Using Light to Make Food Ch. 7 PHOTOSYNTHESIS Using Light to Make Food

Autotrophs Are the Producers of The Biosphere Autotrophs make their own food Photoautotrophs use the energy of light and chloroplasts to produce organic molecules Most plants, algae and other protists, some prokaryotes (cyanobacteria) Copyright © 2009 Pearson Education, Inc.

Figure 7.1A Forest plants.

Chloroplast Outer and inner membranes Thylakoid Intermembrane space Stroma Granum Thylakoid space Figure 7.2 The location and structure of chloroplasts.

Visible Radiation Drives the Light Reactions Sunlight is a type of electromagnetic energy (radiation) Visible light is a small part of the EM spectrum Light travels in waves and is particulate One wavelength = distance between the crests of two adjacent waves (shorter λ’s have more energy) Photons are discrete packets of light energy (they contain a fixed quantity of light energy)

Increasing energy 10–5 nm 10–3 nm 1 nm 103 nm 106 nm 1 m 103 m Micro- Gamma rays Micro- waves Radio waves X-rays UV Infrared Visible light Figure 7.6A The electromagnetic spectrum and the wavelengths of visible light. (A 380 400 500 600 700 750 Wavelength (nm) 650 nm

Visible Radiation Drives the Light Reactions Pigments Are proteins Absorb specific wavelengths of light, transmit others The color of a pigment is the color of light most reflected or transmitted by that pigment Ex. chlorophyll reflects/transmits green and appears green Various pigments are built into the thylakoid membrane of the chloroplast Each type of pigment absorbs certain wavelengths of light because it is able to absorb the

Visible Radiation Drives the Light Reactions Chloroplasts contain several different pigments and all absorb light of different wavelengths Chlorophyll a: absorbs blue violet and red light and reflects green Chlorophyll b: absorbs blue and orange and reflects yellow-green The carotenoids: absorb mainly blue-green light and reflect yellow and orange

Photosynthesis is a process that converts solar energy to chemical energy Plants use water and atmospheric carbon dioxide to produce a simple sugar and release oxygen Light energy 6 CO2 + 6 H2O C6H12O6 + 6 O2 Carbon dioxide Water Glucose Oxygen gas Photosynthesis

Photosynthesis Occurs in Chloroplasts in Plant Cells Leaves have: Stomata = pores that allow CO2 to enter and O2 to exit Veins transport water & nutrients absorbed by roots Chloroplasts = site of photosynthesis Contain the green pigment chlorophyll Vein CO2 O2 Stoma Figure 7.2 The location and structure of chloroplasts. Chloroplasts

Photosynthesis is a Redox Process, as is Cellular Respiration A loss of electrons = oxidation A gain of electrons = reduction Electrons are lost and gained in the form of hydrogen During redox reactions of photosynthesis, H2O donates electrons and is oxidized, CO2 accepts electrons and is reduced Reduction 6 CO2 + 6 H2O C6H12O6 + 6 O2 Oxidation

H2O CO2 Chloroplast Light NADP+ ADP  P LIGHT REACTIONS CALVIN CYCLE (in stroma) (in thylakoids) ATP Electrons NADPH O2 Sugar

The “Photo” in Photosynthesis: THE LIGHT REACTIONS

Photosystems Capture Solar Power in the Light Reactions A photosystem is a functional unit that captures sunlight and helps convert it to the chemical energy of ATP & NADPH A Photosystem has a light-harvesting complex surrounding a reaction center chl. a and a primary e- acceptor are in the reaction center Light-harvesting complexes Reaction center e– Figure 7.7B Light-excited chlorophyll embedded in a photosystem: Its electron is transferred to a primary electron acceptor before it returns to ground state. Pigment molecules Pair of Chlorophyll a molecules

Photosystems Capture Solar Power in the Light Reactions Two photosystems exist: photosystem I and photosystem II Each photosystem has a characteristic reaction center Photosystem II: functions first; the chl. a of this photosystem is called P680 (it best absorbs light at 680nm or red) Photosystem I: functions next; the chl. a of this photosystem is called P700 (it best absorbs light at 700nm, also red)

