Transportation in Plants
All green plants require, along with the food prepared by photosynthesis, other inorganic nutrients. These are absorbed from the soil by the roots as they are in contact with the soil. The materials prepared by photosynthesis and the materials absorbed by the roots should be diffused to other parts of the plant and this process is called transportation in plants. The two main vessels involved in transportation are xylem and phloem vessels. The transportation with the expense of energy is called active transport and that without expense of energy is called passive transport.
Phloem, Sugar and Translocation
Phloem consists of several types of cells: sieve tube cells (aqua sieve elements), companion cells and the vascular parenchyma. Sieve cells are tubular cells with end walls known as sieve plates. Most lose their nuclei but remain alive, leaving an empty cell with functioning plasma.
Companion cells load sugar into the sieve element (sieve elements are connected into sieve tubes). Fluids can move up or down within the phloem and are transported from one place to another. Sources are places where sugars are being produced. Sinks are places where sugar is being consumed or stored.
Food moves through the phloem by a Pressure-Flow Mechanism. Sugar moves from a source to a sink by osmotic pressure. Translocation of sugar into a sieve element causes water to enter that cell, increasing the pressure of the sugar/water mix. The pressure causes the sap to flow towards an area of lower pressure, the sink. In the sink, the sugar is removed from the phloem by another energy-requiring step and usually converted into starch or metabolized.
Transportation of water and minerals in plants
Root hairs are thin-walled extensions of the epidermal cells in roots. They provide increased surface area and thus more efficient absorption of water and minerals. Water and dissolved mineral nutrients enter the plant via two roots. Water and selected solutes pass through only the cell membrane of the epidermis of the root hair and then through plasmodermata on every cell until they reach the xylem: intracellular route. Water and solutes enter the cell wall of the root hair and pass between the wall and plasma membrane until it enters the endodermis, a layer of cells that they must pass through to enter the xylem: extra cellular route.
Only when the concentration inside the endodermal cell falls below that of the cortex parenchyma cells, does the water flow into the endodermis and from there into the xylem.
In the leaf cells, these raw materials are combined in the presence of solar energy by a series of chemical reactions. These reactions take place in the chloroplasts with the help of many enzymes. The end products of these reactions are glucose and oxygen. Oxygen is released in the atmosphere through the stomata. The glucose molecules get converted to another form of sugar called sucrose. Sucrose is easily transported to various parts of the plant body through phloem vessels and is stored in the form of starch.
The chemical process of photosynthesis can be summarized as follows:
The leaf is well equipped for the process of photosynthesis.
Green plants not only produce food for themselves but for the rest of the living world.