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Visual Anatomy & Physiology
First Edition
Martini & Ober
Chapter 22
(Sections 22.1-22.4)
Metabolism & Cellular Respiration
Lecture 8
Lecture Overview
• Enzymes and control of metabolic reactions
• Energy and metabolic reactions
• Cellular respiration
–
–
–
–
Overview
ATP as the biological energy carrier
Oxidation/Reduction
Steps in Cellular Respiration
• Glycolysis
• The Citric Acid (Krebs) Cycle
• The Electron Transport Chain
• Relationship of anabolism to catabolism
2
Enzymes and Metabolic Reactions
Enzymes – Biological catalysts
• control rates of metabolic reactions
• lower activation energy needed to start reactions
• globular proteins with specific shapes
• not consumed in
chemical reactions
• substrate specific
• shape of active
site determines
which substrate(s)
the enzyme can act
on
Figure from: Hole’s Human A&P, 12th edition, 2010
3
Enzymes Lower Activation Energy
Enzymes lower the
barriers that block
chemical reactions, i.e.,
they lower the
activation energy
needed to begin
energetically favorable
reactions
5
Control of Metabolic Reactions
Metabolic pathways
• series of enzyme-controlled reactions leading to formation of a
product
• each new substrate is the product of the previous reaction
Enzyme names commonly
• reflect the substrate
• have the suffix – ase
• sucrase, lactase,
protease, lipase,
hydrolase, oxidase
Factors that alter activity of
enzymes
• heat
• radiation
• substrate concentration
• required cofactors
• changes in pH
6
Cofactors and Coenzymes
Cofactors
• make some enzymes
active
• ions or coenzymes
Coenzymes
• complex organic molecules
that act as cofactors (so
coenzymes ARE cofactors)
• vitamins
• NAD+
Vitamins are essential organic substances that human
cells cannot synthesize, i.e., they must come from the diet
- required in very small amounts
- examples - B vitamins: Thiamine (B1), niacin
The protein parts of enzymes that need a nonprotein part
(coenzymes, cofactors) to work are called apoenzymes
7
Overview of Cellular Metabolism
Metabolism – All
the chemical
reactions that
occur in an
organism
ETS
KEEP THIS
OVERALL
SCHEME IN
MIND AS WE
GO INTO
DETAILS!!
Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001
8
Overview of Glucose Breakdown
9
Figure from: Hole’s Human A&P, 12th edition, 2010
Overview of Catabolism
Figure from: Martini, Visual Anatomy & Physiology, Pearson, 2011
11
A Closer Look at Mitochondria
Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001
(Impermeable to charged or polar molecules)
Strategically
placed in cell
where ATP
demand is high
Concentration of enzymes in the matrix is so high that there is
virtually no hydrating water. Enzyme-linked reactions and
pathways are so crowded that normal rules of diffusion do not apply!
12
Carbohydrate Metabolism
• Most cells generate ATP and other energyyielding compounds via the catabolism of
carbohydrate (and fats)
General Reaction sequence in carbohydrate catabolism
C6H12O6 + 6 O2  6 CO2 + 6 H2O + ENERGY
If the above reaction happened all at once, all the chemical
energy contained in the carbohydrate would be
DISSIPATED AS HEAT.
How does the body harness the energy from carbohydrates?
13
Harnessing Energy - Stepwise Breakdown of Carbohydrates
Figure from: Alberts et al., Essential Cell Biology, Garland Press, 1998
14
Energy for Metabolic Reactions
Energy
• ability to do work or change something (potential, kinetic)
• heat, light, sound, electricity, mechanical energy, chemical
energy
• changed from one form to another, but NEVER destroyed
(law of conservation of energy)
• involved in all metabolic reactions
Release of chemical energy
• most metabolic processes depend on chemical energy
• oxidation of glucose generates chemical energy
• cellular respiration releases chemical energy slowly from
molecules and makes it available for cellular use
15
Oxidation and Reduction Revisited
Oxidation
• gain of O2
• loss of e• loss of H (since a H carries an electron with it)
• increase in oxidation number, e.g., Fe2+ -> Fe3+
Reduction
• loss of O2
• gain of e• gain of H
• decrease in oxidation number, e.g., Fe3+ -> Fe2+
Oxidation Is Loss of electrons; Reduction Is Gain of electrons
“OIL RIG”
16
Energy of Organic Molecules
• Carbohydrates like glucose store a great deal
of chemical energy (as H·)
• As carbohydrates (C6H12O6) are oxidized to
CO2 they liberate their energy and lose
electrons and H (H·)
• But there must be molecules to accept these
electrons, i.e., some molecules must be
reduced.
