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. 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. The Light Reaction – More ATP!
Sunlight O2
Chlorophyll
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+
H2O H+ H+

11. The Dark Reaction – in the Stroma
6 CO2
6 C-C-C-C-C
12 C-C-C
6 CO2 molecules are fixed onto a six carbon long molecule of glucose
10 C-C-C
12 C-C-C~P
The dark reaction is called the Calvin cycle in honor of the biochemist, Melvin Calvin, who figured out most of the cycle.
Carbon dioxide molecules are “trapped” and combined with longer chain carbon molecules floating in the stroma. Energy to drive this reaction comes from the ATP molecules formed 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.
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.
12 :C-C-C~P
C6H12O6

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

13. Photosynthesis – in two parts
Light Reaction
energy
ADP + NADP+ + H2O O2 +ATP+NADPH
Dark Reaction
CO2 + ATP + NADPH C6H12O6+ ADP + NADP+
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.

14. Photosynthesis – In Summary
Light Reaction
energy
ADP + NADP+ + H2O O2 +ATP+NADPH
Dark Reaction
energy
CO2 + ATP + NADPH C6H12O6+ ADP + NADP+
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.
CO2 + H2O C6H12O6+ O2