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CONTENTS
Vol. XVII No. 3 March 2015 Corporate Office:
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CrosswordTRANSLOCATION OF WATER ANd mINERALS (ASCENT OF SAp)
The upward movement of sap (water with dissolved ingredients) from the root towards the top of the plant is known as ascent of sap. Roots are organs concerned with the absorption of water. If the plant is to survive and grow, this water absorbed by roots must reach the top of plants to replace the water lost in transpiration and to be used in photosynthesis as a raw material.
The upward movement of water occurs through the tracheary elements (tracheids and vessels) of xylem. Sap is lifted from near the root tip to the shoot tip against the force of gravity, sometimes to great heights. Some trees are much taller, often attaining a height upto 110-130 m, e.g., Sequoia sempervirens (height 110 m), Picea sitchensis (height 95-100 m),
Pseudotsuga menziesii (height 100 m), Eucalyptus (height above 130 m), etc. The force required to move water to such
heights are substantive. The rate of translocation is 25–75 cm/min (15–45 m/hr).
Several theories have been put forward to explain the mechanism of ascent of sap. The four main theories are: vital force theory, relay pump theory, root pressure theory, capillary force theory and transpiration pull theory.
Vital force theory
A common vital force theory about the ascent of sap was put forward by Sir J.C. Bose in 1923. It is called pulsation theory and it believes that the innermost cortical cells of the root absorb water from the outer side and pump the same into xylem channels.
TransporT in planTs -ii
The content for PMT Biology is very vast and does not allow students to engage in inquiry and develop meaningful knowledge. An essential topic for PMT is presented here to enable students grasp the topic, analyse the type of questions appearing in PMTs, and score HIGH.
Relay pump theory
Relay pump theory was put forward by Godlewski (1884). According to this, upward conduction of water is due to the pumping activity of xylem parenchyma cells and the cells of medullary rays.
Root pressure theory
Root pressure theory was put forward by Priestley (1916). Root pressure refers to positive hydrostatic pressure which sometimes develops in the xylem sap of roots as a result of metabolic activities of roots. It is a manifestation of active water absorption. Root pressure is observed in certain seasons which favour optimum metabolic activity and reduce transpiration.
Root pressure is maximum during rainy season (in tropical regions) and spring season (in temperate habitats). It is retarded or absent under conditions of starvation, low temperature, drought and reduced availability of oxygen. Root pressures are usually not more than + 1 to + 2 bars. Therefore, it could account for the ascent of sap only to a height of about 20 m. Hence, this theory can account for the ascent of sap only in the herbaceous plants. The magnitude of pressure developed is too small to push the water to the apical regions, in the tall trees. Besides, root pressure is not found in all plants. No or little root pressure is found in gymnosperms which have some of the tallest trees in the world. In rapidly transpiring plants, no root pressure is found, instead a negative pressure is found under such conditions.
Capillary force theory
Capillary force theory was given by Boehm (1809). According to this theory, water rises up in narrow tubes of xylem vessels by surface tension or adhesion and cohesion. The upward movement of water will continue till the forces of adhesion and cohesion are balanced by the downward pull of gravity.
The value of capillarity is very small which can raise water to a height of about 1 metre in vessels of normal diameter (0.03 mm). Therefore, if operational, it will be useful to only small sized plants and cannot operate in plants having tracheids due to the presence of end walls.
Transpiration pull or cohesion-tension
theory
The most widely accepted theory for ascent of sap is transpiration pull theory or cohesion-tension theory. This theory was proposed by Dixon and Jolly in 1894. This theory is based on the following points:
Transpirational pull exerted on the water column. –
Cohesive and adhesive properties of water molecules –
so as to form an unbroken continuous column of water in the xylem.
Transpiration pull
Water is lost from mesophyll cells to the intercellular spaces of leaves as a result of transpiration. The water vapours move out of the plant through stomata. As a result of loss of water from mesophyll cells, the diffusion pressure deficit (DPD) increases.
With the increase of diffusion pressure deficit, these cells absorb water from adjoining cells, ultimately the water is absorbed from xylem elements of vascular bundles of leaf.
Since the xylem elements are filled with continuous water column, a tension or pull called transpiration pull develops at the top of the column. This tension or pull is transmitted down from petiole to stem and finally to roots leading to upward movement of water.
Cohesion and adhesion of water in xylem
Xylem tracheids and trachea are long tubular structures filled with water, extending from root to leaf. Thus, one end of xylem (continuous with one another) is in the root and other end is in the leaf. Water molecules remain attached to one another by a strong mutual force of attraction called cohesive force, which is due to the presence of hydrogen bonds amongst adjacent water molecules.
Supplementing the cohesion between water molecules is adhesion between water molecules and the walls of tracheary elements of xylem. Thus, according to this theory, water ascends in the plant because of transpiration pull and this column of water remains continuous because of cohesive and adhesive forces of water molecules.
TRANSpIRATION
The loss of water in the form of vapours from the living tissues of aerial parts of the plant is termed as transpiration. The loss of water due to transpiration is quite high, 2 litres per day in sunflower, 36-45 litres in apple and upto 1 tonne per day in elm tree. Approximately 98-99% of the water absorbed by a plant is lost in transpiration, 0.2% is used in photosynthesis while the remaining is retained in the plant during growth.
