Chemicals for Metabolism Help (page 3)
Introduction Chemicals for Metabolism
Energy is the ability to do work. All cells derive energy from their metabolism. They use this energy to carry out their required processes of life.
Different Types Of Energy
Technically speaking, several types of energy are usually identified. Potential energy is generally defined as energy that is locked-up or stored. This stored energy has the “potential” (possibility) to do work, providing that it is unlocked or released. Chemical bonds, for instance, contain potential energy. When these bonds are broken (as by the action of an enzyme), their stored (potential) energy is released. This released energy is frequently called free energy or kinetic (kih- NET -ik) energy, because it is “free” to help particles “move” (kinet), thereby doing some kind of work. One type of work done within cells is synthesis, making new molecules or organelles.
Free Energy, Work, And The ATP-ADP Cycle
An important example of stored or potential energy is found within the bonds of the ATP, or adenosine (ah- DEN -oh-seen) triphosphate (try- FAHS -fate), molecule. The ATP (adenosine triphosphate) molecule is often symbolized as: A-P~P~P . The letter, A, of course, is an abbreviation for adenosine. Each letter P denotes phosphate ( FAHS -fate), a phosphorus-containing chemical group. Within ATP, there are “three” (tri-) phosphate groups attached to the adenosine. The last two chemical bonds in the ATP are special high-energy bonds. This is indicated using the squiggly line, ~, before each of the last two phosphate groups (~ P ~ P ).
Many cells contain a special type of enzyme called ATPase (ATP-ace), literally meaning “ATP splitter” (-ase). ATPase enzyme, therefore, acts to split the second high-energy phosphate bond within the ATP molecule. When this bond is split, large amounts of previously stored potential energy is converted into free (kinetic) energy and is used to do work, such as synthesizing large proteins or other macromolecules. After losing the end phosphate group, ATP or A-P~P~P, becomes ADP or adenosine diphosphate ( die-FAHS -fate): A-P~P . ADP, therefore, is a reduced version of ATP that contains “two” (di-) phosphate groups, rather than three.
When a person eats, say, a candybar or other foodstuff containing a high number of carbohydrate molecules (such as glucose), the individual carbohydrate molecules are eventually broken down. The potential energy stored in their chemical bonds is released. The free energy released from breakdown of food molecules is often used to re-attach the end phosphate group back onto ADP, thereby recreating more ATP. The food that most organisms either produce or eat, therefore, is eventually used to make more ATP. The cells of the organism then turn to the ATP, breaking it back down into ADP, such that more free energy is released to do the body’s work.
The back-and-forth process between ATP and ADP can technically be called the ATP–ADP cycle. Whenever the cell is deficient in free energy, ATP is broken down into ADP. The released energy fills the gap and does cell work. Whenever the cell has an excess of free energy (such as after an organism eats a heavy meal), however, the opposite half of the cycle takes place. The excess free energy becomes stored as another high-energy phosphate group bond, converting ADP back into ATP. This ATP–ADP cycle is a continual process that goes around and around, for as long as the cell lives.
Anabolism Versus Catabolism: “building-up” Versus “breaking Down”
The ATP-ADP cycle is an important part of the two primary processes of metabolism – anabolism (ah- NAB -oh-lizm) on one side of metabolism, and catabolism (kah- TAB -oh-lizm) on the opposite side. The distinction between these two opposite faces of metabolism is made clear by examining Figure 4.5. Anabolism is literally a “condition of” (-ism) “building up” (anabol), while catabolism, the exact opposite, is a “condition of casting [breaking] down.”
During catabolism (Figure 4.5, A), larger, more complex molecules of eaten food are broken down into simpler, smaller ones, usually resulting in the release of free (kinetic) energy. This free energy is then used to create ATP back out of ADP. And because more complex, larger substances are broken down into simpler, smaller ones, there is an increase in the amount of Biological Disorder present.
During anabolism (Figure 4.5, B), the opposite process occurs. Smaller, simpler particles are brought together by consuming free energy released from ATP. New chemical bonds are formed, and larger, more complex molecules are synthesized. Such anabolic (an-ah- BAHL -ik) processes often result in the creation of new organelles or tissue structures, as well. Anabolism also plays an important role in tissue growth and repair functions. Due to the building up of larger, more complex body structures, anabolism is associated with an increase in the amount of Biological Order and pattern present.
Chlorophyll And Photosynthesis In Plants
One critically important example of anabolism is the photosynthesis that occurs within plants. Plants are classified as autotrophs ( AW -tuh- trohfs ) – organisms that are “self” (auto-) “nourishing” (troph). Most plants are autotrophic or self-nourishing because they contain large numbers of chloroplasts ( KLOR -uh-plasts). Chloroplasts are “pale green” (chlor) organelles “formed” ( plast ) within plant cells. Their green color is due, of course, to the presence of thousands of chlorophyll molecules. (Recall that chlorophyll literally means “green leaf.”) The large amount of chlorophyll within leaves allows plants to produce most of their own free energy by themselves, using photosynthesis. In brief,
Photosynthesis or “synthesis” using “light” (photo-), involves two linked sets of chemical reactions. The first set is called the light-dependent reactions . The second set, however, does the actual synthesizing of chemicals. This second set is known as the light-independent reactions or Calvin cycle . In brief overview,
In the first set of light-dependent reactions, the chlorophyll absorbs lots of energy striking the plant surface from sunlight. This energy absorption agitates the electrons within the chlorophyll. Some of these energized electrons are transferred, along with an H + ion from water, to a special electron acceptor molecule. Water (H 2 O) molecules are split during this process. Thus, the molecules of oxygen (O 2 ) that remain after H + ion transfer are given off into the air. (This is the main reason why plants are oxygen-producing organisms.) The energy released by the transferred electrons helps add a phosphate group to ADP, creating more ATP.
