Class IX, BIOLOGY, "Structure of Eukaryotic Cell"

Structure of Cell

There are two types of cells:

1. Prokaryotic cell
2. Eukaryotic Cell

Prokaryotic cell lacks a membrane bound nucleus and membraned organelles e.g. bacterial cell while eukaryotic cell has a membrane bound nucleus and membraned organelles e.g. cells of plants and animals.
With the help of light microscope and electron microscope, a typical Eukaryotic cell shows the following structural details.

1. Cell Wall
2. Cell Membrane
3. Nucleus
4. Cytoplasm

1. Cell Wall

It is the outer most boundary of plant cells. It is rigid and non-living. It is chemically composed of Cellulose. The cell wall of fungi is made up of Chitin. The walls of some cells are thick and walls of some cells are thin. For example, in plants, xylem vessel elements and tracheids (which transport water and minerals) have thick walls whereas as parenchyma cells (which store water and food) have thin walls. The primary layer of cell wall is known as primary walls which are further strengthened by an additional layer called secondary wall especially in xylem vessels. Secondary wall is thicker than the primary wall. Electron microscope studies reveal that cellulose fibers in primary and secondary walls have a criss cross arrangement.

Functions
Cell wall provides a definite shape, rigidity, protection and support to plant cell.

2. Cell Membrane

It is a thin membrane which is also called Plasma membrane. It is present in cells of all plants and animals. It is outer most boundary of animal cell while in plant cells; it is present inner to cell wall. Both nucleus and cytoplasm are surrounded by cell membrane. According to fluid mosaic model, cell membrane is composed of two layers of lipids in which protein molecules are partially or completely embedded.

Functions
Cell membrane is selectively permeable membrane. It means that it allows some things to pass through easily while some not. Thus, it controls the movement of material inside or outside the cell.

3. Nucleus

It is most important and distinct part of the cell. It is present in center of the animal cell while in plant cell it is pushed on one side due to large central vacuole. It is also surrounded by a membrane which is called membrane. Under microscope, it to be doubled and nuclear electron appears porous.
1. The number of chromosomes is fixed for each species. This number is called diploid number (2n). E.g human’s cell has 46 chromosomes, cell of Radish has 18 chromosomes, and cell of union has 16 chromosomes.
2. Chromosomes are composed of protein and DNA.
3. In the nucleolus (plural, Nucleoli) ribosomal RNA is formed which helps in the formation of ribosome.

4. Cytoplasm

It is viscous opaque substance. It is present between nuclear membrane and cell membrane. In a living cell, many types of organelle of different sizes and shapes are found. It contains many insoluble granules of storage substances. There are also present organic compounds like carbohydrates, proteins, lipids (fats), enzymes and inorganic compounds like water and salts.

Functions
Cytoplasm provides chemicals, site and environment for different biochemical reactions.