Transcript Overview light reaction

PhotosynthesisThe Basis for Life on Earth
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.
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 below 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.
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) allows 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. Yet the action
spectrum shows that a small
amount of light does occur
in green light.
This diagram shows the
location of the chloroplasts
in the plant. Leaves are
specialized to for
photosynthesis. The leaves
are usually arranged on the
plant so that it maximizes
exposure to the sunlight.
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 is 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 comes 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 eare 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 consist 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.
Comparison of
cyclic versus 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 make glucose
This is the Calvin cycle. The various compounds only show the
carbon skeleton, leaving the hydrogen and oxygen atoms off. The
next slide after that includes the names of the molecules used in the
cycle.
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).
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 affects the rate of photosynthesis.
1. Light intensity- At first an increase in the light intensity results in a
corresponding increase in the rate of photosynthesis as the photosystems 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 regulates stomates but as gas
exchange occurs water leaves the stomates via transpiration.
When a plant becomes water stressed, stomates close to conserve water.
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 CO2
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 these mesophyll and
spongy mesophyll. The chloroplast
are different.
The chloroplasts in the mesophyll have well defined thylakoids and
specialize in the light reaction and the Hatch-Slack pathway. The
thylakoids in the bundle-sheath 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 HatchSlack 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. C4
photosynthesis (light reaction, Hatch-Slack, Calvin cycle) is called this
because the first product made has 4 carbons.