General Biology I - BSC 1010C - Chapter 10: Photosynthesis
Questions With Complete Solutions
Describe how scientists used stable isotopes to determine the
fate of oxygen in the photosynthetic equation. Correct Answers
During the 1950s, scientists tested C.B. van Niel's hypothesis
that plants split H2O as a source of electrons from hydrogen
atoms and released O2 as a by-product by using oxygen-18 as a
tracer to follow the path of oxygen atoms during photosynthesis.
The experiments showed that a significant result of the shuffling
of atoms during photosynthesis was the extraction of hydrogen
from water and its incorporation into sugar. The waste product
of photosynthesis, O2, was released into the atmosphere,
confirming the hypothesis.
Describe the action of the electron transport chain between
photosystems I and II. How does it differ with the mitochondria
in terms of the hydrogen ion concentration gradient? Correct
Answers Each photoexcited electron passes from the primary
electron acceptor of PS II to PS I via an electron transport chain,
which is made up of the electron carrier plastoquinone (Pq), a
cytochrome complex, and a protein called plastocyanin (Pc).
Each component carries out redox reactions as electrons flow
down the ETC, releasing free energy that is used to pump H+
protons into the thylakoid space, creating an electrochemical
gradient across the thylakoid membrane. While in cellular
respiration, the H+ concentration builds up in the intermembrane
complex outside of the mitochondrial matrix, the electrons in the
light reactions travel from the outer stroma to the thylakoid
space, creating a high H+ concentration on the interior of the
chloroplast.
, Describe the events that occur at photosystem II. Be sure to
address: water, energy, electrons, oxygen gas, light harvesting
complexes, chlorophyll, reactions center, and primary electron
acceptor. Correct Answers A photon of light strikes one of the
pigment molecules in a light-harvesting complex of PS II,
boosting one of its electrons to a higher level. Energy is relayed
to other pigment molecules until it reaches the P680 pair of
chlorophyll-a molecules in the PS II reaction center complex. It
excites an electron in the pair of molecules to a higher energy
state, which is transferred from the excited P680 to the primary
electron acceptor. An enzyme catalyzes the splitting of a water
molecule into two electrons, two H+ ions, and an oxygen atom.
The electrons are supplied one by one to the P680 acceptor, each
electron replacing one transferred to the primary electron
acceptor. The H+ ions are released into the thylakoid space, and
the oxygen atom combines with an oxygen atom generated by
the splitting of another water molecule, forming the waste
product of oxygen gas or O2.
Explain what "photorespiration" is. How does it affect the plant's
carbon-fixing efficiency? Correct Answers Photorespiration is a
metabolic pathway that consumes oxygen and ATP, releases
carbon dioxide, and decreases photosynthetic output; it is
considered a wasteful process. It generally occurs on hot, dry,
bright days, when stomata close and the O2/CO2 ratio in the leaf
increases, favoring the binding of O2 rather than CO2 by
rubisco.
How does cyclic electron flow differ from linear electron flow?
Does the ATP that is produced go to the Calvin cycle? Correct
Questions With Complete Solutions
Describe how scientists used stable isotopes to determine the
fate of oxygen in the photosynthetic equation. Correct Answers
During the 1950s, scientists tested C.B. van Niel's hypothesis
that plants split H2O as a source of electrons from hydrogen
atoms and released O2 as a by-product by using oxygen-18 as a
tracer to follow the path of oxygen atoms during photosynthesis.
The experiments showed that a significant result of the shuffling
of atoms during photosynthesis was the extraction of hydrogen
from water and its incorporation into sugar. The waste product
of photosynthesis, O2, was released into the atmosphere,
confirming the hypothesis.
Describe the action of the electron transport chain between
photosystems I and II. How does it differ with the mitochondria
in terms of the hydrogen ion concentration gradient? Correct
Answers Each photoexcited electron passes from the primary
electron acceptor of PS II to PS I via an electron transport chain,
which is made up of the electron carrier plastoquinone (Pq), a
cytochrome complex, and a protein called plastocyanin (Pc).
Each component carries out redox reactions as electrons flow
down the ETC, releasing free energy that is used to pump H+
protons into the thylakoid space, creating an electrochemical
gradient across the thylakoid membrane. While in cellular
respiration, the H+ concentration builds up in the intermembrane
complex outside of the mitochondrial matrix, the electrons in the
light reactions travel from the outer stroma to the thylakoid
space, creating a high H+ concentration on the interior of the
chloroplast.
, Describe the events that occur at photosystem II. Be sure to
address: water, energy, electrons, oxygen gas, light harvesting
complexes, chlorophyll, reactions center, and primary electron
acceptor. Correct Answers A photon of light strikes one of the
pigment molecules in a light-harvesting complex of PS II,
boosting one of its electrons to a higher level. Energy is relayed
to other pigment molecules until it reaches the P680 pair of
chlorophyll-a molecules in the PS II reaction center complex. It
excites an electron in the pair of molecules to a higher energy
state, which is transferred from the excited P680 to the primary
electron acceptor. An enzyme catalyzes the splitting of a water
molecule into two electrons, two H+ ions, and an oxygen atom.
The electrons are supplied one by one to the P680 acceptor, each
electron replacing one transferred to the primary electron
acceptor. The H+ ions are released into the thylakoid space, and
the oxygen atom combines with an oxygen atom generated by
the splitting of another water molecule, forming the waste
product of oxygen gas or O2.
Explain what "photorespiration" is. How does it affect the plant's
carbon-fixing efficiency? Correct Answers Photorespiration is a
metabolic pathway that consumes oxygen and ATP, releases
carbon dioxide, and decreases photosynthetic output; it is
considered a wasteful process. It generally occurs on hot, dry,
bright days, when stomata close and the O2/CO2 ratio in the leaf
increases, favoring the binding of O2 rather than CO2 by
rubisco.
How does cyclic electron flow differ from linear electron flow?
Does the ATP that is produced go to the Calvin cycle? Correct
