Lipids

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  • Created by: Jenny Le
  • Created on: 09-04-14 22:27

What are Lipids?

Soluble in organic solvents

e.g. chloroform and methanol

Insoluble in water

Small molecules but they form aggregates held together by weak interactions.

Have no monomers that can combine to make polymers.

Not all chemically related - they are classified together due to physical property 

Considered as macromolecules because they aggregate together but are not large in weight

They cluster together due to weak hydrophobic interactions which hold the structure loosely giving flexibility and fluidity, ideal for membranes.

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Classification of Lipids

SIMPLE

Fats and Waxes

COMPLEX

Glycerophospholipids - from glycerol

Sphingolipids - from sphingosin

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Functions of Lipids

Structural component of membrane

Intracellular storage depots of metabolic fuel - much of our energy comes from fats in our diet, excess energy is stored in fat cells (adipocytes) in adipose tissue.

Transport of metabolic fuel - have to be made soluble in order to be transported in the blood. Lipoproteins (lipids combined with protein) are used to transport metabolic fuel.

Protection - the hydrophobic nature acts as a waterproof agent on bacterial cell walls, leaves, exoskeleton of insects and skin of vertbrates.

Some lipids have intense biological activity including some vitamins and their precursors and a number of hormones.

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Fatty Acids

Have a long hydrocarbon chain and a terminal carboxyl group.

The simplest lipids are fatty acids.

Always number from the carboxyl group

Dieticians number from the opposite end (omega end)

Often refer to omega 3 and omega 6 (3 and 6 represent the positions of a double bond)

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Saturated/Unsaturated Fatty Acids

EXAMPLE: Palmitic acid (Hexadecanoic) - symbol 16:0 (16 carbons : 0 double bonds) - SATURATED fatty acid - no double bonds

EXAMPLE: Stearic acid (Octadecanoic) - symbol 18:0 (18 carbons : 0 double bonds) - SATURATED fatty acid - no double bonds

EXAMPLE: Oleic acid (9-octadecenoic) - symbol 18:1D9 (18 carbons : 1 double bond on carbon 9) - UNSATURATED fatty acid - one or more double bond

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Cis/Trans isomerism

Molecules that form double bonds can occur in two isomeric forms: cis and trans.

In cis-isomers - similar groups are on the same side of the double bond. This is the most naturally occurring isomer. 

In trans-isomer - similar groups are on opposite sides of the double bond

Trans fatty acids have structurs similar to saturated fatty acids and have high melting points.

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Melting Points in Fatty Acids

As the hydrocarbon chain in fatty acids lengthens, the melting points increases 

Saturated fats are solid at room temperature - found in animal fats and dairy products.

Melting points of unsaturated fats are below room temperature and so are liquid. The more double bonds, the lower the melting point.

Most fatty acids are obtain from the diet but can be synthesised - non essential.

Linolenic and Linoleic acid cannot be synthesised by mammals as we don't have the appropriate enzymes. These are essential fatty acids and are required in the diet. They are found in vegetable oils, nuts and seeds. 

A deficiency of these fatty acids can lead to poor wound healing and hair loss.

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Polarity of Fatty Acids

Fatty acids are AMPHIPATHIC (dual sympathy). They have both a polar head and a hydrophobic hydrocarbon chain.

This means that fatty acids will form a monolayer on the surface of water.

Saturated fatty acids have full rotation around every bond - extended chain

Double bonds are rigid and only give a 30 degree bend in the chain. 

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Acylglycerols (Neutral fats or Glycerides)

A fatty acid can be called an acyl group where the R is the long hydrocarbon chain.

Fatty acids have some biological function in signaling but their main role is as precursors (starting material) of other molecules.

Acylglycerols - esters of fatty acids with a sugar alcohol called glycerol derived from glyceraldehyde.

3 fatty acids - triacylglycerol (TAG or triglyceride)

2 fatty acids - diacylglycerol (DAG or diglyceride)

1 fatty acid - monoacylglycerol (MAG or monoglyceride)

Neutral fats have no charge or polar groups - therefore hydrophobic


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Triacylglycerols (Triglycerides)

The fatty acid molecules do not have to be the same, they can be different, unsaturated or saturated.

