Animal, Plant & Soil Science
E4-3
Photosynthesis
Interest Approach •  At the beginning of class, ask the students to list
all they know about photosynthesis, the products
of photosynthesis, and the importance of
photosynthesis to life. After 10 minutes, ask
students to share their thoughts. Allow the
discussion to flow into the objectives of the
lesson.
Objectives
•  1 Examine the role of chloroplasts and plant
pigments in photosynthesis.
•  2 Analyze the major steps involved in
photosynthesis, including the two phases of
photosynthesis.
•  3 Compare and contrast C3, C4, and CAM
plants.
•  4 Identify factors that affect photosynthesis.
Terms
•  _ ATP
•  _ bundle sheath cells
•  _ C3 plants
•  _ C4 plants
•  _ Calvin cycle
•  _ CAM photosynthesis
•  _ carbon fixation
•  _ carotenoids
•  _ chlorophyll
Terms
•  _ chlorophyll a
•  _ chlorophyll b
•  _ chloroplasts
•  _ grana
•  _ light-dependent reaction
•  _ light-independent reaction
•  _ NADPH
Terms
•  _ photons
•  _ photophosphorylation
•  _ photosynthesis
•  _ stroma
•  _ thylakoids
•  _ xerophytes
What is the role of chloroplasts and
plant pigments in photosynthesis?
•  I. Photosynthesis is a chemical process in which
plants absorb light and convert solar energy into
stored chemical energy.
•  A. Photosynthesis takes place in organelles known as
chloroplasts that are located mainly in the cells of
the mesophyll tissue found in leaves. The majority of
the chloroplasts are found in the mesophyll cells of
leaves. However, photosynthesis does occur in other
leaf cells, plant stems, and green flower parts. Even
roots exposed to light sometimes have chloroplasts
where photosynthesis occurs. Typically, there are 20
to 100 chloroplasts in each mesophyll cell.
What is the role of chloroplasts and
plant pigments in photosynthesis?
•  B. Chloroplasts are bound by a membrane.
•  1. A fluid-filled region within the chloroplast is
known as the stroma. The stroma contains
most of the enzymes required for photosynthetic
reactions.
•  2. Inside the chloroplast is a system of
membranes that form an interconnected set of
flat, disk-like sacs referred to as thylakoids.
These stacked sacs, similar in appearance to a
stack of coins, are called grana.
What is the role of chloroplasts and
plant pigments in photosynthesis?
•  C. Pigments within the chloroplast are
responsible for capturing light. Chlorophyll
and carotenoid (yellow and orange) pigments
are the primary collectors of light.
•  1. Chlorophyll is a complex molecule and is
the main pigment of photosynthesis. It has
two primary forms.
▫  a. The most important is chlorophyll a.
Chlorophyll a is bright green and is
responsible for absorbing energy from the
violet-blue to red, red-orange light wavelengths.
Chlorophyll a initiates the light-dependent
reactions in the photosynthetic process.
▫  b. Chlorophyll b is yellow-green and absorbs
wavelengths of light slightly different from
chlorophyll a.
What is the role of chloroplasts and
plant pigments in photosynthesis?
•  2. Carotenoids are yellow and orange
accessory pigments that absorb energy from
green-yellow-orange wavelengths. They are less
efficient than chlorophyll. The energy absorbed
by carotenoids can be transferred to chlorophyll
a. Xanthophylls are yellow pigments from the
carotenoid group.
What are the major steps involved in
photosynthesis?
•  II. Photosynthesis is a very complex process.
•  A. During photosynthesis, light particles (known as photons)
strike atoms in chlorophyll. In the process, light energy is
transferred to electrons. With this energy, chlorophyll makes
the high-energy compounds ATP (adenosine triphosphate)
and NADPH. Chlorophyll uses the energy in these
compounds to split water molecules. Electrons from the
hydrogen are transferred to the chlorophyll and passed
through a series of electron acceptors. As the electrons are
passed along, their level of excitement is increased by
additional light energy. Some of the energy produced from
this process is used to unite the hydrogen atoms from the
water molecule with carbon dioxide to produce simple sugars.
Two oxygen atoms from the water molecules bond to form O2.
The O2 escapes through the stomata into the atmosphere.
What are the major steps involved in
photosynthesis?
•  B. Photosynthesis has two main reactions.
•  1. The light-dependent reaction only occurs in
the presence of light. Chlorophyll absorbs the light
energy, which is converted to electrical energy. Some
of the electrical energy is used to make ATP through
a process known as photophosphorylation.
Some of the light energy trapped by the chlorophyll
is used to split water molecules. Oxygen (O2) from
the water is released. Hydrogen atoms from the
water combine with NADP to form NADPH.
What are the major steps involved in
photosynthesis?
