Biology · Cell Biology · Human Anatomy

The Human Cell
A Field Guide

Every one of the roughly 37 trillion cells in your body is a self-contained living system. This guide breaks down the key organelles and structures that make it work.

Annotated Cell Diagram

nucleolus Cell Membrane Nucleus Mitochondria Rough ER Golgi Apparatus Ribosomes Lysosome Vacuole Centrosome cytoplasm fills the cell interior

Fig. 1 — Simplified cross-section of a generalised human cell

37T
Cells in the body
10µm
Typical diameter
200+
Cell types
3.2B
DNA base pairs

01 — Outer Boundary

Cell Membrane

Also called the plasma membrane — the cell's gatekeeper.

01.1

Plasma Membrane

Boundary

A flexible phospholipid bilayer that encloses the entire cell. Hydrophobic tails face inward, shielding the cell's interior from the watery environment outside. Cholesterol molecules are embedded throughout to regulate fluidity and structural stability.

01.2

Membrane Proteins

Transport · Signalling

Integral and peripheral proteins dot the membrane surface. They act as channels, pumps, receptors, and enzymes — facilitating selective transport of ions and molecules, receiving hormonal signals, and enabling cell-to-cell communication.

01.3

Glycocalyx

Identity · Adhesion

A sugary coat of glycoproteins and glycolipids on the outer membrane surface. It acts as a cellular fingerprint, enabling immune recognition and playing roles in cell adhesion, protection, and communication with the extracellular environment.

02 — Control Centre

Nucleus

The command hub that houses the cell's genetic blueprint.

02.1

Nuclear Envelope

Boundary

A double-layered membrane that surrounds and protects the nucleus. Studded with thousands of nuclear pore complexes, it controls which molecules enter and leave the nucleus — allowing mRNA out and regulatory proteins in.

02.2

Chromatin & DNA

Genetic Material

DNA wound around histone proteins forms chromatin fibres. The human genome contains approximately 3.2 billion base pairs encoding around 20,000–25,000 genes, distributed across 23 pairs of chromosomes. When a cell divides, chromatin condenses into visible chromosomes.

02.3

Nucleolus

Ribosome Factory

A dense region within the nucleus — not bounded by its own membrane — that assembles ribosomal RNA (rRNA) and ribosomal subunits. These are then exported to the cytoplasm where they form complete ribosomes and begin protein synthesis.

03 — Specialised Structures

Key Organelles

Membrane-bound compartments that carry out dedicated functions within the cytoplasm.

03.1

Mitochondria

Energy Production

Often called the powerhouse of the cell, mitochondria generate ATP through cellular respiration. They have a double membrane — the inner membrane is folded into cristae to maximise surface area for the electron transport chain. Mitochondria contain their own circular DNA, a relic of their ancient bacterial ancestry.

03.2

Endoplasmic Reticulum

Synthesis & Transport

A vast network of interconnected membrane tubules and sacs. The rough ER (studded with ribosomes) synthesises and folds proteins destined for export or membrane insertion. The smooth ER, which lacks ribosomes, handles lipid synthesis, calcium storage, and drug detoxification.

03.3

Golgi Apparatus

Processing & Sorting

A stack of flattened membrane sacs that receives proteins and lipids from the ER. It modifies, tags, and packages them into vesicles for delivery to their correct destinations — whether inside the cell, to the membrane, or outside the cell entirely via secretion.

03.4

Lysosomes

Digestion & Recycling

Membrane-bound sacs packed with hydrolytic enzymes that function best at an acidic pH. They break down worn-out organelles (autophagy), cellular debris, and foreign invaders such as bacteria. The products are recycled as raw materials for new cellular components.

03.5

Ribosomes

Protein Synthesis

Tiny, non-membrane-bound complexes of rRNA and protein. They translate messenger RNA (mRNA) sequences into chains of amino acids — i.e. proteins. Ribosomes can float freely in the cytoplasm (for proteins used inside the cell) or be anchored to the rough ER (for proteins destined for export).

03.6

Peroxisomes

Detoxification

Small, membrane-bound organelles that carry out oxidative reactions using molecular oxygen. They break down fatty acids and destroy harmful compounds such as hydrogen peroxide — converting it to water using the enzyme catalase. Particularly abundant in liver and kidney cells.

