2.1: Hierarchy of the Body

The importance of understanding body materials and cells becomes evident when one realizes that the human body has a hierarchy of structure and functioning, with these materials and cells at its foundation. This chapter will take a broad view of this hierarchy and then examine major body substances and cells in greater detail.

Atoms, Ions, and Molecules

Like all physical objects, the body is made of matter composed of units called atoms. Each atom has at its center a nucleus that contains one or more small particles called protons and neutrons. Each atom also has one or more smaller particles called electrons, which move about in regions some distance away from the nucleus (Figure 2.1).

Figure 2.1 The structure of atoms (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission. )

An atom is the smallest unit of an element that has the properties of that element. For example, all the carbon atoms that make up a lump of coal can burn and produce carbon dioxide. Most types of atoms lack a complete set of electrons in their outer regions. Therefore, these atoms may react chemically with one another in ways that produce complete electrons sets. Atoms that engage in one type of chemical reaction may lose or gain electrons and become ions. (Figure 2.2); atoms engaged in other chemical reactions end up sharing electrons with neighboring atoms (Figure 2.3). Ions attract each other because they have opposite charges and atoms that share electrons may be bonded together in specific ratios, thus forming groups called molecules.

Figure 2.2 A chemical reaction with giving and taking of electrons, forming a molecule with ions. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission. )

Figure 2.3 A chemical reaction with sharing of electrons, forming a molecule with ions. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission. )

Furthermore, fragments of some molecules may lose or gain electrons. Such molecular fragments are also called ions (Figure 2.4).

Figure 2.4 Forming ions from molecules and vice-versa (reversible processes). (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission. )

Each type of molecule has its own properties, which may be very different from those of the atoms composing it. For example, hydrogen atoms form a gas that can burn explosively and oxygen atoms support combustion, but when hydrogen atoms are chemically bonded to oxygen atoms in a 2:1 ratio, they form water molecules, which can extinguish fires. Molecules also undergo chemical reactions, which may produce still other molecules. For example, during digestion, water molecules may react with starch molecules and produce sugar molecules.

Besides producing new substances, each chemical reaction either releases or absorbs energy. The energy given off by chemical reactions can be used to power other activities, including other chemical reactions that absorb energy. For example, the energy given off by burning fuel can be used to power an automobile. The energy in foods, measured as calories, is given off when the foods are metabolized. This energy powers all body functions and keeps us warm.

Organelles and Cells

Many atoms, ions, and molecules in the body are arranged in combinations that form structures of various shapes, such as sheets, granules, and tubes. These structures make up components called organelles (e.g., cell membrane, ribosomes, microtubules) (Figure 2.5) . Organelles in turn are in highly complicated and organized structures called cells. The organelles are like the parts of an automobile in that each organelle can carry out only a few specialized functions. However, like the parts of an automobile that is being driven correctly, a complete and properly assembled set of organelles that has proper guidance can operate on its own.

Figure 2.5 The hierarchy of body structure (Images used with permission. Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission. )

Cells are the smallest units of the body that can survive on their own under favorable conditions (i.e., homeostasis) and have all the characteristics of life. These characteristics include organization, constant chemical activity, external or internal movement, an active response to stimulation, and reproduction. By possessing and carrying out the characteristics of life, cells and the substances they produce constitute all the larger components in the body and perform all body functions. Though cells in the body have many features in common, they are also specialized in a variety of ways and therefore can perform different functions. For example, muscle cells can contract to provide gross movements, and bone cells can secrete materials that make hard and rigid bones that provide support and protection.

Tissues, Organs, Systems, Organism

Cells of the same type plus some materials they secrete are found together in organized groups called tissues (Figure 2.5) . Different tissues are organized in groups called organs, organs are organized in groups called systems, and systems are found in an organized group called the organism, a human body. Just as each cell performs certain functions, so also does each tissue, organ, and system. When these functions are combined and coordinated, homeostasis can be maintained and the individual can survive. Combining and coordinating male and female functions can result in reproduction, which maintains the survival of the human species. There are additional hierarchical levels beyond the body (e.g., family, community, nation) which are studied in disciplines other than biology (e.g., sociology and political science).