1. Photosynthesis
Introduction to a fundamental biological concept

2. Photosynthesis
is the name we give to the process by which plants “capture” energy.
Light energy from the sun is transduced into chemical energy held in glucose
takes place in leaves (and stems) of most plants
Photosynthesis is the name we give to a process that involves almost 100 chemical reactions.
During photosynthesis, plants capture energy from the sun and CO2 and H2O from the air and soil and convert it into glucose C6H12O6 and O2. The energy of the sunlight is put into the chemical bonds that hold the carbons, hydrogens and oxygens in place. Oxygen, which we need to breather is only an accidental by-product of the reaction!

3. ? Photosynthesis Sunlight C6H12O6 CO2 O2 H2O
Photosynthesis is the name we give to a process that involves almost 100 chemical reactions.
During photosynthesis, plants capture energy from the sun and CO2 and H2O from the air and soil and convert it into glucose C6H12O6 and O2. The energy of the sunlight is put into the chemical bonds that hold the carbons, hydrogens and oxygens in place in glucose.
When you think about the energy stored in these bonds, think about the amount of heat and light you felt when the hydrogen balloon exploded and then remember the amount of heat released when we burned the marshmallow. Oxygen, which we need to breathe is only an accidental by-product of the photosynthesis! O2
H2O

4. Photosynthesis light reactions the dark reactions the
Biochemists, people who study the chemistry of living things, began to investigate photosynthesis and understand how energy from the sun could be captured in chemical bonds and how plants could really do that.
One of the first things biochemists learned was that photosynthesis occurs in two distinctly different stages, involving different locations in the chloroplast. One set of reactions only happens in the presence of specific wavelengths of sunlight. The second set happens whether or not sunlight is around. The set of reactions that happen only in the presence of light are collectively called the light reactions and those that can happen with or without light are collectively called the dark reactions.

5. Photosynthesis CO2 Sunlight C6H12O6 light reactions the dark reactions
During the light reactions, the plant uses certain wavelengths of light energy from the sun to split water into oxygen and hydrogen ions. (Remember ions are atoms with a net positive or negative charge.)
Oxygen molecules are released from the plant while the hydrogen ions and the energy they carry are used to fuel the dark reactions where carbon dioxide, hydrogen and the energy it carries are used to form glucose. O2

6. What Colors of Sun Light?
Plants “look” green. But do they have color???
Sunlight appears white to our eyes but it is made up of a spectrum of colors, in fact the spectrum is the “rainbow” of colors you might see after a rain storm: red, orange, yellow, green, blue and purple.
As you may have learned in physics, the color something appears to have is really the light that is reflected from its surface, minus the light it absorbs.
Chlorophyll is the molecule in plants that captures the sun’s energy. You may think that chlorophyll is a “green” molecule but what you are really seeing when you look at “green plants” is the green light that is reflected back from the surface of the leaf. Chlorophyll actually captures the blue and red light energy and reflects back green light energy. If you grew plants in green light they would die!
BLUE
RED
Chlorophyll A
Chlorophyll B

7. Where does Photosynthesis Happen?
Leaf Cross section
Photosynthesis takes place in chloroplasts located in specific kinds of cells, mesophyll cells. in plant leaves. Since these cells are so important in a plant’s life, they lie below the leaf’s surface but still “see” enough sunlight to carry out their job of making food.
Mesophyll
Cell

8. Chloroplast
Outer Membrane
Inner Membrane
Stroma
Thylakoid Membranes

9. THE LIGHT Reactions: Photosystem II and I I II Light Light
Sunlight is captured in two centers in the thylakoid membranes. Sunlight falling on Photosystem II energizes an electron. The energized electron releases some of its energy to synthesize ATP (the “energy currency” of the cell) as it travels to Photosystem I. In photosystem I, also in the thylakoid membrane, sunlight energizes a second electron. The two energized electrons travel down the electron transport chain, transferring their energy to the molecule NADPH. NADPH shuttles the energy to the dark reaction where the energy is used to glue carbon dioxide molecules together to make glucose. Remember glucose “stores the sun’s energy in its chemical bonds.” I II

10. The Light Reactions
This slide shows you the exact site in the chloroplast where the light reaction of photosynthesis takes place.
The chlorophyll molecules ( the barrel shapes in the membrane) capture incoming sunlight energy. They use this energy to split water molecules into hydrogen ions and oxygen ions. The oxygen ions reform into oxygen molecules and are released into the air. The energy from the splitting of the water molecules and the hydrogen ions are captured in the “energy carrying molecules” ATP and NADPH that are used to fuel the dark reactions.

11. Overview of the Light and Dark Reactions
This slide shows you the exact site in the chloroplast where the light reaction of photosynthesis takes place.
The chlorophyll molecules ( the barrel shapes in the membrane) capture incoming sunlight energy. They use this energy to split water molecules into hydrogen ions and oxygen ions. The oxygen ions reform into oxygen molecules and are released into the air. The energy from the splitting of the water molecules and the hydrogen ions are captured in the “energy carrying molecules” ATP and NADPH that are used to fuel the dark reactions.

12. The Dark Reactions – in the Stroma
The dark reaction is called the Calvin cycle in honor of the biochemist, Melvin Calvin, who figured out most of the cycle.
The first step of the cycle is shown here. Carbon dioxide molecules are “trapped” and combined with six, 5 carbon long molecules floating in the stroma to form 12 three carbon long molecules. This is the essential first step in the carbon cycle and in making carbon available to all life forms. Remember in ecology we learned than animals cannot use carbon in its gaseous form. We animals rely on plants to capture the carbon in the atmosphere and put it into molecules so that animals and plants can use it.
The end product of step 1: a set of twelve, 3 carbon long molecules (C-C-C).

13. Photosynthesis CO2 + H2O C6H12O6+ O2 Overall Reaction Light Reaction
energy
CO2 + H2O C6H12O6+ O2
Light Reaction
energy
ADP + NADP+ + H2O O2 +ATP+NADPH
When we have written the reaction before, the photosynthesis equation was written as the first equation in this slide.
Now that we have a deeper understanding of the process, we can break the equation down into the light and dark reactions.
As with algebra equations, we could add the light reaction reaction to the dark reaction equation and cross out the terms that appear on both sides of the equation. When you do this, you are left with the overall photosynthesis equation at the top of the slide.
Dark Reaction
CO2 + ATP + NADPH C6H12O6+ ADP + NADP+

14. Concept Check - I
Use this slide to check that you understand the reactants and products of the light and dark reactions in photosynthesis.