NEET Biology Transport in Plants Notes

NEET Biology Chapter 11 Transport in Plants notes covering the means of transport such as diffusion, facilitated diffusion, active transport and types of membranes, plant water relations with water potential, osmosis and plasmolysis, imbibition, long distance transport of water, absorption of water by root hairs and the apoplast and symplast pathways, active and passive absorption, the ascent of sap through root pressure and the cohesion-tension transpiration pull model, transpiration and stomatal mechanism, uptake and translocation of mineral ions, and the pressure flow hypothesis of phloem transport.

At a Glance

• Introduction
• Means of transport
  • Diffusion
  • Facilitated diffusion (passive symports and antiports)
  • Active transport
  • Comparison of different transport processes
• Plant water relations
  • Water potential
  • Osmosis
  • Plasmolysis
  • Imbibition
• Long distance transport of water
  • How do plants absorb water
  • Water movement up a plant
    • Root pressure
    • Transpiration pull
• Transpiration
• Uptake and transport of mineral ions
  • Uptake of mineral ions
  • Translocation of mineral ions
• Phloem transport

Important Terms, Differences and Flow Charts

Terms and Definitions:
1. Diffusion pressure - the pressure exerted by the tendency of particles to diffuse from an area of higher concentration to an area of lower concentration.
2. Membrane permeability - the extent to which a membrane permits or restricts the movement of a substance.
3. Reverse osmosis - the expulsion of pure water from a solution through a semipermeable membrane under external pressure higher than the osmotic pressure of the solution.
4. Solute potential - the reduction in water potential caused by dissolved solute; it is always negative.
5. Pressure potential - the pressure built up by water entering a cell against its wall; it is usually positive.
6. Casparian strips - bands of suberin in the radial walls of endodermal cells that block apoplastic water movement.
7. Turgor pressure - the pressure exerted by the protoplast against the rigid cell wall due to entry of water.
8. Scotoactive stomata - stomata that open in the dark and close during the day, as seen in succulents.

Differences:
1. Transpiration and evaporation - transpiration is a physiological process occurring through stomata and cuticle in plants, while evaporation is a physical process occurring on any free surface.
2. Open and closed stomata - open stomata take in carbon dioxide and lose water vapour, while closed stomata prevent gaseous exchange and transpiration.

Flow Charts:
1. Types of membranes - permeable, impermeable, semipermeable and differentially permeable membranes.
2. Types of transpiration - cuticular, stomatal, lenticular and bark transpiration.

INTRODUCTION

Plants lack both interstitial fluid and a regular circulation system. Even so, they have to transport various substances, such as gases, minerals, water, hormones, photosynthates and organic solutes, over short distances (from one cell to another, or from one tissue to another) and over very long distances (such as water from roots to the tops of plants, or photosynthates from leaves to the tips of roots).

Substances move over short distances through diffusion and active transport, supplemented by cytoplasmic streaming. Movement over longer distances occurs through the vascular system, the xylem and the phloem. This type of movement is called translocation.

An important aspect of translocation is its direction:
1. In rooted plants, transport of water and minerals through the xylem is unidirectional, from roots to stems.
2. Transport of organic compounds and mineral nutrients is multidirectional. Organic compounds, synthesised in leaves, are exported to all other parts, including every living cell, growing points, fruits and storage organs. They are later re-exported from storage organs.
3. Mineral nutrients are primarily picked up by roots, and they are passed upwards to leaves, stems and growing regions through the xylem and phloem.
4. Senescent organs and leaves pass out most of their nutrients, especially minerals, before falling off the plant.
5. Plant hormones and other chemical stimuli are transported in very small amounts.

Thus, a complex traffic of materials goes on in flowering plants, some moving in different directions, some passing out in a polarised manner, with most organs receiving some substances and giving out others.

MEANS OF TRANSPORT

Diffusion

1. Diffusion is passive and may occur from one part of the cell to another, from cell to cell, or over short distances, such as from the intercellular spaces of the leaf to the outside. No energy expenditure takes place.
2. Diffusion is a random movement of individual molecules from a region of higher concentration to a region of lower concentration. It is a slow process and is independent of the living system.
3. A larger difference in concentration means a more rapid flow of molecules.
4. Uptake and distribution of water, gases and solutes in plants occur as a result of diffusion.
5. Diffusion is common in gases and liquids, but diffusion in solids (rather than of solids) is more likely. For example, diffusion of CO₂ from the atmosphere to leaves.
6. Diffusion rates are affected by the concentration gradient, membrane permeability, temperature and pressure.

Diffusion pressure (DP): The term was coined by Meyer (1938). It is the pressure exerted by the tendency of particles to diffuse from an area of higher concentration to an area of lower concentration. Diffusion pressure is directly proportional to the concentration of particles of the diffusing substance and to temperature.

Membrane permeability: The extent to which a membrane permits or restricts the movement of a substance. It depends on the membrane composition and the chemical nature of the solute.

Connecting Concepts:
1. Diffusion pressure of pure water is maximum. Its value at 20 °C is 1336.40 atm.
2. DPD (diffusion pressure deficit) is the reduction in the diffusion pressure of water over its pure state. DPD is the water absorbing capacity of a solution. Therefore, DPD can also be called suction pressure (SP).
3. DPD is related to osmotic pressure (OP), turgor pressure (TP) and wall pressure (WP): DPD / SP = OP - WP. Since WP = TP, DPD = OP - TP.
4. In fully turgid cells, turgor pressure equals osmotic pressure, so OP = TP and DPD = 0.
5. In fully plasmolysed cells, TP becomes zero due to a decrease in TP and an increase in OP. So, SP or DPD = OP.
6. Thus, DPD of a plant cell is not directly proportional to its OP or the concentration of cell sap alone, but depends on both OP and TP.

Types of Membranes (Flow Chart 11.1)
1. Permeable membrane - allows diffusion of both solvent and solute molecules or ions through it. Example: cellulosic cell wall and lignified cell walls.
2. Impermeable membrane - prohibits the diffusion of both solvent and solute molecules or ions. Example: heavily cutinised or suberised cell walls in plants.
3. Semipermeable membrane - allows diffusion of solvent molecules but does not allow the passage of solute molecules. It forms a perfect partition between two osmometers. Examples: membranes of colloidion, parchment paper and copper ferrocyanide. It is essential for the operation of osmosis. In plant-water relations, biomembranes largely function as semipermeable membranes.
4. Differentially permeable membrane - allows only certain substances to pass through them. It allows the passage of solvent as well as some selected solutes using different mechanisms. Example: all biological membranes, such as the plasma membrane, tonoplast and membranes covering cell organelles.

[Figure: Flow Chart 11.1 Types of membranes, showing the four types of membranes branching from permeability, each with a diagram of solvent and solute molecules crossing or not crossing the membrane.]

