Photosynthesis Light Reactions

A little background info… Chloroplasts contain pigments that help photosynthesis occur. They absorb different wavelengths of light. Wavelengths that are not absorbed are reflected or transmitted. Leaves appear green because chlorophyll reflects and transmits green light Animation: Light and Pigments

Pigments Chlorophyll a Chlorophyll b, Carotenoids is the main photosynthetic pigment Participates directly in the light reactions Absorbs blue-violet and red light Chlorophyll b, Accessory pigments Absorb blue and orange light Carotenoids absorb excessive light that would damage chlorophyll

Some FYI extra info… When a pigment absorbs light, it goes from a ground state to an excited state, which is unstable When excited electrons fall back to the ground state, photons are given off, an afterglow called fluorescence If illuminated, an isolated solution of chlorophyll will fluoresce, giving off light and heat

(a) Excitation of isolated chlorophyll molecule (b) Fluorescence Fig. 10-11 Excited state e– Heat Energy of electron Photon (fluorescence) Photon Figure 10.11 Excitation of isolated chlorophyll by light Ground state Chlorophyll molecule (a) Excitation of isolated chlorophyll molecule (b) Fluorescence

Important concept Photosystems These are protein complexes that are surrounded by light harvesting pigments bound to other proteins. It is using these photosystems that the light reactions of photosynthesis occur.

The 2 Photosystems… Photosystem 2 Photosystem 1 Contains a special chlorophyll a molecule called P680 Abbreviated “PS II” Photosystem 1 Contains a special chlorophyll a molecule called P700 Abbreviated “PS I”

Light Reactions The goals of the light reactions are two fold: Make NADPH Using linear (noncyclic) photophosphorylation Make some ATP Using non-linear (cyclic) photophosphorylation

Linear Photophosphorylation Steps in making NADPH: 1.) Light is absorbed by P680 in PS II, which excites the electrons in the pigment. 2.) Two excited e- are passed to a molecule called the primary electron acceptor. 3.) The e- are passed down an electron transport chain (a series of proteins). As they move down the chain, they lose energy. This energy is used to phosphorylate about 1.5 ATP molecules.

Primary acceptor Pigment molecules Photosystem II (PS II) 2 e– P680 1 Fig. 10-13-1 Primary acceptor 2 e– P680 1 Light Figure 10.13 How linear electron flow during the light reactions generates ATP and NADPH Pigment molecules Photosystem II (PS II)

Primary acceptor Pigment molecules Photosystem II (PS II) 2 H2O e– Fig. 10-13-2 Primary acceptor 2 H2O e– 2 H+ + 3 1/2 O2 e– e– P680 1 Light Figure 10.13 How linear electron flow during the light reactions generates ATP and NADPH Pigment molecules Photosystem II (PS II)

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

Linear Photophosphorylation 4.) The e- reach the end of the chain, which terminates at PSI (with P700). 5.) The e- are excited again by sunlight and passed to another primary electron acceptor. 6.) The e- are then passed down a short, second electron transport chain where they combine with NADP+ and H+ to form NADPH.

Electron transport chain Fig. 10-13-4 Primary acceptor Primary acceptor 4 Electron transport chain Pq e– 2 H2O e– Cytochrome complex 2 H+ + 3 1/2 O2 Pc e– e– P700 5 P680 Light 1 Light 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. 10-13-5 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)

ATP NADPH Mill makes ATP Photosystem II Photosystem I e– e– e– e– e– Fig. 10-14 e– ATP e– e– NADPH e– e– e– Mill makes ATP Photon Figure 10.14 A mechanical analogy for the light reactions e– Photon Photosystem II Photosystem I

Splitting of Water The two e- that originated in PS II are now incorporated into NADPH. The loss of these e- from PS II is replaced when H20 is split into two e-, 2 H+ and ½ O The 2 e- replace the e- lost from PS II, one of the H+ gets incorporated into another NADPH molecule and the ½ O contributes to the oxygen gas that is released.

Summary eqn of the Light Rxns: H20 + ADP + Pi + NADP+  ATP + NADPH + O2 + H+

Non-linear Photophosphorylation Occurs simultaneously with linear photophosphorylation. Occurs when the e- from PS I are “recycled” The e- join with protein carriers and generate more ATP due to the high demand for it during the Calvin Cycle. The e- eventually go back to PS I, are re-excited and then either participate in linear or non-linear photophosphorylation.

