Photosynthesis-
The Basis for Life on Earth
Part I
Photosynthesis- is the process that converts light energy into chemical
energy. This chemical energy is usually a carbohydrate. Only
photoautrotrops 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.
Which of the following statements is a correct
distinction between autotrophs and heterotrophs ?
A) Only heterotrophs require chemical compounds
from the environment.
B) Cellular respiration is unique to heterotrophs.
C) Only heterotrophs have mitochondria.
D) Autotrophs, but not heterotrophs, can nourish
themselves beginning with CO2 and other nutrients
that are inorganic.
E) Only heterotrophs require oxygen.
D
Autotrophs, but not heterotrophs, can nourish
themselves beginning with CO2 and other nutrients
that are inorganic. Photoautotrophs use light to do
this and chemoautrophs uses sources of reduced
inorganic chemicals for energy such as H2S.
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 (caroteinoids and
xanthophylls) allows photosynthesis to occur in green light.
The The light reactions of photosynthesis supply
the Calvin cycle with
A) light energy.
B) CO2 and ATP.
C) H2O and NADPH.
D) ATP and NADPH.
E) sugar and O2.
D
The purpose of the light reaction is to provide the
Calvin cycle with the necessary chemical energy
needed to reduce CO2 in the formation of a
carbohydrate in the form of ATP and NADPH.
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 greeen light.
Which of the following conclusions does not make sense from
studying the absorption spectrum for chlorophyll a and the action
spectrum for photosynthesis?
A) Not all wavelengths are equally effective for photosynthesis.
B) There must be accessory pigments that broaden the spectrum
of light that contributes to photosynthesis.
C) The red and blue areas of the spectrum are most effective in
driving photosynthesis.
D) Chlorophyll owes its color to the absorption of green light.
E) Chlorophyll a has two absorption peaks.
D
Green pigment refects green light and does not
absorb 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.
Grass stains are so difficult to remove from
clothing because
A) chlorophyll contains the metallic ion Mg.
B) green pigments reflects blue light and most
liquid detergents are blue.
C) chlorophyll contains a hydrocarbon tail which
repels water.
D) Mg is an ion which repels the polar nature of
detergent.
Chlorophyll is green and reflects green light. The
hydrocarbon tail of chlorophyll repels water making
it difficult to remove grass stains from clothing.
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 embeded 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 preceeding one. Protein Q has the ability
to receive an energized electron from the p680 reaction center. It
then moves to PQ or plastoquione. From there it move to a
cytochrome 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.
Which of the following processes is most directly
driven by light energy?
A) creation of a pH gradient by pumping protons
across the thylakoid membrane
B) carbon fixation in the stroma
C) reduction of NADP+ molecules
D) removal of electrons from chlorophyll molecules
E) ATP synthesis
D
Light energy excites electrons in the photosystem
which eventually reaches a chlorophyll pigment in
the photosystem. This electron then has the
ability to be moved (removed) to the electron
transport chain.
The electron has now left the electron transport chain.
Replacement electrons for PS II comes from the splitting of
water. A managanese 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.
Which of the following sequences correctly
represents the flow of electrons during
photosynthesis?
A) NADPH -> O2 -> CO2
B) NADPH -> chlorophyll -> Calvin cycle
C) H2O -> photosystem I -> photosystem II
D) NADPH -> electron transport chain -> O2
E) H2O -> NADPH -> Calvin cycle
E
The electrons originate from water and are
released when water is split. The electrons are
moved to the electron transport chain where at
the end of the electron transport chain, the
electrons are used to reduce NADP to form
NADPH. From the the NADPH is shuttled to the
Calvin cycle where they are used to reduce CO2 in
the formation of a carbohydrate
The mechanism, photophosphorylation is most
similar to
A) substrate-level phosphorylation in glycolysis.
B) oxidative phosphorylation in cellular respiration.
C) the Calvin cycle.
D) carbon fixation.
E) reduction of NADP+.
B
Both cell respiration and photosynthesis use
chemiosmosis to generate ATP.
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.
Cooperation of the two photosystems is required
for
A) ATP synthesis.
B) reduction of NADP+.
C) cyclic photophosphorylation.
D) oxidation of the reaction center of photosystem
I.
E) generation of a proton-motive force.
B
When two photosystems are involved, NADP is
reduced, water is split, oxygen is produced. This is
noncyclic photophosphorylation. Both cyclic and
noncyclic photophosphorylation involve the
synthesis of ATP with a generation of protonmotive force. Cyclic photophosphorylation only
involves one photosystem.
Which of the following statements is a correct
distinction between cyclic and noncyclic electron
flow?
A) Only noncyclic electron flow produces ATP.
B) In addition to ATP, cyclic electron flow also
produces O2 and NADPH.
C) Only cyclic electron flow utilizes light at 700
nm.
D) Chemiosmosis is unique to noncyclic electron
flow.
E) Only cyclic electron flow can operate in the
absence of photosystem II.
E
Noncyclic photophosphorylation has two
photosystems and cyclic photophosphorylation only
has one photosystem as shown above.
Comparison of
cyclic versus noncyclic
photophosphorylation