Facilitated Diffusion

1. The rate of diffusion depends on the size of the substance: smaller substances diffuse faster.
2. Diffusion also depends on the solubility of the substance in lipids. Lipid soluble substances diffuse faster.
3. Substances that have a hydrophilic part find it difficult to pass through the membrane. Their movement is facilitated by proteins, which provide a site at which these molecules cross the membrane. This is called facilitated diffusion.
4. In facilitated diffusion, a concentration gradient must be present.
5. Special proteins that help in the movement of substances across membranes do not require ATP energy.
6. Facilitated diffusion is sensitive to inhibitors that react with protein side chains.
7. Facilitated diffusion cannot cause net transport of molecules from a low to a high concentration, as this would require energy input.
8. The transport rate reaches a maximum when all the protein transporters are in use (saturation).
9. Facilitated diffusion is very specific, and it allows the cell to select substances for uptake.
10. Some protein channels are always open; others can be controlled.
11. Porins are proteins that form large pores in the outer membranes of plastids, mitochondria and some bacteria.
12. Water channels are made up of eight different types of aquaporins, which are membrane proteins for the passive transport of water.

[Figure: Fig. 11.1 Facilitated diffusion. An extracellular molecule binds to the transport protein, the transport protein then rotates, and the molecule is released inside the cell.]

Passive Symports and Antiports
Some transport proteins or carrier proteins permit diffusion only if two types of molecules move together.
1. In symport, both molecules cross the membrane in the same direction.
2. In antiport, both molecules cross the membrane in opposite directions.
3. In uniport, a molecule moves across the membrane independently of other molecules.

[Figure: Fig. 11.2 Facilitated diffusion by symport and antiport carriers, showing two molecules moving in the same direction in symport and in opposite directions in antiport.]

Check Point

Give a scientific term for the following:
(i) The movement of molecules from the region of their higher concentration to the region of lower concentration.
(ii) The extent to which a membrane permits or restricts the movement of a substance.
(iii) Type of membranes that allow selective passage of solutes along with solvent through them using different mechanisms.
(iv) The movement over longer distances through the xylem and phloem.
(v) The reduction in the diffusion pressure of water over its pure state.
(vi) Both the molecules cross the membrane in opposite directions.
(vii) Proteins that form huge pores in the outer membranes of plastids, mitochondria and some bacteria.

Answer:
(i) Diffusion
(ii) Membrane permeability
(iii) Differentially permeable membrane
(iv) Translocation
(v) Diffusion pressure deficit (DPD)
(vi) Antiport
(vii) Porins

Active Transport

1. Active transport uses energy to pump molecules against a concentration gradient.
2. It is carried out by membrane-bound proteins. Different proteins in the membrane play a major role in both active and passive transport.
3. Cells undergoing active transport have abundant mitochondria to provide ATP, which powers active transport. Pumps are proteins that transport substances.
4. Active transport shows uphill transport, because movement occurs from a lower to a higher concentration.
5. Carrier proteins are highly specific, like enzymes, for the substances they carry across the membrane.
6. Inhibitors can stop the process by reacting with protein side chains.
7. When all the protein carriers are in use or saturated, the transport rate reaches its maximum.

Table 11.1: Comparison of Different Transport Mechanisms

PLANT WATER RELATIONS

1. Water is essential for all physiological activities of plants.
2. Water acts as an excellent solvent and helps in the uptake and distribution of mineral nutrients and other solutes.
3. The protoplasm of cells is water in which different molecules are dissolved and suspended. Watermelon has over 92% water. Most herbaceous plants have only about 10 to 15% of their fresh weight as dry matter.
4. The distribution of water within a plant varies. Woody parts have very little water, while soft parts contain mostly water. A seed may appear dry, but it still has water, otherwise it would not be alive and respiring.
5. Terrestrial plants take up a huge amount of water daily, but most of it is lost to the air through evaporation from the leaves, that is, transpiration.
6. A mature corn plant absorbs three litres of water in a day, while a mustard plant absorbs water equal to its weight in 5 hours for growth and productivity.
7. Water plays an important role in many useful reactions within cells.
8. Water maintains the turgidity of cells, which is essential for cell enlargement, growth and development.
9. Water regulates heat in plants through transpiration, guttation and evaporation.

Water Potential

1. The term water potential was proposed by Slatyer and Taylor. It is used in place of DPD (diffusion pressure deficit) and SP (suction pressure).
2. Water potential is the difference between the free energy of water in a system and the free energy of pure water at atmospheric pressure. It is defined as the chemical potential of water divided by the partial molal volume of water (the volume of one mole of water). It is a measure of the free energy associated with water per unit volume (J m^-3).
3. Water potential (Ψ_w) is the fundamental concept for understanding water movement. Solute potential (Ψ_s) and pressure potential (Ψ_p) are the two main components that determine water potential.
4. The greater the concentration of water in a system, the greater its kinetic energy, or water potential. So, pure water has the greatest water potential.
5. If two systems containing water are in contact, random movement of water molecules results in net movement from the system with higher energy to the system with lower energy. So, water moves from higher water potential to lower water potential.
6. Water potential is denoted by the Greek symbol psi (Ψ) and is expressed in pressure units such as bars or pascals (Pa). 1 MPa = 10 bars.
7. By convention, the water potential of pure water at standard temperature, not under any pressure, is taken to be zero.
8. When a solute is dissolved in pure water, the water concentration falls and the water potential is reduced. This lowering is called solute potential, osmotic potential or Ψ_s. Ψ_s is always negative. The more solute molecules, the lower (more negative) the Ψ_s.
9. For a solution at atmospheric pressure, Ψ_w = Ψ_s.
10. If a pressure greater than atmospheric pressure is applied to pure water or a solution, its water potential increases. This is equivalent to pumping water from one place to another.
11. When water enters a plant cell by diffusion, pressure builds up against the cell wall and makes the cell turgid. This increases the pressure potential. Pressure potential is usually positive, but in plants, negative pressure (tension) in the water column of xylem plays a major role in water transport up a stem.
12. Water potential of a cell is affected by both solute and pressure potential: Ψ_w = Ψ_s + Ψ_p.

Connecting Concepts:
1. The lower the water potential in a plant cell or tissue, the greater its ability to absorb water. Conversely, the higher the water potential, the greater the ability of the tissue to supply water to other more desiccated cells and tissues. Thus, water potential is used to measure water deficit and water stress in plant cells and tissues.
2. Solved problem: Cell A has osmotic potential (Ψ_s) of -16 bars and pressure potential of 8 bars. Cell B has Ψ_s of -12 bars and pressure potential of 2 bars. Ψ of cell A = 8 - 16 = -8 bars. Ψ of cell B = 2 - 12 = -10 bars. Water moves from higher water potential to lower water potential, so the flow of water is from cell A to cell B (-8 bars to -10 bars).
3. Traube (1867) used copper ferrocyanide as a semipermeable membrane to show its utility in the osmosis of plant physiology. Pfeffer (1887) developed the osmoscope by using a semipermeable membrane.

