Photosynthesis- is the process that converts light energy into chemical energy. This chemical
energy is usually a carbohydrate. Only photoautotrophs can do photosynthesis. Heterotrophs
must obtain their high organic nutrients from the environment.
6CO2 + 6H2O
___________>
C6H12O6 + 6O2
Glucose has more energy than carbon dioxide and water. The reaction is endergonic and will
require an energy input of ATP and NADPH2.
The equation describes the Calvin cycle of
photosynthesis and the chemical energy
needed to make the sugar.
In order to keep the reaction going, the cell
must regenerate ATP and NADPH.
Regeneration of NADPH and ATP require light and intact chloroplasts with chlorophyll. This part
of photosynthesis is known as the light reaction. The
hydrogen needed to reduce NADP comes from the
splitting of water. In the overall purpose and products of
the light reaction is
1. ADP + P ____> ATP
2. NADP + H____> NADPHH____> 2H+ + 2e- + ½ O2
Chlorophyll is green and reflects green light
and absorbs red and blue. Chlorophyll is made
from a tetrapyrole ring with Mg in the middle
and a hydrocarbon tail. These pigments form
photosystems found in the thylakoid
membrane. There are some other pigments in
photosystems which are yellow. These yellow
pigments (carotenoids and xanthophylls) allow
photosynthesis to occur in green light.
White light is mixture of different colors of light
with different wave lengths and frequencies.
When white light lands on a blue object, red
and green light is absorbed and blue is
reflected.
The absorption graph shows that the pigments absorb red and blue/violet light best. This is due to
the accessory pigments.
Chloroplasts have 3 membranes. The
outer 2 are
smooth and the inner one makes stacks of
thylakoids
which is a granum. The chlorophyll and
other
pigments are found inside the thylakoid
membrane.
They have the ability to convert light
energy into
chemical energy. A stack of thylakoids is called a granum. The matrix that the grana are
embedded in is the stroma. It contains enzymes for carbohydrate synthesis. Below is a diagram
of the thylakoid and the location of chlorophyll. The chlorophyll molecules and accessory
pigments form photosystems (I & II). Each photosystem has a reaction center (p700& p680
respectively) Once photons are
absorbed by the pigments of the
photosystem, the electron
becomes excited and the
energy is passed on from
molecule to molecule until it
reaches the reaction center
pigment. The p680 and p700
has the ability to pass the
energized electron on to the
electron transport system.
Also embedded in the thylakoid
membrane is a series of
proteins that have the ability to
be reduced an oxidized. Each
one has less reducing power
than the preceding one. Protein Q has the ability to receive an energized electron from the p680
reaction center. It then moves to PQ or plastoquinone. From there it move to a cryptochrome
complex which is a proton pump that when reduced has the ability to pump hydrogen from the
outside of the stroma to the inside of the thylakoid. From there it goes plastocyanin. By this time
the e- has lost much of its free energy and must be energized by photosystem I. The electron
has now left the electron transport chain. Replacement electrons for PS II come from the splitting
of water. A manganese complex associated with PS II has the ability to split water to produce,
H+, e-, and oxygen gas. The e- are shuttled to the photosystem, H+ are used to lower the pH of
the thylakoid, and the oxygen gas is released to the atmosphere. As the electron transport chain
runs, there is an accumulation of H+ on the inside of the thylakoid. This is due to the splitting of
water and the proton pump, PQ. As the H+ collect, the pH of the interior is lowered and there is a
separation of charge across a membrane. This can now do work.
On the thylakoid membrane, there are CF complexes which contain a channel (CF0) and a large
protein head (CF1). On the CF1 head, there is an enzyme ATP synthetase. This enzyme has the
ability to phosphorylate ADP--->ATP as 3 H+ pass through.
This is noncyclic photophosphorylation.
This is noncyclic photophosphorylation.
1. Water is split to make replacement e-, H+ and O2.
2. There are two photosystems involved.
3. NADP is reduced to NADPH.
Cyclic photophosphorylation is
considered to be a more ancient
biochemical pathway. It is found in
most photosynthetic bacteria and all
photosynthetic eukaryotes. It consists
of one photosystem (PSI) and a
simple electron transport chain. At the
end of the electron transport chain,
the electron is returned to PS I. That
being the case, water is not split, nor
is NADP reduced. One part of the
electron carrier does pump H+ across
the thylakoid membrane to make ATP.
Cyclic photophosphorylation does not
provide hydrogens for the reduction of
carbon dioxide to make a
carbohydrate. So therefore quite
often the hydrogens come from H2S.
In photosynthetic, eukaryotic cells, two
photosystems (II & I) work together to
form noncyclic photophosphorylation.
Overview light reaction
1. 18 ATP are made from 18ADP +
18P
2. Water is split. e- + H are used for
#3. 6 O2 are released.
3. 12 NADPH are made.
Overview dark reaction
The carbohydrate is made in the
stroma. It requires enzymes every
step.
