The lipids connected to the glucose pathway include cholesterol and triglycerides. Cholesterol is a lipid that contributes to cell membrane flexibility and is a precursor of steroid hormones. The synthesis of cholesterol starts with acetyl groups and proceeds in only one direction. The process cannot be reversed.
Triglycerides—made from the bonding of glycerol and three fatty acids—are a form of long-term energy storage in animals. Animals can make most of the fatty acids they need. Triglycerides can be both made and broken down through parts of the glucose catabolism pathways. Glycerol can be phosphorylated to glycerol-3-phosphate, which continues through glycolysis. Fatty acids are catabolized in a process called beta-oxidation, which takes place in the matrix of the mitochondria and converts their fatty acid chains into two-carbon units of acetyl groups. The acetyl groups are picked up by CoA to form acetyl CoA that proceeds into the citric acid cycle.
Evolution Connection
Pathways of Photosynthesis and Cellular MetabolismThe processes of photosynthesis and cellular metabolism consist of several very complex pathways. It is generally thought that the first cells arose in an aqueous environment—a “soup” of nutrients—possibly on the surface of some porous clays, perhaps in warm marine environments. If these cells reproduced successfully and their numbers climbed steadily, it follows that the cells would begin to deplete the nutrients from the medium in which they lived as they shifted the nutrients into the components of their own bodies. This hypothetical situation would have resulted in natural selection favoring those organisms that could exist by using the nutrients that remained in their environment and by manipulating these nutrients into materials upon which they could survive. Selection would favor those organisms that could extract maximal value from the nutrients to which they had access.
An early form of photosynthesis developed that harnessed the sun’s energy using water as a source of hydrogen atoms, but this pathway did not produce free oxygen (anoxygenic photosynthesis). (Another type of anoxygenic photosynthesis did not produce free oxygen because it did not use water as the source of hydrogen ions; instead, it used materials such as hydrogen sulfide and consequently produced sulfur). It is thought that glycolysis developed at this time and could take advantage of the simple sugars being produced but that these reactions were unable to fully extract the energy stored in the carbohydrates. The development of glycolysis probably predated the evolution of photosynthesis, as it was well suited to extract energy from materials spontaneously accumulating in the “primeval soup.” A later form of photosynthesis used water as a source of electrons and hydrogen and generated free oxygen. Over time, the atmosphere became oxygenated, but not before the oxygen released oxidized metals in the ocean and created a “rust” layer in the sediment, permitting the dating of the rise of the first oxygenic photosynthesizers. Living things adapted to exploit this new atmosphere that allowed aerobic respiration as we know it to evolve. When the full process of oxygenic photosynthesis developed and the atmosphere became oxygenated, cells were finally able to use the oxygen expelled by photosynthesis to extract considerably more energy from the sugar molecules using the citric acid cycle and oxidative phosphorylation.
25. Apply your understanding of how living
organisms use energy to argue in favor of why it is either beneficial or detrimental for cells to use ATP rather than directly using the energy stored in the bonds of carbohydrates to power cellular reactions. ATP is readily available in the form of a single unit that provides a consistent, appropriate amount of energy. If cells harvested energy from various carbohydrate
compounds, they would need to tailor each reaction to each energy source. ATP energy cannot activate the ROS dependent stress response whereas food molecules are responsible for activating ROS. ATP is low in energy, but food molecules (in the form of carbohydrates) possess higher levels of energy that cells can use. ATP is readily available to cells, unlike the carbohydrate compounds that have to first be phosphorylated in order to release their energy.
26.
What role does \text{NAD}^{+} {\!} play in redox reactions?
\text{NAD}^{+} {\!}, an oxidizing agent, can accept electrons and protons from organic molecules and get reduced to \text{NADH}.
\text{NAD}^{+} {\!}, a reducing agent, can donate its electrons and protons to organic molecules.
\text{NAD}^{+} {\!}, an oxidizing agent, can accept electrons from organic molecules and get reduced to NADH2.
\text{NAD}^{+} {\!}, a reducing agent, can donate its electrons and protons to inorganic molecules.
27.
Which statement best explains how electrons are transferred and the role of each species. Remember that R represents a hydrocarbon molecule and RH represents the same molecule with a particular hydrogen identified.
\text{RH} + \text{NAD}^{+} {\!} \rightarrow \text{NADH} + \text{R}
\text{RH} acts as a reducing agent and donates its electrons to the oxidizing agent \text{NAD}^{+} {\!}, forming \text{NADH} and \text{R}.
\text{NAD}^{+} {\!}, the oxidizing agent, donates its electrons to the reducing agent \text{RH}, forming \text{R} and \text{NADH}.
