Download Chapter 9 Cellular Respiration: Harvesting Chemical Energy

Chapter 9 Cellular Respiration: Harvesting Chemical Energy
I. Releasing Energy from Organic Fuels
- Living cells require transfusions of energy from outside sources to perform their many tasks.
- Energy flows into an ecosystem as sunlight and leaves as heat.
- Catabolic pathways yield energy by oxidizing organic fuels.
A. Catabolic Pathways and Production of ATP
- The breakdown of organic molecules is exergonic.
- One catabolic process, fermentation is a partial degradation of sugars that occurs
without oxygen.
- Cellular respiration, the most prevalent and efficient catabolic pathway, consumes
oxygen and organic molecules, such as glucose, and yields ATP.
- In principle, cellular respiration is similar to combustion of gasoline.
Organic Compound + O2
CO2 + H2O + Energy
- To keep working, cells must regenerate ATP.
B. Redox Reactions: Oxidation and Reduction
- Catabolic pathways yield energy due to the transfer of electrons.
1. The Principle of Redox
- Redox reactions transfer electrons from one reactant to another by oxidation and
reduction.
- In oxidation, a substance loses electrons, or is oxidized.
- In reduction, a substance gains electrons, or is reduced.
- Some redox reactions do not completely exchange electrons but rather change the
degree of electron sharing in covalent bonds.
- Potential energy is released when electrons are moved closer to a more
electronegative atom.
2. Oxidation of Organic Fuel Molecules during Cellular Respiration
- During cellular respiration, glucose is oxidized and oxygen is reduced.
- Organic molecules with many hydrogen atoms (carbohydrates and lipids) are excellent
fuels because their bonds are a source of “hilltop” electrons that release energy as
they “fall” down the energy gradient.
3. Stepwise Energy Harvest via NAD+ and the Electron Transport Chain
- Cellular respiration oxidizes glucose in a series of steps.
- Electrons from organic compounds are usually first transferred (by an enzyme called
dehydrogenase) to NAD+, a coenzyme.
- NADH, the reduced form of NAD+ passes the electrons to the electron transport chain.
- If electron transfer is not stepwise, a large release of energy occurs, as in the reaction
of hydrogen and oxygen to form water.
- The electron transport chain passes electrons in a series of steps instead of in one
explosive reaction
- uses the energy from the electron transfer to form ATP
C. The Stages of Cellular Respiration
- Respiration is a cumulative function of three metabolic stages: glycolysis, the citric acid,
cycle, and oxidative phosphorylation.
1. Glycolysis breaks down glucose into two molecules of pyruvate.
2. The Citric Acid Cycle completes the breakdown of glucose.
3. Oxidative Phosphorylation produces ATP using energy from electron transport and
chemiosmosis.
- Both glycolysis and the citric acid cycle can generate ATP by substrate-level
phosphorylation.
II. Glycolysis , “Splitting of Sugar”
- occurs in the cytosol of the cell
- does not require oxygen
- splits glucose (6-C) into pyruvate (3-C) to release energy
- produces 2 ATP and 2 NADH
- consists of two major phases:
A. Energy Investment Phase
- consumes 2 ATP; splits glucose
1. Glucose enters cell and is phosphorylated by hexokinase which “traps” glucose within
the cell and makes glucose more reactive.
2. Glucose-6-phosphate is converted to fructose-6-phosphate by phosphoglucoisomerase.
3. Fructose-6-phosphate is phosphorylated by phosphofructokinase preparing it to be split
in half.
4. Fructose-1, 6-bisphosphate split in half by aldolase forming 2 3-carbon isomers,
glyceraldehyde-3-phosphate (which enters the second half of gycolysis) and
dihydroxyacetone phosphate.
5. Dihydroxyacetone is converted into glyceraldehyde-3-phosphate by isomerase and
enters the second half of gycolysis.
B. Energy Payoff Phase
- generates 4 ATP (net total of 2 ATP) and 2 NADH
- Produces 2 pyruvate and 2 H2O
6. Glyceraldehyde-3-phosphate undergoes 2 reactions each catalyzed by triose phosphate
dehydrogenase: G3P is oxidized forming 2 NADH, and the energy released is used to
attach another phosphate forming 1, 3-Bisphosphate which gives it much potential
energy.
7. 1, 3-Bisphosphate undergoes substrate phosphorylation by phosphoglycerokinase to
produce 2 ATP.
8. Phosphoglyceromutase moves the phosphate in preparation for the next step.
9. A water molecule is removed from 2-phosphoglycerate to produce
phosphoenolpyruvate which is less stable.
10. Phosphoenolpyruvate undergoes substrate phosphorylation by pyruvate kinase to
produce 2 more ATP and 2 pyruvate molecules which then enter the Citric Acid Cycle.
III. The Citric Acid Cycle
- completes the energy-yielding oxidation of organic molecules
- takes place in the matrix of the mitochondrion
- requires oxygen
- produces (for each original glucose) 2ATP, 6NADH, 2 FADH2, and 4 CO2
A. Conversion of Pyruvate to Acetyl CoA
- Before the citric acid cycle can begin, pyruvate must first be converted to acetyl CoA,
which links the cycle to glycolysis.
