1. Photosynthesis Part II:
The Calvin Cycle, Environmental Conditions, & Preventing Photorespiration

2. Photosynthesis: An Overview
The net overall equation for photosynthesis is:
Photosynthesis occurs in 2 “stages”:
The Light Reactions (or Light-Dependent Reactions)
The Calvin Cycle (or Calvin-Benson Cycle or Dark Reactions or Light-Independent Reactions)
6 CO2 + 6 H2O C6H12O6 + 6 O2
light
Photosynthesis is an endergonic reaction because it requires an input of energy to occur; that energy comes in the form of light.
This is a review from the previous PowerPoint, to activate students’ prior knowledge.
Is photosynthesis an ENDERGONIC or EXERGONIC reaction?

3. Photosynthesis: An Overview
To follow the energy in photosynthesis,
light
Light
Reactions
Calvin
Cycle
Organic compounds (carbs)
ATP
NADPH
light
thylakoids
stroma
It cannot be overemphasized that energy is not MADE, but is instead transformed through the processes of photosynthesis and cellular respiration. Each of the “boxes” (light/ATP & NADPH/organic compounds) contains energy, but in a different form. Photosynthesis is a process that converts energy from an “un-usable form” (light) into a “usable form” (organic compounds), and requires an intermediate step (ATP/NADPH).
This is a review from the previous PowerPoint, to activate students’ prior knowledge.

4. Phase 2: The Calvin Cycle
In the Calvin Cycle, chemical energy (from the light reactions) and CO2 (from the atmosphere) are used to produce organic compounds (like glucose).
The Calvin Cycle occurs in the stroma of chloroplasts.

5. Phase 2: The Calvin Cycle
The Calvin Cycle involves the process of carbon fixation.
This is the process of assimilating carbon from a non-organic compound (ie. CO2) and incorporating it into an organic compound (ie. carbohydrates).
CARBON FIXATION

6. Phase 2: The Calvin Cycle
Step 1: Carbon Fixation
3 molecules of CO2 (from the atmosphere) are joined to 3 molecules of RuBP (a 5-carbon sugar) by Rubisco (an enzyme also known as RuBP carboxylase)
This forms 3 molecules which each have 6 carbons (for a total of 18 carbons!) C C
- Only the carbons are shown in this diagram for clarity, though oxygen and hydrogen are also present. Also, the carbon atoms shown in red and those shown in black are identical, but are color-coded to show where they come from (red are CO2 from the atmosphere, black are the carbons in RuBP).
Rubisco C C C C
3 carbon dioxide molecules
3 RuBP molecules

7. Phase 2: The Calvin Cycle
Step 2: Reduction
The three 6-carbon molecules (very unstable) split in half, forming six 3-carbon molecules.
These molecules are then reduced by gaining electrons from NADPH.
ATP is required for this molecular rearranging
Where did the NADPH and ATP come from to do this?
ATP
- Emphasize to students that the NADPH and ATP required to perform these reactions as part of the Calvin Cycle were produced during the light reactions.
ADP P C C C C C C C C
NADPH C
NADP+

8. Phase 2: The Calvin Cycle
Where did these 3 extra carbons come from?
There are now six 3-carbon molecules, which are known as G3P or PGAL.
Since the Calvin Cycle started with 15 carbons (three 5-carbon molecules) and there are now 18 carbons, we have a net gain of 3 carbons.
One of these “extra” 3-carbon G3P/PGAL molecules will exit the cycle and be used to form ½ a glucose molecule.
Emphasize to students that these 3 extra carbons came from the carbon dioxide, which was obtained through the stomata from the atmosphere; these 3 additional carbons are denoted in red.
Remember to emphasize that this is the Calvin Cycle; we end up where we began. So, since we started with 15 carbons, we will also return to 15 carbons. C C C C C C

9. Phase 2: The Calvin Cycle
Once the Calvin Cycle “turns” twice (well, actually 6 times), those 2 molecules of G3P (a 3-carbon carbohydrate) will combine to form 1 molecule of glucose (a 6-carbon carbohydrate molecule) OR another organic compound.
- We say that the Calvin Cycle turns twice to make one molecule of glucose (6-carbons), but really it turns 6 times; each entering carbon dioxide molecule represents one “turn” of the cycle, and 6 carbon dioxide molecules must be incorporated into organic compounds in order for one 6-carbon glucose molecule to be produced. This PowerPoint (along with many textbooks) shows 3 carbon dioxide molecules entering together for clarity (it’s hard to show 1/3 of a G3P molecule as the product of each turn of the cycle).
- Glucose is what we usually think of as being the major product of photosynthesis; however, G3P (also known as PGAL) is the real product, and though it is often used to make glucose it can also be used as a carbon skeleton to form other organic molecules. C C C C
G3P
(from 3 turns of the Calvin Cycle)
G3P
(from 3 turns of the Calvin Cycle)
glucose

