ZIMSEC O Level 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
  • 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