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45 Cards in this Set

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What is translocation

Over small distances, substances move by diffusion and by cytoplasmic streaming supplemented by active transport. Transport over longer distances proceeds through the vascular system and is called translocation. In rooted plants, transport in xylem is essentially unidirectional, from roots to the stems. Organic and mineral nutrients however, undergo multidirectional transport. Hormones for plant growth regulators and other chemical stimuli are also transported, though in very small amounts, sometimes in a strictly polarized or unidirectional manner from where they are synthesized to other parts.


Movement by diffusion is passive, and maybe from one part of the cell to the other or from cell to cell, or over short distances, say, from the intercellular spaces of the leaf to the outside. Molecules move in a random fashion, net result being substances moving from regions of higher concentration to regions of lower concentration. No energy expenditure takes place. It is a slow process and is not dependent on a living system. It is obvious in gases and liquids, diffusion in solids rather than of solids is more likely. The only means for gaseous movement within the plant body is diffusion. Diffusion rates are affected by the gradient of concentration, the permeability of the membrane separating them, temperature and pressure.

It continues till equilibrium is reached. Direction of diffusion of one substance is independent of the movement of another substance.

Diffusion pressure

The diffusing particle create a certain pressure called as diffusion pressure (DP) which is directly proportional to the number or concentration of diffusion particles. The molecules move from higher diffusion pressure to lower diffusion pressure.

What is facilitated diffusion

Substances soluble in lipids diffuse through the membrane faster. Substances that have a hydrophilic moiety, find it difficult to pass through the membrane; their movement has to be facilitated. Membrane proteins provide sites at which such molecules cross the membrane. They do not set up a concentration gradient; our concentration gradient must already be present for molecules to defuse even if facilitated by the proteins. This process is called facilitated diffusion.

Proteins used in facilitated diffusion

Special proteins help move substances across membranes without expenditure of ATP energy. Facilitated diffusion cannot cause net transport of molecules from a low to high concentration as this would require input of energy. Transport rate reaches a maximum when all of the protein transporters are being used (saturation). Facilitated diffusion is very specific: it allows cell to select substances for uptake. It is sensitive to inhibitors which react with protein side chains. Proteins form channels in the membrane for molecules to pass through it. Some channels are always open whereas others can be controlled. The porins are proteins that forms huge pores in the outer membrane of the plastids, mitochondria and some bacteria allowing molecules up to the size of small proteins to pass through. Extracellular molecule bound to the transport proteins; the transport proteins then rotates and releases the molecule inside the cell, e.g., water molecules- made up of 8 different types of aquaporins.

Passive symport and antiport

In the symport, both molecules across the membrane in the same direction; in an antiport, they move in opposite directions. When a molecule moves across the membrane independent of other molecules, the process is called uniport.

Active transport

Active transport uses energy to pump molecules against the concentration gradient. Active transport is carried out by membrane proteins. Pumps are proteins that use energy to carry substances across the cell membrane. These pumps can transport substances from a low concentration to high concentration (uphill transport). Transport rate reaches a maximum when all the protein transporters are being used or are saturated. It is very specific and sensitive to inhibitors that react with protein side chains.

Comparison of different transport processes

Proteins in the membrane are responsible for facilitated diffusion and active transport and hence so common characteristics of being highly selective; they are liable to saturate, respond to inhibitors and are under hormonal regulation. But diffusion weather facilitated or not- take place only along a gradient and do not use energy.

Plant water relations

Water provides the medium in which more substances are dissolved. The protoplasm of the cells is nothing but water in which different molecules are dissolved and suspended.

What is transpiration

Terrestrial plants take up huge amount of water daily but most of it is lost to the air through evaporation from the leaves, i.e., transpiration. a mature corn plant absorbs almost 3 litres of water in a day, while the mustard plant absorbs water equal to its own weight in about 5 hours. Water is often the limiting factor for plant growth and productivity.

