A cell wall is a tough, protective outer layer that surrounds the cells of plants, fungi, and many bacteria. Think of it like armor that gives these cells their shape and strength. While animal cells have only a thin, flexible membrane on the outside, plant cells have both a membrane and a thick cell wall outside of it.

The cell wall is what makes a carrot crunchy or an apple crisp: you're biting through millions of tiny cell walls. These walls are made mostly of cellulose in plants, which creates a rigid structure that helps trees grow tall and strong. Without cell walls, a mighty oak tree would collapse into a puddle of cellular goo.

Cell walls have tiny pores that let water and nutrients pass through while keeping the cell's contents safely inside. They also help plants stand upright without skeletons or muscles. When you see a wilted plant perk up after watering, that's because water pressure inside the cells pushes against the cell walls, making the plant firm and upright again.

Scientists can identify plant cells under a microscope by looking for their distinctive boxy shape, which comes from the rigid cell wall. Animal cells, lacking this structure, appear rounder and more irregular.