Mechanism of Nutrient UptakeA plant nutrient is a chemical element that is essential for plant growth and reproduction. These elements must be required for a plant to complete its life cycle. There are some 240,000 species of higher plants and not all of those species will have the same mineral needs, at the same scale. Some will require a specific element in much higher concentration than others, and others will be able to tolerate a much higher concentration of an essential element that would, to a different species, be toxic. There are many essential plant nutrients, but they can be divided into two general groups based on the quantities of the nutrient needed for a healthy plant: the macronutrients, which are required in relatively large amounts, and the micronutrients, which are sometimes required in only trace amounts. There are six basic macronutrients required by virtually all plants: Nitrogen (N) Phosphorus (P) Potassium (K) Sulfur (S) Calcium (Ca) Magnesium (Mg)

Micronutrients form a coherent group, including eight core elements: iron (Fe), sodium (Na), chlorine (Cl), boron (B),

manganese (Mn), zinc (Zn), copper (Cu), molybdenum (Mo).

The precursor of plant nutrition is the acquisition of plant essential nutrients by plant roots. In order for uptake to occur, plant nutrients must exist near, and come in contact with, a functional plant root. Up to four processes can be involved.

Root Interception Mass Flow Biological Transport Diffusion

Root InterceptionAs roots elongate, they explore (and come in contact with) greater volumes of soil. With the exception of systems where a physical barrier exists, roots generally elongate downwards. As this occurs, roots touch more soil particles and more soil solution. Furthermore, roots often grow root hairs. Root hairs are unicellular appendages that increase the surface area of roots dramatically. Root elongation and growth results in increasing contact with plant essential nutrients. This natural growth and elongation of plant roots results in the interception of elements in solution and elements adsorbed (attached) to soil particles, hence, root interception.

Mass Flow (also called convection):In soil, water flows in any and every direction, not just down with gravity. Nutrients can remain soluble in soil and flow with water. During periods of light, plant stomata are open and convective flow through the plant occurs. You know, evapotranspiration (ET). Water leaves the leaf blade, and to prevent loss of turgor (wilting), leaf water is replaced with newly acquired soil water delivered by the xylem from the roots. You must recognize soil water often contains nutrients and these nutrients will travel into the plant with water. This is different from root interception. As stated above, root interception occurs when a root physically elongates and acquires a nutrient ion (by invading its space). Mass flow is when a soluble nutrient ion is pulled to the root, with water, all by sheer vacuum. See how this is different from root interception?

Biological Transport:Plants form mutually beneficial relationships with organisms and other plants. These relationships are termed symbiotic, and a relationship of this type exists between mycorrhiza and grasses. The term 'mycorrhiza' literally means fungus root! Mycorrhizas are ubiquitous in most soils and commonly colonize plant roots. The plant benefits through improved fertility conditions, as the fungal hyphae actually transport nutrients to the roots from locations far away. The fungus benefits because it consumes the carbonrich exudates that are excreted by the roots through natural growth and aging processes. Phosphorus, copper, and zinc are the most common nutrients transported through this process.

DiffusionNutrients such as phosphorus and potassium are absorbed strongly by soils and are only present in small quantities in the soil solution. These nutrients move to the root by diffusion. As uptake of these nutrients occurs at the root, the concentration in the soil solution in close proximity to the root decreases. This creates a gradient for the nutrient

to diffuse through the soil solution from a zone of high concentration to the depleted solution adjacent to the root. Diffusion is responsible for the majority of the P, K and Zn moving to the root for uptake. Roots (or more specifically root walls) are porous. They allow water to freely enter, but not much else. Everything else, including all 14 essential plant nutrients, enters the root by any of three possible ways:

