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

Understanding the context

The transport of sugars in plants occurs mainly through the phloem, which consists of sieve tubes. The transport process is commonly referred to as translocation.

Identifying the mechanism of sugar transport

Translocation in the phloem relies on pressure differences between the source (leaves) and the sink (roots or other parts). Sugars produced in the leaves are loaded into the phloem, creating a high-pressure area.

Connecting pressure gradients and options

Option (a) 'a pressure gradient inside sieve tubes' directly relates to the mechanism where sugars move due to pressure flow created by osmotic differences between the source and the sink.

Evaluating other options

Options (b), (c), and (d) are not directly related to the main method of sugar transport in plants. Option (b) refers to different solute concentrations, option (c) to xylem functions, and option (d) to processes involved in water movement, not sugar.

Conclusion

The pressure gradient is the key driver in the movement of sugars from leaves to roots within the phloem, making option (a) the correct choice.

Key concepts

Pressure Gradient

In the process of plant nutrition, the pressure gradient plays a pivotal role. The pressure gradient within the phloem is crucial in the movement of sugars from leaves where photosynthesis occurs, to other parts like roots. In essence, it functions much like a conveyor belt, where the beginning of the belt is a high-pressure area and the end is a low-pressure area.

When sugar is produced in the leaves, it is actively loaded into the sieve tubes, part of the phloem. This increases the concentration of solutes (like sugars), creating a region of high pressure due to osmotic effects. The high solute concentration causes water to enter the sieve tubes, increasing the pressure in this area even further.

Conversely, at the sink regions (such as the roots), sugars are being used up or stored, which reduces their concentration, leading to a lower pressure. Thus, the intrinsic pressure difference between the source and the sink powers the flow of nutrients across the plant's vascular system.

Sieve Tubes

Sieve tubes are specialized structures within the phloem that transport nutrients throughout the plant. They are essentially elongated cells that form a continuous channel from leaves to roots and other plant parts, facilitating efficient transport of sugars.

In these tubes, the living sieve elements are devoid of a nucleus, and their cytoplasm forms a continuous channel through sieve plates, which are perforated structures that connect adjacent sieve tube elements. These cells have companion cells adjacent to them, which aid in metabolic support and loading of sugars.

Sieve tubes rely on the pressure differences between various plant parts to drive nutrient flow. By using a system of pressure-driven flow, sieve tubes efficiently transport large quantities of sugars quickly and effectively from sources like leaves to sinks such as roots.

Translocation

Translocation is the process by which plants transport sugars and other solutes through the phloem from source to sink. It's an essential function for distributing the nutrients produced by photosynthesis to parts of the plant that require energy for growth and storage.

The process begins in the leaves, the primary source of sugars, produced during photosynthesis. These sugars are actively loaded into the phloem sieve tubes. This loading increases the solute concentration inside the tubes, which in turn draws in water from the adjacent xylem due to osmotic forces.

This sets up a high-pressure zone, pushing the sugary sap through the sieve tubes to areas of lower pressure where sugars are consumed or stored, like roots, fruits, or seeds. As the sugars are unloaded at these sinks, the pressure decreases, maintaining the pressure flow. Thus, translocation efficiently balances the distribution of nutrients based on the plant's metabolic needs.

Osmotic Differences

Osmotic differences are central to the transport mechanism of nutrients via the phloem. Essentially, osmosis involves the movement of water across a semi-permeable membrane from areas of lower solute concentration to areas of higher solute concentration.

When sugars are loaded into the phloem, there is an increase in the solute concentration within the sieve tubes, creating an osmotic imbalance. To counter this imbalance, water moves from the xylem into the phloem, driven by osmotic potential. This entry of water increases the turgor pressure in the phloem, enabling the flow of nutrients down the pressure gradient.

The difference in osmotic pressure between the source where sugars are loaded and the sink where sugars are being removed facilitates a smooth flow. This osmotic-driven pressure flow is integral to phloem transport, ensuring that sugars reach all necessary parts of the plant efficiently.