Download Metabolism - ZANICHELLI.it

David Sadava, David M. Hillis,
H. Craig Heller, May R. Berenbaum
La nuova
biologia.blu
Le cellule e i viventi
Energy and Metabolism
What Physical Principles Underlie Biological Energy Transformations?
Metabolism: the sum
total of all chemical
reactions occurring in
a biological system at
a given time.
Metabolic reactions
involve energy
changes.
How Does Glucose Oxidation Release Chemical Energy?
Fuels: molecules whose stored energy
can be released for use.
In cells, energy from fuel molecules is
used to make ATP.
Glucose is the most common fuel in
cells.
C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + free energy
How Does Glucose Oxidation Release Chemical Energy?
Oxidation–Reduction (Redox) reactions: one
substance transfers electrons to another substance.
Reduction: gain of one or more electrons by an atom,
ion, or molecule.
Oxidation: loss of one or more electrons.
How Does Glucose Oxidation Release Chemical Energy?
Coenzyme NAD+ is a key electron carrier in redox
reactions.
How Does Glucose Oxidation Release Chemical Energy?
Three catabolic
processes harvest
the energy from
glucose.
Where do Energy Pathways Work?
What Are the Aerobic Pathways of Glucose Metabolism?
Glycolysis
• Takes place in the cytosol
• Converts glucose into 2 molecules of pyruvate
• Produces 2 ATP and 2 NADH
• Occurs in 10 steps.
Glucose + 2 ATP + 4 ADP + 2 Pi + 2 NAD+
2 pyruvate + 4 ATP + 2 ADP + 2 NADH + 2 H+ + 2 H2O
Glycolysis Converts Glucose into Pyruvate
Steps 1–5 require ATP (energy-investing reactions).
Glycolysis Converts Glucose into Pyruvate
Steps 6–10 yield NADH and ATP (energy-harvesting
reactions).
How Does Oxidative Phosphorylation Form ATP?
Many bacteria and archaea have evolved
pathways that allow them to exist where O2
is scarce or absent, by using other electron
acceptors— anaerobic respiration.
How Is Energy Harvested from Glucose in the Absence of Oxygen?
Without O2, ATP can be produced by glycolysis
and NADH can be oxidased by fermentation.
The Respiratory Chain and ATP Synthase Produce ATP
What Are the Aerobic Pathways of Glucose Metabolism?
Pyruvate Oxidation:
• occurs in the mitochondrial matrix
• produces acetate and CO2
• acetate binds to coenzyme A to form acetyl-CoA
• is a multistep reaction catalyzed by the pyruvate
dehydrogenase complex.
One NAD+ is
reduced to NADH.
What Are the Aerobic Pathways of Glucose Metabolism?
Citric acid cycle:
• Acetyl-CoA is the
starting point.
• The acetyl group is
completely oxidized to
2 molecules of CO2.
• Energy released is
captured by ADP, NAD+,
FAD, and GDP.
What Are the Aerobic Pathways of Glucose Metabolism?
The complete oxidation of one glucose molecule
yields:
• 6 CO2
• 10 NADH
• 2 FADH2
• 4 ATP
How Does Oxidative Phosphorylation Form ATP?
Oxidative phosphorylation: ATP is synthesized by
reoxidation of electron carriers in the presence of O2.
Relationships among the Major Metabolic Pathways of the Cell
What Is Photosynthesis?
Photosynthesis is the “synthesis from light”.
Energy from sunlight is captured and used to
convert CO2 to more complex carbon compounds.
What Is Photosynthesis?
Two pathways occur in different parts of the chloroplast.
• Light reactions:
Convert light energy
to chemical energy as
ATP and NADPH.
• Light-independent
reactions: Use ATP
and NADPH (from the
light reactions) plus
CO2 to produce
carbohydrates.
Photosynthetic Organisms Changed Earth’s Atmosphere
About 2.5 billion years ago, photosynthesis changed
the nature of life on Earth.
Early photosynthetic cells were probably similar to
cyanobacteria (prokaryotes).
The atmosphere of early Earth had no O2, but it began
to increase as photosynthetic prokaryotes increased.
Organisms that could tolerate O2 proliferated.
Abundant O2
opened up new
avenues of
evolution.
Adapted from
Life: The Science of Biology, Tenth Edition, Sinauer Associates, Sunderland, MA, 2014
Inc. All rights reserved