CARBOHYDRATES
Carbohydrates: are the cheapest and most plentiful of all nutrients and are an important part of our daily
diet. Carbohydrates are fromed by plants through the process of photosynthesis. Foods, which contain
carbohydrates, are called energy foods.
Structure of Carbohydrates
Structure of carbohydrates: Pure carbohydrates consist of just three elements: carbon, hydrogen, and
oxygen in the ratio of 1:2:1. The traditional general structure of carbohydrates is: Cx H2x Ox, but many
important carbohydrates, like deoxyribose have more hydrogen. Carbohydrates are classified into
monosaccharides, disaccharides, and polysaccharides.
Monosaccharides (simple sugars): Monosaccharides are the simplest of all sugars and are a base for all
other carbohydrates. Monosaccharides include Glucose, Fructose and Galactose, and share the chemical
formula C6 H12 O6. Even though they are chemically similar, different sugars are formed because the
elements are arranged differently.
The basic structure is of a ring structure containing carbon atoms and one oxygen atom. Glucose is the
most common monosaccharide, and it plays a major role in nutrition because it is a readily available source
of energy. Fructose is only found in fruits and honey. Galactose does not occur naturally in foods, but
results from digestion of a disaccharide.
Disaccharides (sugars): Disaccharides are commonly referred to as sugars. Disaccharides are composed
of two monosaccharides joined together. When the two molecules join together a molecule of water is
released. The disaccharide sugars include Sucrose, Lactose, and Maltose.
Glucose + Fructose = Sucrose
Glucose + Galactose = Lactose
Glucose + Glucose = Maltose
These different disaccharides contribute to various levels of sweetness in foods. Sugars are found
naturally in fruit and vegetables, and are also found in many processed foods. Processed foods usually
contain much more sugar than natural foods. Sugars act as sweeteners in food but they are also very high
in energy. This is a concern because sugars make up a large part of the diet and if the energy isn't
expended, the body stores the energy as fat. Excessive sugar in the diet can lead to nutritional disorders
such as obesity and dental caries.
Polysaccharides: are very large molecules, which are formed from many monosaccharides joined together.
The four most important polysaccharides are Starch, Glycogen, Pectin, and Cellulose.
Starch: is a complex carbohydrate and is a polymer of glucose produced by plants. There are two forms of
starch, they are Amylose and Amylopectin. Amylose is linear in structure and contains between 50 and 500
glucose units. Amylopectin is in a branching structure and can contain up to 100 000 glucose units. Some
sources of starch include wheat, rice, potatoes and oats. Starch is stored in granules of plants and is
broken down to glucose for use as energy.
Glycogen is the body's equivalent to starch. It is stored in the liver and muscles, and is broken down into
glucose when energy is required. The structure of glycogen is very similar to amylopectin but has more
branching occurring every 10 - 15 glucose units.
Pectin: and pectic substances are compounds, which are found in the cells of fruits and some root
vegetables. Pectin isn't significant to human nutrition but are important in the gelling of certain foods e.g.
jams.
Cellulose: is dietary fiber found in plants and it is the undigestible part of plant foods. Cellulose is similar
to starch but is structurally and nutritionally different because of the way the glucose molecules are
joined together. The human body is incapable of digesting cellulose because it lacks the necessary
enzymes. Cellulose is only broken down by the action of chewing.
Sources of Carbohydrates
Food sources of carbohydrates include grain products, potatoes, table sugar, syrups, fruit, cereals, pasta,
flour, bread, soft drinks and vegetables.
Functions of Carbohydrates
The function of carbohydrates includes the glucose and the fiber in the carbohydrate foods.
Glucose: has a major role in nutrition because it is a readily available source of energy. Glucose supplies
the body with heat and energy it needs, providing 16 kilojoules of energy per gram. Glucose is obtained
from food, through the digestion and absorption of sugars and starches. It is also obtained from glycogen,
which is stored in the liver and muscles, and is broken down into glucose when energy is required. Glucose
can also be obtained from amino acids only when other sources fail; the glucose is transported to the cells
of the body via the bloodstream.
Dietary fiber: Fiber isn’t an actual nutrient since the body can't actually digest it. It provides no energy
and no involvement in the growth and repair of the body, and doesn't regulate any metabolic processes.
The dietary fiber passes through the digestive system and is then excreted. However the fiber has
important functions in the digestive system. It slows the process of food passing through the stomach,
which contributes, to the feeling of satiation (fullness). It also slows the passing of food through the
small intestine, which aids in the absorption of many nutrients. Dietary fiber also absorbs moisture, which
softens and increases the bulk of the wastes. This bulk then stimulates the muscles of the colon to
contract, which promotes excretion and avoids constipation.
ANSWER THE FOLLOWING QUESTIONS:
1. What three elements make up the chemical structure of a carbohydrate?
2. Name the three classifications of carbohydrates and explain the difference between each.
3. List three food sources of carbohydrates and give three food product examples of each.
4. Describe the functional role of glucose and dietary fiber.
5. List all the foods and drinks you consumed yesterday and highlight those containing sugar, s