Photosynthesis Chapter 9

Photosynthesis: The Big Picture Source of BOTH matter and energy for most living organisms Captures light energy from the sun and converts it into chemical energy Synthesized organic molecules from inorganic molecule BOTTOM LINE: Makes FOOD

Definitions Autotroph: Organisms that make their own food (energy-rich organic molecules) from simple, inorganic molecules Photoautotroph: Organisms that make their own food through photosynthesis; obtain energy from the sun Type of autotroph Heterotroph: Get carbon and energy by eating autotrophs or one another

Photoautotrophs Capture sunlight energy and use it to carry out photosynthesis Plants Some bacteria cyanobacteria Many protistans algae Plants Algae (spirogyra) Cyanobacteria Algea (Kelp)

Linked Processes Photosynthesis Aerobic Respiration Energy-storing pathway Releases oxygen Requires carbon dioxide Aerobic Respiration Energy-releasing pathway Requires oxygen Releases carbon dioxide

Photosynthesis Equation LIGHT ENERGY 6H2O + 6CO2 6O2 + C6H12O6 Water Carbon Dioxide Oxygen Glucose In-text figure Page 115

Chloroplast Structure Inner and outer membranes Stroma Granum (Grana) Thylakoid Figure 7.3d, Page 116

Two Stages of Photosynthesis Light dependent reactions Converts light energy into chemical energy (ADP ATP) Gathers e- and H+ from water (NADP+  NADPH) Occurs in thylakoid membranes Light independent reactions (Calvin-Benson Cycle) Reduces CO2 to synthesize glucose using energy and hydrogens (i.e. ATP and NADPH) generated in the light dependent reaction Occurs in Stroma Notice that these reactions do not create NADH, but rather NADPH

Two Stages of Photosynthesis sunlight water uptake carbon dioxide uptake ATP LIGHT-DEPENDENT REACTIONS ADP + Pi LIGHT-INDEPENDENT REACTIONS NADPH NADP+ P glucose oxygen release new water In-text figure Page 117

Electromagnetic Spectrum Shortest Gamma rays wavelength X-rays UV radiation Visible light Infrared radiation Microwaves Longest Radio waves wavelength

Visible Light Electromagnetic energy with a wavelength of 308-750nm Light energy is organized into packets called photons The shorter the wavelength the greater the energy carried by the photons

Properties of Light White light (from the sun) contains all of the wavelengths of light When light hits matter, it can be reflected (transmitted) or absorbed White substances reflect all light Black substances absorb all light

Pigments A substance that absorbs light We see the color that is transmitted by pigment The absorbed color disappears into pigment

Plant pigments Plant use a variety of pigments during photosynthesis: Chlorophylls a and b Carotenoids Anthocyanins Phycobilins The main photosynthetic pigment is Chlorophyll a

Wavelength absorption (%) Wavelength (nanometers) Chlorophylls Chlorophyll a absorbs red and blue light, and reflects green light (what we see) Note: The colors that are absorbed are used for photosynthesis Wavelength absorption (%) chlorophyll a chlorophyll b Figure 7.7 Page 120 Figure 7.6a Page 119 Wavelength (nanometers)

Effect of Light on Pigments What happens when light hits pigments? The color disappears, but the energy does not Absorbing photons of light excites electrons (e-), thus adding potential energy Ground state: normal pigment Excited state: pigment absorbing light (e- excited) Photon of light: e- e- Atom in pigment: Ground state Atom in pigment: Excited state

Photosystems In thylakoid membrane, pigments are organized in clusters called photosystems These clusters contain several hundred pigment molecules Two types of photosystems Photosystem I = P700 (absorbs light at 700nm) Photosystem II = P680 (absorbs light at 680nm)

Reaction Center Chlorophyll One of the pigments in each photosystem is known as the reaction center chlorophyll (RCC) if any pigment within the photosystem gets hit by a photon, the energy is transferred to the RCC The RCC will then transfer its excited e- into an electron transport chain

Pigments in a Photosystem reaction center Figure 7.11 Page 122

Light Dependent Reactions Location: the thylakoid membranes Function: to generate ATP (energy!) and NADPH (reducing power!) that will be used in the light independent reaction Two processes: Non-cyclic electron flow Generates ATP and NADPH Cyclic electron flow Generates only ATP

Noncyclic Electron Flow Two-step pathway for light absorption and electron excitation Uses two photosystems: type I and type II Produces ATP and NADPH Involves photolysis - splitting of water

Machinery of Noncyclic Electron Flow H2O second electron transfer chain photolysis e– e– ATP SYNTHASE first electron transfer chain NADP+ NADPH ATP PHOTOSYSTEM II PHOTOSYSTEM I ADP + Pi Figure 7.13a Page 123

Steps of Non-cyclic electron flow Photosystem II gets hit by a photon; electron of RCC gets excited The excited (high energy) e- gets picked up by an electron carrier and taken into an electron transfer chain (ETC) The excited e- provides energy to pump protons (H+) into the thylakoid (tiny space) Through chemiosmosis, ATP is generated

Chemiosmotic Model of ATP Formation Electrical and H+ concentration gradients are created between thylakoid compartment and stroma H+ flow down gradients into stroma through ATP synthase The energy driven by the flow of H+ powers the formation of ATP from ADP and Pi

