Lipids

Question 1

what molecules are grouped under the category of lipids?

Answer 1

fats (triglycerides), phospholipids, glycolipids, sterols

Question 2

what are the characteristics of lipids?

Answer 2

insolubility in water, and high solubility in non-polar solvents

Question 3

what three types of lipids are there, and what are their differences?

Answer 3

simple lipids, compound lipids, and lipid derivatives
simple lipids consist of only fatty acids and an alcohol (usually glycerol)
compound lipids consist of fatty acids, an alcohol, and other groups

Question 4

define simple lipids

Answer 4

simple lipids consist of an alcohol (usually glycerol) linked to one or more fatty acids via an ester linkage (eg. triglycerides)

Question 5

what role do phospholipids and sterols play in biological membranes?

Answer 5

they are major structural elements

Question 6

what is the molecular formula of glycerol?

Answer 6

C3H8O3

Question 7

what are the structural features of glycerol? (number of carbons, functional groups, solubility in water)

Answer 7

it is a three-carbon alcohol, with each carbon bearing a hydroxyl (OH) group. it is soluble in water due to its polar -OH group

Question 8

what are fatty acids? (type of acid, functional group, structure)

Answer 8

carboxylic acids, composed of an acidic carboxyl (COOH) functional group and an attached hydrocarbon chain (long carbon skeletons between 12-20 carbons long)

Question 9

why are fatty acids so hydrophobic?

Answer 9

due to the abundance of non-polar C-H bonds

Question 10

what are the ways in which fatty acids can differ?

Answer 10

1. length of hydrocarbon chain (between 12-20 carbons long)

2. number and location of C=C double bonds along their carbon skeletons

Question 11

what is the difference between saturated and unsaturated fatty acids (and their examples)?

Answer 11

saturated: no double bonds between carbon atoms, every carbon is maximally bonded to hydrogen atoms (eg. palmitic acid)
unsaturated: one or more C=C double carbon bonds, kink (bend) in its tail wherever a double bond occurs (eg. linoleic acid)

Question 12

what is an ester linkage (where is it found, how is it formed)

Answer 12

an ester linkage is found in lipids, between a hydroxyl (OH) group of an alcohol and the carboxyl (COOH) group of a fatty acid
it is formed during a condensation reaction, where one molecule of water is also produced

Question 13

what does the linkage of a glycerol molecule to different numbers of fatty acids produce?

Answer 13

one fatty acid: monoglyceride
two fatty acids: diglyceride
three fatty acids: triglyceride

Question 14

how does the melting point of fats vary with hydrocarbon chain length, and why?

Answer 14

proportionally related.
as hydrocarbon chain length increases, hydrophobic interactions between the chains becomes more extensive, and melting point increases

Question 15

what are hydrophobic interactions

Answer 15

a weak bond that exists between hydrophobic molecules, a force of attraction between non-polar molecules

Question 16

how does the melting point of fats vary with the degree of unsaturation, and why?

Answer 16

inversely related
as the degree of saturation increases, there are more C=C double bonds and thus more kinks that prevent molecules from packing closely, so hydrophobic interactions are less extensive, and less thermal energy is required to break these interactions to liquefy the fats

Question 17

how does the proportion of C and H atoms compare to O atoms in triglycerides? do triglycerides have more or less carbon atoms per unit mass than carbohydrates? how does this structural feature help?

Answer 17

triglycerides have a higher proportion of C and H atoms compared to O atoms, and contain a greater number of carbon atoms per unit mass than carbohydrates.
upon oxidation, triglycerides release a larger amount of energy (38kJ/g, around twice that of carbohydrates), so triglycerides are more efficient energy stores

Question 18

how do the hydrogen atoms per unit mass of triglycerides and carbohydrates compare?

Answer 18

triglycerides are highly reduced, since they contain twice as many H atoms per unit mass as carbohydrates.
they release more METABOLIC water when oxidised during cellular respiration (important to desert animals like camels)

Question 19

what is the nature of triglyceride molecules, and its function?

