Photosynthesis most often takes place in two separate but dependent series of st
ID: 203550 • Letter: P
Question
Photosynthesis most often takes place in two separate but dependent series of steps: the light reactions and the photosynthetic carbon reduction cycle. Herein lies the first misconception. Most textbooks agree on the name of the former series, the light reactions, but refer to the latter series as the "dark reactions" of photosynthesis. This time-honored name is misleading and inaccurate, implying that photosynthesis occurs in the dark in green plants. For example, one highly respected high school biology textbook openly states that the dark reactions of photosynthesis take place at night. Net carbohydrate production from atmospheric CO2 does not occur in the dark in green plants for at least three well-known reasons: 1. The light reactions that supply the ATP and NADPH required for CO2 fixation and sugar production in the chloroplast operate only in the light. 2. Foliar stomates close in the dark shutting off the supply of atmospheric CO2 (except, of course, in the crassulacean acid metabolism plants described below). 3. Several enzymes of the photosynthetic carbon reduction cycle, including rubisco (ribulose 1,5 bisphosphate carboxylase/oxygenase, the enzyme that catalyzes the initial reaction of CO2 fixation into 3-phosphoglyceric acid), are regulated by light-they are active in the light, and much less active, inactive or even deactivated in the dark (see Buchanan 1984; Salisbury & Ross 1985; Kelly 1982). If "dark reactions" is unacceptable, then what is an acceptable term? Physiologists often refer to the process of CO2 fixation, sugar production and carbon cycling in photosynthetic cells as the "photosynthetic carbon reduction cycle" (as used above), the "reductive pentose phosphate pathway" or the "CO2 fixation cycle." When considering higher plants, names like "C-3 photosynthetic pathway" or "C-3 cycle" are often employed. Each of these names presents a problem to teachers and students or are somewhat misleading. The first two names are just too long for most students to remember, adding to an already burdensome load of biological jargon, while their acronyms PCRC and RPPP are but little improvement. "CO2 fixation cycle" slights the remainder of the processes involved. Recognition of the C-3 nature of the cycle in the name is useful but requires that students understand the C-4 pathway of photosynthesis described below. Thus, to avoid these and other pitfalls, the reactions in the vast majority of photosynthetic organisms that incorporate CO2 initially into a threecarbon acid, then form several sugars along a cyclic pathway are best called the "Calvin cycle." This name recognizes the Nobel Prize winning work of Melvin Calvin and associates (including Andrew Benson and James Bassham) at the University of California in Berkeley from 1946 to 1953. The Calvin cycle designation helps humanize the perceived cold, factual world of science, gives recognition where it is due, is easy for students and teachers to remember and use and doesn't mislead them. After (if) students are introduced to alternative pathways of photosynthesis (see below), "C-3 pathway" may be used to describe plants that utilize the Calvin cycle exclusively for carbohydrate production from atmospheric CO2.
The Calvin Cycle should not be called the “Dark Reactions” of photosynthesis because:
a. In most plants, CO2 enters the leaves the day but not at nightExplanation / Answer
ANSWER
The Calvin Cycle should not be called the “Dark Reactions" of photosynthesis because:
c. The reactions that generate NADPH and ATP, which are needed for the Calvin Cycle, only run during the day
This is because in most plants, CO2 enters the leaves both in the day and in the night. Also, none of the the enzymes in the Calvin Cycle are indirectly activated by light, which actually becomes the basis for this name of Dark Reactions.