Why are some substances able to pass through a cell membrane while others are not

Depending on the size, some substances are able to pass through cell membranes

Explanation:

Some molecules, such as hydrocarbons and oxygen can cross the membrane. Many large molecules (such as glucose and other sugars) cannot. Water can pass through between the lipids.

Hope that answers your question.

clumped distribution, because you'de have all of the population gathered in a relatively small are.

it's the most common because it provides protection and a mate for reproduction

example: humans, seeweed, cattles, etc.