Mastering Concepts
5.1
1. What is photosynthesis? Describe the reactants and products in words and in chemical
symbols.
Photosynthesis is the chemical process in which plants, algae, and some microorganisms
convert solar energy into chemical energy. A photosynthetic cell uses light energy to
form glucose and oxygen gas from carbon dioxide and water. The chemical equation is:
light energy
6CO2 + 6H2O >>>>>>>>>>>>>>>> C6H12O6 + 6O2
2. Why is photosynthesis essential to life on Earth?
Photosynthesis is essential to life on Earth because almost all life on the planet ultimately
depends on it for a food source.
3. What happens to the glucose that plants produce?
The glucose that plants produce in photosynthesis is used as fuel for the plant’s own
cellular respiration, which generates the ATP that powers biochemical reactions and
allows the plant to grow. The glucose is also used to produce other chemical compounds
(including amino acids) and to produce cellulose for plant cell walls. Excess glucose is
stored as starch or sucrose.
4. How is an autotroph different from a heterotroph?
Autotrophs can make their own organic compounds, but heterotrophs have to get their
carbon by consuming other organic compounds.
5. How did the origin of photosynthesis alter Earth’s atmosphere and the evolution of
life?
The evolution of photosynthesis filled the atmosphere with O2 gas giving an evolutionary
advantage to those organisms that could use O2 in respiration to produce the most energy.
The oxygen also helped produce the ozone layer, which decreased damage from the sun
and lead to an explosion in life’s diversity.
5.2
1. What are the three main components of sunlight?
The three main components of sunlight are high-energy radiation such as ultraviolet
radiation; visible light; and low-energy radiation such as infrared radiation.
2. Describe the relationship among the chloroplast, stroma, grana, and thylakoids.
Chloroplasts are organelles that contain the other structures or substances. Inside the
chloroplast, the stroma is a fluid-filled space that surrounds grana, which are stacks of
thylakoids.
3. How does it benefit a photosynthetic organism to have more than one type of pigment?
Multiple pigments allow a photosynthetic organism to absorb energy from a broader
range of wavelengths of light.
4. How does the reaction center chlorophyll interact with the antenna pigments in a
photosystem?
The antenna pigments capture light energy and send it to the reaction center chlorophyll,
which uses it for the reactions of photosynthesis.
5.3
1. What happens in each of the two main stages of photosynthesis?
In the first stage of photosynthesis (light reactions), light energy is captured by pigments
and converted to the chemical energy of ATP and NADPH. In the second stage of
photosynthesis (carbon reactions), the energy of ATP and the electrons in NADPH are
used to make glucose from CO2.
2. Where in the chloroplast do the light reactions and the carbon reactions occur?
The light reactions occur in the thylakoid membrane of a chloroplast, and the carbon
reactions occur in the stroma of a chloroplast.
5.4
1. Describe the events in photosystem II, beginning with light and ending with the
production of ATP.
In photosystem II, light strikes a photosynthetic pigment; energy is absorbed; energy
bounces to the chlorophyll molecule in the reaction center, which releases two energized
electrons; the electrons are replaced by two electrons stripped from a water molecule,
forming oxygen gas (O2) and two hydrogen ions (H+); as the energized electrons move
along the proteins of the electron transport chain of photosystem II, hydrogen ions build
up in the space within the thylakoid, forming a reservoir of potential energy; as ATP
synthase moves hydrogen ions back to the stroma, the released energy bonds a phosphate
group onto ADP, forming ATP.
2. How do electrons pass from photosystem II to photosystem I?
Electrons pass from photosystem II to photosystem I in the electron transport chain.
Light strikes antenna pigments in photosystem I; these pigments transfer the energy to the
reaction center chlorophyll molecule of photosystem I. The reaction center chlorophyll
releases two energized electrons, which are replaced by the electrons from photosystem
II.
3. How are the electrons from photosystem II replaced?
The boosted electrons lost from the reaction center in photosystem II are replaced by
electrons stripped from a water molecule.
