MODULE 7:

Photosynthesis

Module 7: Photosynthesis

Objectives:Objectives:

• review photosynthetic processes: review photosynthetic processes: electron transport chain, Calvin electron transport chain, Calvin CycleCycle

• define absorption and action define absorption and action spectraspectra

• know differences between Cknow differences between C33, C, C44 and CAM plantsand 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

PhotosynthesisPhotosynthesis

CARBOHYDRATESCARBOHYDRATES

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 Figure 10.4  An overview of photosynthesis: cooperation of the light reactions and the Calvin cycleCalvin 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 photosynthesisFigure 10.7  Absorption and action spectra for photosynthesis

Methods Box (page 182): Determining an Absorption SpectrumMethods 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, Two ways in which energy (ATP, NADPH) is generated for Calvin NADPH) is generated for Calvin

CycleCycle

Figure 10.11  How noncyclic electron flow during the light reactions generates ATP Figure 10.11  How noncyclic electron flow during the light reactions generates ATP and NADPHand NADPH

Figure 10.13  Cyclic electron flowFigure 10.13  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

•CO2 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 CycleCalvin Cycle

Calvin CycleCalvin 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 CO2, 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 10.16  The Calvin cycleFigure 10.16  The Calvin cycle

•C4 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: C4 plants

Alternative mechanisms of carbon Alternative mechanisms of carbon fixation: CAM plantsfixation: CAM plants

• CAM (crassulacean acid metabolism)

• plants open their stomata at night to take in CO2,

• CO2 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 CO2, 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 10.18  CFigure 10.18  C44 and CAM photosynthesis compared, corn and pineapple and CAM photosynthesis compared, corn and pineapple

Today:Today:

• Part IPart I: Analysis of Absorption : Analysis of Absorption Spectra of Photosynthetic Spectra of Photosynthetic PigmentsPigments

• Part IIPart II: Light Reaction : Light Reaction Photosynthesis in Isolated Photosynthesis in Isolated ChloroplastsChloroplasts