CLASS 8 CELL:-STRUCTURE AND FUNCTIONS

 NOTES-

Cells

Cells are the fundamental structural units of living organisms and the basic units of life.

Robert Hooke discovered the cell in the year 1665 from a thin section of cork under a self-made microscope.

Prokaryotic & Eukaryotic cells

Prokaryotic – Size: generally small ( 1-10 µm) Where 1 µm = 10 $-6$m.

• The nucleus is not well defined and known as a nucleoid.
• Single, circular chromosome.
• Membrane-bound organelles absent. Eg: Bacterial cell

Eukaryotic – Size: generally large ( 5-100 µm).

• The nucleus is well defined and surrounded by a nuclear membrane.
• Possess more than one and linear chromosome.
• Membrane-bound cell organelles are present. Eg: Human cell.

Cell walls in plants

• A cell wall is an additional covering over the cell membrane in plant cells.
• It gives shape and rigidity to these cells.
• It gives a rigid structure to plant cells and that is why they can withstand the stress of high-velocity wind, rain, high temperature etc.

Plastids

• Plant cells have tiny, coloured structures called plastids.
• Plastids carry pigments of various colours like orange, red, green and even colourless.
• The plastids, which carry green-coloured pigments i.e., chlorophyll are called chloroplasts.

Plants vs Animals

Comparison of plant and animals cells

Cell Organelle	Plant Cell	Animal Cell
Cell wall	Yes	No
Cell membrane	Yes	Yes
Cytoplasm	Yes	Yes
Nucleus	Yes	Yes
Chloroplast	Yes	No
Vacuole	Yes	YeS

NCERT EXCERCISE QUESTION AND ANSWERS

Question 1.
Indicate whether the following statements are True (T) or False (F).
(a) Unicellular organisms have a one-celled body.
(b) Muscle cells are branched.
(c) The basic living unit of an organism is an organ.
(d) Amoeba has an irregular shape.
Answer:
(a) True
(b) True
(c) False
(d) True

Question 2.
Make a sketch of the human nerve cell. What function do nerve cells perform?
Answer:
Function of nerve cells: The function of the nerve cell is to receive and transfer messages, it helps to control and coordinate the working of different parts of the body.

Question 3.Write short notes on the following.

(a) Cytoplasm
(b) Nucleus of a cell
Answer:
(a) Cytoplasm: The jelly-like substance found between the nucleus and the cell membrane is called cytoplasm. It is made up of basic elements like C, H, O, N. Various other components or organelles, like mitochondria, Golgi bodies, ribosomes, etc., of cells are present in the cytoplasm.

(b) Nucleus of a cell: Nucleus of a cell is an important component of the living cell. It is located at the centre of the cell. It is separated from the cytoplasm by a membrane called nuclear membrane. It contains genetic material.

Question 4.
Which part of the cell contains organelles?
Answer:
Cytoplasm

Make sketches of animal and plant cells. State three differences between them.
Answer:

Plant cells	Animal cells
(i) The outermost covering is a cell wall and it is made of cellulose.	(i) The outermost covering of animal cell is the plasma membrane.
(ii) Plastids are present in plant cells.	(ii) Plastids are absent in animal cells.
(iii) Large vacuoles are present in plant cells.	(iii) No or very small vacuoles are present in animal cells.
(iv) It lacks centrosomes and lysosomes.	(iv) They have centrosomes or lysosomes.

Question 6.
State the difference between eukaryotes and prokaryotes.
Solution:

Eukaryotes	Prokaryotes
(i) Eukaryotes possess membrane-bound organ­elles.	(i) Prokaryotes lack membrane-bound organelles.
(ii) Nucleus of the cell has nuclear membrane. Example: higher plants and animals.	(ii) Nucleus is not bounded by membrane. Example: bacteria and blue-green algae.

Question 7.
Where are chromosomes found in a cell? State their function.
Answer:
Chromosomes are present in the nucleus. The functions of chromosomes is to carry genes on them and to transfer the character from parents to the next generation.

Question 8.Cells are the basic structural units of living organisms.’ Explain.

Answer:
Different cells combine to form tissues and tissues combine to form organs. Similarly, organs combine to form body. Thus they are termed as the basic structural unit of every living organism.

Explain why chloroplast are found only in plant cells?
Answer:
Chloroplasts are plastids required for the food making process, called photosynthesis, and thus they are only present in plant cells.

Question 10.
Complete the crossword with the help of clues given below.
Across
1. This is necessary for photosynthesis.
3. Term for component present in the cytoplasm.
6. The living substance in the cell.
8. Units of inheritance present on the chromosomes.
Down
1. Green plastids.
2. Formed by collection of tissues.
4. It separates the contents of the cell from the surrounding medium.
5. Empty structure in the cytoplasm.
7. A group of cells.
Solution: