RowQ
The Vault
RowQ
The Vault
CBSE Class 11 Biology · 10 questions · 24 marks
Every gesture, from a blink to a sprint, depends on muscle fibres pulling on a rigid frame according to rules that this chapter lays out in detail, from the sliding of protein filaments at the molecular scale up to the joints and bones that make the whole system move. It also surveys the human skeleton bone by bone and closes with the disorders — from cramp to arthritis to osteoporosis — that show what happens when this finely tuned machinery breaks down.
The protein that binds calcium ions and, together with tropomyosin, controls exposure of myosin-binding sites on actin is:
Answer
Troponin is correct — it is the calcium-binding regulatory protein on the thin filament; when Ca2+ binds troponin, it causes tropomyosin to shift position and expose the myosin-binding sites on actin, allowing cross bridge formation and contraction to begin.
Which type of muscle is striated in appearance yet contracts involuntarily and rhythmically without conscious control?
Answer
Cardiac muscle is correct — it uniquely combines the striated banding pattern typical of skeletal muscle with involuntary, self-generated rhythmic contraction, a combination not found in either purely voluntary skeletal muscle or purely involuntary, non-striated smooth muscle.
The joint between the skull bones, which permits essentially no movement, is an example of a:
Answer
Fibrous joint is correct — skull sutures are held together by dense fibrous connective tissue with no joint cavity, permitting virtually no movement, unlike cartilaginous joints, which allow slight movement, and synovial joints, which allow free movement.
During muscle contraction according to the sliding filament theory, which of the following occurs?
Answer
Actin filaments slide over myosin filaments, shortening the sarcomere is correct — neither the actin nor the myosin filaments change their own length during contraction; instead they slide past one another, pulling the Z-lines closer together and shortening the sarcomere as a whole.
Assertion (A): A skeletal muscle can pull a bone but cannot push it back to its original position on its own. Reason (R): Skeletal muscles typically function in antagonistic pairs, such as the biceps and triceps, so that one muscle's contraction reverses the movement produced by its partner.
Answer
Both A and R are true and R is the correct explanation of A — muscle tissue can only generate force by contracting and pulling, never by pushing, so opposing movements at a joint require a second, antagonistic muscle to pull the bone back the other way, exactly as the triceps extends the arm that the biceps flexed.
Differentiate between the axial skeleton and the appendicular skeleton, giving examples of bones in each.
Answer
The axial skeleton forms the main central axis of the body and includes the skull, the vertebral column made up of 26 vertebrae, the sternum (breastbone) and the twelve pairs of ribs that together form the rib cage protecting the heart and lungs. The appendicular skeleton consists of the bones of the upper and lower limbs, along with the pectoral girdle (shoulder bones) and pelvic girdle (hip bones) that attach the limbs to the axial skeleton, enabling the wide range of movement seen in the arms and legs. Together the two divisions make up the total of 206 bones in the adult human skeleton.
Explain the role of calcium ions and ATP in the contraction and relaxation of a skeletal muscle fibre.
Answer
When a nerve impulse reaches a muscle fibre, it triggers the release of calcium ions from the sarcoplasmic reticulum into the sarcoplasm surrounding the myofibrils. These calcium ions bind to troponin on the thin actin filaments, causing a shift in the position of tropomyosin that exposes the myosin-binding sites previously covered on the actin filament. Myosin heads, energised by ATP hydrolysis, then attach to these exposed sites, forming cross bridges, and undergo a power stroke that pulls the actin filament inward, using further ATP to detach and reset for another cycle; this repeated cross-bridge cycling shortens the sarcomere and produces contraction. Relaxation requires calcium ions to be actively pumped back into the sarcoplasmic reticulum, a process that also consumes ATP; once calcium is removed, tropomyosin returns to its blocking position over the myosin-binding sites, cross bridges can no longer form, and the muscle fibre relaxes and returns to its resting length.
Describe the structure of a sarcomere and explain the sliding filament theory of muscle contraction in detail.