10. Phase 2: The Calvin Cycle
Step 3: Regeneration of RuBP
Since this is the Calvin Cycle, we must end up back at the beginning.
The remaining 5 G3P molecules (3-carbons each!) get rearranged (using ATP) to form 3 RuBP molecules (5-carbons each).
Where does the ATP come from to do this?
Emphasize to students that the other G3P molecule has left the cycle and was used to form glucose (or other organic compounds).
The ATP required to do this rearranging comes from the ATP generated during the light reactions. C C C C C C C
ATP C
ADP
5 G3P molecules Total: 15 carbons
3 RuBP molecules Total: 15 carbons P

11. Phase 2: The Calvin Cycle
CO2
Rubisco
ATP
RuBP
NADPH
- This simple schematic diagram gives a basic overview of what occurs during the Calvin Cycle. Carbon dioxide enters the cycle from the atmosphere and is joined to RuBP by Rubisco. NADPH and ATP are used to “turn” the cycle, and organic compounds (such as G3P/PGAL) are produced.
NADP+
ADP P
ORGANIC COMPOUND

12. Phase 2: The Calvin Cycle
- Though the AP curriculum framework very clearly states that students do not need to memorize the steps in the Calvin cycle, the structure of the molecules and the names of the enzymes involved (except for ATP synthase), some students may find this diagram helpful in understanding the cyclical nature of the Calvin cycle.

13. Phase 2: The Calvin Cycle
Quick recap:
In the Calvin Cycle, energy and electrons from the Light Reactions (in the form of ATP and NADPH) and carbon dioxide from the atmosphere are used to produce organic compounds.
The Calvin Cycle occurs in the stroma inside the chloroplasts (inside the cells…).
Carbon dioxide, ATP, and NADPH are required (reactants).
Organic compounds (G3P) are produced (products).

14. Photosynthesis: A Recap
So, as a broad overview of photosynthesis,
The Light Reactions (Phase 1) capture the energy in sunlight and convert it to chemical energy in the form of ATP and NADPH through the use of photosystems, electron transport chains, and chemiosmosis.
The Calvin Cycle (Phase 2) uses the energy transformed by the light reactions along with carbon dioxide to produce organic compounds.

15. Photosynthesis: A Recap
Based on this equation, how could the rate of photosynthesis be measured?
The photosynthetic equation:
Provides the carbon to produce organic compounds during the Calvin Cycle
The organic compound ultimately produced during the Calvin Cycle
light
6 H2O
6 CO2
6 O2
C6H12O6
Emphasize to students the importance of understanding how and when each component of the photosynthetic equation is used; this is much more valuable (and less intimidating!) than simply having them memorize the equation!
Most realistically, the rate of photosynthesis could be measured by using the:
Decrease in environmental CO2 (in a closed system)
Increase in environmental O2 (in a closed system)
Increase in glucose (perhaps measured using radioactive carbon)
Split during the light reactions to replace electrons lost from Photosystem II
Produced as a byproduct of the splitting of water during the light reactions
Excites electrons during the light reactions

16. Photosynthesis: A Recap
Photosynthesis Animation
(click on “Animation” after clicking the link)
- Online animation of the entire process of photosynthesis, including both the light reactions & the Calvin Cycle
- Animation url:

17. Environmental Factors & Photosynthesis
The rate (or speed) of photosynthesis can vary, based on environmental conditions.
Light intensity
Temperature
Oxygen concentration

18. Environmental Factors & Photosynthesis
Light intensity
As light intensity increases, so too does the rate of photosynthesis.
This occurs due to increased excitation of electrons in the photosystems.
However, the photosystems will eventually become saturated.
Above this limiting level, no further increase in photosynthetic rate will occur.
light saturation point
- Emphasize to students what it means to be saturated – as “full” as an item can be, or at its full capacity. A sponge is a good example to illustrate saturation; if a sponge is fully saturated with water, it can be left in a bucket of water overnight and will not gain any more water. In the same way, electrons in photosystems can be excited more often as light intensity increases, but eventually a “maximum” rate of excitation will be achieved; increasing light intensity beyond this point of light saturation will not yield an increase in photosynthetic rate. Be certain students don’t confused “stopped increasing the rate” with “ceases”!

