Transcript Chapter 7

7
Vital Harvest:
Deriving Energy from Food
Relevant questions to be answered:
1. How do cells convert food into energy? Do
they convert fats differently from
carbohydrates? Do you get fat by eating fat or
calories?
2. Why do we need to breathe? How does
exercise affect metabolism?
3. What about energy supplements? What does it
mean to have a fast or slow metabolism?
ATP is the most important energy storage molecule:
Potential energy from food breakdown is used
to drive the endergonic synthesis of ATP
(like recharging a battery).
2. The charged ATP has energy that can be
released at any time by breaking off the third
phosphate to do a variety of actions).
Electrons from food carry energy to make ATP
1. Electrons from glucose run downhill.
Transferred by carriers, the electron drop
powers uphill synthesis of ATP.
2. Electron transfers to molecules—redox
reactions occur side by side:
a) One molecule is oxidized—loses electrons
b) Another molecule is reduced—gains those
electrons (reduces charge).
Electrons from food carry energy to make ATP
3. Intermediate electron carriers serve to shuttle
electrons through these reactions transferring
energy as they go:
NAD+ (empty city cab) in redox reaction is an oxidizing
agent (removes electrons causing a substance to be
oxidized), accepts a hydrogen atom and one electron
becomes NADH (full cab).
Intermediate electron carriers serve to shuttle electrons through
these reactions transferring energy as they go: NADH can carry
electrons (proceed down energy hill) on to another
acceptor, thus being regenerated (empty cab).
NAD+ is made by cells from the vitamin niacin.
Enzymes coordinate all these transfers by bringing together the
glucose derivatives with energy carrier molecules.
Overview of the three stages:
1. General reaction:
C6H12O6 +6O2 + ADP  6CO2 + 6H2O + ATP
2. Energy coupling: downhill breakdown of glucose releases
electrons, carried along and used to transfer energy to drive
uphill synthesis of ATP.
First Stage of Respiration:
Glycolysis—For eukaryotes this is the first stage. It begins
breakdown of glucose, yielding little energy, but it does transfer
electrons to NAD+. On the plus side, it doesn’t require oxygen
and occurs in the cytoplasm, and some prokaryotes and singlecelled eukaryotes have long used it as the sole source of energy.
Krebs cycle and electron transport chain—later evolved but
generate larger quantities of energy; only problem is they occur
only in mitochondria (only eukaryotes) and the electron
transport chain requires oxygen.
Glycolysis: Steps in the process:
1. Sugar in bloodstream enters cytoplasm, where
this breakdown begins. Enzymes catalyze each
reaction in metabolic pathway (first is
hexokinase, which adds a phosphate from ATP
called phosphorylation).
2. Although breaking sugar apart generates
energy, it requires some activation energy
(another ATP is used to attach another
phosphate: -2ATP total).
Rearrangement eventually leads to splitting the molecule in half
from one 6-carbon sugar into two 3-carbon sugars (pyruvic acid
is end product).
Oxidation by 2NAD+ transfers electrons, and leads to the
attachment of a high-energy phosphate to each sugar. Enough
energy is generated by the eventual release of these four total
phosphates in the next two steps to attach them to ADP to make
ATP (+ 4ATP).
Glycolysis
Plus side—Very fast reactions cut glucose in half, generating
small amount of energy (net 2 ATP), and electrons (2 NADH),
but no oxygen was required.
Minus side—What is next redox reaction where electrons can be
transferred to empty the cab (NADH) for more passengers
(NAD+)? Not much ATP made for all the work.
Glycolysis
3. Sidebar: When Energy Harvesting Ends at
Glycolysis
a) Bacteria and certain eukaryotes may only
use glycolysis. Problem—How can they
recycle the NAD+?
Solution—Alcoholic fermentation, yeast in absence of oxygen
(bread and wine) must regenerate NAD+, so they dump electrons
from NADH onto the acetaldehyde (converted from pyruvic acid
and spewing off CO2), reducing it to ethanol, but regenerating
the NAD+.
Solution—Lactate fermentation, in animals in absence of
oxygen (muscle fatigue), pyruvate accepts electrons from
NADH and regenerates NAD+, but is converted into lactic acid
(muscle burn).
Pyruvate from glycolysis  lactic acid
Energy and Exercise—Huge quantities of ATP are required to
contract skeletal muscle, but ATP is generated in different ways
depending on circumstances.
a) First burst of activity (6 seconds): cells have stockpile of
ATP and phosphocreatine:
b) Look at Sidebar Figure 1—What is greatest source of energy
aerobic or anaerobic at 30 seconds? 10 minutes? Why? Because,
although aerobic respiration is slower, it is much more
productive. The body moves to aerobic respiration if it can, using
anaerobic only under extreme exertion when oxygen drops:
Krebs cycle sugar derivatives are oxidized to yield electrons in interior
of inner membrane of mitochondria:
Each of the two pyruvic acid travels into the mitochondria,
where they combine with coenzyme A to make acetyl
CoA, one NADH and CO2 (breathe out):
Acetyl CoA combines with oxaloacetic acid to make citric acid,
continues around through a series of reactions that finally yield
oxaloacetic acid again (cycle).
Steps in the process: Figure 7.8
b) During the cycle of reactions, as the acetyl CoA is
transformed, it is being oxidized by electron carrier molecules
NAD+ and FAD.
c) Also, ATP and CO2 are produced.
Ledger: 8 NADH, 2 FADH2, and 2 ATP, acetyl CoA completely
broken down into CO2.
Majority of electrons for next stage (electron transport chain)
Electron transport chain—Series of molecules in the mitochondrial
inner membrane that are the destination of the electrons carried
by NADH and FAHD2.
1. Steps in the process:
a) NADH arrives, and it bumps the ETC’s first carrier, which accepts the
electrons, then passes them on along the chain (like a hot potato).
b) Movement of electrons at each transfer releases enough energy to
power the movement of H+ ions from the inner compartment into
the outer compartment (like heat of hot potato dissipating as it is
passed). They are being pumped against their concentration
gradient (uphill).
c) Hydrogen ions are allowed to flow downhill
through an enzyme in the membrane called ATP
synthase, like a water wheel spinning; as the ions
pass, energy is used to transfer phosphate onto
ADP to make ATP.
Greatest amount of ATP is made in this stage
(up to 32 ATP per glucose).
At the end of the ETC , who accepts the electron?
½ O2 + 2 electrons + 2 H+ = H2O
Other Foods, Other Respiratory Pathways
Fats, proteins, and other sugars can also enter pathway to
be converted to energy, but not in exactly the same way:
Food eaten in excess of caloric demands can also be
converted from amino acids, fatty acids, and sugars into
proteins, fats, and carbohydrates for structure or storage
(98 percent of energy reserves of animals are fats).
1
The 3 steps of
Aerobic Respiration:
2
8
3
Up
to
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Summary of Aerobic respiration
From one molecule of glucose
Glycolysis  Krebs Cycle  Electron Transport Chain
Produces:
Produces:
Produces:
Net gain 2 ATP
CO2
Up to 32 ATP
2NADH
8NADH
2FADH2
2ATP
The End