1. Agenda 9/28 and 9/29 Light reactions lecture
Light reactions children’s book
Turn in: workbook and video notes
Homework: work on your IA!!!
Data collection or background research!

2. What happens in photosynthesis? (part 1)

3. Photosynthesis: An Overview
The net overall equation for photosynthesis is:
Photosynthesis occurs in 2 “stages”:
The Light Reactions (or Light-Dependent Reactions)
The Calvin Cycle (or Calvin-Benson Cycle or Dark Reactions or Light-Independent Reactions)
6 CO2 + 6 H2O C6H12O6 + 6 O2
light
Photosynthesis is an endergonic reaction because it requires an input of energy to occur; that energy comes in the form of light energy and is dependent solely on the frequency of the incident light.
Though the Calvin Cycle is sometimes referred to as the Dark Reactions, emphasize to students that this is actually a very poor name since these reactions do NOT ever occur in the dark. The Calvin Cycle requires ATP and NADPH from the light reactions. As soon as the light is gone, so is the supply of ATP and NADPH so the Calvin Cycle stops, too. The AP exam is most likely to refer to the Calvin Cycle as “Light-Independent Reactions” but even that is a misleading term. Emphasize to students that even though these reactions do not use light directly, they do stop as soon as the light (think energetic photons) is gone.
Is photosynthesis an ENDERGONIC or EXERGONIC reaction?

4. Photosynthesis: An Overview
Follow the energy in photosynthesis,
light
Light
Reactions
Calvin
Cycle
Organic compounds (carbs)
ATP
NADPH
light
thylakoids
stroma
- It cannot be overemphasized that energy is not MADE, but is instead transformed through the processes of photosynthesis and cellular respiration. Each of the “boxes” (light/ATP & NADPH/organic compounds) contains energy, but in a different form. Photosynthesis is a process that converts energy from an “un-usable form” (light) into a “usable form” (organic compounds), and requires an intermediate step (ATP/NADPH).
Granum

5. Phase 1: The Light Reactions
The light reactions of photosynthesis involve the use of photosystems.
A photosystem is a cluster of pigment molecules bound to proteins, along with a primary electron acceptor.
2 photosystems are involved:
Photosystem II (P680)
Absorbs light best at a wavelength of 680nm
Photosystem I (P700)
Absorbs light best at a wavelength of 700nm
A photosystem works essentially like an antenna to capture light.
The photosystems were named in order of their discovery; in non-cyclic photophosphorylation, photosystem II actually “comes” before photosystem I.

6. Phase 1: The Light Reactions
Photosystem II [a group of pigment molecules] absorbs the energy in a photon [a particle of light], exciting an electron to a higher energy level.
Thus, PSII is now 1 electron SHORT of what it needs.
This electron is replaced by photolysis – the splitting of water using light.
O2 is released as a byproduct.
- Photolysis – the splitting of water using light – replaces the electron that was “excited out of” photosystem II, and releases oxygen as a byproduct. The H+ ions from water are used to build the concentration gradient during chemiosmosis.

7. Phase 1: The Light Reactions
Non-Cyclic Electron Flow

8. Phase 1: The Light Reactions
The excited electron travels down the electron transport chain, made of increasingly electronegative cytochromes, “losing energy” as it goes. This energy is used to build a concentration gradient of protons (H+ ions)
At the same time, Photosystem I [another group of pigment molecules] also absorbs light energy, exciting one of ITS electrons to a higher energy level.
A good time to emphasize to students the concept of electronegativity – an atom’s ability to attract electrons to itself. Also, since energy cannot be created or destroyed (according to the 1st law of thermodynamics), emphasize that the energy the electron “loses” as it goes down the electron transport chain is harnessed to build a concentration gradient as part of coupled reactions.
The coupling of exergonic reactions to endergonic reactions is a theme seen over and over again in AP biology and is certain to be on the exam. Emphasize this concept whenever discussing examples of it, as here and also in cell respiration.

9. Phase 1: The Light Reactions
The electron lost from Photosystem I is replaced by the electron that was excited and subsequently lost from Photosystem II.
The excited electron from Photosystem I travels down another electron transport chain, “losing energy” as it goes, and ultimately REDUCES NADP+ to NADPH [an electron carrier].
Emphasize that the “loss in energy” is exactly what drives other reactions.

