• Living is work.
• To perform their many
tasks, cells must bring in
energy from outside
sources.
• In most ecosystems, energy enters as sunlight.
• Light energy trapped in organic molecules is available to both photosynthetic organisms and others that eat them.
Introduction
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 9.1
• Cellular respiration is similar to the combustion of
gasoline in an automobile engine.
• The overall process is:
• Organic compounds + O2 -> CO2 + H2O + Energy
• Carbohydrates, fats, and proteins can all be used as
the fuel, but it is traditional to start learning with
glucose as the fuel molecule, because it is the one
most abundantly used.
• C6H12O6 + 6O2 -> 6CO2 + 6H2O + Energy (ATP + heat)
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Benchmark Clarifications
Students will explain how the products of photosynthesis are used
as reactants for cellular respiration and vice versa.
Students will explain how photosynthesis stores energy and
cellular respiration releases energy.
Students will identify the reactants, products and/or the basic
function of photosynthesis.
Students will identify the reactants, products and/or the basic
functions of aerobic & anaerobic cellular respiration.
Students will connect the role of ATP to energy transfers within
the cell.
• In cellular respiration, glucose and other fuel
molecules are oxidized, releasing energy.
• Molecules that have an abundance of hydrogen are
excellent fuels because their bonds are a source of
energetic electrons that give off their energy as they
are transferred to oxygen.
• The energy from these electrons will be transferred
to the energy rich bonds of ATP molecules when
they help form these bonds.
4. Electrons “fall” from organic molecules to
oxygen during cellular respiration
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• Cellular respiration does not oxidize glucose in a single step that transfers all the hydrogen in the fuel to oxygen at one time.
• Rather, glucose and other fuels are broken down gradually in a series of chemical reaction steps, each catalyzed by a specific enzyme.
• At key steps, hydrogen atoms are stripped from glucose and passed first to a coenzyme, like NAD+
(nicotinamide adenine dinucleotide).
5. The “fall” of electrons during respiration
occurs in small steps
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 9.4
• This changes NAD+, to NADH, which carries the
energy of the electrons.
Part of this involves mitochondria in eukaryotic cells
1. Respiration involves glycolysis, the Krebs
cycle, and electron transport:
an overview
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 9.6
• During glycolysis, glucose, a six carbon-sugar, is split into two, three-carbon sugars, pyruvate. Let’s watch the whole thing. 5 min.
• Each of the ten steps in glycolysis is catalyzed by a specific enzyme, and occurs in the cytoplasm.
• These steps can be divided into two phases: an energy investment phase and an energy payoff phase.
2. Glycolysis breaks down glucose to
pyruvate in 10 small steps:
a closer look
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• In the energy investment phase, ATP provides activation energy by phosphorylating glucose.
• This requires 2 ATP per glucose.
• In the energy payoff phase, 4 ATP are produced and NAD+ is reduced to NADH.
• 2 ATP (net) and 2 NADH are produced per glucose.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 9.8
• The net yield from glycolysis is 2 ATP and 2
NADH per glucose.
• No CO2 is produced during glycolysis.
• Glycolysis occurs whether O2 is present or not.
• If O2 is present, pyruvate moves into the
mitochondria to the Krebs cycle and the
energy stored in NADH can be converted to
ATP by the electron transport chain.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• As pyruvate enters the mitochondrion, a
multienzyme complex modifies pyruvate to acetyl
CoA which enters the Krebs cycle in the matrix.
• A carboxyl group is removed as CO2.
• A pair of electrons is transferred from the remaining
two-carbon fragment to NAD+ to form NADH.
• The 2 carbon acetic
acid combines with
coenzyme A to
form acetyl CoA.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 9.10
• More than three quarters of the original energy in
glucose is still present in two 2 molecules of
pyruvate.
• If oxygen is present, pyruvate enters the
mitochondrion where enzymes of the Krebs cycle
complete it’s breakdown to carbon dioxide. Let’s
watch it..
3. The Krebs cycle completes the energy-
releasing breakdown of organic
molecules: a closer look
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• The Krebs cycle is named after Hans Krebs who
was largely responsible for elucidating its
pathways in the 1930’s.
• This cycle begins when acetic acid from acetyl CoA
(2C) combines with oxaloacetate (4C) to form citrate
(6C).
• Ultimately, the oxaloacetate is recycled and the acetate
is broken down to CO2.
• Each cycle produces one ATP, three NADH, and one
FADH2 (another electron carrier) per acetyl CoA.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• The Krebs
cycle consists
of eight steps.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 9.11
• Only 4 of 38 ATP ultimately produced by respiration
of glucose are derived from glycolysis and the Krebs
Cycle
• 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.
4. The inner mitochondrial membrane
couples electron transport to ATP
synthesis: a closer look
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Electrons carried by NADH and FADH are transferred to the molecules in the electron transport chain.
• The electrons continue along the chain, which includes several cytochromeproteins.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 9.13
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 9.15
• Electrons from NADH or FADH2 ultimately pass to
oxygen, the so-called final electron acceptor, causing this
to be called aerobic respiration.
• The electron transport chain generates no ATP directly.
• The movement of electrons along the chain does contribute
to a process called chemiosmosis, which leads to ATP
synthesis by oxidative phosphorylation (or ox-phos as
it’s referred to in small talk at biologists’ cocktail parties).
