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
Question
Movement of insects requires muscles in antagonistic pairs. The diagram shows an insect leg with muscles labelled $\mathrm{X}$ and $\mathrm{Y}$.
What actions in the human arm are equivalent to muscle X contracting and muscle Y relaxing?
A. triceps contracts, biceps relaxes, arm extends
B. biceps contracts, triceps relaxes, arm flexes
C. triceps contracts, biceps relaxes, arm flexes
D. biceps contracts, triceps relaxes, arm extends
Answer/Explanation
Answer: A. triceps contracts, biceps relaxes, arm extends
Explanation:
Look at the insect leg diagram:
- Muscle X is shown attached on the top side (closer to the leg joint).
- Muscle Y is on the bottom side of the leg joint.
How insect leg muscles work:
- When muscle X contracts and muscle Y relaxes, it straightens or extends the leg.
- When muscle Y contracts and muscle X relaxes, it bends or flexes the leg.
Human arm equivalent:
- Triceps is the muscle that extends the arm.
- Biceps is the muscle that flexes the arm.
So:
- Muscle X → equivalent to triceps → causes extension
- Muscle Y → equivalent to biceps → causes flexion
Question
The electron micrograph shows sarcomeres in myofibrils of striated muscle during muscle contraction. The lines P–Q and R–S show two regions of one sarcomere.
How would regions P–Q and R–S change when the muscle relaxes?
Answer/Explanation
Answer: C
Explanation:
What we see in the sarcomere image:
- P–Q represents the I-band (light band with just actin filaments).
- R–S represents the A-band (dark band with myosin filaments).
Muscle contraction vs relaxation:
When the muscle contracts, the actin filaments slide over myosin filaments.
- I-band (P–Q) becomes narrower because actin overlaps myosin more.
- A-band (R–S) stays the same width since myosin length doesn’t change.
When the muscle relaxes, the filaments slide back to the resting position.
- I-band (P–Q) becomes wider (less overlap).
- A-band (R–S) does not change (myosin length is constant).
Analyze the options:
| Option | P–Q (I-band) | R–S (A-band) | Correct? |
|---|---|---|---|
| A | wider | narrower | No, A-band does not narrow |
| B | narrower | wider | No, I-band widens on relax |
| C | wider | no change | Yes, fits muscle relaxation |
| D | no change | wider | No, A-band doesn’t widen |
Final answer with image context:
This means Answer C is correct.