Question

8. The light reactions are made up of two photosystems. Describe each of the photosystems. Where do these reactions occur? When do they occur? 9. How are pigments used in the photosystems? How is it possible for pigments to catch light or absorb light to fuel the reactions of the chloroplast? 10. What is carbon focation? What is nitrogen fixation? The Calvin Cycle is also called the dark reactions. Why? 11. What are the steps in the dark reactions? What role do the stomata play in the dark reactions? What enzyme is primarily involved in capturing or fixing the carbon from CO2? 12. What serious risk does a plant take when it keeps its stomata open during the day? What alternative types of photosynthesis have evolved to compensate for this risk?

Answer 1

8.- These reactions occur in the tilakoid membrane, inside the chloroplast, during light-dependant reactions. Photsystems I and II are two coupled molecular machineries, they both are sensitive to light radiation to start an electron chain. While photosystem II (the first one to exist in evolution) obstains the electrons from water cleavage, photosystem I cannot cleave water and only receives the electrons from the photosystem II chain.

PS II appeared in bacteries first in evolution, then PS I appeared in algae, thats why bacteries have only PS II, while algae and plants have both.

9.- Pigments are sentitive to certain wavelenghts of light radiation. This molecules work by being capable of transfer the resonance energy obtained from light into electrons. This initial energy taken from light is used to break H2O molecule and take its electrons to go run in the electron transport chain. We could say plants take solar energy just to break water, but then the rest of reaction come in place by this.

10.- Carbon fixation is the process in which inorganic carbon (CO2) is transformed into organic carbon (into carbohydrates). Nitrogen fixation is the process in which inorganic nitrogen (N2) is transformed into organic nitrogen (NH3)

11.- Dark reactions refers to calvin-benson cycle's reactions. They take place in the stroma, an it has 3 mayor phases:

a) The first and special stage is the carboxylation stage. This is called like that because it is the event in which inorganic CO2 its introduced in carbohydrates to be part of the organic carbons.

b) Next one is the reduction stage, in which the compounds get to be reduced.

c) And final de regeneration stage, in which the cycle regenerates the compounds needed to receive another CO2 molecule and carboxylate the carbohydrate in question.

Stomata play a mayor role here because, as we can see, this cycle depends on the CO2 supply. Stomata open to let out oxygen (produced by the light reactions) and let in CO2 to undergo dark reactions. If stomata don't open then the concentration of oxygen goes up and concentration of CO2 goes down, leading to no carbon fixation and even to cell respiration.

The enzyme involved in carbon fixation is called rubisco

12.- If the plant keeps stomata open during the day, it gets a risk of getting dehydrated. The hot temperatures tend to transform more water into vapour, if the plant lets water scape then cells will die. Hight temperatures but with lots of humidity/water don't represent a risk (like in tropical forests), but hot temperatures with drought will kill plants with open stomata (like in deserts).

There are two types of photosyntesis to compensate such risk. The C4 metabolism found in Poaceae, and the CAM (crassulacean acid metabolism) found in the succulent plants (cactaceae, agavaceae, orchidaceae, etc.)