Animal and plant cells

A plant cell has three things an animal cell lacks: a cellulose wall, chloroplasts and a large central vacuole.

Two kinds of eukaryotic cell

Bite into a stick of celery and it crunches. Press on a piece of raw meat and it gives. Part of that difference starts at the level of single cells. Animal cells and plant cells are both eukaryotic: each has a nucleus wrapped in a membrane and many other membrane-bound parts called organelles, a word that means “little organs”. Most of the machinery is shared. The differences are few, but they explain a lot about how plants and animals live.

What both have

Both kinds of cell have a plasma membrane, a thin double layer of fat-like molecules with proteins in it that controls what goes in and out. Inside is the cytoplasm: a jelly-like fluid, the cytosol, with organelles floating in it. The cytoplasm is 70 to 80 percent water, yet it feels semi-solid because of all the proteins dissolved in it. Both cells have a nucleus that holds the DNA, ribosomes that build proteins, an endoplasmic reticulum and a Golgi apparatus that finish and ship proteins and lipids, and mitochondria that make ATP, the molecule cells use to carry energy. Both also have small sacs called peroxisomes that break down fatty acids and some poisons.

What only plant cells have

Outside its membrane, a plant cell has a cell wall. The wall is a rigid layer made mostly of cellulose, a long chain of glucose units. It protects the cell, holds it up and gives it a fixed shape, which is why plant cells in the model look like boxes. When you bite celery, your teeth tear through these walls, and that is the crunch.

Plant cells also have chloroplasts, the green organelles that carry out photosynthesis. Using light energy, they turn carbon dioxide and water into sugar and oxygen. That is the main reason plants can make their own food while animals have to eat. In a leaf cell the chloroplasts are lens-shaped bodies about 4 to 6 µm across, often lined up along the wall.

The third difference is the large central vacuole, a sac of water that fills most of a mature plant cell. It controls how much water the cell holds. When the vacuole is full, it presses outward on the wall, and that pressure keeps soft stems and leaves firm. Forget to water a plant for a few days and the vacuoles lose water, the walls lose their support and the plant wilts. In the model, the blue body in the middle is the vacuole; the nucleus and the other organelles are pushed into a thin layer of cytoplasm around it.

What animal cells have instead

Animal cells have a centrosome near the nucleus, with a pair of centrioles set at right angles to each other, and they have lysosomes, small sacs of digestive enzymes that act as the cell’s recycling center. Most plant cells have neither; in plants, much of that digestion happens in the vacuole. With no wall, an animal cell can change shape, which lets white blood cells squeeze between other cells and wrap around germs.