•  2. The light-independent reaction does not
require light and is sometimes referred to as
the dark reaction. In this reaction, ATP and
NADPH formed during the light-dependent
reaction are used to make high-energy
carbohydrates. The carbohydrates function as a
good long-term energy storage system. The
materials used in the process include carbon
dioxide (CO2) from the air and hydrogen (H)
from the NADPH (originally obtained from
water).
How do C3, C4, and CAM plants
compare?
•  III. Carbon fixation is the
process by which carbon dioxide
is converted to organic molecules.
Different groups of plants go
about fixing carbon in different
ways. The three groupings are C3,
C4, and CAM plants.
How do C3, C4, and CAM plants
compare?
•  A. Most plants use the Calvin cycle (C3) to fix
carbon during the light-independent reaction.
The Calvin cycle takes place in the stroma
within mesophyll cells. Initially, a six-carbon
molecule is formed. Instantly, it splits into two
three-carbon molecules. The three-carbon
molecules join to form simple sugars, glucose,
and fructose. It takes six turns of the Calvin
cycle, involving the input of six CO2 molecules
and hydrogen from NADPH and ATP, to result
in one molecule of glucose. In some plants (e.g.,
sugar beets and maples), glucose and fructose
join to produce sucrose. Most plants are called
C3 plants because the first product of carbon
fixation is a three-carbon compound.
How do C3, C4, and CAM plants
compare?
•  B. Many plants with tropical origins
have the ability to fix carbon dioxide
into four-carbon compounds,
sometimes referred to as C4 plants,
which evolved in areas of high
temperatures, high light intensities,
and limited amounts of water. Corn,
sugar cane, and crabgrass are C4
plants. These plants are typically fast
growing and produce yields two to
three times higher than C3 plants.
How do C3, C4, and CAM plants
compare?
•  1. The leaves of C4 plants differ physically from C3
plants. C4 plants have specialized cells (bundle sheath
cells) that are packed tightly around the veins of a leaf.
Mesophyll cells surround them. Reactions of the C4
pathway take place in the mesophyll cells. In the process,
the plants fix CO2 into four-carbon compounds.
•  2. A result of the C4 pathway is an increased
concentration of carbon dioxide in bundle sheath cells.
Levels of carbon dioxide within the bundle sheath cells
reach 10 to 60 times that of cells with only the C3
pathway. During the reaction, compounds produced in
the mesophyll cells migrate to the bundle sheath cells
and go through the Calvin cycle (the C3 pathway). With a
high level of CO2, photosynthesis occurs at a rapid rate.
How do C3, C4, and CAM plants
compare?
•  C. Some plants, typically growing in arid conditions, fix
carbon through a pathway known as CAM (crassulacean acid
metabolism) photosynthesis. Pineapples and cacti are
examples of plants that perform CAM photosynthesis.
•  1. Plants adapted to dry climates are called xerophytes.
Characteristics of xerophytes are small and thick leaves, a
thick cuticle layer, and sunken stomata. CAM plants close
their stomata during the day to reduce water loss and thus
prevent the exchange of gases. The stomata open at night
when temperatures are cooler and the air is more humid.
Carbon dioxide, for use in carbon fixation, enters through the
open stomata. At night the plants fix carbon dioxide into
malate, which is stored in vacuoles. During the day, the
malate is decarboxylated to yield CO2, which is used by the
plant to manufacture sugars via the C3 pathway.
How do C3, C4, and CAM plants
compare?
•  2. CAM may seem similar to the C4 pathway.
However, there are distinct differences. The
processes take place in different locations in the
plants and at different times. C4 plants capture
the CO2 in mesophyll cells and transfer it to
bundle sheath cells where it is used in the C3
pathway. CAM plant processes occur within the
leaf tissues. As long as there is light available, C4
plants fix carbon. Meanwhile, CAM occurs only
at night.
What factors affect photosynthesis?
•  IV. Outside factors influence
the rate of photosynthesis.
•  A. The first requirement for
photosynthesis is a healthy,
living plant. The plant must
also have chlorophyll to
absorb light.
What factors affect photosynthesis?
•  B. There must be an ample supply of carbon
dioxide in the atmosphere. Higher levels of CO2
result in more rapid growth. In some
greenhouses, carbon dioxide is added to the
atmosphere to speed photosynthesis and plant
growth.
What factors affect photosynthesis?
•  C. Water must be available for plant
use. Severe lack of water results in
wilting or a limp appearance.
Stomata close to conserve water.
CO2 levels in the leaf drop, and
photosynthesis shuts down.
•  D. Light or radiant energy drives
photosynthesis, which cannot occur
without light.
REVIEW
•  1. What is the role of chloroplasts and plant
pigments in photosynthesis?
•  2. What are the major steps involved in
photosynthesis?
•  3. How do C3, C4, and CAM plants compare?
•  4. What factors affect photosynthesis?