Phloem Transport Flow From Source to Sink, Difference between Source and Sink in Plants

Definition Of Phloem transport

It's the process of distribution of organic nutrients, mainly sugars produced in photosynthesis, in a plant. This function is attributed to phloem, a vascular tissue. Phloem transport plays a very essential role in plant physiology supplying energy and nutrients to the growing tips and roots from storage organs, hence supporting growth, development, and general health. This is the process that helps in maintaining the metabolic activities of the plant and, at the same time, enables the proper distribution of energy throughout the plant.

Mechanism Of Phloem Transport

The mechanism is described below.

Pressure Flow Hypothesis (Mass Flow Hypothesis)

Explanation of the pressure flow mechanism

The pressure flow hypothesis explains the flow of nutrients, particularly sugars like sucrose through the phloem from the source, the area where the sugars are produced, for instance, the leaf, to a sink, where they are used or stored, as in the case of roots or fruits.

Steps involved in the process

• At the Source, Sugars are actively loaded into the sieve tube elements of the phloem, increasing the osmotic pressure.

• Water is gained into the sieve tubes from surrounding tissues by osmosis because of the high osmotic pressure.

• Bulk flow as turgor pressure increases, pushing the phloem sap along the sieve tubes toward the sink.

• Repression at the Sink: Sugars are actively transported out of the sieve tubes at the sink, decreasing the osmotic pressure.

• Water Movement: Water exits the sieve tubes at the sink. This preserves a pressure gradient that keeps the flow of the sap going.

Active And Passive Transport In Phloem

Role of active transport in loading and unloading of sugars

• Active mechanisms of transport using energy gained from the breakdown of ATP are used in sugar uptake within the sieve tubes at the source and their removal at the sink.

• Proton pumps, H+-ATPase, create a gradient that makes it possible for the active transport of sugars against a concentration gradient.

Role of osmosis and diffusion

• Osmosis establishes the flow of water in and out of the phloem. This is a vital process in the maintenance of the pressure gradient responsible for the flow of sap.

• Diffusion provides for the passive transport of water and solutes across cell membranes, thus accomplishing the overall transportation in the phloem.

Source To Sink Transport

Source: Any vegetative body part or organ of a plant from which the sugars are produced or mobilized for transport. Such as:

• Leaves: The photosynthetic organs where the sugars are produced.

• Storage Organs During Mobilisation: These are such structures as tubers and bulbs when they release stored nutrients to support new growth.

Sink: Any tissue or organ that consumes or stores sugars, such as:

• Roots: The organs themselves that need energy for growth and maintenance.

• Fruits: The developing organs store up sugars for growth and further ripening.

• Seeds: Nutrients are stored in the developing structures for germination.

• Storage Organs During Accumulation: Organs like tubers and bulbs when themselves are accumulating nutrients for future use.

Seasonal Changes In Source And Sink

Spring and Early Summer: Leaves act as sources, synthesizing sugars which get translocated to the growing roots, shoots and developing fruits and sills.

Tubers and bulbs on the other hand act as sinks storing up sugars for winter.

The stored sugars in tubers and bulbs now acting as sources, mobilize for support of new growth in roots and shoots which are now acting as sinks.

Differences Between Source And Sink In Plants

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