Most of the transpiration occurs through foliar surface or surface of the leaves. It is known as foliar transpiration. Young stems, flowers, fruits, etc., also transpire. Transpiration from stems is called cauline transpiration.
Stomatal apparatus
The stomata are tiny pores present in the epidermal surface of leaves, young stems and in certain fruits (e.g., banana, Citrus, cucumber, etc.).
This pore is surrounded by kidney shaped or bean shaped epidermal cells called guard cells. In monocots, the guard cells generally are dumb-bell shaped. These cells are living, having nucleus, chloroplasts and cytoplasm. The walls of guard cells are thickened on inner side. They have one or two pairs of wall extensions or ledges to prevent entry of water drops into stomata.
The outer walls of guard cells are thin and more elastic. These guard cells are surrounded by some specialized epidermal cells called subsidiary cells or accessory cells.
Stomata are meant for gaseous exchange during photosynthesis and respiration, as well as they are the main source of transpiration.
When turgidity increases within the two guard cells flanking each stomatal aperture or pore, the thin outer walls bulge out and force the inner walls into a crescent shape. The opening of the stomata is also aided due to the orientation of the microfibrils in the cell walls of the guard cells. Cellulose microfibrils are oriented radially rather than longitudinally, making it easier for the stomata to open. When the 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 stomata close.
Number and distribution of stomata on leaf
On the basis of number and distribution of stomata, leaves are categorized into following types:
– Apple type (mulberry type): Stomata are present only on the lower surface of leaf (hypostomatic leaf),
e.g., apple.
– Potato type : Stomata are present on both surfaces of leaf but more on the lower surface, e.g., potato, pea, tomato and many other dicot plants.
Oat type:
– The number of stomata is equal on both surfaces of leaf (amphistomatic leaves) e.g., oat and many other monocots.
– Water lily type: The stomata are present only on the upper surface of leaf, e.g., water lily and most floating plants (Nelumbo, Nymphaea, Victoria, Eurale, etc.).
Potamogeton
– type : Stomata are either absent or vestigeal (astomatic leaves), e.g., Potamogeton, Hydrilla,
Najas.
period of stomatal opening
On the basis of periods of opening and closing, stomata are of following types: Alfalfa type:
– Stomata remain open throughout the day, remain closed at night, e.g., beans, pea. – Barley type : Stomata open only for a few hours in day and remain closed in rest period as in cereals.
– Potato type: Stomata remain open throughout the day and night closing only under the period of water stress, when rate of transpiration exceeds the rate of absorption.
Equisetum
– type: Stomata seldom close, usually remain open throughout, as in emergent hydrophytes. Succulent xerophytes like
– Opuntia or Bryophyllum type: Stomata remain closed in day hours to discourage transpiration but open at night for gaseous exchange and CO2 fixation.
mechanism of stomatal movement
Stomata function as turgor operated valves because opening and closing of stomata is governed by change in O.P. or turgidity of guard cells. When guard cells are turgid, stomata open and when guard cells are flaccid, stomata close. Different theories about the mechanism of stomatal movements have been proposed :
Hypothesis of guard cell photosynthesis
– (given by Schwendener, 1881).
Starch-sugar interconversion theory
– (given by Lloyd, 1908 and by Sayre 1926 and modified by Steward
1964). Malate or K
– + ion pump hypothesis (given by Fujino, 1959 and modified by Levitt, 1974) Hypothesis of guard cell photosynthesis
According to this, the photosynthetic activity of the guard cells was believed to be responsible for the stomatal movement. During the day, glucose is photosynthesised and being osmotically active, lowers the water potential of the cell sap, thus there is an influx of water into the guard cells and the stomata open.
At night, sugar is converted to starch and being osmotically inactive, decreases the osmotic concentration of guard cell. Thus, water moves out and guard cells become flaccid and stomata close.
Biggest objection to it was that the photosynthetic activity of guard cell chloroplasts was found to be negligible. Starch-sugar interconversion theory
According to this theory, a change in pH effects the opening and closing of stomata. Starch sugar interconversion is brought about by a pH dependent enzyme starch phosphorylase. The change in the pH of the guard cell’s sap is due to the presence or absence of CO2 which is dependent on light availability.
During day time, in the presence of light, CO2 is utilized for photosynthesis and hence does not accumulate in the guard cells and pH of the guard cells increases to 7. Hence, the enzyme favours the formation of glucose-1-phosphate from starch and inorganic phosphate.
Glucose being osmotically active, lowers the water potential of the cell sap, thus there is an influx of water into the guard cells and the stomata open.
At night, in the absence of light, CO2 evolved in respiration accumulates in the cell sap and dissolves in water to form carbonic acid and pH of the cell sap decreases to 5 (acidic).
At this pH, starch synthesis is favoured from glucose-1-phosphate.
Starch being osmotically inactive, decreases the osmotic concentration of guard cell. Thus, water moves out and guard cells become flaccid and stomata close.
There are many objections to starch-sugar interconversion theory. First of all, it is a slow process and cannot account for rapid stomatal movements. Secondly, enzyme phosphorylase catalyzes the reaction only in forward direction. Moreover, sugar has never been found in free state in guard cells of open stomata and starch has never been reported from guard cells of onion.
malate or K+ ion pump hypothesis
This is the most accepted modern theory. It is also known as hormonal regulation theory (due to ABA secretion). This theory may be represented as follows:
In light
First of all, the starch is converted into organic acids particularly phosphoenol pyruvic acid (PEP). Phosphoenol pyruvic acid then combines with CO2 in the presence of PEP carboxylase enzyme to produce oxalo acetic acid (OAA) and then malic acid.
