Transport in Plants Biology Notes - Diffusion, Water Potential, Osmosis, Transpiration, Mineral Uptake and Phloem Transport

Plants lack interstitial fluid and a circulatory system but need to transport various substances (water, minerals, organic nutrients, growth regulators, etc.) over long distances.

Direction of Transport

• Unidirectional transport: E.g., Transport of water and minerals in xylem (from roots to stems, leaves, etc.).
• Multidirectional transport: E.g., Transport of photosynthates (organic compounds), mineral nutrients, etc.

Sometimes, plant hormones and other chemical stimuli are transported in a polarized or unidirectional manner from where they are synthesized to other parts.

Means of Transport

1. Diffusion

• It is the slow movement of gases, liquids, and solutes from a higher concentrated region to a lower concentrated region without energy expenditure.
• It may occur from one part of the cell to another, from cell to cell, or over short distances.
• It is not dependent on a living system and is the only means for gaseous movement in a plant body.

Factors affecting diffusion rates:

• Concentration gradient.
• Permeability of the membrane.
• Temperature and pressure.
• Size or density. Smaller substances diffuse faster.
• Solubility in lipids of the membrane. Substances soluble in lipids diffuse through the membrane faster.

2. Facilitated Diffusion

• It is the diffusion of hydrophilic substances with the help of membrane protein channels without expenditure of ATP energy.
• It requires a concentration gradient.

• It is very specific, allowing the cell to select substances for uptake. It is sensitive to inhibitors that react with protein side chains.
• Transport rate reaches a maximum when all protein transporters are used (saturation).
• Some protein channels are always open; others can be controlled. Some are large-sized, e.g., porins.
• Porins form huge pores in the outer membranes of plastids, mitochondria, and some bacteria, allowing molecules the size of small proteins to pass through.
• An extracellular molecule binds to the transport protein, which rotates and releases the molecule inside the cell. E.g., water channels made up of eight types of aquaporins.

Passive uniports, symports, and antiports:

• Uniport: A molecule moves alone across a membrane through a transport or carrier protein.
• Symport: Two molecules cross the membrane together in the same direction.
• Antiport: Two molecules move in opposite directions.

3. Active Transport

• It is the transport of molecules against a concentration gradient (from a lower concentrated region to a higher concentrated region) with the expenditure of energy.
• It is carried out by membrane proteins.
• Pumps are proteins that use energy to transport substances across the cell membrane (uphill transport).
• Transport rate reaches a maximum when all protein transporters are used (saturated).
• The carrier protein is very specific and sensitive to inhibitors that react with protein side chains.

Comparison of Different Transport Processes

Plant-Water Relations

• Water is a universal solvent.
• Protoplasm is mainly water in which different molecules are dissolved and suspended.
• Soft plant parts mostly contain water. E.g., watermelon has 92% water.
• Herbaceous plants have only 10-15% dry matter.
• Dry seeds and woody parts also contain little water.
• A mature corn plant absorbs 3 liters of water daily.
• A mustard plant absorbs water equal to its own weight in about 5 hours.

Water Potential (Ψw)

• It is the potential energy of water, measuring the ability of water molecules to move freely in solution.
• It is expressed in pressure units such as Pascals (Pa).
• Water molecules have kinetic energy. In liquid and gaseous forms, they show random, rapid, and constant motion.
• As the concentration of water in a system increases, its kinetic energy (‘water potential’) also increases. Hence, pure water has the greatest water potential.
• Water molecules move from a higher energy system (higher water potential) to a lower energy system (lower water potential). This movement down a gradient of free energy is called diffusion.
• Water potential (Ψw) of pure water at standard temperatures, not under any pressure, is zero.
• If a solute is dissolved in pure water, water potential decreases due to a decrease in the concentration (free energy) of water. Hence, Ψw of solutions is lower than pure water.
• The magnitude of lowering of water potential due to dissolution of a solute is called solute potential (Ψs) or osmotic potential.
• Ψs is always negative. The more solute molecules, the lower (more negative) is the Ψs.
• For a solution at atmospheric pressure, Ψw = Ψs.
• If a pressure greater than atmospheric pressure is applied to pure water or a solution, its water potential increases, equivalent to pumping water from one place to another.
• When water enters a plant cell due to diffusion, it causes pressure against the cell wall, making the cell turgid. This increases the pressure potential (Ψp).
• Pressure potential is usually positive, though negative potential or tension in the water column in the xylem plays a major role in water transport up a stem.
• Water potential of a cell is affected by solute potential and pressure potential. The relationship is:

Ψw = Ψs + Ψp

Osmosis

• It is the spontaneous diffusion of water across a differentially- or semi-permeable membrane.
• Cell membrane and tonoplast (membrane of vacuole) are important determinants of molecule movement in or out of a plant cell. The cell wall is permeable to water and substances in solution, so it is not a barrier.
• Vacuolar sap in the large central vacuole contributes to the solute potential of the cell.
• The net direction and rate of osmosis depend on pressure gradient and concentration gradient.
• Water moves from a region of higher chemical potential (concentration) to a region of lower chemical potential until equilibrium is reached. At equilibrium, both chambers have the same water potential.

