AP Biology Chapter 10

 “photo”=light  “synthesis”=to build  Photosynthesis is the process that converts solar energy to chemical energy.  Directly or indirectly, it nourishes almost all of the living organisms on Earth.

 Autotrophs=the producers in any ecosystem. They produce organic compounds from CO 2 and energy from the sun.  Heterotrophs=the consumers in an ecosystem. They must eat other things to survive.

 Photosynthesis occurs in plants, algae, certain protists, and some prokaryotes.  These organisms feed not only themselves but almost all other living things as well.

 Chloroplasts likely evolved from photosynthetic bacteria.  Their green color is from chlorophyll, a green pigment.  Chlorophyll may be located in stems or other structures as well. Chlorophyta

 Their green color is due to chlorophyll, a green pigment located in chloroplasts.  Light energy is absorbed by the chlorophyll and is used to build organic compounds.

Leaf cross section Vein Mesophyll Stomata CO 2 O2O2 Chloroplast Mesophyll cell Outer membrane Intermembrane space 5 µm Inner membrane Thylakoid space Thylakoid Granum Stroma 1 µm Zooming in on the location of photosynthesis in a plant.

1 µm Thylakoid space Chloroplast Granum Intermembrane space Inner membrane Outer membrane Stroma Thylakoid

 Photosynthesis can be summarized as the following equation: 6 CO H 2 O + Light energy  C 6 H 12 O O H 2 O Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Photosynthesis is a redox reaction where H 2 O is oxidized and CO 2 is reduced. (What does this mean?) Chloroplasts split H 2 O into electrons, H+ ions, and oxygen. The electrons and H+ ions are incorporated into the CO 2 and glucose is made.

 “Photo”=light  The Light Reactions  Light energy is absorbed by the chloroplast and stored in ATP and NADPH  Water is split; Oxygen is released  “Synthesis”=to build  The Light- Independent Reactions (aka: the Calvin Cycle)  Glucose is made

Light Fig H2OH2O Chloroplast Light Reactions NADP + P ADP i + Light Reactions: What goes in……

Light Fig H2OH2O Chloroplast Light Reactions NADP + P ADP i + ATP NADPH O2O2 Light Reactions: What is released…..

Light H2OH2O Chloroplast Light Reactions NADP + P ADP i + ATP NADPH O2O2 Calvin Cycle CO 2 Dark Reactions (Light-independent): What goes in……

Light H2OH2O Chloroplast Light Reactions NADP + P ADP i + ATP NADPH O2O2 Calvin Cycle CO 2 [CH 2 O] (sugar) Dark Reactions (Light-independent): What is released…..

 Light is electromagnetic radiation that travels in waves.  The wavelengths of light determine the energy  Plants use certain wavelengths of visible light for photosynthesis

UV Fig Visible light Infrared Micro- waves Radio waves X-rays Gamma rays 10 3 m 1 m (10 9 nm) 10 6 nm 10 3 nm 1 nm 10 –3 nm 10 –5 nm nm Longer wavelength Lower energyHigher energy Shorter wavelength

 Pigments are molecules that absorb visible light  Different pigments absorb different wavelengths.  Wavelengths that are not absorbed are reflected.

Fig Reflected light Absorbed light Light Chloroplast Transmitted light Granum

Chlorophyll is the main photosynthetic pigment in plants. Other accessory pigments absorb wavelengths of light that are not absorbed by chlorophyll.

 A collection of proteins and chlorophyll molecules make up a photosystem.  These are embedded in the thylakoid membranes of the chloroplast.

 A photosystem consists of a reaction-center complex (a type of protein complex) surrounded by light-harvesting complexes  The light-harvesting complexes (pigment molecules bound to proteins) funnel the energy of photons to the reaction center Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

 Light hits a chlorophyll (or accessory pigment) in a photosystem. The chlorophyll gets “excited” and loses an electron.  The electron is passed from chlorophyll to chlorophyll until it is finally accepted by a special Chlorophyll A. The Chlorophyll A in the Reaction center passes the electron to a Primary Electron Acceptor

 The Primary Electron Acceptor passes the electron (and its’ energy) to the Electron Transport Chain.  The ETC is a series of proteins embedded in the thylakoid membrane.

