Rufus Lumry

The mechanism of the photochemical activity of isolated chloroplasts. I. Effect of temperature.

Abstract 1. 1. The rate-limiting step in the dark process of the Hill reaction has the same activation energy of 10 kcal. for several varieties of the species Beta vulgaris . 2. 2. The loss of Hill-reaction reactivity of isolated chloroplast fragments is due to at least two different processes: one, significant at temperatures greater than 30 °C., is first-order and demonstrates the characteristics of thermal denaturation of proteins; another, significant at both low and high temperatures, is second-order and has an activation energy of 8 kcal. Both reactions appear to involve participation of at least one common chloroplast substance. 3. 3. Some samples of chloroplast fragments prepared under nitrogen rather than air can be activated by incubation to a Hill-reaction reactivity as much as 50% higher than normal.

Inhibition of the photochemical activity of isolated chloroplasts. I. Salts.

Abstract The data and conclusions as listed below are valid only under the experimental conditions used as described in the section on Materials and Methods . 1. 1. Univalent inorganic anions inhibit the Hill reaction of isolated chloroplast fragments in the order: SCN − > I − > F − > NO 3 − > ClO 3 − > Br − > Cl − . 2. 2. Univalent inorganic cations at high concentrations inhibit in the order: NH 4 + > Cs + > Rb + > K + = Na + = Li + 3. 3. Univalent fatty acid ions inhibit in the order: formate > acetate > propionate > n -butyrate n -valerate. 4. 4. A mass-law mechanism is shown to provide a promising initial interpretation for the inhibition by univalent ions. 5. 5. Multivalent anions are less effective than univalent anions, while bivalent cations are more effective than univalent cations as chloroplast inhibitors.

The mechanism of the photochemical activity of isolated chloroplasts. II. Quantum requirements.

Abstract 1. 1. Working under conditions of temperature, pH, light wavelength, and light intensity found to be most favorable, the lowest value of the quantum requirement for the Hill reaction was found to be 7.9 ± 0.3 quanta/oxygen molecule. This reproducible value obtained at 6750 A. is probably not the true minimum since experimental errors were almost uniformly in a direction to give higher than minimum values. 2. 2. It is concluded that, although differences may exist between the Hill reaction and whole-cell photosynthesis, energetically the two processes are so similar that the mechanisms of energy utilization are probably identical.

COLLECTION AND UTILIZATION OF ENERGY IN PHOTOSYNTHESIS

Types of photochemical reactions carried out by photosynthetic systems are reviewed. Topics discussed include evolutionary aspects of photosynthesis, photosynthetic apparatus, trapping of light quanta, studies of the fInorescence yield, the role of chlorophyll and chloroplasts, and the kinetics of photosynthesis and the Hill reaction. 86 references. (C.H.)