Carol Void, age 14,of Bridgeport, Conn., for her question:
WHAT IS LIFE?
Basic activities of life include growth, nutrition, reproduction, responsiveness and movement. These activities are performed by all living things at one time or another. No nonliving thing can perform them all. Growth means more than an increase in size. An organism may grow by increasing the number of its cells or by increasing the size of its cells.
Life has no simple definition. It is one of the great mysteries of the universe. Since the dawn of time, philosophers have been pondering the meaning of life. It is easy to recognize most things as either living or nonliving. You and the members of your family are living. The rug on your living room floor is not.
Scientists have learned how organisms grow, what conditions they need to live, how they respond to their surroundings and how they reproduce. They have classed living things as either plants or animals. But still they cannot say exactly what life itself actually is.
Living things are made up of the same basic elements as nonliving things. The most common elements found in living things are carbon, hydrogen, oxygen, nitrogen, sulfur, iron, phosphorus, potassium, calcium and magnesium. Lots of these same elements are found in nonliving things.
But elements found in living things usually have combined to form compounds. Water, a compound of hydrogen and oxygen, makes up between 65 and 90 percent of the weight of most plants and animals.
Life can exist only under certain conditions. All living things need certain chemical elements and compounds for nutrition and cellular respiration.
Most living things are capable of some kind of movement. You can run and walk. Fish can swim and birds can fly. Plants may seem to be motionless but their growth is actually a slow kind of motion. The protoplasm within living cells is always moving. This movement gets its energy from inside the organism.
Nonliving things may move too, but a stone rolling down a hill and water falling over rocks are examples of movement because of some outside force, such as gravity.
Protoplasm is the substance that makes up all living things. It is a complex mixture of many different materials, including proteins, water, fats, carbohydrates and certain minerals. Many characteristics of life result from the properties of protoplasm.
The cell is the basic unit of life. Protoplasm is organized to form cells and when the organization breaks down, the protoplasm is no longer a living substance.