Topic 2 - Cell Membranes

Question 1

Function of the Plasma Membrane?

Answer 1

• Partially permeable barrier that seperates the cell’s components from the external environment - i.e. compartmentalisation.
• Regulates the transport of materials in and out of the cell.
• May contain enzymes involved in specific metabolic pathways/reactions
• Has antigens for cell recognition
• Has receptors for cell signalling/communication - hormones, drugs and other chemical signals can bind to membrane bound receptors (it can also release chemical signals).
• Site of chemical reactions

Question 2

State the permeability of plasma membranes?

Answer 2

• Plasma membranes are partially permeable.

- Partially permeable means that it allows some specific molecules to pass through but not all.

Question 3

How do things move across the membranes?

Answer 3

• Some very small molecules diffuse through the plasma membrane between structural molecules.
• Some dissolve in the lipid layer and then pass through.
• Some pass through channel proteins or are carried through by carrier proteins.

Question 4

What is the permeability (of a membrane) determined by?

Answer 4

Permeability is determined by the components that make up the plasma membrane.

Question 5

As discussed metabolic processes can occur on membranes and these are usually mebranes within a cell. Give examples?

Answer 5

• Cristae (in mitochondria) have a large SA to catalyse the reactions involved in ATP production (aerobic respiration) + to localise enzymes needed.
• Thylakoid membranes (in chloroplast) house chloropyll + allow some photosynthesis reactions to take place on it.
• Digestive enzymes can be found on the plasma membranes of epithelial cells that line the small intestines - they can catalyse the final stages of sugar breakdown.

Question 6

What is the main component of plasma membranes and explain how it forms the plasma membrane?

Answer 6

The main component of plasma membranes are phospholipids. They consist of a phosphate head which is hydrophilc/polar and two lipid (fatty acid) tails which are hydrophobic/non-polar.

• When in water, the phospholipids automatically form a bilayer made up of two layers of phospholipids. They are organized in such a way that the lipid tails (of the phospholipids in the bilayer) face inwards towards each other, creating a hydrophobic region, while their phosphate heads face outwards, creating a hydrophilic region.
• This phospholipid bilayer arrangement means that the polar phosphate heads are in contact with the internal and external aqueous environment while the non-polar lipid tails are hidden away inside the bilayer, not in contact with water (otherwise the lipids would prevent polar molecules from interacting with the membrane).

Question 7

SO.. what is the arrangement of biological plasma membranes?

Answer 7

A phospholipid bilayer.

Question 8

Plasma membranes contains many different components - what do the different proportions depend on?

Answer 8

The different types of components and the proportion of each component that make up a plasma membrane depends on the function of the membrane.

Question 9

Plasma membranes contain 2 main types of proteins. What are they?

Answer 9

Intrinsic/Integral Proteins

Extrinsic/Peripheral Proteins

Question 10

What are Intrinsic/Integral proteins?

Answer 10

Intrinsic proteins are proteins that span both sides of the phospholipid bilayer/plasma membrane.`

Question 11

What are Extrinsic/Peripheral proteins?

Answer 11

Extrinsic proteins are proteins that span only onse side of the phospholipid bilayer/plasma membrane.

Question 12

Examples of intrinsic proteins (2 types)? Give examples.

Answer 12

Channel Proteins e.g. aquaporin

Carrier Proteins e.g. sodium-potassium pump

Question 13

What are channel proteins?

Answer 13

1) Channel proteins are intrinsic proteins which allow the passive movement of small polar molecules and ions like water (and water-soluble substances) from one side of the membrane to the other by facillitated diffusion
2) This movement occurs down a concentration gradient (high conc to low conc) via a hydrophilic channel.

Question 14

Why is the hydrophilic channel hydrophilic?

Answer 14

This is because it is lined with hydrophilic amino acids.

Question 15

What are carrier proteins?

Answer 15

1) Carrier proteins are intrinsic proteins which actively move polar (and non-polar) molecules and ions across the plasma membrane via active transport using energy in the form of ATP
2) This movement occurs when molecules are moved against the concentration gradient from low conc to high conc.
3) These proteins have an ATP binding site which uses the ATP, produced in aerobic respiration, to trigger conformational change in its shape/structure, thus allowing it to accept, transport and release molecules from one side of the mebrane to the other.
4) Carrier proteins can also be used for facilitated diffusion, a form of passive transport, in which the molecules use their inbuilt kinetic energy to move and bind with the carrier protein triggering conformational change, moving it to the other side. No ATP required.
5) This occurs when molecules are moved down the concentration gradient from high conc to low conc.
6) In both situations, with carrier proteins, the molecules BINDS TO THE SPECIFIC COMPLEMENTARY SITE ON THE PROTEIN.

