Biology TRANSPORT OF WATER

### KEY TOPICS

star Long Distance Transport of Water
star Water Absorption from soil to Root hair

### LONG DISTANCE TRANSPORT OF WATER

● color{violet}("Long distance transport") of substances within a plant cannot be by color{brown}("diffusion alone.")

● color{violet}("Diffusion") is a slow process.

● It can account for only color{brown}("short distance") color{violet}("movement of molecules") .

● For example, the color{violet}("movement of a molecule") across a typical plant cell (color{brown}("about 50 μm") ) takes approximately color{brown}("2.5 s.")

● In large and color{violet}("complex organisms,") often substances have to be moved across very color{violet}("large distances.")

● Sometimes the color{violet}("sites of production or absorption") and sites of storage are too far from each other; diffusion or
active transport would not suffice.

● Special color{brown}("long distance transport systems") become necessary so as to move substances across long distances and at a color{violet}("much faster rate.")

● color{violet}("Water and minerals, and food") are generally moved by a color{brown}("mass") or color{brown}("bulk flow system.")

● color{violet}("Mass flow") is the movement of substances color{brown}("in bulk or 𝑒𝑛 𝑚𝑎𝑠𝑠𝑒") from one point to another as a result of pressure differences between the two points.

● It is a characteristic of color{violet}("mass flow") that substances, whether in solution or in suspension, are swept along at the
same pace, as in a color{violet}("flowing river.")

● This is color{violet}("unlike diffusion") where different substances move independently depending on their concentration gradients.

● color{violet}("Bulk flow") can be achieved either through a color{brown}("positive hydrostatic pressure") gradient (e.g., a garden hose) or a color{brown}("negative hydrostatic pressure") gradient (e.g., suction through a straw).

● The color{violet}("bulk movement") of substances through the conducting or vascular tissues of plants is called color{brown}("translocation").

● The color{violet}("higher plants") have highly specialised vascular tissues – color{brown}("xylem and phloem.")

● color{brown}("Xylem") is associated with translocation of mainly color{violet}("water, mineral salts, some organic nitrogen and hormones") , from roots to the aerial parts of the plants.

● The color{brown}("phloem") translocates a variety of color{violet}("organic and inorganic") solutes, mainly from the leaves to other color{violet}("parts of the plants.")

### WATER ABSORPTION FROM SOIL TO ROOT HAIR

● We know that the color{brown}("roots absorb") most of the water that goes into color{violet}("plants") ; obviously that is why we apply water to the color{violet}("soil") and not on the color{violet}("leaves.")

● The responsibility of color{violet}("absorption of water and minerals") is more specifically the function of the color{violet}("root hairs ") that are present in millions at the tips of the color{violet}("roots.")

● color{violet}("Root hairs") are thin-walled slender extensions of color{brown}("root epidermal cells") that greatly increase the surface area for color{violet}("absorption.")

● Water is absorbed along with color{violet}("mineral solutes"), by the color{violet}("root hairs,") purely by color{brown}("diffusion.")

● Once water is absorbed by the color{violet}("root hairs,") it can move deeper into color{violet}("root layers ") by two distinct pathways:

color{green}(star \ \ " Apoplast pathway")

color{green}(star \ \ "Symplast pathway")