Solution A: High water potential, high solute potential.

Solution B: Low water potential, low solute potential.

Potato Osmometer

• Create a cavity in a potato tuber and pour a concentrated sugar solution into it. This setup is called a potato osmometer.
• Place it in water. Water enters the cavity due to osmosis.

A Demonstration of Osmosis

• A thistle funnel filled with sucrose solution is inverted in a beaker containing pure water.
• The sucrose solution is separated from water by a semi-permeable membrane (e.g., egg shell membrane).
• Water moves into the funnel, causing the solution level to rise until equilibrium is reached (figure a).
• If external pressure is applied from the upper part of the funnel, no water diffuses into the funnel through the membrane (figure b).
• This pressure, required to prevent water diffusion, is the osmotic pressure, a function of solute concentration. Higher solute concentration requires greater pressure to prevent diffusion.
• Numerically, osmotic pressure is equivalent to the osmotic potential, but with opposite signs: osmotic pressure is positive, and osmotic potential is negative.

Plasmolysis

• If an external solution balances the osmotic pressure of the cytoplasm, it is called isotonic.
• In an isotonic solution, there is no net flow of water into or out of the cell (water flow is in equilibrium). Such cells are flaccid.
• If the external solution is more dilute (higher water potential) than the cytoplasm, it is hypotonic. Cells swell and become turgid in hypotonic solutions.
• If the external solution is more concentrated (more solutes) than the cytoplasm, it is hypertonic.

• When a cell is placed in a hypertonic solution, water moves from the cell (area of high water potential) to the outside (area of lower water potential), causing the cell to shrink. This is called plasmolysis. Water is first lost from the cytoplasm and then from the vacuole.
• During plasmolysis, the cell membrane and protoplast shrink away from the cell wall. Such cells are plasmolysed.
• Plasmolysis is usually reversible. When placed in a hypotonic solution, water diffuses into the cell, and the cytoplasm builds up pressure against the wall, known as turgor pressure.
• The pressure exerted by the protoplasts due to water entry against the rigid walls is called pressure potential (Ψp). The cell does not rupture due to the rigidity of the cell wall. Turgor pressure causes enlargement and extension growth of cells.

Imbibition

• It is a type of diffusion in which water is absorbed by solids (colloids), increasing their volume. E.g., absorption of water by seeds and dry wood.
• The pressure due to the swelling of wood can split rocks.
• Seedlings emerge from the soil due to imbibition pressure.
• Imbibition requires:
• Difference in concentration gradient.
• Water potential gradient between the absorbent and the liquid imbibed.
• Affinity between the adsorbent and the liquid.

Long Distance Transport of Water

• Diffusion is a slow process, suitable for short distances. For example, molecule movement across a typical plant cell (about 50 μm) takes approximately 2.5 seconds.
• Long-distance transport systems are necessary to move substances faster over extended distances.
• Movement of substances in bulk (en masse) from one point to another due to pressure differences is called mass (bulk) flow. Examples include the movement of water, minerals, and food.
• In mass flow, substances (in solution or suspension) move together at the same pace, like in a flowing river. In contrast, diffusion involves independent movement of substances based on their concentration gradients.
• Bulk flow is driven by either a positive hydrostatic pressure gradient (e.g., water through a garden hose) or a negative hydrostatic pressure gradient (e.g., suction through a straw).
• Bulk movement of substances over long distances through conducting tissues (xylem and phloem) is called translocation.

Absorption of Water by Plants

• Water and minerals are absorbed by diffusion through millions of root hairs at the root tips, which increase the surface area for absorption.
• Absorbed water moves deeper into root layers via two pathways:
1. Apoplast pathway
2. Symplast pathway

1. Apoplast Pathway

• It consists of a continuous system of adjacent cell walls, interrupted only by the casparian strips in the endodermis of the roots.
• Water moves exclusively through intercellular spaces and cell walls without crossing the cell membrane.
• Movement through the apoplast depends on a gradient and occurs via mass flow.
• The apoplast offers no barrier to water movement.