Photosystems Capture Solar Power in the Light Reactions Light energy is absorbed by pigments of PS II and passed from pigment to pigment within the photosystem The energy passes to chl. a, exciting its electrons An excited e- from chl. a is transferred to the primary electron acceptor An enzyme oxidizes water and gives the electrons to chl. a This step releases oxygen Electron transport chain Provides energy for synthesis of by chemiosmosis NADP+ + H+ NADPH Photon Photon Photosystem II ATP Photosystem I 6 Stroma 1 Primary acceptor Primary acceptor 2 e– e– Thylakoid mem- brane 4 5 P700 P680 e– Thylakoid space e– e– 3 H2O H+ 2  1 O2 + 2

The Electron Transport Chain Each photoexcited e- passes from the primary e- acceptor of PS II to PS I via an electron transport chain (ETC) The exergonic “fall” of electrons down the ETC will help generate a H+ gradient used to generate ATP. The ETC is a bridge between photosystems II and I

The Electron Transport Chain NADP+ + H+ NADPH Photon Photon Photosystem I Photosystem II 6 Stroma 1 Primary acceptor Primary acceptor 2 e– e– Thylakoid mem- brane 4 5 P700 P680 Thylakoid space 3 H2O 2  1 O2 + 2 H+

The Electron Transport Chain The ETC accepts electrons from PS II Electrons are passed along ETC and the 2nd protein complex pumps H+ into the thylakoid space This generates a proton (H+) gradient H+ flows from the thylakoid space to the stroma, down its gradeint, through an ATP synthase. ATP synthase phosphorylates ADP forming ATP This is an energy-coupling process called chemiosmosis, where the exergonic flow of H+ powers the endergonic phosphorylation of ADP

Electron transport chain Stroma (low H+ concentration) H+ H+ ADP + P ATP H+ NADP+ + H+ NADPH PS II H+ PS I H2O H+ H+ H+ Figure 7.9 The production of ATP by chemiosmosis in photosynthesis. 2  1 O2 + 2 H+ H+ H+ H+ H+ H+ Electron transport chain H+ ATP synthase H+ Thylakoid space (high H+ concentration)

The Light Reactions Electrons moving down the ETC are passed to chl. a of PS I NADP+ reductase transfers electrons from PS I to NADP+ forming NADPH

Electron transport chain Stroma (low H+ concentration) H+ H+ ADP + P ATP H+ NADP+ + H+ NADPH PS II H+ PS I H2O H+ H+ H+ Figure 7.9 The production of ATP by chemiosmosis in photosynthesis. 2  1 O2 + 2 H+ H+ H+ H+ H+ H+ Electron transport chain H+ ATP synthase H+ Thylakoid space (high H+ concentration)

The Light Reactions As a result of the light reactions, ATP and NADPH are produced (in the stroma) Photophosphorylation is the process of generating ATP from ADP & phosphate by means of a proton-motive force generated by the thylakoid membrane in the light reactions of photosynthesis

THE CALVIN CYCLE: CONVERTING CO2 TO SUGAR

ATP and NADPH Power Sugar Synthesis in the Calvin Cycle The Calvin cycle makes sugar within a chloroplast Atmospheric CO2, ATP, and NADPH are required to produce sugar Using these three ingredients, a three-carbon sugar called glyceraldehyde-3-phosphate (G3P) is produced A plant cell may G3P to make glucose and other organic molecules CO2 ATP NADPH Input CALVIN CYCLE Figure 7.10A An overview of the Calvin cycle. Output: G3P

ATP and NADPH Power Sugar Synthesis in the Calvin Cycle The Calvin Cycle Has Three Phases: Carbon Fixation- Atmospheric carbon (CO2) is incorporated into a molecule of ribulose bisphosphate (RuBP) by the enzyme rubisco 3 molecules of CO2 are required to make 1 molecule of G3P 2. Reduction Phase – NADPH reduces 3PGA to G3P 3. Regeneration of Starting Material – RuBP is regenerated and the cycle starts again To synthesize one G3P molecule, the Calvin cycle consumes nine ATP and six NADPH molecules, which were provided by the light reactions. Copyright © 2009 Pearson Education, Inc.

