Fold Commons

CellTour

Short, guided tours of the cells that build a body. Pick a cell, step through its parts one stop at a time, and see what each piece does — written for middle- and high-school readers. Every tour cites textbook-grade sources. Nothing is installed; nothing about you is collected.

This is the tool — the readable, shareable core. Explanations are Fold Commons editorial (CC BY 4.0); each tour links to references such as Molecular Biology of the Cell on the NCBI Bookshelf and peer-reviewed reviews. When a native version ships, it adds cell-scale visuals and offline use; it never gates the web.

≈4% a curated starter set of 8 of the ~200 human cell types

A curated starter set of 8 of the ~200 broadly-recognized human cell types (plus two non-human model cells — a plant guard cell and E. coli — for contrast). The human body has on the order of 200 named cell types (and ~37 trillion cells); this edition tours a hand-picked teaching set, not the whole atlas.

What this set does not cover yet (5)
  • No sensory cells (photoreceptor rods/cones, hair cells of the inner ear, olfactory neurons).
  • No epithelial or secretory cells beyond the pancreatic beta cell (e.g. goblet, ciliated airway, keratinocyte).
  • No connective-tissue cells (fibroblast, chondrocyte, osteocyte, adipocyte).
  • No liver detoxification detail, kidney nephron cells, or germ-line stem cells.
  • Coverage is a teaching sample; it does not claim to represent every human cell type or every organelle.
System
Level
Animal Starter

Neuron — the body's wiring

Neuron (nerve cell) · Nervous system · 10–100 µm (soma); axons can reach over a metre

Walk a brain cell from the end that listens to the end that shouts.

  1. Whole cell

    The whole neuron

    A neuron is a cell built for sending messages. It listens at one end and passes a signal out the other, sometimes over a very long distance.

  2. Dendrites

    Dendrites

    These branching arms collect signals from thousands of neighbouring cells at junctions called synapses.

  3. Nucleus

    Nucleus and cell body

    The cell body holds the nucleus, which stores the DNA and directs the proteins the neuron needs to work.

  4. Axon

    The axon

    The axon is a long fibre that carries an electrical pulse away from the cell body. A fatty myelin coat lets the pulse jump along faster.

  5. Axon terminals

    Axon terminals

    At the tip, the neuron releases chemical messengers called neurotransmitters to pass the signal to the next cell.

  6. Mitochondria

    Mitochondria

    Firing and resetting takes a lot of energy, so mitochondria cluster where the work is heaviest to supply ATP.

Sources (2)
Animal Core

Cardiomyocyte — the beating cell

Cardiac muscle cell (cardiomyocyte) · Circulatory system · about 100 µm long, 10–25 µm wide

Meet the muscle cell that contracts about once a second for your whole life.

  1. Whole cell

    The whole cell

    A cardiomyocyte is a heart muscle cell. Millions of them contract together, in rhythm, to pump blood.

  2. Myofibrils / sarcomeres

    Sarcomeres

    The striped pattern is made of sarcomeres — overlapping actin and myosin filaments that slide past each other to shorten the cell.

  3. Mitochondria

    Mitochondria

    Roughly a third of the cell is mitochondria. A heart cell never rests, so it needs a huge, constant supply of energy.

  4. Sarcoplasmic reticulum

    Sarcoplasmic reticulum

    This internal store releases a pulse of calcium ions that acts as the 'go' signal for each contraction, then pumps them back to relax.

  5. Intercalated discs

    Intercalated discs

    Special junctions glue neighbouring heart cells together and let the electrical signal pass directly between them, so the tissue beats as one.

Sources (2)
Animal Core

T cell — the immune patrol

Cytotoxic T lymphocyte · Immune system · 7–12 µm

Follow an immune cell that scans other cells and destroys the infected ones.

  1. Whole cell

    The whole cell

    A T cell is a white blood cell of the adaptive immune system. It learns to recognise one specific threat and hunts for it.

  2. T-cell receptor

    T-cell receptors

    Receptors on the surface read short protein fragments that other cells display on MHC molecules, checking each cell for signs of infection.

  3. Nucleus

    Nucleus

    A large nucleus fills most of the cell and holds the DNA that codes for its receptors and weapons.

  4. Cytotoxic granules

    Cytotoxic granules

    When a target is confirmed infected, the T cell releases perforin and granzymes from these granules to trigger the target's self-destruct program.

  5. Mitochondria

    Mitochondria

    Once activated, the cell rewires its metabolism and uses its mitochondria to power rapid division and the killing response.

Sources (2)
Animal Starter

Red blood cell — the oxygen courier

Erythrocyte (red blood cell) · Circulatory system · 6–8 µm across, about 2 µm thick

See why the simplest human cell is shaped like a doughnut.