10. ATP H+ H+ H+ H+ H+ H+ ADP H+ P thylakoid membrane thylakoid space
stroma
ATP synthase e-
NADPH
NADP+ e-
Photosystem I (P700)
Photosystem II (P680)
5000 e-
4999 e-
5000 e-
5000 e-
5000 e-
4999 e- H+
This “animation” walks through the steps of noncyclic electron flow, as outlined on the previous 3 slides. The “5000 e-” is meant for illustrative purposes only; no matter how many electrons were contained in photosystems II and I, if there was no way to replace those electrons, eventually the number of electrons would be 0. If that were to occur, there would no electrons to be excited by light, and the light reactions would grind to a halt. The electron that was “excited away” from photosystem I is replaced by the electron that was “excited away” from photosystem II; photosystem II’s lost electron is replaced through photolysis – the splitting of water – which releases ½ a molecule of O2 as a byproduct. This is where the oxygen comes from that is produced during photosynthesis, and is why autotrophs need water to perform photosynthesis! The oxygen is released through the stomata. The electron that was excited away from Photosystem I ends up reducing [adding an electron to] NADP+ to form NADPH, an important electron carrier that is needed in the Calvin Cycle.
Make sure to point out to students the coupled reactions that occur; as the electron travels down the electron transport chain, its “lost energy” is used to pump protons from the stroma to the thylakoid space to build a concentration gradient. Then, as those protons diffuse back across the thylakoid membrane through ATP synthase to achieve equilibrium, they cause ATP synthase to spin (like a turbine), which forces ADP and the phosphate group together, forming ATP.
Don’t forget to point out that the membrane is key here! If there was no thylakoid membrane (or if its integrity was disrupted and therefore “leaky”), it would be impossible to build this concentration gradient – not to mention that the cytochromes, photosystems, and ATP synthase would not exist/be functional!
Make sure to make the connection with “osmosis” when discussing “chemiosmosis;” for students who understand the idea of the movement of molecules from an area of high concentration to an area of low concentration (as in osmosis), discuss the idea that this is essentially the same process, just with protons (H+) instead of water molecules. H+ H H H+ O H+ H+ H+ H+ e- H+ O H+
(2 H+ & ½ O2)

11. Phase 1: The Light Reactions
The energy LOST from the electrons as they travelled down the electron transport chain is used in the process of chemiosmosis to make ATP.
Chemiosmosis is the movement of ions across a selectively permeable membrane
What is ATP?!?
Ask students to define “osmosis”. Their answer should say something similar to: “Osmosis is the net movement of solvent molecules through a semipermeable membrane into a region of higher solute concentration, in order to equalize the solute concentrations on the two sides. It may also be used to describe a physical process in which any solvent moves, without input of energy, across a semipermeable membrane (permeable to the solvent, but not the solute) separating two solutions of different concentrations. It is important to note that although osmosis does not require input of energy (a passive process), it does use kinetic energy and can be made to do work.
Help students to better understand that “chemiosmosis” is the movement of ions across a selectively permeable membrane, down their electrochemical gradient. 

12. ATP What are the 3 parts of an ATP molecule? ADENINE
3 PHOSPHATE GROUPS
Why is ATP so unstable?
- Adenosine triphosphate or ATP is the energy currency of the cells and is used to power cellular activity. ATP is made up of adenine (a nitrogenous base – more specifically, a purine), ribose (a pentose sugar), and 3 phosphate groups. ATP is a very unstable molecule because of the negative charges of the 3 neighboring phosphate groups; those negative charges repel, so it is very easy to “pop off” one (or even two) of the phosphate groups to do cellular work. A phosphate group from ATP (which then becomes ADP) is added to a molecule (often a protein) by a kinase, an enzyme responsible for phosphorylation; phosphorylation typically activates the molecule that has gained a phosphate group.
How is ATP used to do cellular work?
RIBOSE

13. Phase 1: The Light Reactions
Protons (H+) are pumped from the STROMA into the THYLAKOID SPACE, across the thylakoid membrane.
This builds up a concentration of H+ in the thylakoid space.
This concentration gradient represents POTENTIAL ENERGY.
In chemiosmosis, protons diffuse back to the stroma through ATP synthase.
This is known as the proton motive force.
This causes ATP synthase to spin (like a turbine) and forces ADP and a phosphate group together.
This forms ATP!
- The word “pumped” implies that energy is required (and it is, since the H+ ions are moving AGAINST their concentration gradient). The energy required to pump protons across the thylakoid membrane against the concentration gradient came from the energy “lost” by the excited electrons as they moved down the electron transport chain after being excited by photons.

