Photosynthesis using light to make food unit 5

Have you thanked a green plant today?

Energy for Life Processes Review: All living things use energy - autotrophs / heterotrophs Autotrophs- organisms that manufacture their own food. Most autotrophs use PHOTOSYNTHESIS to obtain food. Besides plants, algae and some bacteria are photosynthetic organisms

Energy from food monomers

Energy & Chemical Cycling

Organisms that photosynthesize

Overview - What is the general equation for photosynthesis?

Tracking atoms in photosynthesis Reactants: 6 CO2 12 H2O Products: C6H12O6 6 H2O 6 O2 Figure 10.4 Tracking atoms through photosynthesis Photosynthesis is a redox process in which H2O is oxidized and CO2 is reduced

Today’s Focus: Photosynthesis: process of converting solar energy into chemical energy We can break down photosynthesis into two different stages: Light-dependent reactions Light-independent reactions What are examples of transformations of energy we encounter every day? Focus question: How do plants convert solar energy into chemical energy?

Photosynthesis 2-part overview

Not all plants photosynthesize: parasitic Dodder

Where in plants does photosynthesis occur? All green parts 13

All green parts of a plant have chloroplasts! Lots of plant cells

zoom into a plant cell inside a chloroplast Chloroplast – is a light absorbing organelle Features inside: GRANA (granum – singular) - stacks of thylakoids. (thigh) STROMA- solution that surrounds thylakoids “Leaves have about 500,000 chloroplasts per millimeter squared of leaf surface” Green color is due to chlorophyll pigment

Chloroplast: sites of photosynthesis

Stop and Jot In a typical plant cell, what are the different membranes light has to cross to reach chlorophyll? (cell wall, cell membrane, chloroplast membrane, and thylakoid membrane)

Beginning of photosynthesis story (Part I the light reactions): Step 1. Light must be absorbed into the chloroplasts. One cell in a plant leaf can have 50 or more chloroplasts. BASICS OF LIGHT Sunlight -> white light (really the whole visible spectrum) ROYGBIV Wavelength Pigment- compound that absorbs light Absorbs, reflects, transmits. Absorption spectra

Thylakoid In membrane - lots of pigments: most imp. pigment -> chlorophylls Accessory pigments - chlorophyll b, carotenoids, etc. In leaves of plants the chlorophylls are most abundant pigment -> thus leaves green. In Autumn - lose chlorophylls-> other pigments show through. Non-photosynthetic parts of plant?

interacts with hydrophobic regions of proteins inside Fig. 10-10 CH3 in chlorophyll a CHO in chlorophyll b Porphyrin ring: light-absorbing “head” of molecule; note magnesium atom at center Figure 10.10 Structure of chlorophyll molecules in chloroplasts of plants Hydrocarbon tail: interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts; H atoms not shown

Spectroscopy of Chlorophyll a and b

Visible Spectrum

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

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

The Light Reactions

Photosystems: protein complex that contains pigments There are two types of photosystems in the thylakoid membrane Photosystem II (PS II) functions first (the numbers reflect order of discovery) and is best at absorbing a wavelength of 680 nm The reaction-center chlorophyll a of PS II is called P680 Photosystem I (PS I) is best at absorbing a wavelength of 700 nm The reaction-center chlorophyll a of PS I is called P700 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Chlorophyll pigment is found in the thylakoid membrane How does chloroplast structure relate to its function? The stacks of thylakoids help to increase the surface area, allowing for more photosynthesis to occur. Having the thylakoids stacked also helps decrease the volume a chloroplast occupies, allowing numerous chloroplasts to be in a cell and also increase photosynthesis. photosynthetic cells must be intricately set up in order to be able to harvest light energy & not let it dissipate in unproductive ways)

(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)

Electron Resonance Transfer (excitation energy) What does the light energy end up doing in photosystem II? A photon hits a pigment and its energy is passed among pigment molecules until it excites P680 An excited electron from P680 is transferred to the primary electron acceptor Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Analyzing models © 2010 Pearson Education, Inc.

Producing ATP and NADPH

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

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+

Summary of light reactions Light reactions happen in the thylakoids Split water Release oxygen Reduce NADP+ to NADPH Generate ATP from ADP by photophosphorylation

Check For Understanding Where does the original high energy electrons come from? What is the final electron acceptor?

Relating cellular respiration to photosynthesis Mitochondria transfer chemical energy from food to ATP; chloroplasts transform light energy into the chemical energy of ATP

Part II: The Calvin Cycle uses ATP and NADPH to convert CO2 to sugar 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

The Calvin cycle has three phases: Carbon enters the cycle as CO2 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 CO2 The Calvin cycle has three phases: Carbon fixation (catalyzed by rubisco) Reduction Regeneration of the CO2 acceptor (RuBP) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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

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)

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)

3  5C 6  3C Calvin Cycle Regeneration of CO2 acceptor 5  3C Fig. 10-UN2 3 CO2 Carbon fixation 3  5C 6  3C Calvin Cycle Regeneration of CO2 acceptor 5  3C Reduction 1 G3P (3C)

Fig. 10-UN4

You should now be able to: Describe the structure of a chloroplast Trace the movement of electrons in linear electron flow Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Alternative mechanisms

Alternative Mechanisms Stomata: absorb CO2 and release water On a hot day, plants close their stomata (reduce the CO2 present)

Water Saving Adaptations: C4 and CAM Plants

C4

C4

CAM Crassulacean acid metabolism – 4C compound

CAM Plants: Night and Day

Photorespiration CO2 becomes scarce Rubisco binds to O2 instead of CO2 ATP consumed rather than generated! No sugar produced! Consumes organic material from Calvin Cycle! What’s the point? In some case clear evidence that photorespiration protects plants from damaging products of light reaction which build up when CO2 is scarce

Atmospheric CO2 Concentrations