilitated diffusion?
- charged particles
- polar molecules
- large molecules
what is co-transport?
where two different particles are transported through the same protein at the same time
(eg glucose and Na+, which are transported into cells together)
in addition to increasing temperatures, what is the rate of facilitated diffusioj dependent on?
the number of protein carriers
is osmosis a passive process?
yes
is osmosis affected by respiratory inhibitors?
no
because respiratory inhibitors that inhibit ATP production do not affect passive transport methods
the higher the concentration of solute in a solution or cell, the ____ the water potential
lower
if glucose or ions are pumped or diffuse into a vell, what happens to the water potential of that cell?
it gets more negative/lower
what are some issues with using blood products in experiments?
- the risk that the blood coule be infected and this could be passed on
- blood clots when removed from
an animal
is there any net movement in an isotonic solution?
no
which of these water potentials is the highest? -650kPa, -800kPa, -750kPa, -700kPa
-650kPa
what does the water potential of solutions depend on?
- the solute concentration (solute potential)
- the pressure exerted on the solution (pressure potential)
(both water and solute potentials are negative values)
(pressure potential is always a positive value)
when it is not physically possible for any more water to enter the (plant) cell, what is thr water potential of the cell?
0
(thr solute potential and pressure potential cancel each other out)
what does active transport do?
moves substances from low to high concentrations (against a concentration gradient) under specific protein carriers and energy from ATP
what is the rate of active transport affected by?
- respiration rate
as ATP is used to pump the substances across the membrane
does aerobic respiration use oxygen?
yes
what affects the rate of respiration?
- the more oxygen available, the faster the rate of respiration and active transport
- increased temperature can result in higher rates of active transport
how does cyanide prevent active transport?
- it is a respiratory inhibutor
- it inhibits cytochrome oxidase, stopping the production of ATP
- as a result, cyanide prevents active transport
(passive transport is unaffected by cyanide)
what is bulk transport?
the movement of solids or liquids
what are two methods of bulk transport?
- endocytosis
- exocytosis
what are the two types of endocytosis?
- phagocytosis (movement of solids like cells or large proteins)
- pinocytosis (movement of liquids)
what is phagocytosis?
the movement of solids - like cells or large proteins
what is pinocytosis?
the movement of liquids
what happens in endocytosis?
- the cell membrane extends around the particles being taken into the cell
- eventually the particles are surrounder and the membrane of the cell fuses around the particle, forming a vesicle in the cytoplasm containing the particle
what is exocytosis?
the mechanism by which large particles such as enzymes and hormones are secreted from cells/leave the cell
(the vesicles containing the particles migrate to the cell membrane and fuses with it, releasing the particles to the outside of the cell)
what is the main function of the cell membrane?
controlling the movement of substances in and out of the cell
(however also containd receptors for other molecules such as hormones and enables adjacent cells to stick together)
what are the three factors that affect the permeability of a cell membrane?
- heat (denatures the MEMBRANE proteins) + phospholipids move more
- ethanol (dissolves the lipid components of the membrane)
- pH (denatures the membrane’s proteins)
what is cytosis?
- a form of active transport where parts of the plasma membrane form infoldings or outfoldings
- there are two types of cytosjs - exocytosis and endocytosis
what is the solute potential?
the contribution of solute to the water potential
why can lipid-soluble molecules move tbrough the cell membrane more easily than water-soluble substances?
because the phospholipid layer is hydrophobic
what is facilitated diffusion?
the passive transfer of molecules or ions down a concentration gradient, across a membrane, by protein carrier molecules in the membrane
are channel proteins open all the time?
no, the channels open and close according to the needs of the cell
what are examples of molecules that need carrier proteins to cross the membrane?
large polar molecules such as sugars and amino acids
how do carrier proteins work?
- a molecules attaches to its binding site, on the carrier protein
- the carrier protein changes shape and releases the molecule on the other side of the membrane
how does active transport take place?
- the molecule or ion combines with a specific carrier protein on the outside of the membrane
- ATP transfers a phosphate group to the carrier protein on the inside of the membrane
- the carrier protein changes shape and carries the molecule or ion across the membrane, to the inside of the cell
- the molecule or ion is released into the cytoplasm
- the phosphate ion is released from
the carrier molecule back to the cytoplasm and recombines with ADP to form ATP - the carrier protein returns to its original shape
what are some examples of processes involving active transport?
- muscle contraction
- nerve impulse transmission
- reabsorption of glucose in the kidney
- mineral uptake into plant root hais
what does the process of active transport occur through?
intrinsic carrier proteins spanning the membrane
what is sodium-glucose co-transport significant in?
absorbing glucose and sodium ions across cell membranes and into the blood in the ileum and the kidney nephron
how does co-transport work?
(e.g sodium-glucose co-transport)
- a glucose molecule and two sodium ions outside the cell attach to a carrier protein in the cell membrane
- the carrier protein changes shape and deposits the glucose molecule and the sodium ions inside the cell
- the glucose molecule and sodium ions seperately diffuse through the cell to the opposite membrane
(the glucose passes into the body by facilitated diffusion and sodium ions are carried by active transport)
what are the two opposing forces that plants cells are under the influence of?