Water potential

Water potential is a concept fundamental to understanding water movement. Free energy of water is referred to as water potential. It is also defined as chemical potential of water. It is directly proportional to concentration of water and atmospheric pressure. Solute potential and pressure potential are the two main components that determine water potential. Water molecules possess kinetic energy. In liquid and gaseous form, they are in random motion that is both rapid and constant. The greater the concentration of water in a system, the greater is its kinetic energy or water potential. Pure water will have the greatest water potential. If two systems containing water are in contact, random movement of water molecules will result in that movement of water molecules from the system with higher energy to the one with lower energy. Thus water will move from the system containing water at higher water potential to the one having lower water potential. This process of movement of substances down a gradient of free energy is called diffusion. Water potential is denoted by the Greek symbol Psi and is expressed by pressure units such as Pascals. By convention, the water potential of pure water at standard temperature, which is not under any pressure, is taken to be zero.

What is solute potential

If some solute is dissolved in pure water, the solution has fewer free water and the concentration of water decreases, reducing its water potential. Hence, all solutions have a lower water potential than pure water; the magnitude of the lowering due to dissolution of a solute is called solute potential. It is always negative. The more the solute molecules, the lower (more negative) is the solute potential. For a solution at atmospheric pressure, water potential = solute potential. If a pressure greater than atmospheric pressure is applied to pure water or a solution, its water potential increases.

Effects of pressure on cell wall

Pressure can build up in a plant system when water enters a plant cell due to diffusion causing a pressure built up against the cell wall, it makes the cell turgid; this increases the pressure potential. Pressure potential is usually positive, do in plants 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 both solute and pressure potential. Relationship between them is:-

Water potential = solute potential + pressure potential.


Cell wall is freely permeable to water and substances in solution, hence is not a barrier to the movement. Cells usually contain a large central vacuole, whose contents, the vacuole sap, contribute to the solute potential of the cell. The cell membrane and the membrane of the vacuole, the tonoplast together are important determinants of movement of molecules in or out of the cell. Osmosis is the term used to refer specifically to the diffusion of water across a differentially semipermeable membrane. It occurs spontaneously in response to a driving force. The net direction and rate of osmosis depends on both the pressure gradient and concentration gradient. Water will move from its region of high chemical potential to its region of lower chemical potential until equilibrium is reached. At equilibrium, the two chambers should have the same water potential.

What is osmotic pressure

Pressure required to prevent water from diffusing is in fact, the osmotic pressure and this is the function of the solute concentration; more the solute concentration, greater will be the pressure required to prevent water from diffusing in. Numerically osmotic pressure is equivalent to the osmotic potential, but the sign is opposite. Osmotic pressure is the positive pressure applied, while osmotic potential is negative.

Factors affecting osmotic pressure:-

1) concentration of solute particles

2) ionization of solute molecules

3) temperature

4) hydration of the solute particles.

Difference between osmotic potential and osmotic pressure

Osmotic potential is the lowering of free energy of water in a system due to the presence of solute particles. Osmotic pressure is the hydrostatic pressure developed in a solution when it is separated from pure solvent by semipermeable membrane in a rigid vessel. Osmotic potential occurs in a confined system or an open system whereas osmotic pressure develops only in a confined system. The value of osmotic potential is negative whereas that of osmotic pressure is positive.

What is plasmolysis

Behaviour of the plant cells or tissues with regard to water movement depend on the surrounding solution. If the external solution balances the Osmotic pressure of the cytoplasm, it is said to be isotonic. If the external solution is more dilute than the cytoplasm, it is hypotonic and if the external solution is more concentrated, it is hypertonic. Cells swell in hypotonic solutions and shrink in hypertonic solutions. Plasmolysis occurs when water moves out of the cell and the cell membrane of a plant cell shrinks away from its cell wall. This occurs when the cell is placed in a solution that is hypertonic (has more solutes) to the protoplasm. Water moves out; it is first lost from the cytoplasm and then from the vacuole. The movement of water occurred across the membrane moving from an area of high water potential to an area of lower water potential outside the cell.

Stages of plasmolysis

1) limiting plasmolysis- gradual loss of water decreases turgor pressure or pressure potential to zero. Protoplast is in contact with cell wall but does not press it.

