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Chem 352 - Lecture 11

Question for the Day: How is photosynthesis analogous to a combination of , the pentose pathway, and the electron transport chain?

Introduction 2 15.1 The Basic Processes of Photosynthesis 15.2 The 15.3 The Light Reactions 15.4 The Reactions: The Calvin Cycle

Chem 352, Lecture 11 - Photosynthesis 2

Elements and of Living Systems 3 The Origin of Biomolecules and Cells ✦ Timeline • The Earth is around 4.6 billion years old • Evidence for the presence of living on Earth dates back to 3.5 billion years ago. • The appearance of multicellular organisms on earth dates back to “only” 400 to 500 million years ago.

Chem 352, Lecture 1 - Introduction to 3

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15.1 The Basic Process of Photosynthesis

Basic Process of Photosynthesis 5 Concomitant with the ready supply of in our atmosphere, oxidation reactions have become a major source of energy for most organisms.

(CH2O) + O2 CO2 + H2O + energy

Chem 352, Lecture 9, Chemical Logic of 5 Basic Process of Photosynthesis 6 The evolution of photosynthesis provided organisms with using the sun as the source of energy for making energy-rich molecules.

CO2 + H2O + energy (CH2O) + O2

It also allowed … and led to the to be used as the oxygenation of the reducing agent…more oxidized earth’s atmosphere

H H H H

H C H H C OH H C O HO C O O C O

H H more reduced

Chem 352, Lecture 9, Chemical Logic of Metabolism 6

Basic Process of Photosynthesis 7 The

Chem 352, Lecture 11 - Photosynthesis 7

Basic Process of Photosynthesis 8 Photosynthesis can be divided into two subprocess • Light Reactions • Carbon Reactions (formerly the Dark Reactions) The two are connected by ATP and NADPH/H+

Chem 352, Lecture 11 - Photosynthesis 8

Basic Process of Photosynthesis 9 There are other sources of reducing equivalents used by some organisms

CO2 + 2 H2A (CH2O) + H2O + 2 A

Photosystems PS II + PS I PS I PS II

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15.2 The Chloroplast The Chloroplast 11 In higher plants and algae, which use both PS II and PS I, photosynthesis takes place in an organelle called the chloroplast.

• Like mitochondria, contain their own DNA, which codes for some of their proteins, and is believed to have evolved from free-living .

Chem 352, Lecture 11 - Photosynthesis 11

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15.3 The Light Reactions

The Light Reactions 13 Light (electromagnetic radiation) has both wave- like and particle-like properties. • Its wavelike properties are characterized by its wavelength (λ) or frequency (ν) c ν = λ - where c is the speed of light (= 2.9979x108 m/s) • The particle-like properties are characterized by Planck’s equation, which gives the energy of a photon of light having a frequency of ν, hc E = hν = λ - where h is Planck’s constant (= 6.626x10-34 J•s)

Chem 352, Lecture 11 - Photosynthesis 13

The Light Reactions 14 Light interacts with matter.

• Infrared photons are absorbed and lead to molecular vibrations, which are perceived as heat. • Photons in the far-ultraviolet range possess sufficient energy to break covalent bonds. - Which is why they are referred to as ionizing radiation. • Photosynthetic pigments capture light in the near-IR and visible range

Chem 352, Lecture 11 - Photosynthesis 14

The Light Reactions 15 Photosynthetic pigments. • Chlorophyll a and b are the most abundant plant and algal pigments; β- carotene and lutein are accessory pigments • These pigments are located in the membranes

Chem 352, Lecture 11 - Photosynthesis 15 The Light Reactions 16 Photosynthetic pigments.

Chem 352, Lecture 11 - Photosynthesis 16

The Light Reactions 17 Photosystems.

• Light-harvesting pigments and their associated proteins are organized into membrane-embedded multisubunit complexes called photosystems

• Plants use two distinct photosystems (PSI and PSII); PSI and PSII contain antenna pigment molecules (such as chlorophylls) bound to light-harvesting complexes (LHCs) and a pair of special chlorophyll molecules which act as a reaction center

Chem 352, Lecture 11 - Photosynthesis 17

The Light Reactions 18 Photosystems. • When absorbing molecules are tightly packed, the energy of the excited may be transferred to an adjacent molecule (resonance transfer) or the excited electron itself may be transferred to a nearby molecule with a slightly lower excited state (electron transfer).

Chem 352, Lecture 11 - Photosynthesis 18

The Light Reactions 19 Photosystems.

• Photons of light are absorbed by antennae pigment molecules and rapidly transferred to a reaction center • The reaction center excited state energy level is slightly lower, trapping the quanta of energy • Excitation of the reaction center starts a series of electron transfer reactions

Chem 352, Lecture 11 - Photosynthesis 19

The Light Reactions 20 Photosystems.

• Photosystems (PSs) were namedCopyright ©2019, in 2016, order 2013 Pearson of Education, their Inc. Alldiscovery; Rights Reserved. PSI absorbs light up to 700 nm and PSII absorbs light up to 680 nm • In each PS, the primary step is transfer of a light-excited electron to a reaction center.

Chem 352, Lecture 11 - Photosynthesis 20 The Light Reactions 21 Photosystems.

• Both Photosystems contain a number of electron carriers, similar the the electron transport chain.

Chem 352, Lecture 11 - Photosynthesis 21

The Light Reactions 22 Photosystems.

• And Plastoquinone is quite similar to Ubiquinone (Coenzyme Q), which is one of the mobile electron carriers in the electron transport chain.

