Download The Reactions of Respiration

Cell Respiration: Releasing Chemical Energy
 What problem do birds
and other small animals
face in winter?
 What adaptations help them
overcome this problem?
A robin (Erithacus rubecula) perched on
a pine tree
An Overview of Respiration
5.1 Metabolism and Cell Respiration (cont.)
• Cell respiration is a decomposition pathway that
provides the energy cells need to function.
• Respiration releases free energy by oxidizing sugars
or other organic substrates.
• Some of this energy is conserved in ATP
which in turn provides the energy to power
most life processes.
An Overview of Respiration
5.1 Metabolism and Cell Respiration (cont.)
• Cell respiration can occur in the presence or
absence of oxygen.
– In aerobic respiration—occurring in the presence
of oxygen—oxygen is the oxidizing agent that
receives electrons from the decomposed
substrates.
– In anaerobic respiration—occurring without
oxygen—the substrate may be only partly
decomposed, releasing less energy, or a nitrogen
or sulfur compound may substitute for oxygen.
An Overview of Respiration
5.1 Metabolism and Cell Respiration (cont.)
• The raw materials for aerobic respiration are
carbohydrates, fats, and proteins.
• Glucose (C6H12O6) and glucose-phosphate
(C6H11O6—H3PO3) are important substrates for
respiration.
An Overview of Respiration
5.1 Metabolism and Cell Respiration (cont.)
• During aerobic respiration, a great deal of energy
released as glucose gradually oxidizes and breaks
down to carbon dioxide.
• The overall reaction is summarized in the
following equation:
An Overview of Respiration
5.1 Metabolism and Cell Respiration (cont.)
• Oxidizing one molecule of glucose
releases much more energy than
a single reaction needs.
• Cell respiration releases
energy by oxidizing glucose
in a series of small steps that
lead to the production of one
molecule of ATP.
The letters a through g represent intermediate
compounds in the decomposition of glucose to
carbon dioxide and water.
An Overview of Respiration
5.1 Metabolism and Cell Respiration (cont.)
• Cell respiration provides both ATP and the carbon
skeletons needed for biosynthesis.
Products of cell respiration
Click the image to view an animated version.
An Overview of Respiration
5.2 The Stages of Aerobic Respiration
• The respiration of a simple carbohydrate such as
glucose can be divided into three main stages:
– glycolysis
– the Krebs cycle
– the electron transport system
• Each stage involves a series of chemical reactions
catalyzed by enzymes.
An Overview of Respiration
5.2 The Stages of Aerobic Respiration (cont.)
Aerobic respiration occurs in
three stages—glycolysis, the
Krebs cycle, and the electron
transport system. As glucose
and other substrates are
oxidized to carbon dioxide
and water, NAD+ is reduced
to NADH, and FAD is reduced
to FADH2. These reduced
compounds carry hydrogen
ions (H+) and electrons (e–) to
the electron transport system.
An Overview of Respiration
5.2 The Stages of Aerobic Respiration (cont.)
• Glycolysis is the initial breakdown of a
carbohydrate, usually glucose, into smaller
molecules at the beginning of cell respiration or
fermentation.
• The Krebs cycle completes the breakdown of the
intermediate products of glycolysis, releasing energy;
also, a source of carbon skeletons for use in
biosynthesis reactions.
• The electron transport system is the process in
which electrons transfer from one carrier molecule to
another in photosynthesis and in cell respiration. It
results in storage of some of the energy in ATP
molecules.
An Overview of Respiration
5.2 The Stages of Aerobic Respiration (cont.)
• During glycolysis, enzymes partially oxidize glucose
and split it into two 3-carbon molecules releasing
enough energy to form a small amount of ATP.
• An enzyme releases a molecule of carbon dioxide
from each 3-carbon molecule that was produced in
glycolysis.
• The resulting 2-carbon molecules are oxidized
completely to carbon dioxide in the second stage,
called the Krebs cycle, producing additional ATP
molecules.
An Overview of Respiration
5.2 The Stages of Aerobic Respiration (cont.)
• Whenever one substance
is oxidized, another must
be reduced.
