Plant Structure and Function Flashcards
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Read the first 6 Plant Structure and Function flashcards as text
The upward movement of water through the xylem in a 30-meter tall oak tree is best explained by which of the following?
Answer: The cohesion of water molecules and the tension created by transpiration.
The cohesion-tension theory is the primary explanation for water transport over long distances in plants. Transpiration (evaporation of water from leaves) creates a negative pressure, or tension, that pulls the column of water up the xylem. The cohesive properties of water (molecules sticking together via hydrogen bonds) and adhesive properties (molecules sticking to xylem walls) ensure this column remains unbroken.
A houseplant placed on a windowsill bends towards the light. This phenomenon, known as phototropism, is primarily mediated by the redistribution of which plant hormone?
Answer: Auxin
Phototropism is caused by the plant hormone auxin. When light strikes one side of the stem, auxin migrates to the shaded side. This higher concentration of auxin on the shaded side stimulates cell elongation, causing the stem to bend towards the light source.
Which statement accurately compares the functions of xylem and phloem?
Answer: Xylem primarily transports water and minerals from the roots up, while phloem transports sugars from sources to sinks.
Xylem is responsible for the unidirectional transport of water and dissolved minerals from the roots to the rest of the plant. Phloem transports sugars (produced during photosynthesis) from a source (e.g., leaves) to a sink (e.g., roots, fruits), and this movement is bidirectional.
The Casparian strip in the root endodermis is a critical structure for selective nutrient uptake because it:
Answer: forces water and solutes to pass through the selectively permeable membrane of endodermal cells.
The Casparian strip is a waxy, waterproof band that blocks the apoplastic pathway (movement through cell walls). This forces all water and dissolved minerals to cross the plasma membrane of an endodermal cell (the symplastic pathway) before entering the vascular cylinder (xylem). This ensures that the plant has selective control over which substances enter its transport system.
Double fertilization, a process unique to angiosperms, involves the fusion of two sperm nuclei with which female gametophyte cells?
Answer: One sperm with the egg cell, and the other with the two polar nuclei in the central cell.
In double fertilization, one sperm nucleus fuses with the egg cell to form the diploid (2n) zygote, which develops into the embryo. The second sperm nucleus fuses with the two polar nuclei located within the large central cell, forming a triploid (3n) cell that develops into the endosperm, a nutrient-rich tissue.
During a period of prolonged drought, a plant's stomata will close to conserve water. What is the most significant trade-off of this physiological response?
Answer: A reduced intake of carbon dioxide, limiting photosynthesis.
Stomata are pores on the leaf surface that regulate gas exchange. While closing them is an essential adaptation to reduce water loss via transpiration during a drought, it also prevents the plant from taking in atmospheric CO2. Since CO2 is a necessary reactant for photosynthesis, its limited availability will reduce the plant's ability to produce sugars.