Transport in Plants — NCERT Class 11 Biology

The rate of diffusion depends on: 1. Concentration gradient: Steeper the gradient, faster the diffusion. 2. Temperature: Higher temperature increases kinetic energy of molecules, speeding up diffusion. 3. Permeability of the membrane: More permeable membranes allow faster diffusion. 4. Size of molecules: Smaller molecules diffuse faster than larger ones. 5. Solubility in lipids: Lipid-soluble substances diffuse faster through the lipid bilayer. 6. Pressure: An increase in pressure increases the rate of diffusion.

Porins are protein molecules that form large pores in the outer membranes of plastids, mitochondria, and some bacteria. They create hydrophilic channels that allow the passive transport of molecules up to the size of small proteins across the membrane. Porins do not require energy — substances move through them down their concentration gradient by facilitated diffusion.

Facilitated diffusion is the passive movement of substances across a biological membrane through specific protein channels or carrier proteins, down their concentration gradient, without expenditure of ATP. It differs from simple diffusion in that it requires membrane proteins and is selective (specific for particular solutes) and saturable (rate plateaus at high solute concentration). Examples: aquaporins for water, glucose transporters.

Two types of protein channels are involved:

• Channel proteins — form pores (e.g., porins, aquaporins); allow passage of specific ions/molecules.
• Carrier proteins — bind the solute, undergo conformational change, and release the solute on the other side.

Water potential (ψw) is the potential energy of water per unit volume relative to pure water at atmospheric pressure and reference temperature. It determines the direction of water movement — water moves from higher water potential to lower water potential.

Components:

• ψw = ψs + ψp
• ψs (solute/osmotic potential): Always negative; proportional to the concentration of dissolved solutes. More solutes = more negative ψs.
• ψp (pressure potential): Usually positive; the pressure exerted on the water (turgor pressure in cells). Negative in xylem during transpiration (tension).

Pure water at standard conditions has ψw = 0 MPa (by convention). Adding solutes makes ψw negative.

When a pressure greater than atmospheric pressure is applied:

• Pure water: The pressure potential (ψp) becomes positive, so water potential becomes positive (ψw = ψp, since ψs = 0). Water potential is now greater than zero.

When a plant cell is placed in a hypotonic solution (solution with higher water potential than the cell): 1. Water enters the cell by osmosis (water moves from higher ψw outside to lower ψw inside). 2. The protoplast swells and pushes against the cell wall — this pressure is called turgor pressure (ψp increases). 3. The cell wall exerts an equal and opposite wall pressure. 4. The cell becomes turgid (firm and swollen). 5. Eventually, equilibrium is reached when ψw(cell) = ψw(solution).

The pressure flow hypothesis (also called the mass flow hypothesis) was proposed by Munch (1930). It explains the translocation of organic solutes (mainly sucrose) through the phloem:

1. At the source (e.g., leaves): Sucrose is actively loaded into sieve tube elements using ATP. This decreases the solute potential (ψs becomes more negative), causing water potential inside the sieve tube to decrease. Water enters from the adjacent xylem by osmosis, creating a high turgor pressure (ψp) at the source end. 2. At the sink (e.g., roots, fruits): Sucrose is unloaded from the sieve tube. This increases the solute potential (ψs becomes less negative), increasing water potential. Water moves out of the sieve tube into the xylem, creating a low turgor pressure at the sink end. 3. Pressure gradient: The difference in turgor pressure between source (high) and sink (low) drives the mass flow of phloem sap from source to sink.

The hypothesis is supported by: (a) sieve tubes have sap under pressure, (b) concentration of sucrose is higher at source than sink, and (c) phloem transport is bidirectional.

Translocation is the movement of organic substances (primarily sucrose, amino acids, hormones) from the source (leaves or storage organs where food is synthesised) to the sink (roots, fruits, growing tips where food is utilised or stored) through the phloem.

The three means of transport across the membrane are:

1. Diffusion (simple diffusion): Passive movement of molecules from higher concentration to lower concentration without any membrane protein or energy. Small, lipid-soluble molecules (O₂, CO₂) cross easily. 2. Facilitated diffusion: Passive transport through specific membrane proteins (channels or carriers) down the concentration gradient. No ATP required. Selective and saturable. Examples: aquaporins for water, glucose transporters. 3. Active transport: Movement against the concentration gradient using energy (ATP). Requires carrier proteins called pumps. Example: Na⁺/K⁺ pump. Allows cells to maintain concentration gradients.

Water potential (ψw) is the potential energy of water per unit volume relative to pure water under standard conditions. It determines the direction and rate of water movement. Convention: ψw of pure water = 0 MPa.

Factors affecting water potential: 1. Solute concentration (ψs): Adding solutes decreases ψw (makes it more negative). More dissolved substances = lower water potential. 2. Pressure (ψp): Positive pressure (turgor) increases ψw; negative pressure (tension in xylem) decreases ψw. 3. Gravity: Affects water potential in tall plants (gravitational component ψg).

Plasmolysis is the shrinkage of the protoplast away from the cell wall when a plant cell is placed in a hypertonic solution (solution with lower water potential than the cell).

