Photosynthesis: Using Light to Make Food Chapter 7 Photosynthesis: Using Light to Make Food

AN OVERVIEW OF PHOTOSYNTHESIS Photosynthesis is the process by which autotrophic organisms use light energy to make sugar and oxygen gas from carbon dioxide and water Carbon dioxide Water Glucose Oxygen gas PHOTOSYNTHESIS

Autotroph: can use CO2 as a sole source of carbon to make other organic compounds Heterotroph: cannot use CO2 as a sole source of carbon   Autotroph = producer Heterotroph = consumer, e.g., animals Autotroph Photoautotroph : light as the energy source: e.g., plants, algae, photosynthetic bacteria Chemoautotroph : chemical compounds as the energy source : e.g., H2-oxidizing bacteria

7.1 Autotrophs are the producers of the biosphere Plants, some protists, and some bacteria are photosynthetic autotrophs They are the ultimate producers of food consumed by virtually all organisms

In aquatic environments, algae and photosynthetic bacteria are the main food producers Figure 7.1C Figure 7.1D

7.2 Photosynthesis occurs in chloroplasts In most plants, photosynthesis occurs primarily in the leaves, in the chloroplasts 6CO2 + 6H2O  C6H12O6 + 6O2 All green parts of a plant have chloroplasts Chloroplasts are the sites where photosynthesis occurs Green pigment : chlorophyll (absorption of light energy) Chloroplasts are most abundant in the mesophyll cells Gas exchange (CO2/O2) occurs by way of tiny pores called stomata

The location and structure of chloroplasts Figure 7.2_1 The location and structure of chloroplasts Epidermis cell Leaf Cross Section Leaf Mesophyll Vein Mesophyll Cell CO2 O2 Stoma Chloroplast

Inner and outer membranes Figure 7.2_2 Chloroplast Inner and outer membranes Granum Thylakoid Thylakoid space Figure 7.2_2 Zooming in on the location and structure of chloroplasts (part 2) Stroma 9

7.3 Plants produce O2 gas by splitting water The O2 liberated by photosynthesis is made from the oxygen in water Figure 7.3A

Experiment 1: 6 CO2  12 H2O → C6H12O6  6 H2O  6 O2 Reactants: Products:

7.4 Photosynthesis is a redox process, as is cellular respiration Water molecules are split apart and electrons and H+ ions are removed, leaving O2 gas These electrons and H+ ions are transferred to CO2, producing sugar Energy input is required (light energy) Reduction Oxidation Figure 7.4A Oxidation Reduction Figure 7.4B

7.5 Overview: Photosynthesis occurs in two stages linked by ATP and NADPH The complete process of photosynthesis consists of two linked sets of reactions: the light reactions and the Calvin cycle The light reactions convert light energy to chemical energy and produce O2 The Calvin cycle assembles sugar molecules from CO2 using the energy-carrying products of the light reactions

LIGHT REACTIONS (in grana) CALVIN CYCLE (in stroma) An overview of photosynthesis H2O CO2 Chloroplast Light NADP+ ADP + P LIGHT REACTIONS (in grana) CALVIN CYCLE (in stroma) ATP Electrons NADPH O2 Sugar

7.6 Visible radiation drives the light reactions THE LIGHT REACTIONS: CONVERTING SOLAR ENERGY TO CHEMICAL ENERGY 7.6 Visible radiation drives the light reactions Light is a form of electromagnetic radiation Light behaves like wave and particle

10–5 nm 10–3 nm 1 nm 103 nm 106 nm 1 m 103 m Increasing energy Gamma rays Micro- waves Radio waves X-rays UV Infrared Visible light 380 400 500 600 700 750 Wavelength (nm) 650 nm

Visible light : 380 – 750 nm Energy is inversely proportional to wavelength E = hc/l h: Planck’s constant C: velocity of light (3 x 108 m/sec)

When a molecule absorbes a photon, one of its electrons is raised from a ground state to an excited state The excited molecule can serve as a strong reductant to reduce another molecule Pigment : a molecule that absorbs visible light

Pigments that are involved in photosynthesis Chlorophyll : major photopigments in chloroplasts Carotenoid : 광합성시 이용가능한 광범위를 넓혀줌 Protection chlorophylls from photodamage

Light Reflected light Chloroplast Absorbed light Thylakoid Figure 7.6B Light Reflected light Chloroplast Absorbed light Thylakoid Transmitted light

Several pigments are built into the thylakoids of chloroplasts Absorb some wavelengths of light; reflect or transmit others Chlorophyll a Absorbs blue-violet and red light, reflects green light Participates directly in the light reactions

Chlorophyll b Carotenoids Absorbs blue and orange light, reflects yellow-green Conveys absorbed energy to chlorophyll a Carotenoids Yellow-orange pigments that absorb mainly blue-green light May pass energy to chlorophyll a or protect it by dissipating excessive light energy

Chlorophyll b CH3  CHO Porphyrin ring with a Mg2+ Phytol

When an excited chlorophyll molecule returns to the ground state, Emission of heat or fluorescence Reduction of the primary electron acceptor in the reaction center https://www.youtube.com/watch?v=joZ1EsA5_NY

7.7 Photosystems capture solar power The site where light energy is absorbed and light reaction begins Reaction center + light harvesting complex Located in the thylakoid membrane RC: Chl. a + primary electron acceptor LHC: Chl. a and b + carotenoids RC Antenna pigment molecules Photopigment molecules are associated with membrane proteins

Pair of chlorophyll a molecules Figure 7.7B Photosystem Light Light-harvesting complexes Reaction-center complex Primary electron acceptor Thylakoid membrane Pigment molecules Pair of chlorophyll a molecules Transfer of energy

