NCERT Solutions for Class 9 Science The Fundamental of Life – Structure of Cell
NCERT Class 9 Science Notes: The cell is generally composed of three basic components:
(i) Cell wall and cell membrane
- a) Cell membrane or Plasma membrane:
The plasma membrane is the covering of the cell that separates the contents of the cell from its external environment.
It is a living part of the cell and is present in cells of plants, animals, and microorganisms.
It is very thin, delicate, elastic and selectively permeable membrane.
It is composed of lipid and protein.
As it is a selectively permeable membrane, it allows the flow of limited substances in and out of the cell.
2. b) Cell wall:
The cell wall is non-living, thick and freely permeable covering made up of cellulose.
It is present in eukaryotic plant cells and in prokaryotic cells.
- It determines the shape and rigidity of the plant cell.
- It protects the plasma membrane.
- It prevents desiccation or dryness in the cell.
Nucleus is dense and spherical organelle.
The nucleus is bounded by two membranes, both forming nuclear envelope. Nuclear envelope contains many pores known as nuclear pores.
The fluid which presents inside the nucleus is called nucleoplasm.
The nucleus contains chromosomes and chromosomes contain genes which are the centers of genetic information.
- The nucleus controls all the metabolic activities of the cell.
- It regulates the cell cycle.
- The nucleus is the storehouse of genes.It is concerned with the transmission of hereditary traits from the parent to offspring.
It is a jelly-like, viscous, colorless semi-fluid substance that occurs between the plasma membrane and the nuclear membrane.
The aqueous ground substance of cytoplasm is called cytosol that contains a variety of cell organelles and other insoluble waste products and storage products, like starch, glycogen, lipid, etc.
• Protoplasm acts as a store of vital chemicals like amino acids, proteins, sugars, vitamins, etc.
• It is the site of certain metabolic reactions, like glycolysis, synthesis of fatty acids, nucleotides, etc.
Inside the cell, there are different parts performing different activities to keep the cell alive and functional. These part are called Cell Organelles. They are explained below:
- Golgi Apparatus:
Golgi apparatus consists of a set of membrane-bound, fluid-filled vesicles, vacuoles and flattened cisternae (closed sacks).
Cisternae are usually arranged parallel to each other.
- Its main function is to store, modify, package and dispatch the substances.
- It is also involved in the synthesis of the cell wall, plasma membrane, and
- Endoplasmic Reticulum:
It is a membranous network of tube-like structures extending from nuclear membrane to plasma membrane.
It is absent in prokaryotic cells and matured RBCs of mammals.
There are two types of endoplasmic reticulum:
(i) Rough Endoplasmic Reticulum (RER): Here ribosomes are present on the surface for the synthesis of proteins.
(ii) Smooth Endoplasmic Reticulum (SER): Here ribosomes are absent and is meant for secreting lipids.
- It gives internal support to the cell.
- It helps in transport of various substances from nuclear membrane to the plasma membrane or vice versa.
- RER helps in synthesis and transportation of proteins.
These are extremely small, dense and spherical bodies which occur freely in the matrix (cytosol) or remain attached to the endoplasmic reticulum.
These are made up of ribonucleic acid (RNA) and proteins.
They play a major role in the synthesis of proteins.
They are small rod-shaped organelles.
It is a double membrane structure with the outer membrane being smooth and porous whereas inner membrane is thrown into a number of folds called cristae.
They contain their own DNA and ribosomes.
They are absent in bacteria and red blood cells of mammals.
- They are the sites of cellular respiration; hence provide energy for the vital activities of living cells.
- They store energy releases during reactions, in the form of ATP (Energy currency of the cell). Therefore, they are also called ‘powerhouse’ of the cell.
Plastids are present in most of the plant cells and absent in animal cells.
They are usually spherical or discoidal in shaped and double membrane-bound organelles.
They also have their own DNA and ribosomes.
Plastids are of three types:
(a) Chloroplasts: These are the green colored plastids containing chlorophyll. Chloroplasts aid in the manufactured food by the process of photosynthesis.
(b) Chromoplasts: These are the colorful plastids (except green color).
(c) Leucoplasts: These are the colorless plastids.
- Chloroplasts trap solar energy and utilize it to manufacture food for the plant.
- Chromoplasts impart various colors to flowers to attract insects for pollination.
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Difference between Animal Cell and Plant Cell:
Structure of Plant cell and Animal cell:
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