Cellular Biology Reading Comprehension Lesson

What Is a Cell?

Over billions of years, the biological universe has developed an enormous variety of organisms — from towering trees to single-celled bacteria to complex humans. Despite their differences, every one of these organisms is made of the same basic building block: cells. Cells are the smallest living units of life. Inside every cell are countless chemicals and molecules working together to keep the organism alive. The hierarchy of biological organization in living things looks like this: organisms are made of organs, organs are made of tissues, and tissues are made of cells.

Although all living things are made of cells, the cells of different organisms are not identical. Animal cells have a different structure than plant cells. Bacterial cells differ from both. Because cells are so important and so varied, an entire branch of science called cell biology is devoted to studying them. Cell biology investigates a cell’s structure, the small compartments inside it called organelles, what each organelle does, and how cells are born, divide, and die.

All cells fall into one of two broad categories: prokaryotic cells or eukaryotic cells. Prokaryotic cells, such as bacteria and archaea, have very simple structures. None of their inner compartments are surrounded by membranes. Their genetic information — mostly DNA — floats freely in a region called the nucleoid. Prokaryotes are also surrounded by a tough outer layer called the cell wall, which gives them shape and protects the contents inside.

Eukaryotic Cells and Their Organelles

Eukaryotic cells are much more complex than prokaryotic cells. They include the cells of animals, plants, fungi, and protozoans. Like prokaryotic cells, eukaryotic cells are surrounded by a cell membrane, but inside they contain many organelles separated from one another by intricate inner membranes. The largest organelle is usually the nucleus, which holds the cell’s DNA and acts as the control center for everything the cell does.

Other organelles handle specific tasks. Mitochondria are the cell’s power plants — they take in nutrients and produce the energy the cell needs to live. The endoplasmic reticulum builds proteins and lipids (fats), which are then packaged and shipped through small bubble-like structures called Golgi vesicles. Ribosomes are tiny factories that assemble proteins. In animal cells, lysosomes use digestive enzymes to break down old or unwanted materials. Plant cells have a special organelle called the chloroplast, where photosynthesis converts sunlight into food.

Cells come in many shapes and sizes. A chicken egg, for example, is a single enormous cell, while the cells on your fingertip are so small that thousands of them fit on a tiny patch of skin. Cells in a complex organism are usually specialized to do specific jobs. Skin cells protect us from injury and infection. Muscle cells help us move. Nerve cells carry messages around our bodies. Without all these specialized cells working together, complex life as we know it could not exist.

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