Photosynthesis: Light and Carbon Fixation PDF

Photosynthesis Photosynthesis reactions are divided into two major processes: 1. Energy – transduction (or light reactions) - light dependent reactions 2. Carbon fixation reactions- light independent reactions Light reactions  Conversion of light energy to chemical energy.  P...

Photosynthesis Photosynthesis reactions are divided into two major processes: 1. Energy – transduction (or light reactions) - light dependent reactions 2. Carbon fixation reactions- light independent reactions Light reactions  Conversion of light energy to chemical energy.  Photons of light strike photosystem I (PI) and photosystem II (PII) simultaneously.  Energized electrons pass from chlorophyll reaction centre of PII to a series of proteins in the electron transport chain (b6-f complex). 1  At PSII  Electrons lost by PII are replaced by a process called photolysis - Oxidation of water molecule to produce oxygen and free molecules of Hydrogen ions.  Light energy is used to form ATP from ADP and inorganic phosphate -ADP + Pi + light chloroplasts ATP + H2O i.e., phosphorylates ADP to ATP  Light energy is also used to reduce the electron carrier molecule NADP+.  Reduced NADP+ is converted to NADPH which provides energy for biosynthetic pathways - Photosystems I (absorbs long wavelength light < 700nm 2 - Photosystem II absorbs shorter wavelength light i. e. < 680nm -) - PSI and PSII use light energy to oxidize H2O. AND THIS - Occurs by splitting the H2O into Oxygen gas and Hydrogen atoms. - This produces two electrons - The two available electrons are accepted by NADP+ and H+ to form NADPH: H2O 2H+ + ½ O2 2e- H++ NADP+ NADPH 3 Carbon – Fixation Reactions  The ATP and NADPH generated by the light rxs are used to fix & reduce carbon to synthesize simple sugars.  The carbon is available in the form of CO2. Carbon dioxide fixation via the three – carbon pathway (C3 pathway)  Occurs in most plants (i. e. most common)  Occurs in the stroma of chloroplast via a series of rxs known as the Calvin cycle.  The starting & ending compound is a five – carbon sugar with two phosphate groups 4  called Ribulose 1,5 – bisphosphate (RuBP).  The cycle occurs in three stages: -Carboxylation, reduction & regeneration. 5 6  Carboxylation (1st stage)  CO2 enters the cycle and is enzymatically bonded covalently with RuBP. - The involved enzyme is Rubisco (ribulose - bisphosphate carboxylase-oxygenase).  Produces an unstable 6 - carbon compound (2-carboxy-3-ketoarabinitol-1, 5 bisphosphate) – (no need to that is immediately memorise this name) hydrolysed to give 2 molecules of 3- phosphoglycerate/3-phosphoglyceric acid (PGA).  Hence the name the C3 pathway. 7 Reduction (2nd stage) ATP and NADPH work in two stages, to reduce 3 – phosphoglycerate to Glyceraldeyde - 3 – phosphate (PGAL) or 3-phosphoglyceraldehyde (GA3P) (also known as triose phosphate or 3-phosphoglyceraldehyde; abbreviated as G3P, GA3P, GADP, GAP, TP, GALP or PGAL) Regeneration (3rd stage)  The final step is a complex series of reactions, which regenerates the 5-Carbon Acceptor (Ribulose-1-5-bisphosphate [RUBP]). The C4 photosynthetic cycle  Some plants have Carboxylating enzymes that have a higher affinity for CO2, compared to RUBISCO, especially at low CO2 concentrations & high temperatures. 8  These plants use 3-Carbon acceptors like Phosphoenolpyruvate (PEP) rather than the 5-Carbon acceptor RUBP.  Their Carboxylation produces a 4-Carbon Acid like Malic Acid (Malate).  Hence the name C4 photosynthesis.  The Vascular Bundles of C4 leaves are surrounded by large Photosynthetic Bundle Sheath Cells.  This is called Kranz (wreath) Anatomy. 9   This is very distinctive and occurs in Monocots like Corn & Sugarcane plus some Dicots. The Stomata are precisely arranged to provide optimal diffusion paths for the Photosynthetic Mesophyll Cells. This allows for extremely efficient gas exchange with the atmosphere and the Mesophyll when the stomata are open. 10  CO2 crosses the Cell Wall and Plasmalemma of a Mesophyll Cell. 11   It is fixed by PEP Carboxylase to form a 4- Carbon Acid like Malate.  This occurs in the Cytoplasm.  The 4-Carbon Acid is transported via Plasmodesmata to a Bundle Sheath Cell.  It looses one CO2 (Decarboxylation) and enters a Bundle Sheath Chloroplast where CO2 is fixed by RUBISCO using the C3 Calvin Cycle.  PEP is transported into the Mesophyll Cell where it can accept another CO2. Reasons Why C4 is more efficient than C3 photosynthesis  PEP Carboxylase has a much higher affinity for CO2 than RUBISCO.  PEP Carboxylase does NOT have Oxygenase activity. 12   The CO2 concentration in the Bundle Sheath Chloroplasts greatly favors Carboxylation by RUBISCO & virtually eliminates Photorespiration. CAM photosynthesis  Crassulacean Acid Metabolism (CAM) is another ecologically significant way in which plants concentrate CO2.  This does not involve the sophisticated structural specialization seen with C4 plants.  Stomatal opening is regulated temporally so that they may Open at Night when water demand is low, thus avoiding water loss.  CAM plants can show an 80% reduction in water loss compared to C3 plants under the same conditions.  This has obvious adaptive and ecological significance! 13   CAM plants open their stomata at night and fix CO2 via PEP Carboxylase.  They Store Malate (4 Carbon Acid) in their Vacuoles.  Malate is transported to Chloroplasts during the Day where Decarboxylation occurs.  The released CO2 is Fixed by RUBISCO as in C3 plants.  CAM Mesophyll Cells typically have extremely Large Storage Vacuoles and CAM Plants are Often Succulent in appearance.  e. g. the Crassulaceae Agavaceae Aizoaceae Cactaceae Euphobiaceae 14 Orchidaceae  Since the Stomata are Closed during the Day, CO2 & Water can’t escape.  Virtually all of the CO2 is fixed and Photorespiration is negligible. The process 15 16 17  Carbondioxide is fixed at night by adding it to phosphoenol -pyruvate (PEP).  PEP converts to OAA (Oxaloacetate) 18