Pre AP Biology -CELLS 4.1 Organ/Tissue Systems- MCQ Exam Style Questions -New Syllabus

Pre AP Biology -CELLS 4.1 Organ/Tissue Systems- MCQ Exam Style Questions – New Syllabus 2025-2026

Pre AP Biology -CELLS 4.1 Organ/Tissue Systems- MCQ Exam Style Questions – Pre AP Biology – per latest Pre AP Biology Syllabus.

Pre AP Biology – MCQ Exam Style Questions- All Topics

Question

Which of the following is the order of events during bulk flow from source to sink?

a. sucrose loaded into companion cells and then into sieve tube members; water potential in the sieve tube member falls and water enters by osmosis; turgor pressure increased within sieve tube member; phloem sap moved by bulk flow between a source and a sink; sink takes up sucrose

b. turgor pressure increased within sieve tube member; sucrose loaded into companion cells and then into sieve tube members; water potential in the sieve tube member falls and water enters by osmosis; phloem sap moved by bulk flow between a source and a sink; sink takes up sucrose

c. water potential in the sieve tube member falls and water enters by osmosis; phloem sap moved by bulk flow between a source and a sink; sink takes up sucrose; sucrose loaded into companion cells and then into sieve tube members; turgor pressure increased within sieve tube member

d. sink takes up sucrose; phloem sap moved by bulk flow between a source and a sink; turgor pressure increased within sieve tube member; water potential in the sieve tube member falls and water enters by osmosis; sucrose loaded into companion cells and then into sieve tube members

▶️ Answer/Explanation

Detailed solution

Correct Option: a

1. Sucrose is actively loaded into companion cells and then into sieve tube members at the source.
2. High sucrose concentration lowers the water potential ($\psi_s$) inside the sieve tube.
3. Water enters the sieve tube from the xylem by osmosis, following the water potential gradient.
4. The influx of water increases the turgor pressure (hydrostatic pressure) at the source end.
5. This pressure gradient drives the bulk flow of phloem sap toward the sink (lower pressure).
6. Finally, the sink takes up sucrose, causing water to exit and completing the cycle.

Question

Which of these processes results in stomata opening?

a. $K^+$ flows out of guard cells.

b. $CO_2$ concentration inside the leaf increases.

c. Turgor pressure in the guard cells increases.

d. The $H^+$-ATPase pumps in the guard cell membranes stop pumping.

▶️ Answer/Explanation

Detailed solution

The correct option is c. Turgor pressure in the guard cells increases.
Stomata open when guard cells take up water via osmosis, increasing their turgor pressure.
This uptake is driven by the active transport of $K^+$ ions into the guard cells, not out.
Higher internal $CO_2$ concentrations typically signal the plant to close stomata to conserve water.
The $H^+$-ATPase pumps must be active to create the electrochemical gradient needed for $K^+$ entry.
As guard cells become turgid, their unique wall structure causes them to bow outward, opening the pore.
Conversely, a loss of turgor pressure leads to stomatal closure.

Question

Which of the following describes the functions of the Casparian strip in the root?

a. to prevent water from moving via symplast and to promote backflow

b. to prevent water from moving via apoplast and to promote backflow

c. to prevent water from moving via apoplast and to prevent backflow

d. to prevent water from moving via symplast and to prevent backflow

▶️ Answer/Explanation

Detailed solution

The correct option is c.
The Casparian strip is a band of waterproof suberin located in the endodermis.
It blocks the apoplast pathway, forcing water to enter the living protoplast.

This selective barrier ensures that minerals are actively filtered by the cell membrane.
Once water enters the vascular cylinder, the strip acts as a seal to prevent backflow.
This maintains root pressure, allowing water to move upwards toward the leaves.
Therefore, it blocks the apoplastic route and prevents the leakage of water out of the stele.

Pre AP Biology -CELLS 4.1 Organ/Tissue Systems- MCQ Exam Style Questions -New Syllabus