Chemiosmotic Model for ATP Formation H+ is shunted across membrane by some components of the first electron transfer chain Gradients propel H+ through ATP synthases; ATP forms by phosphate-group transfer Photolysis in the thylakoid compartment splits water H2O e– acceptor ATP SYNTHASE ATP ADP + Pi PHOTOSYSTEM II Figure 7.15 Page 124

Non-cyclic electron flow: Photolysis While Photosystem II gets hit by light, etc., water is split: H2O  ½ O2 + 2H+ + 2e- This process is called photolysis The H+ are pumped into the thylakoid to create the proton gradient The e- replace the excited e- that was taken away from the RCC

Non-Cyclic Electron Flow: The saga continues Photosystem I gets excited at the same time as photosystem II Its excited e- gets taken into a second electron transfer chain that attaches the excited e- and the leftover H+ to NADP+ to make NADPH: NADP+ + H+ + e-  NADPH

Non-cyclic electron flow The “electron hole” in photosystem I is then filled with the used up, low energy e- from photosystem II Now everything is back to normal, and we can start all over again

Energy Changes in Non-cyclic electron flow second transfer chain e– NADPH first e– transfer chain Potential to transfer energy (volts) e– e– (Photosystem I) (Photosystem II) H2O 1/2O2 + 2H+ Figure 7.13b Page 123

Non-cyclic electron flow: Summary After two excited photosystems, two ETCs and the splitting of water, both ATP and NADPH are generated!!!

Cyclic electron flow The light independent reactions require more ATP than NADPH Cyclic electron flow is like a short cut to making extra ATP Involves only Photosystem I

Cyclic electron flow Photosystem I gets excited Excited e- is carried into the first ETC; energy goes to pump H+ into thylakoid compartment Chemiosmosis powers formation of ATP The same e- (now low energy) replaces itself in the “electron hole” in Photosystem I

second electron transfer chain first electron transfer chain Cyclic electron flow H2O second electron transfer chain photolysis e– e– ATP SYNTHASE first electron transfer chain NADP+ NADPH ATP PHOTOSYSTEM II PHOTOSYSTEM I ADP + Pi Figure 7.13a Page 123

Light dependent reactions: Summary Non-cyclic electron flow Generates ATP and NADPH Uses both photosystems (P680 and P700) and both electron transport chains Involves photolysis Cyclic electron flow Generates ATP only Uses only P700 and only one ETC

Light-Independent Reactions Synthesis part of photosynthesis Can proceed in the dark Take place in the stroma Also called Calvin-Benson cycle, or Calvin Cycle, or Dark Reactions

Calvin- Benson Cycle Three Phases: Carbon Fixation Reduction Regeneration of RUBP

Calvin-Benson Cycle: Carbon Fixation Capturing atmospheric (gaseous) CO2 by attaching it to RuBP, a 5-carbon organic molecule This process forms two 3-carbon molecules The enzyme that catalyzes this process is called Rubisco

Calvin Benson Cycle: Reduction The captured CO2 has very little energy and no hydrogens In order to make sugar, energy and hydrogens need to be added to the molecules formed by Carbon fixation ATP and NADPH (made in the light dependent reactions) break down to form ADP and NADP+ and, in the process, transfer energy and hydrogens to the 3-carbon compounds formed by carbon fixation, resulting in sugar formation

Calvin-Benson Cycle: Regeneration Some of the sugar created by reduction leaves the Calvin cycle, and is used to build up glucose and other organic molecules The rest of the sugar is used to remake (regenerate) RuBP This process requires ATP (which was made in the light dependent reactions)

Calvin-Benson Cycle: Summary The cycle proceeds 6 times to form each molecule of glucose In the process, ATP and NADPH is used up 6CO2 are converted into C6H12O6 - glucose

The C3 Pathway In Calvin-Benson cycle, as described, the first stable intermediate is a three-carbon PGA Because the first intermediate has three carbons, the pathway is called the C3 pathway

Photorespiration in C3 Plants On hot, dry days stomata (holes in the leaf) close to prevent evaporation of water As a result, within the leaf oxygen levels rise, and Carbon dioxide levels drop Rubisco attaches RuBP to oxygen instead of carbon dioxide Results in a VERY wasteful process known as Photorespiration – uses up ATP without generating sugar

C4 and CAM Plants To avoid photorespiration, plants that live in hot, dry climates evolved mechanisms to separate carbon fixation from the Calvin Cycle The CO2 that enters the Calvin cycle is derived from the breakdown of previously synthesized organic acids In this way, the enzyme that catalyzes the reaction that attaches CO2 to RuBP is not exposed to atmospheric oxygen

C4 and CAM Plants C4 plants (grasses) do carbon fixation in a different location (cell type) than the Calvin cycle CAM plants (succulents and Cacti) do carbon fixation at a different time (night) that the Calvin cycle (day)

Summary of Photosynthesis light 6O2 12H2O CALVIN-BENSON CYCLE C6H12O6 (phosphorylated glucose) NADPH NADP+ ATP ADP + Pi PGA PGAL RuBP P 6CO2 end product (e.g., sucrose, starch, cellulose) LIGHT-DEPENDENT REACTIONS 6H2O LIGHT-INDEPENDENT REACTIONS Figure 7.21 Page 129