Carbohydrates: Structure, Function, and Types

Carbohydrates

Carbohydrates are organic molecules composed of carbon, oxygen, and hydrogen. They serve purposes such as structure, storage, and fuel for cells.

Include sugars (simple carbohydrates), and their polymers (complex carbohydrates or polysaccharides).
Monosaccharides are the simplest sugars.
- They can have a linear structure or form a ring; the ring structure is often favored for chemical equilibrium.
- Glucose (C6H{12}O_6) is a common monosaccharide.
- Essential for cell function; used in cellular respiration to produce energy.
Disaccharides are formed when two monosaccharides combine and are also considered simple sugars.
- The bond between two monosaccharides is formed by a dehydration reaction, where a water molecule is lost.
- Maltose is formed when two glucose molecules combine.
- Sucrose (table sugar) is formed by bonding a glucose and a fructose molecule.

Polysaccharides (complex carbohydrates) consist of many monosaccharides.
- Formed by dehydration reactions, with a water molecule lost each time a monosaccharide is added.
- Used for storage and structure in organisms.
Storage Polysaccharides:
- Starch:
  - Found in plants and made up of only glucose molecules.
  - Plants store starch (often in chloroplasts) and use it as a glucose source for energy.
- Glycogen:
  - Made of only glucose molecules but is highly branched.
  - Animals store glycogen (often in liver and muscle cells) as a glucose source for energy.
Structural Polysaccharides:
- Cellulose:
  - A major component of plant cell walls, providing strength.
  - Made of glucose molecules with a slightly different ring structure (β-glucose) and does not branch.
  - Many glucose polymers run parallel, forming strong cellulose microfibrils that overlap to strengthen cell walls.
  - Difficult to digest due to β-linkages; only certain organisms can break it down.
  - Humans cannot digest cellulose for energy but consume it as fibre for colon health.
- Chitin:
  - Found in the exoskeletons of arthropods (insects, crustaceans, spiders), creating a hard, protective casing.
  - Also found in the cell walls of fungi.
  - Similar structure to cellulose, except each glucose molecule contains nitrogen on a side chain.
  - Can be used to make surgical thread that is strong and biodegradable.

Figure 1: The structure of glucose in linear and ring forms.
Figure 2: The chemical equation for cellular respiration.
Figure 3: Maltose formation by dehydration reaction.
Figure 4: Glycogen's branched structure.
Figure 5: Structural difference between α-glucose and β-glucose.
Figure 6: Cellulose microfibrils in plant cell walls.
Figure 7: Beta glucose forms strands of microfibrils that provide strength to the cell walls of plants.
Figure 8: Chitin's presence in arthropod exoskeletons.
Figure 9: Chemical structure of chitin.
Figure 10: Chitin as a material for surgical thread.