Answers to Mastering Concepts Questions
5.1
1. How is an autotroph different from a heterotroph?
An autotroph uses inorganic molecules to make its own organic compounds, but
heterotrophs must eat other organisms to get their carbon.
2. What is photosynthesis?
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.
3. Why is photosynthesis essential to life?
Photosynthesis is essential to life because almost all life on the planet ultimately depends
on it for a food source.
5.2
1. What is the relationship between visible light and the electromagnetic spectrum?
The electromagnetic spectrum includes the visible light portion in addition to gamma
rays, X-rays, ultraviolet radiation, infrared radiation, microwaves, and radio waves.
2. How does it benefit a plant to have multiple types of pigments?
Multiple pigment types allow a photosynthetic organism to absorb energy from a broader
range of wavelengths of light.
5.3
1. Describe the relationship among the chloroplast, stroma, grana, and thylakoids.
Chloroplasts are organelles that contain the other structures or substances. Inside the
inner membrane of the chloroplast, the stroma is a fluid-filled space that surrounds grana,
which are stacks of thylakoids.
2. 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.
5.4
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 does each stage occur?
The light reactions occur in the thylakoid membranes of a chloroplast, and the carbon
reactions occur in the stroma of a chloroplast.
5.5
1. Describe the events that occur after light strikes photosystem II, 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
accumulate 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 an 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.
5.6
1. What happens in the carbon reactions?
The carbon reactions use NADPH and ATP from the light reactions to fix carbon dioxide,
producing the three-carbon molecule PGAL and, ultimately, glucose.
2. What are the roles of CO2, ATP, and NADPH in the Calvin cycle?
CO2 is the source of the carbon atoms that eventually become a glucose molecule. ATP
provides the energy and NADPH provides the electrons for the Calvin cycle reactions.
5.7
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. 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 four-carbon compound, which 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.
4. 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 a four-carbon compound 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 four-carbon molecule
within a vacuole. In the daytime, CAM plants minimize water loss by keeping their
stomata closed, but CO2 is released from the vacuoles and enters the Calvin cycle.
5.8
1. Explain the most important finding of this study.
A particular species of sea slug contains chloroplasts acquired from its food. The
researchers found that the slug’s DNA contains a gene that is required for photosynthesis
and that is not normally found in animals.
2. What evidence led the researchers to their conclusion?
The researchers first determined that the chloroplasts cannot operate independently in the
slug’s body, and then they searched the slug’s DNA for the required gene.
Answers to Write It Out Questions
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 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: thylakoid membrane;
photosystem; chloroplast; electron transport chain; reaction center.
From smallest to largest: reaction center; photosystem; electron transport chain;
thylakoid membrane; chloroplast.
a) Reaction center: the chlorophyll a molecule that actually uses light energy in
photosynthesis reactions.
b) Photosystem: a unit consisting of chlorophyll a aggregated with other pigment
molecules and proteins that anchor the entire complex in the membrane.
c) Electron transport chain: a group of aligned proteins that shuttle electrons,
releasing energy with each step; an electron transport chain links the two
photosystems.
d) Thylakoid membrane: internal membrane in a chloroplast; contains the
photosystems that capture light energy.
e) Chloroplast: organelle of photosynthesis in plants and algae.
4. Determine whether each of the following molecules is involved in the light reactions,
the carbon reactions, or both and explain how: O2, CO2, carbohydrates, 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 in the light reactions.
5. 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 and the
production of carbohydrates in photosynthesis. The plant used these carbohydrates to
build its tissues.
6. 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.
7. 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 in CO2 should become a part of the glucose (C6H12O6) molecule since it would
be fixed in the carbon reactions.
8. 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)? If so, how?
(a) Increasing CO2 levels should decrease the rate of photorespiration. (b) Increasing
temperatures should increase the rate of photorespiration.
9. 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. When daylight returns, they close
their stomata, which reduces water loss. Their stored CO2 moves into the chloroplasts,
where the carbon reactions occur.
10. 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 use some of the
glucose produced in photosynthesis to undergo respiration, which generates ATP. (c)
Accessory pigments such as carotenoids can contribute to photosynthesis.
Answers to Pull it Together Questions
1. Where does the electron transport chain fit into this concept map?
There are many ways to integrate “electron transport chain” into the concept map.
“Electron transport chain” can be connected to “Light reactions” with the phrase “is
essential to”. “Electron transport chain” can be connected to “ATP” and “NADPH” with
the word “produces”.
2. What specific process in the light reactions gives rise to the waste product, O2?
Water is split to replace the electrons that are ejected from chlorophyll in photosystem II.
The products of this reaction are H+ ions and the waste product, O2.
3. How would you incorporate the Calvin cycle, rubisco, C3 plants, C4 plants, and CAM
plants into this concept map?
The “Calvin cycle” is another name for the carbon reactions. “Rubisco” is the enzyme
that fixes CO2 in the carbon reactions. “C3 plants” use only the Calvin cycle; “C4 plants”
and “CAM plants” use additional pathways to avoid photorespiration.
4. Where do humans and other heterotrophs fit into this concept map?
“Heterotrophs” could attach to “Glucose” with the phrase “require”, as glucose is used to
create ATP energy in all organisms. “Heterotrophs” could also 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 “surrounds”, 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 used by the plants to fuel their own metabolism. Glucose is also used to form
the cellulose of cell walls. If excess glucose is produced, it is stored in the plant tissues
as starch. Herbivores use glucose stored in plants as a food source.