4. What happens in photosystem I?
In photosystem I, energy from sunlight energizes a pair of electrons in the reaction center
(replaced by a pair of electrons from photosystem II) and these are passed to molecules of
NADP+ to reduce them to NADPH. This molecule carries the electrons (and potential
energy) to the carbon reactions of photosynthesis.
5.5
1. What is the product of the carbon reactions?
The carbon reactions produce the three-carbon molecule PGAL.
2. What are the roles of rubisco, RuBP, ATP, and NADPH in the Calvin cycle?
Rubisco is an enzyme used to fix CO2 by combining it with RuBP, which is a five-carbon
sugar. ATP provides the necessary energy and NADPH the necessary electrons for this
reaction.
3. What is the relationship between the light reactions and the carbon reactions?
The light reactions provide the energy and electrons for the carbon reactions (in the form
of ATP and NADPH). The carbon reactions use the energy and electrons from the light
reactions to reduce CO2, forming organic molecules (such as glucose).
5.6
1. Why is the Calvin cycle also called the C3 pathway?
The Calvin cycle is called the C3 pathway because the first stable compound produced is
the three-carbon molecule PGA.
2. How does photorespiration counter photosynthesis?
In photorespiration, the rubisco enzyme uses O2 instead of CO2. The resulting chemical
reaction liberates CO2 that has already been fixed, and thus counters photosynthesis.
3. What conditions maximize photorespiration?
Photorespiration is more likely when plants close their stomata to minimize water loss,
letting CO2 levels fall while O2 levels rise.
5.7
1. Describe how a C4 plant minimizes photorespiration.
The leaves of C4 plants have a distinctive arrangement of mesophyll cells and bundle
sheath cells. Each vein is surrounded by a concentric ring of bundle-sheath cells that, in
turn, are surrounded by a concentric ring of mesophyll cells. In mesophyll cells, CO2 is
converted to a 4-carbon compound, oxaloacetate, which is reduced to malate. The malate
then moves to the bundle sheath cells, where the CO2 is released to the Calvin cycle. The
high CO2 concentration and low O2 concentration in the bundle sheath cells reduces the
effects of photorespiration.
2. How is the CAM pathway like C4 metabolism, and how is it different?
The CAM pathway is like C4 metabolism since it occurs in plants from a hot and dry
climate, and malate is used during the fixation of CO2. CAM plants differ from C4 plants
as they open their stomata at night and CO2 diffuses into spaces within leaves. Mesophyll
cells then store the carbon dioxide in a 4-carbon molecule within a vacuole. In the
daytime, when CAM plants minimize water loss by keeping their stomata closed, CO2
moves from vacuoles and enters the Calvin cycle.
5.8
1. Explain the most important finding of this study, and describe the evidence the
researchers used to arrive at their conclusion.
After observing that the slug cells contained an identical copy of the psbO gene, whereas
the chloroplast did not contain the gene, the researchers concluded that the slug's own
cells provided the necessary photosynthetic proteins and that the chloroplasts were not
independent of the slug.
2. The researchers also looked for the psbO gene in pufferfish (a vertebrate animal) and
slime molds (a nonphotosynthetic protist). The gene was absent in both species. How was
this finding important to the interpretation of the results of this study?
The pufferfish and slime molds served as control groups. The fact that the psbO gene
was found in the sea slugs, but not these other eukaryotic organisms, strengthened the
conclusion that the gene was present as a result of horizontal gene transfer as opposed to
vertical gene transfer from some ancestral species.
Write It Out
1. Photosynthesis takes place in plants, algae, and some microbes. How does it affect a
meat-eating animal?
Photosynthesis produces the plant tissue that herbivores eat. A meat-eating animal that
eats the herbivore therefore relies indirectly on photosynthesis. Photosynthesis also
produces O2, which meat-eating animals require.
2. What color would plants be if they absorbed all wavelengths of visible light? Why?
If plants absorbed all wavelengths of visible light, they would appear black because no
light would be reflected.
