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PART 2
PLANT BIOLOGY
The process of photosynthesis as well as the structure and physiology of plants are detailed in the next
three chapters.
8
9
10
Photosynthesis
Plant Organization
Plant Reproduction, Growth and Development
CHAPTER 8 PHOTOSYNTHESIS
The chapter details how solar energy is incorporated into the photosynthetic process. The standards C3
photosynthesis is examined and the C4 and CAM variations are described. Photosynthesis and cellular
respiration are compared.
LEARNING OBJECTIVES
A student should learn the following concepts:
1. Plants use specific portions of solar energy in the visible light range to carry on photosynthesis.
2. Photosynthesis takes place in chloroplasts, organelles made of membranous thylakoids
surrounded by a fluid-filled stroma.
3. Photosynthesis involves light-dependent and light independent reactions.
4. Photosynthesis has two sets of reactions: solar energy is captured by the pigments in the
thylakoids, and carbon dioxide is reduced by enzymes in the stroma.
5. Solar energy energizes electrons and permits a buildup of ATP.
6. Carbon dioxide reduction requires energized electrons and ATP.
7. Photosynthesis provides most of the food for the biosphere.
8. Plants use either C3, C4, or CAM photosynthesis, named for the manner in which CO2 is fixed.
9. In aerobic cellular respiration, carbohydrate is oxidized to carbon dioxide and oxygen is reduced
to water. In photosynthesis, carbon dioxide is reduced and water is reoxidized, releasing oxygen
gas.
CHAPTER OUTLINE
8.1
Radiant Energy
Producers and Consumers
Plants are producers that make their own food after capturing solar energy. Animals are
consumers that take in pre-formed food. Plants, algae, and some bacteria perform photosynthesis.
Almost all life is dependent on solar energy directly or through stored fossil fuels.
Visible Light
Visible light rays are a small portion of the radiant energy from the sun. The visible spectrum
separates white light into a red, orange, yellow, green, blue and violet continuum. Chlorophyll
and carotenoid pigments absorb more of the violet-blue and orange-red portions of visible light.
8.2
Structure and Function of Chloroplasts
Cellular Organelles
Photosynthesis occurs in chloroplasts. Openings called stomata allow gases to enter the center of
a leaf where mesophyll cells contain chloroplasts. Vessels deliver water to these cells from the
plant roots.
Structure of Chloroplasts
In chloroplasts, thylakoids are stacked to form grana. Grana are surrounded by the stroma.
Function of Chloroplasts in Photosynthesis
During photosynthesis, water molecules are oxidized and CO2 is reduced. In detail,
photosynthesis consists of light-dependent and the light-independent reactions.
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8.3
Solar Energy Capture
Photosystems
The light-dependent reactions require the participation of photosystem I (PS I) and photosystem
II (PS II) which are located within the thylakoid membrane. Photosystems contain pigment
complexes that absorb solar energy and pass it on until it is concentrated in one particular
chlorophyll a molecule, called the reaction center chlorophyll a molecule. Electrons in the
reaction-center chlorophyll a become excited, escape, and are picked up by an electron-acceptor
molecule.
Cyclic Electron Pathway
In the cyclic electron pathway, an acceptor molecule in photosystem I sends the electrons down
an electron transport system that produces ATP before the electrons return to the same reaction
center chlorophyll a molecule from which they started.
Noncyclic Electron Pathway
In the noncyclic electron pathway, electrons, energized by light, leave the reaction-center
chlorophyll a molecule in photosystem II and are picked up by an acceptor. (PS II takes electrons
from water, which splits, releasing oxygen). The acceptor molecule passes the electrons down an
electron transport system that produces ATP. Spent electrons arrive at PS I where they are again
energized by light. This time an acceptor molecule passes the electrons on to NADP+ which
becomes NADPH.
ATP Production
Chemiosmosis explains the linkage between an electron transport system and ATP formation.
