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You may know that all living things are made of tiny units called cells. But did you know that different types of living things have different types of cells? Not all cells are the same. Living things such as people, birds, fish, and insects are made of animal cells. Living things such as flowers, grass, trees, and mosses are made of plant cells. Animal cells are surrounded by a thin and flexible cell membrane. Because the cell membrane is flexible, animal cells can have many different shapes. This is why cells in your body can look so different. Some cells are long with thread-like extensions. Other cells are round and compact. Cells in your body have different shapes so that they can do different jobs. Plant cells are surrounded by a cell membrane. However, they are also surrounded by a strong and thick cell wall. Cell walls are rigid and surround the cell membranes. Because cell walls are not very flexible, they give all plant cells a similar box-like shape. Cell walls give support to plants. They allow plants to grow straight up. Because of cell walls, trees can grow hundreds of feet tall. Animal cells are also different from plant cells because animal cells cannot make food. All animals must eat food to survive. However, plants can make their own food. Plants have structures called chloroplasts inside their cells. Chloroplasts capture sunlight. Plant cells use energy from sunlight to make food. Without sunlight, plants would not be able to make food. Animals do not have chloroplasts inside their cells. Every plant and animal cell also has a part called the nucleus. A nucleus controls all the activities that take place inside a cell. However, the cells of bacteria do not have a nucleus. Bacteria cells are less complicated than plant and animal cells. However, all bacteria cells are surrounded by a cell membrane. Some bacteria cells also have cell walls and chloroplasts.