Cell Anatomy: A Labeled 3D Animal Cell and Its Organelles

An animal cell has three main parts: the plasma membrane around it, the cytoplasm inside it (the jelly-like cytosol plus the organelles), and the nucleus, which holds the DNA. Every cell, from a bacterium to a neuron, shares four components: a plasma membrane, cytoplasm, DNA and ribosomes. (OpenStax Biology 2e, 4.2 Prokaryotic cells; OpenStax Anatomy and Physiology 2e, 3.2 The cytoplasm and cellular organelles.)

Cellular anatomy differs most between prokaryotes and eukaryotes. Animal cells are eukaryotic: they keep their DNA in a nucleus and divide their cytoplasm into membrane-bound organelles, each with its own job. Eukaryotic cells are typically 10–100 µm across, compared with 0.1–5 µm for bacteria. (OpenStax Biology 2e, 4.2 Prokaryotic cells.) Most mammalian cells have a volume of about 1,000–10,000 µm³. (Milo & Phillips, Cell Biology by the Numbers: How many cells are there in an organism?.) The human body is made of about 30 trillion human cells by a 2016 estimate, or 37 trillion by an earlier one, of more than 200 types. (Sender, Fuchs & Milo (2016): Revised estimates for the number of human and bacteria cells in the body, PLOS Biology 14:e1002533; Milo & Phillips, Cell Biology by the Numbers: How big is a human cell?.)

The model shows a generalized animal cell: real cells specialise. A neuron grows long dendrites and an axon (see our neuron anatomy model), a muscle fibre fills with contractile filaments, and a red blood cell loses its nucleus altogether. How cells group into tissues is covered in our tissue types guide.

A labeled animal cell diagram shows the plasma membrane, cytoplasm and nucleus (with nuclear envelope, pores, nucleolus and chromatin), and the organelles of the cytoplasm: rough and smooth endoplasmic reticulum, ribosomes, Golgi apparatus, vesicles, lysosomes, peroxisomes, mitochondria, the centrosome and the cytoskeleton.

In the 3D model, turn on All labels to name everything at once, or click a part to zoom to it. Slide the Plasma membrane control down to see the whole cell interior from any angle.

Each organelle has a main job. This table lists the parts of an animal cell, what they look like and what they do.

The nucleus is usually the largest and most prominent organelle. It is wrapped in a nuclear envelope of two lipid bilayers with a thin space between them, and holds the chromatin and the nucleolus in a gel-like nucleoplasm. (OpenStax Anatomy and Physiology 2e, 3.3 The nucleus and DNA replication; OpenStax Biology 2e, 4.3 Eukaryotic cells.) Typical nuclei are 2–10 µm across. (Milo & Phillips, Cell Biology by the Numbers: How big are nuclei?.)

Traffic in and out passes through nuclear pores, protein complexes about 120 nm across with eightfold symmetry; each human pore weighs about 110 MDa, and molecules smaller than about 40 kDa can diffuse through it freely. (Dultz, Wojtynek, Medalia & Onischenko (2022): The nuclear pore complex: birth, life, and death of a cellular behemoth, Cells 11:1456; Rush et al. (2023): Unveiling the complexity: assessing models describing the structure and function of the nuclear pore complex, Frontiers in Cell and Developmental Biology 11:1245939.) Inside, DNA wraps around histone proteins as nucleosomes, about 147 base pairs per histone core, forming the threads of chromatin; a human cell carries 46 chromosomes and almost 22,000 genes. (OpenStax Anatomy and Physiology 2e, 3.3 The nucleus and DNA replication; Milo & Phillips, Cell Biology by the Numbers: How big are nuclei?; OpenStax Biology 2e, 4.3 Eukaryotic cells.) The nucleolus builds ribosome subunits, which leave through the pores. Just inside the envelope, a mesh of intermediate filaments called the nuclear lamina shapes the nucleus. (OpenStax Microbiology, 3.4 Unique characteristics of eukaryotic cells; Gruenbaum & Aebi (2014): Intermediate filaments: a dynamic network that controls cell mechanics, F1000Prime Reports 6:54.)

The endomembrane system is a group of membranes and organelles that work together to make, modify, package and ship proteins and lipids. It includes the nuclear envelope, endoplasmic reticulum, Golgi apparatus, vesicles, lysosomes and, by interaction, the plasma membrane; mitochondria are not part of it. (OpenStax Biology 2e, 4.4 The endomembrane system and proteins.)

