Tuesday, March 6, 2012

CBSE - Class 8 - Science - Ch8 - Cell Structure and Functions

NCERT Solutions

Q1:Indicate whether the following statements are True (T) or False (F).

Answer: (a) Unicellular organisms have one-celled body. True
(b) Muscle cells are branched. False
(c) The basic living unit of an organism is an organ. False
(d) Amoeba has irregular shape. True

Q2. Make a sketch of the human nerve cell. What function do nerve cells perform?

A neuron or nerve cell is elongated in shape. Nerve cells are branched to receive and transfer messages. Nerve cells help control and coordinate the working of different parts of the body. The brain is made up of approximately 100 billion neurons.

Neurons have specialised projections called dendrites and axons. Dendrites bring information to the cell body and axons take information away from the cell body. Some nerve cells are short in length (less than a millimetre) and some are long up to 1.5 meters.

Q3. Write short notes on the following:
(a) Cytoplasm
(b) Nucleus of a cell

(a) Cytoplasm: The jelly-like substance between the nucleus and the cell membrane is called cytoplasm. Various cell organelles like ribosomes, mitochondria etc. are suspended inside cytoplasm. It helps exchange and storage of substances among cell organelles. Most of the metabolic activities occur inside cytoplasm.

(b) Nucleus of a cell: The central dense round body in the centre is called the nucleus. It is an important component of the living cell. It is generally spherical and located in the centre of the cell. It can be stained and seen easily with the help of a microscope. Nucleus is separated from the cytoplasm by a membrane called the nuclear membrane. This membrane is also porous and allows the movement of materials between the cytoplasm and the inside of the nucleus. A smaller spherical body in the nucleus, is called the nucleolus. Nucleus also contains thread-like structures called chromosomes. These carry genes and help in inheritance or transfer of
characters from the parents to the offspring.

Q4. Which part of the cell contains organelles?
Answer: Cytoplasm, gelatinous fluid inside a cell contains various organelles like mitochondria, golgi bodies, ribosomes etc.

Q5. Make sketches of animal and plant cells. State three differences between them.
Animal Cell Plant Cell
(1) Cell wall absent Cell wall present.
(2) Chloroplasts are absent. Chloroplasts are present..
(3) Centrosome is present and helps  in cell division. Centrosome is absent.
(4) Small vacuoles or absent Large vacuoles to store food.

Q6.State the difference between eukaryotes and prokaryotes.
Prokaryotes: The cells having nuclear material without nuclear membrane are termed as prokaryotic cells. The organisms with these kinds of cells are called prokaryotes. Examples are bacteria and blue green algae.

Eukaryotes: The cells having a well-organized nucleus with a nuclear membrane are termed as eukaryotic cells. The organisms with these kinds of cells are called eukaryotes. Examples are onion cells and cheek cells.

Q7. Where are chromosomes found in a cell? State their function.
Answer: Chromosomes are found in the nucleus of a cell. These thread like structures carry genes and help in inheritance or transfer of characters from the parents to the offspring.

Q8. 'Cells are the basic structural units of living organisms'. Explain.
Answer: All basic functions for the organism survival take place inside cells. As bricks assemble to make a building, cells assemble to make the body of every organism. A group of cells form a tissue which performs a specific function. A group of tissue forms an organ. And in the same way a group of organs form an organism. That is why cells are considered as the basic structural units of living organisms.

Q9. Explain why chloroplasts are found only in plant cells?
Answer: Chloroplasts are found in plant cells only because chloroplasts contain chlorophyll which is essential for photosynthesis. Chlorophyll traps cell energy and use it to manufacture food.