What this chapter is about
This chapter introduces you to the chemical compounds that make up living organisms. All life, from bacteria to banyan trees to humans, is built from the same basic types of molecules: carbohydrates, proteins, lipids and nucleic acids. You will learn what these biomolecules are made of, how they are structured, and what roles they play in cells.
The chapter also covers enzymes, which are special proteins that speed up chemical reactions in living systems. Understanding how enzymes work is essential for grasping how metabolism occurs. You will see how the structure of a molecule determines its function, a principle that runs through all of biology.
By the end of this chapter, you should be able to identify the major classes of biomolecules, describe their building blocks (monomers), explain how polymers form, and understand the basic mechanism of enzyme action. This knowledge forms the foundation for later topics like photosynthesis, respiration and genetics.
Key ideas
- Living tissue contains both organic and inorganic compounds. Organic compounds contain carbon and include carbohydrates, proteins, lipids and nucleic acids. Inorganic compounds include water and mineral ions.
- Carbohydrates are made of carbon, hydrogen and oxygen in the approximate ratio C:H:O = 1:2:1. Simple sugars (monosaccharides) like glucose and fructose join to form disaccharides (sucrose, maltose) and polysaccharides (starch, cellulose, glycogen).
- Proteins are polymers of amino acids joined by peptide bonds. There are about 20 different amino acids in living systems. The sequence and folding of amino acids determine a protein's shape and function.
- Lipids are fats and fat-like substances that are insoluble in water but soluble in organic solvents. They include triglycerides (fats and oils), phospholipids (in cell membranes) and steroids (like cholesterol).
- Nucleic acids (DNA and RNA) store and transmit genetic information. They are polymers of nucleotides. Each nucleotide has a nitrogenous base, a pentose sugar and a phosphate group.
- Enzymes are biological catalysts that lower the activation energy of reactions. They are highly specific and are affected by temperature, pH and substrate concentration.
- Primary, secondary, tertiary and quaternary structures describe the levels of protein organisation, from the amino acid sequence to the arrangement of multiple polypeptide chains.
Formulas and facts to remember
1. General formula for monosaccharides: (CH₂O)ₙ, where n is typically 3 to 7. For glucose, n = 6, giving C₆H₁₂O₆.
2. Peptide bond formation: When two amino acids join, the –COOH group of one reacts with the –NH₂ group of another, releasing one molecule of water. This is a condensation or dehydration reaction.
3. Glycosidic bond: The bond that joins monosaccharides to form di- or polysaccharides, also formed by condensation.
4. Phosphodiester bond: The bond linking nucleotides in nucleic acids, connecting the phosphate of one nucleotide to the sugar of the next.
5. Lock and key model of enzyme action: The substrate fits exactly into the active site of the enzyme, like a key fits a lock.
6. Induced fit model: The enzyme's active site changes shape slightly when the substrate binds, improving the fit.
7. Optimum temperature and pH: Each enzyme works best at a specific temperature and pH. Human enzymes typically have an optimum around 37 °C.
8. Cofactors and coenzymes: Non-protein components that help enzymes function. Cofactors are usually metal ions (Zn²⁺, Mg²⁺); coenzymes are organic molecules often derived from vitamins.
Worked examples
### Example 1: Identifying the type of carbohydrate
A student tests a white powder and finds it gives a positive result with iodine solution, turning blue-black. What type of carbohydrate is likely present?
Solution: The iodine test specifically detects starch. Starch is a polysaccharide made of glucose units. The blue-black colour arises because iodine molecules fit into the helical structure of amylose (a component of starch). Therefore, the powder likely contains starch.
### Example 2: Counting peptide bonds
A small polypeptide chain contains 15 amino acids. How many peptide bonds hold this chain together?
Solution: Each peptide bond links two amino acids. If there are 15 amino acids in a chain, the number of bonds connecting them equals 15 − 1 = 14 peptide bonds. Think of it like a string of 15 beads: you need 14 threads to connect them in a line.
### Example 3: Effect of temperature on enzyme activity
An enzyme extracted from a bacterium found in hot springs shows maximum activity at 75 °C. If the same experiment is done at 37 °C, what would you expect?
Solution: Enzymes have an optimum temperature where they work fastest. For this thermophilic (heat-loving) bacterium's enzyme, the optimum is 75 °C. At 37 °C, the temperature is well below the optimum, so the enzyme molecules have less kinetic energy. The rate of reaction would be lower because fewer enzyme-substrate collisions occur with enough energy. The enzyme is not denatured at 37 °C, just slower.
Common mistakes
- Thinking all carbohydrates taste sweet → Only simple sugars (monosaccharides and some disaccharides) taste sweet; polysaccharides like starch and cellulose do not.
- Confusing peptide bonds with glycosidic bonds → Peptide bonds join amino acids in proteins; glycosidic bonds join sugars in carbohydrates.
- Believing enzymes are used up in reactions → Enzymes are catalysts and are released unchanged after each reaction cycle; they can be reused.
- Assuming all proteins are enzymes → Many proteins have structural, transport or regulatory roles (like collagen, haemoglobin, hormones) and are not enzymes.
- Thinking high temperature always increases enzyme activity → Beyond the optimum, enzymes denature and lose function permanently.
Quick revision
- Carbohydrates: monosaccharides → disaccharides → polysaccharides; joined by glycosidic bonds.
- Proteins: amino acids joined by peptide bonds; structure levels are primary, secondary, tertiary, quaternary.
- Lipids: fats (energy storage), phospholipids (membranes), steroids (signalling); insoluble in water.
- Nucleic acids: nucleotides (base + sugar + phosphate) joined by phosphodiester bonds; DNA stores genetic information, RNA helps express it.
- Enzymes: biological catalysts; specific; affected by temperature, pH and inhibitors; work by lowering activation energy.