Process: 1. Water moves out of the cell by osmosis (from higher ψw inside to lower ψw outside). 2. The protoplast shrinks and pulls away from the cell wall. 3. The cell becomes flaccid.

Plasmolysis is reversible — if the plasmolysed cell is placed back in a hypotonic solution, water re-enters and the cell becomes turgid again. Plasmolysis does not occur in animal cells (no cell wall).

Guttation is the loss of water in the form of liquid droplets from the margins and tips of leaves through special structures called hydathodes. It occurs when root pressure is high and transpiration is low (e.g., early morning, humid conditions).

Root pressure is the positive hydrostatic pressure developed in the root cortical cells that pushes water upward through the xylem. It is generated by the active transport of ions into the root xylem, which lowers the water potential, causing water to enter from the soil by osmosis. This creates a positive pressure that pushes the water column upward.

Role in ascent of sap:

• Pushes water upward through the xylem, especially at night when transpiration is low.
• Responsible for guttation — excess water is pushed out through hydathodes.
• However, root pressure alone is insufficient to push water to the tops of tall trees (it can only raise water by about 1–2 atm, enough for a few metres). The transpiration pull is the major force in tall plants.

The cohesion-tension theory is the most widely accepted explanation for the ascent of sap. It was proposed by Dixon and Jolly (1894).

Key principles: 1. Cohesion: Water molecules are attracted to each other by hydrogen bonds, forming a continuous column in the xylem. The tensile strength of this column is very high (can withstand tension up to 350 atm). 2. Adhesion: Water molecules are attracted to the walls of xylem vessels (capillarity). This prevents the water column from pulling away from the walls. 3. Transpiration pull: As water evaporates from the stomata (transpiration), it creates a negative pressure (tension) at the leaf surface. This tension is transmitted through the continuous water column all the way down to the roots, pulling water upward.

The combination of cohesion, adhesion, and transpiration pull maintains an unbroken water column from roots to leaves, enabling ascent of sap even in trees over 100 m tall.

External factors: 1. Temperature: Higher temperature increases evaporation rate, increasing transpiration. 2. Light: Light opens stomata (phototropic opening), increasing transpiration. 3. Humidity: Higher relative humidity reduces the water vapour gradient, decreasing transpiration. 4. Wind: Wind removes water vapour from the leaf surface, increasing the gradient and increasing transpiration. 5. Water availability: If soil water is deficient, transpiration decreases.

Internal (plant) factors: 1. Stomatal number and distribution: More stomata = more transpiration. Most stomata are on the lower surface (to reduce water loss). 2. Stomatal aperture: Wider opening = more transpiration. 3. Leaf area: Larger leaf area = more transpiration. 4. Cuticle thickness: Thicker cuticle = less transpiration (xerophytes). 5. Leaf orientation: Vertical leaves (in some grasses) reduce direct sunlight and transpiration.

Imbibition is the absorption of water by hydrophilic colloidal substances (like seeds, wood, gelatin) resulting in swelling. The substance that absorbs water is called the imbibant and the water absorbed is the imbibate.

Conditions necessary for imbibition: 1. Water potential gradient: The imbibant must have a lower water potential than the surrounding water. 2. Hydrophilic surfaces: The substance must have affinity for water (e.g., proteins, starch, cellulose). 3. Increase in volume: Imbibition causes swelling and generates considerable pressure (imbibition pressure).

Examples: Seeds swell before germination; wooden doors swell during rainy season; raisins swell in water.

(See Intext Page 191 Q1 — same answer.)

The pressure flow hypothesis (Munch, 1930) explains phloem translocation:

• Source (leaves): Sucrose is actively loaded into sieve tubes using ATP → ψs decreases → water enters from xylem → turgor pressure increases.
• Sink (roots/fruits): Sucrose is unloaded → ψs increases → water leaves to xylem → turgor pressure decreases.
• Pressure gradient between source (high ψp) and sink (low ψp) drives mass flow of phloem sap.

Evidence: (a) Phloem sap is under positive pressure, (b) concentration gradient exists from source to sink, (c) when phloem is cut, sap exudes out.

Transpiration provides several benefits: 1. Transpiration pull: Creates the suction force that pulls water and minerals upward through the xylem (ascent of sap). 2. Cooling effect: Evaporation of water from leaf surfaces cools the plant (similar to sweating in humans), preventing overheating. 3. Mineral transport: Helps in the transport of dissolved minerals from roots to shoots. 4. Maintains turgor pressure: Water movement maintains cell turgidity, keeping cells firm. 5. Concentration of cell sap: Transpiration concentrates cell sap, which aids in osmotic absorption of water.

However, excessive transpiration can lead to wilting and water stress.

1. Primary active transport: Uses ATP directly to pump substances against their concentration gradient. Example: Na⁺/K⁺ ATPase pump — pumps 3 Na⁺ out and 2 K⁺ into the cell using one ATP molecule. 2. Secondary active transport (co-transport): Uses the gradient created by primary active transport (not ATP directly) to move another substance. Two types:

• Symport: Both substances move in the same direction. Example: Sucrose-H⁺ symport in phloem loading (H⁺ moves down its gradient, dragging sucrose along).
• Antiport: Substances move in opposite directions. Example: Na⁺/Ca²⁺ antiport in cardiac muscle cells.