Membrane diffusion Facilitated diffusion Active transport Membrane diffusion: Carbon dioxide (CO2) and oxygen (O2) are neutral molecules that are small enough to diffuse through membranes freely. However, these two molecules (and water [H2O]) are the only ones privileged with total access. Everything else is either facilitated or carried into the symplast (the plant circulatory system). Gaseous ammonia (NH3) may also be capable of membrane diffusion, but is more commonly found as the ammonium ion (NH4+) in solution. This type of plant uptake does not require any energy. Facilitated diffusion: This plant uptake mechanism governs entry of charged ions (different from the neutral ions mentioned above, like CO2, O2, H2O, and gaseous ammonia [NH3]). Root cell membranes have specialized structures embedded in them called transport proteins and channel proteins. When a concentration gradient exists, uptake will occur through facilitated diffusion. For example, a plant is highly deficient in phosphate (H2PO4-) as very little phosphate is contained within its cells. If someone fertilizes this system with a starter fertilizer, many phosphate ions will diffuse through the soil water. Before too long, these ions will diffuse right up to the root membrane and form a concentration gradient. Many phosphate ions outside the root cell, very few

phosphate ions within the cell. A greater concentration of phosphate outside the cell than inside the cell promotes the diffusion of phosphate into the cell. Over time, there will be an equal number of phosphate ions inside the cell and outside the cell. At that point, the concentration gradient will cease to exist and facilitated diffusion will end. This occurs with all the 14 essential plant nutrients and it occurs with the non-essential ones too (Na, Al, etc.). If they are in high concentration in the soil they will likely diffuse from an area of high concentration (soil) to the area of low concentration (root cell), whether the plant wants those ions or not. This type of plant uptake does not require any energy. Active transport: Active transport is different from the two types of diffusion listed above, mainly because it requires energy. If ions move from areas of high concentration to low concentration, active transport prevents ions from leaving root cells and replenishing soil water. Cell membranes use active transport to accumulate solutes (nutrients) against concentration gradients. Active transport is also used to maintain electrical equilibrium between the cell interior and exterior. Again, active transport costs the plant energy. Because energy is expensive, plants only use active transport mechanisms when they are absolutely necessary. It is important to note that active transport is used by root cells as the last ditch measure to either acquire nutrients in which the plant is deficient, or expel nutrients or ions that are overly-abundant and causing a toxicity to the plant. Under optimal conditions, where all nutrients are available in optimal ranges in the soil, plants really don't have to actively transport much at all. This allows plants to reserve that energy for growth, maintenance and reproduction; and makes them what we call healthy. Again, when optimal concentrations of plant nutrients are present (and maintained) in the soil, facilitated diffusion allows for adequate nutrition. When soils are depleted or simply devoid of plant essential nutrients, other ions (non-essentials) will follow concentration gradients into the root cells by facilitated diffusion. This forces the plant to use active transport to achieve optimal conditions within the plant. Again, active transport requires and consumes energy. Whole-plant response to global atmospheric change is the product of a complex set of processes that results from coordinated interactions between root and shoot .As the

main organ involved in water and nutrient uptake and as one of the major sinks for assimilated C, roots play a critical role in determining plant and ecosystem responses to various facets of global change ranging from N deposition and elevated CO2 to elevated ozone and UV-B. Despite the growing recognition that roots play a critical role in determining the overall response of terrestrial vegetation to global change, very little is known about the mechanisms involved. An important root shoot interaction that might determine the overall response of plants is the ability of the root system to adjust nutrient acquisition capacity to meet variations in shoot demand Ca used by environmental changes. Plant roots can alter their nutrient acquisition capacity by adjusting their physiological, longevity, morphological and} or architectural characteristics to meet changes in shoot nutrient demand. Even though physiological capacity of root nutrient uptake is only one of the number of adjustments that influences nutrient acquisition, its response to changes in plant environment might provide a key mechanistic explanation of why some species are more sensitive to global change than others. However, it should be highlighted that the degree to which kinetics of nutrient uptake or other potential adjustments are expressed would ultimately depend on soil nutrient availability and soil factors that determine nutrient transport to the root surface.

