- Xylem and folema
- The circulation of the wise: cohesion-tension theory
- Explanation of the movement of water in a plant
- The brewed sap
- Composition of phloem sap or processed sap
- References
The crude sap is an aqueous solution syrupy flowing through the vascular system of a plant. It is the juice of the plants of any type, especially the ascending and descending juices or circulating fluids essential for the nutrition of the plant.
The ascending sap is the raw sap, the assimilation of which takes place in the leaves, when it becomes the elaborated sap suitable for the growth of the plant. It is composed of phytoregulators (plant-type hormones that regulate plant growth), minerals and water obtained from the soil, which is processed in the leaves and distributed throughout the plant in the form of processed sap.
Raw sap
The sage contains sugars, vitamins, minerals, proteins and fatty acids that allow it to develop all its growth and fruiting processes. Plants also secrete other liquids that are often confused with raw sap; latex, resins or mucilage.
Plants have two different types of tissues to carry sap. The Xylem is the tissue that carries the raw sap or ascending sap from the roots to the leaves, and the phloem carries the elaborated sap from the leaves to the rest of the plant.
Xylem and folema
The xylem is a composite tissue in vascular plants that helps provide support and drives raw sap upward from the roots. It is composed of tracheids, vessels, parenchymal cells, and woody fibers.
The xylem participates in the support and reserve of nutrients, in addition to taking care of the conduction of minerals. Its structure has a tubular shape, without crossed walls that allow a continuous column of water and facilitates faster transport within the glasses.
It is unidirectional (moves the stem of the plant) and responsible for replacing the water lost through transpiration and photosynthesis.
On the other hand, the phloem transports the elaborated sap from the green leaves and stems to the roots. This elaborate sage is composed of minerals, sugars, phytoregulators and water.
The circulation of the wise: cohesion-tension theory
The circulation of raw sap through plants is based on this theory. Cohesion-tension theory is a theory of intermolecular attraction that explains the process of upward flow of water (against the force of gravity) through the xylem of plants.
This theory was proposed by botanist Henry Dixon in 1939. He states that the raw sap in the xylem is drawn upward by the drying power of the air, creating a continuous negative pressure called tension.
The tension extends from the leaves to the roots. Most of the water that a plant absorbs is lost through evaporation, typically from stomata on the plant's leaves, a process called transpiration.
Perspiration places negative pressure (pulls) on continuous columns of water that fill the narrow conductive tubes of the xylem. A column of water resists breaking into droplets as it moves through a narrow conduit such as the xylem tube (water molecules are connected by hydrogen bonding).
Thus, the negative pressure created by perspiration (tension) pulls the entire column of water that fills the xylem tube. It is then, due to osmosis, that the raw sap reaches the xylem of the roots of a plant.
Water molecules are linked together by hydrogen bonds, therefore water forms a chain of molecules during its movement towards the xylem. The water molecules stick together and are stopped by a force called tension. This force is exerted due to evaporation on the surface of the sheet.
There is another theory that explains the transport of raw sap called the root pressure theory.
Root pressure is basically the idea that a plant's roots can maintain a higher or lower pressure based on its environment. It does this in order to promote or discourage nutrient absorption.
In other words, a plant's root system can alter its pressure to: a) help the raw sap build up through the plant, or b) push the raw sap out of the plant.
Explanation of the movement of water in a plant
As the raw sap enters the roots through osmosis, the xylem cells fill and swell, putting pressure on the stiffer outer cells of the root.
This pressure, especially when the levels are low outside the plant, causes the sap to be forced up into the plant, despite the force of gravity.
The electrical charge in these outer root cells creates a kind of "one-way path" that does not allow the raw sap to back up and out of the roots.
Root pressure was determined to be a pressure developed in the tracheal elements of the xylem as a result of the metabolic activities of the root. Root pressure is said to be an active process that is confirmed by the following facts:
-Living cells are essential in the root for root pressure to develop.
-The supply of oxygen and some metabolic inhibitors affect root pressure without affecting the semi-permeability of the membrane systems.
-The minerals accumulated against the concentration gradient by active absorption using metabolically generated energy reduce the water potential of the surrounding cells, leading to the entry of raw sap into the cells.
The transpirational traction is responsible for the rise of the sap in the xylem. This rise in sap depends on the following physical factors:
- Cohesion - Mutual attraction between water molecules or raw sap.
- Surface tension - Responsible for the greatest attraction between molecules of water or raw sap in the liquid phase.
- Adhesion - Attraction of water molecules or raw sap on polar surfaces.
- Capillarity - Ability to raise raw sap in thin tubes.
These physical properties of sap allow it to move against gravity in the xylem.
The brewed sap
The substances taken from the soil through the root (water and mineral salts) form the raw sap. It rises from the roots to the leaves through the stem.
The leaves are responsible for transforming raw sap into the poorer water-poor and nutrient-rich processed sap due to the role of chlorophyll.
The elaborated sap descends to the root to feed the plant. It needs photosynthesis to form, instead, raw sap is created without photosynthesis.
Composition of phloem sap or processed sap
The main components of phloem sap are carbohydrates. Analysis of phloem exudates from various plants has shown that sucrose is the main form of carbohydrate transport.
In some species of Cucurbits, in addition to sucrose, some oligosaccharides such as raffinose, stachyose and verbascose have also been found in the composition of the phloem or elaborated sap.
In some cases, mannitol and sorbitol or dulcitol sugar alcohols have been found in phloem exudates.
Generally, algae produce large amounts of mannitol. Phloem exudate rarely contains hexoses even though glucose and fructose are commonly present in phellogenous tissue.
References
- Sha, R. (2016). Phloem Sap Composition. 1-10-2017, from Biology Discussion Website: biologydiscussion.com.
- TutorVista. (2016). Theories for Ascent of Sap. 10-1-2017, from TutorVista Website: tutorvista.com.
- TutorVista. (2016). Cohesion Adhesion Tension Theory. 10-1-2017, from TutorVista Website: tutorvista.com.
- Diffen. (2015). Phloem vs. Xylem. 1-10-2017, from Diffen Website: diffen.com.