• As glucose is oxidized,
electrons and protons, are
passed to NAD+ (nicotinamide
adenine dinucleotide),
reducing it to form NADH.
An Overview of Respiration
5.2 The Stages of Aerobic Respiration (cont.)
• In the electron transport
system, NADH is oxidized as it
donates protons and
electrons, regenerating the
supply of NAD+.
• The protons and electrons
release energy to form ATP as
the electron transport system
transfers them to oxygen,
forming water.
An Overview of Respiration
5.2 The Stages of Aerobic Respiration (cont.)
• Most of the ATP is synthesized
by the electron transport
system.
• In one step in the Krebs cycle,
two hydrogen atoms derived
from glucose reduce a second
hydrogen-carrier molecule,
FAD (flavin adenine
dinucleotide), instead of NAD+.
• NADH, NADPH, and FADH2 all
carry hydrogen in cells.
An Overview of Respiration
5.2 The Stages of Aerobic Respiration (cont.)
• This process is called
aerobic respiration because
oxygen must accept the
electrons at the end of the
electron transport system.
• The energy released in
this reaction is used to
synthesize ATP.
The Reactions of Respiration
5.3 Glycolysis
• Both aerobic and anaerobic respiration begin
with glycolysis.
• Three important things happen during glycolysis:
1. the glucose molecule breaks into two pieces
2. some ATP forms
3. some NAD+ is reduced to form NADH
The Reactions of Respiration
5.3 Glycolysis (cont.)
Step A: Glycolysis
begins when an enzyme
converts a molecule of
glucose to glucose-6phosphate.
The Reactions of Respiration
5.3 Glycolysis (cont.)
Step B: A molecule of ATP
provides the phosphate and
the energy to power the
reaction. Another enzyme
rearranges the glucose-6phosphate, and a second
ATP molecule donates
another phosphate group.
The Reactions of Respiration
5.3 Glycolysis (cont.)
Step C: The resulting
molecule splits into two
3-carbon sugar-phosphates.
The Reactions of Respiration
5.3 Glycolysis (cont.)
Step D: Other enzymes
catalyze the rearrangement
and partial oxidation of
these molecules to form
the 3-carbon compound
pyruvic acid.
The Reactions of Respiration
5.3 Glycolysis (cont.)
The Reactions of Respiration
5.3 Glycolysis (cont.)
• In plant cells, starch and
sucrose break down to
glucose or glucose-1phosphate, which can
enter glycolysis directly
at step a.
• Three-carbon
sugar-phosphates
formed in
photosynthesis
can enter the
process at step c.
The Reactions of Respiration
5.3 Glycolysis (cont.)
• At the end of glycolysis, the fate of pyruvate depends
on whether oxygen is present.
– If insufficient oxygen is present, animal
cells convert NADH and pyruvate into NAD+
and lactate.
– NAD+ cycles back to glycolysis, in an anaerobic
pathway known as lactic-acid fermentation.
– If sufficient oxygen is present, pyruvate enters
the Krebs cycle.
The Reactions of Respiration
5.4 Mitochondria and Respiration
• Mitochondria are the organelles in prokaryotes in
which the Krebs cycle and the electron transport
system occur.
• Most ATP is synthesized in the mitochondria.
The Reactions of Respiration
5.4 Mitochondria and Respiration (cont.)
• A cell may contain anywhere from ten to several
thousand mitochondria depending on its
energy needs.
• Each mitochondrion is
usually only 2–3 µm long
and about 1 µm thick.
This transmission electron micrograph
shows a mitochondrion in a human
liver cell (x80,000; color added).
The Reactions of Respiration
5.4 Mitochondria and Respiration (cont.)
• A mitochondrion has outer and inner membranes.
• The inner membrane holds the
enzymes of the electron
transport system and the
enzymes for ATP formation.
• Most of the enzymes of the
Krebs cycle are within the
fluid-filled interior matrix.
• The outer membrane regulates
the movement of molecules
into and out of the mitochondrion.
The Reactions of Respiration
5.5 The Krebs Cycle
• The Krebs cycle completes the decomposition and
oxidation of glucose to carbon dioxide.