2) incipient plasmolysis- turgor pressure decreases i.e., becomes negative, contracting protoplast withdraws from cell wall. Contraction initially takes place from the corner.

3) evident plasmolysis- turgor pressure becomes more negative. Protoplast becomes nearly rounded. Cell cannot remain alive for long in this stage.

What does turgor pressure do?

When water flows into the cell and out of the cell and are in equilibrium, the cell are said to be flaccid. Plasmolysis is usually reversible. When the cells are placed in a hypotonic solution water diffuses into the cell causing the cytoplasm to build up a pressure against the wall, that is called turgor pressure. The pressure exerted by the protoplast due to entry of water against the rigid walls is called pressure potential. Because of the rigidity of the cell wall, the cell does not rupture. This turgor pressure is ultimately responsible for enlargement and extension growth of cells.

What is imbibition

Imbibition is a special type of diffusion when water is absorbed by solids-colloids causing them to enormously increase in volume. Absorption of water by seeds and dry wood are the example of imbibition. The pressure that is produced by the swelling of wood had been used by Prehistoric man to split rocks and boulders. Imbibition is also diffusion since water movement is along a concentration gradient; the seeds and other such materials have almost no water hence they absorb water easily. Water potential gradient between the absorbent and the liquid imbibed is essential for imbibition. In addition, for any substance to imbibe any liquid, affinity between the adsorbent and the liquid is also a prerequisite.

Imbibition plays a great role in the germination of seed, rupturing of seed coat and emergence of seedlings is due to higher imbibition pressure developed by seed kernel. Among plant imbibants, phycocolloids are the best imbibants. Imbibition has three important features:- volume change, production of heat and development of pressure. Magnitude of imbibition pressure is:-


How does water and Minerals and food are transported to long distance.

Water and Minerals, and food are generally moved by a mask or bulk flow system. Mass flow is the movement of substances in bulk or en masse from one point to another as a result of pressure differences between the two points. It is a characteristic of mass flow that substances, whether in solution or in suspension, are swept along at the same pace, as in a flowing river. Bulk flow can be achieved either through a positive hydrostatic pressure gradient (e.g., a garden hose) or a negative hydrostatic pressure gradient (e.g., suction through a straw). The bulk movement of substances through the conducting or vascular tissue of plants is called translocation.

What are the highly specialised vascular tissues

Xylem and phloem. Xylem is associated with translocation of mainly water, mineral salts, some organic nitrogen and hormones, from roots to the aerial parts of the plants. The phloem translocates a variety of organic and inorganic solutes, mainly from the leaves to other parts of the plants.

What is diffusion pressure deficit

Meyer called it diffusion pressure deficit and Renner called it suction pressure. Pure water has maximum diffusion pressure. There is always a difference between the diffusion pressure of solvent and its solution. The difference between the diffusion pressure of a solution and a pure solvent, when both are subjected to the same atmospheric pressure is called diffusion pressure deficit. Its value is always positive for a cell. DPD = osmotic pressure - turgor pressure

When a cell is flaccid, DPD = OP. Entry of water into cell causes turgor pressure. When cell is target country of water would stop i.e., when increasing turgor pressure becomes equal to decreasing osmotic pressure OP - TP = 0. Thus, DPD = 0.

How do plants absorb water

Roots absorb most of the water. The responsibility of absorption of water and Minerals is more specifically the function of the root hairs that are present in millions at the tips of roots. Root hairs are thin walled cylinder extension of root epidermal cells that greatly increase the surface area for absorption. Water is absorbed along with general solids, by the root hairs, purely by diffusion. Once water is absorbed by the root hairs, it can move deeper into root layers by two distinct Pathways it can move deeper into root layers by two distinct Pathways:-

1) apoplast pathway-consists of non-living parts.

2) symplast pathway- consists of living parts.

Apoplast pathway

The apoplast is the system of adjacent cell walls that is continuous throughout the plant, except at the casparian strips of the endodermis in the roots. The apoplastic movement of water occurs exclusively through the intercellular spaces and the walls of the cells. Movement through the apoplast does not involve crossing the cell membrane. This movement is dependent on the gradient. The apoplast does not provide any barrier to water movement and water is through mass flow. As water evaporates into the intercellular spaces or the atmosphere, tension develop in the continuous stream of water and apoplast, hence mass flow of water occurs due to the adhesive and cohesive properties of water.

