Ascent of Sap: Definition, Mechanisms and Theories
The ascent of sap is the upward movement of water and minerals through xylem from roots to leaves. It occurs mainly by the cohesion-tension mechanism, supported by root pressure and capillary action. This process is essential for photosynthesis and nutrient distribution in plants.
This Story also Contains
- What is Ascent of Sap?
- Types of Sap in Plants
- Components of Plant Transport System
- Mechanism of Ascent of Sap
- Role of Transpiration in Ascent of Sap
- Factors Affecting Ascent of Sap
- Ascent of Sap NEET MCQs (With Answers & Explanations)
- Recommended video on "Ascent of Sap"
What is Ascent of Sap?
Ascent of sap is a plant physiological process that transports water and dissolved nutrients from the roots towards the leaves and other parts of the plant. The movement goes through the xylem vessels and is mainly driven by root pressure, capillary action, and transpiration pull. This is, in fact, a process crucial to the growth of the plant itself.
It spreads the nutrients and keeps the plants in a hydrated state for running photosynthesis and other metabolic activities. The rise of sap supports the health of the entire plant by circulating nutrients for nourishment as well as providing mechanical support in much the same way photosynthesis provides energy through the production of glucose.
Types of Sap in Plants
Plant sap refers to any fluid that circulates through plants, mainly water, combined with various nutrients and other substances.
Xylem sap which mainly consists of water and dissolved minerals that are absorbed from the soil, flowing from roots to leaves.
Phloem sap contains organic nutrients, especially the sugars produced in photosynthesis, from leaves to other parts of the plant.
Components of Plant Transport System
The components of the plant transport system are:
Xylem: A tissue composed of vessels and tracheids. This tissue is in charge of the upward transport of water and minerals.
Vessels: Tubular organs that allow quick transport of water.
Tracheids: Elongated cells that help in the transport of water by pushing their contents and providing structural support.
Phloem: Consists of sieve tubes and companion cells. Together, they transport organic nutrients throughout the plant.
Mechanism of Ascent of Sap
The mechanism is explained by three major theories:
Cohesion-Tension Theory
According to this theory, the ascent of sap is due to the transpiration pull from the top of the plant and the cohesion and adhesion properties of water. It is a convincing theory for explaining the ascent of sap in tall trees.
Attraction between water molecules is called cohesion. Water molecules remained joined due to the hydrogen bonds. Adhesion property is between water molecules and the walls of xylem, which further ensures the continuity of water in the xylem.
Root Pressure Theory
The pressure exerted by the root tissues that force the water up in the xylem. The basis for root pressure theory is a higher solute concentration and accordingly a more negative water potential, in the xylem sap than in the soil solution.
This water potential draws water into the stele, once there, water can now only move upwards. The e\best evidence of root pressure is guttation and bleeding. However, root pressure cannot account for the ascent of sap in trees.
Capillary Action
The capillary action theory explains the ascent of sap in plants by proposing that xylem vessels act as narrow, capillary tubes. Water rises in these vessels due to the combined forces of adhesion and cohesion. It also mentions the importance of atmospheric pressure.
Although, this theory cannot alone fully explain how sap can travel to the top of the tall trees. This is because the capillary action is not strong enough to lift water at such heights.
Role of Transpiration in Ascent of Sap
The pulling force developed in the water of xylem due to transpiration is called transpiration pull. When transpiration occurs in leaves, water evaporates from the intercellular spaces of the leaves and goes in the atmosphere through stomata.
As a result, more water is released into the intercellular spaces from mesophyll cells. The mesophyll draws water from the xylem, creating water tension. This tension is transmitted to the xylem of the root, causing the water to be pulled upward to the leaves.
Factors Affecting Ascent of Sap
The factors that affect the ascent of sap are:
Environmental Factors
Temperature
As the temperature increases, the rate of transpiration increases and therefore there is a greater pull of the sap.
Humidity
When the humidity is low, the rate of transpiration becomes higher as the gradient between the moisture inside the leaf and the air outside is greater.
Light Intensity
A higher light intensity keeps the stomata open for photosynthesis and raises the transpiration rate and consequently the rise of sap
Internal Factors
Age of Plant
Various plant species differ in the anatomy and efficiency of their water transport mechanism. Various plant species have different anatomy and efficiency in their water transport mechanism.
