In Depth


Cells have so many different parts that it’s easy to confuse them! Let’s look closer at two pairs of cell structures many people get mixed up.

Mitochondria vs. Chloroplasts

Find all of the mitochondria in the diagram. Now, find the chloroplasts. You probably noticed both cells have mitochondria, but only plant cells have chloroplasts! That’s because the chloroplast’s job is to make food. They make sugar molecules using sunlight, carbon dioxide, and water through a process called photosynthesis. Most animals, including humans, don’t make their own food. Instead, they get sugar from the food they eat.

Mitochondria do not make food. In fact, they do the opposite; they break down sugar molecules in a chemical reaction. Chemical energy from the broken sugar molecules is transferred into a new molecule, ATP, in a process called cellular respiration. The newly made ATP molecules leave the mitochondria and travel around the cell. Cells use the energy stored in ATP for the jobs they need to do.

Cell Walls vs. Cell Membranes

While both cell walls and cell membranes surround cells, they do very different jobs. Cell walls provide strength and shape to plant and bacteria cells. Animal cells do not need cell walls, in part because their skeletons provide a similar, strong frame.

However, all cells have and need cell membranes. Cell membranes control what enters and leaves a cell. They allow certain things to pass through, such as oxygen, sugar, and water. Cell membranes also keep things out. They protect cells by blocking harmful things like viruses and toxins from entering. Cell membranes are semipermeable—only certain things can pass through. Permeable membranes allow all materials to pass through.

Fun Fact: Scientists call cell structures like mitochondria and chloroplasts, organelles, which means “tiny-organs.” Why is that term appropriate?