Symplast pathway

The symplastic system is the system of interconnected protoplasts. Neighbouring cells are connected through cytoplasmic stands that extend through plasmodesmata. The water travel through the cells- their cytoplasm; intracellular movement is through the plasmodesmata. Movement is relatively slower. Movement is again down a potential gradient and it may be aided by cytoplasmic streaming.

What is casparian strip and what is its role

Most of the water flow in the roots occurs via the apoplast since the cortical cells are loosely packed and hands over no resistance to water movement. The inner boundary of the cortex, the endodermis, is impervious to water because of a band of suberised matrix called the casparian strip. Water molecules are unable to penetrate the layer, so they are directed to all regions that are not suberised, into the cells proper through the membranes. The water then moves through the simplest and again crosses a membrane to reach the cells of the xylem. The movement of water through the root layers is ultimately symplastic in the endodermis.

Uptake of water

Movement of water is apoplastic. Water is absorbed by the root hair due to diffusion pressure deficit gradient produced by transpiration that develops in the leaf. Root simply acts as a passage or channel for water movement. Water absorption from soil and its involved movement is osmotic pressure dependent. Passage of water from living cells to xylem channel requires accumulation of solute in xylem which is an energy dependent process. Hence, pumping of water in xylem channel is active. This creates a positive pressure in xylem called root pressure.

1) Passive absorption (through roots)

Force for passive absorption lies in shoot. Transpiration pull plays a great role. Negative pressure developed in xylem. Rate of absorption is high. Responsible for 96% of total absorption. Occurs in rapidly transpiring plants. It is apoplastic.

2) Active absorption (by roots)

Force for active absorption develops in root. Osmotic pressure and energy plays the major role. Positive pressure developed in phloem. Rate of absorption is low. Only 4% of water uptake. Occurs in slowly transpiring ones. It is symplastic.


A mycorrhiza is a symbiotic association of a fungus with a root system. The fungal filaments form a network around the young root or they penetrate the root cells. The hyphae have a very large surface area that absorb mineral ions and water from the soil from a much larger volume of soil that perhaps a root cannot do. The fungus provides minerals and water to the roots, in turn the roots provide sugars and nitrogen containing compounds to the mycorrhiza. Pinus seeds can not germinate and establish without the presence of mycorrhiza.

What is root pressure and what is its effect

As various ions from the soil are actively transported into the vascular tissues of the roots, water follows and increases the pressure inside the xylem. This positive pressure is called root pressure, and can be responsible for pushing up water to small heights in the stem. Effects of root pressure is also observable at night and early morning when evaporation is low and excess water collects in the form of droplets around special openings of veins near the tip of glass blades, and leaves of many herbaceous parts. Such water loss in its liquid phase is known as guttation. The greatest contribution of root pressure maybe 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 needs by transpiratory pull.

What is the Cohesion tension transpiration pull model of water transport.

Most researchers agreed that water is mainly pulled through the plant, that the driving force for this process is transpiration from the leaves. It was given by Dixon and Jolly.

What is transpiration

Transpiration is the evaporative loss of water by plants. It occurs mainly through the stomata in the leaves. Exchange of Oxygen and Carbon dioxide in the leaf also occurs through the pores called stomata. Transpiration is affected by several external factors: temperature, light, humidity, wind speed. Plant factors that affect transpiration include number and distribution of stomata, percent of open stomata, water status of the plant, canopy structure etc. The transpiration driven ascent of xylem sap depends mainly 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.


Normally stomata are open in the daytime and closed during the night. The immediate cause of the opening or closing of the stomata is the change in the turgidity of guard cells. The inner wall of each guard cell, towards the pore or stomatal aperture, is thick and elastic. Usually the lower surface of dorsiventral (often dicotyledonous) leaf has a greater number of stomata while in an isobilateral (often monocotyledonous) leaf, they are about equal on both surfaces.

