Essentials of Biology
Sylvia S. Mader
Chapter 6
Lecture Outline
Prepared by: Dr. Stephen Ebbs
Southern Illinois University Carbondale
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
6.1 Overview of Photosynthesis
• Photosynthesis transforms solar energy
into the chemical energy of carbohydrates.
• Photosynthetic organisms include plants,
algae, and cyanobacteria.
• The products of photosynthesis provide
both food and fuel (coal, wood) to humans.
Overview of Photosynthesis (cont.)
Flowering Plants as
Photosynthesizers
• The green portions of plants, such as
leaves, carry out photosynthesis, using
carbon dioxide and water as substrates.
• Carbon dioxide enters leaves through
openings called stomata.
• The carbon dioxide and water diffuse to
the chloroplast, the site of photosynthesis.
Flowering Plants as
Photosynthesizers (cont.)
• The structure of the chloroplasts is
important to photosynthesis.
– The chloroplast has a double membrane that
surrounds the liquid stroma.
– The stroma contains numerous flat thylakoid
disks arranged in stacks called grana.
– The chlorophyll pigments imbedded in the
thylakoid membranes absorb solar energy
during photosynthesis.
Flowering Plants as
Photosynthesizers (cont.)
The Photosynthetic Process
• The overall equation for photosynthesis
can be written in this form.
gain of hydrogen atoms
Solar energy
reduction
CO2 + H2O
(CH2O) + O2
oxidation
loss of hydrogen atoms
The Photosynthetic Process (cont.)
• The equation for photosynthesis can also
be written in another form to show the
formation of the product, glucose.
Solar energy
6 CO2 + 6 H2O
C6H12O6 + 6 O2
Two Sets of Reactions
• Photosynthesis involved two sets of reactions.
– The light reactions
– The Calvin cycle reactions
• The light reactions involve several events.
– Chlorophyll absorbs solar energy, which energizes
electrons.
– ATP is produced using an electron transport chain.
– NADP+, a coenzyme, accepts electrons to become
NADPH.
Two Sets of Reactions (cont.)
• The Calvin cycle reactions involve several
events.
– CO2 is taken up by one of the substrates in
the cycle.
– ATP and NADPH from the light reactions
reduce CO2 to a carbohydrate.
Two Sets of Reactions (cont.)
6.2 Light Reactions
• Solar energy can be described in terms of its
wavelength and energy content.
• While there are several forms of solar, or radiant
energy, that strike the Earth’s atmosphere,
visible light is the form of energy that gets
through.
• The chloroplast pigments absorb solar energy
during the light reactions.
Light Reactions (cont.)
Photosynthetic Pigments
• The two primary pigments used during photosynthesis
are chlorophylls and carotenoids.
• Chlorophylls absorb violet, red, and blue wavelengths of
visible light and reflect green light.
• Carotenoids absorb in the violet-blue-green range but
reflect yellow-orange wavelengths.
• The carotenoids and other pigments become visible in
the autumn as chlorophyll is degraded.
Photosynthetic Pigments (cont.)
The Electron Pathway of the Light
Reactions
• The light reactions consist of an electron
pathway that produces ATP and NADPH.
• The pathway uses two photosystems to
complete the light reactions.
• A photosystem consists of several parts.
– A pigment complex, or light antenna.
– A special chlorophyll pigment, the reaction center.
– Electron acceptor molecules.
The Electron Pathway of the Light
Reactions (cont.)
• When photosystem II (PS II) absorbs solar
energy, energized electrons are passed to
electron acceptors.
• PS II splits a water molecule to recover the
electrons passed to the electron acceptors.
• The electron acceptors send the energized
electrons down an electron transport chain.
The Electron Pathway of the Light
Reactions (cont.)
• As the electrons are passed down an
electron transport chain, energy is
released and stored in the form of a
hydrogen ion (H+) gradient.
• This H+ gradient is used later in
photosynthesis to produce ATP.
The Electron Pathway of the Light
Reactions (cont.)
• When photosystem I (PS I) absorbs solar
energy, energized electrons are passed to
different electron acceptors.
• Electrons from the end of the electron transport
chain replace the electrons from PS I.
• The electron acceptors pass the electrons to
NADP+ to form NADPH.
The Electron Pathway of the Light
Reactions (cont.)
Organization of the Thylakoid
Membrane
• PS II, PS I, and the electron transport
chain are located within the thylakoid
membrane.
• Another component required for
photosynthesis and ATP production is the
ATP synthase complex.