The organic acids, viz., malic acid, dissociate into malate anion and H+ in the guard cells. H+ are transported to epidermal cells and K+ are taken into the guard cells in exchange of H+. The process is called ion exchange. K+ are balanced by organic anions (i.e., malate). Some Cl– ions are also taken in to neutralize a small percentage of K+.
H+– K+ exchange is an active process which requires involvement of energy (ATP) supplied either by respiration or photophosphorylation. Increased concentration of K+ and malate ions in the vacuole of guard cells causes sufficient osmotic pressure to absorb water from surrounding cells. Increased turgor of guard cells due to entry of water causes stomatal pore to open.
In dark
Higher concentration of CO2 in sub-stomatal cavity prevents proton gradient across protoplasmic membrane in guard cells (Sharpe and Zeiger, 1981). As a result, active transport of K+ into guard cells ceases.
Cowan et al. (1982) proposed that closure mechanism involves participation of an inhibitor hormone-abscisic acid, which functions at lower pH. As soon as the pH of guard cells decreases, the abscisic acid inhibits K+ uptake by changing the diffusion and permeability of the guard cells. Malate ions present in the guard cell cytoplasm combine with H+ to produce
malic acid. Excess of malic acid inhibits its own synthesis by decreasing the activity of PEP carboxylase. These changes cause reversal of ion movement so that K+ is transported out of guard cells into surrounding epidermal cells.
The osmotic concentration of guard cells is thus decreased, resulting in the movement of water from guard cells to surrounding cells. The guard cells become flaccid and the stomatal pore gets closed.
Factors affecting stomatal movement
In majority of plants, the stomata open in
– light and close in darkness. Maximum opening occurs in red light and blue light. No opening occurs in green light, UV-light and infrared light.
Increase in
– temperature leads to opening of stomata upto certain extent. Very high temperature leads to closing of stomata even in day (mid-day closure). At 38° – 40°C, stomata can open in complete darkness, while at 0°C they remain closed even in continuous light. Normally high temperature above 30°C, reduces stomatal opening in many species.
Water stress
– (or water deficit) brings about stomatal closure due to rise in DPD of epidermal cells. Mechanical shock
– causes closure of stomata.
High
– CO2 concentration in the intercellular spaces of leaves causes closure of stomata even in day time. Low CO2 concentration usually induces opening of stomata.
Rise in
– pH results in opening of stomata while fall in pH induces closing. Cytokinins
– are essential for opening of stomata while ABA takes part in stomatal closure. A number of
– minerals are essential for stomatal movements e.g., K, P, N, Mg, Ca etc. Oxygen
– is essential for stomatal opening.
Stomatal pore is measured with the help of instrument called porometer.
Rate of transpiration is measured by potometer, e.g., Ganong’s potometer, Farmer’s potometer, Darwin’s potometer, etc. Principle of working of all potometers is that absorption is proportional to transpiration. Cobalt chloride paper method is used to compare rate of transpiration on two surfaces of leaf.
Factors affecting transpiration
External factors Light
– increases transpiration through opening of stomata and increased protoplasmic permeability. Increase in
– temperature brings about an increase in the rate of transpiration. Rate of transpiration is generally doubled with every 10°C rise in temperature.
The rate of transpiration is inversely proportional to the
– relative humidity, i.e., the rate of transpiration is higher
when the relative humidity is lower and vice versa. The
– movement of air increases the rate of transpiration by removing the saturated air around the leaves. Upto 20-30 km/hr, the rate of transpiration increases with the wind velocity. A wind velocity of 40-50 km/hr decreases transpiration by closing the stomata due to mechanical effect, drying and cooling of the transpiring organs. Reduced availability of soil water
– causes wilting or loss of turgidity resulting in drooping and rolling thus reduces transpirational rate.
Internal factors In general, higher the
– root/shoot ratio, more will be the rate of transpiration. It is due to the fact that an extensive root system is more efficient in water uptake from the soil.
More is the
– leaf area, more will be the rate of transpiration. Cuticular transpiration decreases with the thickness of cuticle. –
The sunken (deep seated) stomata, present in xerophytes, are a device to reduce the rate of transpiration. –
Leaf modifications such as formation of prickles, leaf spines, phyllodes etc., help in reducing the rate of –
transpiration.
Compact mesophyll reduces transpiration while a loose mesophyll increases transpiration. –
Presence of mucilage decreases the rate of transpiration by holding water. –
Wilting
Wilting is the loss of turgidity of leaves and other soft aerial parts of a plant causing their drooping, folding and rolling. The symptoms of wilting are not shown by thick-walled tissues. Therefore, they are less conspicuous in sclerophyllous plants. Wilting is of 3 types : incipient wilting, temporary or transient wilting and permanent wilting. In incipient wilting, there are no external symptoms of wilting but the mesophyll cells have lost sufficient water due to transpiration being higher than the availability of water. It occurs during mid day for a brief period in almost all plants even when sufficient water is present in the soil. Temporary or transient wilting is the temporary drooping down of leaves and young shoots due to loss of turgidity during noon. Lower leaves show wilting earlier than the upper ones. Temporary wilting is corrected only after the rate of transpiration decreases in the afternoon accompanied by replenishment of water around the root hairs. A permanent wilting is that state in the loss of turgidity of leaves when they do not regain their turgidity even on being placed in a saturated atmosphere. It occurs when the soil is unable to meet the requirement of plant for transpiration. After permanent wilting, the plant dies.