Some of the produced ATP provides energy to run the second stage of photosynthesis. This second stage is called the Calvin cycle (after the scientist, Melvin Calvin). The Calvin cycle uses the ATP and special electron acceptor molecules that are given off by the first set of reactions.
Light-Independent Reactions - The Calvin Cycle
The Calvin cycle is termed a “cycle” because it begins and ends at exactly the same point – carbon dioxide (CO 2 ) molecules. (This explains the well-known fact that plant cells consume CO 2 .)
The cycle rotates once when it receives an input of one CO 2 molecule. ATP is also split to provide free energy. Several 3-carbon sugar molecules are produced. Some of these enter the cytoplasm ( SIGH -toh-plazm) or fluid “matter” ( plasm ) of the plant “cell” ( cyt ). The 3-carbon sugars entering the cytoplasm are eventually converted into glucose, fats, or amino acids. The cycle has to rotate 6 times, and take in 6 molecules of CO 2 , in order to synthesize one molecule of glucose. Finally, the glucose molecule serves as an important energy source for the plant cell.
The overall equation for photosynthesis is a simple one:
Observe from the equation that photosynthesis releases large numbers of oxygen (O 2 ) molecules. Since ancient times, the green (chlorophyll-containing) organisms of this world have been steadily producing oxygen. Because of its large percentage of O 2 (about 1/5 or 21%), the Earth’s atmosphere has become a true biosphere ( BUY -oh- sfeer ), or “ball of life.” And practically all of the life in the biosphere (other than the green organisms) requires oxygen from photosynthesis for its ultimate survival.
Glycolysis In Heterotrophs
The preceding section discussed photosynthesis as an important example of anabolism or synthesis of glucose that occurred in the world of autotrophs, such as green plants. The glucose did not have to be eaten or ingested by the plants, since the plant cells and their chloroplasts manufactured it ultimately from the energy found in sunlight.
In heterotrophs ( HET -er-oh-trohfs), however, the situation is markedly different. A heterotroph is an organism that is “nourished” ( troph ) by some “other” or “different” ( hetero -) source. Human beings and most animals, for instance, are heterotrophs that eat or consume “other” organic foodstuffs (such as proteins, lipids, or carbohydrates) that come from “different” plants or living creatures. In short, when a human being needs a particular fuel molecule, such as glucose, it generally goes out and eats one as part of its diet! (The cells of heterotrophs such as humans can manufacture glucose or other fuel molecules by anabolism, but ultimately, they must still obtain some foodstuffs from other living, organic sources.)
The focus of this section, then, is not the anabolism or synthesis of glucose. Rather, the emphasis is upon glycolysis (gleye- KAHL -ih-sis) — the catabolism or “process of breaking down” ( lysis ) of “sweets” ( glyc ), such as glucose. Once glucose is present within the cytoplasm of heterotroph cells, its chemical bonds can readily be broken down by glycolysis with the help of enzymes, releasing lots of free energy to make ATP. During glycolysis, one 6-carbon glucose molecule is broken down into two 3-carbon pyruvic (pie- ROO -vik) acid molecules. Overall, as shown in the figure, 2 ATP molecules are produced.
In anaerobic ( an -er- OH -bik) catabolism of a glucose molecule, this 6-carbon compound is broken down “without” ( an ) any oxygen from the “air” ( aer ) being present. During such conditions, the simple process of glucose decomposing into 2 pyruvic acid molecules (that is, glycolysis) is quickly followed by the conversion of the pyruvic acids into lactic ( LAK -tik) acid molecules, as shown in the figure overleaf.
Lactic acid is named for its occurrence in sour “milk” ( lact ). But it is also produced by muscle and other body cells during anaerobic catabolism of glucose. When a human being runs very rapidly, for instance, the fibers (cells) in the leg muscles cannot get oxygen fast enough to allow them to utilize it for metabolism. Hence, glucose is broken down by anaerobic catabolism. Excess lactic acid molecules quickly accumulate, and they irritate local nerve endings within the muscle fibers. This creates leg cramping and pain.
Aerobic Respiration In Heterotrophs
When enough O 2 is present within the cell, however, glycolysis is followed by respiration (res-pir- AY -shun). Respiration literally translates from Latin as the “process of breathing” (spir), which is appropriate when it applies to the entire human or animal body. In the biology of individual heterotroph cells, however, respiration has a far different meaning. It is cellular respiration, not the process of breathing, itself, that is directly tied to cell metabolism. Cellular respiration is the breakdown or catabolism of organic molecules (such as carbohydrates, lipids, or proteins) within the cell, and in the presence of oxygen. Cellular respiration is therefore often called aerobic (air- OH -bik) respiration . This is because the process involves the breakdown of organic molecules within cells when oxygen from the “air” ( aero -) is present.
Cellular respiration (aerobic respiration within cells) most often involves the catabolism of glucose or other simple sugars. Cellular respiration, then, starts up where the process of glycolysis stops. Glycolysis always provides the cell with a net of 2 ATP molecules/1 glucose molecule catabolized. Aerobic respiration provides the cell with many, many more.
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