Chem 352, Lecture 11 - Photosynthesis 22

The Light Reactions 23 Photosystems.

• Cytochrome b6f is closely related to the Complex III in the electron transport chain..

Chem 352, Lecture 11 - Photosynthesis 23

The Light Reactions 24 Photosystems.

• The oxygen-evolving coupled (OEC) is associated with PSII, and contains a cluster of manganese

Chem 352, Lecture 11 - Photosynthesis 24

The Light Reactions 25 Photosystems. • This metal cluster can exist in a series of five oxidation states (S0–S4) • Light-driven cycling through the oxidation states provides the energy for the dismantling of H2O. • PSII is one of the strongest oxidizing agents found in nature.

Chem 352, Lecture 11 - Photosynthesis 25 The Light Reactions 26 Photosystems.

• PSI receives electrons from photosystem II and transfers them to NADP+ to form NADPH • PSI is a multiprotein complex that spans the thylakoid membrane • PSI consists of the core complex and LHCI • In the last stage of transfer, reduced soluble ferridoxin transfers electrons to NADP+:

Chem 352, Lecture 11 - Photosynthesis 26

The Light Reactions 27 Photosystems. • Summary of Reactions:

PSII

PSI

PSII + PSI

Chem 352, Lecture 11 - Photosynthesis 27

The Light Reactions 28 Photosystems.

• About 12 moles protons are pumped from the into the thylakoid lumen per mole of O2 released • This energy in this proton gradient is used to generate ATP synthesis (similar as seen in the mitochondrial respiratory chain)

• About 4 moles of ATP are generated per mole of O2 released

Chem 352, Lecture 11 - Photosynthesis 28

The Light Reactions 29 Photosystems.

• When ATP levels are low and NADPH/H+ levels are high, the ferridoxin will pass the electrons back to Cyt b6f in what is called cyclic photophorylation • This leads to the production of ATP with no concomitant reduction of NADP+.

Chem 352, Lecture 11 - Photosynthesis 29

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15.4 The Carbon Reactions: The Calvin Cycle The Calvin Cycle 31 The carbon reactions are the ones involved in the fixation of carbon, i.e. converting inorganic CO2 into organic carbon. • These reactions take place in the storm of the chloroplast

And Stage ThereII uses are two stages to reactions from Stage I uses the pentose the Calvin cycle reactions from phosphate gluconeogenesis pathway • Stage I comprises CO2 fixation and sugar production. • Stage II comprises regenerating the ribulose-1,5- bisphosphate that is used to accept the CO2.

Chem 352, Lecture 11 - Photosynthesis 31

The Calvin Cycle 32 Stage I: and Sugar Production • CO2 is added to a ribulose-1,5-bisphosphate molecule to produce two 3-phosphoglycerate molecules. • The reaction involves two addition reactions followed by an elimination reaction.

Chem 352, Lecture 11 - Photosynthesis 32

The Calvin Cycle 33 Stage I: Carbon Fixation and Sugar Production • The that carries out this reaction is ribulose-1,5- bisphosphate carboxylase/oxygenase, which is affectionally called Rubisco. - It is the most abundant enzyme in the biosphere - The next time you take walk in the woods, look around you to consider that nearly all of the carbon you see in the plants and animals that surround you has at one point point passed through the active site of Rubisco

Chem 352, Lecture 11 - Photosynthesis 33

The Calvin Cycle 34 Stage I: Carbon Fixation and Sugar Production • Each molecule of 3-Phosphoglycerate formed by the enzyme is phosphorylated and oxidized to glyceraldehyde-3-phosphate using reactions we have seen before in gluconeogenesis.

- Each mole of carbon fixed consumes 2 moles of the ATP and 2 moles of the NADPH/H+ that were produced in the photosynthetic light reactions

Chem 352, Lecture 11 - Photosynthesis 34

The Calvin Cycle 35 Stage I: Carbon Fixation and Sugar Production • Therefore, to fix the 6 moles of carbon needed to produce one mole of hexose requires 12 moles of ATP and 12 moles of NADPH/H+ • Of the 12 glyceraldehyde-3- that are produced, - 2 (2 x 3 = 6 ) will be used to produce the hexose. - And10 (10 x 3 = 30 carbons) will be used to regenerate the 6 ribulose-1,5-bisphosphates (6 x 5 = 30 carbons) needed fix 6 more carbons.

Chem 352, Lecture 11 - Photosynthesis 35 The Calvin Cycle 36 Stage II: Regeneration of the Acceptor • The pathway leading to the regeneration of 6 ribulose-1,5-bisphosphate molecules borrows heavily from the gluconeogenesis and non- oxidative pentose phosphate pathways

+ 6 CO2 + 12 NADPH + 12 H + 18 ATP

2 Glcyeraldehyde 3–phosphate + 12 NADP+ + 18 ADP + 16 Pi

Chem 352, Lecture 11 - Photosynthesis 36

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Stage II of Carbon Reactions

2 fructose-6-P + 2 dihydroxyacetone phosphate + 4 glyceraldehyde-3-phosphate + 6 ATP 6 ribulose-1,5-bisphosphate + 6 ADP + 2 Pi Stage I + II of Carbon Reactions + + 6 CO2 + 12 NADPH + 18 ATP + 12 H2O C6H12O6 + 12 NADP + 18 ADP + 18 Pi + 6 H Light Reactions + Carbon Reactions

6 CO2 + 6 H2O C6H12O6 + 6 O2

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The End