The cohesion-tension theory, a fundamental principle in plant physiology, describes the mechanism of water movement from the roots to the leaves of a plant. This concept incorporates the cohesion of water molecules and the tension generated by transpiration, enabling water transport against gravity.
The Nature of Cohesion and Adhesion
Cohesion
- Definition: Cohesion refers to the attraction between like molecules.
- Hydrogen Bonding: In water, hydrogen bonding between molecules creates a strong, cohesive force.
- Role in Plants: Cohesion enables water molecules to form a continuous column inside the xylem, a crucial factor in water transport.
Adhesion
- Definition: Adhesion is the attraction between unlike molecules.
- Interaction with Xylem Walls: Water molecules adhere to the xylem walls due to polarity.
- Significance in Water Transport: Adhesion contributes to the ascent of water against gravity. This principle is closely linked with the transport of water in plants, demonstrating the interplay between cohesion, adhesion, and plant anatomy.
IB Biology Tutor Tip: Understanding the cohesion-tension theory is crucial for grasping how plants defy gravity to transport water from roots to leaves, integrating principles of physics into biology.
The Cohesion-Tension Theory Explained
Transpiration
- Definition: Transpiration is the loss of water from plant leaves through the stomata.
- Creation of Tension: As water evaporates, it creates tension or negative pressure in the leaf, which is a critical part of the transpiration process.
- Impact on Water Movement: The tension pulls the water column upward, driven by cohesive forces. Understanding osmosis helps in comprehending how water potential differences drive water uptake in the roots.
Water Column in the Xylem
- Continuous Pathway: Cohesion and adhesion create a continuous water column in the xylem vessels.
- Supporting Ascent: This water column facilitates the movement of water from roots to leaves.
Water Uptake in the Roots
- Root Hair Cells: Water is absorbed from the soil by specialized root hair cells through osmosis, influenced by the active uptake of mineral ions.
- Osmosis: Lower water potential in the roots due to mineral ion concentration allows water to enter via osmosis.
Translocation of Water
- Movement Mechanism: The tension created at the leaf level draws this water column upward.
- Role of Cohesion: Cohesive forces keep the water column intact.
- Role of Adhesion: Adhesion assists the water in moving against the gravitational pull.
Capillarity
- Definition: Capillarity refers to the movement of water in narrow spaces.
- In Xylem Vessels: While water properties are fundamental, capillarity serves as a secondary mechanism to transpiration pull.
- Secondary Mechanism: Though significant, capillarity is secondary to transpiration pull.
Regulation of Transpiration
- Stomatal Control: Plants control transpiration by opening or closing the stomata, a vital aspect of transpiration regulation that responds to environmental factors like humidity and light.
- Environmental Response: This regulation responds to environmental factors like humidity and light.
IB Tutor Advice: For exams, visualise the journey of a water molecule through the plant and relate it to cohesion and tension principles to explain water transport in essays or short answers.
Importance of the Cohesion-Tension Theory
- Water Transport: Explains efficient water transport to great heights, ensuring a continuous supply for photosynthesis and nutrient transport.
- Photosynthesis Support: Ensures continuous water supply for photosynthesis.
- Temperature Regulation: Aids in cooling the plant through evaporative cooling.
- Nutrient Transport: Supports the transport of essential nutrients and minerals.
FAQ
The cohesion-tension theory mainly applies to vascular plants that rely on transpiration to move water. In extreme environments, where water might be scarce or pressures unusual, the cohesive forces may not be enough to maintain a continuous water column. In such plants, additional or alternative mechanisms might be in play to facilitate water transport.
In the cohesion-tension theory, hydrogen bonding between water molecules contributes to the cohesive forces. These hydrogen bonds create a network of connections that allow water molecules to stick together, forming a continuous and unbroken water column within the xylem. This cohesive force is essential for the transportation of water against gravity.
The xylem's structure plays a vital role in the cohesion-tension theory. The narrow, hollow, and elongated cells of the xylem create a pathway for water to travel. The walls are made of lignin, which is hydrophilic, allowing for adhesion between the water molecules and the xylem walls, further supporting the cohesive forces within the water column.
During the night, when transpiration rates are low, the cohesion-tension theory still plays a role. The cohesive forces between water molecules keep the water column intact, and soil water potential can cause water to move into the root system. Thus, the xylem can still fill with water, maintaining tension and allowing for water movement even without the significant transpirational pull.
Yes, there are conditions, such as severe drought or freezing temperatures, where the cohesion-tension theory might not adequately explain water transport. In drought, the water column may break due to extreme tension, while in freezing temperatures, ice formation might disrupt the water column. In these situations, other mechanisms or adaptations might be essential for water transport within the plant.
Practice Questions
Cohesion refers to the attraction between water molecules facilitated by hydrogen bonding. This cohesive force enables water molecules to form a continuous column inside the xylem vessels. Adhesion, on the other hand, is the attraction between the water molecules and the xylem walls. Together, cohesion and adhesion maintain the integrity of the water column and aid in its ascent against gravity. The tension created by transpiration pulls this water column upwards, while the cohesive and adhesive forces ensure that the water column remains unbroken, thereby facilitating water transport from roots to leaves.
The cohesion-tension theory is a widely accepted explanation of water transport in plants, focusing on the cohesive forces between water molecules and the tension created by transpiration. However, some criticisms arise, particularly concerning the theory's applicability in extreme conditions such as severe drought or in very tall plants. In such situations, the continuous water column might be vulnerable to breakage, and the theory may not fully explain how plants manage to transport water. The controversy underscores the complexity of plant water transport and suggests that other mechanisms might also be at play, thus demanding further scientific research and validation.
