Lipids

What are lipids?

Lipids are the one class of large biological molecules that does not include true polymers, and they are generally not big enough to be considered macromolecules

• fats (triglycerides)

• phospholipids - structural

• glycolipids

• sterols - structural

—> other lipids play crucial roles as enzyme cofactors, electron carriers, light absorbing pigments, hydrophobic anchors for proteins, emulsifying agents in teh digestive tract, hormones, and intracellular messengers

• Properties: 1. insoluble in water 2. high solubility in non-polar solvents

Classifying lipids

1. Simple lipids (FA + alcohol — triglycerides)

2. Compound lipids (FA + alcohol + other groups - phospholipids)

3. Lipid derivatives

Simple lipids

• consist of an alcohol (usually glycerol) linked to one or more fatty acids via an ester linkage

1 glycerol + 3FA = triglyceride

Triglycerides - structural components - Glycerol

Glycerol

• C3H8O3

• (S) three-carbon alcohol, with each carbon bearing a hydroxyl -OH group

• (P) soluble in water cos of polar -OH group

Triglycerides - structural components - fatty acids

Fatty acids

• (S) carboxylic acids composed of an acidic carboxyl (COOH) functional group at one end and an attached hydrocarbon chain

COOH + hydrocarbon chain = FA

• possess long carbon skeletons (btw 12-20 carbons) with each carbon joined to a hydrogen atom

• (P) abundance of non-polar C-H bonds = hydrophobic

How do fatty acids differ

1. Length of hydrocarbon chain

2. Number and locations of double bonds (C=C) along their carbon skeletons

• a saturated fatty acid has no double bonds (every carbon atom is maximally bonded to hydrogen atoms)

• unsaturated fatty acid has 1 or more double C=C bonds - FA will have a kink in its tail wherever a double bond occurs

table showing commonly occurring fatty acids

Formation of glycerides

• A glycerine is formed when glycerol is linked to one or more fatty acids, resulting in the formation of an ester linkage

• ester linkage between a hydroxyl (OH) group of glycerol and carboxyl group (COOH) group of a fatty acid —> condensation rxn, where one water molecule of water is lost

• each glycerol has 3 -OH groups, has the potential to form up to 3 ester linkages

glycerol link to ? fatty acids

glycerol + 1FA = monoglyceride

glycerol + 2FAs = diglyceride

glycerol + 3FAs - triglyceride = fats

Properties of triglyceride

1. as hydrocarbon chain length increases, MP of fats increases

• longer hydrocarbon chains, more extensive hydrophobic interactions btw the chains = higher MP (more thermal energy is required to break the bonds)

• hydrophobic interaction is a weak bond that exists btw hydrophobic molecules. force of attraction btw non-polar molecules

2. as degree of unsaturation of fatty acids tails increases, (more C=C), MP of fats decreases

• kinks where the double bonds are located prevent the molecules from packing closely

• when fatty acid tails are less closely packed, hydrophobic interactions are less extensive

• less thermal energy is required to break enough of these interaction to liquefy the triglycerides

saturated and unsaturated fats

• made from saturated fatty acids —> saturated fat

most animal fats e.g. butter, are saturated and are solid at room temperature (H/C packed)

• unsaturated fats

e.g. fats of plants and fishes, cooking oil, fish liver oil

consist of one or more types of unsaturated fa, usually liquid at room temp, referred to as oils

Structure of triglycerides & Function

1. Triglycerides have a higher proportion of C and H atoms compared to O atoms

• Triglycerides contain a greater number of carbon atoms per unit mass than carbohydrates

—> Function: Upon oxidation, triglycerides release a large amount of energy

— one gram of fat releases more than twice as much energy (38kJ/g) as a gram of carbohydrates (17kJ/g)

— triglycerides are more efficient energy stores than carbs

2. Triglycerides are highly reduced molecules (contain two-fold more hydrogen atoms per unit mass than carbs)

—> Function: release more water when they are oxidised during cellular respiration compared to carbohydrates

— water, known as metabolic water, is extremely impt to desert animals like camels

3. The C-H bonds are non-polar and hence triglycerides are hydrophobic

• no associated water molecules are stored along with triglycerides and thus triglycerides have no extra weight due to water of hydration

—> Function: triglycerides do not affect water potential of cells when stored in large amounts

— absence of water of hydration in triglycerides fulfils the requirement of an animal’s body mass to be kept to a minimum to facilitate locomotion

— good thermal insulator and hence a layer of fat beneath the skin (subcutaneous fat) insulates the body

— this layer is especially thick in whales, seals and most other marine animals living in cold climates —> blubber

4. hydrocarbon tails are non-polar

• weak hydrophobic interaction occur between triglyceride molecules

—> Function: Triglycerides can slide under pressure

— adipose tissue (contains fats) around vital organs helps to cushion and protect the vital organs against physical impacts

5. triglycerides have a lower molecular weight than water per unit volume

• less dense than water, fats aid buoyancy of aquatic animals (e.g. blubber in whales)

