The cells of your body need a variety of nutrients and compounds to function normally, but only certain compounds can directly cross the plasma membrane of your cell. What types of compounds can cross the plasma membrane directly? How are compounds that cannot cross directly able to enter and exit the cell?
Non-polar substances are able to directly cross the plasma membrane. Polar and ionic substance are able to cross using channels, transporters and carrier proteins.
What is the difference between an isotope and an ion?
Both ions and isotopes have the same number of protons of their neutral atom counterpart (and therefore the same atomic number). However, an isotope has a different number of neutrons, while an ion has a different number of electrons.
It has been suggested that the existence of life itself contradicts the second law of thermodynamics, which states that the natural tendency of the universe is to move towards a state of increased disorder. How do living things both abide by and contradict this law?
This question is best answered through an example: humans ingest food, which is then broken down into nutrients used by the cells of the body to perform their normal functions - it could be argued that this process thus increases the order of the system. However, the breakdown of food and normal cellular processes go on to contribute to the production of heat by the body, a form of energy which can increase the amount of disorder in the universe.