Respiration in the cells

Plant cells are alive, of course, so they use up food for energy. The green plants can provide you with some of the energy you need to live. First you must get this energy to your cells where it can be used. But what happens to food inside the cell? It is broken down, or changed. Respiration – the step in metabolism in which food is broken down - takes place in the mitochondria.

In respiration, food is broken down to give off energy. But the food materials are not broken down all at once. Instead, they are broken down in a series of small steps. Energy is given off with each step (Chart 2.1).

Some of the food materials used by the cell are carbohydrates. Carbohydrates, such as sugars and starches, made up of large molecules, must be broken down into smaller molecules in the mitochondria.

Breaking down molecules means dividing them into smaller, simpler molecules. The smaller, simpler molecules are molecules of materials that are different from the original materials. Energy is given off each time the molecules are divided. Finally, the remaining molecules cannot be divided any more.

In general, the cells in living things break down food molecules to form molecules of water and molecules of a gas called carbon dioxide. The energy given off in breaking down food is used for the life function of the cells.

Respiration occurs during both light and dark hours. On a warm sunny day, plants produce much more food and oxygen through photosynthesis than they can use. At night, plants use up some of this excess food for respiration.

Due to the types of respiration, all the organisms are divided in for:

Aerobic (air + bios – life): requiring free oxygen for respiration.

Anaerobic: applied to the cells (mostly bacterial) that can live without free oxygen; obligate anaerobes cannot live in the presence of oxygen; facultative anaerobes can live with or without oxygen.

Chart 2.1

Comparison of photosynthesis and respiration.

Glycolysis

The first step in cellular respiration is a process of glycolysis. In glycolysis, a glucose molecule is broken in half to form two three- carbon molecules of the substance called pyruvic acid.

Glycolysis takes place in the cytoplasm of the cell as well as in the mitochondria. The process involves a series of 9 enzyme-controlled reactions. Two reactions early in the process are endergonic; each requires the input of one molecule of ATP. The later reactions release enough energy to combine 4 molecules of ATP. Thus, glycolysis produces an overall gain of two molecules of ATP for each molecule of glucose.

In addition to producing two three- carbon molecules of pyruvic acid and two ATPs, glycolysis also releases 4 hydrogen atoms. These hydrogens then combine with a hydrogen acceptor, as in the light reactions of photosynthesis. The following equation (2.2) summarizes the overall reaction:

C₆H₁₂O₆ è 2C₂H₃OCOOH + 4H (2.2)

glucose pyruvic acid

Glycolysis is an anaerobic process—that is, no oxygen is required for the process to take place. Glycolysis is followed by one of two processes. If oxygen is present in the cell, the pyruvic acid is broken down further through the process of aerobic respiration. This process results in an additional gain of ATP molecules. Without oxygen, the pyruvic acid cannot be used to release more energy. However, the hydrogen acceptor molecules must release hydrogen atoms, othervise glycolysis could not continue. Hydrogen atoms are removed through fermentation, an anaerobic process that breaks down pyruvic acid into ethyl alcohol or lactic acid. Together, the processes of glycolysis and fermentation make up anaerobic respiration.

Fermentation

Fermentation occurs in some of the less complex organisms, such as some bacteria and yeasts. Most microorganisms convert pyruvic acid to ethyl alcohol, which some animal cells and other microorganisms convert into lactic acid.

Alcoholic fermentation combines the hydrogen and the pyruvic acid formed during glycolysis to produce ethyl alcohol.

Carbon dioxide is also given off as a byproduct. The general equation (2.3) of the process can be written:

2C₂H₃OCOOH + 4H è 2C₂H₅OH + 2CO₂ (2.3)

pyruvic acid ethyl alcohol

Most of the energy originally stored in the glucose remains in the bonds of the ethyl alcohol molecule. For this reason, alcohol is a good fuel.

