USABO - USA Biology Olympiad Plant Anatomy and Transport Questions and Answers — 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?
- A thick, waxy cuticle on the leaves and stems.
- Stomata located in deep pits or crypts on the leaf surface.
- Large, broad leaves with a high density of stomata. (Correct answer)
- Crassulacean acid metabolism (CAM) photosynthesis.
Correct answer: Large, broad leaves with a high density of stomata.
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?
- To increase the surface area for water absorption.
- To provide structural support to the root cortex.
- To store starch and other carbohydrates for the plant.
- To force water and solutes to pass through the plasma membrane of endodermal cells. (Correct answer)
Correct answer: To force water and solutes to pass through the plasma membrane of endodermal cells.
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?
- The active transport of water from xylem to phloem at the sink.
- The force of gravity pulling the dense, sugary sap downwards.
- The osmosis of water into sieve-tube elements at the source following active loading of sugars. (Correct answer)
- The contraction of companion cells, which squeezes sap through the sieve tubes.
Correct answer: The osmosis of water into sieve-tube elements at the source following active loading of sugars.
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?
- Out of the cell, because the pressure potential is positive.
- Into the cell, because its water potential is -0.3 MPa. (Correct answer)
- There will be no net movement, as the potentials are balanced.
- Into the cell, because its solute potential is more negative than the surrounding water.
Correct answer: Into the cell, because its water potential is -0.3 MPa.
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?
- They form the primary structural tube for translocation.
- They provide metabolic support and load/unload sugars for the sieve-tube elements. (Correct answer)
- They are dead at maturity and provide a hollow conduit for sap flow.
- They regulate the opening and closing of sieve plates.
Correct answer: They provide metabolic support and load/unload sugars for the sieve-tube elements.
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.
Question 6: In the secondary growth of a dicot stem, the vascular cambium is responsible for producing which tissues?
- Pith and cortex.
- Epidermis and periderm.
- Secondary xylem and secondary phloem. (Correct answer)
- Primary xylem and primary phloem.
Correct answer: Secondary xylem and secondary phloem.
The vascular cambium is a lateral meristem that arises between the primary xylem and phloem. Through cell division, it produces new cells inwardly that differentiate into secondary xylem (wood) and cells outwardly that differentiate into secondary phloem. This process increases the girth or diameter of the stem, a characteristic feature of secondary growth.
USABO - USA Biology Olympiad Plant Anatomy and Transport Questions and Answers
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?