Cell Wall and Nucleus: Its Structure and Functions

1.0Cell Wall

It is the outermost rigid, freely permeable layer found outside the plasma membrane in all the plant cells, bacteria, blue-green algae, some protists and all fungi.

The cell wall of plants is made up of cellulose. It is a complex fibrous carbohydrate which cannot be digested by human beings and several other animals but can be digested by ruminant cattle due to presence of bacteria in their gut which secrete cellulase enzyme.

The cell wall is made up of a middle lamella, primary wall and secondary wall.

• The middle lamella is the cementing layer and is common to two cells. It is mainly composed of calcium and magnesium pectate.

• The primary wall is the first formed elastic wall and is thinner. It is formed of cellulose microfibrils and pectin polysaccharides.

• The thicker secondary wall occurs inner to the primary wall in mature and non-dividing cells only.

It is absent in animal cells. Cell walls permit the cells of plants, fungi and bacteria to withstand very dilute (hypotonic) external media without bursting. In such media the cells tend to take up water by osmosis. The cell swells, building up pressure against the cell wall. The wall exerts an equal pressure against the swollen cell. Because of their cell wall, such cells can withstand much greater changes in the surrounding medium than animal cells. When a living plant cell loses water through osmosis there is shrinkage or contraction of the contents of the cell away from the cell wall. This phenomenon is known as plasmolysis.

Shrinking of protoplasm

In plant cell → plasmolysis

In animal cell → crenation

Structure and Function

The cell wall of plant cells is formed of a fibrous polysaccharide called cellulose, while it is formed of peptidoglycan in bacteria and blue-green algae. It is formed of chitin in most of the fungi. The cell wall is a protective and supportive coat. It also provides a definite shape to the cell.

2.0Nucleus (Headquarter of The Cell)

Discovered by - Robert Brown (1831) 

“Nucleus is a double membrane bound dense protoplasmic body, which controls all cellular metabolism and encloses the genetic information of a cell”. Nucleus is considered as controller or director of the cell.

Structure

It is made up of the following four components.

(a) Nuclear membrane/ Nuclear envelope/ Karyotheca

(b) Nuclear matrix/Nucleoplasm

(c) Nucleolus

(d) Chromatin threads

(a) Nuclear envelope: Nucleus is surrounded by two membranes that separates nucleoplasm from cytoplasm. The nuclear membrane has minute pores. These are called nucleopores. Nucleopore take part in exchange of different substances between nucleoplasm and cytoplasm.

(b) Nucleoplasm: The part of protoplasm which is enclosed by a nuclear membrane is called nucleoplasm. It contains chromatin threads and nucleolus.

(c) Nucleolus: Discovered by Fontana. Usually, one nucleolus is present in each nucleus but sometimes more than one nucleolus are present. It is a store house of RNA. It is a membrane less dense body. It is the principal site for the development of ribosomal RNA’s. It is the centre for the formation of ribosomes.

(d) Chromatin threads: A darkly stained network of long and fine threads called chromatin threads is present in the nucleus. Chromatin threads are intermingled with one another forming a network called chromatin reticulum. Whenever the cell is about to divide the chromatin material gets organized into chromosomes.

Chromosomes are chemically made up of DNA and histone proteins. DNA carries all genetic information which is passed on to the next generation. The functional segments of DNA are called genes.

Nucleus is the largest cell structure. Human RBC lacks a nucleus. Mature sieve tubes in plants also lack nucleus.

Functions of Nucleus

The nucleus performs following functions:

(1) It controls all the metabolic activities of the cell.

(2) It brings about growth of the cell by directing the synthesis of structural proteins.

(3) It takes part in the formation of ribosomes.

(4) It regulates cell cycle.

(5) It contains genetic information and is concerned with the transmission of hereditary traits from one generation to another.

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