Animal Cells vs Plant Cells

Animal cells and plant cells are both eukaryotic cells: they both have a nucleus, mitochondria, ribosomes and most of the other organelles. But there are three big features that plant cells have and animal cells do not: a tough cell wall, green chloroplasts, and a giant central vacuole. Those three extras are what make a plant a plant, and they explain almost everything about how plants live and grow differently from animals.

• Both haveNucleus + mitochondriaPlus ribosomes, ER, Golgi, etc.
• Only plant cells haveCell wallMade of cellulose, the source of dietary fibre
• Only plant cells haveChloroplastsWhere photosynthesis happens
• Only plant cells haveBig central vacuoleA water sac that gives a plant its rigidity
• Animal cell shapeRound and flexibleCan change shape easily
• Plant cell shapeBoxy and rigidHeld in shape by the cell wall

What is the same?

Both animal and plant cells are eukaryotes, so they share almost all the basic machinery:

• A cell membrane around the outside.
• A nucleus with DNA inside, controlling the cell.
• Mitochondria, the energy factories.
• Ribosomes, the protein factories.
• An endoplasmic reticulum and Golgi apparatus for transporting and packaging proteins.
• Cytoplasm filling the cell with a jelly-like fluid.

If you swapped many of these organelles between an animal cell and a plant cell, they would still work. The machinery is built from the same chemistry.

The cell wall

Every plant cell is surrounded by a stiff outer box called the cell wall, made of a tough sugar called cellulose. Cellulose fibres are arranged in layers, giving the wall enormous strength while still letting water pass through. The cell wall is what makes plants rigid: trees can stand 100 metres tall partly because every one of their cells is held up by its own little cellulose box.

Animal cells have no cell wall, which is why animals are floppy and can move freely (cells without walls can squish and change shape). It is also why we have evolved a skeleton: we need bones because we do not have cell walls.

Chloroplasts: the green factories

The green colour of plants comes from tiny organelles inside their cells called chloroplasts. Chloroplasts contain a green pigment called chlorophyll, which captures sunlight and uses its energy to combine carbon dioxide and water into glucose (sugar). This process is called photosynthesis, and it is the only thing in the whole biology of cells that can make food from sunlight.

A typical leaf cell has 30 to 100 chloroplasts. Animal cells have none. That is why animals have to eat plants (or other animals that ate plants) to get any energy at all. Plants are the original solar panels.

The big central vacuole

Plant cells contain one giant central vacuole: a single sac of watery fluid that can take up up to 90% of the cell's volume. The vacuole stores water, sugars and waste products, but more importantly, it pushes outwards against the cell wall, holding the cell up like a balloon inside a box. This is called turgor pressure, and it is what keeps a plant standing upright.

When a plant does not get enough water, the vacuoles lose pressure and the whole plant droops (you can see this happen in a few hours if you forget to water a houseplant). When the plant gets water again, the vacuoles refill and the plant perks back up.

Animal cells sometimes have small vacuoles, but never a single big one like in a plant.

Fact Chloroplasts and mitochondria both evolved from ancient bacteria that got swallowed by other cells billions of years ago and ended up living inside them permanently. Both still have their own DNA, separate from the cell's main DNA. So every plant cell in a tree is really three cells living together: a host cell, a mitochondrion (ex-bacterium that does respiration), and chloroplasts (ex-bacteria that do photosynthesis).

Why the differences matter

The three plant-cell extras explain almost every way plants and animals differ as living things:

• Plants make their own food (chloroplasts); animals have to eat.
• Plants are rigid (cell walls); animals can move freely.
• Plants get most of their structure from water pressure (central vacuole); animals get structure from bones and muscle.
• Plants do not move from place to place; animals have evolved specialised cells (muscle, nerve) so they can.

Did you know? Some fungi look like plants (mushrooms grow from the ground, look stationary) but their cells are actually more like animal cells. Fungi do not have chloroplasts (they cannot do photosynthesis, they get energy by absorbing food from dead matter), but they DO have a cell wall (made of chitin, the same stuff as insect shells). Fungi are their own kingdom of life, completely separate from plants or animals.

Deeper dive: why plant cell walls give us dietary fibre

The cellulose in plant cell walls is one of the most common substances on Earth. Every blade of grass, every leaf, every potato, every grain of rice is wrapped in cell walls made mostly of cellulose. So is every cotton fibre and the paper of every book.

What is interesting is that humans cannot digest cellulose. Our digestive enzymes can break apart starch and sugar (which are also chains of glucose molecules), but they cannot break apart cellulose (also a chain of glucose molecules, just joined in a slightly different way). When we eat plants, the cellulose from their cell walls passes straight through our gut and out the other end. This is what nutritionists call dietary fibre. Fibre is essential for healthy digestion: it gives the contents of your gut bulk, feeds the friendly bacteria in your colon and keeps things moving along.

Cows, sheep, termites, koalas and other animals that live on grass and leaves CAN digest cellulose, but not on their own. They host special bacteria in their guts (often in a specialised first stomach called the rumen) that produce enzymes able to break cellulose down. The cow digests the bacteria, not the grass directly. This is also why grazing animals chew their food twice (chewing the cud): they need to give the bacteria plenty of time to do their work.

For more on cells in general, see what is a cell and parts of a cell. For the simplest cells, see single-celled organisms.