Plants, certain protists and photosynthetic bacteria have photosynthetic activities. They make their own food. This makes them Autotrophs
(self-feeders) or Producers (produce their own food).
Photosynthesis is the process by which light energy is converted to chemical bond energy and carbon is fixed into organic compounds.
Photosynthesis involves the uptake of carbon dioxide and water (both low energy compounds). Light energy is the energy source, which drives the process. Products are glucose (a sugar; stored chemical energy), oxygen gas and water. Photosynthesis is the primary energy-storing process on which almost all life, both plant and animal depends.
Complete Photosynthesis Equation
12 + 6
Water

Plants are made up of mostly carbon atoms, which are obtained from carbon dioxide.
Water, an input to photosynthesis is split and oxygen gas is given off, an output of photosynthesis.
Pictured is Elodea sp., an aquatic plant, which is photosynthesizing and giving off O
2 bubbles.

(Plural Grana).

– the site of photosynthesis. Chloroplasts are found in the green parts of plants. They contain the pigment chlorophyll, which takes in the light energy.
Chloroplasts

(a.k.a. Enzymatic Reaction or Dark Reaction)
The light reaction uses light energy to directly produce ATP, while the Calvin Cycle produces sugar. To power the production of sugar, the Calvin Cycle uses the ATP & NADPH formed during the light reaction.
Stage
Light
Reaction
The
Calvin
Cycle
Location in
Chloroplast
Thylakoids
(In Grana)
Energy to
Run Stage
Light
Stroma ATP
NADPH
Other
Chemical
Inputs
H
2
O,
NADP + H +
ADP P +
CO
2
Chemical
Outputs
ATP
NADPH
O
2
C
6
H
12
O
6 (Sugar)
NADP + H +
ADP P +

Chlorophylls (Green in Color)
Chlorophyll a absorbs light in the violet, blue and red ranges. It reflects green, yellow and orange light. Chlorophyll a participates directly in the light reaction. It is a large molecule with a single magnesium (Mg) atom in the middle.
Chlorophyll b absorbs mainly blue and orange light. It reflects yellow-green. Chlorophyll b passes energy to Chlorophyll a .
Chlorophyll b broadens the range of light that a plant can use.
Carotenoids (Yellow, Orange and Red in Color)
Carotenoids absorb light in the blue, green and violet ranges.
Carotenoids are present during the entire life of the leaf. They are easily seen in the fall when chlorophyll is broken down and leaves the leaf. Carotenoids help protect the plant against harmful UV radiation.

Violet Blue Green
Yellow Orange Red
This graph represents the visible light section of the Electromagnetic Energy Spectrum, which is a graph showing the intensity of light (in energy units) as a function of wavelength.

Engelmann’s experiment, demonstrating that red and blue light are effective in photosynthesis. The bacteria (rice-shaped) migrate to that portion of the algal filament (algae In a wide strip) where oxygen is produced.

As covered earlier, the light reaction involves light energy, thylakoids, certain inputs (H
2
O, NADP
+
, H
+
, ADP, P
+
) and certain outputs (ATP, NADPH, O
2
). The photosystems dive deep into specifics about the light reaction.
The light reaction is broken up into 2 pigment systems called
Photosystem I & Photosystem
II. Photosystems are made up of light harvesting complexes in the thylakoid membranes of chloroplasts.
Light harvesting complexes are made up of carotenoids and chlorophyll pigment molecules which capture light energy.
These pigment molecules funnel energy to a chlorophyll
a reaction center. The
chlorophyll a molecule in the reaction center becomes excited and loses an electron to a primary electron acceptor.

The green circles on the below figure symbolize the light-harvesting complex from the previous slide. Do you see how it fits on this slide? Two light-harvesting complexes are shown on this slide. The light harvesting complexes are within photosystems.
Photosystem I’s reaction center is P
700
Photosystem I has more chlorophyll a. because it absorbs light maximally at 700 nm.
Photosystem II’s reaction center is P
680 because it absorbs light maximally at 680 nm.
Photosystem II has equal amounts of chlorophyll a and b, which makes it sensitive to a somewhat shorter wavelength than Photosystem I.
Primary Electron Acceptors

Note: Do you see how the previous slide’s figure fits into this one? We have zoomed out.
Can you find all of the important components of the light reaction on the below figure? Light energy, thylakoid, inputs (H
2
O, NADP
+
, H
+
, ADP, P
+
) and outputs (ATP, NADPH, O
2
), light harvesting complexes and photosystems.

Now to explain the entire photosystem process. Starting at the left, light is absorbed at photosystem II and electrons are excited at the chlorophyll a reaction center and transferred to the primary electron acceptor. Water is split and oxygen is given off. The splitting of water provides electrons to replace those lost from chlorophyll a. The electrons travel from molecule to molecule (electron transport chain) in the thylakoid membrane (down like a slinky), which pumps H + into the thylakoid. Electrons eventually reach photosystem I, where they are again excited and transferred to the primary electron acceptor. Electrons join NADP + and H + and NADPH is formed. A greater concentration of H + is now in the thylakoid and H + travels by diffusion into the stroma and ATP is formed.
(For every 2 photons of light, one water molecule is split, which gives off 2 electrons, which will eventually form one NADPH molecule. It takes two water molecules to make one oxygen gas
(O
2
)molecule). So how many photons of light does it take to make one O
2 molecule?
This protein is called
ATP
Synthase. It synthesizes
ATP.
This production of ATP by light energy is called
Photosphorylation.

This Figure illustrates the photosystems (light reaction) & how the photosystems relate to the Calvin Cycle, which is up next.
Can you see how the previous figures are within this figure?
Light
Reaction
Calvin
Cycle

BIO 141 Botany with Laboratory
• This product is sponsored by a grant awarded under the President’s Community-Based Job Training Grants as implemented by the U.S. Department of
Labor’s Employment and Training Administration. The information contained in this product was created by a grantee organization and does not necessarily reflect the official position of the U.S. Department of Labor. All references to non-governmental companies or organizations, their services, products, or resources are offered for informational purposes and should not be construed as an endorsement by the Department of Labor. This product is copyrighted by the institution that created it and is intended for individual organizational, non-commercial use only.