The Cell and the Biomolecules of Life
The cell is the fundamental unit of life. Robert Hooke first observed and named cells in 1665 while looking at a slice of cork through a microscope. A century and a half later, Schleiden, Schwann and Virchow formalised the cell theory.
(1) All living organisms are composed of one or more cells. (2) The cell is the basic structural and functional unit of life. (3) All cells arise from pre-existing cells through division.
Prokaryotic vs eukaryotic cells
Living cells fall into two grand divisions:
- Prokaryotic cells — bacteria and archaea. No true nucleus; genetic material lies free in the cytoplasm. Small (1–10 μm). No membrane-bound organelles.
- Eukaryotic cells — animals, plants, fungi, protists. Have a nucleus enclosed by a double membrane, plus organelles such as mitochondria and (in plants) chloroplasts. Typically 10–100 μm.
Major organelles
| Organelle | Function |
|---|---|
| Nucleus | Stores DNA, controls cell activity |
| Mitochondria | Site of aerobic respiration; produce ATP — the cell's "powerhouse" |
| Chloroplast (plants only) | Site of photosynthesis; contains chlorophyll |
| Ribosomes | Synthesise proteins from mRNA templates |
| Endoplasmic reticulum | Synthesises lipids (smooth ER) and processes proteins (rough ER) |
| Golgi apparatus | Modifies, sorts and packages proteins for secretion |
| Lysosomes | Contain digestive enzymes; recycle waste |
| Cell wall (plants, fungi, bacteria) | Rigid layer outside the membrane; provides shape and protection |
- All cells share four common features: a plasma membrane, cytoplasm, ribosomes, and DNA.
- Plant cells have a cell wall (cellulose), chloroplasts and a large central vacuole, which animal cells lack.
- Mitochondria and chloroplasts have their own DNA — a clue to their bacterial origin (endosymbiotic theory).
The four biomolecules
Life is built from four major classes of organic molecules:
- Carbohydrates — sugars and starches. Made of C, H, O in a roughly 1:2:1 ratio. Primary energy source. Examples: glucose, sucrose, starch, cellulose, glycogen.
- Lipids — fats, oils, waxes, phospholipids and steroids. Insoluble in water; store energy densely (~9 kcal/g) and form cell membranes.
- Proteins — chains of amino acids (20 standard types) folded into specific 3-D shapes. Roles include catalysis (enzymes), transport (haemoglobin), structure (collagen), defence (antibodies) and signalling (hormones).
- Nucleic acids — DNA and RNA. Store and transmit genetic information. DNA is a double helix of nucleotides, each composed of a sugar (deoxyribose), a phosphate and one of four bases: A, T, G, C.
Photosynthesis and respiration are mirror processes. Photosynthesis: 6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂. Cellular respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP. Together they drive the global carbon and oxygen cycles.
Why this matters
Every disease, drug, crop and biotechnology — from CRISPR gene editing to mRNA vaccines — operates on this molecular landscape. A grasp of cells and biomolecules is the gateway to understanding modern biology in any examination question.