03.7

Vacuoles

Storage

Fluid-filled sacs that store nutrients, waste products, and water. In human cells, vacuoles are typically small and transient — quite unlike the large central vacuole of plant cells. They also participate in endocytosis and exocytosis, helping to move materials across the membrane.

03.8

Centrosome

Cell Division

Composed of two centrioles arranged at right angles, the centrosome is the cell's main microtubule-organising centre. During cell division, it forms the mitotic spindle — a structure of microtubule fibres that pulls chromosomes to opposite poles of the dividing cell.

04 — Internal Framework

Cytoskeleton & Cytoplasm

The scaffold that gives the cell shape and enables movement.

04.1

Microfilaments

Actin · Movement

Thin filaments of polymerised actin protein that provide tensile strength and are involved in cell movement, division, and changes in shape. They work with myosin motor proteins to produce muscle contraction and enable cells to crawl across surfaces.

04.2

Microtubules

Structure · Transport

Hollow tubes of tubulin protein that serve as the cell's railway system. They guide the movement of organelles and vesicles via motor proteins (dynein and kinesin), form the mitotic spindle during division, and build the core of cilia and flagella.

04.3

Intermediate Filaments

Stability

Rope-like protein fibres (e.g. keratin, vimentin, lamin) that provide mechanical stability and help anchor organelles in place. The nuclear lamins — a type of intermediate filament — line the inner nuclear envelope and maintain the nucleus's structural integrity.

04.4

Cytoplasm

Medium

The gel-like fluid (cytosol) that fills the cell and suspends all organelles. It is mostly water, but also contains ions, enzymes, glucose, amino acids, and countless other molecules. Many metabolic reactions — including glycolysis — occur directly in the cytoplasm.

05 — How It All Works

Key Cellular Processes

The major flows of information and energy that keep a cell alive.

  1. Transcription Inside the nucleus, RNA polymerase reads a gene on the DNA template strand and synthesises a complementary strand of pre-mRNA. The pre-mRNA is then processed — introns are spliced out, a cap and poly-A tail are added — before leaving the nucleus.
  2. Translation Mature mRNA travels to the cytoplasm and binds to a ribosome. Transfer RNA (tRNA) molecules bring matching amino acids to each codon on the mRNA. The ribosome links amino acids together into a polypeptide chain, which folds into a functional protein.
  3. Cellular Respiration Glucose is broken down in three stages: glycolysis in the cytoplasm, the Krebs cycle in the mitochondrial matrix, and oxidative phosphorylation along the inner mitochondrial membrane. The net yield is approximately 30–32 ATP molecules per glucose.
  4. Endocytosis & Exocytosis Cells ingest materials by wrapping them in membrane (endocytosis), forming internal vesicles. The reverse — exocytosis — ejects materials when vesicles fuse with the plasma membrane, secreting proteins, hormones, or neurotransmitters into the extracellular space.
  5. Mitosis & Cell Division When a cell divides, chromosomes are duplicated and then separated by the mitotic spindle into two identical nuclei. Cytokinesis then splits the cytoplasm, producing two genetically identical daughter cells. This process is tightly regulated to prevent uncontrolled growth.
  6. Apoptosis Programmed cell death is a controlled process by which damaged, infected, or surplus cells are dismantled from within. Caspase enzymes systematically degrade cellular components; the fragments are then packaged into apoptotic bodies and engulfed by phagocytes without triggering inflammation.

06 — Did You Know

Notable Cell Facts

Scale

A typical human cell is about 10 micrometres across — roughly one-tenth the width of a human hair. Yet within that microscopic space, thousands of chemical reactions occur simultaneously every second. If you stretched all the DNA from a single cell end-to-end, it would measure approximately 2 metres.

Mitochondrial Origins

Mitochondria are believed to have originated about 1.5 billion years ago when a proto-eukaryotic cell engulfed an aerobic bacterium — a symbiosis that proved mutually beneficial. This endosymbiotic theory is supported by the fact that mitochondria have their own circular DNA, replicate by binary fission, and have ribosomes similar to those of bacteria.

Turnover

The body replaces most of its cells over time — red blood cells live about 120 days; gut lining cells, about 5 days. Some neurons in the cerebral cortex, however, are as old as you are. The liver regenerates completely roughly every 150 to 500 days, depending on cell type.

Protein Production

A single cell contains around 42 million protein molecules. A ribosome can add around 15–20 amino acids per second to a growing polypeptide chain. Even so, the cell needs thousands of ribosomes working in parallel to meet its protein demands.