Questions on membrane structure and the fluid mosaic model

Question 1

Explain why phospholipids arrange themselves into a bilayer when they are placed in water, and why a torn fragment of membrane reseals itself into a closed vesicle.

Accepted Answer

B1 a phospholipid is amphipathic: the phosphate head is hydrophilic and the two fatty acid tails are hydrophobic B1 the heads face the water on both surfaces while the tails are buried between them, facing each other B1 no bonds join one phospholipid to the next; this arrangement is simply the one of lowest energy, because it hides the greatest number of tails from water B1 an open edge would leave hydrophobic tails exposed to water, so a fragment closes on itself rather than remaining as a sheet

Question 2

Describe what cholesterol does to a cell surface membrane at high temperature and at low temperature, and describe the effect it has on permeability in each case.

Accepted Answer

B1 at high temperature cholesterol binds to the fatty acid tails around it and restricts their movement B1 so the membrane is less fluid than it would otherwise be and does not become leaky B1 at low temperature cholesterol sits between the tails and stops them packing closely together B1 so the membrane keeps some fluidity instead of setting solid and cracking, and its permeability stays close to normal

Question 3

Compare a channel protein with a carrier protein as routes across a cell surface membrane.

Accepted Answer

B1 a channel protein is a pore lined with hydrophilic amino acids, whereas a carrier protein has a binding site shaped to fit one kind of molecule B1 a substance passes through the open channel, whereas a carrier protein changes shape to move the substance across B1 channels carry water-soluble ions such as sodium, potassium and chloride, whereas carriers take larger polar molecules such as glucose and amino acids B1 channels are used in facilitated diffusion only, whereas carrier proteins are used in both facilitated diffusion and active transport

Question 4

A student cuts discs of beetroot with a cork borer and places them directly into water baths at a range of temperatures without washing them first. Suggest how this affects her absorbance readings, and suggest one control variable she must still keep the same.

Accepted Answer

B1 cutting ruptures the cells at the cut surfaces, which releases pigment that has nothing to do with the temperature being tested B1 absorbance is therefore raised at every temperature, including those at which the membranes are undamaged B1 the readings at low temperature overstate the permeability, so the rise caused by temperature itself is masked B1 any one control variable named: size and number of discs, volume of distilled water, time in the water bath, time before the reading is taken, or the age and variety of the beetroot

Question 5

Compare an intrinsic protein with an extrinsic protein in a cell surface membrane, referring to its position and to the kind of job each commonly does.

Accepted Answer

B1 an intrinsic protein is embedded within the bilayer, often spanning it completely, whereas an extrinsic protein sits on one surface and does not enter the hydrophobic core B1 intrinsic proteins have hydrophobic surfaces facing the core of the bilayer, which is what holds them there, whereas extrinsic proteins are held by binding to the phospholipid heads or to an intrinsic protein B1 intrinsic proteins commonly act as channels or carriers, moving substances across the membrane, whereas extrinsic proteins are often enzymes or proteins that anchor the cytoskeleton B1 both kinds are scattered irregularly through the membrane, which is the mosaic of the fluid mosaic model

Question 6

A cell has receptors for insulin on its surface. Suggest what would happen to that cell's response to a rise in blood insulin concentration if a mutation caused the receptor's binding site to no longer be complementary to insulin's shape, and suggest what this shows about the specificity of cell signalling.

Accepted Answer

B1 insulin would no longer be able to bind to the altered receptor, because binding depends on the receptor's site being complementary in shape to the ligand B1 without insulin binding, the change inside the cell that binding normally triggers would not happen, so the cell would not respond to a rise in blood insulin concentration even though insulin is present B1 this shows that a cell's response to a hormone depends on having a working, complementary receptor and not just on the hormone reaching it, since the hormone passes every cell in the body but only cells with the receptor can respond B1 the effects insulin normally has on the cell would not occur: it would not increase its uptake of glucose from the blood or its conversion of glucose to glycogen

Question 7

The table shows results, typical of the beetroot practical, from discs held in water baths for five minutes each. Explain the pattern in the readings, referring to what happens to each membrane component as the temperature rises.