Primary electron acceptor An excited Chl molecule in the RC serves as a strong reductant PHOTOSYSTEM Photon Reaction center Pigment molecules of antenna Figure 7.7C

7.8 In the light reactions, electron transport chains generate ATP, NADPH, and O2 12H2O  6O2 : synthesis of NADPH and ATP Photosystems in higher plants Photosystem II : water-splitting photosystem Photosystem I : NADPH-producing photosystem

Reduction potential smaller Primary electron acceptor Electron transport Primary electron acceptor Electron transport chain Photons Energy for synthesis of PHOTOSYSTEM I PHOTOSYSTEM II by chemiosmosis Figure 7.8 Electron donor of the ETC : H2O Final electron acceptor of the ETC : NADP+

Figure 7.8A Electron transport chain Provides energy for synthesis of ATP by chemiosmosis NADP  H NADPH Light Light Photosystem I 6 Photosystem II Stroma 1 Primary acceptor Primary acceptor 2 Thylakoid membrane 4 5 P680 P700 Thylakoid space 3 H2O 2 1 O2 H  2

Photosynthetic electron transport Non-cyclic electron transport : synthesis of NADPH and ATP Cyclic electron transport : synthesis of ATP

7.9 Chemiosmosis powers ATP synthesis in the light reactions The electron transport chains are arranged with the photosystems in the thylakoid membranes and pump H+ through that membrane The flow of H+ back through the membrane is harnessed by ATP synthase to make ATP In the stroma, the H+ ions combine with NADP+ to form NADPH

Electron transport chain The production of ATP by chemiosmosis in photosynthesis Chloroplast To Calvin Cycle H+ ATP Light Light ADP P Stroma (low H+ concentration) H+ NADP+ H+ NADPH H+ H+ Thylakoid membrane H+ H+ H+ H+ H2O 1 2 H+ H+ Thylakoid space (high H+ concentration) O2 + 2 H+ H+ H+ H+ H+ H+ Electron transport chain H+ H+ Photosystem II Photosystem I ATP synthase

PQA and B: plastoquinone; cyt: cytochrome b6f complex http://www.youtube.com/watch?v=hj_WKgnL6MI PQA and B: plastoquinone; cyt: cytochrome b6f complex PC: plastocyanine; Fd: ferredoxin

7.10 ATP and NADPH power sugar synthesis in the Calvin cycle THE CALVIN CYCLE: CONVERTING CO2 TO SUGARS 7.10 ATP and NADPH power sugar synthesis in the Calvin cycle The Calvin cycle occurs in the chloroplast’s stroma This is where carbon fixation takes place and sugar is manufactured INPUT CALVIN CYCLE Figure 7.10A OUTPUT:

The Calvin cycle constructs G3P using carbon from atmospheric CO2 electrons and H+ from NADPH energy from ATP Energy-rich sugar (G3P) is then converted into glucose G3P: glyceraldehyde 3-phosphate

Details of the Calvin cycle RuBP carboxylase/oxygenase (Rubisco) 6CO2 C6H12O6 12NADPH + 18ATP

3-phosphoglycerate is the first product of CO2 fixation in Calvin cycle (C3 plants) Glyceraldehyde-3-phosphate is the sugar molecule made by Calvin cycle

7.11 Review: Photosynthesis uses light energy to make food molecules PHOTOSYNTHESIS REVIEWED AND EXTENDED 7.11 Review: Photosynthesis uses light energy to make food molecules A summary of the chemical processes of photo-synthesis Chloroplast Light Photosystem II Electron transport chains Photosystem I CALVIN CYCLE Stroma Electrons Cellular respiration Cellulose Starch Other organic compounds LIGHT REACTIONS CALVIN CYCLE Figure 7.11

Many plants make more sugar than they need The excess is stored in roots, tuber, and fruits These are a major source of food for animals

Photorespiration Rubisco is a bifunctional enzyme carboxylase: RuBP + CO2  2 3-PGA  Calvin cycle oxygenase: RuBP + O2  phosphoglycolate + 3-PGA + H2O In peroxisome mitochondria CO2 CO2와 O2분압의 비율이 Rubisco가 CO2/O2 고정 여부를 결정 Photorespiration이 일어나면 Calvin cycle을 통한 CO2고정이 감소

Photorespiration in a C3 plant CALVIN CYCLE (phosphoglycolate) 2-C compound Figure 7.12A

7.12 C4 and CAM plants have special adaptations that save water Most plants are C3 plants, which take CO2 directly from the air and use it in the Calvin cycle In these types of plants, stomata on the leaf surface close when the weather is hot This causes a drop in CO2 and an increase in O2 in the leaf Photorespiration may then occur

Some plants have special adaptations that enable them to save water CAM PEP : phosphoenolpyruvate PEP+CO2oxaloacetate PEP carboxylase

Figure 7.11 Mesophyll cell CO2 CO2 Night 4-C compound 4-C compound Bundle- sheath cell CO2 CO2 Calvin Cycle Calvin Cycle Figure 7.11 Comparison of C4 and CAM photosynthesis 3-C sugar 3-C sugar Day C4 plant CAM plant Sugarcane Pineapple 45

Rubisco and other Calvin cycle enzymes are present only in bundle sheath cells of C4 plants C4 plant : corn, sugarcane Mesophyll cell Bundle sheath cell

The CAM plants—pineapples, most cacti, and succulents—employ a different mechanism They open their stomata at night and make a four-carbon compound It is used as a CO2 source by the same cell during the day 4-C compound Night Day CALVIN CYCLE CAM : Crassulacean acid metabolism 3-C sugar Figure 7.12C