Enzyme kinetics is a difficult subject for students to learn and for tutors to teach. During the practicals included in the biochemical courses at the Faculty of Chemistry of Universidad Nacional Autó noma de Mé xico, we found that the students acquire good training in the calculations to obtain kinetic parameters such as K m , V max , optimum pH, etc., but, when they are questioned about the significance of the values and their relationship to enzyme catalysis, many confusing ideas arise. To provide extended practice opportunities that could aid in the learning process we developed computer software that simulates an enzyme assay for lactate dehydrogenase, named enzsimil, that was used as well as the practical sessions. We tested three different levels of guidance to work with enzsimil, scripts where the students had to follow detailed (guided), intermediate (semiguided), or minimal (unguided) instructions, and for comparison, one group had a session of solving problems in class extracted from the program (class), and one more group had no additional sessions (control). After their respective sessions, the students either wrote a report or completed their script and undertook a laboratory practical. At the end, an exam was applied to all students. The reports and exams were graded, and the performance of the experimental groups was subjected to a statistical analysis. In addition, we carefully read the answers trying to identify the more common errors and misconceptions. The study revealed that there are statistically significant benefits in the use of the program. The semiguided scheme was more convenient to help the student in the short term for the preparation of better reports, while in the long term, both the semiguided and unguided groups performed equally well. These results are discussed in terms of the convenience of guided or unguided teaching strategies in computer-assisted learning.

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Cell walls of aleurone tissue of developing wheat grains (25 days post anthesis) are degraded by cellulase to give a good yield of protoplasts. As grains become older the aleurone cell walls become increasingly resistant to cellulase, which is completely ineffective on 45-day old material. Resistance to cellulase is provoked even in aleurone cells of young grains by enforced drying of the grains to 12% water content. This suggests that the decreasing effectiveness of cellulase that occurs as grains develop and mature may be caused partly by the natural loss of water.Resistance to degradation by the aleurone cell walls of older or dried grains is to a large extent overcome by the addition of hemicellulase and xylanase to the digestion mixture but completely if acetyl esterase is also included. Changes involving hemicelluloses and xylans are therefore implicated in the effects of drying or maturation, including acetylation processes especially in grains which are almost mature. Arising from these findings a method has been developed for obtaining a high yield of protoplasts from aleurone layers of mature wheat grains, having a high viability and response to gibberellic acid.