- solute potnetial, due to the solutes in the vacuole and cytoplasm pulling water in. the higher these concentrations, the less likely the water is to move out
- the pressure potential, a force which increases the tendency of water to move out
what is the formula for the water potential of the cell?
water potential of cell = pressure potential + solute potential
is it hypotonic or hypertonic to the cell
if thr water potential of the external solution is less negative (higher) than the solution inside the cell?
it is hypotonic to the cell and water flows into the cell
is it hypotonic or hypertonic to the cell
if thr water potential of the external solution is more negative (lower) than the solution inside the cell?
it is hypertonic to the cell and water flows out of the cell
what happens to the water movement if the external solution and cell are isotonic?
there will be no net water movement
when does endocytosis occur?
when material is engulfed by extensions of the plasma membrane and cytoplasm, surrounding it, making a vesicle
are the vesicles produced by phagocytosis or pinocytosis smaller?
pinocytosis
how does phagocytosis work?
- it’s the uptake of solid materia that is too large to be taken in by diffusion or active transport
- when granulocytes engulf bacteria, a lysosome fuses with the vesicle formed and enzymes digest the cells. the products are absorbed into the cytoplasm
are endocytosis and exocytosis passive processes?
no - they are active processess, using ATP
(the cell membrane has to change shape and this requires energy)
what property of the cell membrane is essential for exocytosis and endocytosis to occur?
the propertu of fluidity of cell membrane
what does the water potential of solutions in a plant cell depend on?
the solute concentration (solute potential) and the pressure generated by the cell wall (pressure potential)
what is the equation for the water potential of a plant cell?
water potential = pressure potential + solute potential
is pressure potential always negative or positive?
postitive
is solute potential always negative or positive?
negative
is water potential always negative or positive?
negative
what are the units for all the potentials? (water potential, solute potential, pressure potential)
kPa
why is it that in plant cells, the water potential also depends on the pressure potential as well as the solute potential?
as cells become turgid, the cytoplasm pushes the cell wall which resists further expansion, exerting a pressure on the cytoplasm and preventing further entry of water
what is the water potential of a plant cell when it is not physically possivle for any more water to enter the cell?
0
(this means the solute potential and pressure potentials cancel each other out)
what is the name of the point where 50% of the cells are plasmolysed?
incipient plasmolysis
a tissue was places in a sucrose solution that had a water potential of -600kPa and was left for an hour.
a student concluded that the solute potential of the cell contents was -600kPa. explain why the students reached this conclusion?
- due to the water potential outside the cell being -600kPa
- approximately 50% of the cells showed signs of plasmolysis
- equilibrium is reached
- solute potential ouside is equal to the solute potential inside
-(no pressure on cell wall so no pressure potential)
explain the role of the cell wall in generating pressure potential in the cell
- K is the cell wall which is inelastic/won’t strech
- as water passes into the cell and cell content expands
- pressure potential is generated by the resistance of the cell wall
in one of the solutions, the students concluded that the cells within the tissue were at incipient plasmolysis.
what observation has the student made that allowed her to make this conclusion?
50% of the cells were plasmolysed
the water potential of a cell at incipient plasmolysis was -430kPa. state the value of the solute potential of the cell and explain how you arrived at your answer
-430kPa
- because there is no pressyre potential acting on the cell wall
- and equilibrium has been reached
- so the water potential of the cell wall will be the same as outside
proteins have hydrophilic and hydrophobic portions depending on what?
the location of the R group in the primary sequence
where are hydrophilic parts with polar/charged R groups located?
around the phospholipid heads or on the outside
where are hydrophobic parts with non-polar R groups located?
they are arranged near/between the hydrophobic tails
what are functions of the cell membrane?
- taking up nutrients/uptake of nutrients
- cell recognition
- controls what enters and leaves the cell
- boundary that separates
- adhesion
- receptor sites
- phagocytosis/secreting chemicals
the smaller the molecule, the ___ lipid soluble it is, the ___ the rate of simple diffusion
more
faster
what are the cells that are used to demonstrate osmosis in animals?
red blood cells as they are found suspended in plasma, not conntected together
are the terms hypotonic, isotonic and hypertonic used for discussing plant or animal cells?
only when disscusing animal cells
do cells in a tissue each have its own water potential?
yes - so cells plasmolyse at different sucrose concentrations
what is an example of a respiratory inhibitor?
cyanide
what are examples of large particles that are secreted from the cell by exocytosis?
enzymes and hormones
phospholipids were extracted from a red blood cell and placed on water to form a monolayer. the area of the monolayer is 12.2 u^2. what is the surface area of the red blood cell?
6.1 u^2 bc the area given is for a single layer but in membranes, phospholipids form a bilayer, so the value must be halved
use your knowledge of the structure and properties of plasma membranes to explain why the membrane proteins inside two cells mix after one hour after fusion?
- reference to fluid-mosaic model
- proteins free to move (within membrane/bilayer)
- (after fusion/1hour) there is a new arrangement of proteins
in the beetroot experiment, where beetroot discs are immersed in a solution of 70% ethanol, and the red pigment begins to leak out of the cell turning it red.
why does this leakage of pigment occur?
