26 Inside Living Things-Mid

Inside Living Things Written by Ron Fridell

Introduction Suppose you had microscopic X-ray vision. You would be able to take a deep look inside living things. What would you learn? What would you discover deep inside a rose, for example? Or an elephant? Or a human being like yourself? This book will explore these questions by looking deep inside living things.

Cells All living things are made of cells. Cells are the smallest units of life. They produce the energy that makes living things grow and function. Some living things have only one cell. Tiny algae, diatoms and protozoa are examples of organisms that have only one cell. All of the activities needed for each organism's survival are carried on in its cell. Organisms with one cell live in a wide range of places. This includes ponds, rivers, and soil. Some one-celled organisms are even found inside the bodies of other living things. Other organisms have many cells. All of the cells work together so the organism can live. The bigger the organism, the more cells it usually has. An organism as big as a whale has trillions of cells.

There are two basic types of cells: plant cells and animal cells. Let's look at an animal cell. The outer covering of an animal cell is called the cell membrane. This membrane is like a filter. It allows certain things to pass into and out of the cell. Nutrients are allowed to enter the cell. Waste is allowed to exit. Inside the cell membrane is a jellylike fluid called cytoplasm. Other cell parts float in the cytoplasm One important part is the nucleus. The nucleus is like the cell's brain It controls all the cell's activities.

In many ways, plant cells are like animal cells. Both have a cell membrane, a nucleus, and cytoplasm. But they also have some parts that are different. A plant cell has a cell wall that surrounds the cell membrane. The cell wall supports the cell. Animal cells do not have cell walls. All cells turn food into energy. Mitochondria are cell parts that change energy, into forms that cells can use to do their jobs. Most cells also have several sac-like structures called vacuoles. These structures store food and water until the cell needs them. They also store waste until it can be removed from the cell. PLANT CELLS: FOOD AND STORAGE Animals have to eat food in order to live. However, plants can make their own food. Plants store a special material called chlorophyll. It is found in the plant's chloroplasts. Chlorophyll absorbs energy from light. The plant uses this energy with other materials from air and water to make food. This process is called photosynthesis. Animal cells do not have chlorophyll or chloroplasts.

Different Kinds of Cells Not all cells are alike. An organism's cells differ, depending on their function. To better understand how cells can differ, let's take a look at some kinds of cells found in animals. Most animals have many parts that work together. Each part is made up of different kinds of cells. There are muscle cells, nerve cells, blood cells, and hundreds of other types of cells. The cells must all work together to keep the body working correctly. Do You Know? Scientists estimate that there are about 75 trillion (75,000,000,000,000) cells in the human body, though the exact number changes. Cells are always growing, dying, and being replaced. These cells are very small. It would take ten thousand (10,000) cells just to cover the head of a pin.