19. Environmental Factors & Photosynthesis
Temperature
The effect of temperature on the rate of photosynthesis is linked to the action of enzymes.
As the temperature increases up to a certain point, the rate of photosynthesis increases.
Molecules are moving faster & colliding with enzymes more frequently, facilitating chemical reactions.
However, at temperatures higher than this point, the rate of photosynthesis decreases.
Enzymes are denatured.
-Ask students what type of “situation” is pictured here. They should answer an “optimum” situation is represented by this graph.
-This is an excellent opportunity to review the structure & function of enzymes as 3-D proteins with three or four levels of structure (primary, secondary, tertiary, quaternary) that are subject to external stresses such as temperature extremes. Emphasize to students the increased rate of molecular motion as temperatures increase, as well as the process of denaturation on protein structure and the resultant loss of molecular function.
- Emphasize to students the enzymes involved in photosynthesis, even though their names and specific functions do not need to be memorized. NADP+ reductase, Rubisco, and ATP synthase are all examples of enzymes involved in the process of photosynthesis.

20. Environmental Factors & Photosynthesis
Oxygen concentration
As the concentration of oxygen increases, the rate of photosynthesis decreases.
This occurs due to the phenomenon of photorespiration.

21. Photorespiration
Photorespiration occurs when Rubisco (RuBP carboxylase) joins oxygen to RuBP in the first step of the Calvin Cycle rather than carbon dioxide.
Whichever compound (O2 or CO2) is present in higher concentration will be joined by Rubisco to RuBP.
Photorespiration prevents the synthesis of glucose AND utilizes the plant’s ATP.
- Photorespiration is a negative process for photosynthetic organisms.
Photosynthesis occurs; glucose is produced
Rubisco joins CO2 to RuBP
More CO2
Photorespiration occurs; glucose is NOT produced
More O2
Rubisco joins O2 to RuBP

22. Photorespiration
Photorespiration is primarily a problem for plants under water stress.
When plants are under water stress, their stomata close to prevent water loss through transpiration.
However, this also limits gas exchange.
O2 is still being produced (through the light reactions).
Thus, the concentration of O2 is increasing.
CO2 is not entering the leaf since the stomata are closed.
Thus, as the CO2 is being used up (in the Calvin Cycle) and not replenished, the concentration of CO2 is decreasing.

23. Photorespiration
As the concentration of O2 increases and the concentration of CO2 decreases (due to the closure of the stomata to prevent excessive water loss), photorespiration is favored over photosynthesis.
Some plant species that live in hot, dry climates (where photorespiration is an especially big problem) have developed mechanisms through natural selection to prevent photorespiration.
C4 plants
CAM plants
Again, emphasize to students that photorespiration is unfavorable for photosynthetic organisms. It consumes ATP and does not produce glucose; the strong selective pressure against photorespiration has favored the proliferation of adaptations that increase the evolutionary fitness of those organisms who possess these adaptations.
This is another opportunity to stress how evolutionary adaptations come to exist. A mutation occurs, which may increase or decrease an organism’s chance of survival. If the mutation allows the organism that possesses it to reproduce more than other members of his/her/its population, the mutation will be favored through natural selection and will become more common in the population as organisms that possess the favorable mutation (adaptation) survive and reproduce at higher rates than members of the population which do not possess this adaptation.

24. C3 Plants
C3 plants, which are “normal” plants, perform the light reactions and the Calvin Cycle in the mesophyll cells of the leaves.
The bundle sheath cells of C3 plants do not contain chloroplasts
palisade mesophyll
spongy mesophyll
bundle sheath cells

25. C4 and CAM Plants
C4 plants and CAM plants modify the process of C3 photosynthesis to prevent photorespiration.
Overview:
C4 plants perform the Calvin Cycle in a different location within the leaf than C3 plants.
CAM plants obtain CO2 at a different time than C3 plants.
Both C4 and CAM plants separate the initial fixing of CO2 (carbon fixation) from the using of CO2 in the Calvin Cycle.
Both C4 and CAM plants fix CO2 with an enzyme other than Rubisco (both use PEP carboxylase) so they are able to fix CO2 in spite of the relatively high concentrations of O2. Then they use that CO2 separately in a normal Calvin cycle.