• Here’s how it works:http://highered.mcgraw-
hill.com/sites/0072437316/student_view0/chapter9/animati
ons.html
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• A proton gradient is produced by the movement
of electrons along the electron transport chain,
because several chain molecules can use the
exergonic flow of electrons to pump H+ from the
matrix to the intermembrane space.
• The gradient produced involves more protons in
the intermembrane space than in the matrix.
• This gradient represents a form of potential energy,
very similar to the one in a flashlight battery.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• A protein complex, ATP synthase, in the cristae actually makes ATP from ADP and Pi.
• The energy of theproton gradient is used as the source of power to do the work of ATP synthesis.
• How about a little animation? http://vcell.ndsu.nodak.edu/animations/atpgradient/movie.htm
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 9.14
• The ATP synthase molecules are the only place that
will allow H+ to diffuse back to the matrix.
• This exergonic flow of H+ through the protein complex
is used by the enzyme to generate ATP.
• This coupling of the redox reactions of the electron
transport chain to ATP synthesis is called
chemiosmosis.
• The energy from glucose electrons was used to move
protons across a membrane (uphill, so to speak), and
when they passively flowed back across the membrane
(downhill), their energy was used to do the work of
making ATP.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• During respiration, most energy flows from glucose -> NADH -> electron transport chain -> ATP.
• Considering the fate of carbon, one six-carbon glucose molecule is oxidized to six CO2 molecules.
• Some ATP is produced during glycolysis and the Krebs cycle, but most comes from the electron transport chain.
5. Cellular respiration generates many ATP
molecules for each sugar molecule it
oxidizes: a review
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Assuming the most energy-efficient
shuttle of NADH from glycolysis, a
maximum yield of 34 ATP is produced by
the ETC from one glucose.
• This plus the 4 ATP from substrate-level
phosphorylation gives a bottom line of 38
ATP per glucose molecule broken down.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Glycolysis generates 2 ATP whether oxygen is
present (aerobic) or not (anaerobic).
1. Fermentation enables some cells to produce
ATP without the help of oxygen
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Anaerobic catabolism of sugars can occur by fermentation.
• Fermentation can generate ATP from glycolysis as long as there is a supply of NAD+ to accept electrons.
• If the NAD+ pool is exhausted, glycolysis shuts down.
• Under aerobic conditions, NADH transfers its electrons to the electron transfer chain, recycling NAD+.
• Under anaerobic conditions, various fermentation pathways generate ATP by glycolysis and produce fresh NAD+ by transferring electrons from NADH to pyruvate. This, like glycolysis, happens in the cytoplasm, so it can’t help the Krebs Cycle.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• In alcohol fermentation, pyruvate is converted to
ethanol in two steps.
• First, pyruvate is converted to a two-carbon compound,
acetaldehyde, by the removal of CO2.
• Second, acetaldehyde is reduced by NADH to ethanol.
• Alcohol fermentation
by yeast is used in
baking and alcoholic
beverage making.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 9.17a
• During lactic acid fermentation, pyruvate is
reduced directly by NADH to form lactate (lactic
acid).
• Lactic acid fermentation by some fungi and bacteria is used to make cheese and yogurt.
• Muscle cells switch from aerobic respiration to lactic acid fermentation to generate ATP when O2 is scarce.
• The waste product, lactate, was thought of to be the cause muscle fatigue, but ultimately it is converted back to pyruvate in the liver.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 9.17b
• Carbohydrates, fats,
and proteins can all be
catabolized through
the same pathways.
We usually learn about
the breakdown of
glucose, but at rest,
most of our ATP’s
actually come from the
breakdown of fatty
acids into 2 carbon
acetyl CoA’s.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 9.19
Chloroplasts are found in most plant cells but not in
animal cells. Mitochondria are found in animal
cells and most plant cells. Why are mitochondria
found in most plant cells?
What is the primary purpose of cellular
respiration?
a. To store chemical energy in glucose molecules
b. To store chemical energy in carbon dioxide and
water molecules
c. To use chemical energy from glucose molecules
d. To use chemical energy from carbon dioxide and
water molecules
Cellular respiration is a chemical process and can be
represented by a chemical equation. What are the
products in this chemical process?
a. Hydrocarbons and oxygen
b. Hydrocarbons and carbon dioxide
c. Water, carbon dioxide, and energy
d. Water, carbon dioxide, and oxygen
Which equation shows the reactants and
products of cellular respiration?
a. Carbon dioxide + water → sugar + oxygen
b. Carbon dioxide + oxygen → sugar + water
c. Sugar + carbon dioxide →water + oxygen
d. Sugar + oxygen →water + carbon dioxide
.
All cells need energy. Where does the energy come
from in plants? Briefly trace the energy from the
original source to the “endpoint”.
In animals?
…
Which of these is required for aerobic
cellular respiration?
• A. oxygen
• B. nitrogen
• C. carbon dioxide
• D. sunlight
…
In terms of energy, how are cellular respiration
and photosynthesis related?
a. Energy captured in photosynthesis is used to
power cellular respiration
b. The energy transformed in cellular respiration is
used to power photosynthesis
c. Photosynthesis and respiration perform the same
task in terms of energy transformation
d. Energy is not involved in either photosynthesis
or cellular respiration
Photosynthesis and cellular respiration are
interrelated processes. During which biogeochemical
cycle do the biological processes of photosynthesis
and cellular respiration play key roles?
A. carbon cycle
B. hydrogen cycle
C. nitrogen cycle
D. oxygen cycle
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