Compound lipids

• compound lipids are esters of fatty acids and an alcohol plus other chemical groups, such as phosphate and sugar

• e.g. phospholipids and glycolipids

Phospholipids

• most important function of FA in cells is in the construction of cell membranes

• phospholipids are major constituents of cell membranes

• 1 glycerol + 2 FA + third OH group of glycerol joined to a negatively-charged phosphate group = phospholipid

• additional small molecules, usually charged or polar (e.g. serine, choline and inositol) can be linked to the phosphate group to form a variety of phospholipids

Formation of phospholipids

• 2 FA linked to glycerol by ester linkage

• while phosphate group linked to third OH group of glycerol by a phosphoester linkage

Properties of phospholipids

1. Phospholipids show ambivalent behaviour towards water (hydrophobic and hydrophilic)

• FA hydrocarbon tails are non-polar and hence hydrophobic (but can form bond with hydrophobic molecules or among themselves)

• phosphate group and its attachments form polar/charged hydrophilic head that has an affinity for water

• molecules such as phospholipids with both hydrophobic and hydrophilic regions are described as amphipathic

2. 3 types of lipid aggregates can form when amphipathic phospholipids are situated in aqueous environments - serve to shield the hydrophobic tails from water/the aqueous environment

• micelle - small, spherical droplet consisting of a phospholipid monolayer, with the phosphate heads on the outside, in contact with the aqueous environment

• bilayer - 2 lipid monolayers combine to form a two-dimensional sheet

  • hydrophilic heads are exposed to the polar exterior while the hydrophobic tails are in contact with those of neighbouring molecules but excluded from water in the non-polar interior of the bilayer

• liposome/vesicle - formed when a lipid bilayer folds back on itself to form a hollow sphere

  • by forming vesicles, bilayer sheets avoid exposing their hydrophobic edge regions, achieving maximal stability in their aqueous environment

  • enclose aqueous solutions, creating a separate aqueous compartment

Structure and function of phospholipids

1. Phospholipids are amphipathic molecules each with 2 non-polar, (P) hydrophobic fatty acids tails and a charged (P) hydrophilic phosphate head

—> Function: (bilayer) phospholipids form a selectively permeable cell membrane where hydrophilic heads are exposed to aqueous medium while hydrophobic tails are in contact with those of neighbouring molecules but excluded from aqueous medium in the non-polar interior of the bilayer

= forms effective barrier/boundary between cell and its external environment

—> (liposome/vesicle) phospholipids form liposome/vesicle when a lipid bilayer folds back on itself to form a hollow sphere

— liposomes are used as vesicles for storage and transport of cellular products (e.g. proteins, lipids) as well as for digestion of waste (in vesicles known as lysosomes); liposomes also serve as vesicle for drug delivery in humans

—> (micelle) phospholipids form micelles used for the transport of fats between the gut and the body tissues

— hydrophilic heads of phospholipids are in contact with aqueous environment and hydrocarbon tails are restricted to water-free interior of the micelle

2. hydrophobic interactions exist btw fatty acid tails

—> Function: integrity of the membrane bilayer/liposomes/vesicles/micelles is maintained due to large number of interactions

— individual hydrophobic interactions are weak, permitting lateral movement of phospholipids, which account for membrane fluidity

3. most phospholipids contain choline

—> Function: most abundant phospholipids in c.m. contain choline, represent large proportion of the body’s store of choline

— choline is impt for the synthesis of acetylcholine, a neurotransmitter

Compound lipids - Glycolipids

• glycolipids are composed of 2 hydrophobic hydrocarbon tails + a polar, short carbohydrate chain (less than 15 sugar residues) with no phosphate

• short carbohydrate chain is joined to the glycerol’s -OH group by a glycosidic bond, which is a covalent bond

Structure and function of glycolipids

1. carbohydrate chain is attached to the glycerol

• found at the cell surface membrane facing the exterior environment

• serves as a marker that distinguishes one cell from another in cell-cell recognition - cells recognise other cells by binding to these carbohydrate chains

2. hydrophobic interactions exist between fatty acids tails

• hydrophobic interaction btw fatty acids tails serve to anchor the entire glycolipids at the cell surface membrane

Lipid derivatives

• e.g. steroid hormones, ketone bodies, fatty alcohols, terpenes and carotenoids

• steroid cholesterol***

• cholesterol

  • possesses carbon skeleton made up of 3 fused six-membered and 1 five-membered ring

  • regulate membrane fluidity

  • precursor for synthesis of bile acids, steroid hormones (e.g. oestrogen and testosterone) and vitamin D

emulsion test

Principle: lipids are soluble in organic solvents such as ethanol, but not in water, with which they form emulsions on vigorous shaking

Method:

• add 2cm3 of absolute ethanol to sample and mix well

• decant the ethanol into another test tube containing an equal volume of water

• if lipid is present, dissolves in ethanol to form a clear solution, which then forms an emulsion with water

• if lipid is absent, a clear solution is still formed with ethanol, which remains clear when added to water