The Green World’s Gift
Photosynthesis
8.1 Photosynthesis and Energy
8.2 The Components of Photosynthesis
8.3 Stage 1: The Steps of the Light-Dependent Reactions
8.4 What Makes the Light-Dependent Reactions So Important?
8.5 Stage 2 of Photosynthesis: The Calvin Cycle
8.6 Photorespiration and the C4 Pathway
8.7 Another Photosynthetic Variation: CAM Plants
Essay How Did We Learn? Plants Make Their Own Food, But How?
LECTURE OUTLINE
I.
Introduction: Photosynthesis and Energy
A. Relevance
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.
a. Food is used for 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.
II.
Light Energy Drives 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.
B.
C.
D.
E.
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:
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
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. 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
convert energy into several bonds in a glucose molecule.
b. Electrons from carriers are brought together with CO2 and H2O to make this
glucose.
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:
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 people
passing 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+:
III. Light-Dependent Reactions
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).
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:
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
IV. Light-Independent Reactions
A. The Calvin (C3) Cycle—the “synthesis” of photosynthesis, making food, trapping CO2:
1. Enzyme called rubisco brings together CO2 and sugar, carbon fixation—three lowenergy molecules of CO2 from the atmosphere are combined with three 5-carbon
sugars (RuBP).
2. Six-carbon product is unstable and splits into two 3-carbon products (3-PGA).
3. ATP places a phosphate group on each 3-PGA; 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.
5. Review both stages together:
V.
Photorespiration and Plant Adaptation
A. Glitch in the system—photorespiration
1. Rubisco often combines O2 instead of CO2 with RuBP, unproductively.
2. Occurs with one O2 for every three CO2.
3. Undercuts food production in crops that use C3 cycle.
4. Especially problematic in hot weather because of water evaporation. 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.
B. C4 plants (warm climate adaptation):
1. Grasses, corn, sugarcane, and sorghum
2. Use a different enzyme located in bundle-sheath cells:
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.
VI. CAM Plants—another adaptation that saves water in hot climates
A. Cactus, pineapple, mint, and orchid
B. Close stomata during the day; open them 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: