UNIT 2. MAJOR MOLECULES IN FOOD

Learning objectives After completing this chapter learner should be able to: understand the type of major molecules that make up food become familiar about the sources that supply these macromolecules understand the important functions of each macromolecule.

1.MAJOR MOLECULES IN FOOD

1. Introduction The four major classes of molecules in food are carbohydrates, protein, fat and nucleic acids.  They are made up of relatively small micromolecules called monomers that are linked together to create large macromolecules, which are known as polymers. When monomers are linked together to synthesize a biological polymer, they undergo a process called synthesis. 

2. Functions of macromolecules To provide materials for growth and repair of tissues in order to provide and maintain the basic structure of our bodies.  To supply the body with energy in the form of ATP required to perform external and internal activities To provide materials which regulate the functions of the body To store and decode genetic information To perform structural and catalytic roles in cells

3. Nature of Major molecules The main classes of molecules in food are are called macromolecules  or  polymers. The small micromolecules that are linked together to create these polymers are called monomers.  Monomers are linked together to synthesize a biological polymer by enzyme catalysed reactions.

3.1 Carbohydrates Carbohydrates are the most widely distributed and abundant organic compound on earth. The dry substance of plant is composed of 50-80% of carbohydrates. The structural material in plants is mainly cellulose and hemicellulose. Starch is the major storage polysaccharide in plants. Pectins and sugars such as sucrose ang glucose are also plant components. Glycogen is the storage form of carbohydrates in liver and muscle cells of animls. The blood group substances, mucins and mucopolysaccharides of ground substanes between cells are other important types of carbohydrates in animals. Chitin is an important compound present in the exoskeleton of invertebrates. Carbohydrate biosynthesis in plants starting from carbon dioxide and water with the help of light energy (Photosynthesis) is the basis for the existence of all other organisms which depend on the intake of organic substances with food. 

Functions They are the major source of biological energy through their oxidation in the tissues. They play an important role in the energy metabolism of animals and plant. The energy needed for mechanical work and chemical reactions in all living forms are derived from carbohydrates. The carbon skeletons of almost all carbohydrates are derived from carbohydrates. They provide the basic raw materials for the synthesis of many industrially important compounds.

Sources of carbohydrates Carbohydrate rich foods are abundant and cheap compared with fats and protein.  Food carbohydrates : Starches and sugars found in cereal grains, tubers, vegetables, legumes, fruits, milk and milk products. Fructose is present in Honey. Fiber, consisting largely of cellulose and other non digestible cell-wall polymers of plant origin that are non digestible and plays no metabolic role but helps to maintain proper motility in the intestinal tract.

Sources of Carbohydrate

3.2. Lipids Lipids refer to substances such as fats and oils and fat like substances. They are another group of major molecules found in food. Lipids exhibit greater structural variety than the other classes of biological molecules.   

Functions of lipids Lipids containing hydrocarbon chains serve as energy stores.Fats supply over twice as much energy per unit weight as proteins or carbohydrates. The presence of lipids in diet contributes considerably to palatability. Many enzymes need lipid molecoles for maximal activity. Lipids supply the essential fatty acids which are not synthesised in humen beings but are essential for growth.. They are essential for the absorption of fat soluble vitamins A, D, E and K. In the form of lipid bilayers they are essential components of biological membranes. Many intra-and intercellular signaling events involve lipid molecules. In addition to triglycerides, cholesterol and phospholipids provide structure to the body cells. The subcutaneous lipid deposits act as insulation against excessive heat loss to the environment and insulate vital organs against mechanical trauma. 

Sources Important animal sources are meat, milk , eggs and fish and important plant sources are all oils and vanaspathy.  Fats from animal  sources are relatively rich in saturated fatty acids but contain a low content of polyunsaturated acids. Fats from plant and fish are rich in polyunsaturated fatty acids. Cholesterol is present in significant amounts in animal products, such as egg yolk, butterfat, and meat. It is absent in plant foods.

3. 3 Proteins Proteins are the most abundant substances in all cells forming about  50% of a cell's over all mass. A protein is a unbranched polymer of amino acids.  

Functions Proteins supply the required building blocks for protein biosynthesis. Amino acids are precursors of other nitrogen containing substances such as enzymes, hormones, porphyrins, and many other biomolecules. They also contribute to the flavour of food and are precursors of aroma compounds and colours formed during thermal or enzymatic reactions in production, processing and storage of food.  Oxidation of the carbon skeletons of amino acids also furnishes significant fraction of the total daily energy requirement.  

Animal Sources: Meat, fish, milk, egg, Important sources of protein. Animal Sources: Meat, fish, milk, egg, Plant sources: Cereals, pulses, beans and peas

Sources of protein. Meat, fish, milk, egg, pulses, beans and peas

3.4 Nucleic acids Nucleic acids are polymers composed of nucleotide monomers. The Nucleotide monomers are made up of a five-carbon sugar, a heterocyclic nitrogenous base and phosphate group(s). They are joined to one another by covalent bonds between the phosphate of one and the sugar of another by phosphodiester linkages. Types of nucleic acids Deoxyribonucleic acid (DNA) and Ribonucleic acid (RNA). Nearly all the DNA is found within the cell nucleus. RNA occurs in all parts of a cell. DNA is repository of hereditary character. RNA is essentially required for protein biosynthesis.

Types of nucleic acids Deoxyribonucleic acid (DNA) and Ribonucleic acid (RNA). Nearly all the DNA is found within the cell nucleus. RNA occurs in all parts of a cell. DNA is repository of hereditary character. RNA is essentially required for protein biosynthesis.

Functions of nucleic acids Primary function of DNA is the storage and transfer of genetic information. 2. Information is used (indirectly) to control many functions of a living cell. 3. DNA is passed from existing cells to new cells during cell division. 4. RNA functions primarily in synthesis of proteins, the molecules that carry out essential cellular functions.