Question

Outermost layer of the cell wall is
(A)Plasmalemma
(B)Secondary wall
(C)Middle lamella
(D)Primary wall

Answer 1

Hint: The wall which is present at the outermost of the cell wall acts as a glue between the different cells. . The secondary wall is not present in every cell, it is present in few cells that alter the growth of the primary wall.

Complete answer:
A cell wall is an outer structural layer present in only some types of cells, just outside the cell membrane. It is usually tough, flexible, and sometimes rigid. The cell wall is mostly composed of two layers and sometimes three layers too. The layers of the Cell wall are as follows-
- The primary cell wall is generally a flexible thin, and an extensible layer formed while the cell is growing i.e not fully developed. It is usually made up of cellulose and it is often permeable too.
-The middle lamella, a layer rich in pectins. It is the outermost layer that forms the interface between adjacent plant cells and glues them together.
-The secondary cell wall, if present, is a thick layer formed inside the primary cell wall after the cell is mature. it's not found in all cell types.

Additional Information: The cell wall is majorly composed of the cellulose microfibrils that are linked via hemicellulosic tethers to form the cellulose-hemicellulose network, which is embedded in the pectin matrix. The most common hemicellulose present in the primary cell wall is xyloglucan.
The outer part of the primary cell wall of the plant epidermis is mostly covered with the cutin and wax, forming a permeability barrier known as the plant cuticle.
Secondary cell walls contain a wide range of additional compounds that helps to enhance and modify their mechanical properties and permeability. The major polymers that make up wood (largely secondary cell walls) include cellulose, 35-50% .xylan, 20-35%, a type of hemicellulose, and lignin, 10-25%.
So, the correct answer is ‘Primary wall’.

Note: The major function performed by the cell wall are:-
-Provides definite shape, strength, rigidity.
-Protects against mechanical stress and physical shocks.
-Also helps to control cell expansion due to the intake of water.
- It also helps in preventing water loss from the cell.