Cellular Respiration and Metabolism of Carbohydrates

Cellular Respiration and Metabolism of Carbohydrates

Cellular respiration is a process that releases energy from organic compounds. This process requires three types of reactions: glycolysis , the citric acidcycle, and the electron transport chain. In cellu-lar respiration, glucose and oxygen are needed. The products of these reactions include carbon dioxide, water, and energy. Therefore, in the cellular respiration, the presence­ of oxygen is vital to produce a ­significant amount of energy. During cellular respiration,­ food fuels (­primarily glucose) are bro-ken down in the cells. Glucose enters tissue cells via facilitated diffusion, which is largely enhanced by insulin. It is then immediately phosphorylated to glucose-6-phosphate by transfer of a phosphate groupto its sixth carbon as part of a coupled reaction with ATP. Glucose is ­basically trapped inside the cells because they have a lack of enzymes required to reverse this reaction. Intracellular glucose levels are kept low, which maintains a concentration gradient for glucose entry. The only cells that can reverse this phosphorylation reaction are intestinal mucosacells, kidney tubule cells, and liver cells. For carbo-hydrates, catabolism and anabolism both begin with glucose-6-phosphate.

In cellular respiration, some of the released energy is captured for the formation of ATP, which links catabolic reactions to the work of the cells. Enzymes shift the high-energy phosphate groups of ATP to other molecules, which are then termed phosphorylated. This process causes mol-ecules to increase activity, have movement, or ­perform work. Important steps in metabolic path-ways are catalyzed when phosphorylation activates regulatory enzymes.

Two mechanisms by which the cells capture part of the energy that is released during cellu-lar respiration to manufacture ATP molecules are substrate-level phosphorylation and oxidative phos-phorylation. When high-energy phosphate groupsare directly transferred from phosphorylated sub-strates to ADP, substrate-level phosphorylation occurs. Phosphorylated substrates are identified as metabolic intermediates, of which glyceraldehyde 3-phosphate is an example. This process basically occurs because of high-energy bonds attaching phosphate groups to the substrates. They are highly unstable, even more so than the bonds of ATP. During glycolysis, ATP is synthesized twice by this process and once during each phase of the Krebscycle. The enzymes that catalyze these processes arefound in the cytosol and in the watery matrix of the mitochondria.

The second mechanism is known as oxidative phosphorylation, which also releases most of the energy captured in ATP bonding during cellular res-piration. It occurs because of electron transport pro-teins that form portions of the inner mitochondrial membranes. It is a chemiosmotic process, which cou-ples chemical reactions with substance movements across membranes. Some energy released when food fuels are oxidized helps to pump protons across inner mitochondrial membranes into the intermembrane space. A steep concentration gradient for protons is, therefore, created across the membrane. When hydro-gen ions flow back across it, through the membrane channel protein ATP synthase, a portion of this gra-dient energy is captured and aids in the attachment of phosphate groups to ADP.

Glycolysis

Glycolysis is a process that involves a series of enzy-matically catalyzed reactions in which glucose is broken down to yield lactic acid or pyruvic acid. The breakdown releases energy as ATP (FIGURE 4-4). The six- carbon sugar glucose is broken down in the ­cytosol; it becomes two three -carbon pyruvic acid molecules, gaining two ATP and releasing high-­ energy electrons. Remember that glucose is the most important fuel molecule in the oxidative pathways, which produce ATP.

Glycolysis, also referred to as the glycolytic path-way, begins the process of cellular respiration. Itoccurs in the cytosol (the liquid portion of the cyto-plasm) and has 10 chemical steps. Except for the first step (when glucose that enters the cell is phosphory-lated to glucose-6-phosphate), all steps are reversible. Glycolysis does not require oxygen and is occasionally referred to as the anaerobic phase, because it occurs whether or not oxygen is present. If oxygen is present in the right amounts, pyruvic acid, which is gener-ated by glycolysis, can enter the more energy-efficient pathways of aerobic respiration. These pathways are located in the mitochondria.

The three major phases of glycolysis are sugar­activation, sugar cleavage, and sugar oxidation with ATP formation:

■■ Phase 1 (sugar activation): Glucose is phosphory-lated, converted to fructose-6-phosphate, and phos-phorylated a second time. This process requires two ATP molecules, which are recovered later, and releases fructose-1,6-bisphosphate. The two reac-tions provide activation energy, which is required for the later pathway stages. Therefore, Phase 1 may be described as the energy investment phase

 ■■ Phase 2 (sugar cleavage): Fructose-1, 6-­bisphosphate is cleaved into two three-­carbon fragments,­ which are reversible isomers of dihydroxyacetone phosphate or glyceraldehyde 3-phosphate.

■■ Phase 3 (sugar oxidation with ATP formation): This phase is separated into six steps, during which two major events occur. The first event is the oxidization of the two three-carbon frag-ments (via hydrogen removal), which are picked up by NAD⁺. Some of the energy from glucose is transferred to NAD⁺. The second event is the attachment of inorganic phosphate groups (P₁) to each oxidized fragment via high-energy bonds. When the terminal phosphates are even-tually cleaved off, four ATP molecules can be formed from the captured energy (substrate-level phosphorylation).

When glycolysis is complete, there will be two molecules of pyruvic acid and two molecules of reduced NAD⁺, described as NADH + H⁺ . Each glu-cose molecule has a net gain of two ATP molecules. Four ATP molecules are produced, but two of these are consumed in Phase 1, as explained earlier. What will happen to the pyruvic acid is based on the availability of oxygen. Because of limited NAD⁺, glycolysis con-tinues only if the reduced coenzymes (NADH + H⁺) are relieved of extra hydrogen.

When oxygen is not sufficiently present, NADH + H⁺ releases its hydrogen atoms back onto pyruvic acid, which reduces it. Lactic acid is then yielded, some of which diffuses out of cells for transport to the liver (for processing). When oxygen becomes available, lactic acid is oxidized back to pyruvic acid. It enters the aerobic pathways and is completely oxidized to carbon dioxide and water. The aerobic pathways are defined as the Krebs cycle and electron transport chain within the mitochondria.

Aerobic reactions yield as many as 36 ATP mole-cules per glucose molecule. Completely decomposed glucose molecules can produce up to 38 molecules of ATP. Most result from the aerobic phase, with only two resulting from glycolysis. Approximately half of the released energy is used for ATP synthe-sis, whereas the rest becomes heat. The oxidation of glucose also produces carbon dioxide (which is exhaled) and water (which is absorbed into the internal body environment). The volume of water produced by metabolism is lower than the require-ments of the body, so the drinking of water is neces-sary for survival.

1. Describe the role of oxygen in cellular respiration.

2. What are the three major phases of glycolysis?

3. Under what circumstances does a cell yield lactic acid?