9.2a Plant Transport Systems

09/04/2013 § Leave a comment

So, there’s a lot to cover for this blog, most of which has to do with transpiration and the root systems of plants.

Root systems and mineral ion absorption

The developing embryo is what makes the root of a plant. Once the embryo comes out of its seed, it continues to grow downwards into the soil, developing branches in some plants while other plants form complex networks of fibers and hairs. The point of these root systems is to anchor the plant in the soil, and absorb water and mineral ions with the increased surface area (via the hairs).

Water is absorbed through osmosis, so if the solute concentration inside the root is high, water will naturally move into the roots. The concentration gradients are established by active transport and can be up to 100+ times higher than the solution in the soil. That’s how water gets in. Mineral ions can only be absorbed through diffusion and the appropriate protein pump.

One other thing to note is that some plants absorb minerals and nutrients that benefit both it and a fungus that grows with it (?), and that would be a mutualistic relationship because both parties benefit.

Stomata and Transpiration

So we all know that one of the most important things plants need to live is water (what, really?!) and that an excessive loss of water leads to dehydration, a.k.a. death. Transpiration is the loss of water from the leaves and stems of plants. We’ve learned that the epidermis (the single outer layer of cells) has a “waxy cuticle” that reduces transpiration and prevents an excessive loss of water. However, for the plant to also absorb carbon dioxide (also necessary for survival, whaddup), there need to be breaks or pores in the epidermis and waxy coating – these are stomata! Of course, the problem with this is that though the plant can absorb the carbon dioxide, it could also lose some water in the process.

To avoid that, stomata has guard cells, which are found in pairs and whose function is to control the aperture of the stoma, closing or opening it fully. Basically, the absorption of water opens the guard cells and increases the aperture, and vice versa: the release of water closes the guard cells and reduces the aperture. The actions of the stomata are affected by light – which causes it to open (note: aperture has something to do with taking pictures, if you’re any bit a photographer), low carbon dioxide concentrations (in which the stomata also opens), and a shortage of water (causing the stomata to close).

When a plant suffers “water stress”, the hormone abscisic acid is what causes the guard cells to close even if there is a lot of light and carbon dioxide levels are low – this way, the plant can avoid dehydration (ehem, death, ehem) and continue to photosynthesize with what it has.

The four main abiotic (meaning external factors not controlled by the plant) factors that have major effects on the rate of transpiration (the amount of water vapour a plant loses from its leaves and stems per unit time) are

  1. light – lots of light opens the stomata and speeds up transpiration, darkness closes the stomata and slows down transpiration
  2. humidity – low humidity means a faster transpiration rate (there is water vapour in the environment)
  3. wind – light or still winds doesn’t change take away the water vapour around the plant, therefore humidity is steady, which means transpiration is slow, while strong winds reduce humidity, meaning faster transpiration
  4. temperature – an increase in temperature reduces the humidity outside of the leaf, which speeds up transpiration

Some biotic factors are: the size of the plant, the thickness of the waxy cuticle, how far apart the stomata are, and obviously if the stomata are open or closed.

Water Transport

Water moves upwards through the plant from the roots to the leaves in a flow called the transpiration stream. The xylem vessels are what allow water to move efficiently. The vessels are strengthened by lignin, which keeps the plant from collapsing even in very low pressures. The low pressure inside the xylem vessels are strong enough to pull the water upwards against the force of gravity. This passive process is called the transpiration pull. Adhesion also moves water via the walls of the vessel, at the same time preventing the vessels from breaking.

Wow, I didn’t know if that was ever going to end.

Essay Questions

  1. The main part of growing plants are roots, stems and leaves. Draw a plant diagram to show the arrangement of tissues in the stem of a dicotyledonous plant.
  2. Draw a labelled diagram showing the tissues present in a dicotyledonous leaf.
  3. Explain the functions of the different tissues of a leaf.
  4. Explain the role of auxin in phototropism.
  5. Outline the adaptations of plant roots for absorption of mineral ions from the soil.
  6. Describe the process of mineral ion uptake into roots.
  7. Describe how water is carried by the transpiration stream.
  8. Explain how abiotic factors affect the rate of transpiration in a terrestrial plant.
  9. List three abiotic factors which affect the rate of transpiration in a typical mesophytic plant.
  10. Explain how wind affects the rate of transpiration from a leaf.
  11. Outline adaptations of xerophytes.
  12. Outline the role of the phloem in the active translocation of biochemicals.
  13. Draw the structure of a dicotyledonous animal-pollinated flower
  14. Describe the metabolic events of germination in a starchy seed.
  15. Explain the conditions needed for seed germination.
  16. Explain how flowering is controlled in long-day and short-day plants.

 

DATA BASED QUESTION

Page 127, the Renner experiment

1. Describe the effect of clamping the stem on the rate of water uptake. —> If you clamp the poor thing, the xylem vessels would then be restricted and what was one a passive process (the transpiration pull) would be difficult to do when the xylem vessels aren’t continuous and are being pinched tight. The plant would need to exert more energy (which comes from thermal energy) to pull up the water.

2. Explain the effect of cutting off the top of the shoot on the rate of water uptake. —> The plant’s job is to move the water up to the leaves, which are at the shoot. In removing the leaves and the shoot, we remove the place the water is supposed to go and all of the water would remain stagnant in the xylem vessels of the plant. This could potentially lead to it bursting because it may or may not continue to keep absorbing water. If anything, cutting off the top wouldn’t be good for the plant. The water needs to get somewhere.

3. Calculate the difference between the rate of water uptake caused by the vacuum pump and the rate caused by the leaves immediately before the shoot top was cut off. —> 5.5 – 4.5 = 1.0 cm^3 h^-1 (caused by the vacuum pump), 0 cm^3 h^-1 (immediately before the shoot top was cut off), therefore the difference is 1.0 cm^3 h^-1

4. The water in the potometer was at atmospheric pressure. The vacuum pump generated a pressure of zero. Discuss what the results of the experiment showed about the pressures generated in the xylem by the leaves of the shoot. —> Because the water uptake is generally higher before the vacuum pump is applied, it can be assumed that the pressure inside the xylem vessels was lower compared to its environment. This gradient is what causes the larger uptake of water as opposed to the gradient created by the vacuum pump.

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