LIPIDS NOTES

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These are fats and oils.
Fats are solid at room temperature while oils are liquid.
They are made up of carbon, oxygen and hydrogen atoms.
The structural units of lipids are fatty acids and glycerol.
Fatty acids are made up of hydrocarbon chain molecules with a carboxyl group (-COOH) at one end.
In the synthesis of a lipid, three fatty acid molecules combine with one glycerol molecule to form a triglyceride.
Three molecules of water are lost in the process.
This is a condensation reaction and water is given off.
Lipids are hydrolysed e.g. during digestion to fatty acids and glycerol, water is added.
Condensation = Glycerol + 3 Fatty hydrolysis Lipid + Water acids 

Properties of Lipids

• Fats are insoluble in water but dissolve in organic solvents e.g. in alcohols.
• They are chemically inactive, hence used as food storage compounds.

Functions of Lipids

• Structural materials - as structural material they make up the cell membrane.
• Source of energy - they are energy rich molecules. One molecule of lipid provides more energy than a carbohydrate molecule.
• Storage compound - They are stored as food reserves in plants. In animals e.g. mammals, all excess food taken is converted to fats which are stored in adipose tissue, and around internal organs such as the heart and kidneys.
• Insulation - They provide insulation in animals living in cold climates. A lot of fat is stored under the skin e.g. blubber in seals.
• Protection - Complex lipids e.g. wax on leaf surfaces protects the plant against water-loss and overheating. Fats stored around some internal organs acts as shock absorbers, thus protecting the organs.
• Source of Metabolic Water -:-lipids when oxidised produce metabolic water which supplements water requirements in the body. Desert animals e.g. the camel accumulate large quantities of fat in the hump which when oxidised releases metabolic water.

Proteins

• Proteins are the most abundant organic compounds in cells and constitute 50% of total dry weight.
• Proteins are compounds which are made up of carbon, hydrogen, nitrogen, oxygen and sometimes sulphur and phosphorus.
• The structural units of proteins are amino acids.
• The nature of a protein is determined by the types of amino acids it is made of.
• There are about 20 common amino acids that make up proteins. 

Essential and Non-Essential Amino Acids

• Essential amino acids are those which cannot be synthesised in the body of an organism and must therefore be provided in the diet.
• There are ten amino acids which are essential for humans.
• These are valine, leucine, phenylalanine, lysine, tryptophan, isoleucine, methionine, threonine, histidine and arginine.
• Non-essential amino acids are those which the body can synthesise and therefore need not be available in the diet.
• There are ten of them.
• These are glycine, alanine, glutamic acid, aspartic acid, serine, tyrosine, proline, glutamine, arginine and cysteine.
• Proteins are essential in the diet because they are not stored in the body.
• Excess amino acids are deaminated.

Formation of Proteins

• Proteins are made up of many amino acid units joined together through peptide bonds.
• When two amino acids are joined together a dipeptide is formed.
• The chemical process involved is called condensation and a molecule of water is eliminated .
• When many amino acids are joined together a polypeptide chain is formed.
• The nature of a particular protein depends on the types, number and sequence of amino acids from which it is made. 

Functions of Proteins
As structural materials proteins-

• These are the basic building structures of protoplasms
• Proteins in conjunction with lipid form the cell membrane.

Examples of structural proteins include:

• Keratin (in hair, nails, hoofs, feathers and wool)
• Silk in spider's web.
• Elastin forms ligaments that join bones to each other.

Protective proteins.

• Antibodies that protect the body against foreign antigens.
• Fribrogen and thrombin are involved in clot formation, preventing entry of micro-organisms when blood vessel is cut.

 
As functional chemical compounds.
Examples are hormones and enzymes that act as regulators in the body.
Respiratory pigments.
Examples are haemoglobin that transports oxygen in the blood and myoglobin that stores up oxygen in muscles.
Contractile proteins - make up muscles, i.e. myosin and actin.
Proteins combine with other chemical groups to form important substances e.g. mucin in saliva.
Source of energy.
Proteins are a source of energy in extreme conditions when carbohydrates and fats are not available e.g. in starvation.

Enzymes

• Enzymes are biological catalysts that increase the rate of chemical reaction in the body.
• They are all produced inside cells.
• Some are intracellular and they catalyse reactions within the cells.
• Others are extracellular and are secreted out of the cells where they work. E.g. digestive enzymes.

Properties of Enzymes

• Enzymes are protein in nature.
• Enzymes are specific to the type of reaction they catalyse.
• This is referred to as substrate specificity.
• Enzymes work in very small amounts.
• They remain unchanged after the reaction.
• They catalyse reversible reactions.
• They work very fast (high turnover numbers) e.g. the enzyme catalase works on 600 thousand molecules of hydrogen peroxide in one second. 

Naming of enzymes
Enzymes are named by adding the suffix -ase to:
Name of substrate that they work on e.g.

• Carbohydrates - carbohydrases e.g.sucrase.
• Starch (amylose) - amylase
• Protein - proteinase (protease)
• Lipids -lipases

Type of chemical reaction catalysed e.g.

• Oxidation - oxidase
• Reduction - reductase
• Hydrolysis - hydrolase

Factors Affecting Enzyme Action

Temperature

• Enzymes are sensitive to temperature changes.
• Generally, the rate of an enzyme-controlled reaction doubles with every 10OC increase in temperature.
• However, temperatures above 40°C do not favour enzyme reaction.
• This is because enzymes are denatured by high temperatures.

pH

• Every enzyme has a particular pH range over which it works best.
• Some enzymes work best in acidic media while others function better in alkaline media.
• Many enzymes function well under neutral conditions.

Enzyme Concentration

• Under conditions where the substrate is in excess, the rate of an enzyme-controlled reaction increases as the enzyme concentration is increased.

Substrate Concentration

• If the concentration of the substrate is increased while that of the enzyme remains constant, the rate of the reaction will increase for some time and then become constant.
• Any further increase in substrate concentration will not result in corresponding increase in the rate of the reaction.

Enzyme Inhibitors

• These are substances that either compete with substrates for enzyme active sites or combine with enzymes and hence they inhibit the enzyme reaction; e.g. certain drugs, cyanide and nerve gas.

Co-factors

• Most enzymes require the presence of other compounds known as co-factors which are non-proteins.
• There are three groups of co-factors.
• Inorganic ions - e.g. iron, magnesium, copper and zinc.
• Complex organic molecules known as prosthetic groups are attached to the enzyme e.g. flavin adenine dinucleotide (FAD) derived from vitamin B2 (riboflavin).
• Co-enzymes e.g. coenzyme A is involved in respiration.
• All co-enzymes are derived from vitamins.

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