Step Carbon fixation Input: 3 CO2 Rubisco 3 P P 6 P RuBP 3-PGA CALVIN 1 Input: 3 CO2 Rubisco 1 3 P P 6 P RuBP 3-PGA CALVIN CYCLE Figure 7.10B Details of the Calvin cycle, which takes place in the stroma of a chloroplast.

Step Carbon fixation Input: 3 CO2 Rubisco Step Reduction 3 P P 6 P 1 Input: 3 CO2 Rubisco 1 Step Reduction 2 3 P P 6 P RuBP 3-PGA 6 ATP 6 ADP + P CALVIN CYCLE 2 6 NADPH 6 NADP+ Figure 7.10B Details of the Calvin cycle, which takes place in the stroma of a chloroplast. 6 P G3P

Step Carbon fixation Input: 3 CO2 Rubisco Step Reduction 3 P P 6 P 1 Input: 3 CO2 Rubisco 1 Step Reduction 2 3 P P 6 P RuBP 3-PGA 6 ATP 6 ADP + P CALVIN CYCLE 2 6 NADPH 6 NADP+ Figure 7.10B Details of the Calvin cycle, which takes place in the stroma of a chloroplast. 5 P 6 P G3P G3P Glucose and other compounds Output: 1 P G3P

Step Regeneration of RuBP 6 NADP+ Step Carbon fixation 1 Input: 3 CO2 Rubisco 1 Step Reduction 2 3 P P 6 P RuBP 3-PGA 6 ATP 3 ADP 6 ADP + P 3 ATP CALVIN CYCLE 3 2 6 NADPH Step Regeneration of RuBP 3 6 NADP+ Figure 7.10B Details of the Calvin cycle, which takes place in the stroma of a chloroplast. 5 P 6 P G3P G3P Glucose and other compounds Output: 1 P G3P

Overview of Photosynthesis

PHOTOSYNTHESIS REVIEWED AND EXTENDED Copyright © 2009 Pearson Education, Inc.

H2O CO2 Chloroplast Light NADP+ ADP + P RuBP CALVIN CYCLE Electron Photosystem II RuBP CALVIN CYCLE Electron transport chains 3-PGA (in stroma) Thylakoid membranes Photosystem I ATP Stroma Figure 7.11 A summary of the chemical processes of photosynthesis. NADPH G3P Cellular respiration Cellulose Starch O2 Sugars Other organic compounds LIGHT REACTIONS CALVIN CYCLE

EVOLUTION CONNECTION: Adaptations that save water in hot, dry climates evolved in C4 and CAM plants In hot climates, plant stomata close to reduce water loss so oxygen builds up Rubisco adds oxygen instead of carbon dioxide to RuBP in a process called photorespiration Photorespiration uses oxygen, produces CO2; sugar and ATP are not produced Copyright © 2009 Pearson Education, Inc.

EVOLUTION CONNECTION: Adaptations that save water in hot, dry climates evolved in C4 and CAM plants C4 plants Have a unique leaf anatomy First stable compound is a 4C compound C4 plants partially shut stomata when hot and dry to conserve water Have PEP carboxylase to bind CO2 at low levels (carbon fixation) Allows plant to maintain an adequate concentration of carbon to feed into the Calvin Cycle to continue making sugar Copyright © 2009 Pearson Education, Inc.

EVOLUTION CONNECTION: Adaptations that save water in hot, dry climates evolved in C4 and CAM plants CAM plants open their stomata at night thus admitting CO2 in w/o loss of H2O CO2 enters, and is fixed into a four-carbon compound, (carbon fixation) Carbon is released into the Calvin cycle during the day

CALVIN CYCLE CALVIN CYCLE Mesophyll cell CO2 CO2 Night 4-C compound 4-C compound CO2 CO2 CALVIN CYCLE CALVIN CYCLE Figure 7.12 Comparison of photosynthesis in C4 and CAM plants: In both pathways, CO2 is first incorporated into a four-carbon compound, which then provides CO2 to the Calvin cycle. Bundle- sheath cell 3-C sugar 3-C sugar Day C4 plant CAM plant

Separation of Photosynthetic Pigments in Chloroplasts