RESUMO
Cellulosic ethanol is an alternative for increasing the amount of bioethanol production in the world. In Brazil, sugarcane leads the bioethanol production, and to improve its yield, besides bagasse, sugarcane straw is a possible feedstock. However, the process that leads to cell wall disassembly under field conditions is unknown, and understanding how this happens can improve sugarcane biorefinery and soil quality. In the present work, we aimed at studying how sugarcane straw is degraded in the field after 3, 6, 9, and 12 months. Non-structural and structural carbohydrates, lignin content, ash, and cellulose crystallinity were analyzed. The cell wall composition was determined by cell wall fractionation and determination of monosaccharide composition. Non-structural carbohydrates degraded quickly during the first 3 months in the field. Pectins and lignin remained in the plant waste for up to 12 months, while the hemicelluloses and cellulose decreased 7.4 and 12.4%, respectively. Changes in monosaccharide compositions indicated solubilization of arabinoxylan (xylose and arabinose) and ß-glucans (ß-1,3 1,4 glucan; after 3 months) followed by degradation of cellulose (after 6 months). Despite cellulose reduction, the xylose:glucose ratio increased, suggesting that glucose is consumed faster than xylose. The degradation and solubilization of the cell wall polysaccharides concomitantly increased the level of compounds related to recalcitrance, which led to a reduction in saccharification and an increase in minerals and ash contents. Cellulose crystallinity changed little, with evidence of silica at the latter stages, indicating mineralization of the material. Our data suggest that for better soil mineralization, sugarcane straw must stay in the field for over 1 year. Alternatively, for bioenergy purposes, straw should be used in less than 3 months.
RESUMO
Energy-yielding metabolism is an important biochemistry subject that is related to many daily experiences and health issues of students. An adequate knowledge of the general features of EYM is therefore important, both from an academic and social point of view. In a previous study, we have shown that high-school students present the misconception that carbohydrates, especially glucose, are the sole metabolic fuel for ATP production by human cells. In the present work, we investigated the possible origins of the occurrence of this misconception among students. The analysis of students' answers to questionnaires indicated that the misconception appears as soon as in the 8th grade and remains unchanged throughout subsequent school years. The analysis of grade textbooks showed that the misconception is likely to be a consequence of the teaching of nutrition in the 8th grade, when a single function is emphasized for each nutrient. The energetic function is mainly associated with carbohydrates, while proteins and lipids are considered structural and storage molecules, respectively. An extreme similarity was observed between students' knowledge of nutrient's function and textbook contents. Analysis of high-school textbooks suggested that the misconception would be reinforced because of the detailed teaching only of glucose metabolism, with rare mention of lipids or amino acids as metabolic fuels. The consequences of that approach are discussed and suggestions are made on an alternative teaching of energy-yielding metabolism.