Photosynthesis in dry-climate plants Figure 3 has evolved with adaptations that conserve water. In the harsh dry heat, every drop of water and precious energy must be used to survive.
Two adaptations have evolved in such plants. In one form, a more efficient use of CO 2 allows plants to photosynthesize even when CO 2 is in short supply, as when the stomata are closed on hot days. The other adaptation performs preliminary reactions of the Calvin cycle at night, because opening the stomata at this time conserves water due to cooler temperatures.
In addition, this adaptation has allowed plants to carry out low levels of photosynthesis without opening stomata at all, an extreme mechanism to face extremely dry periods. The two parts of photosynthesis—the light-dependent reactions and the Calvin cycle—have been described, as they take place in chloroplasts. However, prokaryotes, such as cyanobacteria, lack membrane-bound organelles. Prokaryotic photosynthetic autotrophic organisms have infoldings of the plasma membrane for chlorophyll attachment and photosynthesis Figure 4.
It is here that organisms like cyanobacteria can carry out photosynthesis. Figure 4. A photosynthetic prokaryote has infolded regions of the plasma membrane that function like thylakoids.
Although these are not contained in an organelle, such as a chloroplast, all of the necessary components are present to carry out photosynthesis.
Living things access energy by breaking down carbohydrate molecules. However, if plants make carbohydrate molecules, why would they need to break them down? Carbohydrates are storage molecules for energy in all living things. Although energy can be stored in molecules like ATP, carbohydrates are much more stable and efficient reservoirs for chemical energy. Photosynthetic organisms also carry out the reactions of respiration to harvest the energy that they have stored in carbohydrates, for example, plants have mitochondria in addition to chloroplasts.
You may have noticed that the overall reaction for photosynthesis:. Photosynthesis produces oxygen as a byproduct, and respiration produces carbon dioxide as a byproduct. In nature, there is no such thing as waste. Every single atom of matter is conserved, recycling indefinitely. Substances change form or move from one type of molecule to another, but never disappear Figure 5. Figure 5. In the carbon cycle, the reactions of photosynthesis and cellular respiration share reciprocal reactants and products.
CO 2 is no more a form of waste produced by respiration than oxygen is a waste product of photosynthesis. These reactions actually have several names associated with them.
Other names for light-independent reactions include the Calvin cycle, the Calvin-Benson cycle, and dark reactions. The most outdated name is dark reactions, which can be misleading because it implies incorrectly that the reaction only occurs at night or is independent of light, which is why most scientists and instructors no longer use it.
The light-independent reactions of the Calvin cycle can be organized into three basic stages: fixation, reduction, and regeneration. In the stroma, in addition to CO 2 ,two other components are present to initiate the light-independent reactions: an enzyme called ribulose bisphosphate carboxylase RuBisCO and three molecules of ribulose bisphosphate RuBP. RuBP has five atoms of carbon, flanked by two phosphates. This is a reduction reaction because it involves the gain of electrons by 3-PGA. Recall that a reduction is the gain of an electron by an atom or molecule.
Both of these molecules return to the nearby light-dependent reactions to be reused and reenergized. The Calvin cycle is a process that plant s and algae use to turn carbon dioxide from the air into sugar , the food autotroph s need to grow. Every living thing on Earth depends on the Calvin cycle.
Plants depend on the Calvin cycle for energy and food. Other organisms, including herbivore s, also depend on it indirectly because they depend on plants for food.
Even organisms that eat other organisms, such as carnivore s, depend on the Calvin cycle. Without it, they wouldn't have the food, energy, and nutrient s they need to survive. The Calvin cycle has four main steps: carbon fixation , reduction phase , carbohydrate formation, and regeneration phase. Energy to fuel chemical reaction s in this sugar-generating process is provided by ATP and NADPH , chemical compounds which contain the energy plants have captured from sunlight. Carbon dioxide is also the byproduct of burning fossil fuels.
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