• As water evaporates into intercellular spaces or the atmosphere, tension develops in the continuous water stream in the apoplast. Mass flow occurs due to the adhesive and cohesive properties of water.

2. Symplast Pathway

• It involves a system of interconnected protoplasts.
• Water travels through the cytoplasm, with intercellular movement occurring via plasmodesmata (junctions between neighboring cells through which cytoplasmic strands extend).
• Water must enter cells through the cell membrane, making movement slower and dependent on a potential gradient.
• Symplastic movement may be aided by cytoplasmic streaming. For example, in Hydrilla leaves, chloroplast movement due to cytoplasmic streaming is visible.

• Most water flow in roots occurs via the apoplast since cortical cells are loosely packed, allowing movement without resistance. However, the endodermis is impervious to water due to the casparian strip (a band of suberized matrix). Water is directed to non-suberized wall regions, then moves through the symplast, crossing a membrane to reach the xylem.
• Water movement through the root layers is ultimately symplastic in the endodermis, the only pathway for water and solutes to enter the vascular cylinder.
• In young roots, water enters directly into xylem vessels and tracheids, which are non-living conduits and part of the apoplast.
• Some plants have additional structures for absorption. For example, mycorrhiza, a symbiotic association of a fungus with a root system, enhances absorption. Fungal hyphae absorb mineral ions and water from the soil, while roots provide sugars and nitrogen compounds. Some plants, like Pinus, require mycorrhizae for seed germination and establishment.

Water Movement up a Plant

Water moves up a stem against gravity, requiring energy.

Root Pressure

• As ions are actively transported from the soil into the vascular tissues of roots, water follows its potential gradient, increasing pressure inside the xylem. This positive pressure is called root pressure.
• It helps push water to small heights in the stem.

Experiment to Prove Existence of Root Pressure

• In early morning with high atmospheric moisture, cut a soft plant stem horizontally near the base. Drops of solution ooze out due to positive root pressure.
• At night and early morning, when evaporation is low, excess water collects as droplets around special openings of veins near the tips of grass blades and leaves of herbaceous plants. This liquid-phase water loss is called guttation.
• Root pressure provides only a modest push and plays no major role in water movement up tall trees. Its primary role is re-establishing continuous water chains in the xylem, which often break under transpiration-induced tensions.
• In most plants, the majority of water transport occurs via transpiration pull.

Transpiration Pull

• Water flows upward through the xylem at high rates (up to 15 m/hr).
• Water is primarily pulled through the plant due to transpiration pull, the driving force of transpiration. This is explained by the cohesion-tension-transpiration pull model of water transport.

Transpiration

• Transpiration is the evaporative loss of water by plants through the stomata in the leaves.
• Less than 1% of the water reaching the leaves is used in photosynthesis and plant growth. The remainder is lost through transpiration.
• Transpiration can be studied using cobalt chloride paper, which changes color (blue to pink) upon absorbing water.
• During transpiration, the exchange of O₂ and CO₂ in the leaf also occurs.
• Stomata are open during the day and closed at night.
• The opening or closing of stomata is due to changes in the turgidity of the guard cells.

• The inner wall of the guard cell lining the stomatal aperture is thick and elastic, while the outer wall is thin.
• When the turgidity of guard cells increases, the outer walls bulge out, pulling the inner walls into a crescent shape, opening the stoma.
• Cellulose microfibrils in the guard cells are oriented radially, facilitating stomatal opening.
• Guard cells lose turgidity due to water loss (or water stress), and the inner walls regain their original shape, closing the stoma.
• The lower surface of a dicot leaf typically has more stomata, while in monocot leaves, stomata are roughly equal on both surfaces.

Factors Affecting Transpiration

• External factors: Temperature, light, humidity, wind, etc.
• Plant factors: Number and distribution of stomata, number of open stomata, water status of the plant, canopy structure, etc.
• The transpiration-driven ascent of xylem sap depends on the following physical properties of water:
• Cohesion: Mutual attraction between water molecules.
• Adhesion: Attraction of water molecules to polar surfaces (e.g., surfaces of tracheary elements).
• Surface Tension: In the liquid phase, water molecules are more attracted to each other than in the gas phase.

