---

The first level of structural organization of living things is the chemical level. All matter on Earth living and non-living is composed of chemicals. Inorganic chemistry is the chemistry that largely deals with non-living matter such as minerals and metals whereas organic chemistry is the chemistry of life and the study of the molecules that make up living things. Organic chemistry is sometimes called carbon chemistry because the chemicals found in living things center around the atom carbon. This central carbon atom along with three other atoms actually make up 96 percent of the mass of all living things. These are called the major atoms of life and include oxygen which makes up about 65 percent of the mass of all living things, carbon which accounts for 18.5 percent, hydrogen 9.5 percent and lastly nitrogen, accounting for about three percent of the mass of all living things. All other atoms in living things account for less than four percent of their total mass. These four atoms carbon, hydrogen, oxygen, and nitrogen combine to form molecules. The main molecules found in living things include lipids, otherwise known as fats, carbohydrates, proteins, and nucleic acids. These four classes of molecules lipids, carbs, proteins, and nucleic acids, are used to create the main components of the next level of organization of living things, cells.

---

The second level of structural organization of living things is the cellular level. Cells are the basic units of all living things. All cells come from other cells. Some living organisms are composed of only one cell – things like bacteria and protozoa – while others are composed of many many cells such as animals and plants.

---

The third level of organization of living things is tissues. Multiple cells with similar functions form tissues. There are four tissues in the body. They include:

1. Epithelial Tissue – found on surfaces, lining cavities and in glandular tissue.
2. Connective Tissue – a diverse tissue that includes bone, blood, adipose tissue, ligaments, tendons, cartilage and more.
3. Muscle Tissue – Includes cardiac muscle, smooth muscle and skeletal muscle.
4. Nervous Tissue – tissues found in the central and peripheral nervous system.

---

Collections of two or more tissues join together to form organs, the fourth level of structural organization. Simply put an organ is a collection of tissues joined in a structural unit to serve a common function. Organs in the body include structures such as the heart, kidney, lungs, liver, pancreas and brain, to name a few.

---

Organs work together to form the fifth level of structural organization of living things, organ systems, or more simply, sometimes just called systems. There are 12 systems in the body: muscular system, integumentary system, skeletal system, respiratory system, reproductive system, cardiovascular system, lymphatic system, immune system, urinary system, nervous system, digestive system, and last but not least, the endocrine system.

---

All of these systems, organs, tissues, cells, and chemicals, of the body all contribute to the overall function and structure and the final level of structural organization which is the organism.