Cell – Structure and Functions (Class 8, NCERT Science, Chapter 8)

NCERT/CBSE Textbook Exercise

Important Questions Only

Q.1: Indicate True(T) or False(F)

a. Unicellular organisms have one-celled body.

b. Muscle cells are branched structures.

c. The basic living structure of an organism is an organ.

d. Amoeba has irregular shape.

Ans: a)T b)T (spindle shaped structures) c)F d)T.

Q.2: Make a sketch of the human nerve dell. What function do nerve cells perform?

Ans: Functions of human nerve cell:

(i) Nerve cells receive message from different parts of body.

(ii) They further transfer these messages to brain and accordingly brain send commands for functioning of different organs of body.

Q.3: Write short notes on the following:

(a) Cytoplasm (b) Nucleus of a cell

Ans:

(a) Cytoplasm: Cytoplasm is a jelly like substance which is present between the cell membrane and the nucleus. Various other organelles of cells are present in the cytoplasm. Cytoplasm is made up of chemical substances like carbohydrates, proteins and water. These chemical substances are present in cells of all types and sizes. Cytoplasm contains many important tiny substances called Organelles.

(b) Nucleus of a cell: Nucleus is the master of the cell. It commands all the functioning of the cell. It is generally located in the center of the cell and is spherical in shape. A membrane called nuclear membrane separates it from cytoplasm. It contains the genetic material DNA and RNA in it. This porous membrane allows the transfer of material in the nucleus and cytoplasm. Nucleus contains a dense body called Nucleolus which actually contains Chromosomes, the genetic material.

Q.4: Which part of the cell contains organelles?

Ans: Cytoplasm.

Q.5: Make sketches of animal and plant cells. State three differences between them.

Ans: See answer of Q.No.6 (HTTP://CBSEKEY.COMadditional long type questions) below.

Q.6: State a difference between eukaryotes and prokaryotes.

Ans: Prokaryotes do not have a well designed nuclear membrane while, eukaryotes have a well designed nuclear membrane.

Q.7: Where are the chromosomes found in cell? State their functions.

Ans: Chromosomes are found in the nucleus of a cell. Their function is to carry characteristic features of parent cells to the daughter cell means, from parent to offspring.

Q.8: ‘Cells are the basic structural units of living organism’. Explain.

Ans: In Biology, the basic unit of which all living things are composed is known as ‘cell’. The ‘cell’ is the smallest structural unit of living matter that is capable of functioning independently. A single cell can be a complete organism in itself, as in bacteria and protozoans. A unicellular organism also captures and digests food, respires, excretes, grows, and reproduces. Similar functions in multi-cellular organisms are carried out by groups of specialized cells which are organized into tissues and organs such as, the higher plants and animals. Hence, ‘cell’ is known as the basic structural and functional unit of life.

Q.9: Explain why chloroplasts are found only in plant cells.

Ans: Chloroplasts are found only in plant cells because they are required for photosynthesis.

HTTP://CBSEKEY.COM(Additional Important Questions)

Short Questions with their Answers

Q.1: Define Prokaryotes and Eukaryotes.

Ans:

Prokaryote: The cells having nuclear material without nuclear membrane are termed as prokaryotic cells. An organism with these kinds of cells is called a Prokaryote e.g. Bacteria and Blue Green Algae.

Eukaryote: The cells having well organized nucleus with a nuclear membrane are termed as eukaryotic cells. All organisms other than Bacteria and Blue Green Algae are Eukaryotes.

Q.2: What is Protoplasm?

Ans: The entire content of a living cell is known as protoplasm. It includes cytoplasm and the nucleus.

Q.3: Give three examples of unicellular organisms.

Ans: Amoeba, Paramecium and Chlamydomonas.

Q.4: Name the cell organelle which is found only in plant cell.

Ans: Plastids.

Q.5: What is the smallest cell size?

Ans: The smallest cell is 0.1 to 0.5 micrometer in Bacteria.

