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Chapter 12 – Cellular Energetics
Chapter 12 – Cellular Energetics
12.1 First Step of Harvesting Energy from Glucose:
Glycolysis
12.2 The Structure and Functions of Mitochondria
12.3 The Citric Acid Cycle and Fatty Acid Oxidation
12.4 The Electron-Transport Chain and Generation of
the Proton-Motive Force
12.5 Harnessing the Proton-Motive Force to Synthesize
ATP
12.6 Photosynthesis and Light-Absorbing Pigments
12.7 Molecular Analysis of Photosystems
12.8 CO2 Metabolism During Photosynthesis
Cellular Energetics
12.1 First Step of Harvesting Energy from Glucose:
Glycolysis
• Aerobic oxidation – Cells use a four-stage process to
convert energy released by the of glucose/fatty acid
oxidation into ATP terminal phosphoanhydride bond.
• Glycolysis – Stage 1: Cytosolic enzymes convert glucose
to two molecules of pyruvate and generate two
molecules each of NADH and ATP.
• In the absence of oxygen (anaerobic conditions), cells
can metabolize pyruvate to lactic acid or ethanol and
CO2 to convert NADH back to NAD+ required for
glycolysis.
Cellular Energetics
12.2 The Structure and Functions of Mitochondria
• Endosymbiont Hypothesis – Mitochondria and
chloroplasts evolved from bacteria that formed a
symbiotic relationship with ancestral cells containing
a eukaryotic nucleus.
• Mitochondria have two distinct membranes (outer
and inner) and two distinct subcompartments.
• Mitochondria use aerobic oxidation of carboncontaining molecules to generate ATP.
• Mitochondria undergo fusion and fission under
regulation by the state of the cell.
Cellular Energetics
12.3 The Citric Acid Cycle and Fatty Acid Oxidation
• In glucose oxidation stage II, the three-carbon
pyruvate molecule is first oxidized to generate one
molecule each of CO2, NADH, and acetyl CoA, which is
oxidized to CO2 by the citric acid cycle.
• Most of the energy released in glucose oxidation
stages I and II is temporarily stored in reduced NADH
and FADH2, which carry high-energy electrons that
subsequently drive electron-transport chain (stage III).
• Oxidation of short- to long-chain fatty acids occurs in
mitochondria with production of ATP; oxidation of very
long chain fatty acids occurs primarily in peroxisomes
and produces heat not ATP.
Cellular Energetics
12.4 The Electron-Transport Chain and Generation of
the Proton-Motive Force
• Stage III flow of electrons from NADH/FADH2 through
the electron transport chain complexes provides
energy to drive H+ transport across the inner
mitochondrial membrane generating a protonmotive force (voltage and pH gradients).
• Reduction potentials of the electron carriers favor
unidirectional, “downhill,” electron flow from NADH
and FADH2 to O2 to form H2O.
Cellular Energetics
12.5 Harnessing the Proton-Motive Force to Synthesize
ATP
• Chemiosmotic hypothesis – The proton-motive force
across the inner mitochondrial membrane is the
immediate source of energy for ATP synthesis.
• Bacteria, mitochondria, and chloroplasts all use the
same chemiosmotic mechanism and a similar ATP
synthase to generate ATP.
• ATP synthase (F0F1 complex) catalyzes ATP synthesis
as protons flow through the inner mitochondrial
membrane down their electrochemical proton
gradient and rotate the F1 γ subunit.
Cellular Energetics
12.6 Photosynthesis and Light-Absorbing Pigments
• Plant photosynthesis principal end products are O2
and polymers of six-carbon sugars (starch and
sucrose).
• Light-capturing and ATP-generating photosynthesis
reactions occur in chloroplast thylakoid membranes.
• Four photosynthesis stages: (1) light absorption,
generation of high-energy electrons, and O2
formation from H2O; (2) electron transport leading to
reduction of NADP+ to NADPH and pmf generation;
(3) synthesis of ATP; and (4) conversion of CO2 into
carbohydrates (carbon fixation).
Cellular Energetics
12.7 Molecular Analysis of Photosystems
• In the single photosystems, cyclic electron flow from
light-excited special-pair chlorophyll a molecules
generates a proton-motive force used mainly to
power ATP synthesis by the F0F1 complex in the
plasma membrane.
• The two plant photosystems PSI and PSII have
different functions: PSII converts H2O into O2, and PSI
reduces NADP+ to NADPH.
• Light energy absorbed by chloroplast light-harvesting
complexes (LHCs) is transferred to chlorophyll a
molecules in the PSI and PSII reaction centers.
Cellular Energetics
12.8 CO2 Metabolism During Photosynthesis
• Calvin cycle fixes CO2 into organic molecules in a
series of reactions that occur in the chloroplast
stroma.
• In C3 plants, a substantial fraction of the CO2 fixed by
the Calvin cycle can be lost during photorespiration,
which is favored at low CO2 and high O2 levels.
• C4 plants fix CO2 in outer mesophyll cells into fourcarbon molecules that are shuttled to the interior
bundle sheath cells for use in the Calvin cycle,
decreasing the loss in respiration.