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BIOPHYSICS > Lecture 3 (Membrane) > Flashcards

Lecture 3 (Membrane) Flashcards

1
Q

What are the three major classes of membrane lipids?

A
  • Phospholipids
  • Glycolipids
  • Cholesterol
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2
Q

What are the properties of membrane lipids?

A
  • Form a lipid bilayer
  • Amphiphilic -> one end soluble in water (polar) and another end not soluble in water (non-polar)
  • Arrangement of lipids and various proteins control entry and exit of molecules
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3
Q

What is the most common of the three types of membrane lipids?

A

Phospholipids

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4
Q

What are the two major types of phospholipids?

A
  • Sphingolipids (derived from sphingosine)
  • Phosphoglycerides (derived from glycerol)
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5
Q

Describe Phosphoglycerides

A
  • Contain a glycerol backbone, two fatty acid chain, a phosphate group and a polar molecule (serine, choline, ethanol amine etc)
  • Fatty acid tails usually have an even number of carbon atoms (averaging between 14-24)
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6
Q

What is a glycolipid?

A

A lipid with a carbohydrate group attached to its hydrophilic head

[contain a hydrophilic or polar head with one or two hydrophobic tails]

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7
Q

What are the properties of Glycolipids?

A
  • Significantly less abundant compared to phospholipids
  • Found only on extracellular side of plasma membrane
  • Produce GLYCOCALYX (involved in toxin recognition)
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8
Q

What is Cholesterol?

A

A constituent of membranes and the precursor to steroid hormones

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9
Q

What are the properties of Cholesterol?

A
  • Placed in middle of phospholipid bilayer since hydrophobic
  • Hydroxyl group directed towards “heads” of amphiphatic phospholipids
  • Oriented parallel to the lipids carbohydrate chain
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10
Q

What is the difference between saturated fatty acids and unsaturated fatty acids?

A

Unsaturated fatty acids = CONTAIN DOUBLE BONDS

Saturated fatty acids = DO NOT CONTAIN DOUBLE BONDS

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11
Q

How is a Phospholipid formed from a glycerol molecule?

A

1) Three fatty acids are attached to a glycerol molecule to make a TRIGLYCERIDE
2) A fatty acid is removed and replaced with a phosphate group to make a PHOSPHOLIPID

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12
Q

What is the driving force of lipid bilayer formation?

A

Lipid amphipathicity

[two different domains: hydrophilic head and hydrophobic tail]

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13
Q

What is a covalent bond?

A

Chemical bond that is formed when involved atoms share electrons

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14
Q

What are non-covalent bonds?

A

Molecular interactions that do not require the sharing of electrons

(e.g electrostatic interactions, hydrogen bonds and van Der Waals forces)

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15
Q

What is an electrostatic bond?

A

A relatively strong non-covalent interaction between ions or charged molecules. Oppositely charged groups attract one another, while similar groups repel one another.

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16
Q

What is a hydrogen bond?

A

A weak non-covalent force between positively charged hydrogen atom and negatively charged atoms (oxygen, nitrogen, fluorine) in other chemical groups

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17
Q

What is the Valence shell electron pair repulsion (VSEPR) theory?

A

A model used in chemistry to predict the geometry of individual molecules from the number of electron pairs surrounding their central atoms

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18
Q

Describe the Hydrogen bonding phenomena

A

The reason why NH3, H2O and HF have dramatically higher coiling points compared to heavier HCl, H2S is due to hydrogen bonding

[usually the pattern is the heavier the molecule -> the more van Der walls forces -> increase in melting point]

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19
Q

What are van Der Waals forces?

A

Weak non-covalent interactions that occur between neutral molecules

[result from polarisation arising in one molecule due to the presence of another molecule (DIPOLE)]

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20
Q

What are the different types of van Der walls attraction?

A
  • dipole/dipole
  • dipole/induced dipole
  • spontaneous-dipole/induced-dipole (LONDON DISPERSION FORCES)
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21
Q

What are London forces?

A

Large networks of intermolecular forces between non-polar and non-charged molecules and atoms

(aka SPONTANEOUS DIPOLE-INDUCED DIPOLE INTERACTION)

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22
Q

Once lipid bilayers have formed, the membrane lipids are made more cohesive by weak _________ __________

A

monovalent bonds

23
Q

Lipid molecules interact with each other and water molecules via _____ ___ ______ ________

A

Van der Waals forces

24
Q

What is the main mechanism of the hydrophobic effect?

A

Maximising the number of hydrogen bonds

25
Q

What happens when oil and water are mixed?

A

Water molecules are attracted to each other via HYDROGEN BONDING and oil molecules are EXCLUDED -> explains why polar molecules are soluble in water while non-polar molecules are not -> Polar molecules for hydrogen bonds with water, preventing water from excluding them

26
Q

Describe how the amphipathic nature is largely responsible for the stability of the lipid bilayer

A

Amphipathic nature of the membrane lipids causes the formation of a bilayer -> Water molecules tend to maximise the hydrogen bonds among them -> Hydrophobic effect leading to stability of the lipid bilayer

27
Q

What is a micelle?

A

A lipid ball with the hydrocarbon chains pointed in towards the center

28
Q

What is the concentration at which the lipids self-assemble into micelles called?

A

Critical micelle concentration (CMC)

[the conc of lipid necessary for all of these forces combined to produce a -ve Gibbs energy of micelle formation]

29
Q

Describe micelle formation

A

1) Lipid molecules are dispersed among the water molecules -> as conc increases, lipid molecules merge to form MICELLES
2) Once CMC (critical micelle concentration) is reached, any further lipid added to the solution either associates with existing micelles or forms new micelles
3) Conc of micelles increases while conc of free lipid molecules remains constant

30
Q

Which forces contribute to self-assembly in micelle formation?

