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. Carbon dioxide, ATP, and NADPH are required (reactants). Organic compounds (G3P) are produced (products).

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.

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 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

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

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.

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

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. https://www.khanacademy.org/science/biology/photosynthesis-in-plants/photorespiration--c3-c4-cam-plants/a/c3-c4-and-cam-plants-agriculture 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

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.

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 (crassulacean acid metabolism) 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.

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

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.

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.

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.

C4 Plants: Preventing Photorespiration If the Hatch-Slack pathway helps to prevent photorespiration, why wouldn’t ALL plants have this adaptation? 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.

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.

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: 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!

- An excellent overview of the process of CAM photosynthesis

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.