Membranes are barriers and “gatekeepers”, they only let certain molecules pass through them.
they also transport nutrients to the cell
Answer:
meter the mean free path of hydrogen atoms in interstellar space.Explanation:
The mean free path equation is given as:

Where"
d = diameter of hydrogen atom in meters
n = number of molecules per unit volume
We are given: d = 100 pm = 



meter the mean free path of hydrogen atoms in interstellar space.
Yes, remember a chemical reaction is whenever one or more substances combine together to form a different substance. So whatever you put together and form something new is called a chemical change/reaction. Examples such as rusting of Iron, expired milk(going sour), Baking cake and many more to think about
Hope it helps
Of course they are small
Explanation:
The only way you can see them is by a microscope or a lens and can be anywhere.