Capturing and Releasing Energy
Chapter 5
5.1 Impacts/Issues
Green Energy
 We and most other organisms sustain ourselves
by extracting energy stored in the organic
products of photosynthesis
 Photosynthesis
• Metabolic pathway by which photoautotrophs
capture light energy and use it to make sugars
from CO2 and water
Biofuels
Green Energy
 Autotroph
• Organism that makes its own food using carbon
from inorganic sources, such as CO2, and energy
from the environment
 Heterotroph
• Organism that obtains energy and carbon from
organic compounds assembled by other
organisms
Green Energy
 Current biofuel research focuses on ways to
break down abundant cellulose in fast growing
weeds and agricultural wastes
Solar power
5.2 Capturing Rainbows
 Energy radiating from the sun travels through
space in waves and is organized in packets
called photons
 The spectrum of radiant energy from the sun
includes visible light
Capturing Rainbows
 Humans perceive different wavelengths of
visible light as different colors
 The shorter the wavelength, the greater the
energy
 Wavelength
• Distance between the crests of two successive
waves of light
Capturing Rainbows
 Photosynthetic species use pigments to harvest
light energy for photosynthesis
 Pigment
• An organic molecule that can absorb light at
specific wavelengths
 Chlorophyll a
• Main photosynthetic pigment in plants
Wavelength and the
Electromagnetic Spectrum
Some Photosynthetic Pigments
5.3 Storing Energy in Carbohydrates
 Photosynthesis converts the energy of light into
the energy of chemical bonds, which can power
reactions of life and be stored for later use
 Photosynthesis takes place in two stages
• Light-dependent reactions
• Light-independent reactions
The First Stage of Photosynthesis
 Light-dependent reactions (“photo”)
• Convert light energy to chemical energy of ATP
and NADPH, releasing oxygen
• Occur at the thylakoid membrane in plant
chloroplasts
 Photosystem
• Cluster of pigments and proteins that converts
light energy to chemical energy in photosynthesis
Chloroplasts and
the Thylakoid Membrane
 Chloroplast
• Organelle of photosynthesis in plants and some
protists
 Thylakoid membrane
• Chloroplast’s highly folded inner membrane system
• Forms a continuous compartment in the stroma
The Second Stage of Photosynthesis
 Light-independent reactions (“synthesis”)
• ATP and NADPH drive synthesis of glucose and
other carbohydrates from water and CO2
• Occurs in the stroma
 Stroma
• Semifluid matrix between the thylakoid
membrane and the two outer membranes of a
chloroplast
A Many photosynthetic cells in a leaf
B Many chloroplasts
in a photosynthetic
cell
A Leaf: Sites of Photosynthesis
C Many thylakoids
in a chloroplast
Fig. 5-3, p. 83
Sites of photosynthesis
“green spots” are
chloroplast
Summary: Photosynthesis
6CO2 + 6H2O
6O2
→ (light energy) → C
6
A Chloroplast
H12O6 +
light water
lightdependent
reactions
oxygen
carbon dioxide, water
NADPH, ATP
NADP+, ADP
lightindependent
reactions
glucose
Stepped Art
Fig. 5-4, p. 84
Chemical bookkeeping
5.4 The Light-Dependent Reactions
 Chlorophylls and other pigments in the thylakoid
membrane absorb light energy and pass it to
photosystems, which then release electrons
 Energized electrons leave photosystems and
enter electron transfer chains in the membrane;
hydrogen ion gradients drive ATP formation
 Oxygen is released; electrons end up in NADPH
Light-Dependent Reactions
Steps in Light-Dependent Reactions
1. Light energy ejects electrons from a photosystem
2. Photosystem pulls replacement electrons from
water, releasing O2
3. Electrons enter an electron transfer chain (ETC)
in the thylakoid membrane
4. Electron energy is used to form a hydrogen-ion
gradient across the thylakoid membrane
Steps in Light-Dependent Reactions
5. Another photosystem receives electrons from
the ETC
6. Electrons move through a second ETC; NADPH
is formed
7. Hydrogen ions flow across the thylakoid
membrane through ATP synthase and power
ATP formation in the stroma
Electron Transfer Phosphorylation