\text{RH} acts as an oxidizing agent and donates electrons to the reducing agent \text{NAD}^{+} {\!}, producing \text{NADH} and \text{R}.
\text{NAD}^{+} {\!}, the reducing agent, accepts electrons from the oxidizing agent \text{RH}, producing \text{NADH} and \text{R}.
28.
Nearly all organisms on Earth carry out some form of glycolysis. Provide an accurate argument that explains how this fact either supports or refutes the assertion that glycolysis is one of the oldest metabolic pathways.
The presence of glycolysis in nearly all organisms indicates that it is an advanced and recently evolved pathway that has been widely used due to the benefits it provides.
Glycolysis is absent in a few higher organisms, which contradicts the assertion that it is one of the oldest metabolic pathways.
Glycolysis is present in some organisms and absent in others. This inconsistency fails to support the assertion that it is one of the oldest metabolic pathways.
To be present in so many different organisms, glycolysis was probably present in a common ancestor rather than evolved many separate times.
29.
Red blood cells (RBCs) do not perform aerobic respiration, but they do perform glycolysis. Provide the reasoning necessary to explain why all cells need an energy source, and predict what would happen if glycolysis were blocked in a sample of RBCs.
Cells need energy to perform cell division. Blocking glycolysis in RBCs interrupts the process of mitosis, leading to nondisjunction.
Cells require energy to perform certain basic functions. Blocking glycolysis in RBCs causes imbalance in the membrane potential, leading to cell death.
Cells maintain the influx and efflux of organic substances using energy. Blocking glycolysis stops the binding of CO2 to the RBCs, causing cell death.
Cells require energy to recognize attacking pathogens. Blocking glycolysis inhibits the process of that recognition, causing invasion of the RBCs by a pathogen.
30.
What is the primary difference between a circular pathway and a linear pathway?
The reactant and the product are the same in a circular pathway but different in a linear pathway.
The circular pathway components get exhausted whereas those of the linear pathway do not and are continually regenerated.
Circular pathways are not suited for amphibolic pathways whereas linear pathways are.
Circular pathways contain a single chemical reaction that is repeated while linear pathways have multiple events.
31.
Cellular respiration breaks down glucose and releases carbon dioxide and water. Which steps in the oxidation of pyruvate produces carbon dioxide?
Removal of a carboxyl group from pyruvate releases carbon dioxide. The pyruvate dehydrogenase complex comes into play.
Removal of an acetyl group from pyruvate releases carbon dioxide. The pyruvate decarboxylase complex comes into play.
Removal of a carbonyl group from pyruvate releases carbon dioxide. The pyruvate dehydrogenase complex comes into play.
Removal of coenzyme A from pyruvate releases carbon dioxide. The pyruvate dehydrogenase complex comes into play.
32.
What three steps are included in the breakdown of pyruvate?
Pyruvate dehydrogenase removes a carboxyl group from pyruvate, producing carbon dioxide. Dihydrolipoyl transacetylase oxidizes a hydroxyethyl group to an acetyl group, producing NADH. Lastly, an enzyme-bound acetyl group is transferred to CoA, producing a molecule of acetyl-CoA.
Pyruvate dehydrogenase oxidizes hydroxyethyl group to an acetyl group, producing NADH. It further removes a carboxyl group from pyruvate, producing carbon dioxide. Lastly, dihydrolipoyl transacetylase transfers enzyme-bound acetyl group to CoA, forming an acetyl-CoA molecule.
Pyruvate dehydrogenase transfers enzyme-bound acetyl group to CoA, forming an acetyl CoA molecule. It then oxidizes a hydroxyethyl group to an acetyl group, producing NADH. Dihydrolipoyl transacetylase removes a carboxyl group from pyruvate, producing carbon dioxide.
Pyruvate dehydrogenase removes carboxyl group from pyruvate, producing carbon dioxide. Dihydrolipoyl dehydrogenase transfers enzyme-bound acetyl groups to CoA, forming an acetyl-CoA molecule. Lastly, a hydroxyethyl group is oxidized to an acetyl group, producing NADH.
33.
Apply your understanding of the various components of the electron transport chain to evaluate how the roles of ubiquinone and cytochrome c differ from those of the other components.
CoQ and cytochrome c covalently bind electrons, while NADH dehydrogenase and succinate dehydrogenase are bound to the inner mitochondrial membrane.
CoQ and cytochrome c are bound to the inner mitochondrial membrane, while NADH dehydrogenase and succinate dehydrogenase are mobile electron carriers.
CoQ and cytochrome c covalently bind electrons, while NADH dehydrogenase and succinate dehydrogenase are mobile electron carriers.
CoQ and cytochrome c are mobile electron carriers, while NADH dehydrogenase and succinate dehydrogenase are bound to the inner mitochondrial membrane.
