Biology PHLOEM TRANSPORT

KEY TOPICS

star Phloem Transport: From Source to Sink
star Pressure Flow or Mass Flow Hypothesis
star Girdling Experiment

PHLOEM TRANSPORT: FLOW FROM SOURCE TO SINK

● Food, color{brown}("primarily sucrose,") is transported by the vascular tissue color{brown}("phloem") from a color{brown}("source to a sink. ")

● Usually the color{brown}("source") is understood to be that part of the plant which color{brown}("synthesises the food,") i.e., the leaf, and color{violet}("sink,") the part that needs or color{violet}("stores the food.")

● But, the color{violet}("source and sink") may becolor{brown}(" reversed") depending on the color{brown}("season,") or the plant’s needs.

● Sugar stored in roots may be color{brown}("mobilised") to become a color{violet}("source of food") in the color{brown}("early spring") when the buds of trees, act as sink; they need energy for growth and development of the color{violet}("photosynthetic apparatus.")

● Since the color{brown}("source-sink relationship") is variable, the direction of movement in the color{violet}("phloem") can be upwards or downwards, i.e., color{brown}("bi-directional.")

● This contrasts with that of the color{brown}("xylem") where the movement is always color{brown}("unidirectional,") i.e., upwards.

● Hence, unlike color{brown}("one-way flow of water") in color{violet}("transpiration, food in phloem sap") can be transported in any required direction so long as there is a source of sugar and a sink able to use, store or remove the sugar.

● color{violet}("Phloem sap") is mainly color{brown}("water and sucrose,") but other color{brown}("sugars, hormones") and color{brown}("amino acids") are also transported or color{brown}("translocated") through color{violet}("phloem.")

THE PRESSURE FLOW OR MASS FLOW HYPOTHESIS

● The accepted mechanism used for the color{violet}("translocation of sugars") from source to sink is called the color{brown}("pressure flow hypothesis.")

● As color{brown}("glucose") is prepared at the source (by color{violet}("photosynthesis") ) it is converted to color{brown}("sucrose") (a dissacharide).

● The sugar is then moved in the form of color{violet}("sucrose") into the color{brown}("companion cells") and then into the living phloem color{brown}("sieve tube cells") by color{brown}("active transport.")

● This process of loading at the source produces a color{brown}("hypertonic condition") in the color{violet}("phloem.")

● Water in the adjacent color{violet}("xylem") moves into the color{violet}("phloem") by color{brown}("osmosis.")

● As color{violet}("osmotic pressure") builds up the color{violet}("phloem sap") will move to areas of lower pressure.

● At the color{brown}("sink") osmotic pressure must be reduced.

● Again color{brown}("active transport") is necessary to move the color{violet}("sucrose out of the phloem sap") and into the cells which will use the sugar – converting it into color{violet}("energy, starch, or cellulose.")

● As sugars are removed, the color{brown}("osmotic pressure decreases") and water moves out of the color{violet}("phloem.")

● To summarise, the movement of color{violet}("sugars in the phloem") begins at the color{brown}("source"), where sugars are color{brown}("loaded") (actively transported) into a color{violet}("sieve tube.")

● Loading of the color{violet}("phloem sets") up a color{brown}("water potential gradient") that facilitates the mass movement in the color{violet}("phloem.")

● color{violet}("Phloem tissue") is composed of color{brown}("sieve tube cells,") which form long columns with holes in their end walls called color{violet}("sieve plates.")

● color{brown}("Cytoplasmic strands") pass through the holes in the color{violet}("sieve plates"), so forming continuous filaments.

● As color{brown}("hydrostatic pressure") in the phloem sieve tube increases, pressure flow begins, and the sap moves through the color{violet}("phloem.")

● Meanwhile, at the color{violet}("sink"), incoming sugars are color{brown}("actively transported") out of the color{violet}("phloem") and removed as color{violet}("complex carbohydrates. ")

● The loss of solute produces a color{brown}("high water potential") in the color{violet}("phloem"), and water passes out, returning eventually to color{violet}("xylem.")

GIRDLING EXPERIMENT

● A simple experiment, called color{brown}("girdling,") was used to identify the tissues through which color{violet}("food is transported.")

● On the trunk of a tree a ring of bark up to a depth of the color{brown}("phloem layer,") can be carefully removed.

● In the absence of downward movement of color{violet}("food the portion") of the bark above the ring on the stem becomes color{brown}("swollen") after a few weeks.

● This simple experiment shows that color{violet}("phloem") is the color{violet}("tissue responsible") for color{brown}("translocation of food"); and that transport takes place in one direction, i.e., towards the roots.