Question

Sugar transport from leaves to roots occurs by ____________ a. a pressure gradient inside sieve tubes b. different solutes at source and sink regions c. the pumping force of xylem vessels d. transpiration, tension, and cohesion of water

Short answer

a. a pressure gradient inside sieve tubes

Step-by-step solution

Understand the Process

Sugar transport in plants occurs primarily through the phloem, a process known as translocation. The movement of sugars is directed from a source (like leaves) to a sink (like roots). It's important to understand how this transport mechanism functions.

Identify the Key Mechanism

The movement of sugars occurs due to a pressure gradient within the sieve tubes of the phloem. This pressure gradient is created by the active transport of sugars into the phloem at the source, increasing the osmotic pressure.

Recognize the Involved Forces

The generated osmotic pressure causes water to flow into the phloem from the xylem, creating a pressure difference that actively pushes the sap towards the sink regions. This is facilitated by the bulk flow within the sieve tubes.

Eliminate Incorrect Options

The incorrect options are eliminated as follows: different solutes at source and sink points do not drive the transport (b), xylem vessels do not pump sugars (c), and the phenomenon described in (d) refers to water movement, not sugar.

Select the Correct Answer

Given that the process involved is a pressure flow mechanism within the sieve tubes, the correct answer is a) a pressure gradient inside sieve tubes.

Key concepts

Pressure Gradient

In the process of sugar transport through plants, a pressure gradient plays a pivotal role. A gradient refers to a gradual change in a quantity, and in this context, the pressure gradient involves differences in pressure inside the plant.
It occurs within the phloem sieve tubes and is essential for the movement of nutrients. This gradient is established by the active transport of sugars into these tubes at the source, such as leaves, and creates an area of high pressure. Consequently, the sap flows from areas of higher to lower pressure, assisting in the efficient delivery of sugars throughout the plant.
The presence of a pressure gradient is vital because it drives the mass flow of materials, allowing the plant to distribute energy and metabolic resources from the site of photosynthesis to areas requiring nourishment, as well as for storage.

Phloem Transport

Phloem transport is a process in the plant vascular system which circulates nutrients such as sugars. The phloem is a specialized tissue in plants responsible for the distribution of organic compounds made during photosynthesis.
Movement within the phloem is a critical part of a plant's circulatory system. The efficient transport of sugar and other solutes occurs through sieve tubes, a major component of the phloem.
Key aspects of phloem transport include:

• Sieve Tubes: Long tubes with perforated walls that allow easy translocation of sugars and solutes.
• Companion Cells: Cells that assist the sieve tubes, ensuring active transport of nutrients into the phloem.
• Energy Efficiency: The movement is largely maintained by natural pressure differences rather than energy-intensive methods.

Phloem transport is fundamental because it delivers sugars to different parts of the plant, enabling growth and energy distribution.

Osmotic Pressure

Osmotic pressure is crucial in facilitating the movement of water into the phloem from adjacent xylem vessels. This pressure is a result of the difference in solute concentration between the phloem and the surrounding cells.
When sugars accumulate in the phloem at the source, the solute concentration increases, leading to a higher osmotic pressure. This encourages water to flow into the phloem, resulting in increased turgor pressure within the sieve tubes.
The increased pressure helps push the nutrient-rich sap towards the sink areas. Understanding osmotic pressure helps clarify how water movement can drive the process of translocation within the plant's vascular system.

Source and Sink

The terms 'source' and 'sink' describe two critical areas in the plant concerning nutrient transport. A 'source' is any part of the plant that produces more sugars than it requires, typically leaves where photosynthesis occurs. A 'sink', on the other hand, is any region requiring or storing more sugars than it produces, such as roots, buds, or fruits.
The dynamic of source and sink is essential for maintaining a balanced transfer of nutrients within the plant.
Features of sources and sinks include:

• Source: Areas of sugar production and release into the phloem.
• Sink: Regions where sugars are needed for growth or storage.
• Changes Over Time: The roles of source and sink may change; for example, a growing fruit may become a sink during development.

Recognizing the interplay between source and sink helps explain how plants manage the allocation of essential resources.

Translocation Process

Translocation refers to the movement of organic substances, primarily sugars, throughout the plant. This is mainly through the phloem tissue. The process is complex yet efficiently organized, ensuring that nutrients reach all parts of the plant as needed.
Translocation involves several stages, primarily driven by pressure and osmotic gradients.
Steps in the translocation process include:

• Loading at the Source: Sugars enter the sieve tubes and create a high pressure.
• Mass Flow: Sap moves from high-pressure areas (source) to low-pressure areas (sink).
• Unloading at the Sink: Sugars are actively moved out of the phloem for utilization or storage.

Understanding translocation is essential for grasping how plants distribute vital nutrients, enabling sustained growth and development.