Home / IB DP Biology- B3.3 Muscle and motility- IB Style Questions For HL Paper 1A

IB DP Biology- B3.3 Muscle and motility- IB Style Questions For HL Paper 1A -FA 2025

IBDP Biology HL Paper 1 -FA 2025- All Chapters

Question

The diagram shows a sarcomere in its contracted state. Which labelled region changes in length when the muscle relaxes?

▶️ Answer/Explanation
During relaxation, the H zone and I band increase in length because the thin filaments slide back outward, reducing their overlap with thick filaments. The A band remains constant. The labelled part (C) represents the region that changes length most noticeably with contraction and relaxation.
✅ Answer: (C)

Question

A titinopathy is a muscle disorder caused by abnormalities in titin. What could be a consequence of abnormal titin in muscles?

(A) Muscles would detach from tendons.
(B) Muscles would stretch too much during relaxation.
(C) Muscles would have less ATP for contraction.
(D) Antagonistic muscles would contract at the same time.
▶️ Answer/Explanation

Correct Answer: \( \boxed{\mathbf{B}} \)

Titin is a large elastic protein that acts like a molecular spring within the sarcomere. It helps muscles return to their resting length after contraction and prevents excessive stretching. If titin is abnormal, muscles lose this elasticity and stretch too much during relaxation.

Question

The electron micrograph displays a transverse section of a skeletal muscle fiber. Where is the specialized endoplasmic reticulum (sarcoplasmic reticulum) likely located?

▶️Answer/Explanation

Answer: C

Explanation:

Let’s analyze this skeletal muscle fiber image:

  • Label A: Looks like a small vesicle or tubule near the edge could be part of SR or something else.
  • Label B: The thin lines or membranes near A maybe tubules but not the main bulk.
  • Label C: The large, network-like mesh surrounding the darker, denser structures inside (myofibrils).
  • Label D: Large dark, dense blob definitely the nucleus.

Why Label C is the sarcoplasmic reticulum?

  • The sarcoplasmic reticulum (SR) forms an elaborate membranous network wrapping around the myofibrils.
  • It appears as a mesh or web-like structure that surrounds the contractile elements.
  • This network is visible as the light-stained areas wrapping the darker myofibrils in the image.
  • So, C fits perfectly for the SR.

To clear confusion:

  • A and B are likely small tubules or vesicles but not the full SR structure.
  • C is the major network of tubules characteristic of the SR, which stores calcium needed for muscle contraction.
  • D is the nucleus, obvious by its size and density
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