Animal Cell Diagram: Every Labelled Part, How Much Space Each One Really Takes, and What Plants Do Not Have

What does an animal cell diagram show?

It shows a section. An animal cell is a three-dimensional object and the diagram cuts it open, which means the outline you are looking at is a cut edge, not a silhouette.

Three things on the diagram are claims. Which compartments are present. How they nest inside one another. And which membranes are continuous with which — the part almost every diagram draws and almost no reader is told to look for. The rough and smooth ER are one system. The nuclear envelope is continuous with the ER. The plasma membrane is continuous with nothing; it is the edge.

Everything else is a drawing decision. The colours are conventional, not observed. The number of each organelle drawn is whatever fitted. And the sizes are wrong in a specific, correctable direction, which the next two sections are about.

One thing that is not a drawing decision: the shape. A plant cell is drawn as a box because a wall holds it in one. An animal cell has no wall, so the outline is the plasma membrane and nothing else, and it takes whatever shape the cytoskeleton and its neighbours give it. Drawing an animal cell as a neat circle is the first small lie in most diagrams.

The parts of an animal cell, labelled

Each label, and what the structure behind it actually is:

How much of an animal cell is each organelle?

This is the part almost no diagram gets right, and the numbers have been in print since 1983.

Here is the measured breakdown for a liver cell, from Alberts' Table 12-1. A hepatocyte is not every animal cell — a pancreatic exocrine cell is five times smaller and packs sixty per cent of its membrane into rough ER — but it is a real cell with real numbers, which is more than a diagram drawn from another diagram can say.

Relative volumes occupied by the major intracellular compartments in a liver cell, from Alberts et al., Molecular Biology of the Cell, 4th edition, Table 12-1.

Two things fall out of that table.

Mitochondria occupy nearly four times as much of the cell as the nucleus. Not slightly more. Nearly four times. The standard diagram — big central nucleus, two mitochondria off to the side — inverts the relationship, and it is the single most consistent error in the genre.

The membrane-enclosed compartments together take about half the cell. The cytosol is the other half. A diagram that draws the organelles as small islands in a large empty interior has the proportion backwards; the interior is not empty, and there is no more of it than there is of everything else.

The figure above is drawn against the convention. Measured off the drawing, the nucleus covers 10.3% of the section and the nine mitochondria together cover 12.1% — so the mitochondria come out larger, which is the direction the table demands.

It is still an understatement, and the size of the understatement is worth stating. The true ratio is 22 to 6, or 3.7 to 1; the figure manages 1.2 to 1. Nine mitochondria drawn large enough to carry a leader line do not add up to 22% of the picture, and no labelled diagram can — the moment the organelles are small enough to be correctly proportioned, they are too small to label. That is a real limit of the format, and it is better said out loud than quietly drawn around.

What is the difference between a plant cell and an animal cell?

The differences that matter to a diagram are structural, and they run in both directions.

The reversal worth noticing is the last three rows. Plant cells need no anchoring junctions because their walls are already glued together, so the whole apparatus of tight junctions, desmosomes and gap junctions is an animal solution to a problem plants solved with cement. That is why a plant cell diagram and an animal cell diagram look most different where two cells touch — and why the next section draws that.

How two animal cells connect

A plant cell talks to its neighbour through a plasmodesma: a cytoplasmic channel 20–40 nm across that crosses a wall at least 0.1 μm thick, with the plasma membrane of one cell running continuously into the other.

An animal cell has no wall to cross and does it differently. At a gap junction the two plasma membranes come within 2–4 nm of each other, and matched half-channels called connexons dock across that gap to form a continuous pore from one cytoplasm to the other.

The 1.5 nm figure comes from injecting fluorescent molecules of known size into one cell and watching what arrives in the next. Below about 1000 daltons — inorganic ions, sugars, amino acids, nucleotides, vitamins, and second messengers like cyclic AMP — molecules cross freely. Above it, they do not, so coupled cells share their small molecules but keep their proteins and nucleic acids to themselves.

The other two junctions belong on a diagram of an epithelium rather than of a single cell, and they are worth naming because they are what the single-cell diagram leaves out. Tight junctions seal neighbouring cells together into a sheet, so that a tracer applied on one side does not pass beyond them, and they also fence the plasma membrane into an apical and a basolateral domain that do not mix. Desmosomes are button-like contacts that rivet cells together and anchor intermediate filaments from one cell to the next, forming a framework of great tensile strength across a whole tissue.

How do you draw an animal cell?

Six decisions, in the order they matter.

Draw the outline irregular. Not a circle, not a rounded box. Nothing outside the membrane holds the cell to a shape, and an irregular outline says so before a single label is read.

