Wikijunior:Biology/Introduction

Introduction
Biology is the study of life. It helps us understand many things, such as how our body works, how our body keeps warm, and what we are made of. Biology is very important to know. Some things we can learn about in biology are genetics (the study of human traits), zoology (the study of animals), botany (the study of plants), and ecology (the study of relationships between all living things).

Someone who studies biology is called a biologist.

What is life?
Living things are different from things that are not alive. It is usually easy to tell what is living and what is not, but it is sometimes hard to tell, like with very small organisms.

Here are some properties of living things. You might notice that some non-living things can also have some of these properties.


 * Living things can change and grow. However, volcanoes can also change and grow when they erupt.
 * Living things can move. However, the wind is moving air, and water always moves downhill.
 * Just like animals, even plants can move. They can grow, and sometimes move more rapidly than that, in response to things such as the sun or water. One example is that sunflowers will naturally turn during the course of the day so that they are always facing the sun. Similarly, another example is that if a plant gets tipped over, it will want to turn upwards to face the sun.
 * Living things can reproduce, which means that they can produce copies of themselves, over and over. This is the most important difference between living and non-living things.
 * In order to reproduce, living things need nutrition, which are nutrients and energy sources in order to assemble the materials needed to reproduce themselves. In this process, living things must excrete waste. Waste is material which is of no use to living things, or in some cases, material that can be harmful.

Animals, bacteria, and plants are examples of living things. Rivers, mountains, oceans, and soil are examples of non-living things, but they are often homes for living things.

Cars and tables are also not living things, because they cannot reproduce themselves.

Levels of life
Living things can be of many different sizes. Size is very important in biology, since biologists organize the structures and groupings of living creatures according to size. A living creature is called an organism. Organisms can consist of single cells or multiple different types of cells grouped into tissues and organs.

From smallest to largest, these are how living things are grouped:


 * Cells
 * Most cells are only a few microns wide, and are so small that they can only be seen with a microscope. A micron is one thousandth of a millimeter.


 * Tissues
 * Tissues are groups of similar cells that are all doing similar things, like a muscle, which pulls things together.


 * Organs
 * Organs are made of lots of tissues. They all have a special function, like the heart, which pumps blood.


 * Organ systems
 * Organ systems are groups of organs which work together to do something. For example, all the organs which digest your food make up the digestive system.


 * Organisms
 * An organism is a whole living thing, like you, or a tree.


 * Populations
 * A population is a group of organisms which are all the same species and live together.


 * Communities
 * A community is a group of populations of different species, which live together; for example, all the fish in a lake.


 * Ecosystems
 * All the communities of organisms in an area, and the way they interact with non-living things like rivers or the weather in that area, form an ecosystem.


 * Biomes
 * A biome is a large region of Earth that has a certain climate and certain types of living things. Major biomes include tundra, forests, grasslands, and deserts.


 * Biosphere
 * The biosphere is the whole network of living things on planet Earth &mdash; eight thousand miles in diameter, twenty five thousand miles around the equator.

Everything in this list is made up of the things above it. For example, communities are made of many populations and populations are made up of many organisms.