The Energy Conversion Process of Photosynthesis

Chloroplasts: The Sites of Photosynthesis in Plants Leaves are the major locations of photosynthesis Their green color is from chlorophyll, the green pigment within chloroplasts Light energy absorbed by chlorophyll drives the synthesis of organic molecules in the chloroplast CO2 enters and O2 exits the leaf through microscopic pores called stomata Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

A typical mesophyll cell has 30–40 chloroplasts Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf A typical mesophyll cell has 30–40 chloroplasts The chlorophyll is in the membranes of thylakoids (connected sacs in the chloroplast); thylakoids may be stacked in columns called grana Chloroplasts also contain stroma, a dense fluid Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 10-3 Leaf cross section Vein Mesophyll Stomata CO2 O2 Chloroplast Mesophyll cell Outer membrane Figure 10.3 Zooming in on the location of photosynthesis in a plant Thylakoid Intermembrane space 5 µm Stroma Granum Thylakoid space Inner membrane 1 µm

Fig. 10-3a Leaf cross section Vein Mesophyll Stomata CO2 O2 Chloroplast Mesophyll cell Figure 10.3 Zooming in on the location of photosynthesis in a plant 5 µm

Fig. 10-3b Chloroplast Outer membrane Thylakoid Intermembrane space Stroma Granum Thylakoid space Inner membrane Figure 10.3 Zooming in on the location of photosynthesis in a plant 1 µm

Photosynthesis Photosynthesis- Converting solar energy into the usable energy of carbohydrates. Photosynthesis requires: Light Energy Carbon Dioxide Water Chlorophyll Putting together with light

The light reactions (in the thylakoids): Chloroplasts split H2O into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules. Photosynthesis consists of the light reactions (the photo part) and Calvin cycle (the synthesis part) The light reactions (in the thylakoids): Split H2O Release O2 Reduce NADP+ to NADPH Generate ATP from ADP by photophosphorylation

Light Energy Photon- packets of Energy Light travels in waves and is a type of radiation Short wave length radiation has high energy photons Long wave length radiation has lower energy photons

Light Energy Photons of visible light have enough energy to raise electrons to higher levels of energy which is needed for photosynthesis. Visible light energy is absorbed by plants to produce sugar. (C6H12O6) Chlorophyll is the substance that absorbs light energy.

Pop Quiz 1. In your own words, define photosynthesis 2. In your own words, define cellular respiration. 3. What is ATP 4. How is ATP generated or made? 5. Which bond in ATP is loaded with energy?

Chlorophyll (borophyll) Two Types of Chlorophyll Chlorophyll A Chlorophyll B Carotenoids Other pigments that are yellow-orange in plants which absorb violets, blues, and greens. As chlorophyll A and B breakdown they become more noticeable, especially during the fall. Both absorb violet, blue, and red light. Because green is only minimally absorbed, the leaf appears green. The green light is reflected off of the leaf. }

Photosynthesis- the formula 6 CO2 + 6 H2O  C6H12O6 + 6 O2 LIGHT CHLOROPHYLL

i Light NADP+ ADP Light Reactions Chloroplast H2O + P Fig. 10-5-1 Figure 10.5 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle Chloroplast

i Light NADP+ ADP Light Reactions ATP NADPH Chloroplast O2 H2O + P Fig. 10-5-2 H2O Light NADP+ ADP + P i Light Reactions ATP Figure 10.5 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle NADPH Chloroplast O2

i CO2 Light NADP+ ADP Calvin Cycle Light Reactions ATP NADPH Fig. 10-5-3 H2O CO2 Light NADP+ ADP + P i Calvin Cycle Light Reactions ATP Figure 10.5 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle NADPH Chloroplast O2

Chloroplasts – contain chlorophyll Chloroplasts are double membrane organelles. (See figure in book) Note the granum, stroma, thylakoid, and thylakoid space Chlorophyll is found within the membranes of the thylakoids

