25 Inside Living Things-Low

INSIDE LIVING THINGS Written by Ron Fridell

Introduction Suppose you had special vision that let you look deep inside living things. What would you learn? What might you discover deep inside a rose? 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 make the energy that living things need in order to grow and function. Some living things have only one cell. Algae, diatoms, and protozoa are organisms that have only one cell. All of the tasks that each organism needs in order to live are carried on in its cell. Organisms with one cell are found in many places, such as ponds and soil. Some 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. Most of the time, the bigger the organism the more cells it has. For example, a huge whale has trillions of cells.

There are two basic types of cells: plant cells and animal cells. Let's look at an animal cell It has an outer covering called the cell membrane. This membrane is like a filter. It allows certain things to pass into and out of the cell. Things needed by the cell 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 runs 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. It surrounds the cell membrane. The cell wall supports the cell. Animal cells do not have cell walls. All cells turn food into energy the cell can use. Mitochondria are the parts of a cell that have this job. Most cells also have structures called vacuoles. These structures store food and water until the cell needs them. They also hold waste until it can be removed from the cell. PLANT CELLS: FOOD AND STORAGE Animals have to eat food in order to live. Plants can make their own food, though. Plant cells have a special material called chlorophyll. The chlorophyll traps energy from sunlight. This energy is used with other materials from air and water to make food. This method of making food is called photosynthesis. Animal cells do not have chlorophyll.

Different Kinds of Cells Not all cells are alike. An organism's cells are different, depending on what they do. Let's take a look at some cells found in animals. Most animals have many parts that work together. Each part is made up of different kinds of cells. Muscles have muscle cells. Blood has blood cells. Even nerves have cells. There are hundreds of types of cells All types of cells must work together to keep an animal alive and healthy. Do You Know? Scientists think there are about 75 trillion (75,000,000,000,000) cells in the human body. But the number keeps changing. Cells are always growing and being replaced. These cells are very small. It would take ten thousand (10,000) cells just to cover the head of a pin.