Photosynthesis Chapter 7

2 Photosynthesis Overview Energy for all life on Earth ultimately comes from photosynthesis. 6CO H 2 O C 6 H 12 O 6 + 6H 2 O + 6O 2 Oxygenic photosynthesis is carried out by: cyanobacteria, 7 groups of algae, all land plants

Photosynthesis Overview Photosynthesis takes place in 3 stages: –Capturing energy from sunlight –Using the energy to make ATP and reduce NADP+ to NADPH (nicotinamide adenine dinucleotide phosphate) –Using the ATP and NADPH to synthesize organic molecules (glucose) from CO 2 3

4 Photosynthesis Overview Photosynthesis is divided into: light-dependent reactions -capture energy from sunlight -make ATP and reduce NADP + to NADPH carbon fixation reactions (light- independent reactions) -use ATP and NADPH to synthesize organic molecules from CO 2

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6 Photosynthesis Overview Photosynthesis takes place in chloroplasts. thylakoid membrane – internal membrane arranged in flattened sacs –contain chlorophyll and other pigments –Organized into photosystems Capture light and transfer energy (to pigment molecules) grana – stacks of thylakoid membranes stroma – semiliquid substance surrounding thylakoid membranes (houses the enzymes to make organic molecules)

Photosynthesis Overview 7 Photosynthesis takes place in the green portions of plants –Leaf of flowering plant contains mesophyll tissue –Cells containing chloroplasts –Specialized to carry on photosynthesis CO 2 enters leaf through stomata –Diffuses into chloroplasts in mesophyll cells –In stroma, CO 2 fixed to C 6 H 12 O 6 (sugar) –Energy supplied by light

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9 Discovery of Photosynthesis The work of many scientists led to the discovery of how photosynthesis works. Jan Baptista van Helmont ( ) Joseph Priestly ( ) Jan Ingen-Housz ( ) F. F. Blackman ( )

10 Discovery of Photosynthesis C. B. van Niel, 1930’s -proposed a general formula: CO 2 +H 2 A + light energy CH 2 O + H 2 O + 2A where H 2 A is the electron donor -van Niel identified water as the source of the O 2 released from photosynthesis -Robin Hill (in the 1950s) confirmed van Niel’s proposal that energy from the light reactions fuels carbon fixation (making glucose from CO 2 )

11 Pigments photon: a particle of light -acts as a discrete bundle of energy -energy content of a photon is inversely proportional to the wavelength of the light photoelectric effect: removal of an electron from a molecule by light -occurs when photons transfer energy to electrons

12 Electromagnetic Spectrum

13 Pigments Pigments: molecules that absorb visible light Each pigment has a characteristic absorption spectrum, the range and efficiency of photons it is capable of absorbing.

14 Pigments chlorophyll a – primary pigment in plants and cyanobacteria -absorbs violet-blue and red light chlorophyll b – secondary pigment absorbing light wavelengths that chlorophyll a does not absorb

15 A graph of percent of light absorbed at each wavelength is a compound’s absorption spectrum. Action spectrum Oxygen production and therefore photosynthetic activity is measured for plants under each specific wavelength; when plotted on a graph, this gives an action spectrum for a compound. The action spectrum for chlorophyll resembles its absorption spectrum, thus indicating that chlorophyll contributes to photosynthesis. Pigments

16 Pigments accessory pigments: secondary pigments absorbing light wavelengths other than those absorbed by chlorophyll a -increase the range of light wavelengths that can be used in photosynthesis -include: chlorophyll b, carotenoids, phycobiloproteins -carotenoids also act as antioxidants

17 Photosystem Organization A photosystem consists of 1. an antenna complex (light harvesting complex) of hundreds of accessory pigment molecules that gather photons and feeds energy to reaaction center 2. a reaction center of one or more chlorophyll a molecules pass electrons out of photosystem (photochemical reactions) In summary, energy of electrons is transferred through the antenna complex to the reaction center.

18 Photosystem Organization At the reaction center (transmembrane protein complex), the energy from the antenna complex is transferred to chlorophyll a. This energy causes an electron from chlorophyll to become excited. The excited electron is transferred from chlorophyll a to an electron acceptor. Water donates an electron to chlorophyll a to replace the excited electron.

19 Converting light to chemical energy

Photosynthesis Overview

Light Reactions Two electron pathways operate in the thylakoid membrane: the noncyclic pathway and the cyclic pathway. Both pathways produce ATP; only the noncyclic pathway also produces NADPH. ATP production during photosynthesis is called photophosphorylation; therefore these pathways are also known as cyclic and noncyclic photophosphorylation.

