Honors and AP Biology Homework Chapter 6: The Krebs Cycle
Textbook assignment: Chapter 6: How Cells Harvest Energy: 9-16.
- 6.9 The citric acid cycle (smetimes called the Krebs cycle) is a series of reactions that uses most of the same molecules over and over as "machines", kind of like a factory that receives raw cotton and ships thread, using the same machinery over and over to process the raw cotton.
In this case, the 2-carbon molecule attached to AcetylCoA is taken on by a group of molecules which rearrange it, hooking it to a 4-carbon molecule called oxaloacetate. The 6-carbon result, citrate, loses one carbon and some oxygen as carbon dioxide and becomes a 5-carbon chain, alph-ketoglutarate. This molecule loses a carbon as part of carbon dioxide, and the remaining succinate molecle is rearranged back to oxaloacetate -- read to take on another 2-carbon acetyl Coenzym A complex. But at each step of this reaction path, energy is stored in NADH and ATP as the higher-energy electrons in the original 2-carbon chain are siphoned of their energy. At the end, we have oxaloacetate again, two carbon dioxide molecules, some ATP, and some high-energy electrons stored in NADH and FADH2 that can now be harvested in the final process: oxidative phosphorylation.
- 6.10 There are two stages to oxidative phosphorylation. In the electron transport chain sequence of reactions, individual electrons pass through proteins in the membrane of the mitochondrion, and each protein grabs some of the energy and uses it to push H+ ions through the membrane against the concentration gradient. In the chemiosmosis step, the H+ ion "falls" back through a protein gate, ATP synthase, where its energy is stored by bonding phasphate to ADP to make ATP. ATP can move easily around the cell and deliver its stored energy where required to run important metabolic reactions.
- 6.11 SCIENTIFIC THINKING: Brown fat detected in infants has a high mitochondrial count that keeps infants warm, using ion channels that allow H+ ions to cross the concentration gradient releasing heat rather than binding energy in ATP. While brown fat normally disappears as infants grow, PET-CT scans of thinner people and people exposed to colder show the presence of brown fat in the neck and chest of a small percentage of patients, indicating the potential for the body to "activate" brown fat deposits when needed.
- 6.12 This section sums up the preceding sections. Note in particular where ATP is used and where it is produced. If you were confused, go back and review the sections 6.9-6.13 before continuing here.
- 6.13 Now we look at another process of energy release, one which can take place without oxygen (why was oxygen important to cellular respiration?): fermentation. Why do cells need two processes? Not all organisms can do both cellular respiration and fermentation. How does this limit the organism that can only do one of these?
- 6.14 Evolutionists point to the widespread use of glycolysis in most cell forms as evidence that this particular metabolic pathway developed early in the history of life on Earth, and that cellular respiration, which releases O2, is responsible for the Earth's unique atmosphere.
- 6.15 While we focussed on the most common (and best understood) metabolic pathways for harvesting energy, cellular respiration and fermentation, glucose processing is not the only way cells can harvest chemical energy from nutrients. Study Figure 6.15: how does the body break down different fuels to usable components?
- 6.16 Producing energy is only one way a cell can use the nutrients it takes in. The energy produced and stored as ATP is used to create many other molecules required for cell structures and functions.
Read the following weblecture before chat: Cellular Respiration II
This weblecture takes you back to John Kyrk's site to walk through the glycolysis, Krebs, and chemiosmosis cycles -- so plan ahead to spend some time on it!
Take notes on any questions you have, and be prepared to discuss the lecture in chat.
Perform the study activity below:
Use the BioCoach activity Cellular Respiration. Focus on Concepts 3: The Krebs Cycle, Concept 4 Oxidative Phosphorylation, and Concept 5 Fermentation. How are glucose and energy related? (Concept 6)
Optional Web site: The Cellular Respiration page from a University of California biology course has great diagrams of the molecules involved in the steps.
- Required: Complete the Mastery Exercise with a score of 85% or better.
- Optional: Test yourself with the textbook multiple choice questions and note any that you miss that still don't make sense. Bring questions to chat!
- Go to the Moodle and take the quiz for this chapter.
Read through the lab for this week; bring questions to chat on any aspect of the lab, whether you intend not perform it or not. If you decide to perform the lab, be sure to submit your report by the posted due date.
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