USABO - USA Biology Olympiad Plant Anatomy and Transport Questions and Answers

Question 1

A botanist is studying a plant adapted to an arid environment. Which of the following is an anatomical or physiological adaptation she would LEAST expect to find?

Accepted Answer

Large, broad leaves with a high density of stomata.

Answer

Plants in arid environments (xerophytes) need to conserve water. Large, broad leaves with a high density of stomata would maximize water loss through transpiration, which is counterproductive for survival in a dry climate. The other options are common xerophytic adaptations: a thick waxy cuticle reduces evaporative water loss, sunken stomata trap humid air to reduce the water potential gradient, and CAM photosynthesis allows the plant to take in CO2 at night when temperatures are lower and humidity is higher, minimizing water loss.

Question 2

The Casparian strip is a critical component of the root endodermis. What is its primary function?

Accepted Answer

To force water and solutes to pass through the plasma membrane of endodermal cells.

Answer

The Casparian strip is a waterproof band of suberin and lignin that impregnates the cell walls of the endodermis. It blocks the apoplastic pathway (movement through cell walls and intercellular spaces), forcing water and dissolved minerals to cross the selectively permeable plasma membrane of an endodermal cell (the symplastic pathway) before entering the vascular cylinder (xylem and phloem). This allows the plant to regulate which substances enter the transport system.

Question 3

According to the pressure-flow hypothesis, what directly generates the hydrostatic pressure that drives the movement of sap in the phloem?

Accepted Answer

The osmosis of water into sieve-tube elements at the source following active loading of sugars.

Answer

The pressure-flow hypothesis posits that at a sugar source (like a leaf), sugars are actively transported into companion cells and then into sieve-tube elements. This high concentration of solutes decreases the water potential inside the sieve tube, causing water to move by osmosis from the adjacent xylem into the phloem. This influx of water generates a high hydrostatic (turgor) pressure, which pushes the sap in bulk towards a sink (like a root or fruit), where pressure is lower.

Question 4

A plant cell has a solute potential (Ψs) of -0.8 MPa and a pressure potential (Ψp) of +0.5 MPa. The cell is placed in a beaker of pure water (Ψ = 0 MPa). What will be the initial net direction of water movement?

Accepted Answer

Into the cell, because its water potential is -0.3 MPa.

Answer

The total water potential (Ψ) of the cell is the sum of its solute potential (Ψs) and pressure potential (Ψp). In this case, Ψcell = Ψs + Ψp = -0.8 MPa + 0.5 MPa = -0.3 MPa. Pure water has a water potential of 0 MPa. Water always moves from an area of higher water potential to an area of lower water potential. Since the water potential of the surrounding pure water (0 MPa) is higher than the cell's water potential (-0.3 MPa), water will move into the cell.

Question 5

Which of the following correctly describes the function of companion cells in the phloem?

Accepted Answer

They provide metabolic support and load/unload sugars for the sieve-tube elements.

Answer

Sieve-tube elements, which form the main transport conduit in the phloem, are alive but lack a nucleus, ribosomes, and a large central vacuole at maturity. The adjacent companion cells are metabolically active and connected to the sieve-tube elements via plasmodesmata. They perform the necessary life support functions for the sieve-tube elements, including providing the ATP needed for the active transport of sugars during phloem loading and unloading.

USABO - USA Biology Olympiad Practice Test

USABO - USA Biology Olympiad Practice Test

USABO - USA Biology Olympiad Plant Anatomy and Transport Questions and Answers