Anabolism is the process in a living thing that involves putting together complex substances from simpler substances. Plants get their energy from the sunlight through a process called photosynthesis. Animals get their energy from food that is eaten. The food is then broken down through digestion, resulting in a release of energy called catabolism. Living things practice anabolism and catabolism through the whole time they are living. The balance of anabolism and catabolism is called metabolism. The fifth and final characteristic that all living things share is that all living things respond to their environment. Response to their environment can be sudden, through behavior, or gradual, in metabolic process or growth. Stimulus is anything in the environment that causes a living thing to react. Stimuli include light, temperature, odor, gravity, sound, water, and pressure. Plants generally act to stimuli slower than animals. The process in which living things respond to stimuli in ways to keep conditions in their body suitable for life is homeostasis. These five characteristics of living things are just the basics to knowing what makes up living things. Atomic Structure of Living Things The basic unit of matter is the atom. Atoms are extremely small, in fact, if you placed 100 million atoms in a row one after the other, it would be one centimeter long. Even though the atom is small it consists of even smaller particles, called subatomic particles. Scientists believe that there is at least 200 subatomic particles. The three main subatomic particles are the neutron, proton, and electron. In the middle of the atom there is a nucleus. The nucleus makes up 99.9 percent of the atoms weight even though it is a hundred times smaller than the atom itself. The nucleus contains two different kind of subatomic particles, the neutron and the proton. The proton has a positive charge and the neutron is a electrically neutral subatomic particle.