1. Ribosomes on the rough ER insert a new protein into its lumen, where it is folded and may gain sugar chains. (OpenStax Biology 2e, 4.4 The endomembrane system and proteins.)
2. Transport vesicles bud from the ER and fuse with the cis face of the Golgi apparatus. (OpenStax Biology 2e, 4.4 The endomembrane system and proteins; OpenStax Microbiology, 3.4 Unique characteristics of eukaryotic cells.)
3. The Golgi modifies the protein, most often by adding short sugar chains, then sorts and tags it. (OpenStax Biology 2e, 4.4 The endomembrane system and proteins.)
4. Secretory vesicles bud from the trans face and fuse with the plasma membrane, releasing their contents (exocytosis); the Golgi also produces lysosomes. (OpenStax Biology 2e, 4.4 The endomembrane system and proteins; OpenStax Anatomy and Physiology 2e, 3.2 The cytoplasm and cellular organelles.)

The ER is big. In liver and pancreas cells it can hold up to 60% of all the cell's membrane, and in a pancreatic acinar cell its surface area is 10–20 times that of the plasma membrane. (Milo & Phillips, Cell Biology by the Numbers: How big is the endoplasmic reticulum of cells?.) The smooth ER makes lipids and steroid hormones, detoxifies drugs and poisons, and stores calcium; in muscle it is called the sarcoplasmic reticulum. (OpenStax Biology 2e, 4.4 The endomembrane system and proteins; OpenStax Anatomy and Physiology 2e, 3.2 The cytoplasm and cellular organelles.) Lysosomes are the recycling centre: their enzymes work at about pH 4.5–5, far more acidic than the cytosol at about pH 7.2, and digest worn-out organelles (autophagy) and engulfed bacteria. (Dharmasivam & Kaya (2025): Lysosome as a chemical reactor; Weiss (2012): Cross-talk between TRPML1 channel, lipids and lysosomal storage diseases, Communicative & Integrative Biology 5:111; OpenStax Anatomy and Physiology 2e, 3.2 The cytoplasm and cellular organelles.)

Mitochondria are the powerhouses of the cell: double-membraned organelles that make most of its ATP by aerobic respiration. The inner membrane folds into cristae around a matrix that holds the mitochondrion's own DNA and 70S ribosomes. (OpenStax Microbiology, 3.4 Unique characteristics of eukaryotic cells; OpenStax Biology 2e, 4.3 Eukaryotic cells.) A textbook mitochondrion is about 2 µm long and 1 µm wide, and a mammalian cell holds roughly 1,000–10,000 of them. (Milo & Phillips, Cell Biology by the Numbers: How big are mitochondria?.) Numbers vary with energy demand: a neuron has over a thousand, a bone cell a couple of hundred, and muscle cells are packed with them. (OpenStax Anatomy and Physiology 2e, 3.2 The cytoplasm and cellular organelles.) Their bacteria-like DNA and ribosomes are the main evidence that mitochondria descend from bacteria taken up by an ancestral cell (endosymbiotic theory). (OpenStax Microbiology, 3.4 Unique characteristics of eukaryotic cells.) How they make ATP is shown on our cellular respiration diagram.

Peroxisomes are small single-membrane sacs whose enzymes break down fatty acids and toxins such as alcohol, producing hydrogen peroxide that the enzyme catalase then splits into water and oxygen. Liver cells have especially many. (OpenStax Anatomy and Physiology 2e, 3.2 The cytoplasm and cellular organelles; OpenStax Microbiology, 3.4 Unique characteristics of eukaryotic cells; OpenStax Biology 2e, 4.3 Eukaryotic cells.)

The cytoskeleton is a network of protein fibres that gives the cell its shape, holds organelles in place and moves things around. It has three kinds of fibre, which differ in thickness and job. (OpenStax Biology 2e, 4.5 The cytoskeleton; OpenStax Anatomy and Physiology 2e, 3.2 The cytoplasm and cellular organelles.)

Sources differ slightly on the diameters: OpenStax Biology gives 7 nm for actin and 25 nm for microtubules, OpenStax Microbiology gives 6 nm and 23 nm, and Cell Biology by the Numbers gives 6 nm and 25 nm. (OpenStax Biology 2e, 4.5 The cytoskeleton; OpenStax Microbiology, 3.4 Unique characteristics of eukaryotic cells; Milo & Phillips, Cell Biology by the Numbers: How big are the cell's filaments?.) Intermediate filaments are 8–10 nm or about 10 nm. (OpenStax Biology 2e, 4.5 The cytoskeleton; Gruenbaum & Aebi (2014): Intermediate filaments: a dynamic network that controls cell mechanics, F1000Prime Reports 6:54.)

