Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
Translocation is the movement of organic solutes, primarily sucrose and amino acids, from source to sink through the phloem tissue. A source is where sugars are produced (e.g., mature leaves) or mobilized (e.g., storage roots in spring), while a sink is where they are consumed or stored (e.g., roots, fruits, developing seeds).
Phloem loading at the source is an active process. Proton pumps use to transport ions out of the companion cells into the cell wall. This creates a concentration gradient, allowing to diffuse back into the companion cell through a co-transporter protein, bringing a sucrose molecule with it against its concentration gradient.
The high concentration of sucrose in the sieve tube at the source lowers the water potential (). Consequently, water enters the sieve tube from the adjacent xylem via osmosis, creating a high hydrostatic pressure ().
At the sink, sucrose is unloaded into the surrounding cells. This increases the water potential in the phloem, causing water to move back into the xylem via osmosis. This results in a lower hydrostatic pressure at the sink compared to the source.
The pressure gradient between the source and the sink drives the mass flow of phloem sap. This is described by the pressure-flow hypothesis.
Sieve tube elements are specialized cells with reduced cytoplasm, no nucleus, and few organelles to allow for the efficient flow of sap. They are connected by sieve plates with large pores. Companion cells provide the metabolic support (via many mitochondria) for active transport.
Aphid stylets and radioactive labeling with are used to measure the rate of translocation. When an aphid feeds on the phloem, its stylet can be severed, allowing the sap to exude. By measuring the time it takes for -labeled sugars to reach different stylets, the rate of flow can be calculated.
📐Formulae
💡Examples
Problem 1:
In an experiment using labeling and aphid stylets, a scientist determines that radioactive sucrose traveled in minutes. Calculate the rate of translocation in .
Solution:
Explanation:
To find the rate in , convert the time from minutes to hours ( minutes = hours). Then, divide the distance ( ) by the time ( ) to obtain the translocation rate.
Problem 2:
Explain the role of the /sucrose co-transporter in the companion cell membrane.
Solution:
The co-transporter utilizes the electrochemical gradient of ions (established by the -dependent proton pump) to move sucrose into the companion cell against its concentration gradient. This is an example of secondary active transport.
Explanation:
Energy is indirectly used ( for the proton pump) to concentrate sucrose in the phloem, which is necessary to drive osmosis and generate hydrostatic pressure.