Function: Storage of metabolic fuel (energy)

Triacylglycerols are unchared (neutral fat) and not amphipathic

They are a compact molecule hence their role to store energy as they are not hydrated so take up less room and are less oxidised so containt more energy.

Fatty acids are broken down by B-oxidation which removes 2C fragments as acetyl CoA. This acetyl CoA then feeds into the citric acid cycle.

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Metabolism of Lipids and Steroids

Fat is broken down by a metabolic pathway known as beta oxidation which produces coenzymes for the ETC and acetyl CoA for the citric acid cycle.

Glycerol can feed into glycolysis.

Pentose phosphate pathway provides coenzyme NADPH needed to synthesis lipids from acetyl CoA.

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Adipocytes (Fat Storage Cells)

Adipocytes consist of up to 95% triacylglycerols.

This is stored as an energy reserve for times of food shortage. 

Very efficient as it is less oxidised than other forms of energy and therefore will yield more energy on oxidation.

Long lasting storage - up to 2-3 months. Whereas glycogen is only short term - 1 day.

Adipose tissue is also responsible for insulating the body.

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Hydrogenation

Hydrogenatin is an industrial process to hydrogenate plant oils using hydrogen and a catalyst.

Hydrogenating an oil saturates the double bonds and solidifies it - used in the manufacture of margarine.

Some double bonds are converted to trans double bonds.

Hard fats exposed to air are oxidised and double bonds are formed - liquefaction

Hydrogenation produces trans fats which are not very healthy, a new process is now used to emulsify oils in order to produced polyunsaturated spreads e.g. Flora. 

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Waxes

Waxes are similar to triacylglycerols but they are esters of longer chains fatty acids with long chain monohydroxylic alcohols.

Function: external protection, e.g. preventing water loss from surfaces, cuticles of exoskeleton on insects, coating on leaves and fruit, waterproofing fur, feathers and wool - lanolin (wool fat) used in cosmetic industry as moisturiser.

Structural function: Beeswax

R groups - both long hydrocarbon chains one from acid and one from alcohol. 

Very hydrophobic hence its role to prevent water loss

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Phospholipids

Glycerphospholipids (phosphoglycerides) - 1 glycerol, 2 fatty acids, 1 phosphate and alcohol group. More complex.

Similar to acylglycerols but have hydrophilic phosphate and alcohol group.

Phospholipids are amphipathic - polar head (polar groups and charged oxygens) and hydrophobic tails (two fatty acids).

Phosphatidyl cholines (lecithins) - the most abundant phospholipid in animal tissue (membranes)

Also found as an emulsifier in food products and cosmetics.

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Sphingolipids

Sphingolipids - some are phospholipids, some are glycolipids.

Found in membranes

Derived from sphingosine - a long chain amino alcohol

Ceramide - sphingosine joined to a fatty acid - has two long hydrocarbon tails - hydrophobic.

Different polar head groups are joined to the ceramide to create amphipathic structures

There are 3 subclasses of sphingolipids with different head groups.

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Sub class 1 - Sphingomyelin

Sphingomyelin (phospholipid) head group - phosphocholine or phosphoethanolamine.

Have similar srtucture to phosphatidylcholine, a glycerophospholipid.

Present in membranes, especially myelin sheaths of nerve fibres.

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Sub class 2 - Cerebrosides

A galactocerebroside - a sphingolipid and a glycolipid.

Have short carbohydrate chains as their head group.

Head group -1 to 6 sugar units e.g. galactocerebroside has 1 galatose and glucocerebroside has 1 glucose.

Present in membranes, accound for 15% of the lipids in myelin sheath.

Ceramide with a beta linkage to C1 on the sugar.

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Sub class 3 - Gangliosides (glycolipid)

Head group - large oligosaccharides containing several sugar units including N-acetyl neuraminic acid. (sialic acid) 

Present in cell surface membranes, e.g. grey matter of brain tissue.

Cell surface antigens -  Involved in cell recognition andcell/cell interactions e.g. blood group determination on RBC

Inherited defects in ganglioside metabolism found in Tay Sachs disease. Sufferers lack the enzyme to degrade gangliosides which build up in nervous tissue and lead to nerve degradation.

Children suffer retarded development, paralysys, blindness and death by age 3 to 4.