34.
Consider that the number of ATP molecules formed through cellular respiration varies. Identify the claim that accounts for these differences.
The ATPs produced are immediately utilized in the anaplerotic reactions that are used for the replenishment of the intermediates.
Most of the ATPs produced are rapidly used for the phosphorylation of certain compounds found in plants.
Transport of NADH from cytosol to mitochondria is an active process that decreases the number of ATPs produced.
A large number of ATP molecules are used in the detoxification of xenobiotic compounds produced during cellular respiration.
35.
Which of the following best describes complex IV in the electron transport chain?
Complex IV consists of an oxygen molecule held between the cytochrome and copper ions. The electrons flowing finally reach the oxygen, producing water.
Complex IV contains a molecule of flavin mononucleotide and iron-sulfur clusters. The electrons from NADH are transported here to coenzyme Q.
Complex IV contains cytochrome b, c, and Fe-S. Here, the proton motive Q cycle takes place.
Complex IV contains a membrane-bound enzyme that accepts electrons from FADH2 to make FAD. This electron is then transferred to ubiquinone.
36.
.
Review the process of fermentation, as illustrated here by lactic acid fermentation. Which of the following statements best compares fermentation and anaerobic respiration and accurately highlights their differences?
Fermentation uses glycolysis, the citric acid cycle, and the ETC but finally gives electrons to an inorganic molecule, whereas anaerobic respiration sues only glycolysis and its electron acceptor is an organic molecule.
Fermentation uses only glycolysis and its final electron acceptor is an organic molecule, whereas anaerobic respiration uses glycolysis, the citric acid cycle, and the ETC but finally gives electrons to an inorganic molecule other than O2.
Fermentation uses glycolysis, the citric acid cycle, and the ETC but finally gives electrons to an organic molecule, whereas anaerobic respiration uses only glycolysis and its final electron acceptor is an inorganic molecule.
Fermentation uses glycolysis and its final electron acceptor is an inorganic molecule, whereas anaerobic respiration uses glycolysis, the citric acid cycle, and the ETC but finally gives electrons to an organic molecule.
37.
What type of cellular respiration is represented in the following equation, and why?
\text{CO}_2 + \text{H}_2 + \text{NADH} \rightarrow \text{CH}_4 + \text{H}_2\text{O} + \text{NAD}^+
Anaerobic respiration, because the final electron acceptor is inorganic.
Aerobic respiration, because oxygen is the final electron acceptor.
Anaerobic respiration, because NADH donates its electrons to a methane molecule.
Aerobic respiration, because water is being produced as a product.
38.
Would you describe metabolic pathways as inherently wasteful or inherently economical? Justify your answer by explaining why you chose it.
Metabolic pathways are wasteful, as they perform uncoordinated catabolic and anabolic reactions that waste some of the energy that is stored.
Metabolic pathways are economical due to the presence of anaplerotic reactions that replenish the intermediates.
Metabolic pathways are economical due to feedback inhibition. Also, intermediates from one pathway can be utilized by other pathways.
Metabolic pathways are wasteful, as most of the energy produced is utilized in maintaining the reduced environment of the cytosol.
39.
Make a claim to identify the lipids that are connected to glucose catabolism pathways, and support your claim with evidence of how the lipids are connected to those pathways.
Glucagon and glycogen can be converted to 3-phosphoglyceraldehyde that is an intermediate of glycolysis.
Chylomicrons and fatty acids get converted to 1,3-bisphosphoglycerate that continues in glycolysis, forming pyruvate.
Sphingolipids and triglycerides form glucagon that can be fed into glycolysis.
Cholesterol and triglycerides can be converted to glycerol-3-phosphate that continues through glycolysis.
40.
Examine and compare the statements below, each of which proposes the specific mechanism by which citrate from the citric acid cycle affects glycolysis. Which statement offers an accurate description of this effect?
Citrate and ATP are negative regulators of hexokinase.
Citrate and ATP are negative regulators of phosphofructokinase-1.
Citrate and ATP are positive regulators of phosphofructokinase-1.
Citrate and ATP are positive regulators of hexokinase.
41.
Compare the two types of feedback mechanisms, as pictured. Make a claim to explain why negative feedback mechanisms might be more common than positive feedback mechanisms in living cells.
Negative feedback mechanisms maintain homeostasis, whereas positive feedback drives the system away from equilibrium.
Positive feedback mechanisms maintain a balanced amount of substances, whereas negative feedback restricts their accumulation.
Negative feedback mechanisms turn the system off, making it deficient of certain undesired substances. Positive feedback balances out these deficits.
Positive feedback mechanisms bring substance amounts back to equilibrium regardless of environmental input, while negative feedback produces excess amounts of substance.