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

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

4. 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+

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

6. 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

7. 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

8. Cyclic Photophosphorylation

9. 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

10. 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

11. 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

12. 12

13. 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

14. 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

15. 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.

17. 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

19. 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

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

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

23. 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.

24. 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

25. 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.

26. 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.

27. 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

28. 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

29. 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

30. 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