|Root Uptake System|
Fertile soil is a complex mixture of a variety of minerals, many different types of organic matter in different stages of decay, and a host of living microorganisms. This complex medium holds a large quantity of water, which it supplies to plants.
In addition to water, the soil supplies the plants with the thirteen mineral nutrients required for normal growth and development. These nutrients (and the ionic forms taken up by the root) are nitrogen, phosphorus, potassium, sulfur, calcium, magnesium, iron, manganese, boron, chlorine, zinc, copper, and molybdenum.
Varying amounts of the semineral nutrients exist both as constituents of the soil particles and as dissolved ions in the soil water. Root uptake systems are responsible for taking these nutrient ions and water from the soil and moving them into the root tissues.
Most plant roots extend below the soil surface as either a fibrous root system or tap root system. Inafibrous root system, the major root branches numerous times, and each branch divides again and again, until a mesh like network is formed within the soil. This system does not penetrate very deep into the soil, but it does cover considerable area close to the surface.
In a taproot system, only small secondary lateral roots branch off the main root (the taproot) as it grows downward into the soil. The taproot may extend several meters in depth. Along the outer periphery of either of these root systems, large numbers of small filamentous root fibers and root hairs can be found.
The root hairs are filament like projections of the epidermal (outer layer) cells. These root hairs, in conjunction with the cells of the filamentous root fibers, are responsible for the vast majority of water and nutrient uptake.
Each root cell, as is the case in all plant cells, is surrounded by a porous cell wall. Immediately inside the cell wall is a semipermeable membrane, which regulates the movement of ions and molecules into the cytosol of the cell.
This semipermeable membrane is a fluid mosaic structure composed primarily of lipid and protein. A double layer of lipid provides the basic stable structure of the membrane. The protein is then interspersed periodically throughout the lipid bilayer.
Some of these proteins, called peripheral proteins, penetrate only one of the layers of lipid, while integral proteins extend through both lipid layers to interface with the environment both inside and outside the cell. The rate and extent of water and ion movement through membranes are largely determined by this structural configuration.
Ion Uptake Mechanisms
Uptake of ions across membranes is accomplished by three mechanisms: simple diffusion, facilitated diffusion, and active transport. The first two are passive processes, with no direct input of cellular energy required. The latter, as the name indicates, is an active process requiring the cell to expend energy.
In its simplest analysis, diffusion is the net movement of suspended particles down a concentration gradient. Thus, certain ions dissolved in the soil solution will move into the root cell cytosol as long as the external concentration is higher than the concentration inside the cell.
Because of the lipid nature of the membrane, the rate of this movement will be determined by the lipid solubility of the particle. Those particles with high lipid solubility will diffuse across the membrane much faster than those with low lipid solubility.
Many nutrients exhibit low lipid solubility, yet still diffuse across the membrane. This is accomplished by facilitated diffusion. The ion with low lipid solubility combines with a membrane protein, which then facilitates its uptake across the membrane.
In both free and facilitated diffusion, the particles move down a concentration gradient. In numerous instances, however, the ion concentration of the root cell cytosol is greater than the concentration of the ion in the soil solution. The ion will nevertheless continue to accumulate within the root tissues.
Diffusion of any sort cannot account for this "uphill" movement against a concentration gradient. In this instance, the nutrient ion will combine with a membrane protein referred to as a carrier.
This protein carrier will transport the particle across the membrane. Uptake mediated by this protein carrier system is called active transport and requires the input of energy supplied by the hydrolysis of adenosine triphosphate (ATP), the cell’s primary energy currency.
Regardless of the mechanism utilized to transport ions into the root cytosol, the final result is the establishment of an osmotic potential across the membrane. Osmotic potential is a measure of the tendency of water to move across a semipermeable membrane in response to a difference in solute concentration.
The addition of a solute to water lowers its osmotic potential, and water will always move from the side of the membrane containing the solution with the lower solute concentration (higher osmotic potential) to the side of the membrane containing the solution with the higher solute concentration (lower osmotic potential).
Hence, the uptake of the mineral ions by the plant root cells establishes a lower osmotic potential within the cytosol than exists in the soil solution, and water flows across the membrane into the cell.
In order for the water and ions to move across the root from the epidermal layer to the internal transport vessels called xylem, several layers of cortex cells and the endodermis must be crossed. There are two pathways water and ions can take.
One is through the cytosol of the epidermal cells, cortical cells, and endodermal cells by means of the plasmodesmata,which are cytoplasmic strands that pass between plant cells, thereby connecting the cells as microscopic "bridges."
These plasmodesmata provide a means by which the cytosol fromall cells can exist as a continuous mass referred to as the symplast, and transport through this system is called symplastic transport.
A second pathway, referred to as apoplastic transport, occurs through the apoplast (the region of continuous cell walls among cells).
Because this apoplast is freely permeable to water and ions, the cells of the epidermis, cortex, and endodermis are in intimate contact with the soil water. Apoplastic transport, however, cannot occur across the endodermis because of the presence of an impermeablewaxy layer in the cellwalls called the Casparian strip.
Thus, water and ions can travel through the apoplast until reaching the endodermis. There, movement into the endodermal cytosol by one of the three mechanisms mentioned above is required.
After passing the Casparian strip, the water and ions can move back into the apoplast. Although there is some disagreement among plant scientists as to which pathway is most important, it is highly probable that both pathways are involved.
Regardless of whether the transport across the root is apoplastic or symplastic, the water and ions reach the xylem tubes within the interior of the root, where subsequent transport throughout the plant can take place.