Takes place in thylakoid membrane Uses two photosystems, PS-I and PS-II (consists of pigment complexes) PS II captures light energy Causes an electron to be ejected from the reaction center (chlorophyll a) –Electron travels down electron transport chain to PS I –Replaced with an electron from water –causes H + to concentrate in thylakoid chambers –causes ATP production PS I captures light energy (electrons and H) –Transferred permanently to a molecule of NADP + –Causes NADPH production Light Reactions: The Noncyclic Electron Pathway

Light Reactions: Noncyclic Electron Pathway

Uses only photosystem I (PS-I) Begins when PS I complex absorbs solar energy Electron ejected from reaction center –Travels down electron transport chain –Causes H + to concentrate in thylakoid chambers –Which causes ATP production –Electron returns to PS-I (cyclic) Pathway only results in ATP production Light Reactions: The Cyclic Electron Pathway

Light Reactions: Cyclic Electron Pathway

The Organization of the Thylakoid Membrane PS II consists of a pigment complex and electron- acceptor molecules; it oxidizes H 2 O and produces O 2. The electron transport system consists of cytochrome complexes and transports electrons and pumps H + ions into the thylakoid space. PS I has a pigment complex and electron-acceptor molecules; it is associated with an enzyme that reduces NADP + to NADPH. ATP synthase complex has an H + channel and ATP synthase; it produces ATP.

Thylakoid space acts as a reservoir for hydrogen ions (H + ) Each time water is oxidized, two H + remain in the thylakoid space Electrons yield energy –Used to pump H + across thylakoid membrane –Move H + from stroma into the thylakoid space Flow of H + back across thylakoid membrane –Energizes ATP synthase –Enzymatically produces ATP from ADP + P This method of producing ATP is called chemiosmosis ATP Production

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29 Calvin Cycle Reactions: Carbon Dioxide Fixation CO 2 is attached to 5-carbon RuBP molecule –Result in a 6-carbon molecule –This splits into two 3-carbon molecules (3PG) –Reaction accelerated by RuBP Carboxylase (Rubisco) CO 2 now “fixed” because it is part of a carbohydrate

30 Calvin Cycle Reactions: Carbon Dioxide Reduction 3PG reduced to BPG BPG then reduced to G3P Utilizes NADPH and some ATP produced in light reactions

31 Calvin Cycle Reactions: Regeneration of RuBP RuBP used in CO 2 fixation must be replaced Every three turns of Calvin Cycle, –Five G3P (a 3-carbon molecule) used To remake three RuBP (a 5-carbon molecule)

32 The Calvin Cycle: Fixation of CO 2

33 Importance of Calvin Cycle G3P (glyceraldehyde-3-phosphate) can be converted to many other molecules The hydrocarbon skeleton of G3P can form –Fatty acids and glycerol to make plant oils –Glucose phosphate (simple sugar) –Fructose (which with glucose = sucrose) –Starch and cellulose –Amino acids

Other Types of Photosynthesis C 4 Photosynthesis and CAM Photosynthesis

In C 3 plants, the Calvin cycle fixes CO 2 directly; the first molecule following CO 2 fixation is 3PG. In hot weather, stomata close to save water; CO 2 concentration decreases in leaves; O 2 increases. O 2 combines with RuBP instead of CO 2 This is called photorespiration since oxygen is taken up and CO 2 is produced; this produces less 3PG. Most plants are C 3 plants

In a C 3 plant, mesophyll cells contain well ‑ formed chloroplasts, arranged in parallel layers. In C 4 plants, bundle sheath cells as well as the mesophyll cells contain chloroplasts. In C 4 leaf, mesophyll cells are arranged concentrically around the bundle sheath cells. C 4 Photosynthesis

Remember C 3 plants use RuBP carboxylase to fix CO 2 to RuBP in mesophyll; the first detected molecule is 3PG. C 4 plants use the enzyme PEP carboxylase (PEPCase) to fix CO 2 to PEP (phosphoenolpyruvate, a C 3 molecule); the end product is oxaloacetate (a C 4 molecule). In C 4 plants, CO 2 is taken up in mesophyll cells and malate, a reduced form of oxaloacetate, is pumped into the bundle ‑ sheath cells; here CO 2 enters Calvin cycle. In hot, dry climates, net photosynthetic rate of C 4 plants (e.g., corn) is 2–3 times that of C 3 plants. Photorespiration does not occur in C 4 leaves because PEP does not combine with O 2 ; even when stomata are closed, CO 2 is delivered to the Calvin cycle in bundle sheath cells. C 4 plants have advantage over C 3 plants in hot and dry weather because photorespiration does not occur; e.g., bluegrass (C 3 ) dominates lawns in early summer, whereas crabgrass (C 4 ) takes over in the hot midsummer. C 4 Photosynthesis

CAM (crassulacean ‑ acid metabolism) plants form a C 4 molecule at night when stomata can open without loss of water; found in many succulent desert plants including the family Crassulaceae. At night, CAM plants use PEPCase to fix CO 2 by forming C 4 molecule stored in large vacuoles in mesophyll. C 4 formed at night is broken down to CO 2 during the day and enters the Calvin cycle which now has NADPH and ATP available to it from the light ‑ dependent reactions. CAM plants open stomata only at night, allowing CO 2 to enter photosynthesizing tissues; during the day, stomata are closed to conserve water but now CO 2 cannot enter photosynthesizing tissues. Photosynthesis in a CAM plant is minimal, due to limited amount of CO 2 fixed at night; but this does allow CAM plants to live under stressful conditions. CAM Photosynthesis