The Human Body

Cellular Respiration

Cellular Respiration, process in which cells produce the energy they need to survive. In cellular respiration, cells use oxygen to break down the sugar glucose and store its energy in molecules of adenosine triphosphate (ATP). Cellular respiration is critical for the survival of most organisms because the energy in glucose cannot be used by cells until it is stored in ATP. Cells use ATP to power virtually all of their activities—to grow, divide, replace worn out cell parts, and execute many other tasks. Cellular respiration provides the energy required for an amoeba to glide toward food, the Venus fly trap to capture its prey, or the ballet dancer to execute stunning leaps. Cellular respiration occurs within a cell constantly, day and night, and if it ceases, the cell—and ultimately the organism—dies.

Two critical ingredients required for cellular respiration are glucose and oxygen. The glucose used in cellular respiration enters cells in a variety of ways. Plants, algae, and certain bacteria make their own glucose through photosynthesis, the process by which plants use light to convert carbon dioxide and water into sugar. Animals obtain glucose by eating plants, and fungi and bacteria absorb glucose as they break down the tissues of plants and animals. Regardless of how they obtain it, cells must have a steady supply of glucose so that ATP production is continuous.

Oxygen is present in the air, and also is found dissolved in water. It either diffuses into cells—as in bacteria, fungi, plants, and many aquatic animals, such as sponges and fish—or it is inhaled—as in more complex animals, including humans. Cellular respiration sometimes is referred to as aerobic respiration, meaning that it occurs in the presence of oxygen.

Cellular respiration transfers about 40 percent of the energy of glucose to ATP. The rest of the energy from glucose is released as heat, which warm-blooded organisms use to maintain body temperature, and cold-blooded organisms release to the atmosphere. Cellular respiration is strikingly efficient compared to other energy conversion processes, such as the burning of gasoline, in which only about 25 percent of the energy is used and about 75 percent is released as heat.

While most organisms carry out cellular respiration to produce ATP, some cannot produce ATP through this process because they live in anaerobic environments, or environments that lack oxygen. These organisms, typically bacteria, rely on anaerobic processes such as fermentation to generate their ATP.

See also: Converting Food to Usable Energy