Cells — Reading Comprehension Lesson

Reading Passage

Cells

All living things are made of tiny structures called cells. Cells are often called the building blocks of life because every plant, animal, and microbe is built from one or more cells. Some living things, such as bacteria, are made up of just one cell. Others, like trees and humans, are made of trillions of cells working together.

Cells are extremely small — most can only be seen with a microscope. Even though they are tiny, every cell carries out important jobs. Cells take in nutrients, get rid of waste, grow, and reproduce. Different kinds of cells specialize in different jobs to help an organism survive.

Although cells come in many shapes and sizes, most of them have certain parts in common. The cell membrane is a thin outer layer that holds the cell together and controls what enters and leaves. The cytoplasm is a jelly-like fluid that fills the inside of the cell. Floating inside the cytoplasm are tiny structures called organelles, each with a special job.

The nucleus is the control center of the cell. It contains DNA, the instructions that tell the cell how to grow, work, and reproduce. Other important organelles include the mitochondria, which produce energy, and the ribosomes, which build proteins the cell needs.

Plant cells and animal cells are similar in many ways, but they have a few important differences. Plant cells have a stiff outer cell wall that gives them support and shape. They also contain chloroplasts, the green organelles that allow plants to make food from sunlight in a process called photosynthesis. Animal cells do not have cell walls or chloroplasts, but they often have other special structures suited to the jobs they do in the body.

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