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8
The Green World’s Gift:
Photosynthesis
Photosynthesis and Energy (Section 8.1)
1. Try to name something you eat that isn’t from a
plant or from an animal that ate a plant.
2. All food comes from plants. Molecules of our
bodies are made from food we eat, but plants make
their own food from sunlight. Food is used for:
a) Creating macromolecules from monomers like
glucose and amino acids.
b) More importantly, food is used in respiration to generate cellular
energy, ATP. Plants are the nearly universal source of energy for all
living things.
c) Plants take energy-poor reactants (water and carbon dioxide) and use
solar energy to drive the uphill reaction of trapping those reactants in
complex, ordered bonds of glucose.
3. Oxygen needed for respiration is produced as a by-product of
photosynthesis.
A. Nature of Light
1. Energetic rays have different wavelengths in a spectrum from gamma rays
to radio waves, only a portion of which is visible light:
2. Explain what it means to see a plant as red or green in terms of absorption
and reflection, and why a black car is hotter on a sunny day than a white
car. (Black absorbs all light and reflects none, white absorbs little and
reflects almost all.) Explain what it means to absorb light by a pigment.
3. Photosynthesis driven by only part of the visible spectrum (blue
and red); plant pigments in the green plant reflects green and
absorbs blue and red.
Tour of a leaf, where plants absorb light: Figure 8.3
1. Blade
2. Leaf section, epidermis,
stomata, mesophyll
3. Chloroplasts, inner
and outer membranes
4. Grana and stroma
5. Thylakoid membrane,
and compartment
6. Pigments
Stomates
Leaf cross section
C. Photosynthesis occurs in two essential phases.
1. Light-dependent—“photo” of photosynthesis.
a) Power of sunlight excites electrons in pigment molecules.
b) Excited electrons are carried down transport chain of redox
reactions like those in mitochondria.
C. Photosynthesis occurs in two essential phases.
1. Light-dependent—“photo” of photosynthesis.
c) Energy is used to make a gradient of H+ ions to drive synthesis
of ATP, and electrons may be transferred by a carrier molecule
like NAD+, NADP+.
d) Pigment electrons are replaced by electrons stripped from
water, making O2 gas.
2. Light-independent
a) ATP and NADPH are not good permanent storage molecules,
so the plants covert energy into several bonds in a glucose
molecule.
b) Electrons from carriers are brought together with CO2 and H2O
to make this glucose.
D. Photosystems are the working units that absorb solar energy.
1. Aggregates of hundreds of pigment molecules serve as antenna to
absorb solar energy.
2. Reaction center of aggregate contains pair of chlorophyll
molecules with electrons that absorb the energy and jump to
electron carrier molecules: Figure 8.4
E. Energy transfer is possible using redox reactions.
1. One substance loses electrons (oxidized) while another gains
electrons (reduced).
2. Electrons move down the energy hill, losing energy as they go
(analogy of the passing of a hot potato warming each hand as it
drops, giving off some heat as it goes. The last person to get the
potato gets some heat and food as well.) The final recipient of the
electron in this case is NADP+: Figure 8.5
A. Follow the pathway:
1. Photosystem II absorbs solar energy.
2. Electron jumps to the primary electron acceptor.
3. Chlorophyll is left without an electron, making it an oxidizing
agent that grabs an electron from water, splitting it into H+ ions
and O2.
4. Ejected electron falls back down the energy hill through a series of
electron transfer molecules and a series of redox reactions until it
reaches photosystem I (another reaction center also receiving solar
energy).
Light-Dependent Reactions
A. Follow the pathway: Figure 8.5 animated in the
resources for this chapter as figure 8_05.
5. Again, energized electrons from photosystem I are
transferred back down the energy hill, until they are
received by NADP+, an electron carrier that ferries
electrons to the second stage, the light-independent
stage of photosynthesis.
6. Travel took place from thylakoid to stroma: Figure
8.6
B. Importance of the light-dependent phase
1. Oxygen formation
2. Energized electrons being transferred, not just
giving off heat of fluorescing, and ferried in
NADPH.
3. Formation of ATP, which is used to power the
second stage, the light-independent reactions.
A. The Calvin (C3) Cycle: the “synthesis” of photosynthesis, making food,
trapping CO2: Figure 8.7
1. Enzyme called rubisco brings together CO2 and sugar, carbon
fixation—three low-energy molecules of CO2 from the atmosphere
are combined with three five-carbon sugars (RuBP).
2. Six-carbon product is unstable and splits into two three-carbon
products (3-PGA).
3. ATP places a phosphate group on each 3-PGA, and NADPH donates a
pair of electrons yielding a high energy food, G3P.
4. Only one G3P exits the cycle; the other five are used to regenerate the
starting material RuBP.
A. Glitch in the system—Photorespiration
1. Rusbisco often combines O2 instead of CO2 with RuBP,
unproductively.
2. Occurs one O2 for every three CO2.
3. Undercuts food production in crops that use C3 cycle.
4. Especially problematic in hot weather because of evaporation of
water. Plant closes stomata in leaves to prevent evaporation, but as
water is kept in, CO2 is kept out. As the light-dependent reactions
continue, O2 builds up, combining with RuBP unproductively.
1. Grasses, corn, sugarcane, and sorghum
2. Use a different enzyme located in bundle-sheath cells: Figure 8.10
3. Costs ATP to shuttle CO2 to bundle-sheath cells; in sunny climates this
is not an issue, because with abundant sunlight, ATP is plentiful.
4. In northern climates, C4 plants are not as well adapted.
CAM Plants—another adaptation that saves
water in hot climates (Section 8.8)
A. Cactus, pineapple, mint, and orchid
B. Close stomata during the day, open at night
C. Start C4 metabolism at night by fixing CO2 but wait for
day to use abundant ATP to finish.
D. Comparison of three strategies: Figure 8.12
Summary of Photosynthesis in
most kinds of plants
The End