Photosynthesis has two reactions Light dependent Reactions (light RXN) Light capturing reaction absorbs solar energy Occurs in the thylakoid membrane Light independent Reactions (dark RXN) Now called the Calvin Cycle Synthesis reaction Produces glucose Occurs in the stroma

Light Dependent Reactions Takes place in the thylakoid membrane Requires two light gathering systems Photosystems- systems used to gather solar energy which contain chlorophyll A,B, and carotenoids. The photosystems were named in the order that they were discovered, not the order in which they occur. Photosystem I (PSI) The molecules of Chlorophyll a & b which make up the photosystems Photosystem II (PSII) act like antennas gathering solar energy and focus it to a particular spot. }

i CO2 Light NADP+ ADP Calvin Cycle Light Reactions ATP NADPH Fig. 10-5-4 H2O CO2 Light NADP+ ADP + P i Calvin Cycle Light Reactions ATP Figure 10.5 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle NADPH Chloroplast [CH2O] (sugar) O2

PSI and PSII Basically, the function of the photosystems is to convert energy so that glucose can be produced. This is accomplished by generating electron flow. Sometimes PSI occurs exclusively called the cyclic electron pathway. When both PSI and PSII occur PSII occurs first and then PSI. This process is the non-cyclic electron pathway.

Generating electron flow Energy is directed onto a particular molecule of chlorophyll A . The electrons of this molecule are so excited that they escape their orbitals and move through a series of electron acceptors. Reaction center- The molecule of chlorophyll A that loses an electron.

(INTERIOR OF THYLAKOID) Fig. 10-12 Photosystem STROMA Photon Primary electron acceptor Light-harvesting complexes Reaction-center complex e– Thylakoid membrane Figure 10.12 How a photosystem harvests light Pigment molecules Transfer of energy Special pair of chlorophyll a molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID)

Cyclic Electron Pathway (PS1) (not play station 1) An electron leaves the RXN center but eventually returns to it. As the electron is passed from acceptor to acceptor Energy for ATP is released. Occurs when: CO2 levels are extremely low by photosynthetic bacteria

Cyclic pathway – PS1 (cont.) Does not produce NADPH Produces ONLY ATP Probably first to evolve, Because CO2 levels are low no glucose is produced and the organism must survive on the small amount of ATP that is generated

Non-cyclic Pathway (PS 2) (not play station 2) Water is split and an electron enters PSII Causes the reaction center to lose an electron and travel through a series of electron acceptors. As the electron is passed along ATP is generated. This ATP will be used in the Light independent reactions The electron then enters PSI which during the non-cyclic pathway produces the molecule NADPH instead of ATP.

Non-cyclic (cont.) The products of the non-cyclic pathway, NADPH and ATP, enter the Stroma NADP is one of several biological molecules that act as an electron carrier. In the stroma, the light independent reactions occur. The splitting of water results in a H ion (which is basically an electron) and O which is released from the leaf as a waste product.

Electron transport chain Fig. 10-13-5 Electron transport chain Primary acceptor Primary acceptor 4 7 Electron transport chain Fd Pq e– 2 e– 8 e– H2O e– NADP+ + H+ Cytochrome complex 2 H+ NADP+ reductase + 3 1/2 O2 NADPH Pc e– e– P700 5 P680 Light 1 Light 6 6 ATP Figure 10.13 How linear electron flow during the light reactions generates ATP and NADPH Pigment molecules Photosystem I (PS I) Photosystem II (PS II)

Electron transport chain Fig. 10-21 H2O CO2 Light NADP+ ADP + P i Light Reactions: Photosystem II Electron transport chain Photosystem I RuBP 3-Phosphoglycerate Calvin Cycle ATP G3P Figure 10.21 A review of photosynthesis Starch (storage) NADPH Chloroplast O2 Sucrose (export)

Photophosphorylation Occurs when ATP is generated using photosynthesis. Photophosphorylation occurs because of severe differences in the concentration of H+ ions inside the thylakoid space compared to the stroma. The movement of ions allows for a phosphate to be added to ADP.