Primary acceptor Primary Fd acceptor Fd NADP+ Pq + H+ NADP+ reductase Fig. 10-15 Primary acceptor Primary acceptor Fd Fd NADP+ + H+ Pq NADP+ reductase Cytochrome complex NADPH Pc Figure 10.15 Cyclic electron flow Photosystem I Photosystem II ATP

Review ?? #1 Which of the following statements about photosynthetic pigments is true? A.) There is only one kind of chlorophyll B.) Chlorophyll absorbs mostly green light C.) Chlorophyll is required in the Calvin cycle D.) Chlorophyll is found in the membranes of the thylakoids E.) P700 is a carotenoid pigment

Review ?? #1 Which of the following statements about photosynthetic pigments is true? A.) There is only one kind of chlorophyll B.) Chlorophyll absorbs mostly green light C.) Chlorophyll is required in the Calvin cycle D.) Chlorophyll is found in the membranes of the thylakoids E.) P700 is a carotenoid pigment

#2 A product of non-cyclic photophosphorylation is A.) NADPH B.) H20 C.) CO2 D.) ADP E.) AMP

#3 Which of the molecules contain the most stored energy? A.) ADP B.) ATP C.) NADPH D.) Glucose E.) Starch

#4 Which of the following show the correct path for an e- in linear photophosphorylatrion? A.) PS I  ETC  PS II  NADPH B.) PS II  ETC  PSI  ETC NADPH C.) PSI  ETC  PS II  ETC  NADPH D.) P680  ETC  P700  ETC  NADPH E.) More than one of the above is correct

The Calvin Cycle

Some key players RuBP = Ribulose BisPhosphate RUBISCO = an enzyme PGA = Phosphoglycerate G3P = Glyceraldehyde – 3- Phosphate Aka “PGAL” = phosphoglyceraldehyde”

4 steps 1.) Carboxylation 6 CO2 combine with 6 RuBP to produce 12 PGA Enzyme catalyzed: RuBISCO Calvin cycle is called C3 photosynthesis because PGA has 3 carbons.

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

2.) Reduction 12 ATP and 12 NADPH are used to convert 12 PGA to 12 G3P

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)

3.) Regeneration 6 ATP are used to convert 10 G3P to 6 RuBP

4.) Carbohydrate Synthesis 2 of the 12 G3P are used to build glucose.

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)

Summary Eqn for Calvin Cycle 6 CO2 + 18ATP + 12NADPH  18 ADP + 18 Pi + 12 NADP+ + 1 glucose

So, how EXACTLY is the ATP really made? Answer: Chemiosmosis

Chemiosmosis in Chloroplasts Chemiosmosis is the mechanism of ATP generation that occurs when energy is stored in the form of a proton concentration gradient across a membrane. This process in chloroplasts is analogous to ATP generation in the ETC of mitochondria.

Chemiosmosis in Chloroplasts - Steps 1.) H+ Ions accumulate inside thylakoids When water is split in PSII, H+ are released into the lumen of the thylakoid. Also, H+ are carried from the stroma into the lumen by a cytochrome (protein) in the ETC between PS II and PS I.

Chemiosmosis in Chloroplasts - Steps 2.) A pH and electrical gradient across the thylakoid membrane is created. As H+ accumulate inside the thylakoids, pH decreases. Since some of the H+ come from outside the thylakoids (from the stroma), the H+ concentration decreases in the stroma and its pH increases. This creates a pH gradient consisting of differences in the concentration of H+ across the thylakoid membrane Stroma = pH 8; thylakoid = pH 5 (1000x difference) Since H+ are positively charged, their accumulation on the inside of the thylakoid creates an electric gradient as well.

Chemiosmosis in Chloroplasts - Steps 3.) ATP synthase generates ATP - The pH and electrical gradient creates potential energy like water behind a dam. - H+ flow from the thylakoid lumen through a special enzyme called ATP Synthase and out into the stroma. - The energy generated by the passage of the H+ through the enzyme provides energy for the ATP synthase to phosphorylate ADP to ATP.