Nature of Plant
Comparatively young plants have a less developed root system and vascular tissue than mature plants. Hence, they are inefficient in the process of sap ascent.
Availability of Water
Enough water in the soil raises root pressure and hence, enhances the ascent of sap. Water deficiency, on the other hand, inhibits the roots pressure and rise of sap.
Ascent of Sap NEET MCQs (With Answers & Explanations)
The key concepts to be covered under this topic for different exams are:
Mechanism of Ascent of Sap
Factors affecting Ascent of Sap
Practice Questions for NEET
Q1. Xylem translocates:
Water only
Water and minerals only
Water, mineral salts and some organic nitrogen only
Water, minerals salts and some organic nitrogen and hormones
Correct answer: 4) Water, minerals salts and some organic nitrogen and hormones
Explanation:
The xylem is a type of vascular tissue in plants that transports water, mineral salts, some organic nitrogen compounds, and hormones. It carries water absorbed by the roots to various parts of the plant, along with essential nutrients needed for growth and development. Additionally, certain hormones are also transported through the xylem to regulate physiological processes in the plant. The movement of water occurs primarily through transpiration pull, cohesion-tension, and root pressure. The xylem consists of tracheids, vessels, fibers, and parenchyma, each playing a specific role in conduction and support. The lignified walls of xylem cells provide mechanical strength and prevent collapse under negative pressure. This vascular tissue is crucial for maintaining water balance and structural integrity in plants. Xylem also plays a role in drought resistance by adjusting water transport efficiency.
Hence, the correct answer is option 4) water, minerals, salts, and some organic nitrogen and hormones.
Q2. Why is swelling seen above the ring on the stem whose bark has been removed?
Due to infection
Due to accumulation of water
Due to accumulation of food
All of these
Correct answer: 3) Due to accumulation of food
Explanation:
Ringing Experiment: It was demonstrated by Malpighi with a focus on the phloem and Stephan Hales with a focus on the xylem. A ring of bark was removed from the stem. This removed phloem from the stem and stopped food conduction. However, water conduction continued. Accumulation of the food caused swelling in the cut region. Removal of bark caused a break in the phloem continuum which caused accumulation of food above the ring. This caused swelling.
Hence, the correct answer is option 3) Due to accumulation of food.
Q3. With the girdling experiment, it has been observed that:
Girdling has no immediate effect on transpiration.
Photosynthates accumulate above the girdle.
The bark below the girdle dies.
All of the above
Correct answer: 4) All of the above
Explanation:
Girdling experiment demonstrates the translocation of Photosynthates occurs through phloem. In this experiment, the bark of the tree was removed in a ring around the trunk (called girdling). With this experiment, it has been observed that girdling has no immediate effect on transpiration. However, the transport of Photosynthates in the trunk is blocked at the site where the bark has been removed. Photosynthates accumulate above the girdle. Eventually, the bark below the girdle dies, while the bark above swells and remains healthy.
Hence, the correct answer is option 4) All of the above.
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Frequently Asked Questions (FAQs)
Environmental factors such as temperature, humidity, wind, and light intensity affect the rate of transpiration, which in turn influences sap ascent. For example, high temperatures and low humidity increase transpiration and sap flow, while high humidity and darkness reduce them.
The cohesion-tension theory describes the rise of sap as a result of the combination of cohesion—the attraction of water molecules to each other—and adhesion, the attraction between water molecules and the walls of xylem vessels. Transpiration from leaves pulls on top of each water column in the xylem generating negative pressure that pulls water upward and through a plant. This continuous column of water is sustained by cohesive forces between water molecules.
The root pressure is the osmotic pressure developed within the roots due to their active absorption of water from the soil. This pressure pushes up the water through the xylem vessels during most of the night time or when the rate of transpiration is low. However, with the help of root pressure alone, it is not possible to explain the rise of sap to considerable heights in tall plants.
Xylem's sap is mostly water, with dissolved inorganic nutrients or minerals absorbed from the soil, flowing from roots to the rest of the plant. The phloem sap, on the other hand, contains mainly organic nutrients, namely sugars—sucrose—in large amounts, which are formed during photosynthesis in the leaves, and transports these nutrients to various parts of the plant for growth and storage.