Cellular Respiration

I

Introduction

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.

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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.

II

Chemical Reactions and Metabolic Pathways

To understand cellular respiration, it is necessary to understand the nature of chemical reactions. Chemical reactions can occur outside of living organisms—the rusting of a car, for example, is a chemical reaction—or they can occur within organisms, where they are termed biochemical reactions. In a chemical or biochemical reaction, the bonds between atoms that hold molecules together break apart, and the atoms rearrange to form new molecules. Water molecules, for example, are composed of hydrogen and oxygen atoms, and under certain conditions, the bonds between these atoms can break and reform to yield separate molecules of hydrogen and oxygen gas. In living organisms, most biochemical reactions occur with the help of enzymes, specialized proteins designed to carry out specific reactions. All biochemical reactions release energy in the form of heat as they occur.

Cells carry out biochemical reactions to create needed molecules—such as proteins or starch—or to destroy these molecules once they are no longer needed. If certain molecules are built or destroyed in a single biochemical reaction, the reaction may release too much heat, which could incinerate the cell. To control the release of heat, cells build up and break down most molecules in a linked series of small reactions that release only a little bit of heat at a time. The series of linked biochemical reactions is called a metabolic pathway.

Cellular respiration is one of the most important metabolic pathways found in cells. This enzyme-assisted, step-by-step process not only protects the cell from lethal temperature increases but also provides the cell with a mechanism of transferring the energy of glucose to ATP in a controlled manner.

III

How Cellular Respiration Works

The process of cellular respiration occurs in four stages: glycolysis; the transition stage; the Krebs cycle, also known as the citric acid cycle; and the electron transport chain. Each stage accomplishes different tasks.

A

Glycolysis

Glucose is the primary fuel used in glycolysis, the first stage of cellular respiration. This all-important molecule is found in the cell’s cytoplasm, the gel-like substance that fills the cell. Glucose consists of 6 carbon, 12 hydrogen, and 6 oxygen atoms bonded together, along with electrons (negatively charged atomic particles) associated with each atom. Of these components, only the hydrogen atoms and certain electrons participate directly in glycolysis.

In glycolysis, glucose is broken down with the help of enzymes and other molecules found in the cytoplasm. Enzymes first attach two phosphate groups to glucose to make it more reactive. A phosphate group is a cluster of one phosphorus and four oxygen atoms. The addition of the two phosphate groups prepares glucose for the action of another enzyme. This enzyme splits glucose in half to produce two three-carbon molecules, each with one phosphate group attached.

In the next step, an enzyme removes one hydrogen atom and two electrons from each three-carbon molecule. Both hydrogen atoms are modified to hydrogen ions, positively charged particles. A hydrogen ion and two electrons from each three-carbon molecule are transferred as a unit to a large molecule called nicotinamide adenine dinucleotide (NAD⁺⁾ to form two molecules of NADH. The hydrogen ions and electrons stored in each molecule of NADH are used to make ATP in later stages of cellular respiration.

In the final steps of glycolysis, two hydrogen atoms are removed from each three-carbon compound. These hydrogen atoms bond to free-floating oxygen atoms in the cytoplasm to form water. This step prepares the two three-carbon compounds for action by the next enzyme in the pathway. This enzyme removes the phosphate group from each three-carbon compound. Each phosphate group then bonds to a single molecule of adenosine diphosphate (ADP). ADP is composed of three carbon-based rings and a tail of two phosphate groups. The addition of the third phosphate group to the tail forms ATP. In this step, two new ATP molecules are produced. When cells require energy, another enzyme breaks off the third phosphate group, releasing energy that powers the cell. The removal of the third phosphate from ATP converts ATP back to ADP, which is used again in cellular respiration to make more ATP.

When the two three-carbon compounds are separated from the phosphate groups, the three-carbon compounds are converted to two molecules of pyruvate, each composed of three carbon, three oxygen, and three hydrogen atoms. When glycolysis is complete, important products used in the next steps of cellular respiration have been produced: two molecules of NADH and two molecules of pyruvate. The two ATP molecules gained in glycolysis are used for reactions in the cell that require energy.

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