Download 3.5 photosynthesis and the environment

alternative methods of carbon fixation that concentrate CO2 at the site where rubisco is found, thereby suppressing
photorespiration.
6. (a) The most malate in a CAM plant would be found late at night.
(b) The least malate in a CAM plant would be found at the end of the day.
(c) CAM plants close their stomata during the day to prevent water loss, as they live in dry environments.
(d) The gas that is preferentially absorbed during the cool evening is CO2.
(e) During the dark, CAM plants take in CO2 and incorporate it into C4 organic acids using PEP carboxylase. These are
stored in vacuoles until morning.
Applying Inquiry Skills
7. (a) Students should focus on the carboxylation reactions of rubisco and the generation of chemical potential energy, which
can be used in metabolism or consumed by other organisms. They may include the following points:
• Rubisco captures atmospheric carbon dioxide.
• Rubisco is responsible for starting the carbon fixation cycle.
• Without rubisco, plants could not make carbohydrates.
• Without plants, consumers could not survive.
(b) Students should focus on the oxygenase reactions of rubisco and wasting of energy that was captured in the light
reactions of photosynthesis. They should show that photorespiration reduces the amount of carbon that is fixed by a
plant.
• Rubisco takes away PGA that could be used to make sugar.
• Rubisco takes some of the chemical energy that the light reactions of photosynthesis worked so hard to makes and
wastes it.
8. The acid content of a CAM plant kept in the sunlight for 24 hours would be low when compared with a CAM plant kept
in the dark for 24 hours.
Making Connections
10. If a person were looking for CAM plant species, they would look in hot arid environments. CAM plants close their
stomata during the day to prevent water loss through the pores and open them in the night when it’s cooler to fix carbon.
11. If the greenhouse effect continues, C3 plants would benefit the most because they have the lowest light-compensation
point and will therefore photosynthesize the most. Furthermore, C3 plants are more efficient in fixing carbon, as they do
not require the use of intermediate C4 molecules.
12.(a) A weed is a plant growing out of place.
(b) Canada thistle: Cirsium arvense; dandelion: Taraxacum officinale; purslane: Portulaca oleracea
(c) Corn, sorghum, sugar cane
(d) As CO2 levels increase, C3 plants will have a growth advantage over C4 plants. C3 plant numbers are expected to
increase relative to C4 plants, as rubisco is reduced inefficiently in C3 plants because of higher atmospheric CO2
concentrations.
3.5 PHOTOSYNTHESIS AND THE ENVIRONMENT
Section 3.5 Questions
(Page 178)
Understanding Concepts
1.
2.
Irradiance limits the overall rate of photosynthesis under low light conditions. As irradiance increases, photosynthesis
begins.
C4 plants generally exhibit higher rates of photosynthesis when compared with C3 plants because of the fact that although
photorespiration rates increase with increasing temperatures in C3 plants, the loss of CO2 by photorespiration in C4 plants
is almost nonexistent. C4 plants spatially separate the light reactions and the Calvin cycle. This separation means that the
concentration of CO2 is much higher than the concentration of O2 in the bundle sheath cells, which means that the rate of
CO2 fixation does not go down because of increase in photorespiration.
Applying Inquiry Skills
3.
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The amount of oxygen evolved over time (mmol O2/m2/s) is not a good indicator of the rate of photosynthesis of a plant,
because photorespiration and cellular respiration use oxygen as a final electron acceptor.
Unit 1 Metabolic Processes
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4.
The student who measured water absorption would not obtain reliable results because the water absorbed by a plant can
be required to replace that lost by transpiration, not just photosynthesis. Plants require most of the water they absorb to
replace water lost by transpiration, not metabolism.
5. (a) Light compensation point (CO2 uptake = CO2 evolution).
(b) Point A represents the light intensity at which the rate that CO2 is fixed into a carbohydrate is equal to the CO2
produced by photorespiration and cellular respiration.
(c) Light-saturation point.
(d) The light-saturation point occurs when carbon fixation reactions are supplying the Calvin cycle with more ATP and
NADPH than it can use.
6.
7.
Students’ answers will depend on whether they are concerned with highest activity or greatest activity with lowest
irradiance. For example, curve A has the highest photosynthetic activity at high irradiance levels; however, curve B has
the highest photosynthetic activity at low irradiance levels.
8. (a) C3 plants have a higher CO2 compensation point than C4 plants, because they require higher CO2 concentrations to outcompete O2 for the active site of rubisco. In contrast, C4 plants spatially separate their rubisco enzymes to ensure high
CO2 concentration and low O2 concentration.
(b) As CO2 concentrations increase, the rate of photosynthesis increases faster in C4 plants than in C3 plants, because C4
plants can continue to use up ATP and NADPH to form oxaloacetate. However, the rubisco in C3 plants is still in the
presence of oxygen and PGA, which are both reactants for its enzymatic functions. As photosynthetic activity increases
in C3 plants, because of an increase in carbon dioxide concentrations, the concentration of oxygen will increase in the
chloroplasts, because of the light reactions of photosynthesis. The carboxylase and oxygenase functions of rubisco are
in a state of equilibrium depending on substrate concentrations. Therefore, as the concentration of carbon dioxide goes
up, the concentration of oxygen will increase where the rubisco enzyme is localized in C3 plants. Since C4 plants have
their rubisco spatially separated from oxygen production, increased photosynthetic activity does not affect rubisco’s
carboxylation activity.
(c) In an environment where CO2 concentrations are rising, C4 plants will gain an immediate advantage because of their
high rates of CO2 uptake; however, if the CO2 concentrations rise high enough, C3 plants have higher rates of CO2
uptake. This higher rate of CO2 uptake in C3 plants is due to a greater efficiency, because they require no intermediate
molecule to fix CO2.
Making Connections
9.
Higher CO2 concentrations and temperatures will favour the C4 plants over the C3 plants, because C4 plants’ rate of carbon
fixation increases faster as CO2 concentrations go up and remain stable in higher temperatures. C4 plants will out-compete
the C3 plants because of their faster rates of carbon fixation. This will affect the flora of ecosystems, resulting in
unforeseen changes in ecosystem species composition and population sizes. Furthermore, this change in plant species may
increase crop yields.
3.6 COMPARING PHOTOSYNTHESIS AND CELLULAR RESPIRATION
Explore an Issue: Take a Stand: Tropical Rainforest Depletion: Is There Cause for Concern?
(Page 180)
Statement: The governments of developing countries have the right to make room for agriculture and industrialization by
clearing tropical rain forests.
Copyright © 2003 Nelson
Chapter 3 Photosynthesis
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