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Isoprenoids

Minor lipid components - built from 5 carbon units similiar in structure to isoprene - terpenes

Can be linear or cyclic

2 isoprene units - MONOTERPENES

3 isoprene units - SESQUITERPENES

Found in plants - essential oils

Give characteristic odours and flavours, e.g. oil or geranium, lemon oil, mint oil, turpentine, camphor, caraway oil.

Menthol and camphor used in nasal decongestants.

Turpentine used for cleaning paint brushes

Natural rubber - polyterpene - thousands of isoprene units.

Multiple isoprene units - squalene - precursor of steroids.

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Eicosanoids

All contain 20 carbon atoms.

Derived from fatty acid - arachidonic acid

Prostaglandins have important roles has local hormones.

Thromoxanes regulate smooth muscle contraction, nerve transmission.

Leukotrienes - immune response inflammation

Aspirin is an anti-inflammatory drug because it inhibits the pathway of prostaglandin production which is involved in promoting inflammation.

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Steroids

Steroid molecules are also lipids - they are insoluble in water and soluble in organic solvents.

The characteristic steroid ring is produced by secondary folding of a long hydrocarbon chain built up of isoprene units. 

Cholesterol - about the same length as a 16C fatty acid, fits well into membrane structure. Its rigidity moderates the fluidity of the membrane.

Bile acids and steroid hormones are examples of compounds derived from cholesterol. 

Bile acids are emulsifites of fats, they act as detergents due to the hydrophilic side chain at C17.

Steroid hormones:

  • Oestradiol & Testosterone - sex hormones
  • Aldosterone - regulates salt excretion
  • Cortisol - regulates glucost metabolism
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Vitamins

VITAMIN D

Role in phosphate and calcium metabolism

VITAMIN A 

Retinol essential for vision

VITAMIN E

Antioxidant

VITAMIN K

Blood clotting

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Lipid Transport in the body

Lipids are transported bound to carrier proteins because they will not dissolve in blood. 

Free fatty acids are bound to albumin

Other lipids are bound to lipoproteins

Protein component is apolipoprotein or apoprotein

Apoproteins are produced in the liver where the lipoproteins are assembled. 

5 major classes of lipoproteins (based on density):

  • Chylomicrons (remove dietary fat from intestines) 
  • Very low density lipoproteins (VLDL)
  • Intermediate density lipoproteins (IDL)
  • Low density lipoproteins (LDL)
  • High density lipoproteins (HDL)

Lipoproteins have a hydrophobic core of triacylglycerols and cholesterol esters surrounded by amphipathic lipids - phospholipids and free cholesterol.

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Blood cholesterol levels

High blood cholesterol levels lead to:

  • Fatty plaques in arteries
  • Atherosclerosis (hardening of arteries) - high accumluation of LDL cholesterol in the blood vessels, especially coronary arteries.
  • Obstruction of arteries by clots - deposition of lipids leads to inflammation, macrophages becomed engorged with lipid and are known as foam cells and a plaque is deposited in vessel walls. Plaque can then rupture and clots form blocking the arteries. 
  • Myocardial infarction (heart attack) or stroke - blocking of arteries prevents circulation to the heart which can cause a heart attack. If the circulation to the brain is blocked, this can cause a stroke. 

We require cholesterol which is synthesised in the liver for membranes and hormone synthesis. To some extent our cholesterol levels are genetically determined. 

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Cholesterol and Diet

Fats should also be limited insty  diets as well as cholesterol. Diets high in monounsaturated fats and polyunsaturated omega fatty acids decrease the risk of heart disease.

HDL cholesterol transports cholesterol to the liver for disposal and therefore also reduces the risk of heart disease.

Smoking is an important risk factor for the development of CVD. 

The uptake of dietary cholesterol can be reduced by plant sterols e.g. Benecol. Drugs called statins are used to inhibit cholesterol synthesis in at risk groups such as diabetics.

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Genetic disorders

FAMILIAL HYPERCHOLESTEROLAEMIA

Defective LDL cholesterol receptors

Cholesterol doesn't transfer to cell

In liver synthesis is not inhibited, uncontrolled synthesis increases cholesterol futher.

Homozygotes - coronary heart disease in adolescence, cholesterol is 3-5 times greater than normal, may lead to heart attack as early as age 5.

Heterozygotes - 50% chance of heart disease by age 60, cholesterol levels are about twice normal levels.

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