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The following summarizes a few facts to know from the chapter and class about
Understand energy coupling and how it applies to specific rxns during metabolism.
During catabolism (breakdown rxns of metabolism), energy is released in the form of
ATP. Often energy is also released in the form of electrons (i.e. the organic molecule
being broken down is oxidized). These electrons are carried by electron carriers such
as NADH and FADH2 (Note: when not carrying electrons the molecules are in the
form: NAD+ and FAD)
ATP is a central energy transfer molecule in the cell.
- is the breakdown of glucose (a 6 carbon monosaccharide) to 2 molecules of
pyruvate (a 3 carbon compound)
- uses 2 ATP’s to activate the intermediates, then releases 4 ATP’s for a net yield of 2
- in general, under anaerobic conditions glucose is only partially broken down; a few
additional reactions are required; this is known as fermentation
- occurs in the cytosol of the cell
Under aerobic conditions, the breakdown of glucose can continue to completion (i.e.
completely oxidized to CO2) by Cellular Respiration. In cellular respiration,
pyruvate enters the mitochondria where it is completely oxidized to CO2 by going
through two more steps: Acetyl Coenzyme A (AcCoA) formation (also known as
oxidative decarboxylation), then the Citric Acid Cycle (also known as Kreb’s Cycle).
For each pyruvate, one CO2 is lost during AcCoA formation, the other 2 are lost
during the Citric Acid Cycle. From one original molecule of glucose, two more ATPs
are produced during the Citric Acid Cycle.
Electrons removed during each of the three steps of glucose breakdown are carried by
NADH and FADH2 to the Electron Transport Chain (ETC) where additional
molecules of ATP are produced from the oxidation steps of the ETC (~34 additional
ATPs per original molecule of glucose).
Plants and some bacteria can harvest light energy and convert it to a form that is usable
by other living organisms in a process known as Photosynthesis. In plants,
photosynthesis occurs in chloroplasts. In the first half of photosynthesis, light
energy is trapped and converted to chemical energy in the form of ATP and NADPH.
In the second half of photosynthesis, the reactions of the Calvin Cycle use the energy
stored in these two molecules to convert carbon dioxide to carbohydrate, which is a
less temporary energy storage form.
Notice the activation of the precursors for biosynthetic pathways: polysaccharides
(Fig3.20), protein synthesis (Fig 3.23), nucleic acids (Fig 3.25).
Be sure and go back and relate what we talked about in class to the more detailed
description in the book. Although you do not need to know all of the details in the book,
many concepts were interwoven in our class discussion of metabolism and will stick in
your mind better by doing this comparison.