Types of transpiration

1) Stomatal transpiration- occurs through stomata, responsible for 50 to 97% of total transpiration.

2) Cuticular transpiration- water vapours lost directly from outer walls of epidermal cells through the cuticle; about 3 to 10% of total transpiration.

3) Lenticular transpiration- lenticles are aerating pores in the cork of woody stems, twigs, fruits etc. Water vapours are lost through these openings. About 0.1% of total water loss.

4) Bark transpiration- it occurs through the bark of woody stem; responsible for 1% of total transpiration.

Some facts about transpiration and stomata

Guard cells are bordered by one or more modified epidermal cells called subsidiary cells or accessory cells. In monocots, guard cells are ellipsoidal or dumbbell shaped called graminaceous stomata are poaceous stomata. Stomata functions as turgor operated valves. Potometer is the device for measuring the role of transpiration.

Mechanism of opening and closing of stomata

Active potassium transport or potassium pump theory given by S. Imamura and M. Fujino. It showed accumulation of potassium ions in the guard cells during stomatal opening. Later, Levitt explained the influx of potassium ions in the guard cells and their critical role stromatal movement.

Opening of stomata-

In light, starch in the guard cells is incompletely oxidized into PEP and later converted into malic acid, catalysed by PEPcase. Malic acid dissociates into malate ion and protons in the guard cells. Protons are transported to epidermal cells and potassium from the epidermal cells are transported into the guard cells through the agency of hydrogen potassium ion exchange pump in the plasma membrane. In guard cells, potassium ions are balanced by malate anions. Besides, small amount of chlorine ions are also observed with neutralize a small percentage of potassium ions. Process of Ion exchange requires ATP and thus it is an active process. Increased potassium and malate ions forms potassium malate and store it in vacuoles of the guard cells, increasing their osmotic concentration. Hence, water enters the guard cells by endosmosis. Turgor pressure of the guard cells increases due to endosmosis and the stomata are opened.

Closing of stomata-

As carbon dioxide is not utilised in photosynthesis during night, hence its concentration in the sub-stomatal cavity increases. An inhibitor hormone- abscisic acid (ABA), functions in the presence of carbon dioxide. It inhibits potassium ion uptake by changing the diffusion and permeability of the guard cells for positive ions. Potassium ions are transported back to the epidermal or subsidiary cells from the guard cells. Hence, stomata is closed.

What is tensile strength and capillarity.

Tensile strength is an ability to resist a pulling force and capillarity is the ability to rise in thin tubes. Cohesion, adhesion and surface tension give water high tensile strength and high capillarity. In plants, capillarity is aided by the small diameter of the tracheary elements- the tracheids and the vessel elements.

Purpose of transpiration

It creates transpiration pull for absorption and transport of plants. It supplies water for photosynthesis. Transports minerals from the soil to all parts of the plant. It cools leaf surfaces, sometimes 10 to 15 degrees, by evaporative cooling. It maintains the shape and structure of the plants by keeping cells turgid.

Transpiration and photosynthesis

An actively photosynthesizing plant has an insatiable need for water. Photosynthesis is limited by available water which can be swiftly depleted by transpiration. The humidity of rainforests is largely due to this 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 of the Strategies for maximizing the availability of CO2 while minimising water loss. C4 plants are twice as efficient as C3 plants in terms of fixing carbon. However, a C4 plant loses only half as much water as the C3 plant for the same amount of CO2 fixed.

Uptake and transport of mineral nutrients

Plants obtain carbon and oxygen from carbon dioxide. It obtain hydrogen from water and nutrients from minerals in soil.

Factors affecting transpiration

A) External factors

1) light- blue light (more) and red light are effective, constituting its action spectrum.

2) relative humidity is inversely proportional to rate of transpiration.

3) temperature is directly proportional to rate of transpiration.

4) wind blow increases rate of transpiration.

5) available soil water increases transpiration.

B) Internal factors-

1) root shoot ratio is directly proportional to rate of transpiration. Short plants have high root shoot ratio.

2) structure of leaf- thick cuticle, waxy coating, thick-walled hypodermis, sunken stomata reduce the rate of transpiration.

