CBSE Class 11 Biology Chapter 11: Transport In Plants Notes

Means of Transport in Plants

Water Potential: The Driving Force of Water Movement

Water potential (Ψw) is a crucial concept that dictates the direction of water movement. It is defined as the potential energy of water per unit volume relative to pure water in reference conditions. Pure water at standard atmospheric pressure has a water potential of zero (Ψw = 0). Any solution has a negative water potential because dissolved solutes reduce the free energy of water.

Water always moves from a region of higher water potential to a region of lower water potential. This movement continues until equilibrium is reached. Water potential is influenced by two main components:

1. Solute Potential (Ψs): This is due to the presence of dissolved solutes. Solutes reduce the free energy of water, so solute potential is always negative. The more the solute, the more negative the Ψs.
2. Pressure Potential (Ψp): This refers to the physical pressure exerted on water. In a plant cell, turgor pressure (pressure exerted by protoplast against the cell wall) is a positive pressure potential. Tension in the xylem (due to transpiration) creates a negative pressure potential. Pressure potential can be positive or negative.

The overall water potential is the sum of solute potential and pressure potential:

Ψw = Ψs + Ψp

Understanding water potential is key to comprehending processes like osmosis, plasmolysis, and turgidity in plant cells. For instance, when a plant cell is placed in a hypotonic solution, water moves into the cell due to a higher water potential outside, increasing Ψp and making the cell turgid. Conversely, in a hypertonic solution, water moves out, leading to plasmolyosis due to a lower Ψp or even negative pressure on the protoplast.

Important Definitions

Osmosis

The movement of water molecules across a selectively permeable membrane from a region of higher water potential to a region of lower water potential.

Plasmolysis

The shrinking of the protoplast away from the cell wall when a plant cell loses water by osmosis in a hypertonic solution.

Imbibition

A special type of diffusion where water is absorbed by solid colloids, causing them to increase in volume. Example: swelling of seeds.

Transpiration

The evaporative loss of water by plants, primarily through the stomata in the leaves.

Guttation

The loss of water in the form of liquid droplets from the margins of leaves of herbaceous plants, often seen in early morning when root pressure is high and transpiration is low.

Translocation

The long-distance transport of organic substances (food) primarily from leaves (source) to other parts of the plant (sink) through the phloem.

Root Pressure

Positive pressure developing in the xylem sap of roots, pushing water upwards to a small extent, primarily observed at night or early morning.

Long-Distance Transport of Water (Ascent of Sap)

1. Water Absorption by Roots — Water and minerals are absorbed by root hairs (epidermal cells of roots) primarily by diffusion and facilitated diffusion, then moved into the root cortex, endodermis, and finally into the xylem.
2. Transpiration in Leaves — Water evaporates from the surface of mesophyll cells in the leaves into the intercellular spaces, and then diffuses out of the leaf through stomata as water vapor. This creates a negative pressure (tension) in the xylem sap.
3. Cohesion-Adhesion-Surface Tension — Due to the cohesion (mutual attraction between water molecules), adhesion (attraction of water molecules to polar surfaces of xylem walls), and surface tension (water molecules attracted to each other in the liquid phase more than to water in the gas phase), a continuous column of water is maintained in the xylem from roots to leaves.
4. Transpiration Pull — The tension created by transpiration in the leaves pulls the entire water column upwards, from the roots through the xylem vessels, against gravity. This pull is the most significant driving force for water transport in tall trees.

Phloem Transport: Pressure Flow Hypothesis

The transport of food (sugars, primarily sucrose) from the leaves (source) to other parts of the plant (sink) occurs through the phloem. This process is called translocation. The most widely accepted mechanism for this is the Pressure Flow Hypothesis (or Mass Flow Hypothesis).

1. Loading at the Source: Sugars produced during photosynthesis in the source cells (e.g., mesophyll cells of leaves) are actively transported into the sieve tube elements of the phloem. This process is called phloem loading and requires ATP.
2. Osmotic Water Movement: The active loading of solutes (sugars) into the sieve tubes increases the solute concentration within them. Consequently, water moves from the adjacent xylem into the sieve tubes by osmosis, increasing the turgor pressure (hydrostatic pressure) in the sieve tubes at the source end.
3. Mass Flow: This increased pressure at the source causes the sap (water and sugars) to flow in bulk (mass flow) through the sieve tube elements to regions of lower pressure, i.e., the sink.
4. Unloading at the Sink: At the sink (e.g., roots, fruits, growing buds), sugars are actively transported out of the sieve tube elements and utilized or stored. This process is called phloem unloading.
5. Water Recirculation: The removal of sugars at the sink reduces the solute concentration in the sieve tubes, causing water to move back into the xylem by osmosis. This maintains the pressure gradient and allows for continuous recirculation of water.

