Transcript The Process of Cellular Respiration

The Process of Cellular
Respiration
The gory details!!!!
Overview
• Respiration occurs in three metabolic stages:
1. Glycolysis
2. Krebs cycle (Citric acid cycle)
3. Electron transport chain and
oxidative phosphorylation
1. Glycolysis
• Glucose (a six carbon-sugar), is broken down
into two molecules of a compound called
pyruvate.
• Occurs in the cytosol
• 10 steps!! … each catalyzed by an enzyme
• Can occur without oxygen
• If O2 is present, pyruvate moves to the Krebs
cycle
• Payoffs: -2 ATP + 4 ATP = 2 ATP and
2 NADH
Energy Investment Phase
Energy Payoff Phase
2. The Krebs Cycle
• If O2 present: pyruvate enters mitochondrion
• Upon entering pyruvate is converted to a
compound called acetyl coenzyme A (acetyl CoA)
CO2 is removed
1 NADH is made
x2
• enzymes of the Krebs cycle complete the oxidation
of the organic fuel to carbon dioxide
Krebs Cycle Payoffs
• This cycle begins when acetate from acetyl
CoA combines with oxaloacetate to form
citrate.
• Ultimately, the oxaloacetate is recycled and
the acetate is broken down to CO2.
• Each cycle (8 steps!) produces:
• 1 ATP
• 3 NADH
• 1 FADH2 (another electron carrier)
• per acetyl CoA so MULTIPLY BY 2 FOR
TOTAL per glucose molecule
• The conversion of
pyruvate and the
Krebs cycle
produces large
quantities of
electron carriers.
• Where do these
electron carries
go?
3. Electron Transport
Chain and Oxidative
Phosphorylation
• The vast majority of the ATP comes from
the energy in the electrons carried by
NADH and FADH2
• The energy in these electrons is used in
the electron transport system to power
ATP synthesis.
Electron Transport Chain Details
• Thousands of copies of the
electron transport chain are
found in the extensive surface of the
cristae, the inner membrane of the
mitochondrion.
– Most components of the chain are proteins
that can alternate between reduced and
oxidized states as they accept and donate
electrons.
• Electrons drop in free energy as they pass
down the electron transport chain.
• Electrons carried by
NADH are transferred
to the first molecule
in the electron
transport chain,
flavoprotein.
• The electrons carried
by FADH2 are added
to the chain at a
lower energy level.
• Proteins called
cytochromes pass
electrons to O2, which
picks up a pair of H+
ions to form 2 H2O
Does the Electron Transport
Chain make ATP?!
• The electron transport chain does NOT
make any ATP directly.
• It eases the fall of electrons from food to
oxygen, releasing energy in manageable
amounts.
• The energy is used for chemiosmosis (and
ATP synthesis) to occur
• So who makes the ATP and what is the
process called?
Oxidative Phosphorylation
• ATP synthase: protein
complex that makes ATP
– It uses the energy of the
proton gradient that
develops between the
intermembrane space
and the matrix.
• Process is called
oxidative
phosphorylation:
ATP synthesis powered
by the flow of H+ across
the membrane
The Big Picture!
Explanation of The Big Picture:
Chemiosmosis
• Chemiosmosis: an energy-coupling mechanism that
uses energy stored in the form of an H+ gradient
across a membrane to drive cellular work
– In this case: coupling of the redox reactions of the electron
transport chain to ATP synthesis
• The electron transport chain pumps H+ across the
membrane from the matrix into the intermembrane
space
• As the H+ diffuses back to the matrix they must pass
through ATP synthase
• This flow drives the oxidative
phosphorylation of ADP to ATP
• Makes about 34 ATPs
Total: 38 ATPs!
Fermentation
• Occurs if oxygen is NOT present: anaerobic
• Glycolysis generates 2 ATP whether oxygen
is present (aerobic) or not
• But there must be a supply of NAD+ to accept
electrons
• Since NADH can not transfer electrons to oxygen in
the electron transport chain, the electrons are
transferred to pyruvate instead
• Fermentation: glycolysis + reactions that regenerate
NAD+ by transferring electrons from NADH to
pyruvate
• 2 types…
Alcohol Fermentation
Pyruvate is converted
to ethanol in two
steps.
1. pyruvate is converted to
a two-carbon compound,
acetaldehyde by the
removal of CO2.
2. acetaldehyde is reduced
by NADH to ethanol
– Alcohol fermentation
by yeast is used in
brewing and
winemaking
– Also used by some
bacteria
Lactic Acid Fermentation
pyruvate is reduced to form
lactate (ionized form of lactic
acid) - No release of CO2
– Used some fungi and
bacteria is used to make
cheese and yogurt.
– Muscle cells use this when to
generate ATP when O2 is
scarce.
• The waste product
may cause muscle fatigue,
but ultimately it is
converted back to
pyruvate in the liver.
• Fermentation and cellular respiration are
anaerobic and aerobic alternatives,
respectively, for producing ATP from
sugars.
– Both use glycolysis to oxidize sugars to
pyruvate with a net production of 2 ATP by
substrate-level phosphorylation.
– Both use NAD+ as an electron acceptor.
• In fermentation, the electrons of NADH are
passed to an organic molecule,
regenerating NAD+.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• In respiration, the electrons of NADH are
ultimately passed to O2, generating ATP by
oxidative phosphorylation.
• In addition, even more ATP is generated
from the oxidation of pyruvate in the Krebs
cycle.
• Without oxygen, the energy still stored in
pyruvate is unavailable to the cell.
• Under aerobic respiration, a molecule of
glucose yields 38 ATP, but the same
molecule of glucose yields only 2 ATP
under anaerobic respiration.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Review:
1. For each of the following steps of cellular
respiration, explain the reactants, products,
location, and purpose:
a) Glycolysis
b) Krebs cycle (citric acid cycle)
c) Oxidative phosphorylation and chemosynthesis
(Electron transport chain)
2. When does fermentation occur? Give a real
example of this.
3. Compare the different types of fermentation.
4. Summarize all of cellular respiration in one
paragraph.