1. Overview of Photosynthesis
Light energy is converted to chemical energy and carbon is fixed into organic compounds.
Photosynthesis uses the energy of sunlight to convert water and CO2 into O2 and high energy sugars
6 CO2 + 6 H2O + light → C6H12O6 + 6 O2
carbon dioxide + water + light → sugar + oxygen
Plants then use the sugars to produce complex carbohydrates such as starches:
Plants obtain carbon dioxide from the air or water in which they grow.
Carbon Cycle Abiotic to Biotic Transition

2. Photosynthesis first evolved in prokaryotic organisms; scientific evidence supports that prokaryotic (bacterial) photosynthesis was responsible for production of an oxygenated atmosphere;
Prokaryotic photosynthetic pathways were the foundation of eukaryotic photosynthesis. 2

3. Inside a Chloroplast
Chloroplasts are chemical factories powered by the sun…their thylakoids transform light energy into the chemical energy of ATP and NADPH.
Chloroplasts have a double outer membrane that creates a compartmentalized structure – which supports its function. This structure allows cells to capture the energy available in sunlight and convert it to chemical bond energy via photosynthesis.
Thylakoid
saclike structure in chloroplasts made of photosynthetic membranes – these sacs are made up of lipid bilayers (energy-capturing reactions occur here)
Granum
a stack of thylakoids (produce ATP and NADPH2)
Stroma
Aqueous region outside of the thylakoid membranes
Chlorophyll
molecules are embedded in the thylakoid membrane – capture energy from sunlight to power photosynthesis 3

4. Photosynthetic Pigments
Photosynthetic pigments absorb light energy and use it to provide energy to carry out photosynthesis.
Chlorophylls (absorb light in the red, blue, and violet range):
Chlorophyll a - directly involved in transformation of photons to chemical energy
Chlorophyll b - helps trap other wavelengths and transfers it to chlorophyll a
Carotenoids (absorb light in the blue, green, and violet range):
xanthophyll - Yellow
beta carotene - Orange
Phycobilins – Red
Chlorophyll b, the carotenoids, and the phycobilins are known as ANTENNA PIGMENTS – they capture light in other wavelengths and pass the energy along to chlorphyll a.
Chlorophyll a is the pigment that participates directly in the light reactions of photosynthesis!

5. During photosynthesis, chlorophylls absorb free energy from light, boosting electrons to a higher energy level in photosystems I and II.

6. Figure 10.9 Location and structure of chlorophyll molecules in plants
The pigment molecules have a large head section that is exposed to light in the surface of the membrane; the hydrocarbon tail anchors the pigment molecules into the lipid bilayer.
Chlorophyll a is a large molecule with a single magnesium atom in the head surrounded by alternating double and single bonds.
The head is called the porphyrin ring and is attached to a long hydrocarbon tai
The double bonds are the source of the electrons that flow through the electron transport chains during photosynthesis. 6

7. Stages of Photosynthesis
This reaction can be broken into 2 stages:
Light Dependent Reactions
Within the thylakoid membranes inside a chloroplast
“PHOTO” phase – make ATP & NADPH…USE LIGHT ENERGY TO PRODUCE ATP & NADPH
Light Independent Reactions (Calvin Cycle)
Take place in the stroma of the chloroplast
“SYNTHESIS” phase – converts CO2 to SUGAR
BOTH REQUIRE LIGHT (SOMEWHAT):
Even the dark reactions in most plants occurs during daylight because that is the only time the light reactions can operate AND the dark reactions depend on the light reactions!!!
Light reactions are photo part – convert solar energy to chemical energy…make ATP and NADPH
Calvin cycle is synthesis part – incorporates CO2 into organic molecules which are then converted to sugar 7

8. Calvin Cycle Reactions:
Light Reactions:
-carried out by molecules in thylakoid membranes
-convert light E to chemical E of ATP and NADPH
-split H2O and release O2 to the atmosphere
Calvin Cycle Reactions:
-take place in stroma
-use ATP and NADPH to convert CO2 into the sugar G3P
-return ADP, inorganic phosphate, and NADP+ to the light reactions 8

9. Light Dependent Reactions - Overview
require presence of light
occur in thylakoids of chloroplasts
use energy from light to produce ATP and NADPH (a temporary, mobile energy source that helps store even more energy)
water is split during the process to replace electrons lost from excited chlorophyll
oxygen gas is produced as a by-product
The light-dependent reactions of photosynthesis in eukaryotes involve a series of coordinated reaction pathways that capture free energy present in light to yield ATP and NADPH, which power the production of organic molecules in the Calvin cycle (dark reactions).