Transpiration as necessary evil
Transpiration causes loss of huge amount of water absorbed by plants and leads to wilting and injury in plants. It also checks photosynthesis, reduces growth and if too severe, may cause death due to desiccation.
Inspite of various detrimental effects, the plants cannot avoid transpiration due to their peculiar structure of leaves which is basically meant for gaseous exchange during respiration and photosynthesis and also for the required pull for ascent of sap. Therefore, transpiration is also regarded as “necessary evil” by Curtis (1926) or “unavoidable evil” by Steward (1959).
Antitranspirants
Plant antitranspirants are chemical substances, whether naturally synthesised by plants themselves or synthetic materials applied to plants, which reduce the rate of transpiration effectively. An ideal antitranspirant is one which decreases the transpiration rate without affecting CO2 fixation in photosynthesis. There are two types of antitranspirants: metabolic inhibitors and surface films. Metabolic inhibitors reduce transpiration by reducing
the stomatal opening for a period of two or more weeks without influencing other metabolic processes. The most promising of these inhibitors is phenyl mercuric acetate. Another is abscisic acid (ABA).
Film forming chemicals check transpiration by forming a thin film on the transpiring surface. They are sufficiently permeable to carbon dioxide and oxygen to allow photosynthesis and respiration but prevent movement of water vapours through them. The important chemicals of this group are silicon emulsions, colourless plastic resins and low viscosity waxes.
Transpiration and photosynthesis - A compromise
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 minimizing water loss. C4 plants are twice as efficient as C3 plants in terms of fixing carbon (making sugar). However, a C4 plant loses only half as much water as a C3 plant for the same amount of CO2 fixed.
GuTTATION
Loss or excretion of water in the form of liquid droplets from the tips and margins of leaves is called guttation. It was first studied by Bergerstein in 1887.
All plants do not show guttation. It is restricted to about 345 genera of herbaceous and some woody plants. Common examples are garden nasturtium, oat and other cereals, balsam, tomato, cucurbits etc. It occurs in warm, moist soils with humid environment or when warm days are followed by cool nights.
In general, guttation occurs when transpiration rate is very low as compared to rate of water absorption.
Due to this, root pressure is developed and water is pushed out through specialized pores at vein endings called hydathodes. So guttation is not due to activity of hydathodes but due to root pressure. Each hydathode consists of a group of loosely arranged colourless parenchymatous cells called epithem. It lies over the tip of a vascular strand and communicates with the outside through a permanent pore in epidermis called water pore or water stoma.
The guttated liquid is never pure water. It contains 0.6-2.5 gm/litre of solutes– both organic (carbohydrates, organic acids, amino acids, enzymes) and inorganic (Ca2+, Mg2+, K+, Na+, CO
32–, SO42–, Cl–). These salts sometimes are redissolved back into leaves and cause ‘salt injury’.
Gutttion takes place either at night or early in the morning. Dry soils, poorly aerated soils, heavily salted or mineral deficient soils and the atmospheric conditions promoting transpiration inhibit guttation.
Table : Differences between transpiration and guttation
Transpiration Guttation
1. Loss of water is in vapour form. Loss of water is in liquid form. 2. It occurs during day time. It occurs during night or early morning. 3. Loss of pure water. Loss of impure water.
4. Through stomata or epidermis or cuticle or lenticels. Through hydathodes. 5. Controlled phenomenon. Uncontrolled phenomenon.
BLEEdING
It is the exudation of sap or watery solution from the cut or injured parts of the plant, e.g., Agave, Acer, Vitis, etc. It occurs due to root pressure, phloem pressure, local pressure in xylem (stem pressure) and latex or resin.
upTAKE ANd TRANSpORT OF mINERAL NuTRIENTS
uptake of mineral ions
Unlike water, all minerals cannot be passively 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. Like all cells, the endodermal cells have many transport proteins embedded in their plasma membrane; they let some solutes cross the membrane, but not others.
Transport proteins of endodermal cells are control points, where a plant adjusts 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
After the ions have reached xylem of the roots through active or passive uptake, or a combination of the two, their further transport up the stem to all parts of the plants is through the transpiration stream.
Though it is generally considered that xylem transports inorganic nutrients while phloem transports organic nutrients, the same is not exactly true. In xylem sap, nitrogen travels as inorganic ions, as well as organic form of amino acids and related compounds. Small amounts of P and S are passed in xylem as organic compounds. There is also exchange of materials between xylem and phloem. Therefore, mineral elements pass up xylem in both inorganic and organic form. They reach the area of their sink, namely young leaves, developing flowers, fruits and seeds, apical and lateral meristems and individual cells for storage. Minerals are unloaded at fine vein endings through diffusion. They are picked up by cells through active uptake.
