Download 3.6 comparing photosynthesis and cellular respiration

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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Student solutions will vary depending on research. Some points that students may include follow:
• Even if there is concern that cutting down rain forests may contribute to global warming, the rain forests are generally in
impoverished nations. These nations deserve the chance to make money; other parts of the world are profitable, so these
countries deserve this chance as well.
• Even if preservation of rain forests does not help avoid the greenhouse effect, the amount of biodiversity that could be lost
from these areas is tremendous. Even if the rain forests do not help maintain carbon levels, they should be preserved for
other reasons.
Section 3.6 Questions
(Page 182)
Understanding Concepts
1.
Photosynthesis and cellular respiration are exact opposites with respect to reactants and products. The two metabolic
processes are different with respect to the series of reactions that take place in each. Also, photosynthesis absorbs light
energy and cellular respiration transfers energy from glucose to ATP.
2. An electron gains energy as a chlorophyll molecule absorbs a photon, loses some free-energy as it is passed from Q to b6-f
complex to plastocyanin, gains energy in PS I when a chlorophyll molecule absorbs a second photon, and finally, the
electron loses some energy as it passes its energy to the NADPH molecule. The “lost” energy appears as heat.
3. (a) If photosynthesis stopped, almost all life on Earth would cease to exist.
(b) The base of all food webs would disappear. The only organisms that would survive would be those that relied on
alternate forms of energy, such as hydrothermal vents on the ocean floor.
4. (a), (b)
5.
6.
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The energy profile for photosynthesis is a zigzag because of the absorption of light energy twice during the light reactions
of photosynthesis. The energy profile for cellular respiration is straight because the molecules continually lose energy
throughout the process, and no additional energy is added.
A tropical fish aquarium illustrates the relationship between photosynthesis and cellular respiration as plants
photosynthesize and create carbohydrates that the fish consume and use in cellular respiration. Moreover, plants release
oxygen, which is used in cellular respiration, and the fish produce carbon dioxide, which is used by the plants in
photosynthesis.
Unit 1 Metabolic Processes
Copyright © 2003 Nelson
Making Connections
7. (a) The interactions that students include in their answer follow:
• glycolysis
• pyruvate oxidation
• Krebs cycle
• lactate fermentation
• ethanol fermentation
• light reactions of photosynthesis
• Calvin cycle
• C4 pathway
(b) Hundreds of interrelated and complex biochemical pathways have been discovered in living organisms. It is difficult to
memorize them all. Charts illustrate the pathways and record them for future reference.
INVESTIGATION 3.1.1 LIGHT AND PHOTOSYNTHESIS
(Pages 183–184)
Hypothesis
(a) Since light is needed for photosynthesis, light is needed for plants to produce starch.
Analysis
(b) Yes, light is needed for plants to produce starch.
(c) Plants were kept in the dark for 48 hours to deplete them of starch. This controls the presence of starch in the plants’
leaves and allows students to test the hypothesis (it allows them to determine whether light is responsible the production
of starch in plant leaves).
(d) The leaves were placed in boiling ethanol to dissolve chlorophyll and remove it from the leaves.
Evaluation
(e) The experimental evidence supports the hypothesis. The parts of the leaf that were masked had less starch than the parts
exposed to light. The hypothesis is reasonable; light seems to be needed for starch production in plant leaves. Sources of
error include to following:
• The leaves may have been poorly masking by partially transparent paper.
• Plants were not kept in the dark long enough to deplete them of starch.
• Heat produced by the light source may have affected starch production.
(e) Keep plants in the dark longer before starting the experiment. Use opaque screening materials to mask the leaves. If using
artificial light sources, use “cool” sources that limit the amount of heat produced by the light source.
(f) Use different colours of light to determine whether different colours affect starch production. Use different intensities of
light to determine whether varying intensities of light affect the production of starch.
INVESTIGATION 3.2.1 IDENTIFYING PLANT PIGMENTS BY CHROMATOGRAPHY
(Page 184)
Hypothesis
(a) Plant leaves are different colours because they contain different pigments in their cells.
Prediction
(b) There will be several pigments in the leaf extracts of the two different plants.
(c) The pigments in each plant will be different.
Procedure
(d) Spinach
Rf very faint yellow line = 0.98
Rf yellow pigment = 0.5
Rf bright green-blue pigment = 0.28
Rf yellow-green (olive green) pigment = 0.15
Copyright © 2003 Nelson
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