In most forms of carbohydrate respiration, the first major phase takes place in the cytoplasm and requires no oxygen gas (O2). This phase, called glycolysis, involves three main steps and several smaller ones, each controlled by an enzyme. During the process, a small amount of energy is released, and some hydrogen atoms are removed from compounds derived from a glucose molecule. The essence of this complex series of steps is as follows:
1. In a series of reactions, the glucose molecule becomes a fructose molecule carrying two phosphates (P).
2. This sugar (fructose) molecule is split into two 3-carbon fragments called glyceraldehyde 3-phosphate (GA3P).
3. Some hydrogen, energy, and water are removed from these 3-carbon fragments, leaving pyruvic acid (Fig. 10.14).
Two ATP molecules supply the energy needed to start the process of glycolysis. By the time pyruvic acid has been formed, however, four ATP molecules have been produced from the energy released along the way, for a net gain of two ATP molecules. A great deal of the energy originally in the glucose molecule remains in the pyruvic acid. The hydrogen ions and high-energy electrons released during the process
Anaerobic respiration and fermentation are two forms of respiration that were probably carried on in the geological past when there was no oxygen in the atmosphere. These
6. A Kcal is the energy required to raise the temperature of one kilogram of water from 14.5°C to 15.5°C.
Plant Metabolism 187
glucose mitochondrion mitochondrion
(hydrogen acceptor) NAD NADH
to citric acid cycle (aerobic respiration) or fermentation pyruvic acid A NAD
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.