Photosynthesis Dark Reactions Quiz

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• Students can participate in hands-on research during the summer break.
• Opportunity to work with professors at the University of Glasgow's Institute of Molecular, Cellular, and Systems Biology and the Plant Science Research Group.
• £1000 bursary and 4-8 weeks research experience available.
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Photosynthesis - Dark Reactions

• What are the 'dark' reactions?
• How is carbon captured and fixed into sugar?
• What is RuBisCO?
• What is photorespiration?
• What is C4 photosynthesis?

How Were Dark Reactions Identified?

• Fast CO2 injection.
• Rapid 'quench' (perchloric acid) to halt reactions.
• Two-dimensional chromatography improves the separation of intermediates.

Carbon-14 Pulse-Chase Analysis of the C3 Cycle

• Two-dimensional chromatography improves separation of intermediates.
• 5-second and 30-second labeling of photosynthetic products with carbon-14 to track the sequence of events.

Photosynthesis Divided into Light and Dark Reactions

• The Calvin-Benson cycle was identified in 1948 and led to a Nobel Prize.
• The cycle shows the sequence of products being labeled with carbon-14, indicating the sequence of conversions.
• Key enzyme is RuBisCO (Ribulose-bisphosphate carboxylase/oxygenase).

Light and Dark Reactions

• Gaseous CO2 and the 5-carbon sugar (ribulose 1,5-bisphosphate) combine to form two molecules of 3-phosphoglycerate (PGA).
• The reaction is not reversible.
• RuBisCO accounts for ~50% of plant leaf soluble protein.
• RuBisCO has 8 large subunits (nuclear-encoded) and 8 small subunits (chloroplast-encoded).

Chlorophyll Capturing Light

• RuBisCO also catalyzes the competing oxygenation reaction using O2 instead of CO2.
• The oxygenation reaction causes photorespiration which recycles and consumes ATP and NADH, releasing glycine, serine, and CO2.

Alternative Photosynthetic Strategies

• Cyanobacteria contain Carboxysomes to concentrate CO2 around RuBisCO.
• Some algae concentrate CO2 around RuBisCO through pyrenoids.
• Some angiosperms employ C4 or CAM photosynthesis to overcome limitations of RuBisCO.

CAM Photosynthesis

• CAM photosynthesis separates C4 and C3 fixation through time (night and day).
• CAM plants store C4 acids in the vacuole for decarboxylation and refixation by RuBisCO during the day.
• CAM uses an "inverted" stomatal cycle to conserve water and concentrate CO2 in the leaf.

CO2/O2 Limits C3 Cycle Efficiency

• In C3 plants, photorespiration accounts for ~30-50% of RuBisCO turnover.

Is CAM Better for Engineering?

• CAM requires engineering of 35-40 gene targets with their promoters.
• Dark reactions utilize NADPH and ATP to fix CO2 with RuBisCO.
• C3 process is limited by CO2 versus O2 selectivity.
• C4 and CAM overcome this limitation by introducing a 'pre-fixation step' before using RuBisCO to capture CO2 as a four-carbon acid.

Test your knowledge on the dark reactions of photosynthesis, including key concepts like carbon fixation, RuBisCO, and photorespiration. This quiz covers the mechanisms that underpin how plants convert light energy into chemical energy. Perfect for students studying natural sciences or plant biology.