There is remobilization of minerals from older senescing parts. Nickel has a prominent role in this activity. The senescing leaves send out many minerals like nitrogen, sulphur, phosphorous and potassium. Elements incorporated in structural components are, however, not remobilized, e.g., calcium. The remobilized minerals become available to young growing leaves and other sinks.
phLOEm TRANSpORT: TRANSLOCATION OF ORGANIC SOLuTES
Formation of organic food or carbohydrate occurs by green parts of the plant (leaves) as a result of photosynthesis. Non-green parts depend upon green parts for organic food and thus food is transported from green parts to non-green parts of the plant. This movement of organic food or solute in soluble form, from one organ to another organ is called translocation of solutes. For example: from leaves to stem and roots for consumption.
Translocation of solutes takes place always from higher concentration (source or supply end) of its soluble form to lower concentration of its soluble form (sink or consumption end). Translocation of solutes mostly occurs in form of sucrose.
Since the source-sink relationship is variable, the direction of movement in the phloem can be upwards or downwards, i.e., bi-directional. This contrasts with that of the xylem where the movement is always unidirectional, i.e., upwards. Hence, 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.
direction of translocation of solutes
Solutes are translocated in the plants in downward, upward or radial direction. Path of upward and downward translocation is phloem, while path of radial translocation is medullary rays.
Downward translocation : It is of most important type, i.e., from leaves to stem and roots.
Upward translocation : From leaves to developing flowers, buds, fruits and also during germination of seeds and tubers, etc.
Radial translocation : From pith to cortex and epidermis.
Evidences in favour of downward conduction of solutes by phloem Downward conduction of solutes takes place through phloem is evident from the following points.
Suitability : Phloem has long tubes (most suitable for long distance transport) placed one above the other. Thus, phloem seems to be the only suitable tissue for downward conduction of solutes.
Chemical analysis of phloem sap : Chemical analysis of sieve tube sap proves that concentrated solution of sucrose is translocated from the place of synthesis to other parts of the plant body. Glucose and fructose are sometimes found in traces only. The amount of sucrose is more in phloem sap during the day and less in night.
Isotopic studies : If leaf of a potted plant is illuminated in the presence of radioactive C14O
2, it forms radioactive products of photosynthesis which are then transported to stem. It was detected by autoradiographic studies that these substances are translocated through phloem particularly sieve tubes. Radioactivity is found below and above the nodes of the leaf to which radioactive carbon was provided.
Ringing or girdling experiment : In this experiment, all the tissues of the stem outside the vascular cambium are removed in a ring. The upper part of the plant is attached to lower part only by external xylem cylinder and pith. Accumulation of food occurs in the form of swelling just above the ring, which suggests that in absence of phloem, downward translocation of food is stopped. In a girdled plant, roots die first and ultimately shoot dies. This is because the upper part of stem gets ample amount of water supply (as transport of water and minerals occurs through xylem). But as the roots die due to starvation, the upper part of stem also dies in the course of time.
mechanism of translocation of solutes
Several theories have been put forward to explain the mechanism of translocation of organic nutrients through phloem, but Munch mass flow hypothesis or pressure flow hypothesis is most accepted one. It was given by Munch (1930) and supported by Crafts.
According to this theory, translocation of solutes takes place in bulk, through phloem along a gradient of turgor pressure from higher concentration of its soluble form (source) to lower concentration of its soluble form (sink).
Because in leaves (source), osmotic concentration remains always high (due to photosynthesis) and in roots (sink), osmotic concentration remains low, so mass flow of organic food continues from leaves to roots (i.e., along a gradient of turgor pressure).
The organic substances present in mesophyll cells are passed into the sieve tubes through their companion cells by an active process. A high osomotic concentration, therefore, develops in the sieve tubes of the source. Thus, the sieve tubes absorb water from the surrounding xylem and develop a high turgor pressure. It causes the flow of organic solution towards the area of low turgor pressure. A low turgor pressure is maintained in the sink region by converting soluble organic substances into insoluble form. Objections to this theory are as follows:
Vacuoles of the adjacent sieve tube cells are not –
continuous. The cytoplasm present near the sieve plates exerts resistance to the mass flow.
Catalado
– et al (1972) have observed the rate of flow of water (72 cm/hr) and solutes (35 cm/hr) to be different in the same sieve tube.
Phloem transport is not influenced by water deficit. –
The cells at the source end of mass flow should be –
turgid but they are often found to be flaccid in case of germinating tubers, corms, etc.
Various factors affecting translocation of solutes have been summarized in the given flow chart.
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New MCQs
1. Ringing/girdling experiments demonstrate (a) phloem is responsible for translocation of food (b) xylem is responsible for ascent of sap
(c) transpiration pull (d) both (a) and (b).
2. Which of the following theories gives the latest explanation for the opening and closure of stomata? (a) Starch-sugar conversion theory
(b) Munch theory
(c) Transpiration pull theory (d) Active K+ transport theory
3. Match column-I with column-II and select the correct option from the codes given below.
Column-I Column-II
A. Dixon and Jolly (i) Rate of transpiration B. Stomata closure (ii) Transpiration pull C. Potometer (iii) Only water available to
plants D. Capillary water (iv) ABA (a) A-(i), B-(iv), C-(ii), D-(iii) (b) A-(ii), B-(iv), C-(i), D-(iii) (c) A-(ii), B-(i), C-(iii), D-(iv) (d) A-(iii), B-(ii), C-(i), D-(iv)
4. Which of the following statements is incorrect? (a) Guard cells are kidney shaped in dicots and dumb
bell shaped in monocots.
