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BMS238 Molecular and Cell Biology > Lecture 24 > Flashcards

Lecture 24 Flashcards

1
Q

Phospholipids are amphipathic, what is meant by this

A

The contain both hydrophilic and hydrophobic parts. A hydrophilic (polar) phosphate group and the non-polar hydrophobic fatty acid tails

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2
Q

Both lipids and proteins are often tagged by complex sugars in the membrane, T or F

A

T

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3
Q

What is the proposed role of lipid rafts

A

Lipid rafts are thought to play a role in signalling and some forms of endocytosis

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4
Q

Other than lipid rafts, what other ways are there of segregating proteins in the cell membrane

A

Caged behaviours, fenced domains and tight junctions all act to segregate proteins in the membrane

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5
Q

What is the general role of proteins in the cell membrane

A

Mediate interactions with the surrounding environment as well as playing a role in motility and the uptake of nutrients

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6
Q

Which organisms produce omega 3 fatty acids

A

Sea plants, fish and nuts

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7
Q

Describe the nomenclature of omega fatty acids

A

Furthest carbon away from the carboxyl group in a fatty acid is called the omega (?) carbon. The position of the first double bond determines the name of the ?-fatty acid

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8
Q

What percentage of the intracellular membrane is made up of phosphatidylserine molecules and how does this account for its charge

A

4% - however this accounts for most of the intracellular negative charge

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9
Q

What makes fatty acid tails unsaturated

A

The presence of double bonds within the hydrocarbon tails

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10
Q

Why is it that unsaturated fats allow the membrane to be flexible

A

Unsaturated fatty acid tails pack loosely together allowing the bilayer to remain fluid

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11
Q

Give an example of where glycosylation is important

A

Sphingomyelin is often glycosylated forming gangliosides which are very important in neurons. They exhibit progressive structural complexity. Neural stem cells carry simple sugars whereas mature neurons carry highly branched sugars. The presence of gangliosides acts as a signal that the neuron has matured. Myelination of neurons by Schwann cells is determined by the sugar maturation. Defects in ganglioside synthesis can lead to a variety of neurological disorders

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12
Q

What is the general role of lipids in the cell membrane

A

Provide flexibility and continuity

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13
Q

What configuration are the double bonds that make up unsaturated fatty acids usually in

A

This cis conformation which introduces sharp kinks

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14
Q

What causes phospholipids to form sealed compartments when placed in water

A

The repulsion by water of the hydrophobic regions makes the lipid bilayer hide its edges and form a sealed compartment which is energetically favourable

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15
Q

What is meant by the term lipid rafts

A

Cholesterol and sphingolipids can form microdomains called lipid rafts. These lipid rafts are thicker regions of the membrane that form under the influence of membrane proteins that drive this domain formation

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16
Q

Explain the significance of phosphatidylserine in apoptosis and cell death

A

Phosphatidylserine flips to the outer surface only upon apoptosis which takes place during cell death. This acts as a marker of cell death with lipid asymmetry of the plasma membrane breaking down. This in turn causes the cell membrane to become permeable to small molecules.

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17
Q

What methods are there for sugars to be attached to the cell membrane

A

Sugars can either be attached to the membrane via proteins or via lipids

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18
Q

Land plants do not contain the enzyme to insert the double bond in the carbon 3 position – to make ?-3, T or F

A

T

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19
Q

List the four major phospholipids present in cell membranes

A

Phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine and sphingomyelin

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20
Q

Why do disulphide bonds only form in the extracellular environment

A

Cysteines present on the intracellular side of membrane proteins will be in their reduced form and will not form disulphide bonds. Cysteine exposure to the extracellular environment results in oxidisation and the formation of disulphide bridges

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21
Q

Describe the dynamic behaviour of phospholipids in the membrane

A

The behaviour of phospholipid molecules in a lipid bilayer is extremely dynamic. Phospholipids can rotate or exchange and diffuse in the lateral plane of the membrane. Phospholipids can also flip-flop (transfer between leaflets), but this occurs slowly and is very rare

22
Q

One-legged (two tailed) lipid molecules form vesicles encapsulating water, T or F

A

F – two-legged phospholipids do

23
Q

What is the role of phosphatidylserine exposure to the extracellular environment in an intact organism

A

Exposure of phosphatidylserine on the cell surface labels the dead cell and its remnants so that they are rapidly consumed by macrophages

24
Q

What is the role of phosphatidylinositol in the cell membrane

A

Plays an important role in cell signalling (cancer, vesicle traffic etc.). Phosphatidylinositol can be phosphorylated thereby increasing its negative charge

25
Q

What is the role of the negative charge present on the intracellular side of the cell membrane

A

The negative charge repels intracellular molecules and vesicles

26
Q

Which side of the membrane are sugars found attached to

A

The outside of the membrane

27
Q

Myristoyl anchors are used to link peripheral proteins with the cell membrane. What kind of linkage is present in these anchors