Fig. 10-17 STROMA (low H+ concentration) Cytochrome complex Photosystem II Photosystem I 4 H+ Light NADP+ reductase Light Fd 3 NADP+ + H+ Pq NADPH e– Pc e– 2 H2O 1 1/2 O2 THYLAKOID SPACE (high H+ concentration) +2 H+ 4 H+ To Calvin Cycle Figure 10.17 The light reactions and chemiosmosis: the organization of the thylakoid membrane Thylakoid membrane ATP synthase STROMA (low H+ concentration) ADP + ATP P i H+

The evil, ineffective side of photosynthesis Photorespiration The evil, ineffective side of photosynthesis

Rubisco… Is the most common protein on earth. Is the enzyme that fixes CO2 in the Calvin cycle. binds RuBP with CO2 to make PGA. The problem with RUBISCO is two fold: It will fix oxygen to RuBP, which leads to a substance that is not useful at all for photosynthesis. It is not working at full capacity if it spends some of its time fixing oxygen instead of CO2, so the plants are not making as much glucose as they could be making.

The fixing of O2 to RuBP is called Photorespiration. Plants deal with this problem by stationing peroxisomes near the chloroplasts which work to breakdown the products of photorespiration. This takes a lot of energy.

Evolutionary significance Why would plants evolve an enzyme that is highly inefficient? Seems counter productive to have to waste so much energy. Answer: Since the early atmosphere in which primitive plants originated contained very little oxygen, it is hypothesized that the early evolution of rubisco was not influenced by its O2-fixing handicap.

The adaptation of photosynthesis How some plants have evolved to deal with harsh environments

C3 Photosynthesis Normal Photosynthesis: (Draw what’s on the board)

C4 Photosynthesis Some plants have evolved a special “add on” to C3 photosynthesis: When CO2 enters the leaf, it is absorbed by the normal photosynthesizing cells (mesophyll cells). However, instead of being fixed by rubisco into PGA, the CO2 is combined with PEP to form OAA. The enzyme that catalyzes this reaction is called PEP carboxylase. PGA = Phosphoglyceraldehyde PEP = Phosphoenolpyruvate OAA = Oxaloacetic acid

C4 Photosynthesis The OAA is then converted to malate. The malate is shuttled into specialized leaf cells called bundle sheath cells. The malate is converted to pyruvate and CO2. The CO2 combines with RuBP (via rubisco) to form PGA (and begin the calvin cycle (normal C3)) The pyruvate is shuttled back to the mesophyll cells where it is changed back into PEP via ATP.

C4 Drawing….

So why do all this? The purpose of moving the CO2 to the bundle sheath cells is to increase the efficiency of rubisco. The bundle sheath cells surround the veins of the leaf and are themselves surrounded by densely packed mesophyll cells. Since the bundle sheath cells rarely make contact with the intercellular space, very little oxygen reaches them.

Still…why do this? In order for PSYN to occur, leaf pores (stomata) must be open to allow CO2 to enter. PROBLEM: When stomata are open, water can leave the leaf (transpiration). SOLUTION: Because these plants are better able to complete photosynthesis (due to the bundle sheath cells), they only have to leave their stomata open for a short time, thereby reducing water loss.

What plants do this? C4 plants live in hot, dry climates where they have an adaptive advantage over C3 plants. This advantage compensates for the additional energy requirement (1 ATP  AMP). Ex: sugarcane, corn, crab grass

Summary of C4 Provides 2 advantages over C3 psyn: 1.) it minimizes photorespiration. 2.) it reduces water loss.

CAM Photosynthesis Another modification of C3 photosynthesis. CAM = crassulacean acid metabolism Almost identical to C4, with some minor, but significant, differences.

CAM Photosynthesis Steps: 1.) CO2 enters the mesophyll cell through the stomata. 2.) CO2 binds with PEP to form OAA. 3.) OAA is converted to malic acid and stored in a vacuole.

CAM Photosynthesis At night, the stomata are open, allowing the malic acid to be formed and stored in the vacuoles. During the day, the stomata are closed, the malic acid is shuttled out of the vacuoles and is converted back to OAA. The OAA is converted to PEP (requiring 1 ATP), releasing CO2.

CAM Photosynthesis The CO2 then combines with RuBP (as it normally does) and enters the Calvin cycle.

Drawing of CAM Photosynthesis

Advantages of CAM Photosynthesis It allows plants to survive in hot, dry environments (with cool nights), like deserts. Photosynthesis can proceed during the daytime with the stomata closed, thereby reducing water loss.

Examples of CAM plants Cacti, agave, aloe, Spanish Moss, Pineapple.

And…. That’s it! A video to sum it all up….