cell membranes

Name of model proposed by singer and Nicholson 1972

Fluid mosaic model

Explain the name of the accepted plasma membrane structure

Fluid → protein and PL molecules are free to move

Mosaic → protein molecules embedded in the membrane

Explain how the distinction of charged and uncharged parts determine the position a protein will take up in a membrane

Charged particles will associate with hydrophilic heads

Uncharged particles will associate with hydrophobic tails

For lipid soluble molecules mode of transport, component of the membrane through which it passes, factor affecting rate of transport

MoT: diffusion

Comp: PL bilayer

Factor: concentration gradient

For water soluble molecules in high external concentration

MoT: facilitate diffusion

Compon: channel/ carrier proteins

Factor: concentration gradient/ size of molecule/ pH

Water soluble molecules in low external concentration

MoT: active transport

Compon: carrier proteins

Factor: respiratory rate, ATP production/ pH

Explain the pattern of results of temp against colorimeter reading referring to cell membrane structure

Membrane proteins are stable between .. and .. (at low temps)

Appearance of the red pigment means that the tonoplast and cell membrane has been damaged between .. and … (high temp)

Meaning pigment has leaked out of cell/ sap vacuole/ change in membrane permeability/ damaged membrane

Proteins change shape and denature and come out of bilayer

Explain the difference in results if you were investigating the effect of ethanol on the permeability of the membrane

Dissolve phospholipids/ destroys cell membrane structure

So pigment leaks out

Suggest one problem in performing any investigation using blood

Blood clots/ infection

Explain why the time taken for the RBC to burst (haemolysis) occurred quickest when placed in distilled water

in distilled water Water has highest water WP

Water passed down water potential gradient- from high to low water potential

Passes into the cell by osmosis

Explain

Allows results to be comparable

Initial masses would differ

A sample of RBC were placed in a concentration solution of sodium chloride draw a diagram to show the expected appearance of one of the blood cell after 5 minutes

Mark scheme says: crinkled cell, high WP inside cell, water passes out of cell causing shrinkage / distortion

Difference between this model and currently accepted fluid mosaic model

Have PL bilayer, hydrophobic takls and hydrophilic heads , have proteins

However for the F-M model proteins are embedded in the lipid layer, there are instrinsci proteins as well as extrinsic

Also the F-M model there are glycocyx, glycolipids, glycoproteins

An investigation was carried out on the uptake of pottasium ions by root tissue stete with reasons the variables that should be kept constant

Temperature or pH BECAUSE

they change enzyme activity, affect reaction rate

Explain why as the O concentration increases the rate of pottasium ions also increases

Active transport is the main method of transport of pottasium ions

Which is energy dependent, uses energy from ATP and oxygen is required for aerobic respiration for ATP production

Greater O concentration produces greater uptake

State the rate of uptake you would expect of a drop of cyanide solution had been added to each of the four solution

They would all be .. (the starting rate of ion uptake)

cyanide inhibits aerobic respiration which prevents or reduces ATP production when no oxygen is present there is still some uptake by diffusion which is a passive process

Explain why increasing the temp increases the permeability of the cell membrane and result in more .. being realised

PL and protein molecules movement increases as temp increases

Creates gaps between PL molecules

Protein loses tertiary structure as hydrogen bonds weaken/ break

Pores in membrane proteins get larger

More .. can escape more quickly

Methods of transport that transports individual molecules or ions

All methods except endocytosis and exocytosis (bulk) : D, FD, OSM, AT

Don’t understand the last question on the FA on membranes, it is too confusing but mention plasma membrane model: fluid mosaic model

Describe the function of the PL bilayer

Physical barrier, selectively permeable membrane allows diffusion or transport of small non polar molecules

Function of intrinsic or channel/ carrier proteins

Allows FD / AT/ transport of large or polar molecules or ions

Explain why no glycocalyx is present in the inner mitochondrial membrane

No need for recognition/ receptor

Explain why there are more carrier/ channel proteins in inner mitochondrial membrane than cell membrane

More FD/ active transport

Maybe transport of ATP

Evidence that P ions can occur both by AT and Diffusion explain how evidence supports this conclusion

Absorption is highest with oxygen which indicates active transport as this requires ATP

Also occurs in the absence of oxygen which indicates diffusion as this is not dependent on respiration passive process

Explain how the properties of the head and the tail of the chlorophyll molecules results in its position in the PT memrbane

Hydrophobic tail and fatty acid hydrophobic regions of the memrbane

Head is hydrophilx

Suggest an adv of the position of the chlorophyll molecules

Holds chlorophyll head in optimum position for absorption of light

Explain how glucose is reabsorbed from the filtrate into the blood even at very high blood glucose levels

Na+ ions actively transported out of epithelial cell lowering the concentration of Na+ inside

Na+ in filtrate at higher concentrated than inside the cell

Na+ Co transported with glucose into epithelial cell

Glucose concentration in epithelila cell is higher than blood plasma

Glucose moves out of epithelial cell into blood by FD

Circulation maintains plasma concentration below that in epithelial cells maintains concentration gradient