Pigment molecules Light P680 e–e– Primary acceptor 2 1 e–e– e–e– 2 H + O2O2 + 3 H2OH2O 1/21/2 4 Pq Pc Cytochrome complex Electron transport chain 5 ATP Photosystem II (PS II ) The Flow of Electrons Note! The electron that is lost by the chlorophyll passes from the photosystem to the ETC. How does this chlorophyll get another electron?? Answer: It comes from H2O. H2O molecules are split into electrons, H+ ions and oxygen.

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

 Photosystem II  Discovered second  Functions first  The chlorophyll A in the reaction center absorbs best at 680nm.  Photosystem I  Discovered first  Functions second  The chlorophyll A in the reaction center absorbs best at 700nm.

 Electrons can follow one of two pathways:  First, they could flow in a linear way and are eventually put into NADPH. (Linear)  Or, they could recycle back to the system. (Cyclic)

 This is the primary way that plants capture light energy.  Electrons flow through both photosystems (I and II), eventually making ATP and NADPH.

Pigment molecules Light P680 e–e– 2 1 Fig Photosystem II (PS II) Primary acceptor

Pigment molecules Light P680 e–e– Primary acceptor 2 1 e–e– e–e– 2 H + O2O2 + 3 H2OH2O 1/21/2 Fig Photosystem II (PS II)

Pigment molecules Light P680 e–e– Primary acceptor 2 1 e–e– e–e– 2 H + O2O2 + 3 H2OH2O 1/21/2 4 Pq Pc Cytochrome complex Electron transport chain 5 ATP Fig Photosystem II (PS II)

Pigment molecules Light P680 e–e– Primary acceptor 2 1 e–e– e–e– 2 H + O2O2 + 3 H2OH2O 1/21/2 4 Pq Pc Cytochrome complex Electron transport chain 5 ATP Photosystem I (PS I) Light Primary acceptor e–e– P700 6 Fig Photosystem II (PS II)

Pigment molecules Light P680 e–e– Primary acceptor 2 1 e–e– e–e– 2 H + O2O2 + 3 H2OH2O 1/21/2 4 Pq Pc Cytochrome complex Electron transport chain 5 ATP Photosystem I (PS I) Light Primary acceptor e–e– P700 6 Fd Electron transport chain NADP + reductase NADP + + H + NADPH 8 7 e–e– e–e– 6 Fig Photosystem II (PS II)

Mill makes ATP e–e– NADPH Photon e–e– e–e– e–e– e–e– e–e– ATP Photosystem IIPhotosystem I e–e– A mechanical analogy for the light reactions:

 Each electron “falls” down an electron transport chain from the primary electron acceptor of PS II to PS I  Energy released by the fall drives the creation of a proton gradient across the thylakoid membrane  Diffusion of H + (protons) across the membrane drives ATP synthesis Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings The Flow of Electrons

 In PS I (like PS II), transferred light energy excites P700, which loses an electron to an electron acceptor  P700 + (P700 that is missing an electron) accepts an electron passed down from PS II via the electron transport chain Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Where do the electrons go?

 Cyclic electron flow uses only photosystem I and produces ATP, but not NADPH  Cyclic electron flow generates surplus ATP, satisfying the higher demand in the Calvin cycle Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig ATP Photosystem II Photosystem I Primary acceptor Pq Cytochrome complex Fd Pc Primary acceptor Fd NADP + reductase NADPH NADP + + H + Cyclic Electron Flow

 Some organisms such as purple sulfur bacteria have PS I but not PS II  Cyclic electron flow is thought to have evolved before linear electron flow  Cyclic electron flow may protect cells from light-induced damage Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Cyclic Electron Flow

 Cyclic electron flow uses only photosystem I and produces ATP, but not NADPH  Cyclic electron flow generates surplus ATP, satisfying the higher demand in the Calvin cycle Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig Key Mitochondrion Chloroplast CHLOROPLAST STRUCTURE MITOCHONDRION STRUCTURE Intermembrane space Inner membrane Electron transport chain H+H+ Diffusion Matrix Higher [H + ] Lower [H + ] Stroma ATP synthase ADP + P i H+H+ ATP Thylakoid space Thylakoid membrane

Fig Light Fd Cytochrome complex ADP + i H+H+ ATP P synthase To Calvin Cycle STROMA (low H + concentration) Thylakoid membrane THYLAKOID SPACE (high H + concentration) STROMA (low H + concentration) Photosystem II Photosystem I 4 H + Pq Pc Light NADP + reductase NADP + + H + NADPH +2 H + H2OH2O O2O2 e–e– e–e– 1/21/

