Answer: 1. phospholipid bilayer 2. This is carbon-13, an isotope of carbon
Explanation:1. Two layers of phospholipid form the membrane, with their hydrophilic ends inwards. The hydrophobic surfaces protect the cell from osmotic changes by isolating the cytoplasm from the environment - except for embedded transport mechanisms for moving ions and molecules in and out as required.
2. C-13 is a rare, stable isotope of carbon, making up about 1% of natural carbon.
An element is defined by the number of protons in the nucleus, because the number of protons is matched by the number of electrons in the nucleus, and it is the electrons that determine chemical properties. Nuclei with more than one proton (i.e. not H) also have neutrons, which have no charge. They assist in keeping the positive protons together in the nucleus and are important for stability. Atoms of an element can have different numbers of neutrons, more than one of which can be stable. This variants are called isotopes. They behave the same chemically, but have some different physical characteristics due to size difference.
There are minimum and maximum limits to the number of neutrons that form stable isotopes. Beyond those limits, the atoms tend to decay through radioactivity. For example C-12 and C-13 are stable isotopes. Another natural isotope C-14 is radioactive.
There is also a limit to the number of protons that can be held together by any number of neutrons. Uranium, with 92 protons, has only radioactive isotopes, but U-238 (146 neutrons) decays much more slowly than U-235 (143)or U-234 (142).
