Mineral Nutrition (1-3) Flashcards
By what is the ion-uptake of higher plants characterized?
- Selectivity: Certain mineral elements are taken up preferentially, while others are discriminated against or almost excluded
- Accumulation: The concentration of elements can be much higher in cellsap than in the external solution
- Genotype: There are distinct differences between plant species in their ion uptake characteristics
What is Suberin?
- Suberin is present on underground parts, woody stems, and healed wounds
- Suberin is a polymer whose structure is very poorly understood.
- Like cutin, suberin is formed from hydroxy or epoxy fatty acids joined by ester linkages
- It is presence in the Casparian strip of the root endodermis, which forms a barrier between the apoplast of the cortex and the stele
What type of membrane transport proteins are there?
- Channel proteins act as membrane pores, and their specificity is determined primarily by the biophysical properties of the channel.
- Carrier proteins bind the transported molecule on one side of the membrane and release it on the other side.
- Primary active transport is carried out by pumps and uses energy directly, usually from ATP hydrolysis, to pump solutes against their gradient of electrochemical potential.
→Channels and Carriers can mediate the passive transport of solutes across membranes (by simple diffusion or facilitated diffusion), down the solute’s gradient of electrochemical potential.
Please explain the nomenclature of transport proteins?
What are the driving forces behnid the transport?
- the driving force for the facilitated diffusion of an uncharged solute across a membrane is its concentration gradient
- whereas for an ion it is its electrochemical gradient
What is the Nerst equation?
- The Nernst equation allows the direction of the net diffusive flux of an ion at a given membrane potential and temperature to be determined
How high is the energy demand for solute transport in plants?
Why is the concept of one protein carrier per ion to simple?
- The original concept of a single protein-mediated mechanism of ion transport (one carrier system for each ion) did not sufficiently describe the kinetics of when wide concentration ranges were tested
- At concentrations above 1 mM, for example, the kinetics of K uptake differ considerably from those at lower concentrations
What is C min?
- Cmin defines the concentration at which net uptake of ions ceases before the ions are completely depleted
- Cmin is the lowest concentration at which roots can extract an ion from the soil solution
- Cmin for nitrate can vary from between more than 50 AM to less than 1 JuM depending not only on the plant species but also on the environmental conditions
How does the pH of the external solution influence the uptake of nutrients by plant roots?
- effects of solution pH on the chemical species present in solution itself
- effects the apoplasmic pH and so the concentrations of ions present in the apoplasm
- influences the rhizosphere pH for the proton electrochemical gradient and the driving force for proton-coupled solute transport
- solution pH can affect ion transport by protonation/deprotonation of amino-acid residues of transport proteins
How does the pH of the external solution ifluence for example the borum (B) uptake?
- The rate of B uptake decreases strongly when the pH of the external solution is increased
- This closely related to the decrease in the ratio of boric acid, which is the substrate of the transporter catalysing boron uptake by root cells, to the borate anion
What other factors besides the pH affect the ion uptake of the roots?
Main source of energy in non-photosynthesizing cells and tissues (including roots) is respiration. Thus, all factors that affect respiration can influence ion accumulation
- Temperature
- Oxygen: As oxygen lowered, the uptake of ions such as K and P is decreased
- Carbohydrates: carbohydrate concentration, respiration and N uptake decrease within a few hours after excising roots which cuts off the photosynthate supply from the shoot
Please explain the concept of competition in the context of ion uptake!
- the transporters catalysing ion uptake are rarely specific and ions can compete directly for transport
- competition is influenced by the properties of the transporter itself and by the concentrations of differentions in solution
- competition occurs particularly between ions with similar physicochemical properties (valency and ion diameter),
Describe the competition between potassium (K) and ammonium (NH4)?
- NH4 is quite effective in inhibiting K influx, the reverse (inhibition of NH4 uptake by K) is rarely observed
- Ammonium competes with K for transport through both AtAKT1 and AtHAK5 and also reduces the expression of AtHAK5,
- whereas K. does not appear to affect the expression or activity of the major ammonium transporter
- A substantial proportion of ammonium may not be taken up as NH4 but as NH3
What potential effect does the the application of K and Ca fertilizers have?
- Might induce Mg deficiency in crop plants
- This is because the Kations of K+ and Ca2+ inhibit the uptake of Mg2+ by the plant roots