پنجشنبه 21 اردیبهشت 1391 03:30 ب.ظ
The Calvin Cycle
During photosynthesis, light energy is used to generate chemical free energy, stored in ATP and NADPH. The light-independent Calvin ("dark") cycle uses the energy from short-lived electronically-excited carriers to convert carbon dioxide and water into organic compounds that can be used by the organism (and by animals which feed on it). This set of reactions is also called carbon fixation. The key enzyme of the cycle is called RuBisCO. In the following equations, the chemical species (phosphates and carboxylic acids) exist in equilibria among their various ionized states as governed by the pH.
The sum of reactions in the Calvin cycle is the following:
6 CO2 + 12 NADPH + 12 H2O + 18 ATP → C6H12O6 + 12 NADP+ + 18 ADP + 18 Pi
The steps in the Calvin cycle are:
1. RuBisCO reacts with CO2, creating a 3-carbon compound, PGA.
2. One ATP from the light reactions is used, producing an ADP and a Pi .
3. An NADPH from the light reactions combines with an H+ and becomes NADP+.
4. PGAL, a 3-carbon compound, is produced, which stores free energy.
5. Another ATP is consumed, yielding an ADP and a Pi.
6. RuBP is produced, which is a 5-carbon compound.
At high temperatures, RuBisCO will react with O2 instead of CO2 in photorespiration, an apparently-puzzling process, since it seems to throw away captured energy. However it may be a mechanism for preventing overload during periods of high light flux. C4 plants use the enzyme PEP initially, which has a higher affinity for CO2. The process first makes a 4-carbon intermediate compound; hence the name C4 plants.
C4 Pathway of Photosynthesis
C4 Photosynthesis : C4 plants.
- Called C4 because the CO2 is first incorporated into a 4-carbon compound.
- Stomata are open during the day.
- Uses PEP Carboxylase for the enzyme involved in the uptake of CO2. This enzyme allows CO2 to be taken into the plant very quickly, and then it "delivers" the CO2 directly to RUBISCO for photsynthesis.
- Photosynthesis takes place in inner cells (requires special anatomy called Kranz Anatomy)
- Adaptive Value:
- Photosynthesizes faster than C3 plants under high light intensity and high temperatures because the CO2 is delivered directly to RUBISCO, not allowing it to grab oxygen and undergo photorespiration.
- Has better Water Use Efficiency because PEP Carboxylase brings in CO2 faster and so does not need to keep stomata open as much (less water lost by transpiration) for the same amount of CO2 gain for photosynthesis.
- C4 plants include several thousand species in at least 19 plant families. Example: fourwing saltbush pictured here, corn, and many of our summer annual plants
CAMafter the plant family in which it was first found (Crassulaceae) and because the CO2 is stored in the form of an acid before use in photosynthesis.
- Stomata open at night (when evaporation rates are usually lower) and are usually closed during the day. The CO2 is converted to an acid and stored during the night. During the day, the acid is broken down and the CO2 is released to RUBISCO for photosynthesis
- Adaptive Value:
- Better Water Use Efficiency than C3 plants under arid conditions due to opening stomata at night when transpiration rates are lower (no sunlight, lower temperatures, lower wind speeds, etc.).
- May CAM-idle. When conditions are extremely arid,
CAMplants can just leave their stomata closed night and day. Oxygen given off in photosynthesis is used for respiration and CO2 given off in respiration is used for photosynthesis. This is a little like a perpetual energy machine, but there are costs associated with running the machinery for respiration and photosynthesis so the plant cannot CAM-idle forever. But CAM-idling does allow the plant to survive dry spells, and it allows the plant to recover very quickly when water is available again (unlike plants that drop their leaves and twigs and go dormant during dry spells). CAMplants include many succulents such as cactuses and agaves and also some orchids and bromeliads
CAM plants keep their stomata (on the underside of the leaf) closed during the day, which conserves water but prevents photosynthesis, which requires CO2 to pass by gas exchange through these openings. Evaporation through the upper side of a leaf is prevented by a layer of wax.
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