The Thylakoid Membrane
Protein complexes in the thylakoid membrane contain carriers that pump hydrogen ions (due to
water breakdown) into the stroma from the thylakoid space. Hydrogen ions then flow down their
concentration gradient through a protein complex that contains ATP synthase. In the flow of
hydrogen ions down a concentration gradient, ATP molecules are formed from ADP + P.
8.4
Carbohydrate Synthesis
Light-Independent Reactions
Carbohydrate synthesis occurs during light-independent reactions in the stroma of chloroplasts.
The light-independent reactions use the ATP and the NADH formed during the light-dependent
reactions.
Overview of Calvin Cycle
Carbon dioxide is taken up by RuBP. The resulting six-carbon molecule splits producing two
PGA molecules. PGA is reduced to PGAL using ATP and NADH from the light-independent
reactions.
The Role of Glucose
Only one PGAL per turn is used to make glucose, the rest are used to reform RuBP. PGAL is
converted to all the other molecules needed by plants.
Stages of the Calvin Cycle
Fixation of Carbon Dioxide
Fixation of carbon dioxide occurs when the molecule is taken up by RuBP. RuBP carboxylase is
present in huge quantities because it is a slow enzyme.
Reduction of Carbon Dioxide
Reduction of carbon dioxide occurs when PGA is reduced to PGAL. This step requires ATP and
NADPH produced during the light-dependent reactions.
Regeneration of RuBP
Because five PGAL are needed to reform three RuBP, it takes three turns of the cycle to have a
net gain of one PGAL, which can be used to form glucose.
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8.5
Other Types of Photosynthesis
Variations in Photosynthesis Have Evolved
The C3 pathway is only one of several pathways; each has an efficiency under certain
environmental conditions.
C3 Versus C4 Photosynthesis
In C3 plants, the first detectable molecule following carbon dioxide fixation is a C3 molecule. In
these plants mesophyll cells contain chloroplasts. In C4 plants, the first detectable molecule
following carbon dioxide fixation is a C4 molecule and in these plants, the bundle sheath cells
surrounding a leaf vein also contain chloroplasts. The mesophyll cells fix carbon dioxide and then
pass it to bundle sheath cells where the Calvin Cycle is located. C4 photosynthesis is more
complicated but more efficient in hot dry weather when stomate closure is likely. When stomates
close in C3 plants, but not C4 plants, oxygen competes with CO2 for the active site of RuBP.
CAM Photosynthesis
In CAM photosynthesis, C4 formed at night releases CO2 to the Calvin Cycle during the day
when the stomates are closed to conserve water. It is common in cactus and other succulent
plants.
8.6
Photosynthesis Versus Cellular Respiration
Photosynthesis Is Generally the Reverse of Respiration
During photosynthesis, PGA is reduced to PGAL and during aerobic cellular respiration PGAL is
oxidized to PGA. Both cellular respiration and photosynthesis utilize an electron transport system
that produces ATP by chemiosmosis.
NINTH EDITION CHANGES
New/Revised Text:
Section 8.2 Structure and Function of Chloroplasts has been rewritten to clarify the significance of the
light-dependent and light-independent reactions. The section Organization of the Thylakoid (formerly
ATP Production) has been rewritten to simplify the presentation.
