1. Photosynthetic Light Reactions
SBI4U1

2. Photosynthesis Occurs in green plants and autotrophic organisms
Process using light energy
Used to form high energy, complex food molecules called carbohydrates

3. Chloroplasts in leaves are the site of photosynthesis.
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Photosynthesis is divided into two processes, the light reactions and the calvin cycle.
This animation focuses on the light reactions only.
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4. Lumen Stroma NADP+ Photosystem II Chlorophyll Molecules Photosystem IFd PQ Pc H+
H2O
Inside the chloroplast, the light reactions occur in the thylakoids. Let’s start off by labeling the key players in the diagram.
Electron Transport Chain
Electron Transport Chain
Lumen
ATP Synthase
Two photosystems, two electron transport chains, and ATP synthase are the key components of the light reactions of photosynthesis. These parts are embedded in the thylakoid membranes of a chloroplast.
The photosystems in the thylakoid membrane contain chlorophyll molecules. Chlorophyll is the green pigment of leaves and allows plants/photosynthetic organisms to absorb light energy. Absorbed light energy excites electrons to a higher energy level. H+
Stroma H+

5. Why are they called Photosystems II (P680) and I (P700)?
The reason these two photosystems are numbered the way they are is because these are the wavelengths at which their absorption spectrum peaks at that wavelength, In both these cases the colour is orange to red

6. Lumen Stroma LIGHT NADP+ Photosystem II Photosystem I H+ H+ H+ H+
Excited
Electrons!
NADPH
Excited
Electrons! H+
H2O
Excited electrons from photosystem I are passed down an electron transport chain via transport proteins.
The electrons are then added to NADP+ and a hydrogen ion to form NADPH.
Absorbed light energy excites electrons.
Lumen
The chlorophyll molecules absorb light that excite electrons. These energized electrons from photosystem I are passed down an electron transport chain via transport proteins. At the end of the chain, the electrons are added to NADP+ and a hydrogen ion to form NADPH. H+
Stroma H+

7. Lumen Stroma Photosystem II Photosystem I NADPH H+ H+ H+ H+ H2O
The influx of hydrogen ions creates a concentration gradient across the membrane of the chloroplast. Once the electrons pass through the electron transport chain, they are used to replenish photosystem I’s lost electrons.
Simultaneously, excited electrons from photosystem II are passed down its own electron transport chain. Energy from the electrons is used to pump hydrogen ions into the lumen of the thylakoid.
Lumen
At the same time, energized electrons in photosystem II are passed down another electron transport chain. The additional energy gained by the electrons is used to pump hydrogen ions (H+) from the stroma to the lumen in the thylakoid. This action creates a concentration gradient so that there is a higher concentration of hydrogen ions in the lumen. Once the electrons completely pass through the electron transport chain, they enter photosystem I to replenish its lost electrons (which were used to make NADPH from NADP+ and H+). H+
Stroma H+

8. O2 Lumen Stroma Photosystem II Photosystem I NADPH H+ H2O H+ Z H+
The electrons produced by the splitting of water are used to replenish the electrons lost by photosystem II. The oxygen atom and hydrogen ions are released into the lumen.
Water is split by Z protein to produce two electrons, two hydrogen ions, and an oxygen atom. H+
Lumen
Water is split by Z protein to replenish photosystem II’s lost electrons. The products of this reaction, hydrogen ions and an oxygen atom, are released into the lumen. This is the source of the oxygen gas generated by photosynthesis that allows non-photosynthetic organisms to respire. H+ H+ H+
Stroma H+

9. O2 Lumen Stroma Photosystem II Photosystem I NADPH H+ H+
The energy stored by the hydrogen ion concentration gradient is harvested by the enzyme ATP synthase when it photophosphorylates ADP to produce ATP. H+ H+
Lumen
ATP Synthase
The concentration gradient created by the higher concentration of hydrogen ions in the lumen stores potential energy. In order to transfer this stored energy to sugar molecules via the Calvin cycle, the enzyme ATP synthase harvests the energy in the hydrogen ions as they diffuse down their concentration gradient. This energy is captured in the molecule ATP by the photophosphorylation of ADP. After the production of ATP and NADPH is complete, these molecules are used to make sugar molecules in the Calvin cycle. H+ H+ H+
Energized!
ATP
Stroma H+
ADP P

10. But wait there’s still more!
The previous explanation was for noncyclic electron flow
Cyclic electron flow involves Photosystem I (P700) only
This still produces ATP but no NADPH
Why does this happen?
What does it mean?

11. Cyclic Photophosphorylation
Cyclic electron flow of electrons occurs when  NADP+ is present in only small amounts.
Under these conditions, electrons from ferredoxin (Fd) are transferred to the next protein complex, which returns them to the P700 center
When this happens, no oxygen is released and no NADPH is reduced
About one ATP is produced for every two electrons that complete the cycle in a process called cyclic photophosphorylation
Cyclic photophosphorylation thus can produce ATP when little NADP+ is available to accept electrons (i.e., NADPH levels are high). This may be important when the Calvin cycle requires more ATP than can be produced by noncyclic flow.

12. Main Products of the Light Reactions What are these products used for?
NADPH
ATP O2
What are these products used for?
Carbon fixation via the Calvin Cycle
(more to come…)

13. List of Key Terms Photosystem I Photosystem II
Electron Transport Chain
ATP Synthase
Z Protein
Phosphorylation
Concentration Gradient
Electrons
Hydrogen Ions
Oxygen
NADPH
ATP

14. Some helpful websites…

15. References Textbooks Images McGraw-Hill Ryerson Biology 12 © 2008
Benjamin Cummings Biology 6th Ed. © 2002
Di Giuseppe Biology 12 ©2002
Images