(b) Free-floating hydrophytes possess epistomatic leaves. (c) In xerophytes such as Opuntia, stomata are absent. (d) Amphistomatic leaves are generally present in
monocots.
5. The transpiration-driven ascent of xylem sap depends mainly upon _____ property of water.
(a) cohesion (b) adhesion (c) surface tension (d) all of these
6. Which of the following is correct regarding the translocation of substances in vascular tissues of plants? (a) Organic substances, e.g., sugars are transported
upward in the xylem.
(b) Organic substances move up and down in phloem. (c) Salts and other inorganic substances move
downward only through the xylem.
(d) Inorganic substances move upward only through the pholem.
7. A girdled plant (upto bast) may survive for some time but it will eventually die, because
(a) water will not move downwards (b) water will not move upwards
(c) sugars and other organic materials will not move downwards
(d) sugars and other organic materials will not move upwards.
8. Guttation is the release of liquid water from veins at the leaf margins. It is caused by
(a) transpiration (b) high leaf pressure (c) high root pressure
(d) clogged tracheids or vessel elements. 9. Wilting of a plant occurs when
(a) xylem is blocked (b) phloem is blocked
(c) epidermis and few roots are removed (d) pith is removed.
10. Translocation of minerals takes place through (a) phloem (b) xylem
(c) cambium (d) wood parenchyma. 11. The continuity of water column in xylem is maintained
due to
(a) adhesive property of water (b) cohesive property of water (c) presence of air bubbles (d) both (a) and (b).
12. Stomata of a plant open when guard cells show (a) influx of hydrogen ions
(b) efflux of potassium ions (c) influx of potassium ions (d) all of these.
13. Potometer works on the principle of (a) osmotic pressure
(b) amount of water absorbed equals the amount transpired
(c) root pressure (d) capillarity.
14. The rate of transpiration of a plant would gradually increase if
(a) the relative humidity increases (b) the relative humidity decreases
(c) the relative humidity remains unchanged (d) the water potential gradient remains unchanged.
15. The process of guttation takes place when
(a) the root pressure is high and the rate of transpiration is low
(b) the root pressure is low and the rate of transpiration is high
(c) the root pressure equals the rate of transpiration (d) the root pressure as well as rate of transpiration are
high.
16. Select the mismatched pair. (a) Relay pump theory – Godlewski (b) Pulsation theory – Sir J. C. Bose
(c) Transpiration pull theory – Dixon and Jolly (d) Capillary force theory – Munch
17. Unidirectional flow of water, minerals, some organic nitrogen and hormones occurs through
(a) xylem (b) phloem (c) root (d) vascular tissue.
18. Match column-I with column-II and select the correct option from the codes given below.
Column-I Column-II
A. Vein ending (i) Transpiration B. Necessary evil (ii) Osmosis C. Semipermeable (iii) Transpiration pull
membrane
D. Cohesion (iv) Guttation E. Stomata closure (v) ABA (a) A-(iv), B-(i), C-(iii), D-(ii), E-(v) (b) A-(iv), B-(i), C-(ii), D-(iii), E-(v) (c) A-(iii), B-(v), C-(i), D-(ii), E-(iv) (d) A-(i), B-(ii), C-(iii), D-(iv), E-(v)
19. Loss of water in the form of vapours from the living tissues of aerial plant parts is called as
(a) transpiration (b) guttation (c) bleeding (d) precipitation. 20. Rate of transpiration increases with an increase in
(a) temperature (b) light (c) wind velocity (d) all of these.
Exam Section
1. The shade of a tree is cooler than the shade of a roof due to
(a) transpiration (b) guttation (c) photosynthesis (d) green leaves.
(AIIMS 1995)
2. Transpiration can be influenced by interfering with (a) osmotic pressure
(b) guard cell
(c) atmospheric temperature
(d) epidermis of leaf. (AFMC 1995)
3. Sir J.C. Bose proposed which of the following theories to explain the process of ascent of sap ?
(a) Pulsation theory (b) Relay pump theory (c) Transpiration pull theory (d) Capillary force theory
(Karnataka1996)
4. The plants face wilting due to use of excessive fertilizers, because of
(a) exosmosis (b) imbibition (c) endosmosis (d) all of these.
(UP-CPMT 1998)
5. Opening and closing of stomata is due to the (a) hormonal change in guard cells
(b) change in turgor pressure of guard cells (c) gaseous exchange
(d) respiration.
(CBSE-PMT 2002)
6. “Transpiration is a necessary evil”, this statement belongs to
(a) Burgerstein (b) Reschke (c) Curtis (d) Willmer.
(Manipal 2002)
7. Opening or closing of stomata are under the control of
(a) K+ (b) P
(c) Mg (d) none of these.
(DPMT 2002)
8. Water lost by transpiration is (a) rich in dissolved minerals (b) rich in solutes
(c) pure water
(d) rich in dissolved salts
(Karnataka 2002)
9. Guttation is loss of water (a) in the form of vapour (b) in the form of liquid droplets (c) from root
(d) through stomata.
(Manipal 2004)
10. Transpiration is very important for plants because it helps in
(a) the absorption of water from soil (b) the cooling of leaves at high temperature
(c) the movement of water and minerals absorbed by roots to various parts of the plant
(d) all of the above.
