BIOL 101
Photosynthesis
2020
This is the route whereby virtually
all energy enters the biosphere
Introduction
Most producers are photosynthetic autotrophs and use light as an
energy source for manufacturing organic compounds from
carbon dioxide and water.
Autotrophs obtain carbon from carbon dioxide. Photosynthetic autotrophs harness light energy. Chemosynthetic
autotrophs extract energy from chemical reactions involving inorganic substances.
Inside this issue:
Introduction
1
Light energy
2
Photosystems
2
Cyclic photophosphorylation /
Cyclic
2
Noncyclic photophosphorylation / Non-cyclic electron flow
3
Light-independent reactions /Dark
reactions / Calvin cycle / Calvin
Benson cycle / C3 cycle
3
Photorespiration /C3/C4 /
CAM metabolism
4
Heterotrophs feed on autotrophs and acquire carbon and
energy. Heterotrophs include animals, protistans,
bacteria, and fungi.
6CO2 + 12 H2O* ----LIGHT----> C6H12O6 + 6O2 *+ 6H2O
In plants, photosynthesis occurs in chloroplasts, which are located mainly within
mesophyll cells inside the leaf.
Chloroplasts
are organelles bounded by a double membrane; the inner memSecondary Story Headline
brane encloses the stroma and thylakoids. The thylakoids are arranged in stacks
called grana. Thylakoids are highly structured and occur in the chloroplast.
Chlorophyll and other photosynthetic pigments (carotenoids phycobilins etc) are
found within the thylakoid membranes of chloroplasts.
Chlorophyll a (green) is the main pigment
Chlorophyll b (bluish-green)
Carotenoid pigments absorb blue-violet wavelengths but reflect yellow, orange,
and red.
Without plants the food
chain would lose its
source of energy. All animals and humans directly
or indirectly rely on plants.
Not only do we need a
source of energy but also a
source of oxygen.
During photosynthesis, light energy is captured by chlorophyll and used to
chemically combine the hydrogen from water with carbon dioxide to produce
carbohydrates. Oxygen is released as a byproduct.
During the light-dependent reactions of photosynthesis, chlorophyll absorbs light and becomes energized. Some of the energy of the energized chlorophyll is used to make ATP; some is used to split water. Hydrogen from the water is transferred to NADP+, forming NADPH.
The light-independent reactions (dark) of photosynthesis use the energy
of ATP and NADPH formed during the light dependant reactions to manufacture carbohydrate molecules from CO2.
Page 2
Photosynthesis
This is the route whereby
Volume 1, Issue 1
Light energy
Chlorophyll captures light energy with an absorption spectrum similar
to the action spectrum of photosynthesis.
“Packets” of light energy - photons, can excite pigment molecules such
as chlorophyll and boost an electron to a higher energy level The resulting high energy electrons are accepted by electron acceptors. Photons
vary in energy. Chlorophyll pigments absorb blue and red light but reflect green.
Photosystems
Chlorophyll a, b and carotenoids are clustered in the thylakoid membrane assemblies of a few hundred pigment molecules.
Not all pigment molecules can generate sufficient energy to donate excited electrons to the primary electron acceptor
This function is performed by a special chlorophyll molecule (or two)
which acts as a reaction center.
The pigments combined (chlorophyll a, b, carotenoids) function as light
harvesters and pass the energy to the reaction center. The whole system
of pigments and the primary electron acceptor is referred to as a photoPigments normally occur in the ground state. The absorption of photons
boosts electrons to an orbital of higher energy value. The pigment cannot remain in this state. The electron can follow two routes
Cyclic photophosphorylation /Cyclic electron flow
In cyclic photophosphorylation, electrons from Photosystem I are eventually returned to Photosystem I; ATP is produced, but not
NADPH, and no oxygen is generated.

