3 questions

Plants!

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brassica
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3 questions

Post by brassica » Tue Aug 01, 2006 3:25 am

Hello there,

I have been asked these questions by some people; however, my answers are not satisfied. I would like to ask for help and of course I will put my answer in here and see if you all can give me some clues.


First, What are the advantage/disadvantage of using concentration and content? and when people prefer to use content?
Here is my answer. (sorry for the long page....)

Please see the attachment.

Second, since concentration can be expressed in either dry weight basis and fresh weight basis, why dry weight basis is preferred and when people prefer to use freesh weight?

Third, and this one bothers me .....
ion uptake can be either by passive or active and even both. It's kind misleading that passive uptake is no "energy" consumed. What are the driving force for passive uptake and where is the energy coming from for the long term use?



I am not allowed to attach a doc file??

Sorry, I guess whoever is kindly helping me out should read my answer first.

Thank you for the helping.

#1

Concentration is the relative/percentage of amount of a substance in a mixed substance. Content can be thought as a “real value” which exists in a mixed substance. Nutrient levels in plant tissue sometimes given as concentration (g Nutrient/ g leaf) because it can give us an idea the “average” of that particular nutrient accumulated in the leaf tissue. In contrast, content (g Nutrient/ leaf) which gives us an idea “how much” total that particular nutrient accumulated in leaf.
The difference in here is the average vs the total or qualitatively vs quantitatively. Because even in the same concentration, it doesn’t mean both accumulate the same amount of nutrient. For example, in A solution which has 100g NaCl in 1L H2O; in B solution, which has 10 g NaCl in 0.1 L H2O. Both of these two solutions have the same concentration of NaCl, but they do have the different content. A solution has more NaCl content than B solution. Another example, A still has 100 g NaCl in 1 L H2O, and B has 20 g NaCl in 0.1L H2O. In this case, B has higher concentration than A does, but A still has more content than B. Higher content means more substance, but higher concentration means higher relative/percentage of that substance in a mixed matrix. Concentration doesn’t equal the real amount, but it can present how quality is. The same concentration may have the various content depending on the matrix volume.
In a given plant nutrient level, the concentration is used which can be easily to tell the difference of nutrient between plants (tissues). In addition, concentration is also an index to see whether the nutrient level is sufficient or deficient when compare to the standard range.
The disadvantage of using concentration will be b/c of its “average” which can not tell the difference within tissues. For example, concentration is represent as “g Nutrient/g tissue” which is the average of combined total leaves (young and old leaves); therefore, it can not tell the difference between young and old leaves.
On the other hand, using content, which shows us how much of nutrient is accumulated in plant or plant tissues. Higher content means more accumulation. It is a quantitative parameter that plants have such high tolerance or ability to accumulate more nutrient but it can not be used as a good comparison parameter because the different content in two plants or plant tissues may have the same concentration. For example, a well grown plant (5g plant weight) may have 10 mg N, and the other weak plant (2g plant weight) may have 4 mg N. As you can see they are different in content but have the same concentration of N. From the quantitative point of view, we could say, the well grown plant may have the higher capacity of accumulation N. However, from the qualitative point of view, the concentration of N is the same.


#2
Using dry weight basis is more consistency when compare studies and species in any time, while fresh weight basis is more useful for intake considerations because animals, including people, eat organisms in their natural state.
In general, the difference between dry weight basis and fresh weight is the moisture content. However, this could be a confusion in that “dry weight basis x water = fresh weight basis”. Instead, there is a consideration, which is water variation. Water content is 80~95% in plant tissue, which means the variation of water results in the difference once present on a dry weight basis. For example two plants has the same nutrient level on fresh weight basis; however since they have the different water percentage the nutrient level of the dry weight basis are difference. If so, in this case, using fresh weight basis may be a misleading that these two plants (tissues) have the same concentration of nutrient. In fact, using dry weight basis will prefer to present the difference of the nutrient level. Thus, using dry weight basis, the percentage of the element is more consistent because we do not have to consider the variation of water content when compare to different plants/tissues.
As mentioned earlier, fresh weight basis is more appropriate when we consider the intake of nutrient or from the health point of view. Food system is the major way for delivering element to animals and humans; fresh food still has highly percentage in our daily diet; therefore, how much nutrients or toxin we may intake depends on their concentration in food we eat daily (in here, we exclude drinking water). For instance, selenium concentration (ug/g as consumed), the selenium contents may vary from 0.001-0.14 (ug/g) depending on vegetables. As a result, if we eat 100 g broccoli which is a highly accumulation of selenium, we may intake the maximum of 14 ug of selenium. In such a case, I would think that using fresh weight basis is more appropriate.

# 3,
Sorry…. I just know that either active or passive all need energy… please help out on this one!!

Ion fluxes in solution or across the plasma membrane may result from passive or active forces, but they always require a source of free energy. When passive, the flux is driven by the free energy associated with a gradient of concentration or pressure or electrical potential or even a combination of these driving forces. (in here, I kind understanding Ca is passive uptake, however, if passive uptake is based on the “concentration”/ electrical potential, once it makes the equibilium, how do they still do the passive uptake??”

xand_3r
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Post by xand_3r » Tue Aug 01, 2006 1:40 pm

For #3: Usually equilibrum never occurs, since equilibrum is equivalent with maximum entropy (so maximum disorder), and organisms are ordoned systems. The gradients of concentration are maintained through active transport systems. For example, the gradient for Na and K is maintained by the Na/K pump. The gradient for Ca is maintained by the Ca pump and so on. To conclude, even passive ways of transporting substances depend on the active ways. The electrical gradient depends on the distribution of ions. For example, an influx of Na ions could be followed by an influx of Cl ions as a result of electrostatic attraction.

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Post by xand_3r » Tue Aug 01, 2006 1:44 pm

A way for stopping an ion either from entering or from exiting a cell are voltage dependent channels (they open and close only at certain values of the cell potential). So equilibrum can't occur since at a certain voltage value the channel closes, even if the concentration gradients aren't met.

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Post by brassica » Fri Aug 04, 2006 3:20 am

xand_3r, thanks for the reply...

did you mean that the driving force for passive transport is coming from the active transport?

If so, that's also the long -term energy for passive transport??

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