11. The following figure shows the stomatal apparatus. Identify the parts labelled as A, B, C and D and choose the correct answer from the following.
(a) A = Guard cells, B = Stoma, C = Chloroplasts, D = Subsidiary cells (b) A = Subsidiary cells, B = Chloroplasts,
C = Stoma, D = Guard cells (c) A = Guard cells, B = Chloroplasts,
C = Stoma, D = Subsidiary cells (d) A = Subsidiary cells, B = Stoma,
C = Chloroplasts, D = Guard cells
(Karnataka 2005)
12. In which of the following plants, there will be no transpiration?
(a) Aquatic, submerged plants (b) Plants living in deserts
(c) Aquatic plants with floating leaves
(d) Plants growing in hilly regions (Karnataka 2005)
13. Upward movement of water in plants is called (a) sucking (b) ascent of sap (c) translocation (d) none of these.
(Odisha 2006)
14. Sunken stomata are found in
(a) xerophytes (b) hydrophytes (c) mesophytes (d) all of these.
(Odisha 2006)
15. The rupture and fractionation do not usually occur in the water column in vessel/tracheids during the ascent of sap because of
(a) weak gravitational pull (b) transpiration pull (c) lignified thick walls (d) cohesion and adhesion.
(CBSE-PMT 2008)
16. Which of the following chemical serves as an antitranspirant in plants?
(a) Cobalt chloride (b) Dimethyl mercury (c) Potassium iodide
(d) Phenyl mercuric acetate (BHU 2008)
17. Which of the following does not cause opening of stomata?
(a) Light (b) Increased pH (c) Reduced pH
(d) Low CO2 concentration (DPMT 2008)
18. Stomatal opening or closing is assisted by (a) change in the turgidity of guard cells
(b) the inner walls of each guard cells which are thick and elastic
(c) cellulose microfibrils of guard cells, oriented radially
(d) all of the above. (AMU 2010)
19. Which of the following is not a purpose of trans-piration?
(a) Transports minerals from the soil to all parts of the plant
(b) Helps in translocation of sugars from source to sink
(c) Maintains shape and structure of the plants (d) Cools leaf surfaces (Kerala 2010)
20. In land plants, the guard cells differ from other epidermal cells in having
(a) cytoskeleton (b) mitochondria
(c) endoplasmic reticulum
(d) chloroplasts. (AIPMT Prelims 2011)
21. Accumulation of which one of the following acids results into closure of stomata ?
(a) Malic acid (b) Aspartic acid
(c) Phosphoenol pyruvic acid (d) Oxaloacetic acid
(AMU 2011)
22. Radial conduction of water takes place by (a) vessels (b) vessels and trachieds (c) phloem (d) ray parenchyma cells.
(AMU 2012)
23. Force generated by transpiration can create pressure sufficient to lift water even upto the height of
(a) 130 feet (b) 130 metre (c) 230 feet (d) 230 metre.
(AMU 2012)
24. Stomata open at night in
(a) hydrophytes (b) succulents (c) mesophytes (d) halophytes.
25. Which of the following statements is not true for stomatal apparatus?
(a) Guard cells invariably possess chloroplasts and mitochondria.
(b) Guard cells are always surrounded by subsidiary cells. (c) Stomata are involved in gaseous exchange. (d) Inner wall of guard cells are thick.
(NEET - Karnataka 2013)
Assertion & Reason
The following questions consist of two statements each : assertion (A) and reason (R). To answer these questions, mark the correct alternative as directed below :
(a) If both A and R are true and R is the correct explanation of A.
(b) If both A and R are true but R is not the correct explanation of A.
(c) If A is true but R is false. (d) If both A and R are false.
1. Assertion : Exudation of sap from cut or injured parts of plants is called bleeding.
Reason : Bleeding occurs due to root pressure, phloem pressure and local pressure in xylem.
2. Assertion : Loss of water in the form of liquid droplets from leaves of plants is called transpiration.
Reason : Transpiration takes place through special structures called hydathodes.
3. Assertion : Xylem transport is unidirectional. Reason : Phloem transport is bi-directional. 4. Assertion : Transpiration is an unavoidable evil.
Reason : Plants cannot regulate transpiration. 5. Assertion : Guttation drops are restricted to tips or
margins of leaves while dew drops are found all over the plant.
Reason : Both dew drops and guttated water are pure water.
Short Answer Type Questions
1. Read the following statements and fill in the blanks withcorrect words/terms.
(i) Rate of transpiration is _____ when relative humidity is _____.
(ii) _____ and _____ are antitranspirants which reduce transpiration by reducing the stomatal opening for a period of few weeks, without influencing other metabolic activities.
(iii) The movement of ions from soil to interior of root requires _____.
(iv) Mineral elements pass up xylem in both _____ and _____ form.
(v) In_____ there is no external symptom, but mesophyll cells lose sufficient water due to transpiration being higher than the availability of water.