Question 16

What kind(s) of transport occur across channel proteins?

Answer 16

• Facillitated diffusion (passive movement).

Question 17

What kind(s) of transport occur across carrier proteins?

Answer 17

• Active transport (active movement when moving molecules against the concentration gradient).
• Facillitated diffusion (passive movement when moving molecules down the concentration gradient).

Question 18

Can two or more proteins be tranported simultaneously across channel or carrier proteins?

Answer 18

In channel proteins, molecules can move acroos the membrane in both directions at the same time.
Whereas in carrier proteins, molecules can only move in one direction at any given time.
(facillitated diffusion and active transport cannot occur at the same time)

Question 19

How can water move across the plasma membrane?

Answer 19

Sometimes, due to the very small size of water molecules, they can pass through the hydrophobic region of the membrane, between structural components.
However, when there is an osmotic potential difference between the two areas seperated by the membrane, a gradient is created in which water diffuses down it, through a channel protein.

Question 20

Examples of molecules that move through a channel proteins?

Answer 20

Water molecules and any molecule of the appropriate size and charge

Question 21

Examples of molecules that move through a carrier proteins?

Answer 21

• Sugars e.g. glucose (too large for channel proteins)
• Amino acids
• Nucleosides

Question 22

Example of an extrinsic/peripheral protein?

*Extrinisc proteins are usually on the extracellular side

Answer 22

G-protein receptors.

Question 23

What do extrinsic proteins do?

Answer 23

They accept hormones, drugs and minerals causing them to shift, triggering a cascade of enzyme activity.

Question 24

Examples of chemical signalling molecules?

Answer 24

Drugs and hormones

Question 25

What are chemical signals detected by?

Answer 25

Chemicals signals can be detected by receptors or antigens made of:

• protein chains
• glycolipids
• glycoproteins

Question 26

What is the structure of glycolipids?

Answer 26

• Glycolipids consist of a carbohydrate chain attached to a lipid molecule.

Question 27

What is the functions of glycolipids?

Answer 27

Can act as cell receptors for cell signalling.
Can acts as antigen as well for cell recognition
-Glycolipids also play a role in cell adhesion

Question 28

What is the structure of a glycoproteins?

Answer 28

• An intrinsic protein.

- Glycoproteins consist of a carbohydrate chain attached to an intrinisic protein embedded in the plasma membrane.

Question 29

What is the functions of glycoproteins?

Answer 29

They act as cell receptors for cell signalling.
They act as antigens for cell recognition.
They also play a role in cell ashesion.

Question 30

What is cell adhesion?

Answer 30

When cells join together to form tight junctions in certain tissues.

Question 31

How do glycoproteins (and glycoplipids) act as receptors?

Answer 31

The carboydrate chain acts as a recptor binding site which is specific and complementary in a shape to a certain chemical signal e.g. insulin.

Question 32

What is cell recognition?

Answer 32

A process by which antigens are recognised by the cells of the immune system as self or non-slef/foreign.

Question 33

What is the glycocalyx?

Answer 33

The glycocalyx is the outermost region of the membrane where the carbohydrate chains (attached to either proteins or lipids) help the cell to interact with the watery environment ot obtain dissolved substances.

Question 34

How do glycolipids and glycoproteins stabilise the membrane?

Answer 34

Glycolipids and glycoproteins form hydrogen bonds with surrounding water to stabilise the membrane.

Question 35

What is the structure of chlestrol and how is it positioned in the the phospholipid bilayer?

Answer 35

• Cholestrol is a lipid wiht a hydrophilic end and a hydrophobic end (like a phospholipid).
• Cholectrol molecules are positioned between the phospholipids, perpendicular to the membrane plane, with the hydrophilic end interacting with the phosphate heads and the hydrophobic end interacting with the lipid tails.

Question 36

Due to its structure and positioning, what are the functions of cholestrol?

Answer 36

• Gives the membrane extra mechanical STABILITY (without making it too rigid) + regulates the FLUIDITY of the membrane.
• They make the barrier more complete so that molecules such as water cannot pass through when the permeability of the bilayer changes.
• The hydrophobic region of the cholestrol means that the cholestrol acts as a further barrier to polar substance.
• Resists the effects of temperature changes on the structure of the membranes

Question 37

What is the role of cholestrol when the temperature of the surrounding environment decrease?

