Photosynthesis Light Independent Reactions and Limiting Factors IB
Photosynthesis: Light Independent Reactions and Limiting Factors IB HL Biology 1 Modified from S. Frander, J. Naftzinger and G. von Bargen Animations Mc. Graw Hill
Photosynthesis Basics Photosynthesis uses CO 2, water, and light to produce glucose and O 2.
Photosynthesis takes place in the chloroplasts double membrane-bound chloroplasts have stroma and grana, and chloroplasts are permeable to CO 2, 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 lightdependent reactions… O 2 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 Photosystem 2 Electron transport Photoactivation of Photosystem 2 Reduction of NADP+ Photophosphorylati on 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 O 2 gas �Electrons are passed down ETC by electron carriers in thylakoid membrane �Generate ATP through ATP synthase pump �Second photosystem generates NADPH
Overview 2. Light-independent reactions • Enzymes in the stroma synthesize glucose and other organic molecules from CO 2 using the chemical energy stored in ATP and NADPH
Light Independent Reactions aka Calvin Cycle � Why: to make triose phosphate sugars using CO 2 and H+ ions gathered in light-dependent phase • Triose phosphate is a 3 C 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
1. Carbon Fixation - CO 2 is covalently linked or fixed to a 5 carbon skeleton (Ru. BP) • CO 2 enters plants from air, goes into stroma • CO 2 attaches to ribulose bisphosphate (Ru. BP), product of which is immediately split into two molecules of phosphoglycerate (PGA). �Reaction is catalysized by an enzyme called rubisco. • Initial 6 C molecule is unstable, breaks apart into 2 -3 C PGAL molecules
Q: what is the source of these ATP? A: generated from the light reaction Q: what the source of the CO 2 ? A: The atmosphere
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 (G 3 P). • G 3 P siphoned off from this part of the Calvin cycle represents the carbohydrate product of photosynthesis. • G 3 P is a three-carbon sugar-phosphate that can be used to make a range of carbohydrates by other pathways.
3. Regeneration - the CO 2 acceptor Ru. BP reforms at the expense of ATP • The remaining G 3 P is converted into more ribulose-1, 5 -bisphosphate (Ru. BP) so that the Calvin cycle can continue to go around again • Requires an ATP and more than one G 3 P to give the total of five carbons found in Ru. BP • Ru. BP is regenerated by phosphorylation and enzymatic rearrangement of triose sugar G 3 P
Light-Independent Reactions �NADPH and ATP from light-dependent reactions used to power glucose synthesis �Light not directly necessary for lightindependent reactions if ATP & NADPH available �Light-independent reactions called the Calvin-Benson Cycle or C 3 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 CO 2 • An increase in the carbon dioxide concentration increases the rate at which carbon is incorporated into carbohydrate in the lightindependent 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 � 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 � Leaves take in CO 2 from the air or water around them, but this is difficult to measure directly. If CO 2 is absorbed from water, the p. H of the water rises. This can be monitored with p. H indicators or with p. H meters
Increase in 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
- Slides: 30