Explain the distribution of Na+ across the membrane

More Na+ outside than inside

Actively transported out of axon by pump

Relatively impermeable to Na+ CJ channels shut

Explain why Na+ ions don’t diffuse across PL bilayer despite concent gradient

Ions are hydrophilic and memrbane has hydrophobic tails

explain in terms of WP why the transmission was lowest between NaCl concentration 0 to 3

water moves in by osmosis, higher WP inside than outside of cell, down a WP graident into the cell, no cell wall to preevent bursting, most .. is released therefore lowest transmission of light 

as the concentration of NaCl increases the number of bursted (haemolysed) cells decreases explain why there is a range of concent at which haemolysis occurs

different cells have different water potentials or solute potentials so each cell would require a different external, water potential or solute concentration before haemolysis or bursting

what would happen to the cell after it had been placed in a concentrated solution

cell plasmolysis, vacuole and cytoplasm would decrease in volume, cell membrane pulls away from cell wall

state the pressure potential of the cell after it had been placed in a concentrated solution for 30 mins

0 kPa

explain why temp needs to be controlled in experiment of permeability

would affect KE of the molecules which would allow more pigment to escape , proteins would be denatured 

explain the effect of increasing the concent of the NaCl solution on the permeabilty of the membrane

as the concent increases the absorbance decreases , less absorbance means less pigment released, more NaCl reduces pigment released, more NaCl the less permeable the membrane

explain why it is not directly proportional

as the graph is not a straight line it is a curve

explain trend of pigment (absorbance)- pigment is stored within the cell vacuole when the membranes are disrupted the pigment leaks out into surrounding solution

as the temp increases the membrane becomes more fluid as PL gain more KE and more gaps form

proteins then start to denature where there is a large increase in absorption , then at higher temps membrane is fully destroyed so all pigment is released

explain how acid and ethanol affect the membrane there is a lot of red pigment in the solution 

pigment leaks/ diffuses out of cells through the CM as acid denatures the proteins changing their shpae and ethanol dissolves the PLs

why has the beetroot in the water increased in size

water entered the cells by osmosis as the beetroot cells had a lower WP than the water

the movement of glucose from the cell to the bloodstream requires cncent gradient explain how this is maintained

blood flows removing glucose that has been absorbed and lowers glucose concent in bloodstream

the cell mebrnae is adapted to these 3 functions show is mitochond, RER, GB, ribosomes so for active transport of substances from the cell into the blood

mitochondria produces ATP is AR, transporting substances against concent gradient

synthesis of enzymes

ribosomes make proteins, enzymes are proteins

determine the WP of the potato used from experiment results

where it crosses the x axis (% change in mass is 0) this is where the WP of the external solution = the WP of tissue it is the isotonic solution

explain why there was a gain in mass

the external WP is higher than the WP of the cell so water moves in by osmosis gaining mass

explain why there was a decrease in mass

the external WP is lower than the WP of the cell so water moves out by osmosis so decreasing the mass

explain why there was no decrease in mass at any of sucrose concent for sweet potato

sweet potato has a very low WP a very high sugar concent

how to modify this

increase the sucrose concent until a point is reached where water will flow out of the potato 

State which temp provide the most reliable results and explain why

The highest temp where all three experiments gave the same absorbance - the cell had ruptured, membrane proteins were denatured and all pigment has leaked out of cells

Describe how intrinsic proteins would transport a molecule against a concentration gradient

By active transport , using energy from ATP

To determine the structure of the plasma membrane membrane proteins from different cells were labelled using different coloured dyes explain the results after fusion using knowledge of the structure and properties of plasma membrane

Fluid mosaic model, , proteins are free to move within the membrane

After fusion there is a new arrangement of proteins

The value of the membrane width

7-8 nm

Vitamin A is lipid soluble explain how it crosses the membrane

Dissolves in the phospholipid bilayer and is transported by diffusion

Glucose is water soluble, explain how it it crosses the membrane

Cannot pass through the phospholipid layer so it uses transport proteins, transported by facilitated diffusion

Explain the leakage of pigment when the beetroot is placed in ethanol

The ethanol dissolves the phospholipid and makes the proteins denature creating gaps in the membrane making membrane more porous

When the experiment was repeated at a higher temp the time taken for ethanol to turn red decreased

Increased temp increases KE of dye or membrane molecules which increases the rate of diffusion of dye across the membrane , dye diffuses across the e membrane more rapidly

Explain lipid solubility and rate of diffusion through the membrane

As lipid solubility increases the rate of diffusion increases because membrane contains a bilayer of phospholipids , lipid soluble substances can diffuse or move through the membrane more easily than water soluble molecule s