Organization of the Thylakoid
Membrane (cont.)
ATP Production
• During photosynthesis, the thylakoid
space becomes an H+ reservoir.
• The H+ ions that fill this reservoir come
from two sources.
– The oxidation of water by PS II adds H+.
– The flow of electrons through the electron
transport chain releases energy that pumps
H+ into the thylakoid space.
ATP Production (cont.)
• As the H+ are released through an ATP
synthase, the H+ flow down their
concentration gradient and release
energy.
• The ATP synthase couples that release of
energy to the production of ATP.
NADPH Production
• Some enzymes require non-protein helpers to
catalyze their chemical reaction.
• NADP+ is a coenzyme that accepts H+ from a
substrate.
• NADPH is formed during the light reactions by
accepting electrons from the electron transport
pathway and then picks up H+.
6.3 Calvin Cycle Reactions
• The Calvin cycle is a series of reactions that
continually produce a carbohydrate (glucose)
from carbon dioxide during photosynthesis.
• The Calvin cycle has three steps.
– Carbon dioxide fixation
– Carbon dioxide reduction
– Regeneration of ribulose-1,5-bisphosphate (RuBP)
6.3 Calvin Cycle Reactions (cont.)
Fixation of Carbon Dioxide
• During the first step of the Calvin cycle,
CO2 from the air is attached (fixed) to
RuBP.
• The enzyme for this reaction is RuBP
carboxylase oxygenase (rubisco).
• Rubisco splits the resulting 6-carbon
molecule to form two 3-carbon molecules.
Reduction of Carbon Dioxide
• Reduction of CO2 is a series of reactions
that uses NADPH and ATP from the light
reactions to form the carbohydrate.
– NADPH provides electrons for the reduction.
– ATP provides the energy.
Regeneration of RuBP
• The product of the Calvin cycle is
glyceraldehyde-3-phosphate (G3P).
• About 1/6 of the G3P is used to make
glucose.
• About 5/6 of the glucose is used to
regenerate the RuBP required for the
fixation of carbon dioxide.
The Importance of the Calvin Cycle
• The G3P molecules produced by plants
can be used to make a wide variety of
chemicals.
The Importance of the Calvin Cycle
(cont.)
6.4 Other Types of Photosynthesis
• Plants have metabolically adapted
photosynthesis to different climates.
• In areas with moderate temperature,
plants carry out C3 photosynthesis,
meaning that the first detectable molecule
from the Calvin cycle is a 3-carbon
compound.
6.4 Other Types of Photosynthesis
(cont.)
6.4 Other Types of Photosynthesis
(cont.)
• Plants in hot dry climates perform C4
photosynthesis, forming a 4-carbon
compound.
• These other types of photosynthesis are
necessary because O2 competes with CO2
for the binding site on rubisco, decreasing
the efficiency of photosynthesis.
C4 Photosynthesis
• The anatomy of a C4 plant is different from that
of a C3 plant.
• Although chloroplasts are found in both the
mesophyll and bundle sheath cells, the Calvin
cycle occurs primarily in the bundle sheath cells.
• CO2 taken in by the mesophyll cells is combined
with a 3-carbon compound to form a 4-carbon
compound (carbon fixation).
C4 Photosynthesis (cont.)
• The 4-carbon compound is shuttled to the
bundle sheath cell, where it releases the CO2
into the Calvin cycle.
• This spatial separation minimizes the
competition with O2.
• While more complex, C4 photosynthesis is more
advantageous to plants in hot, dry climates.
C4 Photosynthesis (cont.)
CAM Photosynthesis
• Another type of photosynthesis is crassulacean
acid metabolism (CAM), found commonly in
desert plants.
• Similar to C4 photosynthesis, CAM plants
separate CO2 fixation from the Calvin cycle
reaction to minimize competition from O2.
• However CAM plants separate these events by
time.
– CO2 is fixed during the night.
– The Calvin cycle reactions occur during the day.
CAM Photosynthesis (cont.)
• The primary advantage of CAM
photosynthesis involves the conservation
of water.
• When CAM plants open their stomata at
night to obtain CO2, water loss is
minimized.
CAM Photosynthesis (cont.)
Evolutionary Trends
• C4 plants most likely involved in areas with
high light, high temperature, and low
rainfall.
• C3 plants survive better than C4 plants in
temperatures less than 25ºC.
• CAM plants compete well with both C3 and
C4 plants, particularly in arid
environments.