4.1 Biological Molecules

Living organisms are composed of organic molecules that contain specific chemical elements. The three main categories of biological molecules are carbohydrates, fats (lipids), and proteins.

The human body is approximately:

• 62% water
• 16% protein
• 16% fat
• 1% carbohydrate
• 6% other elements (such as calcium and phosphorus)

Carbohydrates are substances that include sugars, starch, and cellulose. They are composed of carbon, hydrogen, and oxygen.

Sugars are carbohydrates with relatively small molecules. They are soluble in water and taste sweet.

Glucose is the most important simple sugar:

• Molecular formula: $\mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6$
• Contains 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms
• Atoms are arranged in a hexagonal shape
• Used in respiration to release energy
• Transported in blood plasma

When many glucose molecules join together, they form polysaccharides (large carbohydrate molecules):

Glycogen is formed when glucose molecules link together:

• Used as an energy store in animal cells
• Stored primarily in the liver
• Made of many glucose molecules linked in long chains

Glucose is converted into two main polysaccharides:

1. Starch

   • Used as an energy store in plant cells
   • Made of many glucose molecules linked in long chains
   • The chains coil into a spiral shape

2. Cellulose

   • Used to make cell walls
   • Made of glucose molecules linked together
   • Forms straight lines rather than coils