Diagram of the Light Dependent Reactions and ATP Synthase

Quiz 1. How is the cyclic pathway different than the non-cyclic pathway? 2. In the non-cyclic pathway, what produces ATP? 3. In the non-cyclic pathway, what produces NADPH? 4. What is the role of NADPH?

Light Independent Reactions AKA- Calvin Cycle (The Dark RXN) The products of the light dependent reactions are NADPH and ATP. Both are used in the light independent reactions. The actual process of producing glucose during the light independent RXN is called the Calvin Cycle.

A Comparison of Chemiosmosis in Chloroplasts and Mitochondria Chloroplasts and mitochondria generate ATP by chemiosmosis, but use different sources of energy Mitochondria transfer chemical energy from food to ATP; chloroplasts transform light energy into the chemical energy of ATP Spatial organization of chemiosmosis differs between chloroplasts and mitochondria but also shows similarities

In mitochondria, protons are pumped to the intermembrane space and drive ATP synthesis as they diffuse back into the mitochondrial matrix In chloroplasts, protons are pumped into the thylakoid space and drive ATP synthesis as they diffuse back into the stroma

H+ Diffusion Electron transport chain ADP + P Fig. 10-16 Mitochondrion Chloroplast MITOCHONDRION STRUCTURE CHLOROPLAST STRUCTURE H+ Diffusion Intermembrane space Thylakoid space Electron transport chain Inner membrane Thylakoid membrane Figure 10.16 Comparison of chemiosmosis in mitochondria and chloroplasts ATP synthase Matrix Stroma Key ADP + P i ATP Higher [H+] H+ Lower [H+]

In summary, light reactions generate ATP and increase the potential energy of electrons by moving them from H2O to NADPH Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The Calvin Cycle (light independent reactions) Has three steps/parts (see fig) 1. CO2 fixation 2. CO2 reduction 3. Regeneration of RuBP RuBP is a 5 carbon compound that will combine with CO2 to form a 6 Carbon Compound. Remember, glucose is a 6 C compound

Step 1- CO2 Fixation Refers to the attachment of CO2 to an organic compound. 3 CO2 bind with 3 RuBP molecules. Results in 3 six Carbon compounds which change to 6 three carbon compounds The three Carbon compound is called PGA The Calvin Cycle is sometimes called the C-3 cycle because of PGA.

Phase 1: Carbon fixation Ribulose bisphosphate Fig. 10-18-1 Input 3 (Entering one at a time) CO2 Phase 1: Carbon fixation Rubisco 3 P P Short-lived intermediate 3 P P 6 P Ribulose bisphosphate (RuBP) 3-Phosphoglycerate Figure 10.18 The Calvin cycle

Step 2 - CO2 Reduction Requires ATP and NADPH (from light RXN) PGA gets converted to G3P (PGAL) using ATP and NADPH. ATP  ADP + P NADPH  NADP PGA  G3P (PGAL) The reduction of CO2 to CH2O results in 6 total PGAL molecules. 5 molecules of G3P (PGAL) will be used to regenerate RuBP. It takes 3 turns of the Calvin Cycle to have a net gain of 1 G3P (PGAL)

Fig. 10-18-2 Figure 10.18 The Calvin cycle Input 3 (Entering one at a time) CO2 Phase 1: Carbon fixation Rubisco 3 P P Short-lived intermediate 3 P P 6 P Ribulose bisphosphate (RuBP) 3-Phosphoglycerate 6 ATP 6 ADP Calvin Cycle 6 P P 1,3-Bisphosphoglycerate 6 NADPH 6 NADP+ 6 P i Figure 10.18 The Calvin cycle 6 P Glyceraldehyde-3-phosphate (G3P) Phase 2: Reduction 1 P Glucose and other organic compounds Output G3P (a sugar)

