Photosynthesis
Copyright © 1999 Rollo Silver
What is photosynthesis?
The ability to convert light energy into chemical energy in
the form of sugar.
PHOTO = light
SYNTHESIS = to make, manufacture
Why is photosynthesis so important to me?
#1: One product of photosynthesis is glucose (sugar) that is
the basis of most food chain.
#2: The second product of photosynthesis is oxygen. Since
you are an aerobic organism, you require oxygen in order to
live.
www.sclero.org
Where does photosynthesis occur?
Autotrophs: self feeders because they make their own food.
Ex: plants
Plants: in leaves that contain organelles called chloroplasts
Where does photosynthesis not occur?
Heterotrophs: organisms that cannot use the sun’s
energy to make its food.
Have to eat other things in order to make energy.
Ex: humans and animals
What raw materials are needed for photosynthesis?
Sunlight: That’s why it is called PHOTOsynthesis
Water
Carbon dioxide (CO2)
Chlorophyll
www.hhcamp.com
How can we summarize photosynthesis?
CO2 + H2O ------Sunlight------ C6H12O6 + 6O2
Carbon dioxide
Glucose (sugar)
oxygen
Water
What else do I need to remember about photosynthesis?
It is important in the oxygen-carbon dioxide cycle.
Carbon dioxide is being removed from the air while putting oxygen
back into the air
Increase in carbon dioxide levels contribute to Global Warming.
Where does the glucose (sugar) go after the leaves make it?
The sugar is stored in various forms in different parts of the plant
Stems: celery, rhubarb
Roots: carrots, turnips, radish
Fruits: apple, banana
Seeds: rice, wheat, barley, corn
Leaves: spinach , lettuce
www.thefeltsource.com
How are plants adapted for photosynthesis?
The leaves of plants are the factories where photosynthesis occurs
Let’s take a look at some structures in the leaves
Photosynthesis occurs
in organelles called
chloroplasts
Inside the
chloroplasts,
photosynthesis
occurs in the grana
and stroma
WHY ARE PLANTS GREEN?
Inside the grana are huge amounts of a pigment called chlorophyll.
Chlorophyll absorbs blue, violet, orange and red light and
reflects green and yellow light
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Photosynthesis occurs in two phases
Light –Dependent Reactions
Light – Independent Reactions or the Calvin Cycle
Light-dependent reactions
Reaction requires sunlight
First phase of photosynthesis
Occurs in the grana
Helps form needed materials for the second phase of
photosynthesis called light-independent reaction or Calvin
Cycle
•NADPH2
•ATP
BUT how does this all happen?
Through a series of complex reactions that looks long on
paper but the reaction occurs in a fraction of a second.
We are going to describe the light-dependent reaction in a
series of steps.
Light-dependent reaction
STEP 1
Chlorophyll is energized
Chlorophyll (the pigment) in chloroplasts absorbs
sunlight and becomes energized or excited.
Chlorophyll passes the energy it captured from the
sun through the electron transport chain and produces
energy for reactions.
Light-dependent reaction
Step 2
Formation of ATP
Chloroplasts contain ADP. Some of the energy
released during the electron transport chain helps
add another phosphate to ADP to form ATP.
A + P + P ------ + P
yields A + P + P + P
ATP stores energy that will be used in the second
phase called the Calvin Cycle
Light-dependent reaction
Step 3
Splitting of water molecules
Some of the other energy produced during the
electron transport chain goes to split water into
hydrogen and oxygen molecules.
Oxygen is released back into the air
The two hydrogen ions are trapped by a coenzyme in
the chloroplasts called NADP and forms NADPH2
which will be used on the Calvin Cycle
Light Independent Reactions or Calvin Cycle
Second phase of photosynthesis
Does not require sunlight
Uses carbon dioxide (CO2) to form glucose
This reaction occurs in the stroma of the chloroplasts
Light Independent Reaction/ Calvin Cycle
Uses the products made in the light dependent reaction (ATP and
NADPH2) These provide the energy to do the following steps.
Combines H+ with CO2 “carbon fixation”
H+ came from the light reaction
CO2 came from the air which entered the leaves thru the stomata
Remember the purpose of this whole mess is to make glucose.
C6H12O6
To build a glucose molecule we need C, H, and O
which we have in our H+ and CO2
A whole series of complicated chemical reactions causes the
H+ and CO2 to form a 3-carbon compound called PGAL
Each PGAL is like a ½ of a glucose molecule. So through
another series of reactions two PGAL combines and presto!
A glucose is made.