Put the nucleus in at about a tenth of the area. A sphere at 6% of a cell's volume cuts a central section around 15% of the cell's area, and a diagram that gives it a third is claiming something the numbers do not support. Draw it off-centre while you are at it — the centrosome sits beside it, and the Golgi is usually close to it.

Draw more mitochondria than feels right. Two is the convention and two is wrong. Enough of them that they collectively look larger than the nucleus is closer to the truth, and still short of it.

Draw the rough and smooth ER as one system. Cisternae with ribosomes on the cytosolic face running into bare tubules. Two separate blobs in different corners is the commonest way of teaching a fact that is not true.

Give each organelle one distinguishing feature. A mitochondrion without cristae and a lysosome are the same oval. The cristae are the distinction, so draw them. Nine ticks around an end-on centriole are the distinction between a centriole and a capsule, so draw those too.

Then label it. Leader lines straight, none crossing another, each ending in a dot on the structure it names. Set the type at a real size while you draw: Nature allows a maximum of 7 pt for figure text and a minimum of 5 pt, with 8 pt bold for panel letters, in a figure 89 mm wide for one column or 183 mm for two, on a page 247 mm deep. A cell diagram drawn at poster scale and dropped into a manuscript at a third of the size is the most common way a good figure becomes an unreadable one.

Vector formats survive that reduction and raster formats do not. If the diagram has to be raster, Nature wants a minimum of 300 dpi at the size it will be printed.

The same diagram without the labels

The same geometry with numbers instead of names. Print it, label it from memory, then check against the table above.

Five things animal cell diagrams get wrong

The nucleus is too big and the mitochondria too few. Covered above, and it is the one that changes what a reader believes about the cell.

Rough and smooth ER drawn as two organelles. They are one continuous membrane system. The only difference is whether ribosomes are bound to the cytosolic face, and a diagram that puts them in opposite corners with separate labels teaches the opposite.

The nucleolus drawn with a membrane. It has none. It is a dense region inside the nucleus, and giving it an outline puts it in the same visual class as the organelles, which is exactly the mistake.

The cell drawn alone in white space. An animal cell in a tissue is pressed against its neighbours, and half of what is interesting about its plasma membrane — tight junctions, desmosomes, gap junctions — only exists where two cells touch. A cell drawn in isolation has nothing for those structures to be between.

Peroxisomes and lysosomes drawn identically. They are different compartments doing different jobs, and at diagram scale the only thing that tells them apart is how they are drawn. Give the lysosome a full, granular lumen and the peroxisome a plain one, or leave one of them off.

Frequently asked questions

What is an animal cell diagram? A labelled section through a single animal cell, drawn to show which compartments exist and what encloses what. It is a cutaway, not a photograph. The outline is the plasma membrane and nothing else.

What are the parts of an animal cell? Plasma membrane, cytosol, nucleus with envelope and nucleolus, rough and smooth ER, Golgi apparatus, mitochondria, lysosomes, peroxisomes, centrosome with its centrioles, ribosomes, and the three cytoskeletal filament systems.

What is the difference between a plant cell and an animal cell? Plants have a wall, plastids, a large central vacuole and plasmodesmata. Animals have centrioles and a set of cell junctions plants have no use for. See the comparison table above for the full list.

What organelles are in animal cells but not plant cells? Centrioles, most clearly. Lysosomes are usually given as the second answer, and it needs a caveat: the plant vacuole is the related compartment and carries the same hydrolytic enzymes.

How do you draw a simple animal cell? Irregular outline, nucleus at about a tenth of the area, more mitochondria than feels right, rough and smooth ER as one system, one distinguishing feature per organelle, then straight non-crossing leader lines.

What is the function of the centrosome? It is the major microtubule-organising centre of an animal cell. It sits beside the nucleus, contains a pair of centrioles at right angles, and becomes the two poles of the mitotic spindle.

What is the animal cell diagram for class 9? The same section with a reduced label set — plasma membrane, cytoplasm, nucleus, nucleolus, mitochondrion, endoplasmic reticulum, Golgi apparatus, lysosome, ribosomes, vacuole and centrosome.

Where to go next

• The plant cell diagram, labelled the same way — the same treatment for the cell on the facing page, including what the wall between two plant cells actually contains and why a plasmodesma is not a gap junction.
• How to make scientific figures — the general rules this diagram follows: type sizes, line weights, colour, and how to check a figure survives reduction.
• Scientific Reports figure requirements — what happens to a diagram like this one when it goes into a manuscript, and which specifications the journal actually publishes.