-
1. As pyruvate enters the mitochondrion, a carboxyl group (-COO ) is removed to
produce a 2-carbon compound and CO2 is released.
2. This 2-carbon compound is oxidized to form acetate and NADH.
3. Coenzyme A, a sulfur-containing compound derived from vitamin B, is attached to
acetate to form acetyl CoA which is very unstable and reactive.
B. Citric Acid Cycle
- aka Krebs (or Tricarboxylic Acid) Cycle
1. The 2-carbon acetyl group from acetyl CoA is joined to oxaloacetate to produce citrate.
2. Citrate converted to isocitrate.
3. Citrate loses a CO2 and is oxidized producing NADH and α-ketogluterate.
4. α-ketogluterate also loses a CO2 and is oxidized producing NADH. Coenzyme A is
added to form an unstable molecule of succinyl CoA.
5. CoA is displaced by a phosphate which is then transferred to GDP, forming GTP, and
then to ADP forming ATP via substrate phosphorylation.
6. Succinate is oxidized (2 hydrogens removed) to form FADH2.
7. H2O is added to fumarate to form malate.
8. Malate is oxidized (hydrogen removed) to form NAPH and oxaloacetate is finally
regenerated completing the cycle.
IV. Oxidative Phosphorylation
- Chemiosmosis couples electron transport to ATP synthesis.
- NADH and FADH2 release electrons to the electron transport chain, which powers ATP
synthesis via oxidative phosphorylation.
A. The Pathway of Electron Transport
- The electron transport chain is a series of molecules (mostly proteins) embedded in
the inner membrane of the mitochondrion.
- In the electron transport chain, electrons from NADH and FADH2 lose energy in several
steps and H+ ions are “pumped” into the intermembrane space creating a H+ gradient.
- At the end of the chain, electrons are passed to oxygen, forming water.
B. Chemiosmosis: The Energy-Coupling Mechanism
- Chemiosmosis is an energy-coupling mechanism that uses energy in the form of a H+
gradient across a membrane to drive cellular work.
- At certain steps along the electron transport chain electron transfer causes protein
complexes to pump H+ from the mitochondrial matrix to the intermembrane space.
- The resulting H+ gradient, referred to as a proton-motive force, drives chemiosmosis
in
ATP synthase and stores energy.
- ATP synthase is the enzyme embedded in membrane of the mitochondrion (and
chloroplast) that actually makes ATP by functioning as an ion pump running in reverse.
C. An Accounting of ATP Production by Cellular Respiration
- During respiration, most energy flows in this sequence: glucose to NADH to electron
transport chain to proton-motive force to ATP
- There are three main processes in this metabolic enterprise which drive oxidative
phosphorylation.
- About 40% of the energy in a glucose molecule is transferred to ATP during cellular
respiration, making approximately 38 ATP.
V. Fermentation
- Cellular respiration relies on oxygen to produce ATP.
- In the absence of oxygen cells can still produce ATP through fermentation.
- Fermentation enables some cells to produce ATP without the use of oxygen.
- Glycolysis can produce ATP with or without oxygen, in aerobic or anaerobic conditions.
- Glycolysis couples with fermentation to produce ATP.
A. Types of Fermentation
- Fermentation consists of glycolysis plus reactions that regenerate NAD+, which can be
reused by glycolysis.
1. Alcohol Fermentation
- Pyruvate is converted to ethanol in two steps, one of which releases CO2.
2. Lactic Acid Fermentation
- Pyruvate is reduced directly to NADH to form lactate as a waste product.
B. Fermentation and Cellular Respiration Compared
- Both fermentation and cellular respiration use glycolysis to oxidize glucose and other
organic fuels to pyruvate.
- Fermentation and cellular respiration differ in their final electron acceptor.
- Cellular respiration produces more ATP.
- Pyruvate is a key juncture in catabolism.
C. The Evolutionary Significance of Glycolysis
- Glycolysis, which occurs in nearly all organisms, probably evolved in ancient prokaryotes
before there was oxygen in the atmosphere.
VI. The Versatility of Catabolism
- Glycolysis and the citric acid cycle connect to many other metabolic pathways.
- Catabolic pathways funnel electrons from many kinds of organic molecules into cellular
respiration.
- Disaccharides and polysaccharides can be converted to glucose.
- Proteins must be digested into amino acids which are then deaminated (amino group
removed) and the products sent to either glycolysis or the citric acid cycle.
- Fats are broken down into glycerol and fatty acids. Glycerol is then converted to G3P.
The fatty acids undergo beta oxidation to be broken down into 2-carbon fragments to
enter the citric acid cycle as acetyl CoA.
A. Biosynthesis (Anabolic Pathways)
- The body uses small molecules to build other substances.
- These small molecules may come directly from food or through glycolysis or the citric
acid cycle.
B. Regulation of Cellular Respiration
- Cellular respiration is controlled by allosteric enzymes at key points in glycolysis and the
citric acid cycle.