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3. Exergonic reactions release energy; endergonic reactions utilize energy.

Components of Metabolic Pathways (Table 6.1)

1. A specific enzyme facilitates each step of a metabolic pathway. (Figure 6.6)

2. ATP is the energy currency of the cell.

3. The energy source is oxidized to release its energy; this oxidation-reduction reaction reduces an electron carrier (Figure 6.8)

4. NAD+, NADP+, and FAD are electron carriers. (Table 6.2)

164 Chapter 6 Metabolism: Fueling Cell Growth

5. Precursor metabolites are building blocks that can be used to make the subunits of macromolecules, and they can also be oxidized to generate energy. (Table 6.3)

Scheme of Metabolism (Figure 6.9)

1. The central metabolic pathways are glycolysis, the pentose phosphate pathway, and the tricarboxylic acid cycle (TCA cycle). Because they play a role in both catabolism and anabolism, they are sometimes called amphibolic pathways.

2. Respiration uses the reducing power accumulated in the central metabolic pathways to generate ATP by oxidative phosphorylation. Aerobic respiration uses O2 as a terminal electron acceptor; anaerobic respiration uses an inorganic molecule other than O2 as a terminal electron acceptor.

3. Fermentation uses pyruvate or a derivative as a terminal electron acceptor rather than oxidizing it further in the TCA cycle; this recycles the reduced electron carrier NADH.

6.2 Enzymes

1. Enzymes function as biological catalysts; they are neither consumed nor permanently changed during a reaction.

Mechanisms and Consequences of Enzyme Action (Figure 6.10)

1. The substrate binds to the active site or catalytic site to form a temporary intermediate called an enzyme-substrate complex.

Cofactors and Coenzymes (Figure 6.11, Table 6.5)

1. Enzymes sometimes act in conjunction with cofactors such as coenzymes and trace elements.

Environmental Factors that Influence Enzyme Activity (Figure 6.12)

1. The factors most important in influencing enzyme activities are temperature, pH, and salt concentration.

Allosteric Regulation (Figure 6.13)

1. Cells can fine-tune the activity of an allosteric enzyme by using an effector that binds to the allosteric site of the enzyme. This binding alters the relative affinity of the enzyme for its substrate.

Enzyme Inhibition

1. Non-competitive inhibition occurs when the inhibitor and the substrate act at different sites on the enzyme.

2. Competitive inhibition occurs when the inhibitor competes with the normal substrate for the active binding site. (Figure 6.14)

6.3 The Central Metabolic Pathways (Table 6.7) Glycolysis (Figure 6.15)

1. Glycolysis is a ten-step pathway that converts one molecule of glucose into two molecules of pyruvate; the theoretical net yield is 2 ATP, 2 NADH + H+, and 6 different precursor metabolites.

Ftentose Phosphate Pathway

1. The pentose phosphate pathway forms NADPH and 2 different precursor metabolites.

Transition Step (Figure 6.16)

1. The transition step results in the decarboxylation and oxidation of pyruvate, and joins the resulting acetyl group to coenzyme A, forming acetyl-CoA. This produces NADH + H+ and 1 precursor metabolite.

Tricarboxylic Acid Cycle (Figure 6.16)

1. The eight steps of the TCA cycle complete the oxidation of glucose; the theoretical yield is 6 NADH + 6H+, 2 FADH2, 2 ATP, and 2 different precursor metabolites.

6.4 Respiration

1. The reducing power accumulated in glycolysis, the transition step and the TCA cycle is used to drive the synthesis of ATP.

The Electron Transport Chain—Generating Proton Motive Force

1. The electron transport chain sequentially passes electrons, and, as a result, ejects protons. Most of the carriers are grouped into large complexes that function as proton pumps.

2. The mitochondrial electron transport chain has three different complexes (complexes I, III and IV) that function as proton pumps. (Figure 6.18)

3. Prokaryotes vary with respect to the types and arrangements of their electron transport components. (Figure 6.19)

4. Some prokaryotes can use inorganic molecules other than O2 as a terminal electron acceptor. This process of anaerobic respiration harvests less energy than aerobic respiration.