Check Point

Fill in the blanks:
(i) The water potential of pure water is __________.
(ii) When a plant absorbs water from the soil, the water potential of the root cell is __________ than the soil.
(iii) Water potential is the sum of two opposing forces in a plant cell, __________ and __________.
(iv) A watermelon has over _____% of water.
(v) Water potential is denoted by the symbol __________ or __________.
(vi) Ψ is always __________.
(vii) Water potential (Ψ_w) can be calculated by __________ + __________.
(viii) Solute potential is also called __________ potential.
(ix) The amount by which the water potential is reduced as a result of the presence of solute is called __________.
(x) If a pressure greater than atmospheric pressure is applied to pure water or a solution, its water potential __________.

Answer:
(i) zero
(ii) lower (more negative)
(iii) solute potential, pressure potential
(iv) 92
(v) psi (Ψ)
(vi) negative (for solutions)
(vii) solute potential (Ψ_s) + pressure potential (Ψ_p)
(viii) osmotic
(ix) solute potential
(x) increases

OSMOSIS

1. The term osmosis was coined by Abbé Nollet (1748).
2. Osmosis is a kind of diffusion that plays a role in the movement of water into and within plants. In plants, it is the diffusion of water from higher concentration to lower concentration across a semipermeable or differentially permeable membrane.
3. Diffusion continues across the membrane until equilibrium is attained.
4. The net direction and rate of osmosis depend on the pressure and concentration gradient.
5. Osmosis can also be defined as the diffusion of water from its pure state into a solution when the two are separated by a semipermeable membrane. It is the diffusion of water or solvent from a dilute solution to a concentrated solution through a semipermeable membrane.
6. A solution that can cause the osmotic entry of water into it is said to be an osmotically active solution.
7. Common experiments for demonstrating osmosis are the thistle funnel experiment and the potato osmoscope.

(i) Thistle funnel experiment: A sucrose solution is taken in a funnel, separated from water by a semipermeable membrane. Water moves into the funnel until equilibrium is reached. Pressure can be applied to stop the water movement into the funnel.

[Figure: Fig. 11.3 Thistle funnel experiment. (a) A thistle funnel containing sucrose solution, with its neck closed by a semipermeable membrane and dipped in a beaker of water. Water diffuses across the membrane. (b) Pressure applied to the funnel stops the water movement into the funnel.]

The experiment proves that sugar solution is an osmotically active solution and can absorb water when separated from water by a semipermeable membrane.

(ii) Potato osmoscope: When a potato tuber with a cavity containing concentrated sugar solution is placed in water, the sugar solution collects water by osmosis. The entry of water into the sugar solution proves that the sugar solution is osmotically active. The cytoplasm of the cells of the tuber between the sugar solution and the water acts as a single semipermeable membrane.

Types of osmosis
1. Endosmosis is the osmotic flow of water into a cell when it is placed in a solution whose solute concentration is less than that of the cell sap. Example: swelling of raisins when placed in water.
2. Exosmosis is the osmotic outflow of water from a cell when it is placed in a solution whose solute concentration is more than that of the cell sap. Example: shrinkage of grapes when placed in concentrated sugar solution.

Connecting Concepts:
1. Saline plants have higher osmotic pressure. Species native to saline soil usually have relatively high osmotic pressure, because such soils are rich in soluble salts and the plants absorb large quantities of mineral salts. The highest recorded osmotic pressure for any species of plant (202.5 atm) was found in the saltbush Atriplex confertifolia, a saline soil species.
2. Reverse osmosis is the expulsion of pure water from a solution through a semipermeable membrane under the influence of external pressure higher than the osmotic pressure of the solution. It is used to remove salts from saline water and for purification of water.

Osmotic pressure

1. Osmotic pressure (term proposed by Pfeffer) is the pressure required to prevent the entry of water into a solution. It is a positive value. Pure solvent has no osmotic pressure.
2. When a solute is added to a pure solvent, it develops osmotic pressure.
3. Osmotic pressure and osmotic potential are numerically equal, but osmotic potential has a negative sign: Ψ_s = -π, where Ψ_s is the osmotic potential of the solution and π is the osmotic pressure.
4. Osmotic pressure can be calculated by OP = CRT, where C is the molar concentration of the solution, R is the gas constant and T is the absolute temperature.
5. The value of osmotic pressure is related to the number of particles, not molecules.
6. Osmosis depends on two factors: (i) concentration of dissolved solutes in a solution, and (ii) pressure difference.

Significance of osmosis
1. Responsible for the absorption of water by roots.
2. Responsible for cell to cell movement of water throughout the plant.
3. Responsible for turgidity of plant organs.
4. Responsible for opening and closing of stomata.
5. Responsible for the resistance of plants to drought, frost, etc.

Check Point

State True or False:
(i) Osmosis involves the selectively permeable membrane.
(ii) Osmosis can operate in any medium.
(iii) An animal cell placed in pure water will swell up and burst.
(iv) Swelling of a wooden door during the rainy season is due to endosmosis.
(v) Bacteria cannot survive in highly salted pickles because they become plasmolysed and death occurs.
(vi) To demonstrate the phenomenon of osmosis, one may use boiled and peeled potato.
(vii) The term plasmolysis was coined by Nollet.
(viii) Osmotic pressure and osmotic potential are numerically equal, but osmotic potential has a negative sign.
(ix) When the protoplast just begins to contract away from the cell wall, it is called evident plasmolysis.
(x) The value of TP becomes zero during incipient and evident plasmolysis.

Answer:
(i) True
(ii) False, osmosis needs a semipermeable membrane.
(iii) True
(iv) False, swelling of wood is due to imbibition.
(v) True
(vi) False, a fresh living potato should be used.
(vii) False, the term plasmolysis was not coined by Nollet, who coined osmosis.
(viii) True
(ix) False, this stage is called incipient plasmolysis.
(x) False, TP is zero only at limiting plasmolysis and below zero during incipient and evident plasmolysis.

Problem: Two chambers P and Q containing solutions of different concentrations are separated by a semipermeable membrane.
(i) Which chamber has a lower water potential? Answer: Chamber Q has a lower concentration of water, so it has lower water potential.
(ii) In which direction will osmosis occur? Answer: Osmosis occurs from higher Ψ to lower Ψ, that is, from chamber P to chamber Q.
(iii) Which solution has a higher solute potential? Answer: The solution in chamber Q has a higher solute concentration, so it has higher solute potential (Ψ_s).
(iv) At equilibrium, which chamber will have lower water potential? Answer: At equilibrium, the water potential is equal on both sides of the semipermeable membrane.
(v) If one chamber has a Ψ of -2000 kPa and the other -1000 kPa, which chamber has higher Ψ, and what is it? Answer: Chamber P has higher water potential, which is -1000 kPa.

Connecting Concepts: The leaf of Tradescantia is used for demonstrating plasmolysis in the laboratory, because its cells contain anthocyanin pigments.

