NCERT Class 11-Biology: Chapter – 7 Structural Organization in Animals Part 4 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Question 4:

Structural organisation in animals attains different levels as cell - organ- organ system. What is missing in this chain? Mention the significance of such an organisation.


Tissue is missing in the given chain. It should be cell – tissue — organ – organ system. A tissue is a collection of similar types of cells which together forms an organ and these organs together form an organ system. Tissues carry out a function together. Similar type of cells ensemble together to form a tissue which is organized in a such a way that it forms an organ like for example stomach. When two or more organs carry out a function of a body together then they are known as an organ system. It is known as an organ system. for example, the digestive system consists of many organs working together.

Question 5:

Stratified epithelial cells have limited role in secretion. Justify their role in our skin.


Stratified epithelial cells help in the protection of the dry skin. It has multi-layers and thus does not have much role in secretion. Stratified epithelial cells are found mainly in the parts of the body which experience mechanical, chemical, osmotic or thermal stresses. It has cell layers made up of dead cells and a protein is known as keratin which is water resistant is present. These cells are often shredded due to various stresses and thus they have a limited role in secretion

Question 6:

How does a gap junction facilitate intercellular communication?


A gap junction which is also known as a communicating junction is used for facilitating intercellular communication. It occurs by the communication between two cells bypassing of small signaling molecules from one cytoplasm to adjacent cytoplasm. These small molecules are generally passed through channels formed in the cytoplasm known as connexus. These channels are opened and closed by the pH and Ca2 + concentration.

Question 7:

Why are blood, bone and cartilage called connective tissue?


Blood, bone, and cartilage are called connective tissues because:

(i) They are involved in the binding and connection of different tissues and organs.

(ii) They help in providing structural rigidity, support and protection of the body.

(iii) They are involved in the transport and exchange of various enzymes, hormones, gases and molecules such as nitrogenous wastes, vitamins, etc.

Question 8:

Why are neurons called excitable cells? Mention special features of the membrane of the neuron?


Neurons are called excitable cells because they have an electrical potential difference i.e.. , they are in a polarised state and they quickly stimulate to an electrical disturbance. There is a change in the potential difference and depolarisation or impulses are observed from the input to output region of the neuron which passes on to the adjacent neuron.

The special features of the membrane of the neuron are:

(i) During the resting stage of the neuron, it has a sodium-potassium ion pump (i.e.. , pump) .

(ii) If there is a change in potential difference then it is spread to the entire neuron.

(iii) Stimulus in normally obtained in a place known as trigger zone in which sodium ions (i.e.. , ions) are abundantly present.

(iv) Neurons are connected to several other adjacent neurons which receive and transmit impulses to various paths collectively.

Question 9:

Why earthworm is called the friend of farmer?


Earthworm is called the friend of farmers because:

(i) Earthworm helps in the aeration of the soil. It occurs when the earthworm moves through the soil and loosens it.

(ii) The excreta which is released by the earthworm helps to increase the fertility of the soil.

(iii) The earthworms which die or decompose and add humus to the soil which increases soil fertility.

Question 10:

How do you distinguish between dorsal and ventral surface of the body of earthworm.


The dorsal surface of the body of earthworm has a dark dorsal median line than the ventral surface of the body of earthworm.

There is the presence of a single median per segment by which coelomic fluid comes out in the dorsal surface of the body of earthworms whereas ventral surface has various openings like genital openings.