 Electron transfer phosphorylation
• Metabolic pathway in which electron flow through
electron transfer chains sets up a hydrogen ion
gradient that drives ATP formation
Light-Dependent Reactions
light energy
to light-independent
reactions
light energy
4
7
5
1
stroma
3
6
2
thylakoid compartment
thylakoid membrane
The Light-Dependent Reactions of Photosynthesis
Fig. 5-5, p. 85
5.5 The Light-Independent Reactions
 Driven by the energy of ATP and electrons from
NADPH, light-independent reactions use carbon
and oxygen from CO2 to build sugars
Carbon Fixation
 In the stroma of chloroplasts, the enzyme
rubisco fixes carbon from CO2 in the Calvin–
Benson cycle
 Carbon fixation
• Process by which carbon from an inorganic
source such as CO2 becomes incorporated into
an organic molecule
Calvin-Benson Cycle
 Calvin-Benson cycle
• Light-independent reactions of photosynthesis
• Cyclic pathway that forms glucose from CO2
• Uses energy from ATP and electrons from
NADPH
 Rubisco
• Enzyme that fixes carbon from CO2 to RuBP in
the Calvin-Benson cycle
Light-Independent Reactions
chloroplast
CO2, H2O
stroma
PGA
ATP
NADPH
RuBP
Calvin–
Benson
Cycle
ATP
sugars
Fig. 5-6, p. 86
Calvin-Benson cycle
Carbon-Fixing Adaptations
 Several adaptations, such as a waterproof
cuticle, allow plants to live where water is scarce
 Stomata
• Gaps that open between guard cells on plant
surfaces; allow gas exchange through the cuticle
 C3 plants
• Use only the Calvin-Benson cycle to fix carbon
• Conserve water by closing stomata on dry days
Photorespiration
 When stomata are closed, oxygen builds up and
interferes with sugar production
 Photorespiration
• Reaction in which rubisco attaches O2 instead of
CO2 to RuBP
Fig. 5-7d, p. 87
5.6 Photosynthesis and Aerobic
Respiration: A Global Connection
 Earth’s atmosphere was permanently altered by
the evolution of photosynthesis
Oxygen and the Atmosphere
 Photoautotroph
• Photosynthetic autotroph
 Anaerobic
• Occurring in the absence of oxygen
 Aerobic
• Involving or occurring in the presence of oxygen
Extracting Energy From Carbohydrates
 Eukaryotic cells typically convert chemical
energy of carbohydrates to chemical energy of
ATP by oxygen-requiring aerobic respiration
 Aerobic respiration
• Aerobic pathway that breaks down carbohydrates
to produce ATP
• Pathway finishes in mitochondria
Photosynthesis and Aerobic Respiration
An Overview of Aerobic Respiration
 Aerobic respiration is divided into three steps
1. Glycolysis
2. Acetyl CoA formation and the Krebs cycle
3. Electron transfer phosphorylation
 In the first two stages, coenzymes pick up
electrons
 In the third stage, electron energy drives ATP
synthesis
Aerobic Respiration Begins
 Glycolysis
• Reactions in which glucose or another sugar is
broken down into 2 pyruvates, netting 2 ATP
 Pyruvate
• Three-carbon product of glycolysis
Aerobic Respiration Continues
 Krebs cycle
• Cyclic pathway that, along with acetyl CoA
formation, breaks down pyruvate to CO2, netting
2 ATP and many reduced coenzymes
Acetyl CoA Formation
and the Krebs Cycle
Mitochondrion
outer membrane
(next to cytoplasm)
inner membrane
inner mitochondrial
compartment
outer mitochondrial
compartment (in
between the two
membranes)
A An inner membrane divides a mitochondrion’s interior
into an inner compartment and an outer compartment. The
second and third stages of aerobic respiration take place
at the inner mitochondrial membrane.
Fig. 5-10a, p. 90
Second Stage of Aerobic Respiration
2 pyruvate
outer membrane
(next to cytoplasm)
inner membrane
2 acetyl–CoA
6 CO 2
2
Krebs
Cycle
ATP
8 NADH
2 FADH2
Breakdown of 2 pyruvate to 6 CO2
yields 2 ATP. Also, 10 coenzymes
(8 NAD+, 2 FAD) combine with
electrons and hydrogen ions,
which they carry to the third and
final stage of aerobic respiration.
B The second stage starts after membrane proteins transport
pyruvate from the cytoplasm to the inner compartment. Six
carbon atoms enter these reactions (in two molecules of
pyruvate), and six leave (in six CO2). Two ATP form and ten
coenzymes accept electrons and hydrogen ions.
