Respiration, photosynthesis and peroxidase activities of living spherical samples, such as algal protoplasts, breast cancer spheroids and bovine embryos, were characterized with scanning electrochemical microscopy (SECM). The concentration profile of the metabolic product around the spherical sample was directly measured by scanning with a probe microelectrode. According to the spherical diffusion theory, the total mass transfer rate per spherical sample is linear to the multiplication of the sample radius and the concentration difference between the sample surface and the bulk of the solution. Therefore, the sample radius is a key parameter to assess the viability of the living samples. For example, we investigated the respiration and photosynthesis activities as a function of the size of the protoplast (Bryopsis plumosa). The respiration rate was linear to the cube of the sample radius. On the contrary, the photosynthesis rate was linear to the square of the sample radius, suggesting that the former is controlled by the volume of the protoplast, and the latter is controlled by the surface area of the protoplast. We will also discuss the size-dependent activity of the breast cancer spheroids and the bovine embryos. Furthermore, relations between the sample size, the concentration difference, and the oxygen consumption rate of the cryo-preserved bovine blastocysts were investigated.
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