Why can small molecules diffuse faster than larger molecules

They have a higher KE

It is easier to pass between phospholipid molecules

Factors affecting the rate of facilitated diffusion

Concentration difference / gradient

Temp

Number of carrier or chem El proteins

Vitamin B is water soluble explain the route taken by it into the cell is determined by the structure of the plasma membrane

polar, cannot pass through the phospholipid bilayer which is hydrophobic, uses transport proteins / channel proteins which are hydrophilic

Vitamin K is fat soluble

It is non polar, so it dissolves in the hydrophobic phospholipid bilayer so it can pass directly through the phospholipid

Explain the name fluid mosaic

Fluid- lipid layer is fluid as it can move

Mosaic of protein molecules are they are randomly arranged

Describe the function of channel proteins

Facilitated diffusion of polar hydrophilic molecules

Explain the meaning of active transport

Movement against a concentration gradient that requires energy from ATP

Why is transport across the membrane vital to the cell

Obtain nutrients and obtain oxygen

Remove CO2 and remove toxic substances

Maintain water potential

Explain what is meant by the term lipid bilayer

Double layer of phospholipids

State the hydrophobic parts of the memrbane

Fatty acids

State functions of the membrane proteins

Form hydrophilic pores

Active transport

Facilitated diffusion

Don’t understand 5b PMT questions by topic

Name the process by which oxygen is ansorbed by the roots given the graph is a straight line (oxygen concentration rate of uptake)

Diffusion as rate is proportional to concentration

Explain why as nitrate ion concentration increases the rate of uptake increases

Increasing ion concentration increases the chance that molecules will collide with carrier protein

When does the rate of ion uptake decrease to 0 during active transport

In the presence of respiratory inhibitor cyanide

Describe endocytosis

The cell membrane engulfs to form a vesicle around the substance

two circle structures on the diagram are involved in the digestion of molecules that have been engulfed name the organelle where these structures are formed

The structures are lysosomes, organelle is Golgi body

Explain why water cannot easily pass through the PL bilayer

Water is a polar molecules and the fatty acids of the PL hi alter are hydrophobic regions

Explain which region is non polar

The region that is adjacent to the fatty acids

Explain why the WP of the cell can be assumed to equal to the solute potential of the solution that causes 50% plasmolysis

Because the cell is at IP which means the presssure potential is 0 as the cell memerbane has pulled away from the cell wall

Use the results in the table to estimate a value for the WP of the cell

Roughly when 50% of the cells are plasmolysed

Outline how the data could be used to determine a more accurate estimate of the cell water potential

Plot solute potential on X axis and % plasmolysed on y axis

Read down from 50% plasmolysed to dilute potential this is also the WP

State how the procedure could be modified to improve reliability

Observe more pieces of onion tissue from each solution , count more cells in each piece of tissue and calculate the mean

State how it could be modified to improve accuracy

Narrower range around 50% plasmolysed between 0.4-0.6

Explain why a cell was plasmolysed after placed in a solute with a SP of -1800 kPa

External water potential was lower than the cell WP so water moves out of the cell by osmosis

When dissolved in water NaCl dissociates to form Na+ and Cl- ions, suggest how interaction between Na+ ions and the phospholipid affects fluidity of membrane

Efeu as AS level paper 1 2023

Describe how oxygens and sodium ions cross the membrane

Oxygeb crosses the membrane by simple diffusion, passes through the PL bilateral

Sodium ions pass through intrinsic/ channel/ carrier proteins by FD, AT

Carp have high % compostipn of ions in the blood plasma, lower than % composition of ions in the habitat suggest what is importing for the survival of the carp

The % ion composition of fresh water/ habitat is greater than that of blood plasma, fresh water is hypotonic to the blood, WP of fresh water is higher than the WP of the carp’s blood, the carp takes in water by osmosis

Therefore

It is important for the carp to produce large volumes of dilute urine, as this prevents bursting of cells, maintaining ion composition, reducing loss of ions

Suggest why cells that maintain ion composition of the blood plasma are important for flounder fish

% ion composition of sea water is greater than that of blood plasma, sea water is hypertonic, WP in the fish is highe than in sea water

Ions will move into the blood plasma by FD down a concent gradient

Therefore ions must be actively transported out of the blood plasma into the surrounding sea water

Sea water has a high % composition of ions , low WP whilst fresh water has a low % composition of ions and a high WP explain which cell has been taken from the sea water

The cell containing more mitochondria to provide ATP for respiration/ for active transport

more highly folded to give larger S.A.

% composition of ions in the blood plasma is higher than in fresh water explain how acidification of this water cause death

The % ion concentration of the blood plasma is higher than the surrounding water, ions will move out of the blood plasma by facilitated diffusion down a CG, acidification will denature enzymes involved in ATP production, change the shape of carrier/ chemo protein in cells

Therefore fewer ions are actively transported into the fish , fewer ions can diffuse out

Explain why as the temp increases the absorbance of the cell increases

As the temp increases the PLs gain more KE, more gaps form

Above 40°C the proteins in the membrane start to denature

Above 70°C the membrane fully destroyed so all the pigment is released