Transport and Energy Demand

Intact membranes are effective barriers to the passage of ions and uncharged molecules. On the other hand, they are also the sites of selectivity and transport against the concentration gradient of solutes. In the experiment recorded in Table 2.2, for example, the potassium concentration in maize root press sap (which is approximately equal to the potassium concentration of the vacuoles) rose to 80 times the value of that in the external solution. In contrast, the sodium concentration in the root press sap remained lower than that in the external solution. It is generally agreed that such selectivity and accumulation requires both, an energy supply as a driving force and specific binding sites, carriers or permeases in the membranes, most likely proteins such as the sulfate-binding protein isolated from microorganisms (Pardee, 1967) or the sulfate permease in roots (Hawkesford and Belcher 1991; Section 2.5.6). A direct coupling of carrier-mediated selective ion transport and consumption of energy-rich phosphates in the form of ATP was proposed in this process. In this model the ATP was supplied via respiration (oxidative phosphorylation in the mitochondria; Section 8.4.3) and required for activation of the carrier, the ion binding to the carrier, the membrane transport of the carrier-ion complex, or the release of the ion from the carrier at the internal surface of the membrane.

Strong support for the involvement of ATP in carrier-mediated ion transport was first presented by Fisher et al. (1970). Studying the uptake of K+ by roots of various plant species, these workers demonstrated a close relationship between K+ uptake and ATPase activity (Fig. 2.6). Furthermore, Mg-ATPases (Section 8.5) of the plasma membrane are strongly stimulated by K+ (Fisher et al., 1970; Briskin and Poole, 1983) so that ions such as K+, when added to the external solution, trigger their own transport across the plasma membrane.

The energy demand for ion uptake by roots is considerable, especially during rapid vegetative growth (Table 2.7). Of the total respiratory energy cost, expressed as ATP consumption, up to 36% is required for ion uptake. With increase in plant age this

Atpase Activity Plant

Fig. 2.6 (A) Potassium ion uptake (influx) and (B) ATPase activity (ATP -> ADP + P;) in isolated roots of different plant species. Key: barley; O, oat; wheat; #, maize. (After

Fig. 2.6 (A) Potassium ion uptake (influx) and (B) ATPase activity (ATP -> ADP + P;) in isolated roots of different plant species. Key: barley; O, oat; wheat; #, maize. (After

Table 2.7

Respiratory Energy Costs for Ion Uptake in Roots of Carex diandra"

Table 2.7

Respiratory Energy Costs for Ion Uptake in Roots of Carex diandra"

Proportion of total ATP demand required for

40

Plant age (days) 60

80

Ion uptake

36

17

10

Growth

39

43

38

Maintenance of biomass

25

40

52

proportion declines in favour of ATP demand for growth and maintenance of biomass. In principle, similar results have been found with maize (Werf et al., 1988).

These calculations at a whole plant level on ATP demand for ion uptake by roots have to be interpreted with care with respect to energy demand for membrane transport of ions in root cells. Firstly, these calculations include energy demand for radial transport through the roots and secretion into the xylem (Sections 2.7 and 2.8). Secondly, a relatively large proportion of carbohydrates supplied from the shoot to the roots are oxidized via the nonphosphorylating mitochondrial electron transport chain ('alternative pathway'; Section 5.3) yielding less ATP synthesized per molecule of carbohydrate oxidized. Taking this shift in respiratory pathway into account, a requirement of one molecule ATP per ion transported across the plasma membrane has been calculated (Lambers et al., 1981). Such calculations are based on net uptake and include energy requirement for re-uptake ('retrieval') of ions from the apoplasm of the root ('efflux costs') which are assumed to be in the range of 20% of the influx costs (Bouma and De Visser, 1993). Thirdly, a direct coupling of ATP consumption and ion transport across membranes is the exception rather than the rule. As discussed below, ATP-driven pumps at the plasma membrane and the tonoplast also have functions other than transport of mineral nutrients and organic solutes across the membranes.

2.4.2 Active and Passive Transport: Electrogenic Pumps, Carriers, Ion Channels

Solute transport across membranes is not necessarily an active process. Solutes may be more concentrated on one side of the membrane (i.e. they may possess more free energy) and thus diffuse from a higher to a lower concentration (or chemical potential). This 'downhill' transport across a membrane is, in thermodynamic terms, a passive transport with the aid of carriers, or across aqueous pores (Clarkson, 1977). In cells, such downhill transport of ions across the plasma membrane may be maintained by a lowering of the ion activity in the cytoplasm, for example, due to adsorption at charged groups (e.g., R ■ COO" or R • NH3") or to incorporation into organic structures (e.g., phosphate into nucleic acids). This is particularly true in meristematic tissues (e.g., root tips).

In contrast, membrane transport against the gradient of potential energy ('uphill') must be linked directly or indirectly to an energy-consuming mechanism, a 'pump' in

External solution

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