CELLULAR RESPIRATION INCLUDES THREE MAIN PROCESSES
The chemical reaction that generates ATP is straightforward: an enzyme tacks a phosphate group onto ADP, yielding ATP. ATP synthesis requires an input of energy. The metabolic pathways of respiration harvest potential energy from food molecules and use it to make ATP.
Like photosynthesis, respiration is an oxidation–reduction reaction.
The pathways of aerobic respiration oxidize (remove electrons from) glucose and reduce (add electrons to) O2. Because of oxygen’s strong attraction for electrons, this reaction is “easy,” like riding a bike downhill. It therefore releases energy, which the cell traps in the bonds of ATP.
This oxidation-reduction reaction does not happen all at once.
If a cell released all the potential energy in glucose’s chemical bonds in one uncontrolled step, the sudden release of heat would destroy the cell; in effect, it would act like a tiny bomb. Rather, the chemical bonds and atoms in glucose are rearranged one step at a time, releasing a tiny bit of energy with each transformation.
According to the second law of thermodynamics, some of this energy is lost as heat.
But much of it is stored in the chemical bonds of ATP.
Biologists organize the intricate biochemical pathways of respiration into three main groups.
glycolysis, the Krebs cycle, and electron transport (figure 6.2).
In glycolysis (literally, “breaking sugar”), a six-carbon glucose molecule splits into two three-carbon pyruvate molecules.
This process harvests energy in two forms. First, some of the electrons from glucose are transferred to an electron carrier molecule called NADH (nicotine adenine dinucleotide). Second, glycolysis generates two molecules of ATP.
Additional reactions, including the Krebs cycle, oxidize the pyruvate and release CO2.
Enzymes rearrange atoms and bonds in ways that transfer the pyruvate’s potential energy and electrons to ATP, NADH, and another electron carrier molecule—FADH2 (flavin adenine dinucleotide).
By the time the Krebs cycle is complete, the carbon atoms that made up the glucose are gone.
liberated as CO2. The cell has generated a few molecules of ATP, but most of the potential energy from glucose now lingers in the high-energy electron carriers, NADH and FADH2. The cell uses them to generate more ATP.
The electron transport chain transfers energy-rich electrons from NADH and FADH2 through a series of membrane proteins.
As electrons pass from carrier to carrier in the electron transport chain, the energy is used to create a gradient of hydrogen ions.
A hydrogen ion is simply a hydrogen atom stripped of its electron.
leaving just a proton. The mitochondrion uses the potential energy stored in this proton gradient to generate ATP.
An enzyme called ATP synthase forms a channel in the membrane.
releasing the protons and using their potential energy to add phosphate to ADP. In the meantime, the “spent” electrons are transferred to O2, generating water as a waste product.
A common misconception is that any ATP-generating pathway in a cell is considered ‘respiration.’
In fact, however, all forms of respiration, aerobic and anaerobic, require an electron transport chain. Fermentation is not respiration because it generates ATP from glycolysis only.
Glycolysis splits one six-carbon glucose into two three-carbon pyruvate molecules. (T or F)
TRUE
ATP is synthesized when a phosphate group is removed from ADP. (T or F)
False (ATP is made by adding a phosphate to ADP)
Cellular respiration is an oxidation-reduction reaction. (T or F)
True
Oxygen acts as the final electron acceptor in aerobic respiration. (T or F)
True
All forms of ATP generation in a cell are considered respiration. (T or F)
False (Fermentation is not respiration)
The electron transport chain directly produces most of the ATP in cellular respiration. (T or F)
True
Glycolysis produces more ATP than the Krebs cycle. (T or F)
False (Krebs cycle contributes more high-energy electron carriers)
Heat energy is completely conserved in cellular respiration. (T or F)
False (some energy is lost as heat)
FADH2 is an electron carrier used in respiration. (T or F)
True
Heat is partly lost in cellular respiration. (T or F)
True
Fermentation requires an electron transport chain. (T or F)
False
Which molecule is split during glycolysis?
A) Pyruvate
B) Glucose
C) ATP
D) NADH
B) Glucose
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THREE MAIN PROCESSES80
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Cells use Energy to use Energy10
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MITOCHONDRIA PRODUCE MOST ATP111
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GLYCOLYSIS BREAKS DOWN GLUCOSE TO PYRUVATE50
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AEROBIC RESPIRATION YIELDS MUCH MORE ATP50
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HOW MANY ATPs CAN ONE GLUCOSE MOLECULE YIELD50
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OTHER FOOD MOLECULES ENTER THE ENERGY-EXTRACTING PATH55
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SOME ENERGY PATHWAYS DO NOT REQUIRE OXYGEN12
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PHOTOSYNTHESIS AND RESPIRATION ARE ANCIENT PATHWAYS50