Answer 19

the C-H bonds are non-polar, so triglycerides are hydrophobic (no associated water molecules, no extra weight of hydration)
do not affect water potential of cells, so animal’s body mass can be kept to a minimum to facilitate locomotion
good thermal insulators

Question 20

nature of hydrocarbon tails, and its function

Answer 20

non-polar tails, with weak hydrophobic interactions between triglyceride molecules
triglycerides slide under pressure, adipose tissue (contains fats) around vital organs cushions and protects them

Question 21

molecular weight of triglycerides compared to water per unit volume, and its function

Answer 21

lower molecular weight than water per unit volume

less dense than water, aid buoyancy

Question 22

define compound lipids (with examples)

Answer 22

esters of fatty acid(s) and an alcohol plus other chemical groups (eg. phosphate and sugar).
examples: phospholipids and glycolipids

Question 23

describe the structure of phospholipids

Answer 23

one glycerol + two fatty acids
third -OH group of glycerol is bonded to a negatively-charged phosphate group
additional small molecules that are usually charged / polar (eg. serine, choline, inositol) may be linked to the phosphate group, making different phospholipids

Question 24

how are phospholipids formed?

Answer 24

the two fatty acids are linked to the glycerol by an ester linkage, negatively-charged phosphate group is linked to third OH group of glycerol by phosphoester linkage
condensation reaction, water molecule as product

Question 25

phospholipids' relationship with water

Answer 25

``` fatty acid (hydrocarbon) are non-polar and hydrophobic tail, phosphate group and its attachments are polar and hydrophilic head phospholipids have hydrophobic and hydrophilic regions, amphipathic ```

Question 26

three types of lipid aggregates formed by amphipathic phospholipids in aqueous environments, and their purpose

Answer 26

purpose is to shield hydrophobic tails from water micelle: small, spherical droplet consisting of phospholipid monolayer bilayer: two-dimensional sheet, in cell membranes liposome / vesicle: lipid bilayer folds back on itself to form hollow sphere, creating a separate aqueous compartment (maximal stability achieved)

Question 27

what is the function of a phospholipid's amphipathic nature? (each of the three types of lipid aggregates)

Answer 27

lipid bilayer: selectively permeable cell membrane, effective barrier between cell and environment liposome / vesicle: for storage and transport of cellular products and digestion of waste, and for drug delivery in humans micelles: transport of fats between gut and body tissues

Question 28

what is the purpose of the hydrophobic interactions between fatty acid tails

Answer 28

integrity of membrane bilayer / liposomes / vesicles / micelles is maintained individual interactions are weak, permitting lateral movement of phospholipids, accounting for membrane fluidity

Question 29

what is the function of most phospholipids containing choline?

Answer 29

choline is important for the synthesis of acetylcholine, a neurotransmitter in humans

Question 30

what is the structure of glycolipids?

Answer 30

glycolipids are composed of two hydrophobic hydrocarbon tails, and a polar, short carbohydrate chain (less than 15 sugar residues) with no phosphate group

Question 31

describe the bonding within glycolipids

Answer 31

the short carbohydrate chain is joined to the glycerol's OH group by a glycosidic bond, which is a type of covalent bond

Question 32

what is the function of a glycolipid, where a carbohydrate chain is attached to the glycerol?

Answer 32

glycolipids are found at the cell surface membrane facing the exterior environment, and serve as markers for cell-cell recognition it is also involved in cell-cell adhesion as a result of binding in tissue formation

Question 33

what function do the hydrophobic interactions in glycolipid's fatty acid tails serve?

Answer 33

they anchor the entire glycolipid at the cell surface membrane

Question 34

what are lipid derivatives (and its examples)?

Answer 34

steroid hormones, ketone bodies, fatty alcohols, terpenes and carotenoids

Question 35

what is the structure of steroid cholesterol?

Answer 35

cholesterol has a carbon skeleton made up of three fused six-membered and one five-membered ring it regulates membrane fluidity and is a precursor for bile acid and steroid hormone and vitamin D synthesis

Question 36

describe the emulsion test for lipids (principle, method, observation)

Answer 36

principle: lipids are soluble in organic solvents such as ethanol, but not in water, with which they form emulsions upon vigorous shaking method: add 2cm^3 of ethanol to the sample and mix well, then add an equal volume of water observation: if lipids are present, it dissolves in ethanol to form a clear solution, which then forms white precipitate with water. if lipids are absent, a clear solution is still formed with ethanol, which remains clear when added to water

Question 37

how are ester linkages formed and broken?

Answer 37

formation: condensation reaction between the H of an OH hydroxyl group and the OH of a COOH carboxyl group, also produces water breakage: hydrolysis reaction that requires water as a reactant