Photosynthesis

Review  Autotrophs – “self-feeders”  producers  Heterotrophs – “other-feeders”  Consumers  Photosynthesis – how plants convert sunlight to usable energy source

Review Cont’d  Chloroplasts – where all the magic happens  Thylakoids – where the chlorophyll is  Chlorophyll gets the party started

More Review  Two processes  Light reactions – it puts the “photo” in photosynthesis  Light energy to chemical energy through the creation of NADPH & ATP  Calvin Cycle – it puts the “synthesis” in photosynthesis  CO 2 to C 6 H 12 O 6 using energy created by light reactions

Light  Electromagnetic Radiation – travels in waves  Electromagnetic Spectrum – the entire range sorted by wavelength  Visible light – what we can see  380 to 750nm  Most important part of spectrum for photosynthesis  Photons – particles of light  Energy is inversely related to wavelength

Electromagnetic Spectrum

Photosynthetic Pigments  Pigment – substance that absorbs visible light  Each pigment absorbs specific wavelengths of light  Spectrometer – shows what wavelengths pigment absorbs via an absorption spectrum graph

Pigments  Chlorophyll a – main pigment in light reactions (blue green)  Violet-blue, red light work best; green is worst  Chlorophyll b – accessory pigment; absorbs similar pigments (olive green)  Carotenoids – absorb violet and blue- green light (yellow or orange)  photoprotection

Photons – Let’s Get Excited!  Absorbed photons of light, cause electrons to jump to higher energy levels  **remember the energy level increases as you get to shells further from nucleus  Once electron is in excited state it has tendency to drop back down very quickly creating heat and fluorescence

Photosystem  Complexes along the thylakoid membrane made up of a reaction-center complex and several light- harvesting complexes  Reaction-center complex: proteins holding a special pair of chlorophyll a  Chlorophyll a transfers electron to primary electron acceptor  Light-harvesting complex: various pigments bound to proteins  Antenna for reaction-center complex  Energy wave

Photosystem II & I  Photosystem II & I – named in order of discovery  Each has characteristic reaction-center complex  Photosystem II is first (confusing I know)  P680 reaction-center  Photosystem I is second  P700 reaction-center

Linear Electron Flow  The flow of electrons through the photosystems and junk in thylakoid membrane to drive the creation of ATP & NADPH  Many steps!!

Steps of L.E.F. 1. Photon excites pigment in light harvesting complex of PS II… chain of excited electron to P680 chlorophyll a molecules 2. Electron to primary electron acceptor 3. Enzyme helps split water into 2 electrons, 2 H +, and oxygen atom. Electron replace lost electrons of P680. H + released into lumen. O goes to make O 2 4. Each excited electron goes through an electron transport chain made up of an electron carrier (Pq), cytochrome complex, and protein (Pc)

LEF Cont’d 5. Fall to lower energy levels provides energy to make ATP. More H + pumped into lumen contributing to proton gradient 6. Light energy transferred to PS I reaction-center, exciting P700 molecules  primary electron acceptor. P700 fills “electron hole” with electron from electron transport chain 7. Electrons then pass through redox reactions in second electron transport chain. No proton gradient = no ATP  8. 2 electrons to reduce NADP+ to NADPH. NADPH is at higher energy level than water… better for Calvin Cycle.

Cyclic Electron Flow  Only uses second part of light dependent reaction (PS I) and first electron transport chain  Photosynthetic bacteria do this (many don’t have PS II)  Some cyanobacteria do this as well  Goal is the same… produce ATP to drive Calvin cycle

Chemiosmosis  The electron transport chain pumps protons across the membrane as electrons pass through.  This creates hydrogen ion gradient  The excess hydrogen has to go somewhere so it is pumped out of thylakoid space by ATP Synthase which phosphorylates ADP creating ATP.

The Calvin Cycle  Light independent cycle  Uses energy from light dependent and CO 2 to create Glyceraldehyde 3- phosphate (not glucose)  Takes 3 cycles to make one molecule of G3P

Calvin Cycle Phases 1. Carbon Fixation: CO 2 is attached to RuBP. Catalyzed by rubisco. Creates 6 carbon unstable compound that splits into two 3-phosphoglycerate 2. Reduction: phosphate is added than lost creating G3P. Only one of the 6 created is the only gain. (number of carbons) 3. Regeneration: The five other G3P created are regenerated into RuBP

Alternative mechanisms  In hot, arid climates it is leaving the stomata open to take in CO 2 also leads to a loss of water  Plants in these types of environments have different mechanisms for dealing with this issue

Photorespiration  C 3 plants use enzyme rubisco for carbon fixation to produce 3 carbon compound  When they close stomata on hot days to conserve water this causes the plants to produce less sugar because there is less CO 2  Rubisco can use O 2 in the place of CO 2 which is not effective as it creates no ATP and no sugar  This  totally counterproductive process is called photorespiration  Protective value for plant  Evolutionary relic

C 4 Plants  Plants that preface the Calvin cycle with an alternate type of carbon fixation that produces 4 carbon compound  2 types of photosynthetic cells  Mesophyll cells (old news)  Bundle-sheath cells (never heard that before) – cells packed around the veins of the leaf under the mesophyll  No PS II, exhibit cyclic electron flow

C4 Pathway 1. Enzyme only in mesophyll cells called PEP carboxylase. Fixes CO 2 to PEP making oxaloacetate. PEP carboxylase is more effective at working with CO 2 than rubisco, so better when CO 2 concentration is low. 2. Four-carbon products shipped to bundle- sheath cells 3. 4-carbon compound releases CO2 then normal Calvin cycle takes place with rubisco. Pyruvate formed goes to mesophyll and converted to PEP.

CAM Plants  Seen in succulent plants, cacti, pineapples, etc.  Only open stomata at night  Incorporate CO 2 at night into various acids  This carbon fixation is called Crassulacean Acid Metabolism (CAM)  Vacuoles store acids until morning until light reactions can occur then they release CO 2

That’s All! So let’s make like a tree and leaf….