iving things, is that all living things need to grow and develop. When an organism is growing, most go through a cycle called development. The single cell that starts the cell divides over and over again to make all the cells that the organism has when in adulthood. As the cycle continues the organism ages. Aging is when the organism becomes less efficient in the process of life. The organism will not be able to reproduce, and death comes as finally too. The fourth characteristic of a living thing is the ability to obtain and use energy. Living things obtain energy from their environment or their surroundings. All living things require energy to live and build their cells. This process is anabolism. 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. Both are almost equal in mass, 1 amu (atomic mass unit). Another subatomic particle in the atom is the electron. It is negatively charged and it\'s mass is about 2000 times smaller than that of a neutron or electron. Usually the number or electron in an atom is the same as the number of protons. So, usually the atoms have not a positive nor negative charge, but they are neutral. Electron are not in the nucleus like the protons and neutrons are. They travel at very high speeds throughout the atom in energy levels. The energy levels are like orbits that surround the nucleus. The number of protons that are in the nucleus of an atom is called the atomic number. The atomic number identifies the atom because no two atoms have the same number of protons in there nucleus. For example, hydrogen has the atomic number of 1, that means all atoms that have one proton in its nucleus is hydrogen. The atomic mass number is the number of neutrons and protons in the nucleus. To find the number on neutrons in an atom, you must round the atomic mass number to the nearest whole number and then subtract the atomic number. Remember the atomic number is the same as the number of protons in the atom. To find out the number or electrons an atom contains, you just need to know the atomic number because there is the same number of prot

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