Phloem transport is bidirectional, meaning food can be transported both upwards and downwards, depending on the location of source and sink. For instance, during early spring, stored food in roots can act as a source for developing buds, making the root the source and bud the sink.

Illustrative Examples

• Calculating Water Potential Q: A plant cell has a solute potential (Ψs) of -0.8 MPa and a pressure potential (Ψp) of +0.5 MPa. What is its water potential (Ψw)? A: Ψw = Ψs + Ψp = -0.8 MPa + 0.5 MPa = -0.3 MPa.
• Direction of Water Movement Q: Cell A has Ψw = -0.6 MPa. Cell B has Ψw = -0.4 MPa. In which direction will water move if they are in contact? A: Water moves from higher water potential to lower water potential. So, water will move from Cell B (Ψw = -0.4 MPa) to Cell A (Ψw = -0.6 MPa).
• Effect of Girdling on Trees Q: If the bark of a tree, including the phloem, is removed in a ring (girdling), what will be the immediate and long-term effects? A: Immediately, downward transport of food to the roots will stop. Long-term, the roots will starve and eventually die due to lack of food, leading to the death of the entire tree, even though water transport through xylem remains intact.

Key Points for Quick Revision

• Short-distance transport (diffusion, facilitated diffusion, active transport) is cell-to-cell, while long-distance transport (mass flow) is through vascular tissues (xylem and phloem).
• Water potential (Ψw = Ψs + Ψp) is the main gradient for water movement; water moves from higher to lower Ψw.
• Pure water has the highest water potential (0 MPa); all solutions have negative solute potential (Ψs).
• Transpiration pull is the primary driving force for water ascent in tall plants, relying on cohesion, adhesion, and surface tension of water.
• Transpiration occurs mainly through stomata and leads to evaporative cooling and absorption of minerals.
• Mineral ions are absorbed actively by roots and transported along with the transpiration stream in the xylem.
• Phloem transports sugars (sucrose) from source (leaves) to sink (storage/growth areas) via the Pressure Flow Hypothesis.
• Phloem loading and unloading are active processes, consuming ATP and creating the osmotic gradient for mass flow.

Exam Pro Tip: Distinguish Between Key Processes

Examiners often test your ability to differentiate between similar-sounding processes or relate them to each other. Pay close attention to the driving force, energy requirement, and direction of movement for:

• Diffusion vs. Facilitated Diffusion vs. Active Transport
• Osmosis vs. Imbibition
• Transpiration vs. Guttation
• Xylem Transport vs. Phloem Transport

Clearly define the terms and describe the underlying mechanisms. Use diagrams if applicable to illustrate pathways. For numerical problems involving water potential, remember that solute potential is always negative, and water moves from less negative to more negative water potential values.

Practice Questions with Solutions

• Q: What is the significance of aquaporins in facilitated diffusion? A: Aquaporins are membrane proteins that specifically facilitate the rapid movement of water molecules across cell membranes, increasing the rate of water transport compared to simple diffusion.
• Q: Why is transpiration considered a 'necessary evil' for plants? A: It's 'necessary' because it creates transpiration pull for water absorption and transport, and provides evaporative cooling. It's 'evil' because it leads to significant water loss, which can be detrimental in water-scarce conditions.
• Q: Explain why phloem transport is considered 'bidirectional' while xylem transport is largely 'unidirectional'. A: Xylem transports water and minerals upwards from roots (source) to leaves (sink), making it unidirectional. Phloem transports food from source to sink, but the source and sink locations can change (e.g., leaves as source to roots as sink, or stored food in roots as source to developing buds as sink), thus allowing bidirectional flow.
• Q: If a plant is kept in an environment with high humidity, how would its rate of transpiration be affected? A: High humidity would decrease the water potential gradient between the leaf's internal air spaces and the outside atmosphere. This would reduce the rate of water vapor diffusion out of the stomata, thus decreasing the rate of transpiration.

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