10. The Light Reactions
Light is absorbed by PS II and PS I in the thylakoid membranes and electrons flow through TWO electron transport chains.
There are 2 possible routes for electron flow:
Noncylic photophosphorylation
Cyclic photophosphorylation
Photophosphorylation is a method of generating ATP by using light to add P to ADP

11. Photosystems
Photosystems are light-harvesting complexes in the thylakoid membranes of chloroplasts.
Each photosystem consists of a reaction center containing chlorophyll a and a region of many atenna pigment molecules that funnel energy into chlorophyll a.
Two types of photosystems cooperate during photosynthesis:
Photosystem I
Photosystem II
Photosystems I and II are embedded in the internal membranes of chloroplasts (thylakoids) and are connected by the transfer of higher free energy electrons through and electron transport chain (ETC).

12. PS I and PS II
Named in the order they were discovered – however, PS II occurs first, followed by PS I.
PS I absorbs light best in the 700nm range (so called P700).
PS II absorbs light best in the 680nm range (so called P680).

13. Cyclic vs. Noncyclic Electron Flow http://highered. mcgraw-hill
uses Photosystem II, and ETC (with the electron carriers B6f comlex and Pq) , Photosystem I, and another ETC using an iron-containing protein called ferredoxin & NADP reductase
produces ATP and NADPH
Cyclic Electron Flow
uses only Photosystem II and the first ETC – no production of NADPH and no release of Oxygen
DOES produce ATP to be used to make up the difference needed due to Calvin cycle demands.
Why Cyclic?
…b/c noncyclic electron flow produces ATP and NADPH in roughly equal quantities…but Calvin cycle consumes more ATP than NADPH. Cyclic flow makes up the difference for more ATP needed. 13

14. Figure 10.11 How a photosystem harvests light
Chlorophyll a
When a photon strikes a pigment molecule, the energy is passed from molecule to molecule until it reaches the reaction center in a chlorophyll a molecule.
Here, an excited electron from the reaction center chlorophyll is captured by a specialized molecule call the primary electron acceptor. 14

15. Figure How noncyclic electron flow during the light reactions generates ATP and NADPH (Layer 1)
Energy is absorbed by PS II. Electrons from the double bonds in the head of chlorophyll a become energized and move to a higher energy level. They are captured by a primary electron acceptor. 15

16. Figure How noncyclic electron flow during the light reactions generates ATP and NADPH (Layer 2)
Photolysis – water gets split apart, providing electrons to replace those lost from chlorophyll a in PS II. Photolysis splits water into two electrons, two protons, and one oxygen atom. Two oxygen atoms combine to produce one O2 molecule, which is released into the air as waste. 16

17. Figure How noncyclic electron flow during the light reactions generates ATP and NADPH (Layer 3)
Electron Transport Chain – Electrons from PS II pass along an electron transport chain consisting of plastoquinone (PQ), and a complex of cytochromes (the cytochrome B6f complex) to plastocyanin and ultimately are shuttled to PS I. This flow of electrons is exergonic and provides energy to produce ATP by chemiosmosis.
Because this ATP synthesis is powered by light, it is called PHOTOPHOSPHORYLATION. 17

18. Figure 8-10 Light-Dependent Reactions
Section 8-3
FOR DIAGRAM:
Protons that were released from water during photolysis (into the stroma) are actively pumped by the thylakoid membrane (B6f complex) into the lumen (thylakoid space) – the energy from the free fall of electrons is used for this active transport mechanism.
ATP is formed as these protons diffuse down the gradient from the thylakoid space, through ATP-synthase channels, and into the stroma.
The ATP produced here provides the energy to power the Calvin cycle.
NADP becomes reduced when it picks up the two protons that were released from water in PS II. Newly formed NADPH carries hydrogen to the Calvin cycle.
Go to Section: 18

