27 Inside Living Things-High

Inside Living Things Written by Ron Fridell

Introduction Suppose you had microscopic X-ray vision, and you took 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 Living things share one basic characteristic: They are all 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 a single cell. Called unicellular organisms, they include tiny algae, diatoms, and protozoa. Each of these single-celled organisms carries on all the activities it needs for survival within its single cell. These organisms live in a wide range of places, including ponds, rivers, soil, and even inside the bodies of other living things. Other organisms large and small, from fleas and mosses to whales and giant redwoods, are multicellular organisms. They are made of trillions of cells that work together. The bigger the organism, the more cells it usually has.

There are two basic types of cells: plant cells and animal cells. Let's look at a typical animal cell. The outer covering of an animal cell is called the cell membrane. The cell membrane is a sac with pores, or small openings. This membrane is like a filter. It allows some things to pass in and out of the cell. The cell membrane allows nutrients to enter the cell and waste 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 directs the cell's activities, much as your brain controls your 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 typical plant cell has a cell wall, which surrounds the cell membrane and gives the cell support. Animal cells do not have cell walls. The support that a cell wall gives the plant cell is important because plants do not have a skeleton for support. All cells turn food into energy. Cells have hundreds or even thousands of mitochondria, which change energy into forms that the cells can use to do their jobs. Most cells also have several sac-like structures called vacuoles, which 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 to survive, but plants can make their own food. Plants store a special substance called chlorophyll in tiny structures called chloroplasts. Animal cells do not have chlorophyll or chloroplasts. A plant uses chlorophyll to make food from water and a gas in the air. The plant absorbs sunlight and uses the Sun's energy to make food from the water and air. This process is called photosynthesis.

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 mammals. Highly complex living things like mammals and reptiles have many parts that work together. Each part is made up of different kinds of cells. These animals have muscle cells, brain cells, nerve cells, blood cells, skin cells, and hundreds of other types of cells. All these cells must work together to keep the body growing and functioning. Do You Know? How many cells are in your body? There is no way to know the exact number. How could we count every single one? The number varies, depending on the size of the person, and the number is constantly changing. Every second you are losing cells, but your body is also manufacturing new ones. The best that scientists can do is estimate, and most say that the average human body contains about 75 trillion (75,000,000,000,000) cells. To get a better idea of how tiny a cell is, think about this. It would take ten thousand (10,000) cells just to cover the head of a pin.