Living things show the characteristics of (1) metabolic processes, (2) generative processes, (3) responsive processes, (4) control processes, and (5) a unique structural organization. The concept of the cell has developed over a number of years. Initially, only two regions, the cytoplasm and the nucleus, could be identified. At present, numerous organelles are recognized as essential components of both prokaryotic and eukaryotic cell types.

In the process of respiration, organisms convert foods into energy (ATP) and waste materials (carbon dioxide and water). Organisms that have oxygen (O₂) available can employ the Krebs cycle and electrontransport system (ETS). Plants use the waste materials of respiration. Therefore, there is a constant cycling of materials between plants and animals. Sunlight supplies the essential initial energy for making the large organic molecules necessary to maintain the forms of life we know.

In the light-capturing reaction stage of photosynthesis, plants use chlorophyll to trap the energy of sunlight. During the light-dependent stage they manufacture a source of chemical energy, ATP, and a source of hydrogen, NADPH. Atmospheric oxygen is released in this stage. In the light-independent reaction stage of photosynthesis, the ATP energy is used in a series of reactions to join the hydrogen from the NADPH to a molecule of carbon dioxide and form a simple carbohydrate. In subsequent reactions, plants use this molecule as a source of energy and raw materials to make complex carbohydrates, fats, and other organic molecules.

The cell is the common unit of life. We study individual cells and their structures to understand how they function as individual living organisms and as parts of many-celled beings. Knowing how prokaryotic and eukaryotic cell types resemble or differ from each other helps physicians control some organisms dangerous to humans.

Cell division is necessary for growth, repair, and reproduction. Cells go through a cell cycle that includes cell division (mitosis and cytokinesis) and interphase. Interphase is the period of growth and preparation for division. Mitosis is divided into four stages: prophase, metaphase, anaphase, and telophase. During mitosis, two daughter nuclei are formed from one parent nucleus. These nuclei have identical sets of chromosomes and genes that are exact copies of those of the parent. Although the process of mitosis has been presented as a series of phases, you should realize that it is a continuous, flowing process from prophase through telophase. Following mitosis, cytokinesis divides the cytoplasm, and the cell returns to interphase.