Photosynthesis Chapter 10

Autotroph vs. Heterotroph Autotrophs “self-feeders” Produce organic materials from CO2 and other inorganic materials Also known as producers Heterotrophs Cannot produce their own food Eat other organisms Also known as consumers

Chloroplasts & Gas Exchange Site of photosynthesis Pigment = chlorophyll Located in the mesophyll cells of plant leaves Has a double membrane Contains thylakoids, grana, and stroma Gases are exchanged through the stomata of leaves

6 CO2 + 6 H2O + light energy  C6H12O6 + 6 O2 Photosynthesis Purpose – to convert light energy into chemical energy stored in sugars or other organic compounds Anabolic pathway Occurs in the chloroplasts of plant cells 6 CO2 + 6 H2O + light energy  C6H12O6 + 6 O2

Oxidizing Agent in Photosynthesis Coenzyme NADP+ Accepts electrons and hydrogen ions from water Transports them to the Calvin cycle Reduced to NADPH by accepting 2 electrons and 1 hydrogen proton

Stages of Photosynthesis Light Reactions “photo” part of photosynthesis Uses solar power to generate ATP and NADPH Occurs on the thylakoid membrane Calvin Cycle “synthesis” part of photosynthesis Produces organic sugars (G3P) through carbon fixation Occurs in the stroma

Photosystems Overview Photosystems are located in the thylakoid membranes of chloroplasts Contain all of the pigment molecules that harvest light energy Contain a reaction-center complex with a pair of chlorophyll a molecules

1. Light Reactions Requires light energy (photons) Photons excite electrons on pigments 3 types of pigments: chlorophyll a and b and carotenoids Uses 2 photosystems: Photosystem II (PS II) and Photosystem I (PS I) PS II and PS I are connected by an electron transport chain which generates ATP through chemiosmosis (this whole process is called photophosphorylation) Electrons are ultimately passed on to NADP+

4 7 2 8 3 5 1 6

A Closer Look At ATP Production

2. Calvin Cycle 3 molecules of CO2 are “fixed” to produce 1 net G3P So by using 6 CO2 molecules you are producing 2 G3P molecules Déjà vu moment (aka: blow your mind moment) G3P molecules are the first molecules formed during glycolysis when glucose splits in half (each G3P is then further processed to get the pyruvate molecules) Therefore, by uniting two G3P molecules, you get GLUCOSE! THE ULTIMATE PRODUCT OF PHOTOSYNTHESIS

2. Calvin Cycle Overview of Phases: Carbon Fixation 3 CO2 enter the cycle and each is attached to a RuBP (a 5-C sugar molecule) by an enzyme called rubisco Each “fixed” RuBP immediately splits in half creating 6 new molecules Reduction ATP adds a phosphate to each of the 6 molecule NADPH adds electrons to reduce each of the 6 molecules Produces 6 G3P molecules of which 1 is released for use Regeneration ATP is used to rearrange the 5 remaining G3P molecules back into the 3 starting RuBP molecules Requires 9 ATP and 6 NADPH to make one usable G3P

This cycle happens twice to utilize all 6 CO2 molecules in the photosynthesis equation b c f d e

Carbon Fixation Problem Photorespiration On hot/dry days the stomata close which decreases the amount of CO2 intake and increases the amount of O2 retained in the leaves This may cause O2 to be added to the Calvin cycle instead of CO2 (RuBP will accept it) Doesn’t produce G3P and consumes ATP The final product (a 2-Carbon sugar) is rearranged and broken down by peroxisomes and mitochondria releasing CO2 (respiration) Would be fine if it also produced ATP, but it doesn’t Normal for C3 plants such as rice, wheat, soybeans

Alternative Carbon Fixation Methods (to reduce photorespiration) C4 Photosynthesis C4 Plants such as corn and sugarcane Contain 2 types of photosynthetic cells: bundle sheath cells and mesophyll cells Use an enzyme called PEP carboxylase to add CO2 to PEP, producing a 4-C sugar in the mesophyll cells PEP carboxylase can not bind to O2 4-C sugar is sent to bundle-sheath cells where it releases CO2 for the Calvin cycle Minimizes photorespiration and enhances sugar production

Alternative Carbon Fixation Methods (to reduce photorespiration) CAM Succulent plants such as cacti and pineapple Live in dry/arid areas and open their stomata at night instead of during the day Crassulacean acid metabolism – take the CO2 and carbon fix it into a variety of organic acids at night During the day when the light reactions are occurring, the organic acids release CO2 for the Calvin cycle