notesof ch5 class 9
science
Answers
Cell
It is the structural and functional unit of life.
Cell is termed as the structural unit of life as it provides structure to our body.
Cell is considered as the functional unit of life as all the functions of the body take place at cell level.
Discovery of cell:Discovered by Robert Hooke in 1665.
Robert Brown in 1831 discovered the nucleus in the cell.
Cell Theory:
Cell theory states that:
All living organisms are composed of cells.
Cell is the fundamental unit of life.
All new cells come from pre-existing cells.
Types of Organisms on the Basis of Number of Cells
There are two kinds of organisms on the basis of cells:
(i) Unicellular Organisms: The organisms that are made up of single cell and may constitute a whole organism, are named as unicellular organisms.For example: Amoeba, Paramecium, bacteria, etc.
(ii) Multicellular Organisms: The organisms which are composed of a collection of cells that assume function in a coordinated manner, with different cells specialized to perform particular tasks in the body, are named as multicellular organisms.
For example: Plants, human beings, animals, etc.
Shape and Size of Cells
Cells vary in shape and size. They may be oval, spherical, rectangular, spindle shaped, or totally irregular like the nerve cell.
The size of cell also varies in different organisms. Most of the cells are microscopic in size like red blood cells (RBC) while some cells are fairly large like nerve cells.
Types of Cells
The cells can be categorized in two types:
1. Prokaryotic Cell 2. Eukaryotic Cell1. Prokaryotic cell
Prokaryotic cells are cells in which true nucleus is absent. They are primitive and incomplete cells. Prokaryotes are always unicellular organisms. For example, archaebacteria, bacteria, blue green algae are all prokaryotes.
structure of prokaryotic cell
2. Eukaryotic Cell
Eukaryobc cells are the cells in which true nucleus is present. They are advanced and complete cells. Eukaryotes include all living organisms (both unicellular and multicellular organisms) except bactera and blue green algae.Difference Between Prokaryotic and Eukaryotic Cells:
S. No.
Prokaryotic cell
Eukaryotic cell
1.
Size of cell is generally small (1-10 mm).
Size of cell is generally large (5-100 mm).
2.
Nucleus is absent.
Nucleus is present.
3.
It contains single chromosome.
It contains more than one chromosome.
4.
Nucleolus is absent.
Nucleolus is present.
5.
Memrane bound cell organelles are absent.
Memrane bound cell organelles such as mitochondria, plastids, endoplasmic reticulum, golgi apparatus, lysosomes, etc., are present.
6.
Cell division takes place by fission or budding.
Cell division takes place by mitotic or meiotic cell division.
Structure of Cell
Cell is generally composed of three basic components:sternae (closed sacks).
Cisternae are usually arranged parallel to each other.
Functions:
Its main function is to store, modify, package and dispatch the substances.
It is also involved in the synthesis of cell wall, plasma membrane and lysosomes.
2. 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.
Functions:
It gives internal support to cell.
It helps in transport of various substances from nuclear membrane to plasma membrane or vice versa.
RER helps in synthesis and transportation of proteins.
SER helps in synthesis and transportation of lipids.
3. Ribosomes:
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.
Function:
They play a major role in the synthesis of proteins.
4. Mitochondria:
They are small rod-shaped organelles.
It is a double membrane structure with outer membrane being smooth and porous whereas inner membrane being thrown into a number of folds called cristae.