PLASMOLYSIS

1. The shrinkage of the protoplast of a living cell from its cell wall, due to exosmosis under the influence of a hypertonic solution, is called plasmolysis. Water is first lost from the cytoplasm and then from the vacuole. The water drawn out of the cell into the extracellular fluid causes the protoplast to shrink away from the walls. The cell is then said to be plasmolysed. Water moves from an area of high water potential (the cell) to an area of lower water potential (outside the cell).
2. Incipient plasmolysis is the stage when the protoplast just begins to contract away from the cell wall.
3. Evident plasmolysis is the stage when the cell wall has reached its limit of contraction and the protoplast has detached from the cell wall, attaining a spherical shape.
4. If a cell with incipient plasmolysis is placed in a hypertonic solution, it shows more plasmolysis.
5. When cells are placed in a hypotonic solution (higher water potential, or a dilute solution compared with the cytoplasm), water diffuses into the cell. The cytoplasm builds up pressure against the wall, called turgor pressure (TP). The pressure exerted by the protoplast due to the entry of water against the rigid walls is called pressure potential (Ψ_p). The rigidity of the cell wall prevents the cell from rupturing. Turgor pressure is ultimately responsible for the enlargement and growth of cells.
6. When a cell or tissue is placed in an isotonic solution, there is no net flow of water into or out of the cell. If the external solution balances the osmotic pressure of the cytoplasm, it is said to be isotonic. When water flows in and out of the cell and they are in equilibrium, the cells are said to be flaccid.
7. The swelling of a plasmolysed protoplast due to endosmosis under the influence of a hypotonic solution or water is called deplasmolysis. Deplasmolysis is possible only immediately after plasmolysis, otherwise the protoplast is permanently damaged.
8. The value of TP becomes zero at the time of limiting plasmolysis and below zero during incipient and evident plasmolysis.

[Figure: Fig. 11.4 Plasmolysis and deplasmolysis: (a) normal cell, (b) plasmolysed cell with protoplast shrunk away from the wall, and (c) deplasmolysed cell with increased turgor pressure.]

Types of solutions
1. Isotonic solution: the concentration of the outer solution equals the concentration of the cell sap, so there is no flow of water.
2. Hypotonic solution: the concentration of the outer solution is lower than that of the cell sap, so water diffuses into the cell and makes it turgid.
3. Hypertonic solution: the concentration of the outer solution is higher than that of the cell sap, so water diffuses out of the cell.

Significance of plasmolysis
1. The osmotic pressure of a cell can be measured by plasmolysis. The osmotic pressure of a cell is roughly equal to the osmotic pressure of a solution that causes incipient plasmolysis in the cell.
2. Salting of pickles, meat and fish, and the addition of sugar to jams, jellies and cut fruits, prevent their decay by microbes, as the microbes are killed by plasmolysis due to high concentration of salt or sugar.
3. By salting, weeds can be killed from tennis courts, and the growth of plants can be prevented in the cracks of walls.
4. Plasmolysis helps in determining whether a particular cell is living or dead, because plasmolysis does not occur in a dead or non-living cell.

IMBIBITION

1. Imbibition is the process of absorption of water by hydrophilic surfaces of a substance without forming a solution. It is a type of diffusion by which the movement of water takes place along a diffusion gradient.
2. The solid particles that adsorb water or any other liquid are called imbibants. The liquid that is imbibed is called imbibate.
3. Examples of imbibition are the adsorption of water by seeds, dry wood, starch, cellulose, agar, gelatin and gum.

Characteristics of imbibition
1. Volume change: During imbibition, imbibants increase in volume. Water molecules arranged on the surface of the imbibant occupy less volume than they do in the free liquid state, so there is an actual compression of water. For example, when a dry piece of wood is placed in water, it swells and increases in volume.
2. Production of heat: As water molecules are adsorbed on the surface of the imbibant, their kinetic energy is released as heat, which raises the temperature of the medium. This is called heat of wetting, or heat of hydration. For example, during kneading, wheat flour gives a warm feeling due to the imbibition of water.
3. Development of imbibition pressure: Imbibition pressure is the maximum pressure that an imbibant develops when it is completely soaked in pure water. It is also called matric potential (Ψ_m), because it exists due to the presence of hydrophilic substances in the cell, including organic colloids and the cell wall.

Factors influencing the rate of imbibition
1. Nature of imbibant: Proteins are the strongest imbibants of water, starch is less strong, and cellulose is the weakest.
2. Surface area of imbibant: The greater the surface area exposed to the liquid, the greater the imbibition.
3. Temperature: Increase in temperature increases the rate of imbibition.
4. Degree of dryness of imbibant: A dry imbibant imbibes more water than a relatively wet one.
5. Concentration of solutes: Increase in the concentration of solutes in the medium decreases imbibition.
6. pH of imbibant: Proteins are amphoteric in nature and imbibe least in a neutral medium. Towards highly acidic or highly alkaline pH, imbibition increases up to a maximum, after which it slows down.

Significance of imbibition
1. Water is first imbibed by the walls of root hairs and then absorbed.
2. Germinating seeds absorb water by imbibition, which helps in rupturing the seed coat (made of cellulose).
3. Water moves into ripening ovules by imbibition, as they develop into seeds.

SIGNIFICANCE OF TURGIDITY IN PLANTS

1. It provides stability to a cell.
2. Turgidity keeps the cell and its organelles (mitochondria, plastids and microbodies) fully distended. This is essential for plants to live and grow normally.
3. Turgor pressure helps in cell enlargement, and consequently in the stretching of stems and in keeping leaves erect and fully expanded.
4. Turgid cells provide the mechanical support necessary for non-woody tissues (such as maize, sugarcane and banana).
5. Loss of turgidity leads to wilting of leaves and drooping of shoots.
6. The opening and closing of stomata are regulated by the turgidity of guard cells.
7. Leaf movements (seismonastic movements) of plants such as bean and sensitive plant (Mimosa pudica) are controlled by the loss and gain of cell turgor.
8. Due to turgor pressure, the plumule and radicle force their way out of seeds at the time of germination.

LONG DISTANCE TRANSPORT OF WATER

1. Transport of substances over longer distances through the vascular tissue, that is, the xylem and phloem, is called translocation. It occurs through mass flow.
2. Mass flow is the movement of substances in bulk, from one point to another, as a result of pressure differences between the two points. In mass flow, substances in solution or in suspension are swept along at the same pace. This is unlike diffusion, where different substances move independently according to their concentration gradients.
3. Bulk flow can be achieved through a positive hydrostatic pressure gradient (as in a garden hose) or a negative hydrostatic pressure gradient (as in suction through a straw).
4. The direction of translocation is unidirectional for water (from root to stem, leaves, flowers and fruits) and multidirectional for minerals and organic solutes. Organic compounds are synthesised in leaves.

HOW DO PLANTS ABSORB WATER?

1. Water may be absorbed through almost any surface of a plant, but most of the absorption of water and mineral salts occurs in the terminal portions of roots.
2. Because of excessive branching of roots, there are numerous root tips that help in obtaining water and minerals from the soil. In higher plants, water is absorbed through root hairs.
3. From the soil, root hairs absorb water by diffusion. Water moves through the root cortex to the xylem elements. Root hairs are thin-walled and increase the absorption area.
4. Water is absorbed along with mineral solutes by root hairs, purely by diffusion.
5. Once water is absorbed by the root hairs, it can move deeper into the root by two distinct pathways: (a) apoplast pathway and (b) symplast pathway. Munch coined the terms apoplast and symplast.