Answer 37

• When temperature decreases, stability decreases, fluidity + permeability of decreases + membrane becomes more brittle.
• The cholestrol can re-stabilise the membrane by acting as spacers between phospholipids, pushing them outwards, and preventing them from compressing each other.
• Cholestrol acts to increase the fluidity + stability + permeability

Question 38

What is the role of cholestrol when the temperature of the surrounding environment increases?

Answer 38

• When temperature rises, stability decrease, permeability and fluidity increases.
• The membrane cholestrol can re-stabilise the membrane by holding together the phospholipids to reduce any large gaps through which unwated large susbtances may enter or cytosol may leave. This will affect the water potential of the cell resulting in lysis.
• Cholestrol acts to increase stability but decrease the fluidity + permeability

Question 39

What is the impact of too much cholestrol in the plasma membrane?

Answer 39

Too much cholestrol = the less permeable and fluid the membrane is (because fewer gaps) = this makes the plasma membrane rigid + inflexible = so more easily ‘shattered’.

Question 40

What is the fluid mosaic model?

Answer 40

• Proposed by Singer and Nicholson
• This states that membranes are composed of a phospholipid bilayer with various proteins and lipid moelcuels floating around within it.

Question 41

In the ‘fluid mosiac model’ what does it refer to when saying ‘fluid’?

Answer 41

The ‘fluid’ part represents how some parts of the membranee such as the phospholipids and other embedded proteins can move around freely (laterally) if they are not attached to other parts of the cell. This is because the phospholipids and the embedded proteins do not form strong or rigid bonds with each other.

Question 42

In the ‘fluid mosiac model’ what does it refer to when saying ‘mosaic’’?

Answer 42

The ‘mosaic’ illustrates the ‘patchwork’ of proteins found in the phospholipid bilayer.

Question 43

EXAM QUESTION: Suggest the likely protein composition of a white blood cell’s plasma membrane?

Answer 43

• It may have a high proportion of antigens that are specific and complementary in shape to proteins found on other cells to determine if they are native or foreign cells.

Question 44

Approxiamate thickness of a phospholipid bilayer?

Answer 44

7.5nm

Question 45

Why do phospholipids form a bilayer of globules in water?

Answer 45

Phospholipids are made of a hydrophilic phosphate head and hydrophobic lipid tails.
As a result, in water, they automatically/spontanously form a bilayer/globule. This arrangement means the polar phosphate heads can interact with its internal and external aqueous environment, whilst the non-polar lipid tails (hydrophobic region) is hidden away to prevent interaction with water.

Question 46

What is cell signalling?

Answer 46

Communication between cells.

Question 47

Function of cell membranes within cells?

Answer 47

• Froms vesicles + allows transport in vesicles.
• Seperates contents of organelles from cytoplasm (compartmentalisation)
• Site of chemical reactions/processes e.g. aerobic repiration in mitochondria.
• Provides surface for attachment (of enzymes/ribosomes).
• Controls what substance enters/leaves organelles.

Question 48

Which factors affect permeability of a plasma membrane?

Answer 48

• Temperature
• Solvent
• PH
• (amount of cholestrol)
• (saturation of phospholipids)

Question 49

What is the effect on membrane permeability there is a increase in temperature?

Answer 49

Increasing the temperature between 0-45 degrees, causes the phospholipid bilayer to move more as a result of increased kinetic energy, thus increasing its permeability. After 45 degrees, the phospholipid membrane breaks down:

• The molecules of the plasma membrane such as phospholipids, channel and carrier proteins, glycoproteins accquire more kinetic energy (converted from thermal energy) cause them to vibrate more and move more (away from each other)
• This increase the size and number of gaps in the plasma membrane. As a result, the fluidity of the bilayer increases and it becomes more leaky.
• In addition to that, the embedded proteins and glycoproteins denature (3D shape/tertiary structure collapses/changes because the weak hydrogen bonds between them break) so gaps are left in the bilayer.
• This causes the PERMEABILITY + FLUIDITY OF THE MEMBRANE TO INCREASE. Further increase in temperature may cause the membrane to be completely destroyed/disrupted.

Question 50

What is the effect on membrane permeability there is a decrease in temperature?

