NADPH. The electrons from NADPH come from the splitting of H 2 O to get
oxygen (O 2 ) and electrons (H+). The two systems of light reactions are called
photosystem Iand photosystem II. Photosystems I and II function together in
the oxygenic process. Under certain conditions many algae and some
cyanobacteria can carry out cyclic photophosphorylation using only photosys-
tem I. These organisms can obtain reducing power from sources other than
water. This requires the presence of anaerobic conditions and a reducing sub-
stance, such as H 2 or H 2 S. Photosystem II is responsible for splitting water to
yield H 2 +O.Dark Phase Reactions
Many photosynthetic bacteria contain carboxysomes in their cytoplasm. These
carbosomes contain many copies of the complex enzyme Rubisco (the most
abundant and probably the most important enzyme on the planet) to start the
Calvin Cycle.
The fixation of CO 2 by most photosynthetic and autotrophic organisms in-
volves the biochemical pathway called the Calvin Cycle. The Calvin Cycle is a
reductive, energy-demanding process in which reducing equivalents from
NADPH and energy from ATP are used to reduce CO 2 to small carbohydrates that
are metabolized to glucose and ultimately to more complex carbohydrates such as
starch, sucrose and glycogen. Other substances with carbon can be used but CO 2
is preferred.Quiz
- What is the energy storage molecule?
(a) ATP
(b) Adenosine oxidate
(c) Phospholipids
(d) NAAD - What reaction releases energy as large molecules are broken down into
small molecules?
(a) Anabolic reaction
(b) Catabolic reaction
(^100) CHAPTER 5 The Chemical Metabolism