Biology Notes: Chemicals of Life: Carbohydrates

• In humans, mammals and a lot of other animals carbohydrates are the body’s main source of energy
• These may be simple, soluble sugars or complex materials like starch and cellulose
• But they all  are bio-molecules consisting of carbon (C), hydrogen (H) and oxygen (O)
• Carbohydrates come in two main types:
• Simple sugar (Monosaccharide) are the simplest forms of carbohydrates
• Complex sugar (Disaccharide and Polysaccharide)

Simple sugars (Monosaccharides)

Structure of Monosaccharides

• Sugars with a single carbon ring are called monosaccharides
• Examples include glucose and fructose
• Like disaccharides, monosaccharides are readily soluble in water
• Simple sugars can provide a lot of energy for immediate usage
• However, they contain no other useful nutrients

Complex sugars (Disaccharides and Polysaccharides)

The formation of Maltose

• Disaccharides are thosesugars with two carbon rings in their molecules are called disaccharides
• They are formed when two monosaccharides form a bond with removal of water
• Examples include maltose, lactose and sucrose
• Disaccharides like monosaccharides are easily soluble in water
• Carbohydrates called polysaccharides  areformed when many glucose molecules are joined together
• Polysaccharides are polymerized monosaccharides, or complex carbohydrates
• They have multiple simple sugars
• Because of their size, polysaccharides are not water-soluble
• Individual molecules can however become hydrated when exposed to water
• Glycogen is an example of a polysaccharide
• forms a food storage substance in many animal cells
• Another common polysaccharide is starch
• The starch molecule is made up of hundreds of glucose molecules joined together to form long
  chains
• Starch is an important storage substance in the plastids of plant cells
• Plastids are important organelles in plant cells
• An organelle is a tiny cellular structure that performs specific functions within a cell
• An example of a plastid is chloroplast
• They are the sites where molecules like starch are made and stored
• Cellulose is another example of a polysaccharide
• It is an important structural component of the primary cell wall of green plants
• Cellulose consists of even longer chains of glucose molecules
• The chain molecules are grouped together to form microscopic fibres, which are laid down in layers to form the cell wall in plant cells
• Complex sugars are a good source of energy for both plants and animals
• The body can easily store this form of energy for rapid use in future
• Animal cells store complex sugars in the form of glycogen
• Plant cells store complex sugars in the form of starch

Foods containing carbohydrates

Part of glycogen molecule showing the individual glucose chains

• Examples of foods that contain a lot of carbohydrates are cereal based foods
• Such as maize, flour and rice
• By extension this includes bread, sadza, pasta, spaghetti etc
• Other sources includes potatoes and cassava
• In humans  energy from carbohydrates is converted into glycogen and is stored in the liver and in our muscles
• When energy is needed, the body changes the glycogen into glucose which is used by the body during aerobic respiration
• If a lot of carbohydrate is eaten, it will be stored as fat

To access more topics go to the O Level Biology Notes page