1. 18ADP + 18P are made from 18
ATP. Energy is released
2. NADPH is oxidized to make NADP.
The hydrogens are transferred making
a carbohydrate.
3. 3 CO2 are used to make a triose G3P (glyceraldehyde 3-phosphate) or PGAL
phosphoglyceraldehyde. 2 of these are
used to make glucose.
The Calvin cycle will make one extra
PGAL. PGAL is a triose. It takes 2
PGALs to make glucose, the hexose. So
therefore the Calvin cycle needs to be
"turned" twice in order to make a molecule
of glucose. (Actually 6 times).
Steps of the Calvin Cycle
1. Carbon dioxide combines with ribulose
biphosphate. Ru-Bp is a pentose
monosaccharide with 2 phosphate groups
2. It will form an unstable intermediate.
3. The intermediate will form 2 molecules
of phosphoglyceric acid.
4. PGA will be phosphorylated by ATP to form DPGA
5. DPGA is reduced by NADPH to form the triose, PGAL. A phosphate group is removed in this
reaction.
6. In the last step, 5 molecules of glyceral aldehyde phosphate (G3P) or PGAL are needed to
remake 3 molecules of Ru-BP. 3 ATP are needed to make this happen. 1 G3P is left over. PGAL
is a triose. In order to make glucose, the Calvin cycle must be turned twice.
This shows how 2 molecules of
G3P or PGAL are turned into a
molecule of glucose and how it
can be turned into starch.
While the glucose is needed for energy,
there is a second reason why the Calvin
cycle evolved; to provide a carbon skeleton
so that other organic molecules or
structures can be made.
Environmental
factors affect the
rate of
photosynthesis.
1. Light intensityAt first an
increase in the
light intensity
results in a
corresponding
increase in the rate of photosynthesis as the photo-systems are activated. As the photosystems
become saturated, an increase in light intensity will not increase the rate of photosynthesis.
2. Temperature- At first an increase in temperature results in an increase in the rate of
photosynthesis because the molecules are moving faster, but at a higher temperature the
reaction rate decreases because enzymes denature.
3. If a plant is given an increase in oxygen, the rate of
photosynthesis decreases because of phenomenon of
photorespiration. The enzyme that puts the CO2 onto
ribulose biphosphate is rubisco. Sometimes rubisco can
make a mistake and put oxygen on to ribulose
biphosphate. This happens when the concentration of
oxygen gas is greater than carbon dioxide. This happens
when the plant is water stressed and the stomates are
closed. Gas exchange takes through pores on the
bottom of the leaf called stomates. Guard cells regulate
stomates but as gas exchange occurs water leaves the stomates via transpiration. When a plant
becomes water stressed, stomates close to conserve
water, but this will stop gas exchange. This will increase
the O2 and decrease CO2. Photorespiration begins. C3
photosynthesis is a plant that does the Calvin cycle and
the light reaction. There are plants that modify C3
photosynthesis by adding an additional pathway-
The leaf of a C3 plant (normal leaf). Chloroplasts are located in the palisade and spongy
mesophyll. There are no chloroplasts in the bundle sheath cells. C4 photosynthesis includes the
light reaction, the Calvin cycle and the Hatch-Slack pathway. These C4 plants also have a
different anatomy. This Hatch-Slack pathway is able to deliver dwindling supplies of CO 2 when
the stomates are closed. The enzyme (PEP carboxylase) that fixes the CO2, combines it with a
three carbon compound, phosphoenol pyruvate (PEP) to form a four carbon compound. This
enzyme does not make a mistake like rubisco. The name of this enzyme is PEP carboxylase.
The leaf of a C4 plant. There are no palisade
mesophyll cells. Instead there is a layer of
mesophyll around the bundle sheath cells.
Chloroplasts are located in the mesophyll and
spongy mesophyll. The chloroplasts are
different. The chloroplasts found in the
mesophyll have well defined thylakoids and
specialize in the light reaction and the HatchSlack pathway. The thylakoids in the bundlesheath chloroplast do not have defined
thylakoids, are larger and store starch. This
indicates the light reaction is not prevalent, and
they do specialize in the Calvin cycle after the
Hatch-Slack pathway delivers the CO2. Plants
that use C4 photosynthesis include corn, sugar
cane, and sorghum.
Another variation of photosynthesis is CAM
(crassulacean acid metabolism). These CAM
plants include succulent plants and pineapples.
Because of the intense heat and arid
conditions, these plants only open up the
stomates at night for gas exchange. Plants that
use C4 photosynthesis include corn, sugar
cane, and sorghum.
The CO2 (like C4 photosynthesis) is fixed to
PEP by PEP carboxylase. It is then converted
to an organic acid and stored until the day. During day stomates are closed and the cell releases
the CO2 from the organic acid and the Calvin cycle occurs. C3 photosynthesis (light reaction and
Calvin cycle) is called this because the first stable product has 3 carbons. C 4 photosynthesis
(light reaction, Hatch-Slack, Calvin cycle) is called this because the first product made has 4
carbons.