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

What is Phloem Transport?

Phloem transport is 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.

Structure of Phloem

The structure of phloem includes following components:

Mechanism of Phloem Transport

The mechanism is described below.

Pressure Flow Hypothesis (Mass Flow Hypothesis)

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 are:

• 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.

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

• 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

The source to sink transport includes:

Source (Producer Sites)

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 are such structures like tubers and bulbs when they release stored nutrients to support new growth.

Sink (Consumer/Storage Sites)

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

• Roots are the organs themselves that need energy for growth and maintenance.

• Fruits, the developing organs store up sugars for growth and further ripening.

• Seeds have the nutrients stored for developing structures for germination.

• Organs like tubers and bulbs when themselves are accumulating nutrients for future use.

Seasonal Changes

• In 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

The difference between source and sink in plants include:

Factors Affecting Phloem Transport

Several factors affect phloem transport:

Temperature

Higher temperatures increase the rate of metabolic activities and therefore increase phloem loading and translocation.

Water Availability

Adequate water maintains the turgor pressure which helps in the pressure flow of solutes. Water scarcity reduces transport capacity.

Rate of Photosynthesis

The greater the rate of photosynthesis, the more will be the production of sugars in the tissues and ultimately enhancing the rate of phloem transport.

Plant Growth Stage

During the active growth stage, transport is directed towards the sink. In mature plants, storage organs and reproductive tissues receive more nutrients.

Phloem Transport NEET MCQs (With Answers & Explanations)

The key concepts to be covered under this topic for different exams are:

• Structure of Phloem

• Source to sink transport

• Sink vs Source

Practice Questions for NEET

Q1. The loading at source in the phloem is

1. Diffusion

2. Pressure flow

3. Active transport

4. Imbibition

Correct answer: 3) Active transport

Explanation:

Sucrose is actively transported into the companion cells from the source tissues, such as leaves, where photosynthesis occurs. In the companion cells, sucrose is then loaded into the sieve tube elements of the phloem through plasmodesmata, using energy-dependent processes. The movement of sucrose into the sieve tubes creates a pressure gradient, allowing for the bulk flow of the phloem sap, a process known as translocation. This pressure flow mechanism helps move sugars and other nutrients from source regions (like leaves) to sink regions (such as roots, fruits, or growing tissues). Active transport ensures that sucrose is efficiently loaded into the phloem, even against concentration gradients, making it an essential part of plant nutrient distribution.

Hence, the correct answer is option 3) Active transport.

Q2. Translocation in the phloem is defined with respect to:

1. Gravity

2. Source to sink

3. Apoplast and symplast pathway

4. Both a and b

Correct answer: 2) Source to sink

Explanation:

Translocation in the phloem is the term used to describe the movement of various molecules, such as sucrose, amino acids, and other compounds, through the phloem of a plant. Gravity does not affect the translocation of nutrients in the phloem system. As opposed to this, sucrose is translocated from the point of supply (the leaf), which is the source to the point of metabolism or storage, which is referred to as a sink. Depending on the requirements of the plant, the source and sink points can be reversed.

Hence, the correct answer is option 2) Source to Sink

Q3. The most abundant solute in the phloem sap is :

1. Potassium ions

2. Sucrose

3. Glucose

4. Starch

Correct answer: 2) Sucrose

Explanation:

Phloem sap is primarily composed of water and sucrose, which serve as the main form of carbohydrate transported within the plant. In addition to sucrose, the phloem sap also contains other sugars, hormones, amino acids, and various organic and inorganic molecules. These substances are translocated through the phloem from source tissues (like leaves, where they are produced) to sink tissues (such as roots, fruits, and growing regions) to support growth, development, and storage.

Hence, the correct answer is option 2) Sucrose.

Also Read:

• MCQs on Phloem Transport

• Minerals

• Nitrogen Cycle

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