• These properties give water high tensile strength (ability to resist a pulling force) and capillarity (ability to rise in thin tubes), aided by the small diameter of tracheary elements—tracheids and vessel elements.
• Xylem vessels supply water from the roots to leaf veins. A continuous thin film of water covers the cells. As water evaporates through the stomata, more water is pulled into the leaf from the xylem. The lower water vapor concentration in the atmosphere compared to the substomatal cavity and intercellular spaces facilitates water diffusion into the surrounding air, creating a ‘pull’.
• Transpiration forces can generate pressures sufficient to lift a xylem-sized column of water over 130 meters high.

Transpiration and Photosynthesis – A Compromise

• Photosynthesis is limited by available water, which is rapidly depleted by transpiration.
• The humidity of rainforests is largely due to the cycling of water from roots to leaves to the atmosphere and back to the soil.
• The C4 photosynthetic system maximizes CO₂ availability while minimizing water loss.
• C4 plants are twice as efficient as C3 plants in fixing carbon (making sugar) and lose only half as much water as C3 plants for the same amount of CO₂ fixed.

Uses of Transpiration

• Creates transpiration pull for absorption and transport.
• Supplies water for photosynthesis.
• Transports minerals from soil to all parts of the plant.
• Cools leaf surfaces, sometimes by 10–15°C, through evaporation.
• Maintains the shape and structure of plants by keeping cells turgid.

Uptake and Transport of Mineral Nutrients

Uptake of Mineral Ions

• Most minerals are actively absorbed by the roots because:
• Minerals occur in the soil as charged particles (ions) that cannot move across cell membranes.
• The concentration of minerals in the soil is lower than that in the root.
• Active uptake of ions contributes to the water potential gradient in roots, facilitating water uptake by osmosis.
• Some ions are absorbed passively.
• Specific membrane proteins in root hair cells actively pump ions from the soil into the epidermal cells.
• The endodermal cell membrane also contains transport proteins that selectively allow certain solutes to cross, acting as control points where the plant adjusts the quantity and types of solutes reaching the xylem.
• The suberin in the root endodermis ensures active transport of ions occurs in one direction only.

Translocation of Mineral Ions

• Ions that reach the xylem are transported to all parts of the plant through the transpiration stream.
• The chief sinks for mineral elements include:
• Growing regions such as apical and lateral meristems.
• Young leaves.
• Developing flowers, fruits, and seeds.
• Storage organs.
• Unloading of mineral ions at fine vein endings occurs through diffusion and active uptake by cells.
• Mineral ions are remobilized, particularly from older, senescing parts (e.g., older dying leaves) to younger leaves.
• Elements most readily mobilized are phosphorus, sulfur, nitrogen, and potassium. Structural components like calcium are not remobilized.
• Nitrogen is primarily transported in organic forms, such as amino acids and related compounds, though some travels as inorganic ions. Phosphorus and sulfur are also carried as organic compounds.
• There is an exchange of materials between xylem and phloem, so xylem does not exclusively transport inorganic nutrients, nor does phloem transport only organic materials.

Phloem Transport: Flow from Source to Sink

• Phloem transport is the long-distance movement of organic substances (primarily sucrose) from a source (region of synthesis, e.g., leaves) to a sink (region of storage or utilization).
• The source and sink may reverse depending on the season or plant needs. For example, in early spring, sugar stored in roots moves to tree buds for growth, making roots the source and buds the sink.
• Movement in the phloem can be upward or downward (bi-directional), unlike xylem, which is always upward (unidirectional). Thus, phloem sap can transport food in any direction.
• Phloem sap mainly consists of water and sucrose, but also includes other sugars, hormones, and amino acids.

The Pressure Flow (Mass Flow) Hypothesis

• This hypothesis explains the mechanism of sugar translocation in the phloem.
• Glucose produced at the source (via photosynthesis) is converted to sucrose.
• Sucrose is actively transported (loading) into companion cells and then into living phloem sieve tubes, creating a hypertonic condition in the phloem (lowering water potential).
• Sieve tube cells form long columns with holes in sieve plates. Cytoplasmic strands pass through these holes, forming continuous filaments.
• Water from adjacent xylem moves into the phloem by osmosis. As osmotic/hydrostatic pressure builds, phloem sap moves to areas of lower osmotic pressure (sink).

• Sucrose from phloem sap is actively transported into sink cells, which convert it into energy, starch, or cellulose.
• As sugars are removed, osmotic pressure decreases (water potential increases), and water moves out of the phloem.

Identification of the Tissue that Transports Food (Girdling)

• Carefully remove a ring of bark (including the phloem layer) from a tree trunk.
• After a few weeks, the portion above the ring swells due to the blocked downward movement of food.
• This demonstrates that phloem is responsible for food translocation and that transport occurs in one direction, toward the roots.