Answer 50

The proportion of unsaturated and saturated fatty acids that make up phospholipids in the plasma memrane determine its permeability at low temperatures:

• Plasma membranes that are mainly composed of saturated fatty acids compress in colder temperatures, pushing against each other and exerting a force on each other. This causes the plasma membrane to become brittle very easily. This is because the phospholipids have less kinetic energy, so vibrate and move less. This also reduces fluidity and permeability as it decreases the number and size of gaps in the membrane
• Increased brittleness makes the membrane susceptible to being easily ruptured at which point any substance can move in our out of the specific cell. Reduced fluidity and permeablity = may prevent the entry of O2 and glucose.
• The reason that phospholipid bilayer membranes mainly composed of unsaturated phospholipids are less affected by colder temperatures is because as they become compressed, kinks in their tails push the adjacent phospholipids molecules apart maintaining the membrane fluidity,Therefore even lower temperatures are required to have the same effect (of brittleness) when compared with a membrane mainly composed of saturated fatty acids.

Question 51

If the temeprature is too high and the carrier/channel proteins denature, what can happen?

Answer 51

These are the proteins involved in transporting substances across the membrane, so the cell may not be able to recieve nutrients, minerals or ions or release enzymes, waste products required for its survival.
The permeability and fluidity will also increase as it may create gaps in the plasma membrane

Question 52

When a membrane loses its structure, how do we describe it?

Answer 52

• The membrane has been disrupted or destroyed (NOT DENATURE as this is when proteins lose their structure).

Question 53

What are saturate and unsaturated fatty acids?

Answer 53

Saturated fatty acids are moelcules which have no double bonds whereas unsaturated fatty acids have at least one double bond.

Question 54

Why are unsaturated fatty acids more ‘fluid-like’ compared to saturated fatty acids?

Answer 54

• The double bond in an unsaturated fatty acid produces a kink in the chain which makes it more fluid-like. The more double bonds = the more fluid-like the phospholipid is (increased fluidity of the membrane).

Question 55

How can high temperatures indrectly affect he permeability?

Answer 55

High temperatures can denature the cytoskeleton, causing the cell to collapse in on itself, which causes the membrane to be ruptured thus affecting permeability

Question 56

What is a solvent?

Answer 56

Solvents are liquids that dissolve solutes.

Question 57

Types of solvents?

Answer 57

• Polar solvents such as water (important for the formation of a phospholipid bilayer)
• Less/non-polar solvents such as alcohols/acetone
• Organic solvents such as benzene

Question 58

Do phospholipids dissolve in water?

Answer 58

Phospholipids are not very polar - this is because despite the phosphate head being polar, the lipid chain is not. Therefore, it does not dissolve completely in water because unlike phospholipid molecuels, water is very polar. This difference in polarity prevents it from dissolving in it.

Question 59

Do phospholipids dissolve in alcohols and acetate?

Answer 59

Alcohols and acetate have none (long chains) or low polarity and thus due to such similar properties, phospholipids can dissolve in it.

Question 60

What happens when phospholipids dissolve in solvents?

Answer 60

When phospholipids dissolve in solvents, the plasma membrane loses its structure/ is disrupted. This results in increased gaps within the membrane which means increased fluidity and permeability.

Question 61

Why is alcohol used as an antiseptic?

Answer 61

Alcohol is used as an antiseptic because it dissolves the plasma membranes of bacteria in a wound killing them.

Question 62

The amount by which the permeability is increased using a solvent depends on…

Answer 62

…the type of solvent used [ethanol < methanol] and the concentration of the solvent

Question 63

What happens if cells are placed in pure alcoholic solutions.

Answer 63

Pure alcoholic solutions are toxic so completely destroys cells.

Question 64

What can some non-polar alcohol molecules do?

Answer 64

Non-polar alcohol molecules can entrer the plasma membrane between phospholipids, creating gaps (increased fluidity and permeability)

Question 65

What happens in neuronal membranes (membrane of the myelin sheath) increases in fluidity?

Answer 65

The rate of nerve impulse transmission slows down.

Question 66

Why do root hair cells in plants have many carrier proteins?

Answer 66

This is to actively transport nitrate ions from the soil into the cells.

Question 67

Why is the inner membrane of the mitochondria 76% protein and 24% lipid?

Answer 67

This is because their inner membrane contain many electron carriers and hydrogen ion channels, which are both made of proteins, and are associated with ATP synthase enzymes.

Question 68

What is the role of the channel proteins and carrier proteins in the plasma membrane covering the long axon in neurones?

Answer 68

Controls the exit and entry of ions to bring about the conduction of electrical impulses along their length.