• As the breakdown products of glucose are oxidized,
NAD+ and FAD are reduced, and a small amount of
energy is saved as ATP.
The Reactions of Respiration
5.5 The Krebs Cycle (cont.)
Step A: pyruvate is transported
into the mitochondria where
enzymes release a molecule of
carbon dioxide from each pyruvate
molecule, leaving a molecule of
acetate.
A carrier molecule,
coenzyme A (CoA), binds to
the acetate and delivers the
acetate to the Krebs cycle.
The Reactions of Respiration
5.5 The Krebs Cycle (cont.)
Step B: Acetate enters the
Krebs cycle, as an enzyme
combined the acetate group of
acetyl CoA with a 4-carbon
acid (oxaloacetate) to form a 6carbon acid (citrate).
Coenzyme A is released and
recycled to deliver more
acetate.
The Reactions of Respiration
5.5 The Krebs Cycle (cont.)
Steps C and D: Other enzymes
catalyze the rearrangement and
oxidation of citrate. Two of the
carbon atoms in citrate are
oxidized to carbon dioxide.
The hydrogen atoms that
these carbon atoms lose
reduce two molecules of
NAD+.
The Reactions of Respiration
5.5 The Krebs Cycle (cont.)
Steps E and F: A 4-carbon
organic acid rearranged and
further oxidized.
The result is a new molecule
of oxaloacetate that begins
another round of the cycle.
The Reactions of Respiration
5.5 The Krebs Cycle (cont.)
The Reactions of Respiration
5.6 The Electron Transport System
• The oxidation of glucose in glycolysis and the Krebs
cycle reduces NAD+ to NADH and FAD to FADH2
which carry hydrogen atoms to the electron
transport system.
• The electron transport system consists of a
series of enzymes and other proteins known
as cytochromes that are embedded in the
inner membranes of mitochondria.
The Reactions of Respiration
5.6 The Electron Transport System (cont.)
• The electron transport system separates hydrogen
atoms into electrons and protons.
• The cytochromes transfer the electrons step by step
through the system.
• The last, or terminal, cytochrome is an enzyme that
combines the electrons and protons with oxygen,
forming water.
The mitochondrial electron transport system
Click the image to view an animated version.
The Reactions of Respiration
5.6 The Electron Transport System (cont.)
• At each transfer in the electron transport chain,
the electrons release free energy that enables
enzymes in the inner mitochondrial membrane to
actively transport protons from the matrix to the
intermembrane space.
• As protons become highly concentrated they tend
to diffuse back into the matrix of the mitochondrion
passing through the ATP-synthetase enzyme
complex, where ATP is synthesized.
As glucose is oxidized in
glycolysis and the Krebs
cycle, NAD+ and FAD are
reduced to NADH and
FADH2. These carriers
pass electrons to the
electron transport system.
ATP forms as these
electrons lose energy in
reducing oxygen. Each
molecule of NADH
generates three ATP
molecules, and each
molecule of FADH2
generates two ATP
molecules. The resulting
oxidized NAD+ and FAD
are recycled as more
glucose is oxidized.
The Reactions of Respiration
5.6 The Electron Transport System (cont.)
• Bacteria do not have mitochondria. Their cell
membranes contain their electron transport systems.
• In some bacteria, in a process called anaerobic
respiration, electrons flow through the system to
oxidizers other than oxygen, such as sulfate
(SO4–2) or nitrate (NO3–) .
The Reactions of Respiration
5.6 The Electron Transport System (cont.)
• Bacteria that can survive for long periods with or
without oxygen, switching between fermentation and
aerobic respiration, are called facultative aerobes.
• Bacteria that are poisoned by oxygen and generate
ATP entirely from fermentation or anaerobic
respiration are called obligate anaerobes.
• Most organisms, such as animals and plants, are
obligate aerobes; they cannot survive for long
without oxygen.
The Reactions of Respiration
5.6 The Electron Transport System (cont.)
The Reactions of Respiration
5.7 Oxygen, Respiration, and Photosynthesis
• Oxygen is needed to oxidize glucose.