- ethanol dissolves lipids/denatures membrane protein
- creates gaps/pores/holes in the membrane/makes more porous
- pigment leaks/diffuses out of CELLS
what are some reasons why transport across the membrane is vital to the cell?
- maintain water potential
- obtaun nutrients/metabolites e.g glucose
- obtain oxygen/remove carbon dioxide
- secrete molecules
- remove toxic substances
what is meant by the term ‘lipid bilayer’?
two layers/double layer of phospholipids
what are some functions of membrane proteins?
- transport
- cell recognition/receptors
what is a disadvantage of decreased fluidity of the hydrophobic parts of the lipid bilayer?
- cells/membranes more easily damaged / cannot pass through capillaries so easily
what are real life examples of active transport?
- uptake of glucose in small intestines
- uptake of soil minerals in plant roots
what is a real life example of facilitated diffusion?
uptake of mineral ions
what is a real life example of simple diffusion?
- gas exchange in the lungs
- steroid absorption
what protein is used in active transport?
carrier proteins
by who was the fluid mosaic model of the membrane structure proposed by? when?
Singer and Nicholson in 1972
what do glycoproteins act as in the phospholipid bilayer?
antigens
what do glycolipids act as in the phospholipid bilayer?
receptor sites for molecules such as hormones
what does a gradient of facilitated diffusion look like for rate of diffusion on y-axis and conc difference across membrane on x-axis?
diffusion?
why?
- facilitated diffusion causes more rapid movement of molecules through a membrane than simple diffusion
- it relies on protein carriers/channel proteins
- and the rate is limited by the number of proteins in the membrane
- therefore the curve flattens out
diffusion - no carriers involved then just limited by conc, so gradient straight line (less steep to begin with but ends higher)
outline the function of intrinsic proteins:
- structural support
- carry water-soluble molecules across the phospholipid bilayer
- form ion channels to enable active transport
outline the functions of extrinsic proteins:
- receptors
- act as antigens, enabling cell recognition
- help cells adhere to each other
what is the glycocalyx?
- a glycoprotein and glycolipid coating surrounding the cell membrane of some cells
what are the factors affecting the permeability of the plasma membrane:
- temperature - as temp increases, the PHOSPHOLIPIDS have more KE and move more, increasing fluidity and permeability of plasma membrane + membrane proteins denature
- organic solvents - they dissolve membranes, disrupting cells. this increases the fluidity and permeability of the plasma membrane
- pH
what happens to the plasma membrane if the temperature becomes too high?
- channel and carrier proteins will become denatured, affecting membrane permeability
- the cell will eventually break down completely
- increased KE, phospholipids vibrate more and move further apart. increases permeability
- proteins within membrane denature
what is incipient plasmolysis?
- the effect produced by placing plant cells in an isotonic solution
- causes the cell membrane to pull away from the cell wall in some areas
- the cell is neither plasmolysed nor turgid
name and describe the process by which secretory products are passed out if the cell [2]
- exocytosis
- transport vesicles fuse with plasma membrane
- break in membrane to allow expulsion of secretion
suggest one problem in performing any investigation using blood [1]
blood clots/infection
describe how an intrinsic protein would transport a molecule against a concentration gradient [2]
- active transport / reference to CARRIER protein
- changing shape
- using ATP/energy
what is the average membrane width?
7/8 nm
for one question, accept 6-10
describe how molecular size affects the rate of diffusion [2]
- small molecules diffuse faster
- higher KE / easier to pass between phospholipid molecules
suggest two reasons why transport across the membrane is vital to the cell [2]
- maintain water potential
- obtain nutrients/metabolites or named e.g glucose
- obtain oxygen/remove carbon dioxide
- secrete molecules
- remove toxic substances or named (not waste products)
explain why the rate of uptake of nitrate ions into the roots increases as the nitrate ion concentration increases [1]
- increasing ion concentration increases chance that molecule will pass through protein/carrier
describe one cellular function of a phospholipid [1]
- component of cell membranes / control of molecules entering/leaving cell
the graph shows that as the concentration of sodium chloride increases from 3 to 6gdm^-3, the number of haemolysed cells decreases. explain why there is a range of concentrations at which haemolysis occurs [2]
- different cells have different ((solute) concentrations / solute potential / water potential (not water concentration)
- each cell would require a different (external water potential/solute concentration) before haemolysis/bursting
describe what happens to a plant cell that was placed in a concentrated salt solution for 30mins [3]
- cell plasmolysis
- cytoplasm decreases in volume
- cell membrane pulls away from cell wall
- vacuole decreases in volume
(pressure potential after 30mins = 0kPa)
permeability definition
the ease/tendency of molecules to pass through a membrane
is cholesterol found in plants??
no only in animals (phospholipid bilayer)
what is the function of glycolipids?
make membranes stable and help cells attach to one another
what is the function of glycoproteins?
recognition sites for substances such as insulin and acetylcholine and act as antigens
how do the phospholipids ensure that the membrane remains fluid? [2]
- not bound within the membrane
- free to move within bilayer/not bonded to one another