Microtubules grow out from the centrosome, a pair of centrioles at right angles, each built from nine triplets of microtubules. (OpenStax Biology 2e, 4.3 Eukaryotic cells; OpenStax Microbiology, 3.4 Unique characteristics of eukaryotic cells.) One centriole can dock under the membrane and become the basal body of a primary cilium, a single non-moving 9 + 0 "antenna" found on most vertebrate cells; motile cilia, such as those that sweep mucus from the airways, have a 9 + 2 pattern. (Bai et al. (2022): Primary cilium in kidney development, function and disease, Frontiers in Endocrinology 13:952055; OpenStax Anatomy and Physiology 2e, 3.2 The cytoplasm and cellular organelles.) Microvilli, finger-like folds of the membrane, increase the surface area of absorptive cells such as those lining the small intestine. (OpenStax Biology 2e, 4.3 Eukaryotic cells.)

Organelles can be sorted by their membranes. The nucleus and mitochondria are wrapped in two membranes; the ER, Golgi apparatus, vesicles, lysosomes and peroxisomes in one; and ribosomes, centrioles and the cytoskeleton in none, so they sit directly in the cytosol. (OpenStax Biology 2e, 4.3 Eukaryotic cells; OpenStax Microbiology, 3.4 Unique characteristics of eukaryotic cells; Pearson Channels, Anatomy & Physiology study guide: Cellular anatomy, structure and function of the cell.)

Animal and plant cells are both eukaryotic and share a nucleus, plasma membrane, cytoplasm, mitochondria, ER, Golgi apparatus and ribosomes. Only animal cells have centrosomes and lysosomes; only plant cells have a cellulose cell wall, chloroplasts and a large central vacuole. (OpenStax Biology 2e, 4.3 Eukaryotic cells.)

Look inside a chloroplast on our chloroplast structure model, and see plant cells in place in the leaf cross-section.

Prokaryotic cells (bacteria and archaea) have no nucleus and no membrane-bound organelles; their DNA lies in a region called the nucleoid. Eukaryotic cells, including animal and plant cells, have both. (OpenStax Biology 2e, 4.2 Prokaryotic cells.)

Sources: OpenStax Biology 2e and Microbiology. (OpenStax Biology 2e, 4.2 Prokaryotic cells; OpenStax Microbiology, 3.4 Unique characteristics of eukaryotic cells.) Explore a bacterium in 3D on our bacterial cell diagram.

Cells stay small because of their surface area-to-volume ratio. Everything a cell takes in or gets rid of must cross its plasma membrane, but as a cell grows its volume rises faster than its surface area. The ratio falls, and past a certain size the membrane can no longer pass enough material by diffusion to serve the whole cell. (OpenStax Biology 2e, 4.2 Prokaryotic cells.)

A worked example with spherical cells shows why. Surface area is 4πr² and volume is 4/3πr³, so the ratio is 3 divided by the radius: each tenfold increase in width gives 100 times the surface but 1,000 times the volume to supply. The same rule helps explain the size gap between the two great kinds of cell: prokaryotic cells are only 0.1–5 µm across, while eukaryotic cells, at 10–100 µm, divide their interior into membrane-bound organelles. (OpenStax Biology 2e, 4.2 Prokaryotic cells.)

The 20 µm cell's volume of about 4,000 µm³ sits in the usual 1,000–10,000 µm³ range for mammalian cells. (Milo & Phillips, Cell Biology by the Numbers: How many cells are there in an organism?.) Cells beat the limit in the ways the 3D model shows: they divide, fold their membrane into microvilli, grow thin or long, and build internal membranes as organelles. (OpenStax Biology 2e, 4.2 Prokaryotic cells.) In a pancreatic cell the ER alone has 10–20 times the area of the plasma membrane, and a neuron can be over a metre long yet only about 10 µm wide. (Milo & Phillips, Cell Biology by the Numbers: How big is the endoplasmic reticulum of cells?; Milo & Phillips, Cell Biology by the Numbers: How big is a human cell?.)

The text version of the 3D model: every labeled part of the cell, grouped by system, with its function. Use it as a study list for a cell labeling worksheet.