Concept:The cell membrane is selectively permeable. It allows small, non-polar molecules to pass through but blocks large molecules.
Explanation:The cell membrane (plasma membrane) has a phospholipid bilayer structure. This layer is semi-permeable. Small molecules like oxygen (
O2​) and carbon dioxide can easily diffuse through the lipid bilayer. They are tiny and non-polar, so they slip between the phospholipids without help.
Large molecules like starch and proteins are too big and often polar or charged. They cannot cross the lipid bilayer directly. The membrane uses special transport proteins (carrier or channel proteins) to move such molecules, but only if needed. Starch must first be broken down by enzymes (like amylase) into glucose before it can enter. Similarly, proteins need to be digested into amino acids. This selective nature keeps the cell’s internal environment stable (homeostasis).
Thus, oxygen passes freely, but starch and proteins are blocked due to their size and the membrane’s selective permeability.
Answer:C. The cell membrane is permeable to selective substances.