– Without oxygen, cells must ferment glucose,
forming only two ATP molecules per glucose
molecule.
– With oxygen present, organisms gain much more
energy from their food.
The Reactions of Respiration
5.7 Oxygen, Respiration, and Photosynthesis
(cont.)
• In general, the products of photosynthesis—
oxygen and carbohydrates—are the raw materials
for cell respiration.
• Cell respiration, in turn, provides the raw materials
for photosynthesis (carbon dioxide and water).
The Reactions of Respiration
5.7 Oxygen, Respiration, and Photosynthesis
(cont.)
Respiration releases chemical energy by using the reduction of oxygen to
water to drive the oxidation of sugar to carbon dioxide. Photosynthesis
stores chemical energy by using the oxidation of water to oxygen to drive
the reduction of carbon dioxide to sugar.
Respiration and Cellular Activities
5.8 The Krebs Cycle in Fat and Protein Metabolism
• The release of energy from fats and proteins also
involves the Krebs cycle.
• When cells use the fatty acids of fats for energy,
enzymes in the mitochondria break down the fatty
acids to acetate which coenzyme A transfers to the
Krebs cycle.
• Without oxygen, most of the energy in fat cannot be
transferred to ATP.
Respiration and Cellular Activities
5.8 The Krebs Cycle in Fat and Protein Metabolism
• When cells use proteins in respiration,
digestive enzymes first break down the proteins
to amino acids.
(cont.)
• Other enzymes remove the amino groups and
convert the ammonia this produces to safer nitrogen
compounds.
• The carbon skeletons remaining from some amino
acids can undergo reactions that form 4- or 5-carbon
acids (oxaloacetate or ketoglutarate), which can
enter the Krebs cycle.
The reactions of cell respiration and
particularly of the Krebs cycle contribute
to both the decomposition and
biosynthesis of carbohydrates, fats, and
proteins. Certain amino acids can be
synthesized from the carbon skeletons
by adding amino groups (—NH2)
derived from ammonia (NH3). Carbon
skeletons can be formed from amino
acids by removing the amino groups.
Most organisms cannot convert fat to
carbohydrate.
Respiration and Cellular Activities
5.8 The Krebs Cycle in Fat and Protein Metabolism
• The Krebs cycle and glycolysis also provide
building blocks for biosynthesis.
– In autotrophs, these pathways, along with the
Calvin cycle, lead to the synthesis of every
organic compound the organism needs.
– In heterotrophs, these pathways lead to the
synthesis of most, but not all, of the necessary
organic compounds.
(cont.)
Respiration and Cellular Activities
5.8 The Krebs Cycle in Fat and Protein Metabolism
• Most synthesis pathways are not the reverse of
decomposition pathways.
(cont.)
• Separate enzymes and pathways for synthesis and
decomposition help cells operate efficiently and
control the activities of these pathways.
• Most biological decompositions involve hydrolysis,
a type of decomposition that inserts the components
of water (H and OH) into a bond to break it.
In each example of a hydrolysis reaction, bonds are
broken in a water molecule and another molecule.
Respiration and Cellular Activities
5.9 Respiration and Heat Production
• Cell respiration releases heat energy, which helps
many organisms keep warm.
• Some mammals have brown fat which contains more
mitochondria than any other body tissue and is
adapted for rapid production of thermal energy.
The arctic ground squirrel, Spermophilus parryi,
spends its summer on the Arctic tundra. In the
winter, it hibernates in its nest. Brown fat
enables the ground squirrel to quickly elevate
its body temperature at the end of hibernation.
Respiration and Cellular Activities
5.9 Respiration and Heat Production (cont.)
• Many plants have also evolved a form of respiration
that produces a great deal of heat energy using an
alternate branch of the electron transport system.
• In this pathway, some of the energy of electron flow
results in the production of more heat energy and
less ATP than in normal respiration.
Skunk cabbage, Symplocarpus foetidus,
releases heat energy that melts snow as
it emerges from the ground.
