BOYS’ HIGH SCHOOL AND COLLEGE

CLASS – IX

BIOLOGY

CHAPTER-2

CELL : THE UNIT OF LIFE

Explanation

1) Cell: Every single living body is made up of cells. They make the structure of the body, both internal and external. When similar cells combine, they form tissues and tissues combine to make organs (structural unit). They are also responsible for doing the various functions that the body performs. Organs have different functions to do in the body. So when any organ does its job, actually the cells which have made that organ are doing it (functional unit).

2) Cell Theory: It explains the various facts which have been given by different scientists, in a collective manner. It tells about the basics of a cell. Three points are given which actually sum up the entire cell theory. The study was made on both animal and plant cells. It explains that-

(a) Cell is the structural and functional unit of any living body. (b) It does all the functions in any living body. (c) Every cell is born from a pre-existing cell by cell division.

3) A cell basically has three essential parts – (a) Cell membrane (b) Nucleus

(c) Cytoplasm, which are found in both plant and animal cells. Cells can be of varied shapes and sizes but these three parts are common to all.

(a) Cell Membrane: The outermost membrane in a plant cell is cell wall which is completely or freely permeable (allows all the substances to pass through).It is made up of cellulose. It gives shape and strength to the cell. It is absent in animal cell. But, cell membrane is common in both plant and animal cell. It is semi-permeable or selectively permeable (allows certain substances to pass through and prevents others). It is made up of lipids and proteins. It surrounds the cytoplasm and holds all the cellular material inside the cell.

(b) Nucleus : It is a spherical mass found in most of the cells. Alike cell, it is also surrounded by a membrane called nuclear membrane and filled with a dense fluid called nucleoplasm. It has a nucleolus in its centre. Thread like structures called chromatin fibres lie in the nucleoplasm. These fibres become thick to form chromosomes, which take part in cell division. Genes, the hereditary units, are located on these chromosomes. The number of chromosomes is fixed in each species.

(c) Cytoplasm: It is a semi-liquid filled in between the cell membrane and nuclear membrane. All the cell organelles like mitochondria, endoplasmic reticulum, ribosomes, golgi bodies, plastids, lysosomes etc, lie in it. It is always moving. Many chemical reactions including the first phase of cellular respiration, take place in it.

4) Mitochondria: They are organelles which are found both in plant and animal cells. They are responsible for releasing energy in the form of ATP which is an energy rich compound. In the process of cell respiration, the first phase goes on in the cytoplasm whereas the second phase goes on in the mitochondria. The cells are packed with many mitochondria. The energy produced is used in all the functions that the body does. Therefore, they are called 'powerhouses of the cell'.

5) Protoplasm: It is the sum of all the living substance found in the cell. It includes both cytoplasm and nucleoplasm and all that is in them. Physically, under the microscope, the protoplasm seems to be a clear homogenous, transluscent, colourless fluid. But chemically,cell to cell, protoplasm is different. It has certain common elements which combine to form different compounds in different ratios. Other elements are not commonly seen in every cell. They are called Trace elements. Protoplasm is very unstable and as soon as it is taken out from the cell, it decomposes. So it is impossible to do the exact chemical analysis of the protoplasm.

Work to be done in the notebook:-

All the questions encircled on page 19 to be answered in the notebook.

Extra questions to be answered in the notebook-

1) State any five activities of the cells in a body. 2) Draw well labeled diagrams of animal cell and plant cell.

Note:-

a) Learn the differences given in table 2.2 and 2.3. b) Practice the questions which have not been marked on page 18 and 19.