14. This is another animation of noncyclic electron flow.
This “animation” walks through the steps of noncyclic electron flow, as outlined on the previous 2 slides.
However in cyclic electron flow, NADPH is not made, water is not split, but ATP is generated through chemiosmosis. 14

15. Phase 1: The Light Reactions
Light-Reactions Animation
Animation web address:

16. ATP H+ H+ H+ H+ H+ H+ ADP H+ P thylakoid membrane thylakoid space
stroma
ATP synthase e-
NADPH
NADP+ e-
Photosystem I (P700)
Photosystem II (P680)
5000 e-
4999 e-
5000 e-
5000 e-
5000 e-
4999 e- H+
This “animation” walks through the steps of noncyclic electron flow, as outlined on the previous 3 slides. The “5000 e-” is meant for illustrative purposes only; no matter how many electrons were contained in photosystems II and I, if there was no way to replace those electrons, eventually the number of electrons would be 0. If that were to occur, there would no electrons to be excited by light, and the light reactions would grind to a halt. The electron that was “excited away” from photosystem I is replaced by the electron that was “excited away” from photosystem II; photosystem II’s lost electron is replaced through photolysis – the splitting of water – which releases ½ a molecule of O2 as a byproduct. This is where the oxygen comes from that is produced during photosynthesis, and is why autotrophs need water to perform photosynthesis! The oxygen is released through the stomata. The electron that was excited away from Photosystem I ends up reducing [adding an electron to] NADP+ to form NADPH, an important electron carrier that is needed in the Calvin Cycle.
Make sure to point out to students the coupled reactions that occur; as the electron travels down the electron transport chain, its “lost energy” is used to pump protons from the stroma to the thylakoid space to build a concentration gradient. Then, as those protons diffuse back across the thylakoid membrane through ATP synthase to achieve equilibrium, they cause ATP synthase to spin (like a turbine), which forces ADP and the phosphate group together, forming ATP.
Don’t forget to point out that the membrane is key here! If there was no thylakoid membrane (or if its integrity was disrupted and therefore “leaky”), it would be impossible to build this concentration gradient – not to mention that the cytochromes, photosystems, and ATP synthase would not exist/be functional!
Make sure to make the connection with “osmosis” when discussing “chemiosmosis;” for students who understand the idea of the movement of molecules from an area of high concentration to an area of low concentration (as in osmosis), discuss the idea that this is essentially the same process, just with protons (H+) instead of water molecules. H+ H H H+ O H+ H+ H+ H+ e- H+ O H+
(2 H+ & ½ O2)

17. Phase 1: The Light Reactions
Quick recap:
The light reactions occur in/across the thylakoid membrane inside the chloroplasts (inside the cells…)
Light and water are required (reactants).
Light: excites electrons and begins the whole process
Water: Replaces lost electrons
Oxygen, ATP, and NADPH are produced (products).
- Be sure students can articulate where/how light and water are used in the light reactions, along with where/how oxygen, ATP, and NADPH are produced.

18. Phase 1: The Light Reactions
Putting it all together…
The light reactions (light-dependent reactions) transfer the energy in sunlight into chemical energy in the form of ATP and NADPH, which are used to power the Calvin Cycle.
Light and water are required for the light reactions to occur (reactants).
ATP, NADPH, and oxygen gas (O2) are produced through the light reactions (products).
Emphasize to students that chlorophyll and intact thylakoid membranes are also required for the light reactions; they aren’t listed here since they aren’t “consumed” or “produced,” per se.
Try to get students to figure out what the red asterisks mean prior to revealing that text box.

19. My simplified steps
1. Photosystem 2 absorbs light and excites an electron. That electron is replaced by splitting water with light which forms oxygen
2. That electron flows down an electron transport chain forcing H+ ions outside the thylakoid membrane
3. Photosystem 1 absorbs light and excites an electron. This electron is replaced by the electron from photosystem 2
4. This electron flows down the chain, causing H+ ions to build up outside of the thylakoid membrane until the electron bumps into NADP+ to form NADPH
5. The build up of H+ outside the thylakoid membrane causes the H+ ions to flow back through ATP synthase, creating ATP

20. Exit Ticket-Who, what, when , where
Identify the main players in the light dependent reactions of photosynthesis:
Example:
Who- ATP
What- Energy needed for the Calvin Cycle
When- After ETC in light dependent reactions
Where- Thylakoid membrane in ATP synthase protein
Do this for ATP, Light, Oxygen, NADPH and Water