• In cellular respiration, O2 becomes the final
electron (H·) acceptor and is reduced to H2O
17
Harnessing Energy from Carbohydrates
General Reaction sequence in carbohydrate catabolism
OXIDATION
C6H12O6 + 6 O2  6 CO2 + 6 H2O + ENERGY
REDUCTION
Electrons (H·) “fall” in energy from organic molecules
to oxygen during cellular respiration.
That is, e- LOSE potential energy during this process
and this energy is captured to make ATP
However, electrons CANNOT be transferred directly
from glucose to the electron transport chain. There
are intermediates – activated carrier molecules
18
Activated Carrier Molecules
Some activated carriers: ATP, NADH, FADH2, GTP, NADPH
Figure from: Alberts et al., Essential Cell Biology, Garland Press, 1998
19
ATP – An Activated Carrier Molecule
Figure from: Hole’s Human A&P, 12th edition, 2010
• each ATP molecule has three parts:
• an adenine molecule These two components
together are called a ?
• a ribose molecule
• three phosphate molecules in a chain
• ATP carries its energy in the form or P
(phosphate)
• ATP is a readily interchangeable form
of energy for cellular reactions
(“common currency”)
High-energy
bonds
20
NAD(H) – An Activated Carrier Molecule
NAD+
NAD (and NADP) are
specialized to carry
high-energy e- and H
atoms
A “packet” of energy =
H·
NADH + H+
NAD+
NADH
These packets of energy will be passed to oxygen in
the electron transport chain, and their energy used
to drive the synthesis of ATP
Important carriers of e- in catabolism: NADH, FADH2
Figure from: Alberts et al., Essential Cell Biology, Garland Press, 1998
21
Overview of Cellular Respiration
Anaerobic
Cellular
respiration
(aerobic)
Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001
22
Overview of Glucose Breakdown
Figure from: Hole’s Human A&P, 12th edition, 2010
23
Overview of Glucose Breakdown
Occurs in three major of reaction series…
1. Glycolysis (glucose to pyruvate; in cytoplasm)
2. Citric acid cycle (finishes oxidation begun in glycolysis;
in the matrix of mitochondria)
3. Electron transport chain (uses e- transfer to make ATP;
on inner membranes of mitochondria)
Produces
• carbon dioxide
• water
• ATP (chemical energy)
• heat (energy has changed form from chemical)
Includes
• anaerobic reactions (without O2) - produce little ATP
• aerobic reactions (requires O2) - produce most ATP
24
Glycolysis
• series of ten reactions
• breaks down glucose (6C) into 2 molecules of pyruvic acid
(pyruvate) (3C)
• occurs in cytosol
• anaerobic phase of cellular respiration (that is, it can
continue to work with OR without O2)
• yields 2 ATP and 2 NADH molecules per glucose
ADP + Pi
Glucose (6C)
ATP
2 Pyruvate (3C)
NAD+
NADH
25
Overview of Glycolysis
glucose (6C) → 2 pyruvate (3C)
Figure from: Hole’s Human A&P, 12th edition, 2010
Products of glycolysis
- ATP
- NADH
- Pyruvate
NOTE what happens with and
without O2 being available…
26
Metabolism of Pyruvate Without O2
• process of forming lactate from glucose is anerobic glycolysis
• important for regenerating NAD+ so glycolysis can continue
to generate ATP for the cell
O2
Pyruvic acid (pyruvate)
Lactic acid (lactate)
NADH + H+
NAD+
NAD+
NADH + H+
Glucose (6C)
2 Pyruvate (3C)
ADP + Pi
ATP
27
Overview of Aerobic Reactions
Figure from: Hole’s Human A&P, 12th edition, 2010
If oxygen is available –
• pyruvic acid is used
to produce acetyl CoA
• citric acid (Krebs)
cycle begins
• electron transport
chain functions
• carbon dioxide and
water are formed
• maximum of 36
molecules of ATP
produced per glucose
molecule
28
Citric Acid Cycle
Figure from: Hole’s Human A&P, 12th edition, 2010
In mitochondria…
What happens…
- Acetyl CoA enters cycle
- Citric Acid is converted to
various intermediates
- Several important products
are produced in these
interconversions of citric
acid…
• ATP is produced
•NAD+ is reduced to
NADH and FAD is
reduced to FADH2
• CO2 produced
29
Source of e- for the Electron Transport Chain
Figure from: Hole’s Human
A&P, 12th edition, 2010
Notice the flow
of electrons to
the Electron
Transport Chain
30
Electron Transport Chain
• NADH and FADH2 carry electrons to the ETC
• ETC series of electron carriers located in cristae of
mitochondria
• energy from electrons transferred to ATP synthase
• ATP synthase catalyzes the oxidative phosphorylation of
ADP to ATP
• water is formed
*Chemiosmosis
Figure from: Hole’s Human A&P, 12th edition, 2010
31
Oxidative Phosphorylation
Chemiosmosis, Chemiosmotic coupling,
or Chemiosmotic phosphorylation
Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001
32
Summary of Cellular Respiration
Figure from:
Martini,
Anatomy &
Physiology,
Prentice Hall,
2001
33
Is This All the
Metabolism
There Is?
