Question

Explain, in your own words, the tension-cohesion model and the pressure-flow model of plant vascular transport. How are they similar? How are they different?

Answer 1

Ans : Plant vascular transport consists of two major types :

1. Transport through Xylem tissue : water and minerals from the soil are taken up by the roots and transported to the leaves, restoring water lost through evapouration in leaves.

2. Transport through Phloem tissue : food prepared from atmospheric CO2 and water is transported from leaves to other parts of the plant

Tension-cohesion model, also known as transpiration pull was stated by Dixon and Jolly. It describes the mechanism by which water is transported through xylem from the roots to the leaves. According to this model, water is pulled up the roots to the leaves through small capillaries in xylem called trachea. This is enabled by the transpiration of water from the surface of leaves.

This upward movement of water is depends on the following physical properties of water :

• Cohesion : water molecules are mutually attracted to eachother through hydrogen bonds which helps them stick together during the upward movement throught trachea of xylrm
• Adhesion : water is also attracted to the hydrophillic walls of trachea
• Surface tension : hydrogen bonding of water molecules along the surface are streched which causes a surface tension  

These properties combinely render the waster or sap being pulled up the xylem high tensile strength and capillarity which is also attributed by the small diameter of the trachea.

Step 1 : Transpiration : It is the evapouration of water from the stroma located at the bottom surface of the leaves. During this process, the hydrogen bonds between the evapourating water molecules are broken.

Step 2 : water chain pulling : as the free water molecules begin to leave the sureface of the leaf as gas, there is tension created on the surface of water. The hydrogen bond attraction, just before breaking, pulls the water molecules preceding them to the surface to the leaf. This action causes the upward movement of subsequent water molecules along the trachea to the leaf surface. The process continues as water molecules from the leaf surface keeps evapourating.

Pressure-flow model describes the passage of solutes from the leaves to other parts of the plant through phloem. This passage can be upward or downward, depending on the concentration of the solutes. Two types of cells play a role in this transport : Source cells and Sink cells

Source cells are the photosythetic leaf cells , where sucrose is produced from atmospheric CO2 and root cells, where large amounts of sucrose are stored. They release these carbohydrates into the phloem which are taken up by Sink cells.

Sink cells are any cell that unloads carbohydrates from the phloem to meet their own requirement for carbohydrates. They are found mainly in roots, devoloping fruits and shoot tips.

Passage of solute through Phloem :

• When the source cells pumps sucrose into the phloem, the solute concentration in the phloem increases which causes water from the surroundings of lower solute concentration, including xylem to be pulled into phloem through osmosis. This increases the turger pressure in the sieve tubes of phloem which in turns allows the passage of the sugar solution in the sieve tubes to regions of lower concentration.
• Thus, solute reaches the sink cells where it is taken up. Thus, the solute concentration in the phloem decreases, which causes water to flow out into the surroundings. This decreases the turger pressure and makes the solution in phloem flow back upwards.

Similarities : Both tension-cohesion transport and pressure-flow transport takes place in roots, stems and leaves.

Differences :

Tension-cohesion model:

• Takes place in phloem

• Bidirectional

• Transorts solute by turger pressure and osmosis

Pressure-flow model :

• Takes place in phloem

• Unidirectional

• Transports water and minerals by cohesion and surface tension