Autotrophs Are the Producers of The Biosphere  Autotrophs make their own food without using organic molecules derived from any other living thing –Photoautotrophs : Autotrophs that use the energy of light to produce organic molecules –Most plants, algae and other protists, and some prokaryotes are photoautotrophs –The ability to photosynthesize is directly related to the structure of chloroplasts Copyright © 2009 Pearson Education, Inc.

Chloroplast Outer and inner membranes Intermembrane space Granum Stroma Thylakoid space Thylakoid

Carbon dioxide C 6 H 12 O 6 Photosynthesis H2OH2O CO 2 O2O2 Water + 66 Light energy Oxygen gas Glucose + 6  Photosynthesis: a process that converts solar energy to chemical energy  Plants use water and atmospheric carbon dioxide to produce a simple sugar and release oxygen

CO 2 O2O2 Stoma Vein Chloroplasts  Leaves contain:  Stomata = tiny pores in the leaf; allow CO 2 to enter and O 2 to exit  Veins deliver water & nutrients absorbed by roots  Chlorophyll= a light absorbing pigment  responsible for the green color of plants  located on thylakoid membrane Photosynthesis Occurs in Chloroplasts in Plant Cells

Photosynthesis is a Redox Process, as is Cellular Respiration  Photosynthesis is a redox (oxidation-reduction) process  A loss of electrons = oxidation  A gain of electrons = reduction *(OIL RIG)  In photosynthesis: Water is oxidized producing oxygen and CO 2 is reduced producing sugar

6 CO H 2 O C 6 H 12 O O 2 Reduction Oxidation Photosynthesis is a Redox Process, as is Cellular Respiration

H2OH2O ADP P LIGHT REACTIONS (in thylakoids) Light Chloroplast NADPH ATP O2O2 CALVIN CYCLE (in stroma) Sugar CO 2  NADP +

Visible Radiation Drives the Light Reactions  Sunlight is a type of electromagnetic energy (radiation)  Visible light is a small part of the electromagnetic spectrum  Light exhibits the properties of both waves and particles –One wavelength = distance between the crests of two adjacent waves (shorter λ’s have greater energy) –Light behaves as discrete packets of energy called photons (photons contain a fixed quantity of light energy)

Wavelength (nm) 10 –5 nm Increasing energy Visible light 650 nm 10 –3 nm 1 nm10 3 nm10 6 nm 1 m 10 3 m Radio waves Micro- waves Infrared X-rays UV Gamma rays

 Pigments are proteins that absorb specific wavelengths of light and transmit others  Various pigments are built into the thylakoid membrane  We see the color of the wavelengths that are transmitted (not absorbed) –Ex. chlorophyll transmits green 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 Visible Radiation Drives the Light Reactions

Reaction center complex e–e– Light-harvesting complexes Photosystem Pigment molecules Pair of Chlorophyll a molecules  Solar energy can be released as heat or light but instead is conserved and passed from one pigment to another  All of the components to accomplish this are organized in thylakoid membranes in clusters called photosystems  A Photosystem consists of a light-harvesting complex surrounding a reaction center complex Photosystems Capture Solar Power

 Two types of photosystems exist: photosystem I and photosystem II –Each type of photosystem has a characteristic reaction center –Photosystem II: functions first; is called P680 (chl a best absorbs light w/ a wavelength of 680, red) –Photosystem I: functions next; is called P700 (chl a absorbs wavelength of 700, also red) Photosystems Capture Solar Power in the Light Reactions

Stroma O2O2 H2OH2O 1212 H+H+ NADP + NADPH Photon Photosystem II Electron transport chain Provides energy for synthesis of by chemiosmosis + 2 Primary acceptor 1 Thylakoid mem- brane P Thylakoid space e–e– e–e– 5 Primary acceptor P700 6 Photon Photosystem I ATP H+H+ +  When light strikes the photosystem energy is passed from pigment to pigment within the photosystem –Energy from the pigments is passed to chl. a in the reaction center where it excites electrons of chl. a –Excited e- from chl. a are transferred to the primary electron acceptor (reducing it) –This solar-powered transfer of an electron from the reaction center chl. a to the primary electron acceptor is the first step of the light reactions Photosystems Capture Solar Power in the Light Reactions

The Light Reactions –The primary e- acceptor passes electrons to an electron transport chain (ETC) –To fill the electron void that chl. a (P680) now has, chl. a (P680) oxidizes H 2 O and takes electrons from water –This is the step where water is oxidized and oxygen released –The ETC is a bridge between photosystems II and I. The ETC also generates ATP.

