Question

Why is the plasma membrane called a selectively permeable membrane?

Answer 1

Hint: The cell wall is a type of membrane that is freely permeable to all substances moving in and out of the cell which means that the cell wall does not provide any resistance to the movement of substances across the cell.

Complete answer:
The plasma membrane is known as a selectively permeable membrane because it has the ability to decide whether to allow substances inside and outside the cell or not. The plasma membrane is able to regulate the movement of substances across the cell because of its phospholipid structure. Not all substances can move across this phospholipid membrane as lipids are hydrophobic. Hence special carrier proteins are required to carry these substances across the cell, and these carrier proteins are under the control of the cell membrane. Thus, the plasma membrane can decide what to allow and what to not inside the cell.

Additional Information:
-The structure of the plasma membrane was deduced after extensive studies in red blood cells of humans.
-Lipids arranged in bilayer and proteins embedded in this bilayer make up the plasma membrane.
-The plasma membrane of the erythrocyte of human beings shows that it is made up of 52% protein and 40% lipids.
-The currently accepted structure of the plasma membrane is the Singer and Nicolson Model which is also known as the Fluid Mosaic Model.
- This model states that the components of the membrane are free to move to give the membrane a fluid-like nature.

So, the correct option is ‘Carbon dioxide’.

Note: -Substances that are hydrophobic in nature, can diffuse across the plasma membrane without any expenditure of energy. Such a process is known as passive transport.
-Substances that are hydrophilic like water cannot pass through the lipid bilayer and thus require proteins to carry them. There is no expenditure of energy and it is known as facilitated transport.
-Some substances are transported inside the cell against the concentration gradient that is from lower to higher concentration with the utilization of energy. This process is known as active transport.