3) number and distribution of stomata, number of open stomata, plant water status, canopy structure also affect the transpiration.

Uptake of mineral ions

Unlike water, all minerals cannot be possibly absorbed by the roots. Two factors account for this: (I) minerals are present in the soil as charged particles ions which cannot move across cell membranes and (ii) the concentration of minerals in the soil is usually lower than the concentration of minerals in the root. Therefore, most minerals must enter the root by active absorption into the cytoplasm of epidermal cells. This needs energy in the form of ATP. The active uptake of Ions is partly responsible for the water potential gradient in roots, and therefore for the uptake of water by osmosis. Some ions also move into the epidermal cells passively. Ions are absorbed from the soil by both passive and active transport. Specific proteins in the membranes of root hair cells actively pump ions from the soil into the cytoplasm of the epidermal cells. Transport proteins of endodermal cells are control points, where a plant adjust the quantity and types of solutes that reach the xylem. Note that the root endodermis, because of the layer of suberin, has the ability to actively transport ions in one direction only.

Translocation of mineral ions

Further transport up the stem to all parts of the plant is through the transpiration stream. The chief sinks for the mineral elements are the growing regions of the plant, such as the apical and Lateral Meristem, young leaves, developing flowers, fruits and seeds, and the storage organs. Unloading of mineral ions occurs at the fine vein endings through diffusion and active uptake by these cells. Mineral ions are frequently immobilized, particularly from older, senescing parts. Older dying leaves export much of their mineral content to younger leaves. Similarly, before leaf fall in deciduous plants, mineral are removed to other parts. Elements most readily mobilized are Phosphorus Sulphur, nitrogen and potassium. Some elements that are structural components like calcium are not remobilized. Some of the nitrogen Travels as inorganic ions, much of it is carried in the organic form as amino acids and related compounds. Small amount of phosphorus and sulphur are carried as organic compounds. In addition, small amount of exchange of material does take place between xylem and phloem. Mineral elements are translocated through xylem in both inorganic and organic form.

Phloem transport

Food, primarily sucrose, is transported by the vascular tissue of phloem from a source to a sink. Source is understood to be that part of plant which synthesizes the food i.e., the leaf, and sink, the part that needs or stores the food. Source and sink may be reversed depending on the season, or the plant's needs. Sugar stored in roots maybe mobilized to become a source of food in the Early Spring when the buds of trees, act as sink; they need energy for growth and development of the photosynthetic Apparatus. The movement in the phloem is bidirectional which contrast with that of the xylem where the movement is always unidirectional. Unlike one way flow of water in transpiration, food in phloem SAP can be transported in any required direction so long as there is a source of sugar and a sink able to use, Store or remove the sugar. Phloem SAP is mainly water and sucrose, but other Sugars, hormones and amino acids are also transported or translocated through phloem.

The pressure flow or mass flow hypothesis

The accepted mechanism used for the translocation of sugar from source to sink is called the pressure flow hypothesis. As glucose is prepared at the source, it is converted to sucrose. The sugar is then moved in the form of sucrose into the companion cells and then into the living phloem sieve tube cells by active transport. This process of loading at the source produces a hypertonic condition in the phloem. Water in the adjacent xylem moves into the phloem by osmosis. As osmotic pressure builds up the phloem SAP will move to areas of lower pressure. At the sink, osmotic pressure must be reduced. Again active transport is necessary to move the sucrose out of the phloem SAP and into the cells which will 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. The movement of sugar in the phloem begins at the source, where Sugars are loaded into a sieve tube. Loading of the phloem sets up a water potential gradient that facilitates the mass movement into the phloem. Phloem tissue is composed of sieve tubes cells, which form long columns with holes in there end walls called seive plates. Cytoplasmic strands passed through the holes in the seive plates, so forming continuous filaments. As hydrostatic pressure in the phloem and seive tube increases, pressure flow begins, and the SAP moves through the phloem. Meanwhile, 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, returning eventually to xylem. Girdling, was used to identify the tissue through which food is transported. Phloem is the tissue responsible for translocation of food and that transport takes place in One Direction, i.e., towards the roots.