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Uptake of Mineral Nutrients into Cells
Nutrients cannot be used by cells while they are in the apoplasm. For metabolism they need to be transferred to the inside of the cell. This process occurs in several ways, all of which are dependent in some way upon the electrochemical gradient across the plasma membrane separating the cell cytoplasm and the cell wall. The same is true of transport across the tonoplast into the vacuole.
Transport into the cell (or vacuole) can be divided into two main types: passive which doesn’t require any energy input, and active that does. An example of passive transport would be when an ion is present in a higher concentration outside of a cell than inside. This would lead to the ion diffusing across the membrane along the chemical gradient set up by the difference in concentration. This passage would be facilitated either by carrier proteins or through aqueous pores in the membrane. Further accumulation of such elements beyond the equilibrium point can be achieved by de-activating them e.g. by incorporating them into organic molecules or by attaching them to charged molecules.

All of the other known methods of transport across the plasma membrane are active processes although some of these use the energy to set up an appropriate electro-chemical gradient rather than to directly transport the ion in question.
One of the most significant processes involved in the active transport of ions is the acidification of the apoplasm by pumping H+ ions into the cell wall from the cytoplasm. This sets up both a pH and a charge gradient across the plasma membrane. Ions can then use this potential difference in various ways to aid their crossing of the membrane.
This gradient is set up by a membrane bound, ATP consuming (ATPase) H+ pump. The importance of this process is highlighted by the proportion of ATP used in the roots to import ions using such processes which can be as high as 36% (Werf et al., 1988). Another gradient across the tonoplast is also set up. Here H+ ions are pumped from the cytoplasm into the vacuole. This leaves the cytoplasm with a pH of 7.3-7.6, with the apoplasm and vacuole having pHs of approx 5.5 and 4.5-5.9 respectively.
The power of the potential differences set up across membranes in higher plants is demonstrated by an equation called the Nerst equation:

This shows that at a potential difference of -59mv (real differences are often in excess of this), monovalent ions have to be present in the cell at concentrations ten times higher(cations) or ten times lower(anions) in order for electrochemical equilibrium to be reached. The factor of ten is increased to a factor of a hundred in divalent ions. This means that cations that are present in the external solution in reasonable amounts are able to use the electrochemical gradient set up across the membrane to travel across it.

This passage is mediated by proteins within the membrane. These are either carrier proteins which allow relatively small numbers of ions across, or channel proteins which allow large number of ions to pass through there central pore. Channel proteins spend most of their time in the closed position, opening in response to environmental stimuli.
Carrier proteins have three different methods of transport: - uniport, symport, and antiport. Uniport is when an ion is transferred across the membrane on its own. This method is likely to be used in the case of ions passing down an electrochemical gradient i.e. cations whose external concentration is sufficiently large.
Symport is when an ion which may not be able to travel along an electro-chemical gradient is attached to an ion that is able to travel along the gradient, and so travels through the membrane with it. K+ ions which sometimes have a prohibitively low external concentration for crossing the plasma membrane can pass across in this way by attaching to H+ ions, which are able to travel along the electrochemical gradient set up by the ATPase pump. It also believed that anions such as nitrate are transported across the membrane using this method.

Counter transport is transporting an ion against a concentration gradient by attaching it to a transporter which is transporting an ion in the opposite direction along a concentration gradient. This method is believed to be used to transport ions from the cytoplasm into the vacuole. All three methods can be viewed in the diagram below.