Respiration and Cellular Activities
5.10 Control of Respiration
• Organisms must control their rate of respiration in
order to direct energy and carbon skeletons
accurately to where they are needed.
• Control is critical to organization, and cells must be
organized to survive.
• The mechanisms that control whether glucose is
broken down in respiration or converted to starch or
fat operate by supply and demand.
Energy regulation in animals
Click the image to view an animated version.
Summary
• Metabolism consists of all the chemical reactions in an
organism, including biosynthesis and degradation.
• Cell respiration involves reactions that oxidize carbohydrates,
fats, or amino acids, with a release of energy that is conserved
as ATP.
• Cell respiration also provides carbon skeletons for the
biosynthesis of macromolecules the cell requires.
• Supply and demand in the cell determine whether carbon
skeletons in this pathway are oxidized or used in biosynthesis.
• Three stages of aerobic respiration are: glycolysis, the Krebs
cycle, and the electron transport system.
Summary (cont.)
• Glycolysis, the first stage of both aerobic respiration and
fermentation, produces a small quantity of ATP and NADH.
• In the absence of oxygen, fermentation occurs.
• Fermentation results in the net synthesis of only two
molecules of ATP per molecule of glucose.
• In the presence of oxygen, aerobic respiration occurs.
• Pyruvate from glycolysis is transported into a mitochondrion
(except in bacteria).
Summary (cont.)
• Coenzyme A carries the acetyl group to the Krebs-cycle
enzymes and produces ATP, NADH, and FADH2.
• Hydrogen atoms carried by NADH and FADH2 are used to
synthesize ATP.
• The complete aerobic respiration of a molecule of glucose
forms 6 molecules of carbon dioxide and a maximum of
38 molecules of ATP.
• Cell respiration and energy production are closely regulated.
• Some plants and animals have special forms of cell
respiration that release larger quantities of heat energy
and produce less ATP.
Reviewing Key Terms
Match the term on the left with the correct description.
___
facultative aerobes
b
___
hydrolysis
e
___
coenzyme A
c
___
cytochromes
a
___
obligate anaerobes
d
a. an electron carrying
pigment in electron
transport systems
b. bacteria that can survive
with or without oxygen
c. a small molecule that
delivers acetate to the
Krebs cycle
d. organisms that cannot
survive in the presence
of oxygen
e. the splitting of a molecule
by reaction with water
Reviewing Ideas
1. Which is more efficient, aerobic or anaerobic
respiration? Explain.
Aerobic respiration is much more efficient than
anaerobic respiration. Aerobic respiration
generates up to 38 ATP molecules from a single
glucose molecule. Anaerobic respiration generates
only two ATP molecules per glucose molecule.
Reviewing Ideas
2. How are photosynthesis and respiration
symbiotically related?
The products of photosynthesis—oxygen and
carbohydrates—are the raw materials for cell
respiration. Cell respiration, in turn, provides the
raw materials—carbon dioxide and water—for
photosynthesis.
Using Concepts
3. Why is oxidizing glucose a in a series of small
steps important?
Oxidizing one molecule of glucose releases much
more energy than a single reaction needs,
however, so glucose is not useful as a direct
source of energy. To release all the energy in
glucose at once would waste most of the energy
and could heat the organism until it cooked itself.
Using Concepts
4. Premature human infants often lack a normal
layer of brown fat. What complications can
might this cause? Why?
The infant may have trouble regulating its body
temperature. Brown fat contains more mitochondria
than any other body tissue and is adapted for the
production of thermal energy. Respiration of stored
fat in brown fat cells produces much heat energy
and little ATP.
Synthesize
5. What similarities exist between photosynthesis
and respiration?
The process of evolution has organized cell
respiration and photosynthesis in similar ways. As
in photosynthesis, the electron transport system of
cell respiration is embedded in a membrane. Just
as chloroplasts contain and organize the enzymes
of photosynthesis in nonbacterial cells, specialized
structures called mitochondria provide efficiency
and organization to cell respiration. Like a
chloroplast, a mitochondrion has an outer and an
inner membrane.
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Chapter Animations
Products of cell respiration
The mitochondrial electron transport system
Energy regulation in animals
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