Question

A method of intercalation of new wall material in between the old component of the cell wall is called as
A. Layering
B. Apposition
C. Intussusception
D. None of the above

Answer 1

Hint: All organisms are composed of cells. Some are composed of a single cell and are called unicellular organisms while others are composed of many cells and called multicellular organisms. Cells that have membrane bound nuclei are called as eukaryotic organisms whereas cells that lack membrane bound nucleus are prokaryotic organisms. In both prokaryotic and eukaryotic cells, a semi fluid matrix called the cytoplasm occupies the volume of the cell. Besides the nucleus, the eukaryotic cells have other membrane bound distinct structures called organelles. The prokaryotic cell lacks such membrane bound organelles.

Complete answer: All the cells, whether prokaryotic or eukaryotic, unicellular or multicellular are bound by a membrane called the cell membrane.
Among the eukaryotic cells, plant cells are different in structure as compared to animal cells. Both have cell membranes but plant cells have an additional layer outside the membrane called the Cell wall. Cell wall is a non living, rigid structure which forms an outer covering for the cell membrane of plants, bacteria, algae and fungi. All of these have different compositions of cell walls. In plants, the cell wall consists of cellulose, hemicellulose, pectin and proteins.
Cell wall is generally a layered structure having primary and secondary layers. Sometimes, a tertiary layer is also seen in some plants.
Primary cell wall is the first formed layer of the cell wall in young plants. It is a thin and elastic layer and is capable of growth. The other layer called the secondary layer is formed inside the primary layer towards the cell membrane.
Two plant cells are held together via a common layer between them called a middle lamella. This middle lamella is formed when the matrix of cell walls is distributed between two cells during the expansion of the primary cell wall layer.
Deposition of substances such as pectin and cellulose in a lamellar membrane between the two cells forms the middle lamella layer that glues the neighbouring cells together. This process is called intussusception. Whereas the formation of secondary walls takes place by the process called as apposition. In this process, cellulose is deposited layer by layer inside the primary cell wall to form a secondary cell wall.
Thus the correct answer is option C.

Note: Cell wall provides mechanical and structural strength to the cell. It also gives shape and rigidity to the cell. As compared to the primary layer, the secondary cell wall is rigid and thick and is mainly composed of cellulose and pectin. In between the cell wall and middle lamella, cytoplasmic connections called plasmodesmata are present for the exchange of materials from one cell to another.