G3P (PGAL) Glucose Glucose is used to produce ATP Plants need several other organic compounds. G3P (PGAL) can be converted to many other compounds such as: fatty acids, Amino Acids, and of course Glucose. Technically, the Calvin cycle ends with the production of G3P (PGAL)

Step 3 - RuBP Regeneration 5 (PGAL) G3P 3 RuBP 3 ATP  3 ADP + P ATP comes from the light dependent reactions

Fig. 10-18-3 Figure 10.18 The Calvin cycle Input 3 (Entering one at a time) CO2 Phase 1: Carbon fixation Rubisco 3 P P Short-lived intermediate 3 P P 6 P Ribulose bisphosphate (RuBP) 3-Phosphoglycerate 6 ATP 6 ADP 3 ADP Calvin Cycle 6 3 P P ATP 1,3-Bisphosphoglycerate 6 NADPH Phase 3: Regeneration of the CO2 acceptor (RuBP) 6 NADP+ 6 P i Figure 10.18 The Calvin cycle 5 P G3P 6 P Glyceraldehyde-3-phosphate (G3P) Phase 2: Reduction 1 P Glucose and other organic compounds Output G3P (a sugar)

Electron transport chain Fig. 10-21 H2O CO2 Light NADP+ ADP + P i Light Reactions: Photosystem II Electron transport chain Photosystem I RuBP 3-Phosphoglycerate Calvin Cycle ATP G3P Figure 10.21 A review of photosynthesis Starch (storage) NADPH Chloroplast O2 Sucrose (export)

Variations in Photosynthesis The C-3 Pathway – CO2 is fixed during the Calvin cycle, and the first detectable molecule is a 3 Carbon compound (PGA). The C-4 Pathway – CO2 is fixed to a 3 C compound (forming a 4 C compound) prior to the Calvin Cycle. CAM Plant Pathway – Similar to the C- 4 Pathway, but at a slightly different time.

C-3 Pathway Occurs in the mesophyll of the leaf RuBP  PGA P3G (PGAL) See Figure 10-18.

C-4 Pathway CO2 is attached to a 3 C compound called PEP with an enzyme called PEP-carboxylase. This process forms oxaloacetate and occurs in the mesophyll. Oxaloacetate enters the bundle sheath cells where the Calvin Cycle takes place Occurs in warm dry climates Partition PSS in space See Figure 10.14

The C4 pathway Mesophyll cell CO2 PEP carboxylase Oxaloacetate (4C) Fig. 10-19b The C4 pathway Mesophyll cell CO2 PEP carboxylase Oxaloacetate (4C) PEP (3C) ADP Malate (4C) ATP Pyruvate (3C) Bundle- sheath cell CO2 Calvin Cycle Figure 10.19 C4 leaf anatomy and the C4 pathway Sugar Vascular tissue

CAM plant Pathway A C4 compound is produced similar to the C-4 pathway. That compound is produced during the night, and stored in the vacuole until day. Occurs in the mesophyll Occurs in hot, dry and stressful environments Allows Stomates to open at night when it is cooler and less water is lost to evaporation.

(a) Spatial separation of steps (b) Temporal separation of steps Fig. 10-20 Sugarcane Pineapple C4 CAM CO2 CO2 Mesophyll cell 1 CO2 incorporated into four-carbon organic acids (carbon fixation) Night Organic acid Organic acid Figure 10.20 C4 and CAM photosynthesis compared Bundle- sheath cell CO2 CO2 Day 2 Organic acids release CO2 to Calvin cycle Calvin Cycle Calvin Cycle Sugar Sugar (a) Spatial separation of steps (b) Temporal separation of steps

The Importance of Photosynthesis: A Review The energy entering chloroplasts as sunlight gets stored as chemical energy in organic compounds Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic molecules of cells Plants store excess sugar as starch in structures such as roots, tubers, seeds, and fruits In addition to food production, photosynthesis produces the O2 in our atmosphere