Biology 123 SI- Dr. Raut’s Class
Session 12-02/29/15
1. Draw out the linear electron flow for the light reactions of photosynthesis. Include: parts of
both photosystems, electron transport chains, and the flow of electrons. Basically, include
everything you can.
When light hits pigments in the light harvesting complex of PSII, a pair of the pigment’s
electrons are excited, when the electrons come back down, they excite the electrons of a
neighboring pigment. This continues until the P680 electrons are excited in the reaction-center
complex. A primary electron acceptor then accepts these electrons, not allowing them to return to
the P680 chlorophyll. The electrons then move from the primary acceptor down an electron
transport chain, causing ATP to be made. The P680 chlorophyll is now electron deficient. Water
splits forming two protons and ½ O2 and giving its electrons to the P680 chlorophyll to fill the
electron hole, so the system can run again. Meanwhile, in PSI the same thing has been
happening, the pigments inside the light harvesting complex are excited, exciting the electrons in
the P700 chlorophyll. The primary electron acceptor then accepts the P700’s electrons. These
electrons are then given to an electron transport chain with NADP+ as the final electron acceptor
forming NADPH. Now the P700 chlorophyll is electron deficient. The electrons from the P680
that were passed down the electron transport chain then fill this electron hole in the P700.
2. Draw out the cyclic electron flow. Include everything. How is this different from linear
electron flow? Why would the cell use cyclic electron flow?
Cyclic electron flow only uses PSI. Light excites pigments in the light harvesting complex,
which then excite the P700 in the reaction-center complex. The P700 electrons are captured by
the primary acceptor, which gives the electrons to an electron transport chain, which gives the
electrons back to the P700. ATP is created. PSII is not active at all. No NADPH is made. This is
only used when the Calvin cycle needs a little bit of extra ATP.
3. How is ATP synthesized in the light reactions? Describe this part of the process.
ATP is synthesized using photophosphorylation, which is very similar to oxidative
phosphorylation in cellular respiration. The electron transport chain that connects PSII and PSI is
embedded in the thylakoid membrane. It causes the build up of protons in the thylakoid space,
making the thylakoid space have a higher proton concentration than the stroma, building a proton
motive force. Protons then move through ATP synthase which is also embedded in the thylakoid
membrane, and causes the formation of ATP in the stroma.
4. Draw out the Calvin cycle. Label each phase.
Remember the Calvin cycle requires 3 CO2, 9 ATP, and 6 NADPH just to make one 3-carbon
sugar! This process requires lots of energy. Also remember that this is a cycle and must
regenerate itself.
5. What is produced by each run of the light reactions? What is produced by each run of the
Calvin cycle?
The linear electron flow light reactions produce 1 ATP and 1 NADH and require 1 water
molecule and sunlight. The cyclic electron flow light reactions produce 1 ATP and require
sunlight. The Calvin cycle requires 3 CO2, 9 ATP, and 6 NADPH and produces 1 G3P.
6. In plants living in hot climates, evaporation through stomata is a huge problem. How is this
dealt with by C3 plants, C4 plants, and CAM plants?
These plants must close their stomata to prevent the evaporation of water. In C3 plants, when
their stomata are closed they have a low concentration of CO2 and a high concentration of O2.
These plants will use rubisco to couple RuBP and O2 , which results in a two carbon molecule
which goes to the mitochondria and peroxisomes and ends up forming CO2 which can then be
used in the Calvin cycle. This process is called photorespiration.
C4 plants use spatial separation and alternate carbon fixation to bind CO2. These plant use the
enzyme PEP carboxylase to bind CO2 and form a four carbon molecule. This binding occurs in
the mesophyll cell. PEP carboxylase has a much higher affinity for CO2 than rubisco and has no
affinity for O2. This four carbon molecule will then move into the bundle sheath cell which is
more interior and is where the Calvin cycle happens. This process costs ATP because after CO2
is given to the Calvin cycle, pyruvate must be formed back into PEP which costs ATP.
CAM plants use temporal separation between the fixation of carbon and the Calvin cycle. The
cells of CAM plants fix carbon into organic acid during the night when stomata are open. These
organic acids are stored until the day when the light reactions happen and the Calvin cycle
proceeds. These two processes occur in the same cell, but at different times.
7. What are the two types of cells in multicellular organisms? What are the processes by which
these cells are replicated?
The two types of cells in multicellular organisms are somatic (body) cells and gamete (sex) cells.
Somatic cells are created through mitosis. Gametes are created through meiosis.
8. What are the purposes of mitosis?
Mitosis can be used for reproduction in unicellular organisms, growth and development, and
tissue renewal.
9. Describe the characteristics of a chromosome.
DNA must be organized into chromosomes by proteins called histones. The DNA and proteins
combined is called chromatin. The centromere is the small waist of a chromosome; the portions
that extend from the centromere are called the arms. A duplicated chromosome forms two sister
chromatids which are connected by cohesion proteins which hold them together.
10. What are the two main phases of the cell cycle? What are the smaller phases within these
main phases?
The two main phases of the cell cycle are the interphase and the mitotic phase. The interphase
includes G1, S, and G2. The G1 and G2 phases are gap phases and involve the cell making
proteins and organelles, so that both cells will have enough protein and organelles after they
divide. The S is the DNA synthesis phase in which the DNA is replicated. The mitotic phase
includes both mitosis and cytokinesis. The mitotic phase includes prophase, prometaphase,
metaphase, anaphase, and telophase. Cytokinesis and telophase partially overlap. Cytokinesis is
the process of the division of the cytoplasm forming two whole cells.
11. Draw out the mitotic phase, and describe what happens at each step.
Prophase: Microtubules begin to polymerize into aster and mitotic spindles. Sister chromatin
begin to condense. The nucleoli disappears. The centrosomes begin to move to opposite poles.
Prometaphase: The nuclear envelope begins to break apart. Chromosomes are completely
condensed. Kinetochore proteins attach to the centromere to the spindles. Kinetochore
microtubules attach to chromosomes. Overlapping non-kinetochore microtubules do not establish
contact with chromosomes. Kinetochore microtubules help to move chromosomes towards the
middle of the cell.
Metaphase: Sister chromatid align on the metaphase plate.
Anaphase: Sister chromatid are split into daughter chromosomes by separase which cleaves the
cohesion proteins. The microtubules begin to depolymerize pulling the chromosomes to opposite
poles.
Telophase: The nuclear envelope forms. Nucleoli reappear. Cytoplasm increases. Mitosis is now
complete. Cytokinesis begins.
Cytokinesis: The cytoplasm begins to divide. This forms a cleavage furrow in animal cells and a
cell plate in plant cells.
12. Which of the following processes or structures is mismatched with its location?
A. light reactions—grana
B. electron transport chain—thylakoid membrane
C. Calvin cycle—stroma
D. ATP synthase—double membrane surrounding chloroplast
E. splitting of water—thylakoid space
13. Accessory pigments within chloroplasts are responsible for
A. driving the splitting of water molecules.
B. absorbing photons of different wavelengths of light and passing that energy to P680 or P700.
C. providing electrons to the reaction-center chlorophyll after photoexcited electrons pass to
NADP+
D. pumping H+ across the thylakoid membrane to create a photon-motive force.
E. anchoring chloropyll a within the reaction center.
14. A cell in which of the following phases has the least amount of DNA?
A. G0
B. G2
C. prophase
D. metaphase
E. anaphase