Chapter Six: How Cells Harvest Chemical Energy

Questions:

1. What are the three different catagories of cellular poisons that affect cellular respiration?
2. What are the two common types of fermentation? Why are they important?
3. What are obligate anaerobes?

Answers:

1. The first category blocks the electron transport chain.  Examples of these are rotenone, cyanide and carbon monoxide.  The second inhibits ATP synthase (oligomycin).  The third makes the membrane leaky to hydrogen ions (dinitrophenal).
2. The two common types of fermentation are alcohol fermentation and lactic acid fermentation.  Fermentation is important because it allows a cell to continue to produce ATP without the use of oxygen, that is, under anaerobic conditions.
3. Obligate Anaerobes are prokaryotes that live in stagnant ponds or deep in the soil because they require anaerobic conditions and are poisoned by oxygen.

Important Facts:

1. Cellular respiration equation shows the changes in hydrogen atom distribution.  Glucose loses its hydrogen atoms and is converted to CO2.  As this happens, O2 gains two hydrogen atoms, while losing an oxygen atom.  This means that this reaction is a redox reaction.
2. Glycolysis begins respiration by breaking glucose, a six carbon molecule, into two molecules of a three-carbon compound called pyruvate.  This stage occurs in the cytoplasm.  The second stage is the Citric Acid cycle or Krebs cycle.  It breaks down pyruvate into carbon dioxide and supplies the next stage with electrons.  This stage occurs in the mitochondria.  During Oxidative phosphorylation, electrons are shuttled through the electron transport chain.  It also produces ATP with chemiosmosis.  This stage occurs in the membrane of the mitochondria.
3. Catabolic pathways occur when molecules are broken down and their energy is released.  Two types of catabolism are fermentation (the partial degradation of sugars that occurs without the use of oxygen) and cellular respiration.
4. Hydrogen ions flow back down their gradient through a channel in the transmembrane protein known as ATP synthase.  ATP synthase harnesses the proton motive force (the gradient of hydrogen ions) to phosphorylate ADP, forming ATP.  The proton motive force is in place because the inner membrane of the mitochondria is impermeable to hydrogen ions.  Electrons are held behind the inner membrane with their only exit the ATP synthase.
5. In alcohol fermentation, pyruvate is converted to ethanol, releasing CO2 and oxidizing NADH in the process to create more NAD+.

Key Terms:

Cellular Respiration- the aerobic harvesting of energy from sugar molecules by cells.

Oxidative Phosphorylation- Step 3 of cellular respiration.  Involves the electron transport chain and chemiosmosis.  NADH and FADH2 shuttle electrons to the electron transport chain imbedded in the inner mitochondrion membrane.  This is where most of the ATP for a cell is produced.  The energy released by the downhill fall of electrons from NADH and FADH2 to O2 is used to phosphorylate ADP.

ATP Synthases- protein complexes built into the inner membrane that synthesize ATP. 

Dehydrogenase- the enzyme that removes hydrogen from an organic molecule.  Requires a coenzyme, NAD+.

Intermediates- Compounds that form between the initial reactant and the final product.

Glycolysis- the multistep chemical breakdown of a molecule of glucose into two molecules of pyruvate.

Chemiosmosis- The energy coupling mechanism.  The production of ATP using the energy of hydrogen ion gradients across membranes to phosphorylate ADP.

Substrate Level Phosphorylation- This is a form of ATP synthesis that occurs when when an enzyme transfers a phosphate group from a substrate molecule to ADP.

Redox Reaction- The movement of electrons from one molecule to another.  Oxidation reduction reaction.

Lactic Acid Fermentation- A process  by which muscle cells, some other cells, and certain bacteria generate NAD+.  NADH is oxidized to NAD+ as pyruvate is reduced to lactate.  

Diagram: This shows cellular respiration which is the set of the metabolic reactions and processes that take place in organism's cells to convert energy from nutrients into ATP, and then release waste products. The reactions involved in respiration are catabolic reactions that involve the oxidation of one molecule and the reduction of another. Respiration is one of the key ways a cell gains useful energy to fuel cellular reformations.

Relevant Video:
http://www.youtube.com/watch?v=vlZZUtpyCgQ


Summary: In this chapter, redox reactions, and the stages of fermentation and cellular respiration are covered.  The major steps of each of the processes , as well as the results are focused on.  Glycolysis, the Citric acid cycle, and oxidative phosphorylation are discussed in great detail.