Capturing and Releasing Energy

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Capturing and Releasing Energy

• Chapter 5

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5.1 Impacts/IssuesGreen 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

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Biofuels
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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

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Green Energy

• Current biofuel research focuses on ways to break
  down abundant cellulose in fast growing weeds and
  agricultural wastes

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Solar power
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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

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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

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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

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Wavelength and theElectromagnetic Spectrum
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Some Photosynthetic Pigments
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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

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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

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Chloroplasts and the Thylakoid Membrane

• Chloroplast
• Organelle of photosynthesis in plants and some
  protists
• Thylakoid membrane
• Chloroplasts highly folded inner membrane system
• Forms a continuous compartment in the stroma

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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

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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
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Sites of photosynthesis
green spots are chloroplast
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Summary Photosynthesis

• 6CO2 6H2O ? (light energy) ? C6 H12O6 6O2

A Chloroplast
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Stepped Art
Fig. 5-4, p. 84
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Chemical bookkeeping
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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

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Light-Dependent Reactions
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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

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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

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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

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Light-Dependent Reactions
to light-independent reactions
light energy
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
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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

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Carbon Fixation

• In the stroma of chloroplasts, the enzyme rubisco
  fixes carbon from CO2 in the CalvinBenson cycle
• Carbon fixation
• Process by which carbon from an inorganic source
  such as CO2 becomes incorporated into an organic
  molecule

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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

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Light-Independent Reactions
chloroplast
CO2, H2O
stroma
PGA
RuBP
ATP
Calvin Benson Cycle
ATP
NADPH
sugars
Fig. 5-6, p. 86
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Calvin-Benson cycle
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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

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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

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Fig. 5-7d, p. 87
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5.6 Photosynthesis and Aerobic Respiration A
Global Connection

• Earths atmosphere was permanently altered by the
  evolution of photosynthesis

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Oxygen and the Atmosphere

• Photoautotroph
• Photosynthetic autotroph
• Anaerobic
• Occurring in the absence of oxygen
• Aerobic
• Involving or occurring in the presence of oxygen

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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

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Photosynthesis and Aerobic Respiration
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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

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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

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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

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Acetyl CoA Formation and the Krebs Cycle
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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 mitochondrions
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
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Second Stage of Aerobic Respiration
2 pyruvate
outer membrane (next to cytoplasm)
inner membrane
6 CO 2
2 acetylCoA
2
ATP
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.
8 NADH
Krebs Cycle
2 FADH2
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
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The Krebs Cycle - details
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Electron Transfer Phosphorylation
Third Stage of Aerobic Respiration Electron
Transfer Phosphorylation
Stepped Art
Fig. 5-11, p. 91
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Summary Aerobic Respiration

• C6H12O6 (glucose) 6O2 (oxygen) 36 ADP ?
• 6CO2 (carbon dioxide) 6H2O (water) 36 ATP

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Summary Aerobic Respiration
Stepped Art
Fig. 5-9, p. 89
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Overview of aerobic respiration
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Where pathways start and finish
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Third-stage reactions
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Mitochondrial chemiosmosis
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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

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Alcoholic Fermentation

• Alcoholic fermentation
• Anaerobic pathway that converts pyruvate to
  ethanol and produces ATP
• Examples baking, wine production

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NADH
NAD

carbon dioxide
pyruvate
acetaldehyde
ethanol
Fermentation pathways
NADH
NAD
pyruvate
lactate
Fig. 5-12b, p. 92
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Lactate Fermentation

• Lactate fermentation
• Anaerobic pathway that converts pyruvate to
  lactate and produces ATP
• Examples cheese, pickles

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5.8 Alternative Energy Sources in the Body

• Carbohydrates
• Fats
• Proteins

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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

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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

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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

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Alternative Energy Sources in the Body
Stepped Art
Fig. 5-14, p. 95
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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

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Fossil Fuel Emissions
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Biofuels of the Future
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Digging Into DataEnergy Efficiency of Biofuel
Production