How do I know when to use por or para?
Both of these words can mean "for," but both of them have other meanings too. Think of "por" as a means of doing something, or being more temporary; "Yo camino por la biblioteca" means "I walk through the library" but it doesn't mean I'm staying there-- just passing through. If I used "para" in this sentence-- "Yo camino para la biblioteca"-- it means I am walking toward the library or for the library. It is my end goal or destination or the more permanent object of my walking. "Para" can be used to indicate that something will be given to someone else, like "Compro las flores para mi madre." Here I am buying flowers to give to my mom. When I say "Compro las flores por mi madre," I am buying flowers in my mom's place because she can't make it to the store. This reveals another use of "por," which is indicating an exchange. I swapped out for my mom to go get some flowers. "Por" is also used to indicate cost, which you can think of an exchange of money for an item, and time, which you can also think of in terms of exchange. "Estudio por dos horas cada noche": I study for 2 hours every night. I exchange 2 of my hours for studying. "Pago cuatro dólares para el cafe": I pay 4 dollars for coffee. I am exchanging 4 of my dollars for coffee. These are the basic rules that I use to decide between using por and para: exchange and destination.
How do I know when to use passato prossimo or passato remoto?
Passato prossimo is what we normally think of for the past tense in Italian; it's the closest equivalent of the -ed ending in English (call-ed, talk-ed, walk-ed, etc.). It can also be used in the places where we would say "have called, have talked, have walked," which in English is the present perfect. Passato remoto, however, is really only used in situations where we are talking about history or something that happened in the remote past, as the name would suggest, and can be considered more formal. Reading a textbook or some kind of historical account would use passato remoto. In speech the passato remoto is less used, and the same story being told by a friend in passato prossimo would be written in a textbook in passato remoto. They can have the same meaning but the passato remoto reads more like a narration of events where the passato prossimo is part of the everyday language.
What is surface-area-to-volume-ratio and why is it important?
Surface-area-to-volume-ratio describes the relationship between the outside and inside of a cell. In smaller cells, there is more cell membrane in comparison to the volume of the inside of the cell, which is more efficient for things like exchange of materials with the environment and the amount of receptors on the membrane. As cell size increases, the volume increases at a much faster rate than the surface area so it is harder to supply the cell with everything it needs to carry out normal functions. This explains why having multi-cellular organisms, like humans for example, is better than having just one big human-sized cell. It also explains why some cells have protuberances like villi or other membrane folds that help to increase the surface area.