19. Figure How noncyclic electron flow during the light reactions generates ATP and NADPH (Layer 4)
Photosystem I – energy is absorbed by PS I. Electrons from the head of chlorophyll a become energized and are captured by a primary electron receptor. Electrons that escape from chlorophyll a are replaced with electrons from PS II (instead of water) – these electrons are shuttled from PSII to PSI by the enzyme plastocyanin.
This electron chain contains ferrodoxin and ends with the production of NADPH, not ATP. 19

20. Figure How noncyclic electron flow during the light reactions generates ATP and NADPH (Layer 5)
Remember: the light reactions use solar power to generate ATP and NADPH, which provide chemical energy and reducing power, to the sugar-making reactions of the Calvin Cycle.
Read # 5 & 6 page 186. 20

21. Figure Comparison of chemiosmosis in mitochondria and chloroplasts
Remember:
In mitochondria…oxidative phosphorylation.
In chloroplast…photophosphorylation.
The thylakoid membrane of the chloroplast pumps protons from the stroma into the thylakoid space, which functions as the H+ reservoir.
The thylakoid membrane makes ATP as the hydrogen ions diffuse from the thylakoid space back to the stroma through ATP synthase.
Thus…ATP forms in the stroma, where it is used to help drive sugar synthesis during the Calvin cycle. 21

22. Light Independent Reactions - Overview
Do not require light directly (Dark Reactions or the Calvin Cycle)
Stroma of chloroplasts
ANABOLIC process – and therefore requires ENERGY ATP and NADPH (light dependent reactions)
Divided into 3 phases:
Phase 1: Carbon Fixation
Phase 2: Reduction
Phase 3: Regeneration of CO2 Acceptor (RuBP)

23. Figure 10.17 The Calvin cycle (Layer 1)
Phase 1: Carbon Fixation
CO2 is incorporated and attached to RuBP (catalyzed by enzyme rubisco).
Product of reaction is 6-carbon intermediate so unstable that it splits in half to form two molecules of 3-phosphoglycerate.
Phase 1: Carbon Fixation
CO2 is incorporated and attached to RuBP (catalyzed by enzyme rubisco).
Product of reaction is 6-carbon intermediate so unstable that it splits in half to form two molecules of 3-phosphoglycerate. 23

24. Each molecule of 3-phosphoglycerate receives additional
Phase 2: Reduction
Each molecule of 3-phosphoglycerate
receives additional
phosphate group from
ATP to become 1,3 bisphosphoglycerate.
A pair of electrons donated from NADPH reduces 1,3 bisphosphoglycerate into Glyceraldehide-3-phosphate (a sugar).
One of the G3P molecules is exported and used to build glucose.
Phase 2: Reduction
Each molecule of 3-phosphoglycerate receives additional phosphate group from ATP to become 1,3 bisphosphoglycerate.
A pair of electrons donated from NADPH reduces 1,3 bisphosphoglycerate into G3P (a sugar)….notice for every 3 molecules of CO2 there are six molecules of G3P 24

25. Figure 10.17 The Calvin cycle (Layer 3)
Phase 3: Regeneration of CO2 Acceptor (RuBP)
In a series of complex reactions, the carbon skeletons of 5 molecules of G3P are rearranged by the last steps of the Calvin cycle into three molecules of RuBP….the RuBP is now prepared again to receive CO2…and the cycle continues. This rearrangement is endergonic and therefore requires energy (provided by ATP).
For the synthesis of ONE G3P
Calvin cycle consumes 6 ATP during reduction PLUS 3 ATP during regeneration (9 total)
Calvin cycle consumes 6 NADPH during reduction (6 total)
So…cyclic v/s noncyclic light reactions!!!
Phase 3
The carbon skeletons of 5 molecules of G3P are rearranged by the last steps of the Calvin cycle into three molecules of RuBP.
The RuBP is now prepared again to receive CO2…and the cycle continues.
The regeneration phase requires ATP. 25

26. Conserved Core Processes
Photosynthesis first evolved in prokaryotic organisms;
Scientific evidence supports that prokaryotic (bacterial) photosynthesis was responsible for the production of an oxygenated atmosphere;
Prokaryotic photosynthetic pathways were the foundation of eukaryotic photosynthesis.