 The Calvin cycle, like the citric acid cycle, regenerates its starting material after molecules enter and leave the cycle  The cycle builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

 Carbon enters the cycle as CO 2 and leaves as a sugar named glyceraldehyde-3-phospate (G3P)  For net synthesis of 1 G3P, the cycle must take place three times, fixing 3 molecules of CO 2  The Calvin cycle has three phases: ◦ Carbon fixation (catalyzed by rubisco) ◦ Reduction ◦ Regeneration of the CO 2 acceptor (RuBP) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig Ribulose bisphosphate (RuBP) 3-Phosphoglycerate Short-lived intermediate Phase 1: Carbon fixation (Entering one at a time) Rubisco Input CO 2 P P P P P

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

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

 Dehydration is a problem for plants, sometimes requiring trade-offs with other metabolic processes, especially photosynthesis  On hot, dry days, plants close stomata, which conserves H 2 O but also limits photosynthesis  The closing of stomata reduces access to CO 2 and causes O 2 to build up  These conditions favor a seemingly wasteful process called photorespiration Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

 In most plants (C 3 plants), initial fixation of CO 2, via rubisco, forms a three-carbon compound  In photorespiration, rubisco adds O 2 instead of CO 2 in the Calvin cycle  Photorespiration consumes O 2 and organic fuel and releases CO 2 without producing ATP or sugar Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

 Photorespiration may be an evolutionary relic because rubisco first evolved at a time when the atmosphere had far less O 2 and more CO 2  Photorespiration limits damaging products of light reactions that build up in the absence of the Calvin cycle  In many plants, photorespiration is a problem because on a hot, dry day it can drain as much as 50% of the carbon fixed by the Calvin cycle Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

 C 4 plants minimize the cost of photorespiration by incorporating CO 2 into four-carbon compounds in mesophyll cells  This step requires the enzyme PEP carboxylase  PEP carboxylase has a higher affinity for CO 2 than rubisco does; it can fix CO 2 even when CO 2 concentrations are low  These four-carbon compounds are exported to bundle-sheath cells, where they release CO 2 that is then used in the Calvin cycle Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig C 4 leaf anatomy Mesophyll cell Photosynthetic cells of C 4 plant leaf Bundle- sheath cell Vein (vascular tissue) Stoma The C 4 pathway Mesophyll cell CO 2 PEP carboxylase Oxaloacetate (4C) Malate (4C) PEP (3C) ADP ATP Pyruvate (3C) CO 2 Bundle- sheath cell Calvin Cycle Sugar Vascular tissue

Fig a Stoma C 4 leaf anatomy Photosynthetic cells of C 4 plant leaf Vein (vascular tissue) Bundle- sheath cell Mesophyll cell

Fig b Sugar CO 2 Bundle- sheath cell ATP ADP Oxaloacetate (4C) PEP (3C) PEP carboxylase Malate (4C) Mesophyll cell CO 2 Calvin Cycle Pyruvate (3C) Vascular tissue The C 4 pathway

 Some plants, including succulents, use crassulacean acid metabolism (CAM) to fix carbon  CAM plants open their stomata at night, incorporating CO 2 into organic acids  Stomata close during the day, and CO 2 is released from organic acids and used in the Calvin cycle Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig CO 2 Sugarcane Mesophyll cell CO 2 C4C4 Bundle- sheath cell Organic acids release CO 2 to Calvin cycle CO 2 incorporated into four-carbon organic acids (carbon fixation) Pineapple Night Day CAM Sugar Calvin Cycle Calvin Cycle Organic acid (a) Spatial separation of steps (b) Temporal separation of steps CO 2 1 2

 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 O 2 in our atmosphere Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig Light Reactions: Photosystem II Electron transport chain Photosystem I Electron transport chain CO 2 NADP + ADP P i + RuBP 3-Phosphoglycerate Calvin Cycle G3P ATP NADPH Starch (storage) Sucrose (export) Chloroplast Light H2OH2O O2O2

Fig. 10-UN1 CO 2 NADP + reductase Photosystem II H2OH2O O2O2 ATP Pc Cytochrome complex Primary acceptor Primary acceptor Photosystem I NADP + + H + Fd NADPH Electron transport chain Electron transport chain O2O2 H2OH2O Pq