NUTRITION PART 1: FOOD AND CARBOHYDRATE

This chapter discusses one of the characteristics of living things, feeding. All living things need food but the difference between plants and animals is that the green plants can make food by photosynthesis but animals have to to rely on other organisms (plants and other animals) as their source of food. Why do living organisms need food?

THE NEED FOR FOOD

• It provides the building materials for making new cells and tissues which are later used for growth.
• It provides energy for the chemical reactions (such as building complex molecules like proteins) which take place in living organisms to keep them alive. Apart from this, the energy is also used for activities such as movement, heart beat and transmission of nerve impulses. Animals also use energy for maintaining their body temperature.
• It provides substances for replacing worn out and damaged or wounded tissues.
  

CLASSES OF FOOD
Food can be divided into two main classes: Macro-nutrients (carbohydrates, proteins and fats) and Micro-nutrients (minerals, vitamins, dietary fibre and water). Macro-nutrients are those needed in large quantities whereas Micro-nutrients are those needed in minutes quantities.

CARBOHYDRATES
Carbohydrates are energy giving food. In fact, they are the most readily available source of energy. They contain the elements carbon, hydrogen and oxygen. When they are oxidized during respiration to provide energy, they are broken down into carbon dioxide and water. One gram of carbohydrate can provide an average of 16kJ of energy. If we eat more carbohydrates more than we need to, the excess is converted in the liver to glycogen for storage in the muscles and the liver itself. If the glycogen is not used for a very long time, it will eventually converted into fats and stored in the fat depots in the abdomen, round the kidneys or in the adipose tissues under your skin. SO BEWARE!!!


Carbohydrates can be further divided into three groups:

A. Polysaccharides
Made up of a large number of glucose molecules joined together. Examples: Starch, Glycogen and Cellulose.

• STARCH can be found in vegetable foods such as cereals, rice, potatoes and tapioca. It is not formed or stored by animals. In digestion, starch is broken down into maltose by enzyme, amylase.
• GLYCOGEN is a storage form of carbohydrate in animals (and also in fungi). For this reason, glycogen is also referred to as an "animal starch". In mammals, glycogen is commonly stored in the liver and muscles. Like starch, glycogen is suitable as storage materials because they are insoluble in water (so they do not change the osmotic pressure of the cells), they are large molecules (so unable to diffuse through cell membrane), they can be converted easily into glucose when needed, and they have compact shape (hence they can occupy less space in the cytoplasm).
• CELLULOSE is one of the component of the cell walls of plant cells. Unlike starch and glycogen, it is inert and very few organisms (those that can produce the enzyme, cellulase) can digest it. Even though, human cannot digest cellulose, plant materials must be included in the diet because it forms the bulk of undigested matter. Hence it prevents constipation.
  

B. Disaccharides (Complex sugars)
Each molecule of disaccharide is made up of two molecules of monosaccharides (simple sugars) condensed together. The general formula for all disaccharides is C12H22O11 (Remember that one molecule of water is removed when two molecules of monosaccharides (formula = C6H12O6) condensed.

• SUCROSE or cane sugar occurs in sugarcane stems, sweet fruits and certain roots like sugar beet and carrots. It consists of a glucose molecule and a fructose molecule combined together by condensation. It is not found in mammals. It can be digested by an enzyme called sucrase.

GLUCOSE + FRUCTOSE --> SUCROSE + WATER

• LACTOSE or milk sugar is found in the milk of all mammals. It is formed from a condensation process between a molecule of glucose and a molecule of galactose. Lactose can be broken down into glucose and galactose by an enzyme called Lactase.
  

GLUCOSE + GALACTOSE --> LACTOSE + WATER

• MALTOSE or malt sugar occurs in malted cereals and sprouting grains. It is formed from a condensation process between two molecules of glucose. Maltose can be broken down into glucose by an enzyme called maltase
  

GLUCOSE + GLUCOSE --> MALTOSE + WATER

C. Monosaccharides
Monosaccharides are also known as simple sugars. Examples: GLUCOSE, FRUCTOSE and GALACTOSE. They all have the same general formula - C6H12O6.

REDUCING AND NON REDUCING SUGARS

• Reducing sugars are glucose, fructose, galactose, maltose and lactose.
• Non-reducing sugar is sucrose.
• The presence of reducing sugar in food can be detected by means of Benedict's solution.
• When the blue Benedict's solution changed to brick red (or brown or yellow or orange), it indicates the presence of reducing sugar in the food.