MODULE 7: Photosynthesis Module 7: Photosynthesis

Objectives: review photosynthetic processes: electron transport chain, Calvin Cycle review photosynthetic processes: electron transport chain, Calvin Cycle define absorption and action spectra define absorption and action spectra know differences between C 3, C 4 and CAM plants know differences between C 3, C 4 and CAM plants

takes place in chloroplasts 1.Light dependent reactions – -plants use light energy to generate ATP,NADPH and Oxygen -occurs in thylakoid membranes 2. Light independent reactions (Calvin cycle)— -ATP and NADPH from the light dependent reactions are required for carbon dioxide fixation in the Calvin Cycle. -occurs in chloroplast stroma Photosynthesis

CARBOHYDRATES Carbohydrates are sugars and their derivatives TYPES: 1. MONOSACHARIDES: single sugar. (i.e., triose sugars - three-carbon sugars such as glyceral dehydes and dihydroxyacetone pentose sugars - five carbon sugars such as ribose and ribulose; hexose sugars - six carbon sugars such as glucose, galactose, and fructose). Sugars may be aldehydes or ketones, depending on the location of the carbonyl group ( -C=O ) Figure 10.4 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle

Photosynthetic pigments: -chlorophyll a, chlorophyll b, and carotenoids -absorb light at different wavelengths. absorption spectrum - range of wavelengths of light a pigments can absorb action spectrum (depicted in terms of photosynthetic rate) - shows the effectiveness of different wavelengths at driving photosynthesis.

Figure 10.7 Absorption and action spectra for photosynthesis

Methods Box (page 182): Determining an Absorption Spectrum

Non-cyclic electron flow Primary pathway for electron flow Uses photosystem I and II to generate ATP and reduce NADP + Cyclic electron flow Secondary pathway for electron flow Uses photosystem I to generate ATP—no NADP + is reduced; instead, electrons cycle back to the cytochrome complex Two ways in which energy (ATP, NADPH) is generated for Calvin Cycle

Figure How noncyclic electron flow during the light reactions generates ATP and NADPH

Figure Cyclic electron flow

Calvin cycle - metabolic pathway during which carbon dioxide is converted into organic sugars. This pathway has three main phases: Phase 1: Carbon fixation CO 2 is attached to the 5-carbon sugar, RuBP. RuBP catalyzed by rubisco (RuBP carboxylase). product is a 6-C sugar that immediately splits into 2 molecules of 3-PGA (phosphoglycerate). Calvin Cycle

Phase II: Reduction 3-PGA is phosphorylated, forming 1,3- bisphosphoglycerate, which is then reduced to produce a sugar G3P (glyceraldehyde 3-phosphate) For every 3 CO 2, there are 6 G3P produced One G3P goes on to be converted into other organic compounds other 5 continue on to Phase III. Phase III: Regeneration 5 G3P molecules are rearranged to regenerate 3 RuBP molecules (used in Phase I)

Figure The Calvin cycle

C 4 plants successful in moderate drought conditions because of special leaf anatomy and physiology more efficient fixation of carbon dioxide using two types of photosynthetic cells: mesophyll cells and bundle-sheath cells examples: corn, sugarcane Alternative mechanisms of carbon fixation: C 4 plants

Alternative mechanisms of carbon fixation: CAM plants CAM (crassulacean acid metabolism) plants open their stomata at night to take in CO 2, CO 2 is incorporated into organic acids and stored in the plants vacuoles during the day, the stomata are closed and the light-dependent reactions use the stored CO 2, along with ATP and NADPH to drive the Calvin cycle closing the stomata during the hottest part of the day prevents water loss from the plant. examples: cacti, pineapple

Figure C 4 and CAM photosynthesis compared, corn and pineapple

Today: Part I: Analysis of Absorption Spectra of Photosynthetic Pigments Part I: Analysis of Absorption Spectra of Photosynthetic Pigments Part II: Light Reaction Photosynthesis in Isolated Chloroplasts Part II: Light Reaction Photosynthesis in Isolated Chloroplasts