Overview of Photosynthesis

Overview of Photosynthesis • Light energy is converted to chemical energy and carbon is fixed into organic compounds. • Photosynthesis uses the energy of sunlight to convert water and CO2 into O2 and high energy sugars • 6 CO2 + 6 H2O + light → C6H12O6 + 6 O2 • carbon dioxide + water + light → sugar + oxygen • Plants then use the sugars to produce complex carbohydrates such as starches: • Plants obtain carbon dioxide from the air or water in which they grow.

Inside a Chloroplast

Photosynthetic Pigments • Photosynthetic pigments absorb light energy and use it to provide energy to carry out photosynthesis. • Chlorophylls (absorb light in the red, blue, and violet range): • Chlorophyll a - directly involved in transformation of photons to chemical energy • Chlorophyll b - helps trap other wavelengths and transfers it to chlorophyll a • Carotenoids (absorb light in the blue, green, and violet range): • xanthophyll - Yellow • beta carotene - Orange • Phycobilins – Red • Chlorophyll b, the carotenoids, and the phycobilins are known as ANTENNA PIGMENTS – they capture light in other wavelengths and pass the energy along to chlorphylla. • Chlorophyll a is the pigment that participates directly in the light reactions of photosynthesis!

During photosynthesis, chlorophylls absorb free energy from light, boosting electrons to a higher energy level in photosystems I and II.

Figure 10.9 Location and structure of chlorophyll molecules in plants The pigment molecules have a large head section that is exposed to light in the surface of the membrane; the hydrocarbon tail anchors the pigment molecules into the lipid bilayer.

Stages of Photosynthesis This reaction can be broken into 2 stages: Light Dependent Reactions Within the thylakoid membranes inside a chloroplast “PHOTO” phase – make ATP & NADPH…USE LIGHT ENERGY TO PRODUCE ATP & NADPH Light Independent Reactions (Calvin Cycle) Take place in the stroma of the chloroplast “SYNTHESIS” phase – converts CO2 to SUGAR BOTH REQUIRE LIGHT (SOMEWHAT): Even the dark reactions in most plants occurs during daylight because that is the only time the light reactions can operate AND the dark reactions depend on the light reactions!!!

Light Reactions: -carried out by molecules in thylakoid membranes -convert light E to chemical E of ATP and NADPH -split H2O and release O2 to the atmosphere Calvin Cycle Reactions: -take place in stroma -use ATP and NADPH to convert CO2 into the sugar G3P -return ADP, inorganic phosphate, and NADP+ to the light reactions

Light Dependent Reactions - Overview • require presence of light • occur in thylakoids of chloroplasts • use energy from light to produce ATP and NADPH (a temporary, mobile energy source that helps store even more energy) • water is split during the process to replace electrons lost from excited chlorophyll • oxygen gas is produced as a by-product

The Light Reactions • Light is absorbed by PS II and PS I in the thylakoid membranes and electrons flow through TWO electron transport chains. • There are 2 possible routes for electron flow: • Noncylic photophosphorylation • Cyclic photophosphorylation • Photophosphorylation is a method of generating ATP by using light to add P to ADP

Photosystems Photosystems I and II are embedded in the internal membranes of chloroplasts (thylakoids) and are connected by the transfer of higher free energy electrons through and electron transport chain (ETC).

PS I and PS II • Named in the order they were discovered – however, PS II occurs first, followed by PS I. • PS I absorbs light best in the 700nm range (so called P700). • PS II absorbs light best in the 680nm range (so called P680).

Cyclic vs. Noncyclic Electron Flowhttp://highered.mcgraw-hill.com/olc/dl/120072/bio12.swf Noncyclic Electron Flow uses Photosystem II, and ETC (with the electron carriers B6f comlex and Pq) , Photosystem I, and another ETC using an iron-containing protein called ferredoxin& NADP reductase produces ATP and NADPH Cyclic Electron Flow uses only Photosystem II and the first ETC – no production of NADPH and no release of Oxygen DOES produce ATP to be used to make up the difference needed due to Calvin cycle demands.

Figure 10.11 How a photosystem harvests light Chlorophyll a

Figure 10.12 How noncyclic electron flow during the light reactions generates ATP and NADPH (Layer 1)

Figure 8-10 Light-Dependent Reactions Section 8-3 Go to Section:

Figure 10.15 Comparison of chemiosmosis in mitochondria and chloroplastshttp://bcs.whfreeman.com/thelifewire/content/chp08/0802002.html

Light Independent Reactions - Overview • Do not require light directly (Dark Reactions or the Calvin Cycle) • Stroma of chloroplasts • ANABOLIC process – and therefore requires ENERGY ATP and NADPH (light dependent reactions) • Divided into 3 phases: • Phase 1: Carbon Fixation • Phase 2: Reduction • Phase 3: Regeneration of CO2 Acceptor (RuBP)

Figure 10.17 The Calvin cycle (Layer 1) • Phase 1: Carbon Fixation • CO2 is incorporated and attached to RuBP (catalyzed by enzyme rubisco). • Product of reaction is 6-carbon intermediate so unstable that it splits in half to form two molecules of 3-phosphoglycerate.

Phase 2: Reduction • Each molecule of 3-phosphoglycerate • receives additional • phosphate group from • ATP to become 1,3 bisphosphoglycerate. • A pair of electrons donated from NADPH reduces 1,3 bisphosphoglycerate into Glyceraldehide-3-phosphate (a sugar). • One of the G3P molecules is exported and used to build glucose.

Figure 10.17 The Calvin cycle (Layer 3) • Phase 3 • The carbon skeletons of 5 molecules of G3P are rearranged by the last steps of the Calvin cycle into three molecules of RuBP. • The RuBP is now prepared again to receive CO2…and the cycle continues. • The regeneration phase requires ATP.

Conserved Core Processes • Photosynthesis first evolved in prokaryotic organisms; • Scientific evidence supports that prokaryotic (bacterial) photosynthesis was responsible for the production of an oxygenated atmosphere; • Prokaryotic photosynthetic pathways were the foundation of eukaryotic photosynthesis.

Overview of Photosynthesis