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Light Reactions of
Photosynthesis
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Printed: November 9, 2014
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C HAPTER
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Chapter 1. Light Reactions of Photosynthesis
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Light Reactions of
Photosynthesis
Describe the structure of a chloroplast.
Write the chemical reaction of photosynthesis.
Name the reactants and products of photosynthesis.
Summarize the process of photosynthesis.
Are plants the only organisms that perform photosynthesis?
Although we generally discuss plants when learning about photosynthesis, keep in mind that plants are not the only
organisms that can make their own food. Some bacteria and some protists, such as the algae pictured here, also
perform photosynthesis. This alga has chloroplasts and photosynthesizes just like a plant.
The Process of Photosynthesis
In the Presence of Sunlight, Carbon Dioxide
Photosynthesis takes place in the organelle of the plant cell known as the chloroplasts. Chloroplasts are one
of the main differences between plant and animal cells. Animal cells do not have chloroplasts, so they cannot
photosynthesize. Photosynthesis occurs in two stages. During the first stage, the energy from sunlight is absorbed
by the chloroplast. Water is used, and oxygen is produced during this part of the process. During the second stage,
carbon dioxide is used, and glucose is produced.
Chloroplasts contain stacks of thylakoids, which are flattened sacs of membrane. Energy from sunlight is absorbed
by the pigment chlorophyll in the thylakoid membrane. There are two separate parts of a chloroplast: the space
inside the chloroplast itself, and the space inside the thylakoids ( Figure 1.1).
• The inner compartments inside the thylakoids are called the thylakoid space (or lumen). This is the site of the
first part of photosynthesis.
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• The interior space that surrounds the thylakoids is filled with a fluid called stroma. This is where carbon
dioxide is used to produce glucose, the second part of photosynthesis.
FIGURE 1.1
The chloroplast is the photosynthesis factory of the plant.
The Reactants
What goes into the plant cell to start photosynthesis? The reactants of photosynthesis are carbon dioxide and water.
These are the molecules necessary to begin the process. But one more item is necessary, and that is sunlight. All
three components, carbon dioxide, water, and the sun’s energy are necessary for photosynthesis to occur. These three
components must meet in the chloroplast of the leaf cell for photosynthesis to occur. How do these three components
get to the cells in the leaf?
• Chlorophyll is the green pigment in leaves that captures energy from the sun. Chlorophyll molecules are
located in the thylakoid membranes.
• The veins in a plant carry water from the roots to the leaves.
• Carbon dioxide enters the leaf from the air through special openings called stomata ( Figure 1.2).
The Products
What is produced by the plant cell during photosynthesis? The products of photosynthesis are glucose and oxygen.
This means they are produced at the end of photosynthesis. Glucose, the food of plants, can be used to store energy
in the form of large carbohydrate molecules. Glucose is a simple sugar molecule which can be combined with other
glucose molecules to form large carbohydrates, such as starch. Oxygen is a waste product of photosynthesis. It is
released into the atmosphere through the stomata. As you know, animals need oxygen to live. Without photosynthetic
organisms like plants, there would not be enough oxygen in the atmosphere for animals to survive.
The Chemical Reaction
The overall chemical reaction for photosynthesis is 6 molecules of carbon dioxide (CO2 ) and 6 molecules of water
(H2 O), with the addition of solar energy. This produces 1 molecule of glucose (C6 H12 O6 ) and 6 molecules of oxygen
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Chapter 1. Light Reactions of Photosynthesis
FIGURE 1.2
Stomata are special pores that allow
gasses to enter and exit the leaf.
(O2 ). Using chemical symbols, the equation is represented as follows: 6CO2 + 6H2 O → C6 H12 O6 + 6O2 . Though
this equation may not seem that complicated, photosynthesis is a series of chemical reactions divided into two stages,
the light reactions and the Calvin cycle ( Figure 1.3).
