CONTENTS INTRODUCTION CHAPTER 1 THE CHARACTERISTICS OF LIVING THINGS 10 CHAPTER 2 CELLs TissUES AND ORGANIS CHAPTER 3 NuTRITION AND ENERGY PRODUCTION f CHAPTER 4 EvoLUTION AND THE HiSTORY OF LIFE ON EARTH 45 CHAPTER 5 THE CLASSIFICATION OF LIVING THINGS 58 75 CONCLUSION GosSARY 76 For MORE INFORMATION BILIOGRAPHY 82 83 INDEX
he world is filled with living and nonliving things In most cases fig uring out which of them are alive is easy For instance it s obvious that a fuzzy woodland creature scampering across a field is alive while a rock plunked down in the A middle of that same field is not Science t however doesn't leave such things to casual observation In order for an organism to be is alive it must pass a series of seven tests One test concerns movement Living things are able to get from one location to another or they at least have movement within themselves For an example of the second type of movement consider a tree CA It can t move from one place to another but it does have the ability to move water nutrients and other materials internally Whatever the type of motion it must occur under the organism's own power Being pushed pulled or otherwise forced to move doesn't count A second test is sensitivity Living organisms respond to conditions in their environment using whatever senses they have Plants stretch and grow toward sun light Animals and people flinch when they hear a loud sound or feel a sharp
The ability to gather nutrients the source of energy from food sources is another test that proves something is alive Eating other organisms is how most animals including humans obtain energy Plants make their own food through a chemical process known as as photosynthesis Some food energy is used right away while some is stored for later use The process used to create energy from food is another essential characteristic of living things It s called respiration In most cases respiration involves the exchange of oxygen and carbon dioxide a process that releases energy Humans and land animals sale oxygen and exhale carbon dioxide from the air using lungs Fish and many other are aquatic animals don't have lungs but are able to absorb oxygen and expel carbon dioxide through structures called gills Plants absorb and push out gases through stomata or pores in their leaves Living things grow meaning new cells are added on to or replace old cells that make up an organism s body Examples of growth include legs or stems getting longer teeth sprouting through the gums of youngsters and new layers of skin or wood that cover and heal wounds In a few special cases plants