A

Amide linkage between the terminal amino group of a protein and myristic acid

28
Q

Explain what is meant by the hydration layer and its tole in the cell membrane

A

The hydrophilic head group of the phospholipids usually have several water molecules surrounding it creating the hydration layer. The hydration layer presents a physical obstacle for random vesicle fusion with the plasma membrane and each other

29
Q

Which side of the membrane are sugars usually attached

A

The extracellular surface

30
Q

What is the role of sphingolipids in the formation of lipid rafts

A

Fatty acid chains of sphingolipids are longer and straighter than other phospholipids these rafts are thicker than rest of plasma. Sphingolipids carry long saturated fatty acids and thus segregate with cholesterol into distinct lipid rafts

31
Q

Other than integral or transmembrane proteins, what other way is there of proteins integrating with the cell membrane

A

Peripheral membrane proteins can associate with the membrane by inserting via a covalently bound lipid modification. Similarly, they can interact with integral membrane proteins via interactions such as disulphide bridges. Finally peripheral proteins can directly bind to lipid present in the phospholipid bilayer

32
Q

What is the significance of carbohydrate modification of cell membrane constituents in organ transplantation

A

Limitations in organ transplantation are caused by a simple sugar modification. Human cells carry beta galactose while other animals carry alpha-galactose. Humans produce antibodies against alpha-galactose and thus reject transplants.

33
Q

Which leaflet is phosphatidylinositol found in

A

The inner leaflet

34
Q

Phosphatidylinositol is only a minor component of the cell membrane, what percentage composition does it make up

A

0.01

35
Q

Cholesterol doesn’t make the membrane less fluid as a whole but makes the membrane less deformable at the surface, T or F

A

T

36
Q

Outline the basic structure of a phospholipid

A

Phospholipid consists of a glycerol molecule, phosphate group, lipid head group and two hydrocarbon tails. The two fatty acids are attached to the oxygen groups in the glycerol molecule via an ester bond

37
Q

Which phospholipid is strictly found on the inner surface of the membrane

A

Phosphatidylserine is strictly found in the intracellular surface of the cell membrane because of its negative charge

38
Q

What attribute of cholesterols structure allows it to stabilise phospholipid bilayers

A

Cholesterol contains a hydroxyl group, a tiny polar head group and a rigid hydrophobic tail that provides the rigidity

39
Q

Describe the structure of cholesterol

A

Cholesterol has a very solid core of 4 carbon rings. Phospholipids and cholesterol together form functional cell membranes

40
Q

What features of the phospholipid unique to the intracellular surface of the cell membrane make it suitable

A

Phosphatidylserine contains a negative phosphate group as well as a serine group containing both a positively charge amino group and a negatively charge carboxyl group. This means that the phospholipid has a net negative charge

41
Q

Unsaturated fatty acids provide flexibility to cell membranes, whereas saturated lipid are too rigid and not compatible with cell membranes, T or F

A

T

42
Q

Give an example of another type of fatty acid linkage used to associate peripheral proteins with the membrane other than palmitoyl and myristoyl anchors

A

Farnesyl anchors – a thioester linkage between cysteine residue and a prenyl group

43
Q

Explain the different behaviours of single and double tailed phospholipids when placed in solution

A

Single tailed phospholipids will form micelles when placed in water. Two tailed phospholipids will form bilayers in water due to geometric restriction of additional fatty acid tail prevent micelle formation

44
Q

Why do phosphatidylethanolamine and phosphatidylcholine have no overall charge

A

Because the negative phosphate head group charge is cancelled out by positive side groups, ethanolamine and choline

45
Q

What is the role of disulphide bonds in membrane fluidity

A

Disulphide bridges in the extracellular domains make the protein more rigid and resistant to degradation

46
Q

One type of protein often associated with the cell membrane is the integral or transmembrane protein. What four ways can these proteins integrate with the membrane

A

As single ? helices spanning both leaflets, multiple transmembrane spanning ? helices, a rolled ? barrel or as an ? helix inserted into only one leaflet

47
Q

What is the significance of sugar modification in botulinum and tetanus toxins

A

Tetanus and botulinum bacteria produce toxin which recognize the complex sugars. These toxins bind only to fast myelinated neurons and hence cause efficient muscle paralysis. BOTOX binds to the ganglioside on the neuronal membrane to cause muscle paralysis and will then release an enzyme which will proteolyze its SNARE target and prevent neurotransmission

48
Q

What is the general role of carbohydrates in the cell membrane

A

Involved in cell tagging and protection

49
Q

What is the role of cholesterol in the cell membrane

A

Cholesterol makes membranes less permeable. Its affects the way hydrophobic tails interact with each other and makes the membrane less permeable to water soluble molecules by packing between the phospholipids

50
Q

Palmitoyl anchors are another way of linking peripheral proteins with the cell membrane. What kind of linkage is present in these anchors

A

Thioester linkage between cysteine in the protein and palmitic acid

BMS238 Molecular and Cell Biology (31 decks)

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