CHAPTER 7-PHOTOSYNTHESIS
I. PHOTOSYNTHESIS-a set of reactions in which plants convert light energy into glucose.
A. Plants use this glucose to generate their supply of ATP. Like animals, plants breakdown
the glucose to generate ATP.
B. During periods of active photosynthesis, plants can store extra supplies of glucose as starch.
This starch can be used as an energy source during periods of stress.
C. Photosynthesis occurs in all plants, algae, and in some bacteria.
D. Autotrophs-organisms that are capable of converting light or chemical energy into sugars
which are then broken down to generate ATP.
1. What types of organisms are classified as being autotrophs?
E. Heteroptrophs-organisms that must ingest food materials to obtain their sugars.
1. What are these sugars used for?
2. What types of organisms are classified as being heterotrophs?
F. Joseph Priestly and Antoine Lavoisier-
II. OVERVIEW OF PHOTOSYNTHESIS
A. Equation for Photosynthesis
B. In photosynthesis, plants capture the energy associated with sunlight. This energy is then
used to convert atmospheric carbon dioxide into glucose. Oxygen is released as a by-product
of plant photosynthesis. Animals depend on this oxygen for their metabolism.
C. Light-serves as the major energy source for photosynthesis.
1. Light occurs in many forms:
2. Light travels in waves.
3. Photons-packet of light energy.
a. The amount of energy in light depends on the wavelength for that particular
type of light.
1) In general, light with
b. In photosynthesis, plants capture photons and use this energy to produce their
supply of glucose.
c. Plants are actually able to absorb many types of light. What happens to light
that is not absorbed by the plant?
4. Absorption Spectrum-the set of wavelengths of light that a molecule absorbs. This can
tell us how much energy the particular molecule can absorb from light.
a. Plants have the ability to absorb photons from many different types of light.
As a matter of fact, plants capture much of their energy from visible light-
b. Absorption Spectrum for Visible Light:
1) All other colors of light that we see are mixtures of the above colors.
2) Violet light has the shortest wavelength. What does this tell about the
amount of energy that violet light contains?
3) Red light has the longest wavelength. What does this tell us?
4) Plants are highly adapted for capturing violet and blue light. Why is
this so?
5) How do plants capture the energy in light?
5. Types of Plant Pigments:
a. Chlorophyll-green pigment found in the chloroplasts of ALL plants. This is the
primary photosynthetic pigment.
1) Why does it appear green to our eyes?
2) It is very highly adapted to capturing
b. Carotenoids-in many plants. These pigments capture photons and pass the
energy on to chlorophyll so that photosynthesis can occur.
1) These range in color from
2) Beta-carotene-orange pigment, common carotenoid found in many
plants.
c. Phycobilins
1) Phycocyanin
2) Phycoerythrin
D. Chloroplasts-the photosynthetic organelles found in all plant cells.
1. Major Parts of a Chloroplast:
a. Double-layered membrane
b. Thylakoids-flat discs in chloroplasts. These are covered by chlorophyll.
c. Granad. Photosystems-collections of chlorophyll found on the thylakoids. There are
two types of photosystems (labeled as PS I and PS II). These capture different
wavelengths of light.
e. Stroma-the liquid portion of a chloroplast.
E. Photosynthesis occurs through two major sets of reactions:
III. THE LIGHT REACTIONS
A. In these reactions, plants capture photons which are used to produce:
B. Both ATP and NADPH are used in the Dark Reactions of photosynthesis.
C. What is responsible for capturing photons during the Light Reactions?
D. Oxygen-is produced as a by-product of the Light Reactions. Why is this oxygen important?
E. Steps/Events in the Light Reactions
1. Sunlight, with its photons, hits PS II in chloroplasts.
2. The photons strike electrons in the chlorophyll of PS II.
a. The photons are strong enough to remove the electrons from the chlorophyll at
PS II.
b. Once removed, from PS II, the electrons contain a small amount of energy.
3. Next, these free electrons are picked up by an electron acceptor (a protein). The
electron acceptor passes the electron, along with its energy, to PS I. This involves
the use of a series of electron carriers.
4. As the electrons are being passed from one carrier to another on their way to PS I,
they lose some of their energy. This energy is used by the plant to generate ATP
via Phosphorylation. What is phosphorylation?
a. This series of events is known as Noncyclic Photophosphorylation.
5. While this is occurring, sunlight is also hitting PS I.
a. As before, light energy removes electrons from PS I.
b. The energy associated with these electrons is used to produce the coenzyme
NADPH.
c. This is known as Cyclic Photophosphorylation.
6. Photolysis-
a. The hydrogen ions produced by this process are used to replace the electrons
lost at PS II (at the beginning of the Light Reactions).
b. The oxygen atoms produced by this process bond together to produce oxygen
gas (O2). This gas is released by the plant into the atmosphere.
F. Last Points on the Light Reactions:
1. ATP and NADPH are immediately transported to the stroma of the chloroplast where
they are used during the Dark Reactions.
2. Oxygen is released as a by-product of the Light Reactions.
IV. THE DARK REACTIONS (CALVIN CYCLE)
A. In these reactions, plants use atmospheric carbon dioxide to produce glucose.
1. What is this glucose used for?
2. The Dark Reactions are often referred to as Carbon-Fixation. What does this mean?
3. The carbon dioxide that is used in the Dark Reactions enters the leaves of plants
through small openings known as stomates.
B. The Dark Reactions occur in the stroma of chloroplasts.
C. These reactions do not capture photons. However, the Dark Reactions are dependent on ATP
and NADPH from the Light Reactions to occur. The Dark Reactions can occur as long as
ATP and NADPH are available.
D. Steps/Events in the Dark Reactions
1. 6 molecules of carbon dioxide combine with Ribulose bisphosphate (a 5
carbon sugar) to produce phosphoglycerate (PGA). This reaction is catalyzed
by the enzyme Rubisco.
2. PGA undergoes a series of reactions to form Phosphoglyceraldehyde (PGAL).
ATP and NADPH from the Light Reactions participate in these reactions.
3. The majority of PGAL is used to resynthesize Ribulose bisphosphate. A small
amount of PGAL is used to generate Glucose for the plant.
4. Extra supplies of glucose generated during the Dark Reactions is often stored by
plants as starch.
V. FACTORS THAT INFLUENCE THE PRODUCTIVITY OF PHOTOSYNTHESIS
A. Light Intensity
B. Water Availability
C. Photorespiration-a situation in which plants lose carbon dioxide without glucose being formed.
1. This is a very inefficient process for plants. Plants can die from this.
2. This usually occurs in warm, humid environments. What actually happens in this
process?
3. Some plants have evolved techniques to avoid problems associated with
photorespiration:
a. C-4 Plants
1) Contain special Bundle Sheath Cells which are impermeable to
carbon dioxide loss. Plants store carbon dioxide in these cells until
it can be used.
2) What are some examples of C-4 Plants?
b. CAM Photosynthesis
1) Examples of plants that undergo CAM photosynthesis: