Chapter 7 Energy-Acquiring Pathways

I. Organisms, Carbon, and Energy
A. All life forms must obtain carbon and energy.
B. Autotrophs are “self-nourishing.”
1. They obtain carbon from carbon dioxide.
2. Photoautotrophs harness light energy.
3. Chemoautotrophs extract energy from chemical reactions.
C. Heterotrophs feed on organisms or organic wastes.
1. Heterotrophs acquire carbon and energy from autotrophs.
2. Heterotrophs are animals, protistans, bacteria, and fungi.

II. Photosynthesis
A. Simplified Picture of Photosynthesis
1. The light-dependent reactions convert light energy to
chemical energy stored in ATP and NADPH.
2. The light-independent reactions assemble organic
molecules using energy from ATP and NADPH.
3. Overall, for glucose formation:
12H₂O + 6CO₂ ---sunlight--> 6O₂ + C₆H₁₂O₆ + 6H₂O
B. Chloroplast Structure and Function
1. Light-dependent reactions occur in the thylakoid system.
a. The thylakoids are folded into grana (stacks of disks).
b. The interior spaces of the thylakoid disks are filled with
H⁺ needed during ATP synthesis.
2. Carbohydrate formation occurs in the stroma area that
surrounds the grana.











III. Light-Dependent Reactions
A. Overview
1. Pigments absorb light energy and give up electrons.
2. Electron and hydrogen transfers form ATP and NADPH.
3. Electrons are replaced in the pigment molecules.
B. Light Absorption
1. Light-Trapping Pigments
a. Chlorophyll pigments absorb blue and red but reflect
green.
b. Carotenoid pigments absorb violet and blue but reflect
yellow, orange, and red.
2. Photosystems
a. A photosystem is a cluster of 200 to 300 light-absorbing
pigments located in the thylakoid.
b. The pigments “harvest” sunlight.
1) Absorbed light boosts electrons to a higher level.
2) The electrons quickly return to the lower level and
release energy.
3) Released energy is trapped by chlorophylls, which act
as a sink for energy harvested by all pigments.
c. The trapped energy is then used to transfer a chlorophyll
electron to an acceptor molecule.
C. How ATP and NADPH Form in Chloroplasts
1. Overview
a. Electrons expelled from chlorophyll go through one or
two electron transport systems in the thylakoid
membranes.
b. As the electron passes from one molecule to another in
each system, phosphate is added to ADP to form ATP by
photophosphorylation.
2. Cyclic Pathway—Photosystem I Only
a. In cyclic photophosphorylation, electrons are excited,
pass through an electron transport system (forming
ATP), and then return to the original photosystem.
b. This photosystem is characterized by the presence of
chlorophyll P700.
c. The cyclic pathway is an ancient way to make ATP from
ADP; it was used by early bacteria.
3. Noncyclic Pathway—Photosystem II, then Photosystem I
a. Noncyclic photophosphorylation transfers electrons
through two photosystems and two electron transport
systems (ETS) in the thylakoid membranes.
b. Pathway begins when chlorophyll P680 in photosystem II
absorbs energy and has an e^- become excited (enters an
ETS, forming ATP).
c. Water is split by light (photolysis), and an e^- jumps from
the water to fill up P680.
d. P700 in photosystem I absorbs energy, and has an e^-
become excited (enters an ETS, forming NADPH). e. The e^- from P680 fills up P700.
f. ATP and NADPH contain energy that can be used to form
organic molecules.
D. A Closer Look at ATP Formation
1. Hydrogen ions from photolysis of water accumulate inside the thylakoid compartment of chloroplasts to set up concentration and electric gradients.
2. As the hydrogen ions flow out through channels into the stroma, enzyme action links phosphate to ADP to form ATP.


















IV. Light-Independent Reactions (Carbon Fixation)
A. Overview
1. The participants and their roles in the synthesis of
carbohydrates are:
a. ATP, which provides energy,
b. NADPH, which provides hydrogen atoms and electrons,
c. Atmospheric air, which provides carbon dioxide.
2. The reactions take place in the stroma of chloroplasts and
are not dependent on sunlight directly.
B. Calvin-Benson Cycle
1. Light energy is now stored as chemical energy in organic
compounds.
2. The cyclic pathway operates as follows:
a. Carbon dioxide becomes attached to ribulose
bisphosphate (RuBP) to form a six-carbon intermediate.
b. The intermediate splits at once to form two PGA
molecules.
c. Each PGA then receives a phosphate from ATP plus H+
and electrons from NADPH to form PGAL.
d. Two PGAL join to form a sugar phosphate, which will be
modified to sucrose, starch, or cellulose.
3. Final tally: 12H₂O + 6CO₂ + 18ATP + 12NADPH -->
C₆H₁₂O₆ + 18ADP + 18phos. + 12NADP⁺ + 6H₂O + 12H⁺

C. How Autotrophs Use Intermediates and Products of
Photosynthesis
1. Sugar phosphates are used as cellular fuel and as building
blocks in synthesis of carbohydrates.
2. Sucrose is transported from leaves to all parts of the plant.
3. Starch is the main storage form of carbohydrate in plants.
4. Photosynthesis also yields intermediates and products
that can be used in lipid and amino acid synthesis.
D. C₄ Plants
1. Plants in hot, dry environments close their stomata to
conserve water but in so doing slow carbon dioxide entry
and permit oxygen buildup inside the leaves.
2. Photosynthesis continues improperly; this nonproductive
process is called photorespiration.
3. To overcome this fate, crabgrass, sugarcane, corn, and
other plants fix carbon twice to produce oxaloacetate (a
four-carbon, hence C₄) compound, which can then donate
the carbon dioxide to the Calvin-Benson cycle.

V. Chemosynthesis
A. Chemosynthetic autotrophs obtain energy from oxidation of
inorganic substances.
B. Some soil bacteria strip protons and electrons from
ammonia.