Fig. 5-10b, p. 90
The Krebs Cycle - details
Electron Transfer Phosphorylation
Third Stage of Aerobic Respiration: Electron Transfer Phosphorylation
4
2
3
5
1
Stepped Art
Fig. 5-11, p. 91
Summary: Aerobic Respiration
C6H12O6 (glucose) + 6O2 (oxygen) + 36 ADP
→
6CO2 (carbon dioxide) + 6H2O (water) + 36 ATP
Summary: Aerobic Respiration
Aerobic Respiration
glucose
Cytoplasm
2 ATP
ATP
Glycolysis
4 ATP ATP
(2 net)
2 NADH 2 pyruvate
A The first stage, glycolysis, occurs in the
cell’s cytoplasm. Enzymes convert a glucose
molecule to 2 pyruvate for a net yield of 2 ATP. 2
NAD + combine with electrons and hydrogen
ions during the reactions, so 2 NADH also form.
Mitochondrion
Krebs
Cycle
6 CO2
2 ATP ATP
B The second stage occurs in mitochondria.
The 2 pyruvate are converted to a molecule that
enters the Krebs cycle. CO2 forms and leaves
the cell. 2 ATP, 8 NADH, and 2 FADH2 form
during the reactions.
C The third and final stage, electron transfer
ATP phosphorylation, occurs inside mitochondria.
10 NADH and 2 FADH2 donate electrons and
ATP
ATP hydrogen ions to electron transfer chains.
Electron flow through the chains sets up
H2O
hydrogen ion gradients that drive ATP
32 ATP
formation. Oxygen accepts electrons at the end
of the chains.
8 NADH, 2 FADH2
oxygen
Electron Transfer
Phosphorylation
Stepped Art
Fig. 5-9, p. 89
Overview of aerobic respiration
Where pathways start and finish
Third-stage reactions
Mitochondrial chemiosmosis
5.7 Fermentation
 Fermentation
• Anaerobic pathway that harvests energy from
carbohydrates
• Alcoholic fermentation and lactate fermentation
 In fermentation, ATP is formed by glycolysis only
• Net yield of 2 ATP per glucose molecule
• Coenzyme NAD+ is regenerated, which allows
glycolysis to continue
• Fermentation pathways finish in the cytoplasm
Alcoholic Fermentation
 Alcoholic fermentation
• Anaerobic pathway that converts pyruvate to
ethanol and produces ATP
• Examples: baking, wine production
NADH
NAD+
+
pyruvate
carbon
dioxide
acetaldehyde
ethanol
Fermentation pathways
NADH NAD+
pyruvate
lactate
Fig. 5-12b, p. 92
Lactate Fermentation
 Lactate fermentation
• Anaerobic pathway that converts pyruvate to
lactate and produces ATP
• Examples: cheese, pickles
5.8 Alternative Energy Sources in the Body
 Carbohydrates
 Fats
 Proteins
Energy from Carbohydrates
 Glucose is absorbed from the intestines into the
blood and broken down by glycolysis
 Blood glucose levels are regulated by the
pancreatic enzymes insulin and glucagon
 Excess glucose intake stimulates storage as
glycogen and fatty acids
Energy from Fats
 The body stores most fats as triglycerides
 When blood glucose falls, enzymes break
triglycerides into glycerol and fatty acids
• Glycerol enters glycolysis
• Fatty acids enter the Krebs cycle as acetyl-CoA
 Fatty acids yield more energy (ATP) than carbs
Energy from Proteins
 Proteins enter the bloodstream as amino acids
 Amino acids can be used for energy by
removing the amino group (as ammonia) and
converting the carbon backbone to acetyl-CoA,
pyruvate, or an intermediate of the Krebs cycle
Food
Complex Carbohydrates
Fats
fatty acids
glycerol
Proteins
glucose, other simple sugarsamino acids
acetyl–CoA intermediate
of glycolysis
acetyl–CoA
Glycolysis
NADH pyruvate
intermediate
of Krebs cycle
Krebs
Cycle
Alternative Energy
Sources in the Body
NADH, FADH2
Electron Transfer
Phosphorylation
Stepped Art
Fig. 5-14, p. 95
5.9 Impacts/Issues Revisited
 Human activities are disrupting the global
cycling of carbon dioxide; we are adding more
CO2 to the atmosphere than photoautotrophs
are removing from it
 The resulting imbalance fuels global warming
Fossil Fuel Emissions
Biofuels of the Future
Digging Into Data:
Energy Efficiency of Biofuel Production