Apoplast pathway
1. Apoplastic movement of water occurs exclusively through the cell walls and intercellular spaces, without crossing any membrane.
2. It is continuous throughout the plant, except at the Casparian strips of the endodermis in roots.
3. This movement depends on the gradient. The apoplast does not provide any barrier to water movement, and water moves by mass flow. As water evaporates into the intercellular spaces or the atmosphere, tension develops in the continuous stream of water in the apoplast. So, mass flow of water occurs due to the adhesive and cohesive properties of water.

Symplast pathway
1. Symplastic movement occurs from cell to cell through plasmodesmata.
2. During symplastic movement, water travels through the cytoplasm of cells, and intercellular movement is through plasmodesmata.
3. Movement is relatively slower and aided by cytoplasmic streaming.

Casparian strips and the endodermis
1. Major proportion of water flow in the root cortex occurs through the apoplast.
2. Beyond the cortex, apoplastic water movement is blocked by the Casparian strip. Beyond the endodermis, water is forced to move through the cell membrane, that is, the symplast.
3. Once water reaches the root xylem, transpiration drives water to move to the leaves through the stem.

Connecting Concepts:
1. Casparian strips are bands of radial cell walls in the endodermis that are impregnated with the wax-like, hydrophobic substance suberin. Suberin acts as a barrier to water and solute movement.
2. Cytoplasmic streaming, also called cyclosis, is an autonomous vital movement that occurs continuously in the cytoplasm of eukaryotic cells around the central vacuole. It can be observed in the cells of the leaf of Hydrilla (an aquatic plant). The movement of chloroplasts due to cytoplasmic streaming can be seen under the microscope.

Mycorrhizal water absorption
1. Some plants have additional structures associated with their roots that help in the absorption of water and minerals.
2. In mycorrhiza, a large number of fungal hyphae are associated with young roots and also extend into the soil. The hyphae have a large surface area for absorption. They absorb water and minerals and pass them to the root. The root provides the fungus with sugar and nitrogenous compounds.
3. Some plants have an obligate association with mycorrhizae. For example, Pinus seeds cannot germinate and establish without the presence of mycorrhizae.

[Figure: Fig. 11.5 Absorption of water, showing the pathway of water movement in a root through the epidermis, cortex, endodermis (with Casparian strip), pericycle and xylem. Apoplast and symplast pathways are marked.]

[Figure: Fig. 11.6 Symplastic and apoplastic pathways of water and ion absorption and movement in roots.]

Check Point

Fill in the blanks:
1. Tissues associated with translocation are __________.
2. Organic compounds are synthesised in __________.
3. __________ coined the term apoplast and symplast.
4. Symplastic movement occurs from cell to cell through __________.
5. __________ breaks the continuity of the apoplast pathway and forces water and solutes to cross the endodermis by passing through __________.
6. The hyphae have large surface area for __________.
7. From soil, root hairs absorb water by __________.
8. The direction of translocation is __________ in case of water and __________ in case of mineral and organic solutes.

Answer:
1. xylem and phloem
2. leaves
3. Munch
4. plasmodesmata
5. Casparian strip; plasma membrane
6. absorption (of water and minerals)
7. diffusion
8. unidirectional; multidirectional

MECHANISM OF WATER ABSORPTION

Two distinct mechanisms operate independently in the absorption of water by plants: active absorption and passive absorption. Renner (1912, 1915) coined these terms.

Active absorption: Active absorption takes place through the activity of the root itself, particularly the root hairs. The factor responsible for water absorption is present within the roots, and it utilises metabolic energy.
1. There are two theories of active absorption: osmotic and non-osmotic.
2. The osmotic theory was proposed by Atkins (1916) and Priestley (1922). It is a purely physical process that does not directly require expenditure of energy. The osmotic pressure of the cell sap of the root hair is higher than the osmotic pressure of the soil water. The DPD and suction pressure in the root hair become higher than those of the soil water, so water enters through the plasma membrane by osmotic diffusion. As a result, the OP, suction pressure and DPD of the root hair fall, while TP increases.
3. The non-osmotic theory was proposed by Thimann (1951) and Kramer (1959). Water absorption still occurs even if the concentration of cell sap in the root hair is lower than that of the soil water, or when water is absorbed against a concentration gradient, that is, from higher DPD to lower DPD. This absorption occurs at the expense of energy obtained from respiration.

[Figure: Fig. 11.7 Passive absorption of water through a root hair.]

Passive absorption: Passive absorption is the most common and rapid method of water absorption. The factor responsible for water absorption is present somewhere other than the roots. It accounts for about 98% of the total water uptake by the plant.
1. The forces responsible for the absorption of water originate not in the root cells but in the cells of the transpiring shoots. The root cells remain passive.

WATER MOVEMENT UP A PLANT (ASCENT OF SAP)

1. Water absorbed by plants is also called sap, because it is actually a dilute solution of minerals. Ascent of sap, or translocation of water, is the upward movement of water or sap from the root towards the top of the plant. It occurs through tracheary elements of the xylem.
2. The rate of ascent of sap is 10 to 75 cm per minute.
3. Ringing experiments were proposed by Malpighi and Stephen Hales to demonstrate that the ascent of sap takes place through xylem vessels.
4. In herbaceous plants, almost all tracheary elements take part in the process. In large woody trees, only the tracheary elements of the sapwood are functional.
5. It has been proved experimentally that sap moves up the stem through the lumen of xylem vessels and tracheids, and not through their walls.
6. Two main theories explain the mechanism of ascent of sap: the root pressure theory and the cohesion-tension transpiration pull model.

Connecting Concepts: The root pressure theory is not acceptable, because root pressure is absent in gymnosperms and in actively transpiring plants, though ascent of sap occurs in them.

Root pressure theory
1. When ions from the soil are actively transported into the vascular tissue of the root, water follows along its potential gradient, and pressure increases inside the xylem. This positive pressure is called root pressure. It can push water up to small heights in the stem.
2. The term root pressure was coined by Stephen Hales. Root pressure can be measured by a manometer.
3. The root pressure theory was proposed by Priestley (1916). It explains the mechanism of ascent of sap in herbs.
4. Water absorbed by the root hairs collects in the cells of the cortex, which become fully turgid. The elastic walls of these cortical cells exert pressure on their fluid contents and force them towards the xylem vessels. Due to this loss of water, the cortical cells become flaccid, absorb water again, become turgid and force out their fluid contents again. The cortical cells of the root carry on this intermittent pumping action, and considerable pressure is set up in the root. This pressure pushes water up the xylem vessels.
5. The effects of root pressure can be observed at night and in the early morning, when evaporation is low. Excess water collects in the form of droplets around special openings of veins near the tip of grass blades and the leaves of many herbaceous plants. This loss of water in liquid phase is called guttation. The term was coined by Bergerstein.
6. Guttation takes place through hydathodes, which are stomata-like pores generally present at the top margin of leaves of plants that grow in moist, shady places.
7. Root pressure provides only a modest push in the overall process of water transport. It does not play a major role in water movement in tall trees. Its greatest contribution may be to re-establish the continuous chains of water molecules in the xylem, which often break under the enormous tensions created by transpiration. Root pressure does not account for the majority of water transport. Most plants meet their need by transpiration pull.