Answer
A myofibril within a skeletal muscle fibre is made up of a repeating series of units called sarcomeres, each bounded on either side by a Z-line. Within a sarcomere lie thin filaments made mainly of the protein actin, along with the regulatory proteins troponin and tropomyosin, and thick filaments made of the protein myosin, whose heads project outward to form potential cross bridges with actin. The region containing only thin filaments appears as a light band called the I-band, the region containing thick filaments (with or without overlapping thin filaments) appears as a darker A-band, and the central region of the A-band containing only thick filaments is called the H-zone. According to the sliding filament theory, contraction begins when a nerve impulse depolarises the muscle fibre membrane and triggers release of calcium ions from the sarcoplasmic reticulum. These ions bind troponin, shifting tropomyosin away from the myosin-binding sites on actin. Myosin heads, already energised by ATP hydrolysis into a high-energy configuration, now bind these exposed sites, forming cross bridges; the heads then bend, pulling the thin actin filaments toward the centre of the sarcomere in a 'power stroke.' A fresh molecule of ATP then binds the myosin head, causing it to detach from actin, after which ATP hydrolysis re-energises the head for another cycle of binding and pulling. As this cycle repeats across many cross bridges simultaneously, the thin filaments are progressively drawn further into the space between the thick filaments; the filaments themselves do not change length, but the zone of overlap increases, so the I-band and H-zone narrow while the Z-lines are pulled closer together, shortening the whole sarcomere and, when repeated across every sarcomere in every myofibril, shortening the whole muscle fibre.
Classify the joints of the human body based on the degree of movement they permit, and describe one example of each with its location.
Answer
Joints, the points where two or more bones meet, are classified according to how much movement they allow. Fibrous joints permit essentially no movement because the bones are held together directly by dense fibrous connective tissue with no cavity between them; the sutures joining the flat bones of the skull are a classic example, remaining fixed once fully ossified in an adult. Cartilaginous joints permit only slight, limited movement because the bones are connected by a pad or disc of cartilage rather than a fluid-filled cavity; the joints between successive vertebral bodies of the vertebral column, cushioned by intervertebral discs, are an example, allowing the spine as a whole to bend and twist even though movement at any single joint is small. Synovial joints permit free movement because the adjoining bone ends are separated by a fluid-filled synovial cavity enclosed in a capsule, with the fluid lubricating the joint and cartilage cushioning the bone surfaces; these are further divided by the type of movement allowed, such as the ball-and-socket joint at the hip and shoulder, which permits movement in almost every direction, the hinge joint at the knee and elbow, which permits movement largely in one plane like a door hinge, and the pivot joint between the first two neck vertebrae, which permits rotation of the head. This range, from immovable to freely movable, matches the mechanical demands placed on each region of the skeleton.
An elderly woman experiences a hip fracture after a minor fall, and a bone density scan reveals significantly reduced bone mass, attributed by her doctor to hormonal changes following menopause. A younger relative, a competitive athlete, develops sudden joint pain and swelling in the big toe after a period of high-protein dieting, and blood tests show elevated levels of a particular waste compound. (a) Name the bone condition affecting the elderly woman and explain the hormonal link her doctor described. (b) Why does reduced bone density make fractures more likely even from minor falls? (c) Name the joint condition affecting the athlete and identify the compound responsible for it. (d) Suggest one dietary or lifestyle measure that could help each individual manage their condition.
Answer
(a) The elderly woman has osteoporosis, a condition in which bone mass and density decline, making bones porous and fragile. It is strongly linked to declining oestrogen levels after menopause, since oestrogen normally helps maintain the balance between bone formation and bone breakdown, and its fall allows bone loss to outpace bone formation. (b) Bone density reflects how much mineralised, load-bearing tissue is present in the bone; when density falls, the internal structure of the bone becomes thinner and more porous, so it can withstand far less mechanical stress before cracking, meaning even a fall that a healthy bone would absorb without damage can be enough to fracture a weakened one. (c) The athlete's condition is gout, caused by the accumulation of uric acid crystals within the joint, most classically the joint of the big toe, producing sudden, intense inflammation and pain; a high-protein diet can raise uric acid production and precipitate an attack. (d) For the elderly woman, adequate dietary calcium and vitamin D along with weight-bearing exercise can help slow further bone loss; for the athlete, moderating protein intake and staying well hydrated can help reduce uric acid accumulation and lower the risk of further gout attacks.
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