Ascent of Sap
The movement of water and minerals from roots to all the other parts of the plant is referred to as the ascent of sap. The xylem vessels in the plants are responsible for the conduction of water and minerals throughout the plant.
The process of transport of water is, however, pressure driven. Water and minerals move from a location of high concentration (root) to a location of low concentration (xylem vessels). This movement increases the pressure in the xylem, called root pressure. Root pressure is a positive pressure that drives the movement of water and ions in the plant.
Another factor that works for the movement of water is a theory called the cohesion-tension transpiration pull model. This model says that water molecules are attracted to each other (cohesion), water molecules are attracted to polar surfaces (adhesion), and water molecules are attracted to each other more in the liquid phase than in the gaseous phase (surface tension). These three characteristics give tensile strength and capillarity to water, resulting in water rising up in the plants.
Note: Many other theories have been touted for the transport of water.
Phloem Translocation
The transport of photosynthates from leaves to other parts of the plant is referred to as phloem translocation. Phloem vessels are responsible for the conduction of food prepared by photosynthesis throughout the plant.
The part which prepares food is referred to as the source, and the part where the food is transported is referred to as the sink. Food movement is multidirectional, depending on the source and sink relationship.
The transport of food follows a theory called the mass flow hypothesis. This hypothesis says that the loading of phloem elements with the sugars creates a hypertonic condition, as a result of which the adjacent xylem cells transport water to the phloem cells via osmosis. This osmotic pressure buildup in the phloem leads to the conduction of the sugars in areas with low pressure.
Ascent of Sap vs Phloem Translocation
| Transport of water and minerals in the plants (upward movement from roots to upper plant body) is referred to as the ascent of sap. | The transport of sugars from the area of the source to the area of sink is referred to as phloem translocation. |
| The route of transport is from roots to other plant parts. | The route of transport is from source to sink. For example: leaves to roots. |
| Unidirectional | Multidirectional |
| Water and other dissolved minerals. | Carbohydrates, proteins and amino acids. |
Explore BYJU’S Biology to learn more interesting concepts.
Also Read:
- Difference Between Xylem And Phloem
- What are the constituents of phloem?
- What are the components of the xylem?
Frequently Asked Questions
What is the difference between the ascent of sap and transpiration?
The ascent of sap is the upward movement of water and ions from roots to different parts of the plant. On the other hand, transpiration is the outward movement or loss of water by evaporation from leaf surfaces.
What is the relationship between transpiration and ascent of sap?
Due to water loss by transpiration, a vacuum is created in the cells that creates a transpirational pull. This transpirational pull acts as a driving force for ascent of sap.
Who proposed the transpiration pull theory of ascent of sap?
The transpiration pull theory was proposed by Dixon and Jolly.
Does gravity help in the ascent of sap?
No, ascent of sap is an upward rise of water that is against the direction of gravity.
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