Definitely NOT!
Notice how
many lines
connect
pyruvate
and Acetyl
CoA to the
rest of
metabolism
34
Intermediates of Metabolism
Acetyl CoA (2C) and
Pyruvate (3C) are
important:
Allow interconversion
of different types of
molecules so cell’s needs
can be met
Figure from: Martini, Visual Anatomy & Physiology, Pearson, 2011
35
Figure from: Hole’s Human A&P, 12th edition, 2010
Summary of
Catabolism of
Proteins,
Carbohydrates,
and Fats
Acetyl CoA is a
common intermediate
in the breakdown of
most fuels.
Acetyl CoA can be
generated by
carbohydrates, fats,
or amino acids
Acetyl CoA can be
converted into fatty
acids
36
Pyruvate is a Key Junction in Metabolism

Lipogenesis
Lipolysis

*
Glycogenolysis


Glycogenesis
Pyruvate is used
to synthesize
amino acids and
Acetyl CoA
Pyruvate can
also be used to
synthesize
glucose via
gluconeogenesis.
Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001
37
Carbohydrate Storage
Excess glucose can be
• stored as glycogen by glycogenesis (liver and muscle cells)
• stored as fat by lipogenesis
• converted to amino acids
38
Figure from: Hole’s Human A&P, 12th edition, 2010
Terms to Know…
-olysis  breakdown of
-genesis  creation of
-neo  new
• Glycolysis – metabolism of glucose to pyruvate
• Gluconeogenesis – metabolism of pyruvate to
glucose (making CHO from non-CHO source)
• Glycogenesis – metabolism of glucose to glycogen
• Glycogenolysis – metabolism of glycogen to
glucose
• Lipolysis – breakdown of triglyceride into
glycerol and fatty acids
• Lipogenesis – creation of new triglyceride (fat)
39
Review
• Enzymes are biological catalysts
–
–
–
–
–
Highly specific for their substrate
Lower activation energy needed to start a reaction
Are not consumed during reaction
May require cofactors/coenzymes
Effectiveness is greatly affected by temperature,
pH, and the presence of required cofactors
• The goal of metabolism is to provide the cell
with energy (catabolism) and materials for the
manufacture of cellular components
(anabolism)
40
Review
• Cells derive energy mainly from carbon
compounds like carbohydrates and fats
– These substances contain a great deal of energy
stored in their chemical bonds
– This energy must be liberated in stepwise
fashion
– Activated carriers serve as intermediates to
capture the energy liberated at each step
• Energy is the ability to do work
– May be potential or kinetic
– Changes form
– Is never destroyed (only converted to another
form)
41
Review
• Anabolism is intimately tied to catabolism
– Energy derived from catabolism is used to drive anabolic
reactions
– Some molecules are important junctions between
catabolism and anabolism
– Acetyl CoA
• Generated from pyruvate, fatty acids, and amino acids
• Can be used to synthesize fatty acids and other molecules
• CANNOT be used to generate pyruvate
– Pyruvate
• Can be synthesized from glucose and amino acids
• can be used to synthesize amino acids, glucose and acetyl CoA
– NOTE, however, that in humans fatty acids cannot be
converted to glucose
44