The cell is the basic structural and functional unit of all living organisms. Every process in the human body — movement, sensation, thought, digestion, immunity, reproduction — is performed by cells or depends on what cells produce.
Structure
A human cell has three essential components:
The cell membrane — a thin, flexible barrier made of a phospholipid bilayer (two layers of fat molecules) with embedded proteins. The membrane separates the cell’s interior from its environment and controls what enters and exits. It is selectively permeable: small, uncharged molecules (oxygen, carbon dioxide) pass through freely; larger or charged molecules (glucose, sodium ions, drugs) require transport proteins to cross. This membrane is why lipophilic drugs (which dissolve in fat) cross cell membranes easily while hydrophilic drugs (which dissolve in water) do not — a distinction that drives much of pharmacokinetics.
The cytoplasm — the gel-like interior of the cell, containing water, dissolved molecules, and specialized structures (organelles) that carry out specific functions:
- Mitochondria produce energy (ATP) from nutrients and oxygen — the cell’s power source
- Ribosomes build proteins from genetic instructions
- Endoplasmic reticulum processes and transports proteins; the smooth endoplasmic reticulum in liver cells houses the cytochrome P450 enzymes responsible for drug metabolism
- Lysosomes break down waste and foreign material
The nucleus — contains the cell’s DNA (deoxyribonucleic acid), the molecular instructions for building every protein the cell needs. When a gene is “expressed,” its DNA sequence is copied into RNA, which is translated into a protein by ribosomes. Genetic variation — differences in DNA sequence between individuals — explains why the same drug dose produces different effects in different people: polymorphisms in genes encoding drug-metabolizing enzymes (CYP2D6, CYP3A4) alter enzyme structure and activity.
Cell types
The human body contains approximately 200 distinct cell types, each specialized for a particular function:
- Neurons — generate and transmit electrical signals; the basis of the nervous system
- Muscle cells — contract to produce movement; three types (skeletal, cardiac, smooth)
- Epithelial cells — line surfaces (skin, gut, airways, blood vessels); form barriers and absorb nutrients
- Immune cells — detect and destroy pathogens (neutrophils, macrophages, lymphocytes)
- Red blood cells (erythrocytes) — carry oxygen bound to hemoglobin
- Glial cells — support neurons; microglia and astrocytes play roles in neuroinflammation and chronic pain
- Hepatocytes — liver cells; metabolize drugs, produce bile, store nutrients
- Fibroblasts — produce connective tissue (collagen, fascia); relevant to structural integration and wound healing
Cells as the unit of drug action
Drugs do not act on “the body” in the abstract. They act on cells — binding to receptors on cell membranes, inhibiting enzymes inside cells, or blocking ion channels that control electrical signaling. Understanding cellular structure is what makes pharmacology intelligible: a drug must reach its target cell, cross (or interact with) the cell membrane, and interact with a specific molecular target within or on that cell. Every step in this process is governed by the cell’s structure.