Cohesion-tension transpiration pull model
1. This is the most widely accepted theory. It was put forward by Dixon and Jolly in 1894, and further supported by Renner (1911, 1915), Curtis and Clark (1951), Bonner and Galston (1952), and Kramer and Kozlowski (1960).
2. It is also known as Dixon's cohesion theory or cohesion-tension theory.
3. This theory relies on three factors: (i) formation of a continuous water column in the xylem vessels, (ii) cohesive force between water molecules, and (iii) transpiration pull, which brings about the ascent of sap.
4. Evaporation of water from the leaf to the atmosphere decreases the water potential of the epidermal cells. Water moves from adjacent cells along the water potential gradient.
5. Water lost from the leaf cells is replaced by water moving from the xylem elements in the leaf veins. Thus, a continuous water column forms from root to leaves, due to the cohesive and adhesive properties of water molecules.
6. Uptake of water in this way is also referred to as transpirational pull. Transpiration pull is a force developed in the mesophyll cells due to transpiration. This tension or pull results in upward movement of water.
7. In this way, water rises due to transpiration pull and the cohesive power of water molecules, from the lowest parts of the roots to the highest peaks of trees. The osmotic pressure in the transpiring leaf cells often reaches 30 atmospheres, whereas only 20 atmospheres are needed to raise water to the tops of the tallest known trees.

[Figure: Fig. 11.8 Path of ascent of sap showing transpiration pull through the vessels and tracheids of the xylem.]

Connecting Concepts:
1. Potometer: an instrument for measuring the rate of transpiration by measuring the rate of water absorption by a shoot.
2. Porometer: an instrument for measuring the degree of stomatal opening.

Check Point

Give the scientific term for:
(i) The upward transport of water along with dissolved minerals from roots to the aerial parts of the plant.
(ii) The most common and rapid method of water absorption.
(iii) The tissue concerned with ascent of sap in plants.

Answer:
(i) Ascent of sap
(ii) Passive absorption
(iii) Xylem

Check Point

Fill in the blanks:
(i) The upward transport of water along with dissolved minerals from roots to the aerial parts of the plant is called __________.
(ii) Root hair absorbs water from soil through __________.
(iii) The most accepted theory of ascent of sap is __________.
(iv) Root pressure is maximum when transpiration is __________ and absorption is __________.

Answer:
(i) ascent of sap
(ii) diffusion
(iii) cohesion-tension transpiration pull model
(iv) low (or absent); high

Check Point

State True or False:
(i) Ascent of sap will be maximum during night.
(ii) Root pressure is sufficient for the transport of water in slowly transpiring herbaceous plants.
(iii) Ascent of sap was demonstrated by the girdling (ringing) experiment.
(iv) Transpiration pull is a force developed in spongy cells due to transpiration.

Answer:
(i) False, ascent of sap is maximum during the day, when transpiration is highest.
(ii) True
(iii) True
(iv) False, transpiration pull is developed in the mesophyll cells of the leaf.

TRANSPIRATION

1. Loss of water in the form of water vapour from plants, through the small pores (stomata) present on leaves, is called transpiration.
2. Maximum transpiration occurs in mesophytic plants. About 98% of the water absorbed by land plants evaporates from the aerial parts and diffuses into the atmosphere.

Table 11.2: Differences between Transpiration and Evaporation

Types of Transpiration (Flow Chart 11.2)
1. Cuticular transpiration: loss of water in vapour form from the general surface of leaves and young stems through the cuticle. It is continuous throughout the day and night, and about 10% of water is lost by this.
2. Stomatal transpiration: loss of water in vapour form from stomata present on the surface of leaves, and to a lesser extent from the surface of flowers and young stems (cauline transpiration). It occurs when stomata are open. About 90% of water is lost by this.
3. Lenticular transpiration: loss of water vapour through lenticels, which are small loosely arranged cells called complementary cells, found in regions of bark. Negligible amount of water is lost through this. The main method of water loss from deciduous trees after leaf fall. It is continuous throughout day and night.
4. Bark transpiration: occurs through the corky covering of the stem. It is very little, but its measured rate is often more than lenticular transpiration. The amount is 0.5% of the total respiration.

[Figure: Flow Chart 11.2 Types of transpiration, branching into cuticular, stomatal, lenticular and bark transpiration.]

Structure of Stomata

1. Stomata are minute pores present on the leaf epidermis. Besides the loss of water vapour in transpiration, exchange of oxygen and carbon dioxide in the leaf also occurs through stomata. Normally, stomata are open in the day time and close during the night.
2. Each stoma (stomatal pore) is bordered by two kidney-shaped guard cells. Guard cells are surrounded by epidermal cells called subsidiary cells. The inner wall of each guard cell, towards the pore or stomatal aperture, is thick and elastic.
3. When turgidity increases in the two guard cells flanking the stomatal aperture, the thin outer walls bulge out and force the inner walls into a crescent shape. The opening of the stoma is also aided by the orientation of the microfibrils in the cell walls of guard cells. Cellulose microfibrils are oriented radially rather than longitudinally, which makes it easier for the stoma to open.
4. When guard cells lose turgor due to water loss (or water stress), the elastic inner walls regain their original shape. The guard cells become flaccid, and the stoma closes.
5. Dorsiventral leaves (dicots) have more stomata on the lower surface, while isobilateral leaves (monocots) have an equal number of stomata on both surfaces.

Types of stomata
1. Barley or cereal type (dumb-bell shaped, for example, maize and wheat): open for a few hours during the day.
2. Alfalfa or leucerne type (for example, pea, bean and grape): remain open throughout the day and close at night.
3. Potato type (for example, potato, cucurbita and banana): open throughout the day and night, but close for a few hours in case of water deficiency.
4. Equisetum type (for example, emergent hydrophytes): remain open and seldom close.
5. Scotoactive stomata open in the dark and close during the day. These occur in succulents.

[Figure: Fig. 11.9 A stomatal aperture with guard cells and subsidiary cells.]

[Figure: Fig. 11.10 Stomatal apparatus, showing the guard cells with the stomatal pore between them and the surrounding subsidiary cells, with open and closed stomata.]

Table 11.3: Differences between Activities during Opening and Closing of Stomata

Main Theories about the Mechanism of Stomatal Movement

(i) Guard cell photosynthesis theory: Given by Schwendener (1881), and also accepted by Von Mohl (1856).
1. Chloroplast in guard cell, with photosynthesis in day time.
2. Formation of sugar in guard cell.
3. Osmotic concentration in guard cell increases.
4. Water is absorbed from adjacent cells.
5. Guard cells become turgid, and the stomata open.