Q.6: What is the largest cell?

Ans: The largest cell is 170mm x 130mm, is the egg of an Ostrich.

Long Questions with their Answers

Q.1: Why cell is known as structural and functional unit of life?

Ans: Refer to the answer of Q.No.8 (NCERT Textbook Exercise) above.

Q.2: Why are the mitochondria known as the power house of the cell?

Ans: Mitochondria are rod shaped and very minute bodies present in cytoplasm. They are concerned with release of energy from food during respiration. Because of this they are often referred to as the power house of the cell.

Q.3: Why plasma membrane is called a selectively permeable membrane?

Ans: A cell bound by a semi-permeable membrane called plasma membrane that enables it to exchange only certain materials with its surroundings. Plasma membrane permits the entry and exit of some material in the cell. It also prevents movement of some other material. Therefore, ‘Plasma Membrane’ is called as ‘selectively permeable membrane’.

Fig: (a) Spherical red blood cells (b) Spindle shaped muscle cells (c) Long branched nerve cells

Q.4: Write a short note on the ‘shape of cells’ or ‘cell shape’.

Ans: Cells exhibit a variety of shapes. Some cells have a definite shape while some keep on changing its shape. For example- White Blood Cell (WBC) present in our bodies, Amoeba continuously changes their shape. However, most of the cells maintain a constant shape and the different shapes are related to their specific functions. For example- blood cells are spherical, muscle cells have spindle shape, and nerve cells are long and branched. It is mainly the cell membrane which provides the shape to the cells of plants and animals. [See figure]

Q.5: Write short notes on the following:

(a) Gene

(b) Chromosomes

(c) Organelles

(d) Vacuole

(e) Tissues

(f) Plastids.

Ans:

(a) Gene: Gene is a unit of inheritance in living organisms. Nucleus contains thread-like structures called chromosomes which carry genes in them. Genes are composed of DNA (Deoxyribonucleic acid), except in some viruses. They achieved their effects by directing the synthesis of proteins.

(b) Chromosome: These are the microscopic thread-like parts present in the nucleus of a cell that carries hereditary information in the form of genes.

(c) Organelles: The various tiny components of a cell present in the cytoplasm are known as organelles. These are – Mitochondria, Golgi bodies, Ribosome etc.

(d) Vacuole: A vacuole is a clear space generally stored in the cytoplasm. Big size vacuole is found in the plant cell whereas; in animal cells they are very small. In protozoa, vacuoles are cytoplasmic organs performing many functions such as digestion, excretion etc.

(e) Tissues: Each organ is further made up of smaller parts called tissues. A tissue is a group of similar cells performing a specific function.

(f) Plastids: Plastids are found in the plant cells but are absent in animal cells. They are found scattered in the cytoplasm of the leaf cells. Plastids are of three types -

(i) Chloroplast

(ii) Leucoplast and

(iii) Chromoplast

Among these three types chloroplast is the most important as they contain chlorophyll which is a necessary element for photosynthesis. Because of this reason plastids are also said as ‘Kitchen of plant cells’.

Q.6: Why Plastids are said as ‘Kitchen of plant cells’?

Ans: See the answer of the Q.No.5 (f).

Q.6: Differentiate between Plant cell and animal cell.

Ans: The differentiation between plant and animal cells is given in the following table:

Fig: Structures of Animal and Plant Cells

Animal Cell Plant Cell

(a) Cell wall is absent.

(b) These cells do not contain Chloroplasts.

(c) Chromosomes are present in the nucleus.

(d) Vacuoles are less and of smaller size. (a) Plant cells have rigid cell walls.

(b) Chloroplasts are present.

(c) Chromosomes are present.

(d) Vacuoles are larger and more in numbers.

Q.6: Write a short note on ‘Nucleus of a cell’?

Ans: Refer to the answer of Q.No.3 (b) (NCERT Textbook Exercise) above.

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