Lipids Flashcards
what molecules are grouped under the category of lipids?
fats (triglycerides), phospholipids, glycolipids, sterols
what are the characteristics of lipids?
insolubility in water, and high solubility in non-polar solvents
what three types of lipids are there, and what are their differences?
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
define simple lipids
simple lipids consist of an alcohol (usually glycerol) linked to one or more fatty acids via an ester linkage (eg. triglycerides)
what role do phospholipids and sterols play in biological membranes?
they are major structural elements
what is the molecular formula of glycerol?
C3H8O3
what are the structural features of glycerol? (number of carbons, functional groups, solubility in water)
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
what are fatty acids? (type of acid, functional group, structure)
carboxylic acids, composed of an acidic carboxyl (COOH) functional group and an attached hydrocarbon chain (long carbon skeletons between 12-20 carbons long)
why are fatty acids so hydrophobic?
due to the abundance of non-polar C-H bonds
what are the ways in which fatty acids can differ?
- length of hydrocarbon chain (between 12-20 carbons long)
2. number and location of C=C double bonds along their carbon skeletons
what is the difference between saturated and unsaturated fatty acids (and their examples)?
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)
what is an ester linkage (where is it found, how is it formed)
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
what does the linkage of a glycerol molecule to different numbers of fatty acids produce?
one fatty acid: monoglyceride
two fatty acids: diglyceride
three fatty acids: triglyceride
how does the melting point of fats vary with hydrocarbon chain length, and why?
proportionally related.
as hydrocarbon chain length increases, hydrophobic interactions between the chains becomes more extensive, and melting point increases
what are hydrophobic interactions
a weak bond that exists between hydrophobic molecules, a force of attraction between non-polar molecules
how does the melting point of fats vary with the degree of unsaturation, and why?
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
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?
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
how do the hydrogen atoms per unit mass of triglycerides and carbohydrates compare?
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)
what is the nature of triglyceride molecules, and its function?
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
nature of hydrocarbon tails, and its function
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
molecular weight of triglycerides compared to water per unit volume, and its function
lower molecular weight than water per unit volume
less dense than water, aid buoyancy
define compound lipids (with examples)
esters of fatty acid(s) and an alcohol plus other chemical groups (eg. phosphate and sugar).
examples: phospholipids and glycolipids
describe the structure of phospholipids
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
how are phospholipids formed?
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
phospholipids’ relationship with water
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
three types of lipid aggregates formed by amphipathic phospholipids in aqueous environments, and their purpose
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)
what is the function of a phospholipid’s amphipathic nature? (each of the three types of lipid aggregates)
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
what is the purpose of the hydrophobic interactions between fatty acid tails
integrity of membrane bilayer / liposomes / vesicles / micelles is maintained
individual interactions are weak, permitting lateral movement of phospholipids, accounting for membrane fluidity
what is the function of most phospholipids containing choline?
choline is important for the synthesis of acetylcholine, a neurotransmitter in humans
what is the structure of glycolipids?
glycolipids are composed of two hydrophobic hydrocarbon tails, and a polar, short carbohydrate chain (less than 15 sugar residues) with no phosphate group
describe the bonding within glycolipids
the short carbohydrate chain is joined to the glycerol’s OH group by a glycosidic bond, which is a type of covalent bond
what is the function of a glycolipid, where a carbohydrate chain is attached to the glycerol?
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
what function do the hydrophobic interactions in glycolipid’s fatty acid tails serve?
they anchor the entire glycolipid at the cell surface membrane
what are lipid derivatives (and its examples)?
steroid hormones, ketone bodies, fatty alcohols, terpenes and carotenoids
what is the structure of steroid cholesterol?
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
describe the emulsion test for lipids (principle, method, observation)
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
how are ester linkages formed and broken?
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
