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Biomedical subjects

B Kok

Publications and source records attributed to B Kok.

At least 37 records · Page 2Linked to original sources
PubMed1972

Photon trapping in photosystem II of photosynthesis. The fluorescence rise curve in the presence of 3-(3,4-dichlorophenyl)-1,1-dimetnhylurea.

Using isolated chloroplasts in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), an analysis was made of the rise of the fluorescence yield effected by weak light. Depending on the pretreatment, the time-course of the rapid photochemical part of the rise varied between nearly first-order and quadratic kinetics, i.e., reflected either a one-quantum or a two-quantum conversion. We consider the occurrence of two photoreductants per system II unit, which are reoxidized in different dark reactions. The data further showed that the "first-order process" is also inhomogeneous.

Chloroplasts↗
PubMed1972

An integrated multi-purpose biology instrument utilizing a single detector, the mass spectrometer.

A mass spectrometer is used to analyze the gas phase in a number of reaction vessels filled with Martian soil. By choosing appropriate incubation conditions this instrument can be used to perform a wide spectrum of experiments ranging from the observation of general indices of life, i.e. processes and patterns unexplainable by physico-chemical mechanisms, to assays utilizing isotopes which probe for specific metabolic processes. Of particular interest is the in situ incubation in which a Martian soil sample is maintained at a constant temperature and its gas phase composition analyzed with time. Properly interpreted, this is a very general life-detection probe which makes minimal assumption as to the nature of Martian biology. Other assays and measurements concerning the soil and the atmosphere compatible with this method are also described.

Atmosphere↗
PubMed1966

Photoinhibition of chloroplast reactions. I. Kinetics and action spectra.

A study was made of photoinhibition of spinach chloroplast reactions. The kinetics and spectral characteristics of the photoinhibition over a range between 230 and 700 mmu have been examined. The decline of activity due to preillumination was independent of wavelength, and dependent upon the number of quanta applied, not upon the rate of application. The effectiveness spectra of photoinhibition indicate that active ultraviolet light is absorbed by a pigment which is not a normal light absorber for photosynthesis and acts with a high quantum efficiency (> 0.1) for photoinhibition.Active visible light is absorbed by the pigments which sensitize photosynthesis (chlorophyll, carotenoids). A very low quantum efficiency (about 10(-4)) was observed for the photoinhibition with visible light.The action spectrum of the photoinhibition of dye reduction by chloroplasts and lyophylized Anacystis cells indicated that the damage caused by visible light is due to quanta absorbed by photosystem II. However, since system I might not be involved in dye reduction, the spectra may reflect only damage to photosystem II.

Journal Article↗
PubMed1966

Photoinhibition of Chloroplast Reactions. II. Multiple Effects.

Ultraviolet light inhibits the photoreduction of 2,6-dichlorophenolindo-phenol or nicotinamide adenine dinucleotide phosphate with water as the electron donor (evolution of oxygen) but not the photoreduction of nicotinamide adenine dinucleotide phosphate with ascorbate as the electron donor. It inhibits photophosphorylation associated with either system. Experiments undertaken to test whether plastoquinone is the site of UV inhibition yielded inconclusive results.Visible light (> 420 mmu) causes the loss of all chloroplast activities, photosystem I being more sensitive than system II. The data suggests 2 modes of action for visible light. The one sensitized by system II results in damage resembling that of UV light. The other, sensitized by system I, results in the destruction of the reaction center of this system.

Journal Article↗