Stroma O2O2 H2OH2O 1212 H+H+ NADP + NADPH Photon Photosystem II Electron transport chain Provides energy for synthesis of by chemiosmosis + 2 Primary acceptor 1 Thylakoid mem- brane P Thylakoid space e–e– e–e– 5 Primary acceptor P700 6 Photon Photosystem I ATP H+H+ +

 As the first protein of the ETC accepts electrons it pumps H + into the thylakoid space. This generates a proton (H + ) gradient.  H + are concentrated in the thylakoid space and less concentrated in the stroma  Protons flow down their gradient through an enzyme called ATP synthase.  As H + flow through the ATP synthase it phosphorylates ADP forming ATP  The phosphorylation of ADP is endergonic; the flow of protons down their gradient is exergonic. The Light Reactions

 Chemiosmosis is a mechanism where the cell couples exergonic and endergonic reactions  Ex. An ATP synthase couples the flow of H + down their gradient (exergonic) to the phosphorylation of ADP (endergonic)  The chemiosmotic production of ATP in photosynthesis is called photophosphorylation  This step produces ATP in the stroma Chemiosmosis Powers ATP Synthesis in the Light Reactions

Two Photosystems Connected by an Electron Transport Chain Generate ATP and NADPH –Electrons moving down the ETC are passed to P700 of Photosystem I, and ultimately to NADP + by the enzyme NADP + reductase –This step produces NADPH in the stroma

+ O2O2 H2OH2O 1212 H+H+ NADP + H+H+ NADPH + 2 H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ Photosystem II Photosystem I Electron transport chain ATP synthase Light Stroma (low H + concentration) Thylakoid space (high H + concentration) ADP + P ATP

THE CALVIN CYCLE: CONVERTING CO 2 TO SUGARS

CO 2 ATP NADPH Input C ALVIN CYCLE G3P Output:  The Calvin cycle makes sugar within a chloroplast  Atmospheric CO 2, 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 then use G3P to make glucose and other organic molecules ATP and NADPH Power Sugar Synthesis in the Calvin Cycle

 The Calvin Cycle Has Three Phases:  1. Carbon Fixation-Atmospheric carbon in the form of CO 2 is incorporated into a molecule of ribulose bisphosphate (RuBP) via rubisco  2. Reduction Phase – NADPH reduces 3PGA to G3P (requires 3 molec’s. of CO 2 for 1 G3P)  3. Regeneration of Starting Material –RuBP is regenerated and the cycle starts again Copyright © 2009 Pearson Education, Inc. ATP and NADPH Power Sugar Synthesis in the Calvin Cycle

NADPH ATP RuBP 3 P G3P P Input: CO 2 1 Rubisco 3 P Step Carbon fixation 3-PGA 6 P C ALVIN CYCLE P Step Reduction 2 2 G3P 5 P P Glucose and other compounds Output: Step Release of one molecule of G3P 1 Step Regeneration of RuBP 4 4 ATP 3 3 ADP NADP + 6 ADP +

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

NADP + NADPH ATP CO 2 + H2OH2O ADP P Electron transport chains Thylakoid membranes Light Chloroplast O2O2 C ALVIN C YCLE (in stroma) Sugars Photosystem II Photosystem I L IGHT R EACTIONS RuBP 3-PGA C ALVIN C YCLE Stroma G3P Cellular respiration Cellulose Starch Other organic compounds

EVOLUTION CONNECTION: Adaptations that save water in hot, dry climates evolved in C 4 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 consumes oxygen, produces CO2, and produces no sugar, or ATP Copyright © 2009 Pearson Education, Inc.

EVOLUTION CONNECTION: Adaptations that save water in hot, dry climates evolved in C 4 and CAM plants  Some plants have evolved a means of carbon fixation that saves water during photosynthesis –C 4 plants partially shut stomata when hot and dry to conserve water –This reduces CO 2 levels, so contain PEP carboxylase to bind CO 2 at low levels (carbon fixation); They’re called C 4 plants because they first fix CO 2 into a four-carbon compound. –This allows plant to still make sugar by photosynthesis Copyright © 2009 Pearson Education, Inc.

EVOLUTION CONNECTION: Adaptations that save water in hot, dry climates evolved in C 4 and CAM plants  Another adaptation to hot and dry environments has evolved –CAM open their stomata at night thus admitting CO 2 in w/o loss of H 2 O –CO 2 enters, and is fixed into a four- carbon compound, (carbon fixation) –It is released into the Calvin cycle during the day Copyright © 2009 Pearson Education, Inc.

Mesophyll cell CO 2 C ALVIN C YCLE CO 2 Bundle- sheath cell 3-C sugar C 4 plant 4-C compound CO 2 C ALVIN C YCLE CO 2 3-C sugar CAM plant 4-C compound Night Day

Chlorophyll molecule Excited state Ground state Heat Photon (fluorescence) e–e–