Accepted Answer

B1 between 20 and 40 °C absorbance rises gently because the phospholipids gain kinetic energy and move more, so slightly more pigment escapes while the membrane stays largely intact B1 above about 45 to 50 °C the intrinsic proteins denature: hydrogen and ionic bonds break and tertiary structure is lost, so the proteins no longer control what crosses, and absorbance climbs steeply B1 the phospholipids do not denature, because they are not proteins; the two gradients in the data come from two different changes, one to each component

Question 8

Describe the three steps of cell signalling, from a signalling molecule leaving one cell to a distant target cell responding.

Accepted Answer

B1 one cell secretes a signalling molecule, usually by exocytosis B1 the molecule is transported to the cells it is meant for, carried in the blood if the target is distant, as a hormone is B1 it binds to a receptor on the target cell's membrane whose binding site is complementary to it, and only cells carrying that receptor respond

Question 9

Explain why a steroid hormone can bind to a receptor inside a target cell, rather than needing a receptor on the cell surface membrane.

Accepted Answer

B1 a steroid hormone is small and non-polar, so it is lipid-soluble B1 such a molecule can dissolve in, and diffuse across, the hydrophobic core of the phospholipid bilayer without needing a channel or a carrier protein B1 having crossed the membrane, it can bind directly to a receptor inside the cell, so it does not need a complementary receptor on the cell surface

Question 10

A person with blood group A receives blood from a group B donor by mistake. Suggest, referring to glycoproteins and glycolipids, why the wrong blood type is recognised as foreign.

Accepted Answer

B1 the ABO blood groups are set by differences in the short branched carbohydrate chains carried by glycoproteins and glycolipids on the outer surface of red blood cell membranes B1 many of these chains act as antigens: molecules the immune system uses to distinguish the body's own cells from foreign ones B1 a group B donor carries different chains from the recipient's own cells, so the recipient's immune system does not recognise them as self and responds to them as foreign

Question 11

Explain why increasing the concentration of ethanol around a cell increases the permeability of its membrane.

Accepted Answer

B1 ethanol is an organic solvent, and it dissolves lipids, which is exactly what the phospholipid bilayer is made of B1 increasing the concentration of ethanol dissolves more phospholipids out of the membrane and disrupts the packing of what remains B1 with the bilayer increasingly disrupted, permeability rises as the ethanol concentration increases

Question 12

Explain why a small non-polar molecule such as oxygen can cross the phospholipid bilayer directly, while a polar molecule such as glucose cannot.

Accepted Answer

B1 the hydrophobic core of the bilayer is made of the fatty acid tails of the phospholipids, and it excludes water and anything charged or strongly polar B1 oxygen is small and non-polar, so it can dissolve in this hydrophobic core and diffuse straight through it B1 glucose is a large polar molecule, so it cannot dissolve in the hydrophobic core and must cross instead through a membrane protein

Question 13

Name the two kinds of molecule found on the outer surface of a cell surface membrane that carry short branched carbohydrate chains.

Accepted Answer

B1 glycoprotein: a membrane protein with a carbohydrate chain attached B1 glycolipid: a lipid with a carbohydrate chain attached

Question 14

State what the word fluid describes in the fluid mosaic model of the membrane, and state what the word mosaic describes.

Accepted Answer

B1 fluid: the phospholipids and most of the proteins move sideways within their own layer B1 mosaic: the proteins are scattered through the phospholipids in an irregular pattern, like tiles set into a floor, rather than forming a layer of their own

Question 15

State what is meant by the term ligand, and state one example of a ligand named in this topic.

Accepted Answer

B1 a ligand is a molecule that binds specifically to a receptor B1 any one correct example: a hormone, a neurotransmitter, or a drug designed to imitate or block one

Temperature / °C	Absorbance of the surrounding water
20	0.05
30	0.08
40	0.12
50	0.35
60	0.78