Biomolecules

Living cells are composed of water, inorganic ions, and organic biomolecules. The major organic classes are carbohydrates, lipids, proteins, and nucleic acids. Carbon, hydrogen, oxygen, and nitrogen dominate the elemental composition of living matter.

Small molecules like sugars, amino acids, fatty acids, and nucleotides are called micromolecules, whereas proteins, nucleic acids, and polysaccharides are macromolecules (also called polymers).

When biological tissue is ground in trichloroacetic acid, the filtrate contains the acid-soluble pool (micromolecules such as glucose, amino acids, ATP, and vitamins), while the residue is the acid-insoluble fraction (macromolecules such as proteins, DNA, RNA, and polysaccharides).

Carbohydrates are aldoses or ketoses made of C, H, and O in the approximate ratio 1:2:1. They are classified as:

• Monosaccharides: glucose (C₆H₁₂O₆), fructose, galactose, ribose (C₅).
• Disaccharides: sucrose (glucose + fructose), maltose (glucose + glucose), lactose (glucose + galactose) — joined by glycosidic bonds.
• Polysaccharides: starch (storage in plants), glycogen (storage in animals), cellulose (structural in cell walls), chitin (fungal cell walls and insect exoskeletons).

Lipids are not polymers in the strict sense. Triglycerides (fats/oils) consist of glycerol + 3 fatty acids via ester bonds. Phospholipids form the bilayer of cell membranes. Steroids (including cholesterol and hormones) and waxes are also lipids.

Proteins are polymers of amino acids joined by peptide bonds (formed between carboxyl group of one amino acid and amino group of the next, releasing water). There are 20 standard amino acids.

Protein structure is organised at four levels:

• Primary: linear sequence of amino acids.
• Secondary: α-helix or β-pleated sheet (hydrogen bonds).
• Tertiary: 3D folding (disulphide bonds, hydrophobic interactions).
• Quaternary: assembly of two or more polypeptide subunits (e.g., haemoglobin).

Enzymes are mostly proteins (exception: ribozymes = RNA catalysts). They lower activation energy without being consumed. Key terms:

• Apoenzyme: protein part alone (inactive).
• Cofactor: non-protein part (metal ions or coenzymes).
• Holoenzyme: apoenzyme + cofactor (fully active).
• Active site: where substrate binds; explains specificity.

Enzyme activity is affected by temperature (optimum ~37 °C in humans), pH, substrate concentration, and inhibitors (competitive vs non-competitive).

DNA and RNA are polymers of nucleotides. Each nucleotide = a nitrogenous base + a pentose sugar + a phosphate group.

• DNA: deoxyribose sugar; bases — adenine (A), guanine (G), cytosine (C), thymine (T). Double-stranded helix.
• RNA: ribose sugar; bases — A, G, C, uracil (U). Usually single-stranded.

DNA base pairing: A = T (2 hydrogen bonds), G ≡ C (3 hydrogen bonds). Adjacent nucleotides are connected by phosphodiester bonds.

Types of RNA and functions:

• mRNA: carries genetic code from nucleus to ribosome.
• tRNA: adaptor molecule; carries specific amino acids to ribosome.
• rRNA: major structural and catalytic component of ribosome.

Metabolism is the sum total of all chemical reactions in the cell. It has two components:

• Anabolism: synthesis of complex molecules from simpler ones (energy-consuming, e.g., protein synthesis, photosynthesis).
• Catabolism: breakdown of complex molecules (energy-releasing, e.g., cellular respiration, digestion).

ATP (adenosine triphosphate) is the universal immediate energy currency of the cell. Energy is stored in high-energy phosphate bonds. Electron carriers NADH and FADH₂ transfer reducing power during respiration.

The living state is a continuous state of chemical flux — steady-state reactions maintain order. A dead cell still has the same molecules for a while, but reactions stop because ATP is no longer regenerated.