chapt 7-photosynthesis

What were the key experiments that led us to understand that plants fix atmospheric carbon and that the oxygen they respire is derived from water?

• Joseph Priestly’s experiment:

  • Plants can “restore” air that has been “injured” by a burning candle or a small animal

  • Mint plant placed in sealed container with candle and observed

  • Demonstrate that plants release oxygen, although source of oxygen was not understood yet

• Jan Ingenhousz’s experiment:

  • Expanded on priestly’s work

  • Plants produce oxygen only in presence of sunlight and green parts of plant

• Robert Hill Experiment:

  • Showed that isolated chloroplasts could produce oxygen when exposed to light in presence of suitable electron acceptor

  • Confirmed water is source of oxygen in photosynthesis

• Ruben and Kamen:

  • Radiolabeled oxygen in water and showed that O2 released during photosynthesis is also radiolabeled

Why might photosynthesis have evolved to take advantage of such a narrow band of the electromagnetic spectrum?

• Absorption spectrum: of a pigment, how much is absorbed at different wavelengths of light (what wavelengths its electrons are excited by)

• Action spectrum: how much biological activity for a process there is at different wavelengths

  • energy efficiency (right amount of energy to excite electrons)

  • emits most of its energy in visible spectrum

  • photosynthetic pigments finely absorb visible light efficiently (maximize energy capture and minimize waste)

What are the contributions of the different pigments that are involved in photosynthesis?

chlorophyll a: primary pigment in photosynthesis, absorb light most efficiently in red and blue wavelengths, directly participates in light reactions [light→chemical]

chlorophyll b: accessory pigment, captures blue and red-orange spectrum

carotenoids: include carotenes and xanthophylls, absorb blue and green, meant to pass absorbed light energy to chlorophyll a and protect plant by dissipating excess light energy

How do photosystems I and II absorb light energy and transform it into chemical bonds? 

• Light dependent reactions

  • inputs: light, water, NADP+, ADP+

  • Outputs: O2, NADPH, ATP

photosystem II (PSII):

• PSII absorbs light that excites electrons at reaction center P680

• splits water molecules, release oxygen, protons, and electrons

• electrons passed through ETC, pump proton into lumen and creates the prroton gradient

photosystem I (PSI)

• absorb light exciting P700

• electrons lost are replenished by those from the PSII by ETC

• exctied electrons from PSI used for NADPH formation

What are the roles of Rubisco (in C3 plants) and PEP (in C4 and CAM plants) in the dark reactions, where carbon fixation occurs?  What are the pros and cons of separating the light and dark reactions spatially temporally?

• “Dark reactions” = calvin or calvin-benson cycle

• (C3 plants) Rubisco is the enzyme that catalyzes the linkage of CO2 to 5C RuBP to form a 6C sugar

• fixes CO2 in calvin cycle

• PEP carboxylasae fixes CO2 into oxaloacetate in C4 and CAM

• high affinity for CO2 and does not bind O2, minimizing photorespiration

• process occurs in mesophyll cells in C4 plants and fixed CO2 is transported to bundle sheath cells for calvin cycle

• CO2 fixation occurs at night for CAM plants, storing as malate for use during the day

C4—

pros: reduce photorespiration by concentrating CO2 in bundle sheath cells and improves efficiency in hot/cold env

cons: require specialized leaf anatomy (Kranz) and additional energy for CO2 transport

CAM—

pros: minimize water loss by fixing CO2 at night when stomata are open, ideal for arid conditions

cons: limit rate of photosynthesis due to storage capacity of malate

kranz anatomy

specialized arrangement found in C4 plants where mesophyll cells form a ring around bundle-sheath cells

What are the inputs and outputs of the light and and dark reactions of photosynthesis, and how are they coupled?

• Light reactions

  • Input: light, ADP, NADP, and H2O

  • Outputs: ATP and NADPH

• Dark reactions (calvin cycle)

  • Inputs: CO2, ATP, NADPH

  • Outputs: the 6-carbon sugar PGAL, NADH, ADP, H2O 

• Photorespiration:

  • When rubisco catalyzes the fixation of O2 rather than CO2 to RuBP

  • The glycolate is toxic and must be processed, with a net loss of energy

  • The result is that some CO2 is lost and neither net ATP nor NADPH are generated

Pros and cons of separating (or not) the light and dark reactions temporally or spatially?

C3- temperate, moist

• Rice and tomato

C4 (spatial)- tropics/semitropics, high light, high temp, occasional droughts

• Corn, sugarcane

CAM (temporal): deserts and other xeric habitats

• Many cacti and other succulents