por What is raw sap?
The raw sap It is a thick aqueous solution that circulates through the vascular system of a plant. It is a substance that all plants have, which is transported by the conductive vessels, and whose main function is their nutrition.
The ascending sap is the raw sap, the assimilation of which takes place in the leaves, when it becomes the elaborated sap suitable for plant growth.
It is composed of phytoregulators (plant 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 elaborated sap.
The sap 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 transport sap. The xylem is the tissue that transports the raw sap or ascending sap from the roots to the leaves, and the phloem transports the elaborated sap from the leaves to the rest of the plant, up to its roots.
The xylem and the phloem
Xylem is a composite tissue in vascular plants that helps provide support and conducts raw sap upward from the roots. It is composed of tracheids, vessels, parenchyma cells, and woody fibers.
The xylem participates in the support and reserve of nutrients, in addition to dealing with the conduction of minerals. Its structure is tubular in shape, without crossed walls, which allow a continuous column of water and facilitate faster transport inside the vessels.
It is unidirectional (it moves the stem of the plant) and is responsible for replacing the water lost through transpiration and photosynthesis.
On the other hand, the phloem transports the elaborated sap from the leaves and green stems to the root. This elaborated sap is composed of minerals, sugars, phytoregulators and water.
Sap circulation: cohesion-tension theory
The circulation of raw sap through plants is based on this theory. The 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.
Transpiration places negative pressure (heaves) in continuous columns of water that fill the narrow conducting tubes of the xylem. A column of water resists breaking up into droplets as it moves through a narrow conduit like the xylem tube (water molecules are connected by hydrogen bonding).
Thus, the negative pressure created by transpiration (tension) pulls on 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 to each other by hydrogen bonds, therefore water forms a chain of molecules during its movement towards the xylem. Water molecules stick together and are held together by a force called tension. This force is exerted due to evaporation on the leaf surface.
There is another theory that explains the transport of raw sap, called root pressure theory.
Root pressure is basically the idea that a plant’s roots can maintain a higher or lower pressure based on their 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 raw sap build up throughout the plant, or b) push 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 more rigid outer cells of the root.
This pressure, especially when levels are low outside the plant, causes the sap to be forced up into the plant, despite the force of gravity.
The electrical charge of those outer root cells creates a kind of “one-way road”, which does not allow 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 root metabolic activities. Root pressure is said to be an active process which is confirmed by the following facts:
– Living cells are essential in the root for root pressure to develop.
– Oxygen supply and some metabolic inhibitors affect root pressure without affecting the semi-permeability of membrane systems.
– 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 crude sap into the cells.
Transpirational traction is responsible for the rise of sap in the xylem. This rise of sap depends on the following physical factors:
– Cohesion: Mutual attraction between water molecules or raw sap.
– Surface tension: Responsible for the greater attraction between water molecules or raw sap in 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 the 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 the raw sap into the processed sap that is poorer in water and richer in nutrients due to the function 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 the phloem sap or processed sap
The main components of phloem sap are carbohydrates. Analyzes of phloem exudates from various plants have shown that sucrose is the major transport form of carbohydrates.
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 sugar alcohols, mannitol and sorbitol or dulcitol have been found in phloem exudates.
Generally, algae produce large amounts of mannitol. Phloem exudate rarely contains hexoses, despite the fact that glucose and fructose are commonly present in phellogenous tissue.
References
Phloem Sap Composition. Retrieved from biologydiscussion.com.
Theories for Ascent of Sap. Retrieved from tutorvista.com.