1. Photosynthesis
Introduction to a fundamental biological concept

2. Photosynthesis
is the name we give to ________________________________________________
Light energy from the sun is ____________into chemical energy held in glucose
takes place in ___________ 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 ?
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!

4. Photosynthesis _____reactions the _____ 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 _____reactions the _____ 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.

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!

7. Where does Photosynthesis Happen?
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.

8. Chloroplast

9. Mesophyll Cell Chloroplast Grana
Leaf Cross section
Mesophyll
Cell
Chloroplast
Thylakoid
membrane
This slide shows you where photosynthesis takes place in the cell. The mesophyll cell contains lots of chloroplasts. They look like those “bubbly” green blobs.
Each chloroplast has a very complicated structure. Each chloroplast is made up of stacks of membranes called grana. Each “pancake” in the stack is called a thylakoid. The membrane around each thylakoid is called the thylakoid membrane.
The chlorophyll molecule that captures the sunlight sits in the thylakoid membrane.
Grana

10. Light RX - Photosystem II
___ Light excites Chlorophyll __
An excited electron jumps to ________
___ is cleaved to replace the lost e- in chlorophyll __
Light e-
Sunlight falling on Photosystem II “energizes” an electron that moves to a reaction center in a primary electron acceptor. When it does, water is split apart and one of its electrons moves in to provide a replacement electron in chlorophyll b.

11. Light RX - Photosystem II  I
The energized __ is transported to photosystem __ via ETC
__ loses some of its energy as it travels
The “lost” energy is used to make _____________
The energized electron releases some of its energy to drive the synthesis of ATP (the “energy currency” of all cells) as it (the electron) travels from Photosystem II to Photosystem I.
energy

12. Photosystem I e- Light excites Chlorophyll __
Excited electron goes to ___
Lost __ is replaced by __ from photosystem __
Light e-
Sunlight falling on Photosystem I “energizes” an electron. The energized electron travels to the reaction center.

13. Photosystem I Excited _______ releases _____ as
_ and _ combine with ____ to form _____ e-
As the energized electron travels down the ETC, the energized electron releases some of its energy to synthesize NADPH.
energy

14. Photosystem II to I – ___ ___ 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.”
___
___

15. The Light Reaction – More ATP!
_________
___
__________
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.
to the dark side! H+ H+
____ H+ H+

16. The Dark Reactions – in the Stroma Step 1 – Carbon Fixation
______
6 CO2
6 C-C-C-C-C
12 C-C-C
__ CO2 molecules are fixed onto ___ __ carbon long molecules
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).

17. Step 2 – Energizing Carbon Molecules
The Dark Reactions – in the Stroma
Step 2 – Energizing Carbon Molecules
__ CO2
______
__ C-C-C
The ___ carbon long chains are ________ and receive ________
__ C-C-C~P
In the second step of the cycle, the 3 carbon long chains are “energized” and given pairs of electrons. This step sets the stage for the formation of the 6 carbon glucose (sugar) molecule to be formed. The energy is donated by the ATP molecules formed during the light reactions. The electron pairs are donated by the NADPH molecules made during the light reaction. Now you see just how closely coupled the light and dark reactions are. Without light, the dark reaction can happen until the chloroplast (and the cell) runs out of ATP and NADPH.
__ :C-C-C~P

18. Step 3 – Glucose Leaves the Cycle
The Dark Reactions – in the Stroma
Step 3 – Glucose Leaves the Cycle
__ CO2
______
__ C-C-C
Glucose is formed and ___ __ carbon molecules remain in the cycle
__ C-C-C
__ C-C-C~P
In the third step of the cycle, “glucose” is formed and leaves the cycle. Ten 3 carbon long molecules remain behind
__ :C-C-C~P
________

19. The Dark Reactions – in the Stroma
Step 4 – Carbon molecules are regenerated
__ CO2
______
__ C-C-C-C-C
__ C-C-C
The remaining __ carbon molecules are rearranged into __, __ carbon molecules
__ C-C-C
__ C-C-C~P
In the final step of the cycle, the remaining ten 3 carbon long molecules are rearranged into six, 5 carbon long chains. The rearrangement requires energy in the form of ATP, generated during the light reaction.
__ :C-C-C~P
_______

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

21. Photosynthesis Overall Reaction Light Reaction Dark Reaction
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