The Light Reactions
Photosynthesis begins with the light reactions. It is during these reactions that the energy from sunlight is absorbed
by the pigment chlorophyll in the thylakoid membranes of the chloroplast. The energy is then temporarily transferred
to two molecules, ATP and NADPH, which are used in the second stage of photosynthesis. ATP and NADPH are
generated by two electron transport chains. During the light reactions, water is used and oxygen is produced.
These reactions can only occur during daylight.
The Calvin Cycle
The second stage of photosynthesis is the production of glucose from carbon dioxide. This process occurs in a
continuous cycle, named after its discover, Melvin Calvin. The Calvin cycle uses CO2 and the energy temporarily
stored in ATP and NADPH to make the sugar glucose.
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FIGURE 1.3
Photosynthesis is a two stage process.
As is depicted here, the energy from sunlight is needed to start photosynthesis.
The initial stage is called the light reactions as they occur only in the presence
of light.
During these initial reactions,
water is used and oxygen is released.
The energy from sunlight is converted
into a small amount of ATP and an energy carrier called NADPH. Together with
carbon dioxide, these are used to make
glucose (sugar) through a process called
the Calvin Cycle. NADP+ and ADP (and
Pi, inorganic phosphate) are regenerated
to complete the process.
Vocabulary
• Calvin cycle: Second stage of photosynthesis in which carbon atoms from carbon dioxide are combined,
using the energy in ATP and NADPH to make glucose.
• chlorophyll: Pigment that absorbs sunlight and gives plants their green color.
• chloroplast: Organelle that carries out photosynthesis in plants.
• electron transport chain: Series of electron-transport molecules that pass high-energy electrons from molecule
to molecule and capture their energy.
• glucose: Simple sugar with the chemical formula C6 H12 O6 ; a product of photosynthesis.
• light reactions: First stage of photosynthesis in which light energy from the sun is captured and changed into
chemical energy that is stored in ATP and NADPH.
• products: End results of a chemical reaction.
• reactants: Molecules that come together to start a chemical reaction.
• stomata: Special pores in leaves; carbon dioxide enters the leaf and oxygen exits the leaf through these pores.
• stroma: Fluid that fills the interior space that surrounds the thylakoids in the chloroplast.
• thylakoid: Stack of flattened sacs of membrane in the chloroplast.
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Chapter 1. Light Reactions of Photosynthesis
Summary
• Photosynthesis occurs in the chloroplast of the plant cell.
• Carbon dioxide, water, and the sun’s energy are necessary for the chemical reactions of photosynthesis.
• The products of photosynthesis are glucose and oxygen.
Explore More
Use the resources below to answer the following questions.
Explore More I
• Photosynthesis at http://www.youtube.com/watch?v=RNufj-64OO0 (7:08)
MEDIA
Click image to the left for use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/57345
1. How do autotrophs differ from heterotrophs? How are they the same?
2. What do plants do with most of the sugar they produce during photosynthesis?
3. How do decreasing levels of CO2 affect plants? How do you think increasing levels of CO2 affect plants?
Explore More II
• Photosynthesis at http://www.youtube.com/watch?v=mpPwmvtDjWw (2:41)
MEDIA
Click image to the left for use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/4707
1. Where do plants get the raw materials for photosynthesis?
2. What do plants take up through their roots? Which of these substances are used for photosynthesis?
3. Where does the chemical reactions of photosynthesis take place?
Review
1. Describe the structures of the chloroplast where photosynthesis takes place.
2. What would happen if the stomata of a plant leaf were glued shut? Would that plant be able to perform
photosynthesis? Why or why not?
3. What are the reactants needed to perform photosynthesis? The products?
4. What happens to the products of photosynthesis?
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References
1. Ollin. Diagram of a chloroplast. Public Domain
2. Dartmouth Electron Microscope Facility; Jon Sullivan. Picture showing the stomata in a leaf. Public Domain
3. Hana Zavadska. Diagram of the steps of photosynthesis. CC BY-NC 3.0
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