The Green World’s Gift
Photosynthesis
Introduction: Photosynthesis and Energy
• Relevance.
– Try to name something you eat that isn’t from a plant, or
from an animal that ate a plant.
– 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 creating macromolecules from
monomers like glucose and amino acids.
Introduction: Photosynthesis and Energy
• Relevance.
– All food comes from plants.
• More importantly, food is used in respiration to generate
cellular energy, ATP. Plants are the nearly universal
source of energy for all living things.
• 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.
– Oxygen needed for respiration is produced as a by-product
of photosynthesis.
Light Energy Drives Photosynthesis
• Nature of Light.
– Energetic rays have different wavelengths in a spectrum
from gamma rays to radio waves, only a portion of which is
visible light.
– 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.
– 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.
Light Energy Drives Photosynthesis
• Tour of a leaf, where plants absorb light.
– Blade.
– Leaf section, epidermis, stomata, mesophyll.
– Chloroplasts, inner and outer membranes.
– Grana and stroma.
– Thylakoid membrane and compartment.
– Pigments.
Photosynthesis
• Two Phases.
– Phase 1: Light-dependent.
• “Photo” of photosynthesis.
• Power of sunlight excites electrons in pigment
molecules.
• Excited electrons are carried down transport
chain of redox reactions like those in
mitochondria.
Photosynthesis
• Two Phases.
– Phase 1: Light-dependent.
• 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+.
• Pigment electrons are replaced by electrons
stripped from water, making O2 gas.
Photosynthesis
• Two Phases.
– Phase 2: Light-independent.
• ATP and NADPH are not good permanent storage
molecules, so the plants convert energy into
several bonds in a glucose molecule.
• Electrons from carriers are brought together with
CO2 and H2O to make this glucose.
Photosynthesis
• Photosystems are the working units that absorb solar
energy.
– Aggregates of hundreds of pigment molecules serve
as antenna to absorb solar energy.
– Reaction center of aggregate contains pair of
chlorophyll molecules with electrons that absorb
the energy and jump to electron carrier molecules.
Photosynthesis
• Energy transfer is possible using redox reactions.
– One substance loses electrons (oxidized) while
another gains electrons (reduced).
– 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+.
Light-Dependent Reactions
• Follow the pathway.
– Photosystem II absorbs solar energy.
– Electron jumps to the primary electron acceptor.
– Chlorophyll is left without an electron, making it an
oxidizing agent that grabs an electron from water,
splitting it into H+ ions and O2.
Light-Dependent Reactions
• Follow the pathway.
– 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).
– 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.
– Travel took place from thylakoid to stroma.
Light-Dependent Reactions
• Importance of the light-dependent phase.
– Oxygen formation.
– Energized electrons being transferred, not just
giving off heat of fluorescing, and ferried in NADPH.
– Formation of ATP, which is used to power the
second stage, the light-independent reactions.
Light-Independent Reactions
• The Calvin (C3) Cycle.
– The “synthesis” of photosynthesis, making food,
trapping CO2.
– Enzyme called rubisco brings together CO2 and
sugar, carbon fixation—three low-energy molecules
of CO2 from the atmosphere are combined with
three 5-carbon sugars (RuBP).
– Six-carbon product is unstable and splits into two
3-carbon products (3-PGA).
Light-Independent Reactions
• The Calvin (C3) Cycle.
– ATP places a phosphate group on each 3-PGA;
NADPH donates a pair of electrons, yielding a highenergy food, G3P.
– Only one G3P exits the cycle; the other five are
used to regenerate the starting material, RuBP.
Photorespiration and Plant Adaptation
• Glitch in the system—photorespiration.
– Rubisco often combines O2 instead of CO2 with RuBP,
unproductively.
– Occurs with one O2 for every three CO2.
– Undercuts food production in crops that use C3 cycle.
– 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.
Photorespiration and Plant Adaptation
• C4 plants.
– warm climate adaptation.
– Grasses, corn, sugarcane, and sorghum.
Photorespiration and Plant Adaptation
• C4 plants.
– Use a different enzyme located in bundle-sheath
cells.
– Costs ATP to shuttle CO2 to bundle-sheath cells; in
sunny climates this is not an issue, because with
abundant sunlight, ATP is plentiful.
– In northern climates, C4 plants are not as well
adapted.
CAM Plants
• Adaptation that saves water in hot climates.
• Cactus, pineapple, mint, and orchid.
CAM Plants
• Close stomata during the day; open them at night.
• Start C4 metabolism at night by fixing CO2 but wait for
day to use abundant ATP to finish.