Unit 2: Cellular Structure & Chemistry
Indiana State Standard 3 & 4
Cells & Energy: Photosynthesis
Concept 3/Photosynthesis
What can a tiny plant do that you can't do?
This tiny plant can use the energy of the sun to make its
own food. You can't make food by just sitting in the sun.
Plants are not the only organisms that can get energy
from the sun, however. Some protists, such as algae, and
some bacteria can also use the energy of the sun to make
their own food.
What is Photosynthesis?
If a plant gets hungry, it cannot walk to a local restaurant and buy a slice of pizza. So, how does a plant
get the food it needs to survive? Plants are producers, which means they are able to make, or produce,
their own food. They also produce the "food" for other organisms. Plants are also autotrophs.
Autotrophs are the organisms that collect the energy from the sun and turn it into organic compounds.
So once again, how does a plant get the food it needs to survive?
Through photosynthesis. Photosynthesis is the process plants use to make their own “food” from the
sun's energy, carbon dioxide, and water. During photosynthesis, carbon dioxide and water combine
with solar energy to create glucose, a carbohydrate (C6H12O6), and oxygen.
The process can be summarized as: in the presence of sunlight, carbon dioxide + water → glucose +
oxygen.
Glucose is a sugar that acts as the "food" source for plants. The glucose is then converted into usable
chemical energy, ATP, during cellular respiration. The oxygen formed during photosynthesis, which is
necessary for animal life, is essentially a waste product of the photosynthesis process.
Actually, almost all organisms obtain their energy from photosynthetic organisms. For example, if a
bird eats a caterpillar, then the bird gets the energy that the caterpillar gets from the plants it eats. So
the bird indirectly gets energy that began with the glucose formed through photosynthesis. Therefore,
the process of photosynthesis is central to sustaining life on Earth. In eukaryotic organisms,
photosynthesis occurs in chloroplasts. Only cells with chloroplasts—plant cells and algae (protist)
cells—can perform photosynthesis. Animal cells and fungal cells do not have chloroplasts and,
therefore, cannot photosynthesize. That is why these organisms, as well as the non-photosynthetic
protists, rely on other organisms to obtain their energy. These organisms are heterotrophs.
The Process of Photosynthesis
Photosynthesis takes place in chloroplasts. Chloroplasts are one of the main differences between plant
and animal cells. There are two separate parts of a
chloroplast (Figure to left).
 The inner compartments formed by the flattened
sacs, or thylakoids, are called the thylakoid space. Energy
from sunlight is absorbed by the pigment chlorophyll in
the thylakoid membrane.
 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 chloroplast is the photosynthesis factory of the
plant. [Figure1]
The Reactants
What goes into the plant cell? The reactants of photosynthesis are carbon dioxide and water, and the
energy from sunlight. This means that carbon dioxide, water, and the sun's energy are necessary for the
chemical reactions of photosynthesis.
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Chlorophyll is the green pigment in leaves that captures energy from the sun.
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 below).
Stomata are special pores that allow gasses to enter and exit the leaf. [Figure2]
The Products
What is produced by the plant cell? The products of photosynthesis are glucose and oxygen. This
means they are produced at the end of photosynthesis.
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Glucose, the food of plants, can be used to store energy for later in the form of carbohydrate
molecules.
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Oxygen is a plant waste product. 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 (H20), with the addition of solar energy. This produces 1 molecule of glucose
(C6H12O6) and 6 molecules of oxygen (O2) (Figure below). Using chemical symbols the equation is
represented as follows:
6CO2 + 6H2O
C6H12O6+ 6O2.
As is depicted here, the energy from sunlight is needed to start photosynthesis. The initial steps are
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. [Figure3]
Summary
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All the energy used by living things on earth came from the process of photosynthesis.
During photosynthesis, carbon dioxide and water combine with solar energy to create glucose
and oxygen.
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The net reaction for photosynthesis is that carbon dioxide and water, together with energy from the
sun, produce glucose and oxygen
Practice
Use the resource below to answer the following questions.
