Properties of Life

In its broadest sense, biology is the study of living things the science of life. Living things come in an astounding variety of shapes and forms, and biologists study life in many different ways. What makes something “alive”? Anyone could deduce that a galloping horse is alive and a car is not, but why? We cannot say, “If it moves, it’s alive,” because a car can move, and gelatin can wiggle in a bowl. They certainly are not alive. What characteristics do define life? All living organisms share five basic characteristics:

1. Order. All organisms consist of one or more cells with highly ordered structures: atoms make up molecules, which construct cellular organelles, which are contained within cells. This hierarchical organization continues at higher levels in multicellular organisms and among organisms.
2. Sensitivity. All organisms respond to stimuli. Plants grow toward a source of light, and your pupils dilate when you walk into a dark room. 
3. Growth, development, and reproduction. All organisms are capable of growing and reproducing, and they all possess hereditary molecules that are passed to their offspring, ensuring that the offspring are of the same species. Although crystals also “grow,” their growth does not involve hereditary molecules.
4. Regulation. All organisms have regulatory mechanisms that coordinate the organism’s internal functions. These functions include supplying cells with nutrients, transporting substances through the organism,and many others.
5. Homeostasis. All organisms maintain relatively constant internal conditions, different from their environment, a process called homeostasis.