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 + WATER → GLUCOSE + OXYGEN
6CO2
+ 6H2O
→ C6H12O6 +
6O2
Photosynthesis is a process that uses light energy to produce sugar in plants which will be used to produce energy for
the plant. 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 below).
 The inner compartments inside the thylakoids are called the thylakoid space (or lumen). This is the site of the first
part of photosynthesis.

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.
(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 below).
Stomata are
special pores
that allow
gasses to enter
and exit the leaf.
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 (H2O),
with the addition of solar energy. This produces 1 molecule of glucose (C6H12O6) and 6 molecules of oxygen (O2). Using
chemical symbols, the equation is represented as follows: 6CO2 + 6H2O → C6H12O6+ 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 below).
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.
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.
Why do you need food?
The main reason you need to eat is to get energy. Food is your body's only supply of energy.
However, this energy must be converted from pizza (or any other food you eat) into an
energy source that your body can use. The process of getting energy from your food is called
cellular respiration.
What is Cellular Respiration?
How does the food you eat provide energy? When you need a quick boost of energy, you might reach for an apple or a
candy bar. But cells do not "eat" apples or candy bars; these foods need to be broken down so that cells can use them.
Through the process of cellular respiration, the energy in food is changed into energy that can be used by the body's
cells. Initially, the sugars in the food you eat are digested into the simple sugar glucose, a monosaccharide. Recall that
glucose is the sugar produced by the plant during photosynthesis. The glucose, or the polysaccharide made from many
glucose molecules, such as starch, is then passed to the organism that eats the plant. This organism could be you, or it
could be the organism that you eat. Either way, it is the glucose molecules that hold the energy.
ATP
Specifically, during cellular respiration, glucose is converted into ATP (Figure below). ATP, or adenosine triphosphate, is
chemical energy the cell can use. It is the molecule that provides energy for your cells to perform work, such as moving
your muscles as you walk down the street. But cellular respiration is slightly more complicated than just converting
glucose into ATP. Cellular respiration can be described as the reverse or opposite of photosynthesis. During cellular
respiration, glucose, in the presence of oxygen, is converted into carbon dioxide and water. The process can be
summarized as: glucose + oxygen → carbon dioxide + water. During this process, the energy stored in glucose is
converted into ATP.
Energy is stored in the bonds between the phosphate groups (PO4-) of the ATP molecule. When ATP is broken down into
ADP (adenosine diphosphate) and inorganic phosphate, energy is released. When ADP and inorganic phosphate are
joined to form ATP, energy is stored. During cellular respiration, about 36-38 ATP molecules are produced for every 1
glucose molecule.
The structural formula for adenosine triphosphate (ATP). During cellular respiration, energy from the chemical bonds of
the food you eat must be converted into ATP.
Summary
Through the process of cellular respiration, the energy in food is converted
into energy that can be used by the body's cells.
During cellular respiration, glucose and oxygen are converted into ATP,
carbon dioxide, and water.
How do trees help you breathe?
Recall that trees release oxygen as a byproduct of photosynthesis. And you need oxygen to breathe. Do you know why?
So your cells can perform cellular respiration.
Connecting Cellular Respiration and Photosynthesis
Photosynthesis and cellular respiration are connected through an important relationship. This
relationship enables life to survive as we know it. The products of one process are the
reactants of the other. Notice that the equation for cellular respiration is the direct opposite
of photosynthesis:

Cellular Respiration: C6H12O6 + 6O2 → 6CO2 + 6H2O

Photosynthesis: 6CO2 + 6H2O → C6H12O6+ 6O2
Photosynthesis makes the glucose that is used in cellular respiration to make ATP. The glucose is then turned back into carbon
dioxide, which is used in photosynthesis. While water is broken down to form oxygen during photosynthesis, in cellular respiration
oxygen is combined with hydrogen to form water. While photosynthesis requires carbon dioxide and releases oxygen, cellular
respiration requires oxygen and releases carbon dioxide. It is the released oxygen that is used by us and most other organisms for
cellular respiration. We breathe in that oxygen, which is carried through our blood to all our cells. In our cells, oxygen allows cellular
respiration to proceed. Cellular respiration works best in the presence of oxygen. Without oxygen, much less ATP would be
produced.
Cellular respiration and photosynthesis are important parts of the carbon
cycle. The carbon cycle is the pathways through which carbon is recycled
in the biosphere. While cellular respiration releases carbon dioxide into
the environment, photosynthesis pulls carbon dioxide out of the
atmosphere. The exchange of carbon dioxide and oxygen during
photosynthesis (Figure below) and cellular respiration worldwide helps to
keep atmospheric oxygen and carbon dioxide at stable levels.
Cellular respiration and photosynthesis are direct opposite reactions. Some
of the ATP made in the mitochondria is used as energy for work, and some
is lost to the environment as heat.
Summary
The equation for cellular respiration is the direct opposite of
photosynthesis.
The exchange of carbon dioxide and oxygen thorough photosynthesis or
cellular respiration worldwide helps to keep atmospheric oxygen and
carbon dioxide at stable levels.