Lactic acid fermentation combines pyruvic acid and hydrogen from glycolysis to form lactic acid. The general equation for this process can be written (4.4):

2C₂H₃OCOOH + 4H è 2CH₃CHOHCOOH (2.4)

pyruvic acid lactic acid

Lactic acid fermentation occurs in animal muscle cells. When oxygen is available, these cells carry on aerobic respiration. During strenuous exercise, oxygen concentration diminishes, and muscle cells are forced to use lactic acid fermentation. This process uses the hydrogen stored by the hydrogen acceptor molecule and makes the acceptor molecule available for reuse in glycolysis to obtain more energy. The accumulation of lactic acid in the cells is one of the causes of muscle soreness. When the oxygen concentration returns to normal, the lactic acid is converted back to pyruvic acid, aerobic respiration begins, and the soreness fades.

Aerobic Respiration

Like fermentation, aerobic respiration begins with the pyruvic acid produced through glycolysis. Although glycolysis takes place in the cytoplasm of the cell, aerobic respiration takes place on the folded membranes inside the mitochondria. Associated with these folded membranes are all the enzymes and coenzymes needed in the reactions that make up the process of aerobic respiration.

The equation for aerobic respiration (2.5) is essentially the reverse of that for photosynthesis:

enzymes

C₆H₁₂O₆ + 6O₂ è 6CO₂ + 6H₂O + energy (2.5)

Glucose

Aerobic respiration results in a maximum energy gain of 38 molecules of ATP from each molecule of glucose—that is, 36 ATPs are produced in the mitochondria in addition to the 2 gained from cytoplasmic glycolysis. Some cells, because of a high metabolic rate, must produce a greater amount of energy. Most species of living things carry on aerobic respiration because of its high energy yields. The highly active and complex organisms that inhabit the earth today could not have evolved without the energy provided by aerobic respiration.

The complete breakdown of one molecule of glucose results in a maximum of 38 molecules of ATP.

The same process of respiration that occurs in humans also occurs in the cells of all living things. Cabbage, mushrooms, and bacteria all carry out respiration.

The metabolism of sugars is important not only in making alcoholic beverages but in providing the energy that organisms store in ATP—the energy you use all the time to fuel both conscious actions such as turning the pages of this book and automatic ones such as the beating of the heart.

Energy and Electrons from Glucose

We are all familiar with fuels and their uses. Petroleum fuels contain stored energy that is harvested to move cars and heat homes. Wood burning in a stove or campfire releases energy as light and heat. Living organisms also need fuels, which must be obtained from foods. This is true whether we are speaking of organisms that make their own foods through photosynthesis or organisms that obtain foods by eating other organisms. The most common fuel for living cells is the sugar glucose (C6H12O6). Many other compounds serve as foods but almost all of them are converted to glucose or to intermediate compounds in the step-by-step metabolism of glucose. As you will see in this section, cells obtain energy from glucose by the chemical process of oxidation which is carried out through a series of metabolic pathways. Before we examine that process, let’s take a brief look at how metabolic pathways operate in the cell. Several principles govern metabolic pathways:

Complex chemical transformations in the cell do not occur in a single reaction, but in a number of separate reactions that form a metabolic pathway.

Each reaction in a pathway is catalyzed by a specific enzyme.

Metabolic pathways are similar in all organisms, from bacteria to humans.

Many metabolic pathways are compartmentalized in eukaryotes, with certain reactions occurring inside an organelle.

The operation of each metabolic pathway can be regulated by the activities of key enzymes.

Cells trap free energy while metabolizing glucose

The familiar process of combustion (burning) is very similar to the chemical processes that release energy in cells. If glucose is burned in a flame, it reacts with O₂, rapidly forming carbon dioxide and water and releasing a lot of energy. The balanced equation for this combustion reaction is C₆H₁₂O₆ + 6 O₂ ®6 CO₂ + 6 H₂O + energy (heat and light).

The same equation applies to the metabolism of glucose in cells. The metabolism of glucose, however, is a multistep, controlled series of reactions. The multiple steps of the process permit about one-third of the energy released to be captured in ATP. That ATP can be used to do cellular work such as movement or active transport across a membrane, just as energy captured from combustion can be used to do work. The change in free energy (∆G) for the complete conversion of glucose and O₂ to CO₂ and water, whether by combustion or by metabolism, is –686 kcal/mol (–2,870 kJ/mol). Thus the overall reaction is highly exergonic and can drive the endergonic formation of a great deal of ATP from ADP and phosphate. It is the capture of this energy in ATP that requires the many steps characteristic of glucose metabolism. Three metabolic processes play roles in the utilization of glucose for energy: glycolysis, cellular respiration, and fermentation. All three involve metabolic pathways made up of many distinct chemical reactions.

