NCERT Solutions for Class 11 Biology Chapter 20: Locomotion and Movement

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1. Draw the diagram of a sarcomere of skeletal muscle showing different regions.

Ans. The diagram of a sarcomere of skeletal muscle is as below:

2. Define the sliding filament theory of muscle contraction.

Ans. According to sliding filament theory, muscle’s fibre contraction occurs due to the sliding of the thin actin filaments over the thick myosin filaments.

3. Describe the important steps in muscle contraction.

Ans. Mechanism of muscle contraction:

Step-1.Depolarization of Sarcolemma: At this stage, the Central Nervous System sends a signal through a motor neuron which arrives at the junction of muscle and neuron i.e., neuromuscular junction. When the signal arrives, a neurotransmitter acetylcholine is triggered and the action potential in the sarcolemma is set by this release.

Step-2. Release of Ca++ ions: The sarcolemma transmits the action potential to the sarcoplasmic reticulum in order to release the calcium ions in the sarcoplasm.

Step-3. Conformational changes occur in the Actin filaments: The released calcium ions tend to bind to the troponin and tropomyosin located on the active filaments. This causes a modification in the three-dimensional structure of the actin-troponin-tropomyosin complex. The active site for myosin which is located on the actin filament is exposed.

Step-4. Activation of Myosin heads: The calcium ions release causes the myosin heads to get activated which in turn causes the release of energy in the form of ATP. Hydrolysis of ATP releases energy which causes the binding of myosin heads to the active sites found on the actin filaments hence forming a crossbridge.

Step-5. Actin Filaments slide over Myosin: The myosin head rotates due to the formation of a cross-bridge, pulling the actin filaments to the centre of the A-band which is the H-zone. The Z-line which is attached to the actin filaments is pulled inwards too. Contraction of sarcomere occurs when the actin filaments are pulled in the opposite ends. During the process of contraction, the I-band shortens whereas the A-band maintains its length causing the muscles to contract.

4. Write true or false. If false, change the statement so that it is true.

(a) Actin is present in thin filament.

(b) H-zone of striated muscle fibre represents both thick and thin filaments.

(c) Human skeleton has 206 bones.

(d) There are 11 pairs of ribs in man.

(e) Sternum is present on the ventral side of the body.

Ans. (a) True.

(b) False.

Corrected statement: H-Zone of striated muscle fibres represents only thick filaments.

(d) False.

Corrected statement: There are 12 pairs of ribs in man.

(e) True.

5. Write the difference between:

(a) Actin and Myosin

(b) Red and White muscles

(c) Pectoral and Pelvic girdle

Ans. (a) Difference between Actin and Myosin:

Actin	Myosin
This is made up of thin and lighter protein called actin.	This is made up of thick and heavier protein called myosin.
Shorter than myosin.	Longer than actin.
It consists of three proteins: Actin, tropomyosin and troponin.	It consists of two proteins: myosin and meromyosin.
It is found in both A and I band of sarcomere.	It is found only in A band of sarcomere.
Cross bridges are absent, hence have smooth surface.	Cross bridges are present, hence have rough surface.
It slides into Z-zone during contraction.	It does not slide during contraction.

(b) Difference between Red and White muscles:

Red muscles	White muscles
They are thinner.	They are thicker.
The numbers of mitochondria are more and glycogen content is low.	The numbers of mitochondria are less and glycogen granules are abundant.
Due to the presence of abundant myoglobin pigment, they are dark.	Due to the presence of very few myoglobin pigments, they are white.
It can store oxygen as oxymyoglobin.	It can’t store oxygen.
Get energy for contraction by aerobic respiration.	Get energy for contraction mainly by anaerobic respiration.
Not get fatigued with work.	Get fatigued with work.
Can accumulate little amount of lactic acid.	Accumulates lactic acid when gets strained due to excessive work.

Pectoral girdle	Pelvic girdle
It has two bones: clavicle and scapula.	It has three bones: Ilium, ischium and pubis.
Glenoid cavity found.	Acetabulum cavity found.
It’s positioned at the shoulder region.	Its position is in the hip region.
It’s articulated to forelimb bones.	It’s articulated to hindlimb bones.
Its function is to provide articulation to arm bones.	Its function is to provide articulation to leg bones.

6. Match column I with column II.

(a) Smooth muscle	(i) Myoglobin
(b) Tropomyosin	(ii) Thin filament
(c) Red muscle	(iii) Sutures
(d) Skull	(iv) Involuntary

7. What are the different types of movements exhibited by the cells of human body?

Ans. The different types of movements exhibited by the cells of the human body are as follows:

(a) Amoeboid movement: The leucocytes present in the blood show amoeboid movement. During tissue damage, these blood cells move in an amoeboid manner (by forming temporary pseudopodia) from the circulatory system towards the site of injury to initiate an immune response. Cytoskeletal elements like microfilaments are also involved in the amoeboid movement.

(b) Ciliary movement: The reproductive cells such as sperms and ova show ciliary movement. The passage of the ova through the fallopian tube towards the uterus is facilitated by this movement. Cilia in the trachea helps in removing dust particles and foreign substances inhaled along with atmospheric air.

(c) Muscular movement: The muscle cells of the human body show muscular movement. These contract and relax to bring about the movement. E.g., Movements of limbs, tongue and jaws are examples of muscular movements.

8. How do you distinguish between a skeletal muscle and a cardiac muscle?

Ans. Difference between a Skeletal muscle and a Cardiac muscle:

Skeletal muscle	Cardiac muscle
It is attached to the bone.	It's from the heart.
The cells of skeletal muscles are unbranched.	The cells of cardiac muscles are branched.
Alternate light and dark bands are present.	Faint bands are present.
Its size is longer than cardiac muscle.	Comparatively smaller than skeletal muscle.
It is controlled by the somatic nervous system.	It is controlled by the autonomic nervous system.
Many nuclei are found in the cells.	Single nucleus is found in the cell.
Endomysium is less dense.	Endomysium is more dense than skeletal muscle.