Question

Why do ions have a difficult time getting through plasma membranes despite their small size?

Short answer

Ions are charged particles present in a solution. The charge is either positive or negative. As the cell membrane is semipermeable, it doesn't allow charged, hydrophilic ions to pass through it.

Step-by-step solution

Step 1. Introduction

Ions being charged and hydrophilic cannot enter easily through the plasma membrane.

Step 2. Explanation

As the cell membrane is semipermeable thus it doesn't allow charged, hydrophilic ions to pass through it. Thus charged ions need special Proteins through which they can pass through and enter cell plasma.