In each of the following cases (A, B, C and D), choose the correct option as given below: Class 9
In each of the following cases (A, B, C and D), choose the correct option as given below: Class 9
Question 1.
In each of the following cases (A, B, C and D), choose the correct option as given below: Class 9
(i) Both (A) and (R) are true, and (R) is the correct explanation of (A).
(ii) Both (A) and (R) are true, but (R) is not the correct explanation of (A).
(iii) (A) is true, but (R) is false.
(iv) (A) is false, but (R) is true.
A. Assertion: Epithelium is well-suited for gas exchange in the lungs.
Reason: It consists of multiple layers of tall cells that slow down diffusion.
Answer:
(iii) (A) is true, but (R) is false.
The assertion is correct - the epithelium lining the lungs (alveoli) is indeed well-suited for gas exchange. However, the reason given is incorrect. The epithelium in the lungs actually consists of a single layer of very thin, flat cell's .(squamous epithelium), NOT multiple layers of tall cells. This single thin layer allows rapid diffusion of oxygen and carbon dioxide between air and blood. Multiple layers would actually slow down gas exchange.
B. Assertion: Cardiac muscle can contract continuously without fatigue.
Reason: Cardiac muscle cells have a high number of mitochondria and an abundant blood supply.
Answer:
(i) Both (A) and (R) are true, and (R) is the correct explanation of (A).
Cardiac muscles are found only in the heart and they beat tirelessly and rhythmically throughout life without ever getting tired. This is because cardiac muscle cells have an exceptionally high number of mitochondria - these are the 'energy factories' of the cell that produce ATP (energy). Additionally, the heart muscle has a very rich blood supply (coronary arteries), ensuring constant delivery of oxygen and nutrients needed for energy production. Together, these features allow cardiac muscles to work continuously.
C. Assertion: Tendons connect bone to bone and allow joint movement.
Reason: Tendons are made of tough connective tissue that transmits force from muscle to bone.
Answer:
(iv) (A) is false, but (R) is true.
The assertion is incorrect - tendons do NOT connect bone to bone. Tendons connect muscle to bone. It is ligaments that connect bone to bone and help prevent dislocation at joints. The reason, however, is correct - tendons are made of tough connective tissue and their function is to transmit the pulling force generated by muscle contraction to the bone, resulting in movement at a joint.
D. Assertion: In a hinge joint, movement occurs primarily in one plane.
Reason: The bone ends are shaped to allow sliding in all directions.
Answer:
(iii) (A) is true, but (R) is false.
The assertion is correct - hinge joints like the elbow and knee allow movement in one plane only (bending and straightening). However, the reason is incorrect. The reason describes a ball and socket joint, not a hinge joint. In hinge joints, the bone ends are shaped like a hinge (interlocking ridges and grooves) that restrict movement to one direction only. It is the ball and socket joint where the rounded top of one bone fits into a hollow, allowing movement in multiple directions.