Quick Question on Plants.

Patri

I'm in leaving cert and just did a question on biology from 2008. It's in regards to plants and their uptake of minerals. I'm a bit confused just hope you can shed some light.

"Describe how minerals such as nitrates enter the plant from the soil?"

Now, according to my book minerals enter via the root hairs through active transport. I don't think this is right? If they are indeed dissolved in water surely the answer should be osmosis or passive transport or something?? Does anyone know the answer?? Thanks!

IrishKnight

They are indeed dissolved in water, but the problem there is a higher conc of minerals inside the plant than outside. As such, if the plant were to only have passive transport, all those wonderful minerals would leave the root for the soil.

So the plants has to use energy (ATP) to pump in minerals into the root.

Patri

IrishKnight wrote: »

They are indeed dissolved in water, but the problem there is a higher conc of minerals inside the plant than outside. As such, if the plant were to only have passive transport, all those wonderful minerals would leave the root for the soil.

So the plants has to use energy (ATP) to pump in minerals into the root.

Oh ok thanks very much. I'm just at odds here because the marking scheme on examinations.ie says the only accepted answers were passive transport and diffusion. My bio teacher says via osmosis and the book strongly emphasises the need for ATP as it's active transport. Now osmosis would be passive transport right? lack of ATP, I'm not sure about active transport.

Improbable

Ok, it's been a long time since I did the Leaving Cert. Biology exam. But:

Uptake and use
Before it can be taken up by plants, nitrogen in the soil needs to reach the root zone. It does this by two mechanisms — diffusion and mass flow. In diffusion, the nitrogen compounds move towards the plant roots along a concentration gradient. As plants take up either ammonium or nitrate, the concentration of these compounds near to the roots decreases, and to balance this, more of the compound flows in from the surrounding area.

The second uptake mechanism — mass flow — only applies to nitrate, but it accounts for around 80 percent of the nitrogen uptake by plants. In mass flow, nutrients are transported to the roots with the soil solution, as part of the flow of water used in transpiration.

Plants can take up nitrogen in three forms — nitrate, ammonium and urea.

Nitrate
Nitrate is the preferred form of nitrogen for most plants. It can be taken up through the roots or through surface components of the plants (e.g. shoots). Nitrate has the chemical formula NO3-, so when a molecule of nitrate is taken up by plant roots, a molecule of OH- is excreted in order to maintain the electrical balance within the plant.

Once in the roots, nitrate can be stored in vacuoles for future use, or it can be translocated to the leaves through the xylem, or it can be reduced to ammonia (NH3). Two enzymes — nitrate reductase and nitrite reductase — are responsible for this process. Where this process occurs — root, shoot, or root and shoot — varies between plant species.

Once formed, ammonia is used for making amino acids or other nitrogen products such as amides. This use is called nitrogen assimilation.

Ammonium
When ammonium (NH4+) is taken up by plant roots, hydrogen ions (H+) are excreted. The relationship between ammonium uptake and hydrogen ion expulsion is not one to one — for every mole of ammonium taken up, between 1.1 and 1.6 moles of hydrogen ions is expelled. As a result of this, the pH of the root zone falls when ammonium is taken up by plant roots.

Once in the plant, ammonium is immediately converted into amino acids or amides. Ammonium cannot be stored in the plant as it is toxic, even at relatively low concentrations.

Urea
Urea can be taken up by plant roots and leaves. However, in the soil, urea is rapidly converted to ammonium, so uptake by roots is rarely observed. In leaves, urea enters the plant through pores by a passive process. Once inside the plant, it is converted to ammonium and assimilated as amino acids and amides.