Problem 1 - Biology: Science for Life with Physiology

Question

Why does a column of water in a xylem tube remain intact despite being under tremendous tension?

Step-by-Step Solution

Answer

A column of water in a xylem remains intact due to cohesion, adhesion, and the transpiration pull.

Step 1: Understanding Cohesion

Water molecules are polar, which means they have a slight charge at each end. This allows them to form hydrogen bonds with each other, a property known as cohesion. In the xylem, these cohesive forces help the water molecules stick together, forming a continuous column.

Step 2: Exploring Adhesion

Water molecules also adhere to the walls of the xylem due to another property called adhesion. This is where the water molecules stick to other surfaces, such as the walls of the xylem tubes, preventing the column from breaking, especially when it's under tension.

Step 3: Considering Capillary Action

The combination of cohesion and adhesion contributes to capillary action, which helps drive water up the plant against gravity. This is another force keeping the water column intact as tension increases due to the transpiration pull.

Step 4: Transpiration Pull

When water evaporates from the leaves' surface during transpiration, it creates a negative pressure (suction effect), pulling more water up through the xylem. This tension pulls the continuous column of water upward without breaking, as cohesion and adhesion work together to maintain its integrity.

Key Concepts

Cohesion

Cohesion is a vital property of water that allows it to stick to itself. This occurs because water molecules are polar, meaning they have a slight positive charge on one side and a slight negative charge on the other. Because of this polarity, water molecules form hydrogen bonds with one another. This bond creates a strong attraction between the molecules, almost like water molecules holding hands in a long chain.
This cohesive property is essential in the xylem, the plant's transportation tubes that carry water from the roots to the leaves. In these tubes, cohesion aids in maintaining a continuous column of water, even under tension. By sticking together, the water molecules can move upward as a solitary unit, allowing plants to transport water efficiently across substantial heights.

Adhesion

Adhesion is another important property that assists in water transport within plants. While cohesion involves water molecules sticking to each other, adhesion refers to water molecules sticking to other surfaces. In the context of the xylem, adhesion helps water molecules adhere to the walls of the xylem tubes.
This adherence is crucial because it prevents the water column from collapsing under the tension created by transpiration in the leaves. By sticking to the xylem walls, water molecules can keep moving upward instead of merely falling back down due to gravity. This adhesion plays a complementary role to cohesion, together keeping the water column unbroken throughout its journey through the plant.

Capillary Action

Capillary action is the phenomenon where water rises up narrow spaces, like the tubes in a plant's xylem, without the assistance of external forces. This effect results from the combination of cohesion and adhesion. Cohesion helps water molecules stick together, while adhesion helps them stick to the xylem walls, creating an upward pull.
Think of capillary action like a straw in a drink. When you sip, the liquid moves upward; similarly, in plants, water climbs the small tubes due to this joint effort of cohesion and adhesion. This capillary effect is vital as it helps plants transport water from the roots to the top against the force of gravity, ensuring that even the tallest plants can receive the moisture they need from the soil.

Transpiration Pull

Transpiration pull is the driving force behind the movement of water through a plant, especially in the xylem. It starts when water evaporates from the tiny openings on a leaf’s surface in a process called transpiration. This loss of water creates negative pressure inside the leaf, like a vacuum.
This negative pressure acts like suction, pulling water up from the roots through the xylem. With the combined strength of cohesion and adhesion, this transpiration pull supports a continuous column of water moving up towards the leaves. This unbroken water stream ensures the plant gets the necessary nutrients transported along with the water, reaching even the highest leaves without disruption.