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Guard cells control stomatal opening:
Each stoma (singular of stomata) is made up of two kidney bean shaped cells called guard cells, which respond to differences in water. Guard cells must control gas exchange through these stomata (oxygen, carbon dioxide, water vapour).
Plant has high water concentration:
Water moves into the guard cells by osmosis. This causes them to swell and become turgid. This turgid conformation opens up the stoma. And encourages gas exchange.
Plant has low water concentration:
Water moves out of guard cells by osmosis and cells become flaccid. This closes the stoma, reducing gas exchange.
Stomata are found almost exclusively on the lower epidermis, apart from in hydrophytes, which have stomata on the upper epidermis.
Driving Transpiration:
Low pressure at stomata: Evaporation of water molecules from the stomata lowers the pressure. This causes water to be drawn up the xylem due to a pressure gradient.
High pressure at roots: Active loading of roots with inorganic ions and nutrients lowers the RHC water potential. Water moves into the roots via osmosis, increasing the pressure exerted by water at the roots.
Cohesion-tension theory: Numerous hydrogen bonds form between water molecules (cohesion) as well as between the water molecules and xylem wall (adhesion). The water moving through the xylem of a plant is usually described as an 'unbroken column held under tension'
Driving Translocation:
Low WP at source: Loading of sugars into the phloem at the source side will lower the water potential of the phloem due to the presence of sugars. This causes water to move from the xylem to the phloem by osmosis.
High WP at roots: After the sugars move out into the respiring tissue (such as RHC), the water potential in the phloem increases, so water moves back into xylem by osmosis.
Structure of the leaf:
-The waxy cuticle covers the upper epidermis and protects the uppder epidermis. It is water resistant and contains a polymer called cutin, as well as suberin.
-The palisade mesophyll in the upper epidermis is composed of palisade cells, which have a high SA:vol ratio and a large number of chloroplasts for efficient photosynthesis. It is on the upper side of the leaf, and is the main site of photosynthesis.
-The spongy mesophyll contains air spaces to encourage gas exchange through diffusion.
Palisade cell
Waxy cuticle
UE
LE
Stoma
Spongy Mesophyll
Phloem loading:
Phloem loading describes how sugars produced through photosynthesis are moved from the photosynthesising cell into the phloem. This involves co transport mechanisms and the use of pH gradients, as well as the presence of a companion cell.
-Sucrose produced by the source cell during photosynthesis (sucrose = glucose + fructose).
-H+ ions actively transported out of companion cell, increasing its pH and establishing a proton gradient.
-H+ ions then move down concentration gradient into companion cell from source cell. This movement includes the co transport of sucrose from the source to the companion cell.
-From here the sucrose can move by facilitated diffusion through plasmodesmata into the phloem.
H+
H+
X
P
Companion Cell
Source Cell
H+
H+
Suc
Suc
[
[
[
[
Xylem cell structure:
-Xylem cells are dead, hollow cells joined end-to-end.
-They are hollow (have no cytoplasm)
-They are lignified (a protein which supports the plant structurally as well as acting as a waterproofing layer.
Formation:
-Top and bottom cell wall and membrane break down
-Nucleus, organelles and cytoplasm leak out
-Cell is lignified in rings of lignin
Water moving in the root:
Apoplast pathway:
Water moves through cells in the root through the spaces in the cell walls filled with cellulose. Since water is not passing through the plasma membrane, the water can carry dissolved metal ions/mineral ions/salts.
Symplast pathway:
Water moves through the cytoplasm of cells joined to one another through plasmodesmata. This method of water movement cannot carry any additional ions
Evidence for mass flow: Ringing experiments
This experiment describes a section of bark being taken from a tree, all the way round. This bark contains the xylem and the phloem, so by removing it the scientists are removing that which they believe is responsible for mass transport. Cells above the cut will swell as sucrose accumulates in them (and water) whilst cells below the cut will die owing to the fact that they will not be able to receive the sugars for respiration.
Evidence for mass flow: Radiolabelled 14C
This experiment describes a plant being grown in a radioactive isotope of 12C, 14C. This can be in the form of radiolabelled CO2, which the plant will take up and assimilate in photosynthesis. Exposure of a cross section of the plant will show radioactivity in a dark form. This technique is known as autoradiography and is widely used in tracing experiments.
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