(ii) Starch hydrolysis theory: First given by Lloyd. Formulated by J.D. Sayre (1923) and modified by Steward (1964).
In day time:
1. CO₂ in intercellular spaces is used in photosynthesis, forming starch.
2. H⁺ concentration falls, and the enzyme phosphorylase is activated.
3. Starch is converted to glucose 1-phosphate, then to glucose 6-phosphate (by phosphoglucomutase), then to glucose and phosphate.
4. The OP and DPD of guard cells increase.
5. Water is absorbed by the guard cells from adjacent cells.
6. Guard cells become turgid and swell, and the stomata open.
In night time (dark):
1. CO₂ accumulates in the substomatal cavity, and the pH decreases.
2. Glucose is converted to glucose 1-phosphate, and then to starch.
3. The OP and DPD of the cell sap decrease.
4. Guard cells become flaccid, and the stomata close.

(iii) Malate or K⁺ ion pump theory: Proposed by Levitt (1974), elaborated by Raschke (1975) and Bowling (1976).
In day time:
1. The pH rises.
2. Starch is hydrolysed to PEP (phosphoenolpyruvate).
3. PEP is converted to OAA (oxaloacetic acid) by the enzyme PEPcase.
4. Malic acid is formed in the guard cells.
5. H⁺ ions and malate ions are formed. K⁺ ions move in from the subsidiary cells in exchange, forming K⁺ malate.
6. The OP of the guard cells increases, and endosmosis occurs, so the stomata open.
In night time:
1. CO₂ concentration increases in the substomatal cavity.
2. ABA participates, and K⁺ ion exchange stops.
3. K⁺ ions move back into the subsidiary cells.
4. The pH decreases, starch is synthesised, and the OP decreases.
5. Exosmosis occurs, and the stomata close.

[Figure: Flow charts of the three theories of stomatal movement, showing the sequence of steps in the guard cell photosynthesis theory, the starch hydrolysis theory (day and night) and the malate or K⁺ ion pump theory (day and night).]

Check Point

State True or False:
(i) In darkness, high pH and high CO₂ concentration govern the closing of stomata.
(ii) Stomata function as turgor operated valves.
(iii) The walls of kidney-shaped guard cells are thickened on the outer side.
(iv) In barley or other cereals, guard cells of stomata are kidney shaped.
(v) Stomatal opening depends upon the availability of K⁺ ions from the adjacent epidermal cells.
(vi) Excessive rate of transpiration leads to faster growth of plants.

Answer:
(i) True, in darkness high pH and high CO₂ concentration favour the closing of stomata.
(ii) True
(iii) False, the inner walls are thick.
(iv) False, cereal guard cells are dumb-bell shaped.
(v) True
(vi) False, excessive transpiration causes stunted growth.

Check Point

Name two plants in which dumb-bell shaped guard cells are found.

Answer: Maize and wheat.

MECHANISM OF TRANSPIRATION AND FACTORS

The transpiration-driven ascent of xylem sap depends on the following physical properties of water:
1. Cohesion: mutual attraction between water molecules.
2. Adhesion: attraction of water molecules to polar surfaces, such as the surface of tracheary elements.
3. Surface tension: water molecules are attracted to each other in the liquid phase more than to water in the gas phase.
4. These properties give water high tensile strength, that is, the ability to resist a pulling force, and capillarity, that is, the ability to rise in thin tubes.

The process of photosynthesis requires water. The system of xylem vessels from the root to the leaf vein can supply the needed water. As water evaporates through the stomata, the thin film of water over the cells is continuous, so water is pulled, molecule by molecule, into the leaf from the xylem. Because water vapour is at a lower concentration in the atmosphere than in the substomatal cavity and intercellular spaces, water diffuses into the surrounding air. This creates a 'pull'. Measurements show that the forces generated by transpiration can create pressures sufficient to lift a xylem-sized column of water over 130 metres high.

[Figure: Fig. 11.11 Water movement in the leaf. Evaporation from the leaf sets up a pressure gradient between the outside air and the air spaces of the leaf. The gradient is transmitted into the photosynthetic cells and on to the water-filled xylem in the leaf vein.]

Factors Affecting Transpiration

External factors
1. Humidity: when humidity is high, the rate of transpiration is low.
2. Temperature: the rate of evaporation doubles with a rise in temperature of 10 °C. However, high temperature reduces transpiration.
3. Wind speed: high wind speed increases transpiration.
4. Light: light affects transpiration through its effect on temperature and on stomata. During the day, stomata open wide, but at night they close. Increased temperature and wide-open stomata increase the rate of transpiration. Light is the most important factor in the regulation of transpiration. Maximum opening of stomata occurs in red light (660 nm), followed by blue light (445 nm). No opening occurs in green light, UV light or far-red light.

Anti-transpirants
Anti-transpirants are chemical substances which reduce transpiration, by increasing leaf resistance to water vapour diffusion, without affecting gaseous exchange. They are of two types:
1. Metabolic inhibitors reduce transpiration by causing partial closure of stomata, without influencing other metabolic processes. The most important are phenyl mercuric acetate (PMA) and abscisic acid (ABA).
2. Film-forming anti-transpirants check transpiration by forming a thin, transparent film on the transpiring surface. They are sufficiently permeable to carbon dioxide and oxygen to allow photosynthesis and respiration, but they prevent the movement of water vapour. Important chemicals are silicon emulsion, colourless plastic resins and low-viscosity waxes.

Internal factors
1. Leaf area: if leaf area is more, transpiration is faster. However, the rate of transpiration per unit area is more in smaller leaves, due to the high number of stomata in a small leaf. The number of stomata per unit area of leaf is called stomatal frequency.
2. Stomatal index: I = S / (E + S) × 100, where I is the stomatal index, S is the number of stomata per unit area and E is the number of epidermal cells per unit area.
3. Leaf structure: sunken or vestigial stomata, hairs, cuticle or waxy layer on the epidermis, hydrophilic substances such as gums and mucilage in cells, and compactly arranged mesophyll cells all help in reducing the rate of transpiration.
4. Root-shoot ratio: according to Parker (1949), the rate of transpiration is directly proportional to the root-shoot ratio.
5. Age of plants: germinating seeds show a slow rate of transpiration. It becomes maximum at maturity, and decreases at the senescence stage.
6. Orientation of leaves: if leaves are arranged transversely on the shoot, they lose more water, because they are exposed to direct sunlight. If they are placed perpendicularly, they transpire at a slower rate.

Significance of Transpiration

Advantages:
1. Transpiration directly influences the absorption of water from the soil.
2. Transpiration exerts a tension or pull on the water column in the xylem, which is responsible for the ascent of sap.
3. Transpiration helps in the movement of water and minerals absorbed by the roots to other parts of the plant.
4. Evaporation of water during transpiration cools the leaves and the surrounding air, and protects leaves from heat injury under high temperature and intense sunlight.

