AP Biology Questions on Phloem: Sieve-Tube Elements and Companion Cells

📚 Quick Study Guide

• 🔬 Phloem: The vascular tissue in plants responsible for transporting sugars and other nutrients from source tissues (e.g., leaves) to sink tissues (e.g., roots, fruits).
• 🌱 Sieve-Tube Elements: Living cells that conduct sugars. They are connected end-to-end to form long sieve tubes. They lack a nucleus and ribosomes at maturity.
• 🤝 Companion Cells: Specialized parenchyma cells closely associated with sieve-tube elements. They support the sieve-tube elements, providing them with proteins and ATP.
• 🔗 Sieve Plates: Porous structures found at the ends of sieve-tube elements, facilitating the flow of phloem sap between cells.
• 🚀 Pressure Flow Hypothesis: The mechanism by which phloem transport occurs. It involves the loading of sugars into sieve tubes at the source, creating a high pressure that drives the flow of phloem sap to the sink, where sugars are unloaded.
• ⚡ Active Transport: Often required to load sugars into sieve-tube elements against a concentration gradient, especially at the source.
• 🌡️ Factors Affecting Translocation: Temperature, water availability, and metabolic activity of source and sink tissues all affect the rate of translocation in the phloem.

Practice Quiz

1. What is the primary function of phloem in plants?
   1. Transporting water and minerals from the roots
   2. Transporting sugars from source to sink
   3. Providing structural support
   4. Absorbing sunlight for photosynthesis
2. Which cells are the main conducting cells of the phloem?
   1. Xylem vessels
   2. Companion cells
   3. Sieve-tube elements
   4. Parenchyma cells
3. What is the role of companion cells in phloem?
   1. To transport water to the sieve-tube elements
   2. To provide metabolic support to the sieve-tube elements
   3. To store excess sugars
   4. To protect the phloem from pathogens
4. What structures connect sieve-tube elements end-to-end?
   1. Plasmodesmata
   2. Sieve plates
   3. Cell walls
   4. Middle lamella
5. According to the pressure flow hypothesis, what drives the movement of phloem sap?
   1. Capillary action
   2. Transpiration pull
   3. A pressure gradient between source and sink
   4. Root pressure
6. Which of the following processes is often required to load sugars into sieve-tube elements at the source?
   1. Osmosis
   2. Passive diffusion
   3. Active transport
   4. Facilitated diffusion
7. What happens to the pressure in the phloem at the sink?
   1. It increases due to water entering
   2. It decreases as sugars are unloaded
   3. It remains constant
   4. It fluctuates randomly