Studies of shoot responses to global change commonly examine physiological parameters such as photosynthetic rate and stomatal conductance. By contrast, studies of root responses to global change often focus on root growth and morphological characteristics, seldom addressing changes in physiological characteristics such as hydraulic conductivity and kinetics of ion uptake. Active root nutrient absorption is a highly adaptive plant characteristic that influences acquisition of N and other nutrients in response to environmental factors. Therefore, knowledge of changes in the kinetics of nutrient uptake and the relative species differences is critical in predicting ecosystem responses to global change. Although in recent years a number of investigators have underscored the need for a better understanding of how, for example, elevated CO2 changes root uptake kinetics.

Nutrient Uptake Mechanisms in PlasmodiumThe growth and replication of the intracellular malaria parasite within the erythrocytes of

its vertebrate host is fuelled by the uptake of essential nutrients from the extracellular medium. Nutrients enter infected cells across the erythrocyte membrane and are taken up from the host cell compartment, into the intraerythrocytic parasite, through the membranes at the parasite surface and/or via the endocytotic process by which the parasite takes up haemoglobin from the host cell cytosol. On entering the parasite the nutrients are metabolised within the parasite cytosol and/or within organelles. The metabolism often entails the nutrients becoming phosphorylated, and this serves to trap them within the compartment in which phosphorylation takes place. Integral membrane proteins that span the membrane lipid bilayer mediate the passage of nutrients across the various membrane systems of the Plasmodium-infected erythrocyte. It is these membrane transport proteins that are the major focus of this review. A number of other recent reviews and commentaries have dealt with one or more aspects of the same subject. MEMBRANE TRANSPORT PROTEINS Channels are proteins which, when open, provide what is essentially a diffusion pathway for small hydrophilic solutes to pass from one side of the membrane to the other. Many types of channel are selective for one or more inorganic ion and are named on the basis of this selectivity (e.g. K+ channels, Na+ channels, Ca2+ channels). There are, however, proteins of this class that are permeable to larger molecules and which provide an effective means for the rapid movement of organic solutes across cell membranes. For example the porins that are present in the outer membrane of gramnegative bacteria are high-capacity channels that render this membrane highly permeable to low molecular weight solutes and which mediate the passage of nutrients from the external environment into the periplasmic space between the outer and inner membranes. Transporters are proteins which, like enzymes, bind their substrates at an active site and which mediate the movement of the substrates from one side of the membrane bilayer to the other via a mechanism, which involves the protein undergoing a conformational change. Transporters are classified in a number of different ways. Uniporters are transporters, which move a single type of solute at a time across the membrane. Symporters and antiporters are transporters that move two or more types

of solute at a time across the membrane; symporters move the different classes of solute in the same direction, whereas antiporters move them in the opposite direction. In both cases the movement of one class of solute down an electrochemical gradient can energise the movement of another class of solute against an electrochemical gradient. Symporters and (to a lesser extent) antiporters thereby provide an effective mechanism for the uptake of nutrients by cells. There is a general tendency for animal cells to use Na+: nutrient symporters (energised by an inward Na+ electrochemical gradient) to facilitate the uptake of nutrients, and for cells of plants and lower eukaryotes to use H+: nutrient symporters (energised by an inward H+ electrochemical gradient), though there are counter examples in each case. A pump is, in this context, a transporter that harnesses the energy derived from a biochemical reaction (commonly, though not always, involving the hydrolysis of ATP) to move solutes across a membrane, against an electrochemical gradient. Such proteins serve to generate transmembrane ion gradients, which are, in turn, used to energise the uptake of nutrients via coupled transporters (i.e. symporters and antiporters). Bacteria make frequent use of ATP-driven pumps to accumulate often-sparse nutrients from the extracellular environment. Eukaryotes also have a range of ATP-driven pumps for organic solutes (most notably members of the ATP Binding Cassette family) but these tend to be used for effluxing solutes from the cell cytosol, either into the extracellular space or into internal organelles, rather than the uptake of nutrients from the environment.

The figure provides a schematic overview of the different types of processes involved in nutrient uptake, including channels (shown as being in an outer membrane, as in a gram negative bacterium), uniporters, Na+ and H+coupled symporters, ATP-driven pumps, and plasma membrane endocytosis. It also shows substrate phosphorylation, which, by rendering the substrate charged and therefore membrane-impermeant,

prevents it from escaping from the cell.