  1. Whole cell

    The whole cell

    There are tens of trillions of red blood cells in your body, and every one does a single job: carry oxygen.

  2. Cell membrane

    Biconcave shape

    The cell is shaped like a doughnut without a hole. That dished shape gives lots of surface for gas exchange and lets the cell bend to squeeze through the narrowest capillaries.

  3. Cytoplasm (haemoglobin)

    Haemoglobin

    The inside is packed with the protein haemoglobin, which grabs oxygen in the lungs and lets it go in the tissues that need it.

  4. Absent nucleus

    No nucleus

    A mature human red blood cell throws away its nucleus and most other organelles, freeing up room to carry more haemoglobin. The trade-off is that it cannot repair itself and lives only about 120 days.

Sources (2)
Animal Core

Pancreatic beta cell — the sugar sensor

Pancreatic islet beta cell · Endocrine system · about 10 µm

Learn how one cell measures your blood sugar and releases insulin.

  1. Whole cell

    The whole cell

    Beta cells live in clusters called islets inside the pancreas. Their job is to keep blood sugar steady by releasing the hormone insulin.

  2. Cell membrane

    Sensing glucose

    Glucose enters the cell and is broken down for energy. Rising energy closes channels in the membrane and makes the cell fire an electrical signal.

  3. Rough endoplasmic reticulum

    Rough endoplasmic reticulum

    This is the factory floor where the insulin protein is first built, studded with ribosomes that read the insulin gene's instructions.

  4. Golgi apparatus

    Golgi apparatus

    The Golgi folds and packages insulin into storage vesicles, ready to be released the moment blood sugar rises.

  5. Secretory granules

    Insulin granules

    The calcium pulse from the electrical signal makes these granules fuse with the membrane and dump insulin into the blood.

Sources (2)
Animal Advanced

Hepatocyte — the liver's chemistry lab

Hepatocyte (liver cell) · Digestive system · 20–30 µm

Tour the busiest chemical factory in the body.

  1. Whole cell

    The whole cell

    Hepatocytes do hundreds of jobs at once: storing sugar, making proteins for the blood, and breaking down toxins and old cells.

  2. Nucleus

    Nucleus (often two)

    Many hepatocytes carry two nuclei, or extra copies of their DNA, which helps them run so many chemical reactions at high volume.

  3. Smooth endoplasmic reticulum

    Smooth endoplasmic reticulum

    This network is the detox bench. Enzymes here chemically alter drugs, alcohol and waste so the body can get rid of them safely.

  4. Mitochondria

    Mitochondria

    Hundreds of mitochondria per cell power all this chemistry and help process fats and amino acids.

  5. Glycogen granules

    Glycogen stores

    The cell stockpiles spare sugar as glycogen and releases it between meals to keep your blood sugar from dropping.

Sources (2)
Plant Core

Plant guard cell — the leaf's mouth

Stomatal guard cell · Plant leaf epidermis · 20–40 µm

Watch two bean-shaped cells open and close a pore so a plant can breathe.

  1. Whole cell

    The whole cell

    Guard cells come in pairs on the underside of a leaf. Together they open or close a tiny pore called a stoma to let gases in and out.

  2. Cell wall

    Uneven cell wall

    The wall is thicker on the inner edge, so when the cell swells it bends into a curve — that is what opens the pore between the pair.

  3. Vacuole

    Vacuole

    Water rushing into this large central sac makes the cell firm and puffed up, forcing the stoma open.

  4. Chloroplast

    Chloroplasts

    Unusually for the leaf surface, guard cells contain chloroplasts, which capture sunlight and help signal when the pore should open.

  5. Nucleus

    Nucleus

    The nucleus combines signals about light, carbon dioxide and water to decide the timing of opening and closing.

Sources (2)
Bacteria Starter

E. coli — life without a nucleus

Escherichia coli (bacterium) · Bacterium (prokaryote) · 1–2 µm long

Compare a bacterium with your own cells and see what it does without.

  1. Whole cell

    The whole cell

    E. coli is a bacterium that lives in the gut. It is far simpler than any human cell — no nucleus and no membrane-wrapped organelles.

  2. Cell wall and membranes

    Cell envelope

    A tough peptidoglycan wall sits between two membranes, giving the rod its shape and protecting it from bursting.

  3. Nucleoid

    Nucleoid

    The DNA is a single circular chromosome that floats free in a region called the nucleoid — there is no nucleus to enclose it.

  4. Ribosomes

    Ribosomes

    Thousands of ribosomes read messenger RNA copied from the DNA and build proteins. Many antibiotics work by jamming the bacterial ribosome.

  5. Flagellum

    Flagellum

    A rotating whip-like flagellum spins like a propeller, letting the cell swim toward food and away from danger.

Sources (2)

Methods & limits