Question

Why is the cell membrane described as Semi-permeable?

Answer 1

Hint: Cell membrane is also termed a plasma membrane because the layer protecting the cell is semi-permeable to certain constituents. Lipid is nothing but fat. Lipid structures separate the components between cellular compartments and the outer environment. It separates the organelles from the cellular matrix. The plasma membrane, present at the cellular surface membrane or plasmalemma. It is a phospholipid bilayer.

Complete answer:
A plasma membrane is said to be a semipermeable membrane because it allows certain molecules but not all to pass or enter into the cell. It regulates the movement of materials in and out of cells and also facilitates the electrical signaling between them. It has major functions like it serves the purpose of passing nutrients and flow of ions into and out of the cells. It shows interaction with other cell boundaries and surfaces.
The biochemical composition of plasma membrane consists of lipid bilayers which are inserted in a protein bed. They are approximately $5nm$ thickness. Some other membranes also carry carbohydrates, which can be different from cell to cell.
The phospholipids contain a polar head group which is attached to two hydrophobic tails which are fatty acids in nature. The head faces towards the inner side which is aqueous in nature and the fatty acid is the tail interior of the bilayer.

Note:
Apart from the phospholipid layer, the plasma membrane has other different types of proteins such as- the embedded proteins function as gated channels transports the movement of compounds across the membrane. They act as neurotransmitters. The membrane proteins present at the periphery provide mechanical and structural support to the membrane. GPI is present in the neuronal membrane.