IB DP Biology- B3.3 Muscle and motility- IB Style Questions For HL Paper 1A -FA 2025
▶️ Answer/Explanation
✅ Answer: (C)
Question
▶️Answer/Explanation
Answer: C. They produce lactate in order to quickly supply the energy for muscle contraction.
During intense exercise, when oxygen is limited, human muscle cells switch from aerobic respiration to anaerobic respiration, specifically lactic acid fermentation. This allows them to continue producing ATP, though in smaller amounts, without using oxygen. The by-product of this process is lactate (lactic acid).
Option Evaluation:
A. Incorrect – Glycolysis is not replaced by lactic fermentation. In fact, glycolysis continues and is followed by lactic acid fermentation when oxygen is scarce. So, fermentation does not replace glycolysis; it happens after glycolysis when there is no oxygen.
B. Incorrect – Muscle cells don’t intentionally reduce ATP production to lower oxygen use. Instead, they switch to a method (anaerobic respiration) that produces less ATP because it’s the only available option under low oxygen. This is not a strategic reduction.
C. Correct – During intense exercise, muscle cells produce lactate through lactic acid fermentation. This allows ATP to be made quickly (although not efficiently), giving muscles enough energy to keep contracting for a short time.
D. Incorrect – Ethanol and carbon dioxide are by-products of anaerobic respiration in yeast and some bacteria, not in human muscle cells. Human muscles produce lactate, not ethanol.
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