involves only Photosystem I

generates ATP (CYCLIC PHOTOPHOSPHORYLATION)

produces NO reduced NADPH

evolves NO O2

the electrons that depart from P700 eventually return

Occurs in the grana /thylakoid membrane
Energy associated with the electron flow drives the formation of ATP.
ATP is formed by a proton motive force (chemiosmotic) in the
thylakoid.
Photosynthesis
This is the route whereby
Volume 1, Issue 1
Page 3
Noncyclic photophosphorylation / Non-cyclic electron flow
In noncyclic photophosphorylation, the electrons emitted by Photosystem I are passed through a chain of
electron acceptors to NADP+. In this process electrons from P680 in Photosystem II are donated to P700
in Photosystem I. Water is split and its electrons are donated to P680 in Photosystem II. Oxygen is released in the process. A series of redox reactions takes place as energized electrons are passed along a
chain of electron acceptors. Energy associated with the electron flow drives the formation of ATP. ATP is
formed by a proton motive force (chemiosmotic) in the thylakoid.

both photosystems are involved

the electrons of photosystem I do not return to the reaction center P700

the electrons are stored as high energy electrons in NADPH

P700 is now a strong oxidizing agent deficient in electrons, these are supplied by photosystem II

the electrons from P680 cascade down a chain of reactions losing potential energy until they reach
P700 of photosystem I

as the e- cascade down the chain H+ are pumped across the thylakoid membrane. This sets up a proton motive force which can drive the synthesis of ATP (NON CYCLIC PHOTOPHOSPHORYLATION)

P680 now has a shortage of e-, (strong oxidizer) and obtains the electrons from H2O

electron removal splits water into 2 H ions and an oxygen atom to form O2

occurs in the grana /thylakoid membrane
Summary Equations for Photosynthesis The light-dependent reactions:
12 H2O + 12 NADP+ + 18 ADP + 18 Pi
ATP
———> 6O2 + 12 NADPH + 18
Light-independent reactions /Dark reactions /
Calvin cycle / Calvin Benson cycle / C3 cycle
During the light-independent reactions, energy from ATP (which provides
energy) and NADPH (which provides hydrogen and electrons) generated by
the light reactions are used to chemically combine carbon dioxide with hydrogen (carbohydrate).
Carbon dioxide diffuses from the air, across the plasma membrane of the
plant cell and into the stroma.
In the Calvin cycle, carbon dioxide is combined with ribulose bisphosphate
(RuBP), a five-carbon sugar. The enzyme that catalyzes this reaction is RuBp carboxylase (RUBISCO). It is the most abundant protein in chloroplasts
(perhaps the world!)
The Calvin cycle results in the synthesis of PGAL a C3 sugar, glyceraldehyde3-phosphate. Two molecules of PGAL are used to produce one molecule of
glucose; the remaining PGAL molecules are modified to regenerate the CO2
acceptor molecules (RuBP).
Light-independent reactions /Dark reactions Calvin cycle /
Calvin Benson cycle / C3 cycle cont.
ATP and NADPH molecules are consumed in the conversion Of CO2 into carbohydrates.
12H2O + 6CO2 + 18ATP + 12NADPH ———> C6H12O6 + 18ADP + 18Pi + 12NADP+ + 6H2O
+ 12H+
Photorespiration /C3/C4 /CAM metabolism
In photorespiration, C3 plants consume oxygen and generate carbon dioxide. This process, which decreases photosynthetic efficiency, occurs on bright, hot, dry days when plant cells close their stomata,
preventing the passage of CO2 into the leaf and a buildup of O2. Oxygen not carbon dioxide becomes
attached to RuBP.
To prevent photorespiration plants fix carbon twice. In the C4 pathway, the enzyme PEP carboxylase
binds CO2 effectively, even when CO2 is at a low concentration. The initial reactions take place within
mesophyll cells. The carbon dioxide is fixed in oxaloacetate, which is then converted to malate.The malate moves into a bundle sheath cell and CO2 is removed from it. The released CO2 then enters the Calvin cycle.
CAM (Crassulacean acid metabolism) plants such as succulents open their stomata and fix CO2 only at
night, storing the intermediate product an organic acid for use the next day. These plants are known as
CAM plants because the fix their carbon at different time in the same cell unlike the C4 pathway that fixes carbon in different cells.