The energy in all biological systems originates in solar energy.
Plants (and certain
bacteria) convert sunlight into chemical energy by photosynthesis, which
makes carbohydrates such as glucose.
Reactions that require light (light-dependent reactions) use chlorophyll &
other pigments to capture light energy, split water molecules, and release oxygen.
"Dark" (light-independent) reactions include the fixing of CO2 and formation
of RUBP and later glucose.
- Anabolism: the building up of larger, energy-rich
molecules from smaller ones. All anabolic processes require energy.
- Photosynthesis: an anabolic process by which plants, some bacteria, and
cyanobacteria use sunlight to make glucose and other carbohydrates.
Photosynthesis requires a light-capturing pigment, usually chlorophyll.
In true plants, photosynthesis only takes place inside chloroplasts.
The overall reaction can be summarized as:
6 CO2 + 12 H2O + energy ——>
C6H12O6 (glucose) + 6 O2
+ 6 H2O
(All of the oxygen on the right comes from the water on the left)
- Light-dependent reactions (light-sensitive, requiring light of certain
wavelengths):
H2O + NADP+ + ADP + Pi + energy
——> ½O2 + ATP + NADPH + H+
The light energy needed for this reaction must be trapped by pigments,
especially chlorophyll a and chlorophyll b;
oxygen is released and ATP is formed.
- Pigments: Chlorophyll a has an light-absorption maximum in the
400-460 nm range (blue) and another between 650-700 nm (red). Chlorophyll
looks green because it absorbs these other wavelengths. Accessory pigments
(including carotenes, xanthophylls, and phycobilins) capture light energy
at other wavelengths.
- Hill reaction: releases electrons to the photosystems:
H2O + NADP+ + energy
——> ½O2 + NADPH + H+
- Photosystems I and II: Photosynthetic pigments occur in two complexes
called photosystem I and photosystem II. Photosystem II must be
excited by a light quantum (also called a photon), boosting the energy
of an electron from the hydrogen of H2O. The electron passes from one
pigment to another and then to photosystem I, which must be excited by another light
quantum in order for the process to continue. Photosystem I transfers the electron
to a second series of pigments and ultimately to NADP+, which then
enters the light-independent ("dark") reactions.
- Light-independent ("dark") reactions (light-insensitive, do not require light):
ATP and NADPH from the light reactions are used in the dark reactions to incorporate
("fix") CO2 into plant tissues and produce sugars.
- C3 plants and the Calvin cycle: Most plants use the
Calvin cycle to fix CO2 into a 3-carbon compound; they are therefore
called C3 plants
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- C4 plants: Certain plants like corn and sugarcane incorporate
CO2 into a 4-carbon compound and are therefore called C4 plants.
C4 plants are mostly tropical. They can make glucose faster and grow faster
than C3 plants and use less water, but they use more ATP to make each
molecule of glucose.
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