Hello I´m a bit confused by the concept of cellular respiration. I know that primarily, we start out with glucose - but I know that other nutrients can be used. Which are these? Does these molecules need to be transformed into glucose before they can start cellular respiration? And in the case of glucose: As I have understood it, only monomeric (monosaccharides) carbohydrates can pass from the intestine to the epithelial lining and on to the blood stream. But how is that possible without some sort of membrane protein like the kind H2O uses? Carbohydrates are generally very polar molecules. And why can monosaccharides diffuse across membranes, but not disaccharides and polysaccharides? Thank you for your help. Sorry for all the questions - these are though not homework questions. I´m just a hobby biologist.