7.1: Energy in Living Systems
Energy production within a cell involves many coordinated chemical pathways. Most of these pathways are combinations of oxidation and reduction reactions. Oxidation and reduction occur in tandem. An oxidation reaction strips an electron from an atom in a compound, and the addition of this electron to another compound is a reduction reaction. Because oxidation and reduction usually occur together, these pairs of reactions are called oxidation reduction reactions, or redox reactions.
The energy currency used by cells is ________.
A reducing chemical reaction ________.
- reduces the compound to a simpler form
- adds an electron to the substrate
- removes a hydrogen atom from the substrate
- is a catabolic reaction
Why is it beneficial for cells to use ATP rather than energy directly from the bonds of carbohydrates? What are the greatest drawbacks to harnessing energy directly from the bonds of several different compounds?
ATP provides the cell with a way to handle energy in an efficient manner. The molecule can be charged, stored, and used as needed. Moreover, the energy from hydrolyzing ATP is delivered as a consistent amount. Harvesting energy from the bonds of several different compounds would result in energy deliveries of different quantities.
Glycolysis is the first step in the breakdown of glucose to extract energy for cellular metabolism. Nearly all living organisms carry out glycolysis as part of their metabolism. The process does not use oxygen and is therefore anaerobic. Glycolysis takes place in the cytoplasm of both prokaryotic and eukaryotic cells.
During the second half of glycolysis, what occurs?
- ATP is used up.
- Fructose is split in two.
- ATP is made.
- Glucose becomes fructose.
Nearly all organisms on earth carry out some form of glycolysis. How does that fact support or not support the assertion that glycolysis is one of the oldest metabolic pathways?
If glycolysis evolved relatively late, it likely would not be as universal in organisms as it is. It probably evolved in very primitive organisms and persisted, with the addition of other pathways of carbohydrate metabolism that evolved later.
Red blood cells do not perform aerobic respiration, but they do perform glycolysis. Why do all cells need an energy source, and what would happen if glycolysis were blocked in a red blood cell?
All cells must consume energy to carry out basic functions, such as pumping ions across membranes. A red blood cell would lose its membrane potential if glycolysis were blocked, and it would eventually die.
7.3: Oxidation of Pyruvate and the Citric Acid Cycle
If oxygen is available, aerobic respiration will go forward. In eukaryotic cells, the pyruvate molecules produced at the end of glycolysis are transported into mitochondria, which are the sites of cellular respiration. There, pyruvate will be transformed into an acetyl group that will be picked up and activated by a carrier compound called coenzyme A (CoA). The resulting compound is called acetyl CoA. CoA is made from vitamin B5, pantothenic acid.
What is removed from pyruvate during its conversion into an acetyl group?
- B vitamin
- carbon dioxide
What do the electrons added to NAD+ do?
- They become part of a fermentation pathway.
- They go to another pathway for ATP production.
- They energize the entry of the acetyl group into the citric acid cycle.
- They are converted to NADP.
GTP or ATP is produced during the conversion of ________.
- isocitrate into α-ketoglutarate
- succinyl CoA into succinate
- fumarate into malate
- malate into oxaloacetate
How many NADH molecules are produced on each turn of the citric acid cycle?
What is the primary difference between a circular pathway and a linear pathway?
In a circular pathway, the final product of the reaction is also the initial reactant. The pathway is self-perpetuating, as long as any of the intermediates of the pathway are supplied. Circular pathways are able to accommodate multiple entry and exit points, thus being particularly well suited for amphibolic pathways. In a linear pathway, one trip through the pathway completes the pathway, and a second trip would be an independent event.
7.4: Oxidative Phosphorylation
You have just read about two pathways in glucose catabolism—glycolysis and the citric acid cycle—that generate ATP. Most of the ATP generated during the aerobic catabolism of glucose, however, is not generated directly from these pathways. Rather, it is derived from a process that begins with moving electrons through a series of electron transporters that undergo redox reactions. This causes hydrogen ions to accumulate within the matrix space.
What compound receives electrons from NADH?
