Question

The process of bulk flow transports fluids in a plant. Describe the two main bulk flow processes.

Short answer

Transpiration and Translocation.

Step-by-step solution

Step-1 Introduction

In the case of plants, the transportation for longer distances is carried out by the bulk flow of substances. Bulk-flow transport is influenced by hydrostatic pressure gradients that can be positive or negative. Two main types of bulk-flow transports are; transpiration and translocation. In general, translocation is a bidirectional transportation process whereas transpiration is a unidirectional mechanism.

Step-2 Explanation

Transpiration is the mechanism of water loss from the surface of a leaf through stomata in the form of vapors. Transpirational movement mainly occurs through the xylem. The movement of water from the root to upward is mediated by capillary motion. After the loss of water from the stomata, the water from spongy parenchyma again accumulated at the leaf epidermis due to a change in concentration gradient. The balance in concentration gradient is maintained as the root absorbs water from the soil and the water moves from the root to the upward region of the plant.

Translocation is the process of transportation of organic substances or photosynthates from the site of photosynthesis (otherwise called source) to storage organs like roots and other storage parts (otherwise called the sink) via the phloem. A hypertonic condition arises when glucose is broken down into sucrose and loaded into a sieve-element by the use of energy (adenosine triphosphate). This results in the movement of water from the adjacent xylem into the phloem by osmosis. Sugars are released to the sink from the phloem in the form of complex carbohydrates. The release of solute from the phloem causes high water potential inside the phloem and subsequently water moves back to the adjacent xylem.