Biology APOLPLAST AND SYMPLAST PATHWAY

### KEY TOPICS

star Apoplast
star Symplast
star Water Transport to Vascular Cylinder
star Mycorrhiza

### APOPLAST

● The color{brown}("apoplast") is the system of adjacent cell walls that is continuous throughout the plant, except at the color{brown}("casparian strips") of the color{violet}("endodermis") in the color{violet}("roots.")

● The color{violet}("apoplastic movement of water") occurs exclusively through the color{brown}("intercellular spaces") and the walls of the cells.

● Movement through the apoplast does not involve crossing the color{brown}("cell membrane.")

● This movement is dependent on the color{brown}("gradient. ")

● The apoplast does not provide color{brown}("any barrier") to water movement and water movement is through color{violet}("mass flow.")

● As water evaporates into thecolor{violet}(" intercellular spaces") or the atmosphere, color{brown}("tension develop") in the continuous stream of water in the apoplast, hence color{brown}("mass flow of water") occurs due to the color{brown}("adhesive and cohesive properties") of water.

### SYMPLAST

● The color{brown}("symplastic") system is the system of color{brown}("interconnected protoplasts.")

● color{brown}("Neighbouring cells") are connected through color{violet}("cytoplasmic strands") that extend through color{brown}("plasmodesmata. ")

● During symplastic movement, the water travels color{brown}("through the cells") – their cytoplasm; intercellular movement is through the plasmodesmata.

● Water has to enter the cells through the color{brown}("cell membrane") , hence the movement is relatively slower.

● Movement is again color{brown}("down a potential gradient.")

● color{violet}("Symplastic movement") may be aided by color{brown}("cytoplasmic streaming.")

● One have observed cytoplasmic streaming in cells of the color{violet}("𝐻𝑦𝑑𝑟𝑖𝑙𝑙𝑎") leaf; the color{brown}("movement of chloroplast") due to streaming is easily visible.

### WATER TRANSPORT TO VASCULAR CYLINDER

● Most of the color{violet}("water flow") in the roots occurs via the apoplast since the color{brown}("cortical cells") are loosely packed, and hence offer color{violet}("no resistance to water movement. ")

● However, the inner boundary of the cortex, the color{brown}("endodermis,") is impervious to water because of a band of color{brown}("suberised matrix") called the color{brown}("casparian strip.")

● color{violet}("Water molecules") are unable to penetrate the layer, so they are directed to wall regions that are not suberised, into the cells proper through the membranes.

● The water then moves through the color{violet}("symplast") and again crosses a membrane to reach the color{violet}("cells of the xylem.")

● The color{violet}("movement of water") through the root layers is color{brown}("ultimately symplastic") in the color{violet}("endodermis.")

● This is the only way water and other solutes can enter the color{brown}("vascular cylinder.")

● Once inside the color{violet}("xylem, water") is again free to move between cells as well as through them.

● In young roots, water enters directly into the color{brown}("xylem vessels") and/or color{brown}("tracheids.")

● These are color{brown}("non-living conduits") and so are parts of the color{violet}("apoplast.")

### MYCORRHIZA

● Some plants have color{brown}("additional structures") associated with them that help in color{violet}("water (and mineral)") absorption.

● A color{brown}("mycorrhiza") is a color{violet}("symbiotic association") of a fungus with a root system.

● The color{violet}("fungal filaments") form a network around the color{violet}("young root") or they penetrate the root cells.

● The color{violet}("hyphae") have a very color{brown}("large surface area") that absorb mineral ions and water from the soil from a much color{violet}("larger volume of soil") that perhaps a color{violet}("root") cannot do.

● The color{violet}("fungus") provides color{brown}("minerals and water") to the roots, in turn the roots provide sugars and N-containing compounds to the color{violet}("mycorrhizae.")

● Some plants have an color{brown}("obligate association") with the color{violet}("mycorrhizae.")

● For example, color{brown}("𝑃𝑖𝑛𝑢𝑠 seeds") cannot germinate and establish without the presence of mycorrhizae.