ATP Synthase—Harvesting the Proton Motive Force to Synthesize ATP

1. ATP synthase permits protons to flow back across the membrane, harvesting energy that is released to fuel the synthesis of ATP.

ATP Yield of Aerobic Respiration in Prokaryotes (Figure 6.21)

1. The theoretical maximum yield of ATP of aerobic respiration is 38 ATP.

6.5 Fermentation

1. In general the only ATP-yielding reactions of fermentations are those of the glycolytic pathway; the other steps provide a mechanism for recycling NADH. (Figure 6.22)

2. Some end products of fermentation are commercially valuable. Lactic acid is important in the production of foods such as cheese and yogurt. Ethanol is used to make alcoholic beverages and breads. (Figure 6.23)

3. Because a given type of organism uses a specific fermentation pathway, the end products can be used as markers that aid in identification.

6.6 Catabolism of Organic Compounds Other than Glucose

1. Hydrolytic enzymes break down macromolecules into their respective subunits.

Polysaccharides and Disaccharides

1. Amylases digest starch, releasing glucose subunits, and are produced by many organisms. Cellulases degrade cellulose.

2. Sugar subunits released when polysaccharides are broken down can then enter glycolysis to be oxidized to pyruvate.

Lipids

1. Fats are hydrolyzed by lipase, releasing glycerol and fatty acids.

2. Glycerol is converted to the precursor metabolite dihydroxyacetone phosphate; fatty acids are degraded by a ^-oxidation, generating reducing power and the precursor metabolite acetyl-CoA.

Proteins

1. Proteins are hydrolyzed by proteases.

2. Deamination removes the amino group; the remaining carbon skeleton is then converted into the appropriate precursor molecule.

6.7 Chemolithotrophs

1. Prokaryotes, as a group, are unique in their ability to use reduced inorganic compounds such as hydrogen sulfide (H2S) and ammonia (NH3) as a source of energy. (Figure 6.25)

2. Chemolithotrophs are autotrophs.

6.8 Photosynthesis

1. The light-dependent reactions of photosynthesis are those used to capture energy from light and convert it to chemical energy in the form of ATP. The light-independent reactions use that energy to synthesize organic carbon compounds.

Capturing Radiant Energy

1. Various pigments including chlorophylls, bacteriochlorophylls, carotenoids, and phycobilins may be used to capture radiant energy.

2. Reaction center pigments function as the electron donor in the photosynthetic process; antennae pigments funnel radiant energy to the reaction center pigment.

Converting Radiant Energy into Chemical Energy

1. The high-energy electrons emitted by reaction center chlorophylls are passed to an electron transport chain, which uses them to generate a proton motive force. The energy of proton motive force is harvested by ATP synthase to fuel the synthesis of ATP. (Figure 6.26).

2. Photosystems I and II of cyanobacteria and chloroplasts raise the energy level of electrons stripped from water to a high enough level to be used to generate a proton motive force and produce reducing power; this process evolves oxygen.

3. Purple and green bacteria employ only a single photosystem; they must obtain electrons from a reduced compound other than water and therefore do not evolve oxygen.

6.9 Carbon Fixation

Calvin Cycle (Figure 6.28)

1. The most common pathway used to incorporate CO2 into an organic form is the Calvin cycle.

6.10 Anabolic Pathways—Synthesizing Subunits from Precursor Molecules (Figure 6.29)

Lipid Synthesis

1. The fatty acid components of fat are synthesized by progressively adding 2-carbon units to an acetyl group. The glycerol component is synthesized from dihydroxyacetone phosphate.

Amino Acid Synthesis

1. Structurally related families of amino acids share common pathways of biosynthesis.

2. Synthesis of glutamate from a-ketoglutarate and ammonia provides a mechanism for cells to incorporate nitrogen into organic molecules. (Figure 6.30)

3. Synthesis of aromatic amino acids requires a multistep branching pathway. Allosteric enzymes regulate key steps of the pathway. (Figure 6.31)

Nucleotide Synthesis

1. Purine nucleotides are synthesized on the sugar-phosphate component; the pyrimidine ring is made first and then attached to the sugar-phosphate. (Figure 6.32)

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