Glycolysis begins glucose metabolism in all cells and produces two molecules of the three-carbon product pyruvate. Asmall amount of the energy stored in glucose is captured in usable forms. Glycolysis does not use O₂.

Cellular respiration uses O₂ from the environment and completely converts each pyruvate molecule to three molecules of CO₂ through a set of metabolic pathways. In the process, a great deal of the energy stored in the covalent bonds of pyruvate is released and transferred to ADP and phosphate to form ATP.

Fermentation does not involve O₂. Fermentation converts pyruvate into products such as lactic acid or ethyl alcohol (ethanol), which are still relatively energy-rich molecules. Because the breakdown of glucose is incomplete, much less energy is released by fermentation than by cellular respiration, and no ATP is produced. Glycolysis and fermentation are anaerobic metabolic processes — that is, they do not involve O₂. Cellular respiration is an aerobic metabolic process, requiring the direct participation of O₂.

Redox reactions transfer electrons and energy

In preceding section described the addition of phosphate groups to ADP to make ATP as an endergonic reaction that can extract and store energy from exergonic reactions. Another way of transferring energy is to transfer electrons. A reaction in which one substance transfers one or more electrons to another substance is called an oxidation–reduction reaction or redox reaction.

Reduction is the gain of one or more electrons by an atom, ion or molecule.

Oxidation is the loss of one or more electrons.

Although oxidation and reduction are always defined in terms of traffic in electrons, we may also think in these terms when hydrogen atoms (not hydrogen ions) are gained or lost because transfers of hydrogen atoms involve transfers of electrons (H = H+ + e–). Thus, when a molecule loses hydrogen atoms, it becomes oxidized. Oxidation and reduction always occur together: As one material is oxidized, the electrons it loses are transferred to another material, reducing that material. In a redox reaction, we call the reactant that becomes reduced an oxidizing agent and the one that becomes oxidized a reducing agent. In both the combustion and the metabolism of glucose, glucose is the reducing agent and oxygen gas is the oxidizing agent. In a redox reaction, energy is transferred. Much of the energy originally present in the reducing agent becomes associated with the reduced product. (The rest remains in the reducing agent or is lost.) As we will see, some of the key reactions of glycolysis and cellular respiration are highly exergonic redox reactions.

The coenzyme NAD is a key electron carrier in redox reactions

In preceding Chapter, we described the role of coenzymes, small molecules that assist in enzyme-catalyzed reactions.

ADP acts as a coenzyme when it picks up energy released in an exergonic reaction and uses it to make ATP (an endergonic reaction). In a similar fashion, the coenzyme NAD (nicotinamide adenine dinucleotide) acts as an energy carrier, in this case in redox reactions.

NAD exists in two chemically distinct forms, one oxidized (NAD+) and the other reduced (NADH + H+). Both forms participate in biological redox reactions. The reduction reaction NAD+ + 2 H ®NADH + H+ is formally equivalent to the transfer of two hydrogen atoms (2 H+ + 2 e–). However, what is actually transferred is a hydride ion (H–, a proton and two electrons), leaving a free proton (H+). This notation emphasizes that reduction is accomplished by the addition of electrons. Oxygen is highly electronegative and readily accepts electrons from NADH. The oxidation of NADH + H+ by O₂, NADH + H+ + 1⁄2 O₂ ®NAD+ + H₂O is highly exergonic, with a ∆G = –52.4 kcal/mol (–219 kJ/mol). Note that the oxidizing agent appears here as “1⁄2 O₂” instead of “O.” This notation emphasizes that it is oxygen gas, O₂, that acts as the oxidizing agent. Just as ATP can be thought of as packaging free energy in bundles of about 12 kcal/mol (50 kJ/mol), NAD can be thought of as packaging free energy in bundles of approximately 50 kcal/mol (200 kJ/mol). NAD is the most common, but not the only, electron carrier in cells. As you will see, another carrier, FAD (flavin adenine dinucleotide), is also involved in transferring electrons during glucose metabolism.

Date: 2014-12-22; view: 1158