A
  • DISPERSION FORCES (strong attractive force pulling hydrocarbon tails together)
  • HYDROPHOBIC EFFECT (as tails drawn together, water molecules are disrupted and pushed away)
31
Q

How does micelle formation decrease water restriction?

A
  • hydrophobic effect causes water to be pushed aside -> increases ENTROPY of water by allowing it to rotate freely
  • possibility for hydrogen bond formation increases
32
Q

Why is a micelle spherical shaped?

A

Negatively charged phosphate head groups creates a REPULSIVE FORCE between them -> To reduce repulsive forces nature puts the head groups on the outside of the micelles and shapes the micelle like a sphere -> Maximises distance between head groups, reduces REPULSIVE INTERACTION

[also allows space between head groups for WATER MOLECULES and COUNTERIONS -> reduce the repulsive force]

33
Q

Explain why as the concentration of positive ions increases, the CMC decreases

A

Positive ions neutralise the negative charge which eliminates the need for ENTHALPY to overcome the REPULSIVE FORCE -> The GIBBS ENERGY of micelle formation becomes even more negative, making it easier to form micelles -> Fewer lipid molecules required means the CMC is lower

34
Q

Explain why micelles formed from two-chain phospholipids in energetically unfavourable

A
  • Extra width causes STERIC INTERACTIONS that increase the distance between hydrocarbon chains of adjacent molecules -> reduces strength of DISPERSION FORCES
  • Extra width blocks hydrocarbon chains from reaching as deep into the micelle
  • Void + increased size = increase the energy cost of micelle formation
  • Increased size decreases the CURVATURE -> repulsive phosphate heads are closer together, unfavourable
35
Q

What type of micelles do two-chain phospholipids form?

A

Liposomes

[a lipid bilayer turned into a micelle]

36
Q

What does the ease of molecules passing through the lipid bilayer depend on?

A
  • Gibbs energy change of getting the molecule past the charged phosphate head groups and into the hydrophobic interior of the bilayer
  • Gibbs energy change for removing from the hydrophobic interior and getting past the charged phosphate head groups on the other side of the bilayer

[both depend on the STRENGTH of forces holding bilayer together + interaction of forces with molecule attempting to pass through the bilayer]

37
Q

Which molecules can pass through the lipid bilayer with ease?

A

Small, uncharged, non-polar (or slightly polar) molecules

38
Q

Explain why it is difficult for large molecules to make it across the bilayer

A

Large molecules have to push the lipids aside -> Pushing lipids aside requires disrupting the DISPERSION FORCES that attract the hydrocarbon tails together -> This takes energy + the larger the molecule, the more lipid molecules it has to push aside

39
Q

How do unsaturated chains make the bilayer less stable than saturated ones?

A

Double bonds restrict FREE ROTATION so each chain will be stiff at the location of the double bond (with a particular angle/kink in the chain at that location) -> Prevent lipid molecules from packing as closely together as saturated lipids can

40
Q

What are the main characteristics of membrane phospholipids?

A
  • Selective solubility
  • Ability to form closed spaces
  • Low permeability for the water molecules and charged ions
  • High permeability for gaseous and small lipid molecules
  • High electric resistance
  • High mobility
  • Asymmetric property
41
Q

Describe the asymmetry of plasma membranes

A
  • Phospholipids that contain CHOLIN (phosphatidylcholine, sphingomyelin) are mostly positioned on OUTER
  • Phospholipids that contain AMINE (phosphatidylethanolamine, phosphatidylserine) are mostly positioned in the INNER
42
Q

What are the different phases of membrane phospholipids?

A
  • Solid phase
  • Crystal phase
  • Liquid phase
43
Q

What are the five main factors that contribute to membrane fluidity?

A
  • Temperature
  • Membrane lipid tail length
  • The degree of unsaturation of membrane lipid tails (no. of double bonds)
  • Phospholipids polar “heads” charge
  • The cholesterol content
44
Q

What happens when the number of double bonds increases?

A

The melting points of unsaturated fatty acid chains decreases

45
Q

What lipid movements can be seen in the bilayer?

A
  • Vibrational (due to rotation around C-C bond)
  • Rotational
  • Lateral movement
  • “Flip Flop” (from one layer to another)
46
Q

What makes vibrational movements in the lipid bilayer feasible?

A

Because the energetic barrier for internal rotation around the C-C bond is low

47
Q

What are the effects of eating trans fats?

A
  • Increase the risk of CORONARY HEART DISEASE
  • Raising levels of LDL (bad cholesterol)
  • Lowering levels of HDL (good cholesterol) -> Increases triglycerides in the bloodstream an promotes systemic inflammation
48
Q

What is LDL cholesterol?

A

“bad” cholesterol that can lead to plaque buildup in your arteries resulting in HEART DISEASE + STROKE

49
Q

What is HDL cholesterol?

A

“good” cholesterol that absorbs cholesterol and carries it back to the liver which flushes it from the body -> high levels can REDUCE the risk of HEART DISEASE + STROKE

50
Q

What are Triglycerides?

A

A type of fat found in your blood that your body uses for energy -> Combination of high levels of triglycerides with low HDL or high LDL cholesterol can increase the risk for HEART ATTACK + STROKE

51
Q

Why are saturated fatty acids never called trans fats?

A

Because they have no double bonds

52
Q

What is Crepenynic acid?

A

Contains a triple bond and is rare with no nutritional significance

53
Q

What are techniques used to measure trans fat levels?

A
  • Chromatography
  • Gas chromatography
  • Mid-infrared spectroscopy
54
Q
A

BIOPHYSICS (15 decks)

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