8.4 Carbohydrate Synthesis now better emphasizes the formation of glucose as an end product.
New/Revised Figures:
The chloroplast figure was revised, and a new icon of the chloroplast was added to many figures in
this chapter. Figure 8.1 Photosynthetic organisms; 8.2 The electromagnetic spectrum; 8.6 Organization of
thylakoids. Fig. 8.7 The light-independent reactions: the Calvin cycle (simplified); 8.8 The lightindependent reactions: the Calvin cycle (in detail); 8.10 Photosynthesis versus cellular respiration
New Bioethical Focus: Food for the Human Population
TECHNOLOGY CONNECTIONS
Annual Review of Plant Physiology and Plant Molecular Biology addresses a wide range of plant
physiology topics in journal review articles with a website at
http://photoscience.la.asu.edu/photosyn/Default.html
Oxygen: Toxin for Aquatic Plants? is the question pursued at this website that presents data related to
whether oxygen is harmful to submerged plants at http://www.dupla.com/e042.htm
Photosynthesis Center at Arizona State University provides extensive information and links to
photosynthesis research at http://photoscience.la.asu.edu/photosyn/Default.html
Photosynthesis and the Web: 2001 provides extensive information and links to photosynthesis research at
http://www.life.uiuc.edu/govindjee/photoweb/
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TECHNOLOGY RESOURCES [BOLD = RECOMMENDED]
Absorbing the Light (FH), 10 min. video
Advanced Photosynthesis (PLP), Mac
Architecture of Cells: Special Structure, Special Function (HRM), filmstrip
C3 and C4 Plants (FH), 10 min. video
Cell Structure and Energy Production (IM), 60 min. video
The Chloroplast and Photosynthesis (PLP), 11 min. video
The Chloroplast and Photosynthesis (IM), 14 min. video
Computer Investigations: Plant Growth (PLP), Mac
Concepts in Science: Photosynthesis (CBSC), 60 min. video
Cycles of Life: Exploring Biology–Plant Structure (A-CPB), 30 min. video
The Dark Reaction (FH), 10 min. video
An Electronic Companion to Molecular Cell Biology (NHBS), Mac, Win CD
Energy Flow at the Cellular Level (FH), 10 min. video
Gardens of Biology: Vol. 1, Energetics and Metabolism (CSG), Mac
Harvesting the Sun–Sunlight and Plants (Biology: Form and Function) (CPB) (IM), 24 min. video
How Cells Obtain Energy (PLP), 17 min. video
Introduction to Photosynthesis (IM), 30 min. video
The Kingdom of Plants (IM), 45 min. video
The Leaf (PLP), Mac and Win CD
Leaf Internal Anatomy (JLM), slide set (20)
Leaf: Structure and Function (PLP), Win
The Light Reaction (FH), 10 min. video
Light: Wave and Quantum Theories (PHO), 16 min. video
The Living Cell (H&R), 27 min. video
Metabolic Pathways (INT), Mac
Metabolism: The Fire of Life (IM) (PLP), 35 min. video
Molecular Structures in Biology (CD-ROM version) (NHBS), Mac, Win CD
Photosynthesis (CAM) (CBSC) (PLP), Mac, Win
Photosynthesis (CBSC), 35 min. video
Photosynthesis (IM), 20 min. video
Photosynthesis (IM), 60 min. video
Photosynthesis (CAM) (CYBER), Mac, Win CD
Photosynthesis (EBE), 20 min. video
Photosynthesis (PLP), Mac and Win CD
Photosynthesis (SK&BL), 6 10-min. videos
Photosynthesis (SK&BL), Mac, Win CD
Photosynthesis and Assimilative Transport (FH), 15 min. video
Photosynthesis: Chemistry of Food Making (PHO), 18 min. video
Photosynthesis: Energy from Light (CH), slide, video
Photosynthesis: Life Energy (IM) (NGS), 22 min. video
Plant Biology Tutor CD-ROM (NHBS), Mac, Win CD
The Plant: Nature’s Food Factory (PLP), Mac
Plants: Problems with Water (IM), 24 min. video
Seeing the Light (FH), 10 min. video
Visualizing Cell Processes: Photosynthesis and Cell Respiration (CBSC), 15 min. video
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LECTURE ENRICHMENT INCLUDING TOPICS AND PROJECTS UTILIZING
SCIENTIFIC REASONING
8.1
Radiant Energy
1. Have students consider that on a spaceship greenhouse, if it was necessary to block out one general
wavelength (color) to decrease light intensity, what would be the least damaging color of glass pane
to install to preserve maximum photosynthesis? If you could install panes that only admitted those
wavelengths that the plant leaves absorbed, what color would these leaves appear to a person inside
this greenhouse?
2. Consider two new plant pigment molecules, one that absorbs light at the red end of the spectrum and
one that absorbs light at the violet end. Given the same light intensity, which would be more energy
productive?
