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IB DP Biology- B3.3 Muscle and motility- IB Style Questions For HL Paper 2 -FA 2025

IBDP Biology HL Paper 2 -FA 2025- All Chapters

Question

The diagram below illustrates the bones and muscles that make up the human elbow joint.

(a) Identify the anatomical structures labeled X and Y.
(b) Describe how the muscles in the elbow act as an antagonistic pair.
(c) Give one reason why striated muscle cells are considered unusual.
(d) Explain how specialized cardiac muscle cells are able to initiate each heartbeat.
▶️ Answer/Explanation
Detailed solution

(a)
X: Ulna
Y: Biceps muscle

(b)
• The elbow muscles work as antagonistic partners—when one muscle contracts, the other relaxes.
• The biceps contracts to flex (bend) the arm at the elbow.
• The triceps contracts to extend (straighten) the arm at the elbow.

(c)
Striated muscle cells are multinucleated; each fiber contains multiple nuclei, which is uncommon for most cell types.

(d)
• Cardiac muscle is myogenic, meaning it contracts without requiring nerve stimulation.
• The sinoatrial (SA) node serves as the heart’s natural pacemaker, initiating each heartbeat.
• It releases an electrical impulse (action potential) that spreads through the heart muscle.
• This impulse triggers coordinated contraction of the atria followed by the ventricles.
• Heart rate can still be influenced by nervous signals and hormones such as epinephrine.

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