Disadvantages:
1. Transpiration often results in a water deficit, which causes injury to plants by desiccation.
2. Rapid transpiration causes mid-day leaf water deficit (temporary wilting). If the condition continues, permanent water deficit (permanent wilting) may develop, which injures plants.
3. Many xerophytes have to develop structural modifications to reduce transpiration. These modifications are an extra burden on plants.
4. Excessive rate of transpiration leads to stunted growth of plants.
5. About 90% of absorbed water is lost through transpiration, so the energy used in absorption and conduction of water is wasted.
6. A plant cannot avoid transpiration, because the leaf structure is meant for gaseous exchange during respiration and photosynthesis. Therefore, transpiration is regarded as a 'necessary evil' by Curtis (1926) and an 'unavoidable evil' by Steward (1959).

Transpiration and Photosynthesis
Transpiration has more than one purpose. It:
1. creates the transpiration pull for absorption and transport of water.
2. supplies water for photosynthesis.
3. transports minerals from the soil to all parts of the plant.
4. cools leaf surfaces, sometimes by 10 to 15 degrees, by evaporative cooling.
5. maintains the shape and structure of the plant by keeping cells turgid.

An actively photosynthesising plant needs water. Photosynthesis is limited by the available water, which can be quickly depleted by transpiration. The humidity of rainforests is largely due to the vast cycling of water from root to leaf to atmosphere and back to the soil. The evolution of the C₄ photosynthetic system is probably one strategy for maximising the availability of CO₂ while minimising water loss.

UPTAKE AND TRANSPORT OF MINERAL NUTRIENTS

Plants obtain most of their carbon and oxygen from the CO₂ of the atmosphere, and hydrogen from water. Minerals are picked up individually from the soil.

Uptake of mineral ions
1. Minerals exist in the soil as ions, which cannot directly cross cell membranes.
2. The concentration of ions is about 100 times higher in the root interior than in the soil.
3. Ions are absorbed from the soil by both passive and active transport.
4. The movement of ions from soil to the interior of the root is against the concentration gradient and requires active transport.
5. Specific ion pumps occur in the membrane of root hairs, which pump mineral ions from the soil into the cytoplasm of epidermal cells of root hairs.
6. Energy is provided by ATP. Respiratory inhibitors such as cyanide, which inhibit ATP synthesis, generally reduce ion uptake.
7. Ions are again checked and transported inwards by transport proteins present over the endodermal cells. The endodermis allows the passage of ions inwards but not outwards. It also controls the quantity and type of ions passed into the xylem.
8. Inward flow of ions from epiblema to xylem is along the concentration gradient.
9. The collection of ions in the xylem creates a water potential gradient in the root, which helps in osmotic entry of water and its passage to the xylem.
10. In the xylem, minerals are carried up along with the flow of xylem sap in the transpiration stream. In leaves, cells absorb minerals selectively through membrane pumps.

Translocation of mineral ions
1. Generally, the xylem transports inorganic nutrients, while the phloem transports organic nutrients.
2. In xylem sap, nitrogen travels as inorganic ions as well as in organic form (amino acids) and related compounds. Small amounts of P and S are passed in the xylem as organic compounds. There is also exchange of materials between xylem and phloem.
3. Minerals reach their sinks (places of utilisation), namely young leaves, developing flowers, fruits and seeds, apical and lateral meristems, and individual cells for storage.
4. Minerals are unloaded at the fine vein endings by diffusion, and they are picked up by cells through active uptake.
5. Minerals such as nitrogen, sulphur, phosphorus and potassium are remobilised from older senescing parts. Elements incorporated in structural components, such as calcium, are not remobilised.
6. The remobilised minerals become available to young growing leaves and other sinks.
7. Nickel has a prominent role in remobilisation.

TRANSPORT IN PHLOEM

1. Phloem transport is the long distance movement of organic substances from the source (region of manufacture or storage) to the sink (region of utilisation).
2. The source and sink may be reversed depending on the season or the need of the plant. For example, sugar stored in roots may be mobilised as a source of food in early spring, when the buds of trees act as sinks and require energy for growth.
3. Since the source-sink relationship is variable, the direction of movement of organic solutes in phloem can be upwards or downwards, that is, bidirectional.
4. This contrasts with the xylem, where movement is always unidirectional, that is, upwards. Food in phloem sap can be transported in any required direction, as long as there is a source of sugar and a sink able to use, store or remove the sugar.
5. Phloem sap is mainly water and sucrose, but other sugars, hormones and amino acids are also transported through the phloem.

Pressure flow or mass flow hypothesis
1. The accepted mechanism for the translocation of sugars from source to sink is called the pressure flow hypothesis. It was put forward by Münch (1930).
2. According to this hypothesis, organic substances move from the region of high osmotic pressure to the region of low osmotic pressure in a mass flow, due to the development of a gradient of turgor pressure.
3. Glucose prepared at the source by photosynthesis is converted to sucrose, a disaccharide. Sucrose is moved into the companion cells and then into the living phloem sieve tube cells by active transport. This loading at the source produces a hypertonic condition in the phloem. Water from the adjacent xylem moves into the phloem by osmosis. As osmotic pressure builds up, the phloem sap moves to areas of lower pressure.
4. At the sink, osmotic pressure must be reduced. Active transport moves sucrose out of the phloem sap and into the cells that use the sugar, converting it into energy, starch or cellulose. As sugars are removed, the osmotic pressure decreases and water moves out of the phloem.
5. The movement of sugars in the phloem begins at the source, where sugars are loaded into a sieve tube by active transport. Loading sets up a water potential gradient that facilitates mass movement in the phloem.
6. Phloem tissue is composed of sieve tube cells, which form long columns with holes in their end walls called sieve plates. Cytoplasmic strands pass through the holes in the sieve plates, forming continuous filaments.
7. As hydrostatic pressure in the sieve tube increases, pressure flow begins, and sap moves through the phloem. At the sink, incoming sugars are actively transported out of the phloem and removed as complex carbohydrates. The loss of solute produces a high water potential in the phloem, and water passes out, eventually returning to the xylem.

Girdling experiment
1. A ring of bark up to the depth of the phloem layer is carefully removed from the trunk of a tree.
2. In the absence of downward movement of food, the portion of the bark above the ring becomes swollen after a few weeks.
3. This shows that phloem is the tissue responsible for the translocation of food, and that transport takes place in one direction, towards the roots.

[Figure: Fig. 11.12 Pathway and mechanism of phloem translocation, showing sucrose loading at the source, the rise in osmotic pressure and water entry in the phloem, the pressure flow to the sink, and unloading at the sink with water returning to the xylem.]

Check Point

Fill in the blanks:
1. Element which has a prominent role in remobilisation is __________.
2. Minerals exist in the soil in __________ form.
3. Inward flow of ions from epiblema to xylem is __________ the concentration gradient.
4. The direction of phloem transport is __________.
5. Tissue responsible for translocation of food is __________.
6. The hypothesis accepted for the translocation of sugar from source to sink is __________.

Answer:
1. nickel
2. ionic
3. along
4. bidirectional
5. phloem
6. pressure flow (mass flow) hypothesis