3. Define these terms and arrange them from smallest to largest:
a. thylakoid membrane
b. chloroplast
c. reaction center
d. photosystem
e. electron transport chain
From smallest to largest: reaction center; photosystem; electron transport chain;
thylakoid membrane; chloroplast.
a) Thylakoid membrane: internal membrane in a chloroplast; contains the
photosystems that capture light energy
b) Chloroplast: organelle of photosynthesis in plants and algae
c) Reaction center: the chlorophyll a molecule that actually uses the energy in
photosynthesis reactions.
d) Photosystem: a unit consisting of chlorophyll a aggregated with other pigments
molecules and proteins that anchor the entire complex in the membrane.
e) Electron transport chain: a group of aligned proteins that shuttle electrons,
releasing energy with each step; an electron transport chain links the two
photosystems.
4. Determine whether each of the following molecules is involved in the light reactions,
the carbon reactions, or both and explain how: O2, CO2, carbohydrate, chlorophyll a,
photons, NADPH, ATP, H2O.
O2 is produced by the light reactions. CO2 is fixed during the carbon reactions and
provides the carbon source for sugar. Carbohydrates are the product of the carbon
reactions. Chlorophyll a is used in the light reactions to absorb light energy. Photons of
light provide the energy input for the light reactions. NADPH is the electron carrier
produced in the light reactions, which provides the source of electrons for the reduction
of CO2 in the carbon reactions. ATP is the chemical energy produced in the light
reactions and used in the carbon reactions for the synthesis of PGAL. H2O provides the
replacement electrons for photosystem II and the H+ for chemiosmotic phosphorylation in
the light reactions.
5. The light reactions described in this chapter are sometimes called “noncyclic
photophosphorylation” because the electron transport chain that generates ATP does not
return the electrons to chlorophyll. Some photosynthetic bacteria, however, generate ATP
by cyclic phosphorylation. In these cells, light energy boosts chlorophyll’s electrons,
which pass through an electron transport chain and then return to the chlorophyll
molecule. Would the light reactions of cyclic photophosphorylation produce ATP?
NADPH? O2? Explain your answers.
Cyclic phosphorylation would directly use the excited electrons from chlorophyll a to
generate the ATP for the carbon reactions. Because water is not necessary to replace the
lost electrons in photosystem II, neither oxygen nor NADPH would be produced.
6. Of the many groups of photosynthetic bacteria, only cyanobacteria use chlorophyll a.
How does this observation support the hypothesis that cyanobacteria gave rise to the
chloroplasts of today’s plants and algae?
In plants, the main photosynthetic pigment is chlorophyll a. Out of all of the existing
photosynthetic bacteria, the only possible source of this pigment is cyanobacteria.
7. One of the first investigators to explore photosynthesis was Flemish physician and
alchemist Jan van Helmont. In the early 1600s, he grew willow trees in weighed amounts
of soil, applied known amounts of water, and noted that in 5 years the trees gained more
than 45 kg, but the soil had lost only a little weight. Because he had applied large
amounts of water, van Helmont concluded (incorrectly) that plants grew solely by
absorbing water. What is the actual source of the added biomass? Explain your answer.
The actual source of the added biomass was attained through carbon fixation into plant
material.
8. One of the classic experiments in photosynthesis occurred in 1771, when Joseph
Priestley found that if he placed a mouse in an enclosed container with a lit candle, the
mouse would die. But if he also added a plant to the container, the mouse could live.
Priestley concluded that plants “purify” air, allowing animals to breathe. What is the
biological basis for this observation?
The plant released O2 as a product of photosynthesis; in turn, the mouse used the O2 in
aerobic respiration.
9. In 1941, biologists exposed photosynthesizing cells to water containing a heavy
oxygen isotope, designated 18O. The “labeled” isotope appears in the O2 gas released in
photosynthesis, showing that the oxygen came from the water. Where would the 18O
have ended up if the researchers had used 18O-labeled CO2 instead of H2O?
The 18O that was used in the CO2 should become a part of the glucose (C6H12O6)
molecule since it was fixed in the carbon reaction.