26. C4 Plants: Preventing Photorespiration
Plants that use C4 photosynthesis include corn, sugar cane, and sorghum.
In this process, CO2 is transferred from the mesophyll cells into the bundle-sheath cells, which are impermeable to CO2.
This increases the concentration of CO2.
Thus, the Calvin Cycle is favored over photorespiration.
The bundle-sheath cells of C4 plants do contain chloroplasts.
- Remember, Rubisco will join whichever compound is present in highest concentration (O2 or CO2) to RuBP; by shuttling CO2 into the bundle-sheath cells from which CO2 cannot escape, the concentration of CO2 is increased, which leads to the joining of CO2 to RuBP and the resultant production of organic compounds through the Calvin Cycle.

27. C4 Plants: Preventing Photorespiration
C4 plants use the Hatch-Slack pathway prior to the Calvin Cycle:
PEP carboxylase adds carbon dioxide to PEP, a 3-carbon compound, in the mesophyll cells.
This produces a 4-carbon compound (which is why it’s known as C4 photosynthesis).
This 4-carbon molecule then moves into the bundle-sheath cells via plasmodesmata.
- The Hatch-Slack pathway is described on this slide, and in the accompanying diagram. The movement of carbon dioxide into the bundle sheath cells from the mesophyll cells through its binding to PEP by PEP carboxylase facilitates the “stockpiling” of CO2 in the bundle-sheath cells and favors the Calvin Cycle over photorespiration.
In the bundle sheath cells, the CO2 is released and the Calvin Cycle begins.

28. C4 Plants: Preventing Photorespiration
If the Hatch-Slack pathway helps to prevent photorespiration, why wouldn’t ALL plants have this adaptation?
Prompt students to look at the diagram to answer the question. Two answers are important and should be covered:
This biochemical pathway exists only in plants whose ancestors had a mutation that caused this adaptation (which turned out to be favorable for them); organisms cannot simply “choose” which pathway to use – they are at the mercy (for better or worse) of their species’ evolutionary history.
The Hatch-Slack pathway utilized by C4 plants requires the use of additional ATP (note the ATP required for the conversion of pyruvate to PEP); this is in addition to the ATP required to drive the Calvin cycle. Thus, plants who utilize the Hatch-Slack pathway must “pay” for its use through the use of additional energy that is not required by C3 plants; however, in these plants the “cost” of additional ATP to prevent photorespiration is “worth it” due to their location in dry environments.

29. CAM Plants: Preventing Photorespiration
Plants that use CAM photosynthesis include succulent plants (like cacti) and pineapples.
In CAM (crassulacean acid metabolism) photosynthesis, plants open their stomata at night to obtain CO2 and release O2.
This prevents them from drying out by keeping their stomata closed during the hottest & driest part of the day.

30. CAM Plants: Preventing Photorespiration
When the stomata are opened at night, the CO2 is converted to an organic acid (via the C4 pathway) and stored overnight.
During the day – when light is present to drive the Light Reactions to power the Calvin Cycle – carbon dioxide is released from the organic acid and used in the Calvin Cycle to produce organic compounds.
Remember:
- Emphasize to students that the Calvin Cycle is not performed at night by CAM plants (or any other!). It is impossible for the Calvin Cycle to occur while it is dark because ATP and NADPH (from the Light Reactions) are required to run the Calvin Cycle. Instead, CAM plants store their CO2 as part of malic acid overnight until it can be released and used when the Light Reactions start again during the lighted hours.
Even though the CO2 is taken in at night, the Calvin Cycle cannot occur because the Light Reactions can’t occur in the dark!

31. - An excellent overview of the process of CAM photosynthesis

32. Avoiding Photorespiration
Both C4 and CAM plants – which are primarily found in hot, dry climates – have evolutionary adaptations which help prevent photorespiration.
C4 plants perform the Calvin Cycle in the bundle-
sheath cells.
CAM plants open their stomata at night and store the CO2 until morning.
- Again, stress the location difference of the Calvin Cycle between C3 and C4 plants, and the temporal difference of the uptake of CO2 between C3 and CAM plants. Both mechanisms are adaptations that promote adequate CO2 levels to promote the Calvin Cycle over photorespiration while preventing desiccation.

33. Created by:
Cheryl Boggs
Richmond, VA