Photosynthesis: Light Independent Reactions and Limiting Factors IB HL Biology 1 Modified from S. Frander, J. Naftzinger and G. von Bargen Animations McGraw Hill

Photosynthesis Basics Photosynthesis uses CO2, water, and light to produce glucose and O2.

Photosynthesis takes place in the chloroplasts double membrane-bound chloroplasts have stroma and grana, and chloroplasts are permeable to CO2, O2, ATP, sugars and other photosynthetic products Stroma: contains enzymes for the Calvin Cycle Site for fixation, reduction and regeneration Thylakoids: Large surface area for light absorption Small inner space for H+

Overview Photosynthesis occurs in a two-pathway process. Light Dependent Reactions Light Independent Reactions

At the end of the light-dependent reactions… O2 is released NADP is reduced to NADPH ATP is generated from ADP NADPH and ATP are used in the light independent reaction

Light Reactions: Light energy is used for Photolysis (splitting water molecules). The energy released is used to produce ATP and NADPH. Photoactivation of Photosystem2 Electron transport Reduction of NADP+ Photophosphorylation Noncyclic and cyclic

Cyclic Photophosphorylation

Noncyclic Electron Flow Light strikes pigments on photosystem, excites electrons there Electrons jump to higher energy level Water is split, releasing O2 gas Electrons are passed down ETC by electron carriers in thylakoid membrane Generate ATP through ATP synthase pump Second photosystem generates NADPH

Overview Light-independent reactions Enzymes in the stroma synthesize glucose and other organic molecules from CO2 using the chemical energy stored in ATP and NADPH

Light Independent Reactions aka Calvin Cycle Why: to make triose phosphate sugars using CO2 and H+ ions gathered in light-dependent phase Triose phosphate is a 3C compound derived from PGA and serves as an intermediate to formation of other organic molecules (i.e., sugars, lipids, amino acids Where: stroma of chloroplast Happens in 3 phases: Carbon fixation Reduction of carbon dioxide Regeneration of building block molecule 11

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Carbon Fixation - CO2 is covalently linked or fixed to a 5 carbon skeleton (RuBP) CO2 enters plants from air, goes into stroma CO2 attaches to ribulose bisphosphate (RuBP), product of which is immediately split into two molecules of phosphoglycerate (PGA). Reaction is catalysized by an enzyme called rubisco. Initial 6C molecule is unstable, breaks apart into 2-3C PGAL molecules

Q: what is the source of these ATP? Q: what the source of the CO2 ? Q: what is the source of these ATP? A: The atmosphere A: generated from the light reaction

2. Reduction - carbohydrate is formed at the expense of ATP and NADPH the PGA is energized and reduced by ATP and NADPH from the light reactions to make glyceraldehyde-3-phosphate (G3P). G3P siphoned off from this part of the Calvin cycle represents the carbohydrate product of photosynthesis. G3P is a three-carbon sugar-phosphate that can be used to make a range of carbohydrates by other pathways.

3. Regeneration - the CO2 acceptor RuBP reforms at the expense of ATP The remaining G3P is converted into more ribulose-1,5-bisphosphate (RuBP) so that the Calvin cycle can continue to go around again Requires an ATP and more than one G3P to give the total of five carbons found in RuBP RuBP is regenerated by phosphorylation and enzymatic rearrangement of triose sugar G3P

Light-Independent Reactions NADPH and ATP from light-dependent reactions used to power glucose synthesis Light not directly necessary for light-independent reactions if ATP & NADPH available Light-independent reactions called the Calvin-Benson Cycle or C3 Cycle

Limiting factors in photosynthesis A limiting factor is something that controls the rate of a process

Factors that limit photosynthesis: Amount of light As light intensity increases, the rate of the light- dependent reaction, and therefore photosynthesis generally, increases proportionately.

Factors that limit photosynthesis: Wavelength of light PSI absorbs energy most efficiently at 700 nm and PSII at 680 nm. Light with a high proportion of energy concentrated in these wavelengths will produce a high rate of photosynthesis.

Factors that limit photosynthesis: Amount of water Plants shut stomata to avoid loosing water However, shutting the stomata will also deprive the plant of CO²

Factors that limit photosynthesis: Amount of CO2 An increase in the carbon dioxide concentration increases the rate at which carbon is incorporated into carbohydrate in the light- independent reaction and so the rate of photosynthesis generally increases until limited by another factor.

Factors that limit photosynthesis: Temperature At low temperatures the enzymes responsible for photosynthesis have very little energy so the rate of photosynthesis is very slow. As the temperature increases, the enzymes get more energy so the rate of photosynthesis increases. If it gets too hot the enzymes begin to lose their shape (denature). They are unable to function properly and the rate of photosynthesis decreases again. At higher temperatures the stomata close to prevent water loss, this also stops gas exchange which slows photosynthesis even further.

How can photosynthetic activity be measured? Think about the products of the process… Photosynthesis involves the production of oxygen, the uptake of carbon dioxide, and an increase in biomass Any of these can be used as a measure of the rate of photosynthesis

Production of Oxygen Rate of photosynthesis measured by oxygen production Aquatic plants release bubbles of oxygen when they carry out photosynthesis. If these bubbles are collected, their volume can be measured

Uptake of Carbon Dioxide Rate of photosynthesis measured by carbon dioxide uptake Leaves take in CO2 from the air or water around them, but this is difficult to measure directly. If CO2 is absorbed from water, the pH of the water rises. This can be monitored with pH indicators or with pH meters

Increase in Biomass Rate of photosynthesis measured by biomass If batches of plants are harvested at a series of times and the biomass of the batches is determined, the rate of increase in biomass gives an indirect measure of the rate of photosynthesis in the plants