10. How does photorespiration counteract photosynthesis?
In photorespiration, the rubisco enzyme uses O2 instead of CO2. The resulting chemical
reaction liberates CO2 that has already been fixed, and thus counters photosynthesis.
11. When vegetables and flowers are grown in greenhouses in the winter, their growth
rate greatly increases if the CO2 concentration is raised to two or three times the level in
the natural environment. What is the biological basis for the increased rate of growth?
Assuming that carbon (and not some other essential element) limits growth, the increased
availability of CO2 means more carbon can be incorporated into glucose, which in turn
contributes to plant growth.
12. Over the past decades, the CO2 concentration in the atmosphere has increased.
a. Predict the effect of increasing carbon dioxide concentrations on photorespiration.
b. Scientists suggest that increasing CO2 concentrations are leading to higher average
global temperatures. If temperatures are increasing, does this change your answer to part
(a)?
(a) Increasing CO2 levels should decrease the rate of photorespiration. (b) Increasing
temperatures should increase the rate of photorespiration.
13. How is the CAM pathway adaptive in a desert habitat?
CAM photosynthesis is adaptive in desert environments because CAM plants open their
stomata only at night to take in large amounts of CO2. They incorporate it into malate and
store it into vacuoles. When daylight returns, they close their stomata, which reduces
water loss. Their stored CO2 moves into the chloroplasts, where the carbon reactions
occur.
14. Explain how C4 photosynthesis is based on a spatial arrangement of structures,
whereas CAM photosynthesis is temporally based.
C4 plants separate the light and carbon reactions of photosynthesis into different cells.
The Calvin cycle occurs in the bundle sheath cells, which are not exposed directly to the
CO2 in air. For CAM plants, CO2 is acquired and temporarily fixed at night, but then the
light reactions and the Calvin cycle occur during the day.
15. Explain why each of the following misconceptions about photosynthesis is false:
a. Only plants are autotrophs.
b. Plants do not need cellular respiration because they carry out photosynthesis.
c. Chlorophyll is the only photosynthetic pigment.
(a) Some bacteria, archaea, and protists are also autotrophs. (b) Plants also undergo
respiration to generate ATP they need for endergonic reactions. (c) Accessory pigments
such as carotenoids can contribute to photosynthesis.
Pull It Together
1. Where does the electron transport chain fit into this concept map?
(OMIT image)
“Electron transport chain” could be connected to “Photosynthesis” by “requires”, and
could connected to “Light reactions” by the phrase “is critical to”.
2. What specific process in the light reactions gives rise to the waste product, O2?
The specific event in the light reactions that gives rise to the oxygen begins with photons
exciting a chlorophyll, which releases an electron from a water molecule. The released
electron breaks up the water (H2O) into reactive atoms. The free hydrogen atoms create
the gradient across the membrane, and the freed oxygen atoms pair into oxygen gas.
3. How would you incorporate the Calvin cycle, rubisco, C3 plants, C4 plants, and CAM
plants into this concept map?
“CO2” connects to “Calvin Cycle” with “enters”. “O2” connects to “Calvin Cycle” with
“(in the case of a C3 plant) can enter”. “Calvin Cycle” leads to: “Glucose” with
“produces”, “C4 plants” with “occurs in the bundle-sheath cells of”, and “C3 and CAM
plants” with “occurs in the mesophyll cells of”.
4. Where do humans and other heterotrophs fit into this concept map?
“Heterotrophs” could lead to “photosynthesis” with the phrase “rely on autotrophs that
perform”
5. Build another small concept map showing the relationships among the terms
chloroplast, stroma, grana, thylakoid, photosystem, and chlorophyll.
“Chloroplasts” leads to “Stroma” with the phrase “contain”, which leads to “Grana” with
the phrase “contains”, which leads to “Thylakoids” with the phrase “are made up of
stacks of”, which leads to “Photosystems” with the phrase “membranes are embedded
with”, which leads to “Chlorophyll” with the phrase “contain”.
6. What happens to the glucose produced in photosynthesis?
Glucose is either stored as glycogen, assimilated into structural support as cellulose, or
used by mitochondria to make chemical energy.