Plants and Photosynthesis

Photosynthesis Organisms Autotrophs: “Self Feeders” Photo-: Light Chemo-: Oxidize inorganics (Ex: Sulfur, Ammonia), unique to bacteria Heterotrophs: “Other Feeders”

History Jean-Baptiste van Helmont (1600’s) grew willow tree Weighed soil before and after Added only water Tree gained 75 kg No change in mass of soil Concluded: mass in plants comes from water

Site of Photosynthesis Upper Epidermis Mesophyll Cells Lower Epidermis Vein Stoma

Site of Photosynthesis Thylakoids Stroma Granum Inner & Outer Membranes

Photosynthesis Conversion of Light E into Chem E Light E Travels in waves (photons) Wavelength (): crest to crest (measured in nm)  inversely related to frequency Higher frequency = more E Different  = different properties

Wavelength (nanometers) Nature of Light Visible spectrum is ~380–750 nm Gamma Rays X-Rays UV Infrared Micro- waves Radio Waves Visible Light 400 450 500 550 600 650 700 750 Wavelength (nanometers)

Nature of Light Pigments absorb certain  and reflect or transmit others

Nature of Light Spectrophotometers measure amount of Light pigments absorb or reflect

Nature of Light Pigments Absorb and reflect light Specific pigment = specific light Chlorophylls a and b – both absorb blues and reds a is 1 pigment for photosynthesis – focuses solar E onto a pair of e-s

Nature of Light Accessory pigments – funnel the E they collect to a central Chlorophyll A Carotenoids Carotenes – reflect oranges Xanthophylls – reflect yellows Phycocyanins – reflect blues Some accessory pigments provide photoprotection against excess light Carotenoids in human eyes serve same function

Absorption/Action Spectra Visible Light 400 450 500 550 600 650 700 750 20 40 60 80 100 % Light Absorption Collectively Chlorophyll Carotenoids Phycocyanin 400 450 500 550 600 650 700 750 Wavelength (nanometers)

Engelmann’s Experiment Simple experiment in 1883 Compare to action spectrum

Photosynthesis Can be divided into Light-dependent rxn Makes E storing compounds NADPH and ATP to fuel L-i rxn Occurs in thylakoids Light-independent rxn Uses NADPH and ATP to produce glucose, a more stable form of E Occurs in stroma

Photosynthesis

Light-dependent rxn

Light-dependent rxn Light is absorbed in photosystem II, an “antenna complex” of hundreds of pigments that funnel E to a reaction center Rxn Center: central chlorophyll a molecule next to a protein, the 1° e- acceptor

Light-dependent rxn Chemi- osmosis

Photosynthesis

Light-dependent rxn

Light-dependent rxn The e-s from the broken bonds slide down the ETC, slowly losing E The e-s are recharged by sunlight in photosystem I and are passed along more carrier proteins to NADP+, reducing it to NADPH

sun O2 Light-dependent H+ H+ H20 H+

Light-dependent sun sun O2 ADP ATP H+ H+ H20 H+

Light-dependent rxn summary H2O is broken up by sunlight O2 is released as waste e-s flow down ETC, pump H+ ions, and finally make NADPH H+ ions diffuse across thylakoid membrane and help form ATP ATP and NADPH move on to the light-independent rxn

Photosynthesis

L-i rxn – C fixation

L-i rxn – Reduction 12 ATPs phosphorylate the 12 3PGs to form 12 1,3 bisphosphoglycerates A pair of e-s from NADPH reduces each 1,3 bisphosphoglycerate to glyceraldehyde-3-phosphate (G3P) The electrons reduce a carboxyl group to a carbonyl group

L-i rxn – Reduction

L-i rxn – Reduction Two G3Ps can now be removed from the cycle to make glucose or be used for as any other carb the plant cell needs

Light-independent rxn summary Carbon Fixation CO2 needed to begin the process Synthesis of G3P (Glyceraldehyde 3 phosphate) ATP and NADPH are used Regeneration of 5C compound Need more ATP to reset the cylce

Photorespiration Stomata not only allow gas exchange, but transpiration also Hot, dry day – stomata close Problem: CO2 , O2  Rubisco can bind either CO2 OR O2 to RuBP When O2 binds, no useful cellular E is produced

Photorespiration When rubisco adds O2 to RuBP, RuBP splits into a 3-C piece and a 2-C piece The 2-C fragment is exported from the chloroplast and degraded to CO2 by mitochondria and peroxisomes Photorespiration decreases photosynthetic output by siphoning organic material from the Calvin cycle Up to 50% of the C fixed by Calvin cycle can be drained away on a hot, dry day

C4 Plants Mesophyll cells use PEP carboxylase to fix CO2 to phosphoenolpyruvate, forming oxaloacetate (4C) PEP carboxylase has a very high affinity for CO2 and can fix CO2 efficiently when rubisco cannot - on hot, dry days with the stomata closed

C4 Plants Oxaloacetate then dumps the extra CO2 into the Calvin cycle in bundle-sheath cells Rubisco can then work with a high concentration of CO2, thus minimizing photorespiration C4 plants thrive in hot regions with intense sunlight Examples: sugar, corn

C4 Plants

CAM Plants Crassulacean Acid Metabolism CO2 is fixed at night, but NO photosynthesis takes place at night During the day, the light reactions supply ATP and NADPH to the Calvin cycle and CO2 is released from the organic acids

CAM Plants Allows plants to keep their stomata closed during the hot, dry hours of day and open in the cooler hours of night Less water is lost in the process Less photorespiration occurs Ex: succulent plants, cacti, pineapples, and several other plant families

CAM Plants

Both C4 and CAM plants add CO2 into organic intermediates before it enters the Calvin cycle In C4 plants, carbon fixation and the Calvin cycle are spatially separated In CAM plants, carbon fixation and the Calvin cycle are temporally separated Both eventually use the Calvin cycle to incorporate light energy into the production of sugar