Question

What is the means of transport through which sucrose moves into sieve tube of source and out of sicve tube at sink? (a) Simple diffusion (b) Facilitated diffusion (c) Active transport (d) Passive transport

Short answer

The correct answer is (c) Active transport.

Step-by-step solution

Analyzing the options

Let's analyze all the given alternatives: \nSimple diffusion is the process where molecules move from a region of higher concentration to a region of lower concentration, without the assistance of any external energy or carrier proteins. \nFacilitated diffusion is a type of passive movement of molecules across the cell membrane led by transport proteins.\nActive transport requires energy to move molecules against the concentration gradient with the aid of carrier proteins. \nPassive transport is a movement of biochemicals from areas of high concentration to areas of low concentration, with no energy expenditure. It includes simple diffusion, facilitated diffusion, and osmosis.

Determining the correct transport mechanism

In plant biology, sucrose is actively transported into sieve-tube members at the source (usually leaves). This process is energy-requiring, hence we can eliminate simple diffusion, and passive transport from our options. Facilitated diffusion also doesn't fit in, as it doesn't require energy. Thus the sucrose utilizes active transport to move into the sieve tube of source and out of sieve tube at the sink.

Key concepts

Sucrose Transport

Sucrose is an essential sugar in plants, serving as a major transportable form of energy. Within the plant's vascular system, sucrose moves from "source" areas, where it is produced (typically the leaves), to "sink" areas, where it is needed for growth or storage (like roots or fruits). This movement is crucial for the plant's growth and development.
To transport sucrose, plants use a process called "active transport." Unlike passive methods, active transport requires energy input, in the form of ATP, to move the sucrose against its concentration gradient. This means moving sucrose from an area of lower concentration in leaf cells into areas of higher concentration within the phloem sieve tubes or from sieve tubes out to storage tissues.
The energy-dependent nature of this process ensures efficient and directed movement of nutrients where they are most needed within the plant. Understanding this energy requirement helps to clarify why simple diffusion or other passive processes are insufficient for the task of sucrose transport.

Sieve Tube

Sieve tubes are specialized structures found in the phloem of vascular plants, specifically adapted for the transport of nutrients like sucrose. These tubes are composed of cells called sieve-tube elements, which are elongated and connected end-to-end. This arrangement forms a continuous channel through which sap can flow.
Each sieve tube element is characterized by the presence of sieve plates, porous end walls that facilitate the movement of sucrose-laden sap from one cell to the next. Despite their vital role, sieve tube elements lack a nucleus and other organelles; they rely on companion cells for metabolic functions required for active transport.
The sieve tube's ability to transport sucrose efficiently is integral to overall plant health. By understanding the structural and functional aspects of sieve tubes, one can appreciate how plants maintain their internal nutrient distribution system.

Passive Transport

Passive transport refers to the movement of substances across cell membranes without the need for energy input. It takes advantage of the natural kinetic energy of molecules and their desire to move from areas of high concentration to areas of low concentration.
There are various forms of passive transport, including:

• Simple diffusion: The direct movement through the lipid bilayer without the need for transport proteins.
• Facilitated diffusion: Movement assisted by transport proteins embedded in the cell membrane.
• Osmosis: The specific diffusion of water molecules across a semi-permeable membrane.

While passive transport is vital for numerous physiological processes, it is not suitable for carrying sucrose into sieve tubes. This is because sucrose often needs to be moved against its concentration gradient, which passive transport cannot achieve.

Facilitated Diffusion

Facilitated diffusion is a subtype of passive transport where molecules diffuse across cell membranes with the help of transport proteins. These proteins provide specific pathways through the membrane, making it easier for certain molecules to pass.
This process is crucial for transporting substances that cannot easily cross the lipid bilayer, such as ions or large polar molecules. Carrier proteins or channel proteins are employed, providing a selective passageway that fits particular molecules.
While facilitated diffusion helps many crucial substances enter and exit cells without energy, it cannot be used for sucrose transport into sieve tubes in plants. This is due to its limitation of not moving substances against a concentration gradient, a requirement met instead by active transport in the case of sucrose.