IB DP Biology- B3.2 Transport - IB Style Questions For SL Paper 1A -FA 2025
▶️ Answer/Explanation
Veins contain valves to prevent the backward flow of blood and have thin walls because blood pressure is low. ✅ Answer: (D)
Question
What is the state of the atrio-ventricular and semilunar valves when the left ventricle contracts?
Answer/Explanation
Answer: D
Explanation:
During left ventricular contraction (ventricular systole), pressure inside the ventricle rises. This causes the atrioventricular (bicuspid/mitral) valve to close and the aortic semilunar valve to open, allowing blood to exit into the aorta while preventing backflow into the left atrium.
Answer Evaluation:
A. Incorrect – AV valves are open and semilunar valves are closed, which happens during ventricular filling (diastole), not contraction.
B. Incorrect – Both valves are open, which never occurs in a normal cardiac cycle. This would allow backflow and disrupt one-way flow.
C. Incorrect – Both valves are closed, which only happens briefly during isovolumetric contraction, not the full contraction phase.
D. Correct – AV valves are closed, and semilunar valves are open—this matches the ejection phase of ventricular systole when the ventricle contracts and pumps blood into the aorta.
▶️ Answer/Explanation
Answer: (D) Water moves up the stem in the xylem.In a healthy plant, evaporation of water from mesophyll cells (transpiration) generates a tension that pulls water upward through the xylem. This upward movement of water is known as the transpiration stream and occurs continuously as long as evaporation is happening.
Why this is always true:
• Evaporation creates a pull (tension).
• Water molecules show cohesion and adhesion inside the xylem.
• This maintains constant upward flow from roots to leaves.
Option Evaluation:
A. Incorrect — Plasmolysis only occurs in severely dehydrated or stressed plants.
B. Incorrect — Photosynthesis continues as long as other factors are available; normal transpiration does not stop it.
C. Incorrect — Stomata close only under excessive water loss, not automatically in healthy plants.
D. Correct — Water is always pulled upward in the xylem when evaporation occurs.
✅ Answer: (D)