- cytochrome c1
Chemiosmosis involves ________.
- the movement of electrons across the cell membrane
- the movement of hydrogen atoms across a mitochondrial membrane
- the movement of hydrogen ions across a mitochondrial membrane
- the movement of glucose through the cell membrane
How do the roles of ubiquinone and cytochrome c differ from the other components of the electron transport chain?
Q and cytochrome c are transport molecules. Their function does not result directly in ATP synthesis in that they are not pumps. Moreover, Q is the only component of the electron transport chain that is not a protein. Ubiquinone and cytochrome c are small, mobile, electron carriers, whereas the other components of the electron transport chain are large complexes anchored in the inner mitochondrial membrane.
What accounts for the different number of ATP molecules that are formed through cellular respiration?
Few tissues except muscle produce the maximum possible amount of ATP from nutrients. The intermediates are used to produce needed amino acids, fatty acids, cholesterol, and sugars for nucleic acids. When NADH is transported from the cytoplasm to the mitochondria, an active transport mechanism is used, which decreases the amount of ATP that can be made. The electron transport chain differs in composition between species, so different organisms will make different amounts of ATP using their electron transport chains.
7.5: Metabolism without Oxygen
In aerobic respiration, the final electron acceptor is an oxygen molecule, O2. If aerobic respiration occurs, then ATP will be produced using the energy of high-energy electrons carried by NADH or FADH2 to the electron transport chain. If aerobic respiration does not occur, NADH must be reoxidized to NAD+ for reuse as an electron carrier for the glycolytic pathway to continue.
Which of the following fermentation methods can occur in animal skeletal muscles?
- lactic acid fermentation
- alcohol fermentation
- mixed acid fermentation
- propionic fermentation
What is the primary difference between fermentation and anaerobic respiration?
Fermentation uses glycolysis only. Anaerobic respiration uses all three parts of cellular respiration, including the parts in the mitochondria like the citric acid cycle and electron transport; it also uses a different final electron acceptor instead of oxygen gas.
7.6: Connections of Carbohydrate, Protein, and Lipid Metabolic Pathways
All of the catabolic pathways for carbohydrates, proteins, and lipids eventually connect into glycolysis and the citric acid cycle pathways (see Figure 7.6.2). Metabolic pathways should be thought of as porous—that is, substances enter from other pathways, and intermediates leave for other pathways. These pathways are not closed systems. Many of the substrates, intermediates, and products in a particular pathway are reactants in other pathways.
A major connection for sugars in glycolysis is ________.
- dihydroxyacetone phosphate
Beta-oxidation is ________.
- the breakdown of sugars
- the assembly of sugars
- the breakdown of fatty acids
- the removal of amino groups from amino acids
Would you describe metabolic pathways as inherently wasteful or inherently economical, and why?
They are very economical. The substrates, intermediates, and products move between pathways and do so in response to finely tuned feedback inhibition loops that keep metabolism balanced overall. Intermediates in one pathway may occur in another, and they can move from one pathway to another fluidly in response to the needs of the cell.
7.7: Regulation of Cellular Respiration
Cellular respiration must be regulated in order to provide balanced amounts of energy in the form of ATP. The cell also must generate a number of intermediate compounds that are used in the anabolism and catabolism of macromolecules. Without controls, metabolic reactions would quickly come to a stand still as the forward and backward reactions reached a state of equilibrium. Resources would be used inappropriately.
The effect of high levels of ADP is to ________.
- increase the activity of the enzyme
- decrease the activity of the enzyme
- have no effect on the activity of the enzyme
- slow down the pathway
The control of which enzyme exerts the most control on glycolysis?
How does citrate from the citric acid cycle affect glycolysis?
Citrate can inhibit phosphofructokinase by feedback regulation.
Why might negative feedback mechanisms be more common than positive feedback mechanisms in living cells?
Negative feedback mechanisms actually control a process; it can turn it off, whereas positive feedback accelerates the process, allowing the cell no control over it. Negative feedback naturally maintains homeostasis, whereas positive feedback drives the system away from equilibrium.