3. Some students may have a tendency to interpret the fact that both vision and photosynthesis are
“tuned” to the middle of the spectrum to be an indication of either supernatural planning or the world
being built for human use. Use class discussion to focus on how organisms must work with what is
available and is most efficient.
8.2
Structure and Function of Chloroplasts
1. Plant physiology books provide tables listing the number of stomates on the top and/or bottom of
various leaves; tree leaves often have all stomates on the bottom, floating leaves may have all on the
dry top, and many grasses will have stomates distributed equally top and bottom. If petroleum jelly
sealed off and suffocated a plant leaf, how would you need to apply it in these cases to kill the leaf?
8.3
Solar Energy Capture
1. Relate the work of von Helmont long before the discovery of gases in the air. He attributed the
weight gain of his plant totally to the water added. After having lectured on photosynthesis, ask
students to explain where von Helmont was correct and where he was wrong, using correct chemical
terminology.
2. This is also a good opportunity to illustrate how some science questions must await developments in
other fields and advanced instrumentation.
3. Term paper topics: von Helmont’s original work with plant mass; current research with efficient plant
forms of possible use in supplying oxygen and food on long-distance space flights.
8.4
Carbohydrate Synthesis
1. PGAL is the raw material for many plant products, as shown in Figure 8.9; for students familiar with
chemistry, fuller chemical formulae can illustrate the cell metabolic pathways.
2. Students read in Chapter 6, page 109, that “Only a small amount of enzyme is actually needed in a
cell because enzymes are not used up by the reaction.” Nevertheless, RuBP carboxylase makes up
20–50% of the protein content of chloroplasts. This is a good point to chart various enzyme speeds,
from peroxidase that can process over 100,000 reactions per second to more average enzymes.
8.5
Other Types of Photosynthesis
1. Pose the question: If there are differences in efficiencies of C3 and C4 plants, why doesn’t one or the
other come to totally dominate a continent? This does require some knowledge of variation in climate
etc.
2. Point out how the anatomy of C3, C4, and CAM plants is intimately involved with the biochemistry.
3. Term paper topic: how is research conducted to prove if C4 plants are more efficient than C3 plants in
hot, dry climates.
8.6
Photosynthesis Versus Cellular Respiration
1. Since carbon is neither created nor destroyed, is there any requirement that the amount of
photosynthesis and cellular respiration be exactly equal over the short term of hours, days, years, or
geological ages? Be sure students consider the carbon tied up in fossil fuels.
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ESSAY QUESTIONS AND ANSWERS
1. Compare the differences in cellular respiration with photosynthesis.
Answer: The overall equation for aerobic cellular respiration is the opposite of that for
photosynthesis:
energy + 6 CO2 + 6 H2O — — — — >C6H12O6 + 6 O2 (photosynthesis)
energy + 6 CO2 + 6 H2O <— — — — C6H12O6 + 6 O2 (aerobic cellular respiration)
Also, cellular respiration occurs in the mitochondria, oxidation of glucose occurs, and energy and
carbon dioxide are released. It requires oxygen and occurs in both plants and animals, day or night.
Photosynthesis occurs in the chloroplast, reduction occurs, energy and oxygen are released while
requiring carbon dioxide. Photosynthesis occurs only during daytime in plants.
2. As days get shorter as winter approaches, what happens to the levels of oxygen and carbon dioxide
produced by a plant such as a pine tree during both day and night?
Answer: Cellular respiration and CO2 production continues during the night, while oxygen
production dwindles as the daylight period shortens.
3. Von Helmont was unaware of gases in the atmosphere; therefore, he measured only the amount of
soil and presumed the plant tissue all came from the water added. Today we can label molecules with
radioisotopes and trace where they travel. Where would we find carbon dioxide labeled with
radiocarbon?
Answer: Carbon dioxide taken into a leaf is converted in photosynthesis into sugars and then other
structural compounds. A percentage of this is then used in respiration, and the carbon would be
released in carbon dioxide although in daytime, that in turn could be reincorporated by
photosynthesis.
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