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or NADH at the plasma membrane-cell wall interface. Toxicity by activated oxygen species and

its derivates is caused, for example by oxidation of thiol groups of en ymes and peroxidation of

polyunsaturated fatty acids in membranes. Aerobic organisms including plants posses a range of

defense system for detoxification of oxygen radicals and hydrogen peroxide, including superoxide

dismutase and peroxidases!catalase. "ineral nutrition of plants may affect at various level both, generation of toxic oxygen species

and hydrogen peroxide, mechanisms for detoxification. These may be summari ed as follows#

$. As a component of detoxifying en ymes%e.g. inc, copper, manganesr, or iron in

superoxide dismutases& iron in peroxidases and catalase'

(. )y the accumulation of precursors for rdical formation % e.g. phenols and *uinones' under

nutrient deficiency %e.g. boron deficiency'

+. Through a decrease in sin activity %i.e demand' and accumulation of photosynthates and

correspondingly elevated levels of toxic oxygen species in source leaves under mineral

nutrient deficiencies %e.g. potassium and magnesium'.

These various aspects are discussed in more detail in tehe relevant section on photosynthesis

%chapter ' and function of mineral nutrients %chapters and /'.

(.0 1olute Transport Across "embranes

(.0.$ Transport and 2nergy Demand

3ntact membranes are effective barriers to the passage of ions and uncharged molecules. 4n the

other hand, they are also the sites of selectivity and transport against the concentration gradient of

solutes. 3n the experiment recorded in table (.(, for example, the potassium concentration in mai e

root pres sap%which is approximately e*ual to the potassium concentration of the vacuoles' rose to5 times the value of that in the external solution. 3n contrast, the sodium concentration in the root

press sap remained lower than that in the external solution. 3t is generally agreed that such

selectivity and accumulation re*uires both, an energy supply as a driving force and specific

binding sites, carriers or permeases in the membranes, most li ely proteins such as the sulfate-

binding protein isolated from microorganisms or the sulfate permease in roots. A direct coupling

of carrier-mediated selective ion transport and consumption of energy-rich phospates in the form

AT6 was proposed in this process. 3n this model the AT6 was supplied via respiration%oxidative

phosphorylation in the mitochondria' and re*uired 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.

1trong support for the involvement of AT6 in carrier-mediated ion transport was first

presented by 7isher et al.%$/85'. 1tudying the upta e 9 : by roots of various plant species, these

wor ers demonstrated a close relationship between 9 : upta e and AT6ase activity. 7urthermore,

"g AT6ases of the plasma membrane are strongly stimulated by 9 : so that ions such as 9 : , when

added to the external solution, trigger their own transport across the plasma membrane.

The energy demand for ion upta e by roots is considerable, especially during rapid vegetative

growth. 4f the total respiratory energy cost, expressed as AT6 consumption, up to +; is re*uired

for ion upta e.

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interpreted with care with respect to energy demand for membrane transport of ions in root cells.

7irstly, these calculations include energy demand for radial transport through the roots and

secretion into the xylem. 1econdly, a relatively large proportion of carbohydrates supplied from

the shoot to the roots are oxidi ed via the nonphosporylating mitochondrial electron transport

chain yielding less AT6 synthesi ed per molecule of carbohydrate oxidi ed. Ta ing this shift inrespiratory pathway into account, a re*uirement of one molecule AT6 per ion transported across

the plasma membrane has been calculated. 1uch calculations are based on net upta e and include

energy re*uirement for re-upta e %retrieval' of ions from the apoplasm of the root %efflux costs'

which are assumed to be in the range of (5 of the influx costs. Thirdly, a direct coupling of AT6

consumption and ion transport acroos membranes is the exception rather than the rule. As

discussed below, AT6 driven pumps at the plasma membrane and the tonoplast also have function

other than transport of mineral nutrients and organic solute across the membranes.

(.0.( Active and 6assive Transport # 2lectrogenic 6umps, =arriers, 3on =hannels

1olute transport across membranes is not necessarily an active process. 1olute may be more

concentrated on one side of the membrane %i.e. they may posses 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

a*ueous pores. 3n cells, such downhill transport of ions acroos the plasma membrane maintained

by a lowering of the ion activity in the cytoplasm, for example, due to adsorption at charged

groups %e.g. @. =44 - or @. NH +: ' or to incorporation into organic structures %e.g., root tips'.

3n contrast, membrane transports against the gradient of potential energy %>uphill' must be

lin ed directly or indirectly to an energy- consuming mechanism, a >pump? in the membrane.