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Photosynthesis at http://www.youtube.com/watch?v=hj_WKgnL6MI (5:04)
1. Where does the energy for photosynthesis come from?
2. In photosynthesis, how does the movement of electrons along the electron transport chain affect
hydrogen ions (H+)? How does this compare to what happens in the mitochondria during
cellular respiration?
3. Do all organisms which carry out photosynthesis have chloroplasts? Explain your answer as
fully as you can.
4. What is the function of mobile electron carriers? What is their relationship to the embedded
protein complexes in the membrane? Which ones are involved in photosynthesis?
A review of photosynthesis can be viewed at http://www.youtube.com/watch?v=mpPwmvtDjWw
(2:41).
Review Questions
Recall
1. What are the reactants required for photosynthesis?
2. What are the products of photosynthesis?
Apply Concepts
3. What happens to the glucose produced from photosynthesis?
4. Why is it important to animals that oxygen is released during photosynthesis?
5. Describe the structures of the chloroplast where photosynthesis takes place.
Critical Thinking
6. 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?
Summary
Photosynthesis.
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6CO2 + 6H2O + Light Energy → C6H12O6 + 6O2
Autotrophs store chemical energy in carbohydrate food molecules they build themselves. Most
autotrophs make their "food" through photosynthesis using the energy of the sun.
Photosynthesis occurs in the chloroplast, an organelle specific to plant cells.
The light reactions of photosynthesis occur in the thylakoid membranes of the chloroplast.
Electron carrier molecules are arranged in electron transport chains that produce ATP and NADPH,
which temporarily store chemical energy.
The light reactions capture energy from sunlight, which they change to chemical energy that is stored
in molecules of NADPH and ATP.
The light reactions also release oxygen gas as a waste product.
The reactions of the Calvin cycle add carbon (from carbon dioxide in the atmosphere) to a simple fivecarbon molecule called RuBP.
The Calvin cycle reactions use chemical energy from NADPH and ATP that were produced in the light
reactions.
The final product of the Calvin cycle is glucose.
FAQs
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What is photosynthesis?
The process of using the energy in sunlight to make food (glucose). But of course it is much more
complex than that simple statement. Photosynthesis is a multistep biochemical pathway that uses the
energy in sunlight to fix carbon dioxide, transferring the energy into carbohydrates, and releasing
oxygen in the process.
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What is NADPH?
Nicotinamide adenine dinucleotide phosphate, an energy carrier molecule produced in the light
reactions of photosynthesis. NADPH is the reduced form of the electron acceptor NADP+. At the end
of the light reactions, the energy from sunlight is transferred to NADP+, producing NADPH. This
energy in NADPH is then used in the Calvin cycle.
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How do you distinguish between the Calvin cycle and the Krebs cycle?
The Calvin cycle is part of the light-independent reactions of photosynthesis. The Calvin cycle uses
ATP and NADPH. The Krebs cycle is part of cellular respiration. This cycle makes ATP and NAPH.
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Do photosynthesis and cellular respiration occur at the same time in a plant?
Yes. Photosynthesis occurs in the chloroplasts, whereas cellular respiration occurs in the mitochondria.
Photosynthesis makes glucose and oxygen, which are then used as the starting products for cellular
respiration. Cellular respiration makes carbon dioxide and water (and ATP), which are the starting
products (together with sunlight) for photosynthesis.
Common Misconceptions
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A common student misconception is that plants photosynthesize only during daylight and conduct
cellular respiration only at night. Some teaching literature even states this. Though it is true the light
reactions can only occur when the sun is out, cellular respiration occurs continuously in plants, not just
at night.
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The “dark reactions” of photosynthesis are a misnomer that often leads students to believe that
photosynthetic carbon fixation occurs at night. This is not true. It is preferable to use the term Calvin
cycle or light-independent reactions instead of dark reactions.
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Though the final product of photosynthesis is glucose, the glucose is conveniently stored as starch.
Starch is approximated as (C6H10O5)n, where n is in the thousands. Starch is formed by the
condensation of thousands of glucose molecules.