2. How do surface films check transpiration?
3. What adaptations are found in xerophytic plants to reduce the rate of transpiration?
Answer Key
New MCQs 1. (d) 2. (d) 3. (b) 4. (c) 5. (d) 6. (b) 7. (c) 8. (c) 9. (a) 10. (b) 11. (d) 12. (c) 13. (b) 14. (b) 15. (a) 16. (d) 17. (a) 18. (b) 19. (a) 20. (d) Exam Section1. (a) 2. (c) 3. (a) 4. (a) 5. (b) 6. (c) 7. (a) 8. (c) 9. (b) 10. (d) 11. (c) 12. (a) 13. (b) 14. (a) 15. (d) 16. (d) 17. (c) 18. (d) 19. (b) 20. (d) 21. (a) 22. (d) 23. (b) 24. (b) 25. (b)
Assertion & Reason
1. (a) 2. (d) 3. (b) 4. (c) 5. (c) Short Answer Type Questions 1. (i) higher/lower, lower/higher
(ii) Phenyl mercuric acetate, ABA (iii) active transport
(iv) inorganic, organic (v) incipient wilting
2. Film forming chemicals check transpiration by forming a thin film on the transpiring surface. They are sufficiently permeable to carbon dioxide and oxygen and allow photosynthesis and respiration but prevent movement of water vapours through them.
3. The sunken (deep seated) stomata and leaf modifications such as formation of prickles, leaf spines, phyllodes etc., are some adaptations in xerophytes which help in reducing the rate of transpiration.
1.
The given graph shows the concentration of urine produced in three different mammals, A, B and C.Three types of kidney tubules, X, Y and Z, are also given below.
Length of loop of Henle may vary in different kidney tubules and is related to urine concentration.
i. What is the reason for the production of different concentrations of urine in these mammals?
---. ii. Refer the graph and match the kidney tubules (X, Y and Z)
with the mammals (A, B and C) to which they belong. ---.
3.
One way to express some aspects of the age distribution characteristics of a population is through a survivorship curve. The given graph shows the percentage of maximum life span and survival per thousand of three organisms i.e., humans, Hydra and oyster.
Given below are the steps involved in two processes P and Q of an assay.
i. Explain the principle involved in the assay and identify the processes P and Q.
---. ii. Explain the significance of the two processes of the assay.
---.
2.
i. The type I, type II and type III survivorship curves in the given graph, belong to which organisms respectively? ---. ii. Explain why, these three organisms have different
survivorship curves.
---.
Survivorship is defined as percentage of an original population that survives to a given age.
There are several species of North American hares. The given figure shows the correlation between external ear size in different species of hares and latitude.
4.
5.
i. Explain why arctic hare has smallest external ear size? ---. ii. Identify the rule, associated with given figure?
---.
Given figure shows the protein structure of the enzyme cellulase, which is found in grass eating animals so that they can digest the cellulose found in grass.
i. Name structure A and describe how it is formed.
---. ii. State the differences between the structures of cellulase and its substrate.
---.
Arctic hare is adapted largely to cold habitats.
sOLuTIOns
1. (i) Mammals are capable of forming urine, that is more concentrated than blood plasma, thus maximising the amount of water retained. The nephron of mammals is able to do this because it possesses a loop of Henle. The loop of Henle has a key role in water conservation, as good amount of water is reabsorbed from the glomerular filtrate over here. Based on lengths of the loop of Henle, there are two types of nephrons: (i) cortical nephrons: they have short loops of Henle which only just extend into the medulla and function most importantly when the water supply is normal to excessive, (ii) juxtamedullary nephrons : they have long loops of Henle that extend deep into the medulla and function most importantly when water is in short supply. The lengths of loops of Henle and of the collecting ducts, and the general thickness of the medulla region of the kidneys, increase progressively in animals best adapted to the drier habitats. (ii) Kidney tubule X belongs to mammal B, kidney tubule
Y belongs to mammal A and kidney tubule Z belongs to mammal C. According to the graph, mammal C is producing most concentrated urine which indicates that it is adapted to arid habitats and possesses kidney with juxtamedullary nephrons. Mammal A is producing least concentrated urine which implies that it thrives well in aquatic habitat so it possesses kidney with cortical nephrons. Mammal B is producing intermediate concentration of urine which indicates that it is well adapted to terrestrial habitats, with adequate water supply hence, possesses kidney tubules intermediate between the two.
2. (i) Type I survivorship curve belongs to humans, type II belongs to Hydra and type III belongs to oyster. (ii) Type I surviorship curves are characterised by high
age-specific survival probability in early and middle life, followed by rapid decline in survival rate in later life. They are typical for species that produce few offspring but care for them well, e.g., humans. In Type III curves, greatest mortality is experienced in early life with relatively low rates of death for those surviving. This type of curve is characteristic of species that produce large number of offspring.
Oysters, produce vast number of offspring, only a few of which, live to reproduce. However, once they become established and grow into reproductive
individuals, their mortality rate is extremely low, so they have type III survivorship curve.
Type II curves are an intermediate between types I and III, where roughly constant mortality rate and survival probability is experienced regardless of age,
e.g., Hydra.
3. (i) The given processes P and Q belong to ELISA (enzyme-linked immunosorbent assay). P is indirect ELISA and Q corresponds to sandwich ELISA. The assay makes use of an enzyme that reacts with a colourless substrate to produce a coloured product. The enzyme is covalently linked to a specific antigen/antibody that recognizes a target antibody/antigen. If the target is present, the antibody-enzyme complex will bind to it and, on addition of the substrate, the enzyme will catalyse the reaction, generating the coloured product. Thus, the presence of the coloured product indicates the presence of the antigen or antibody. (ii) The indirect ELISA is used to detect the presence of
antibody and is the basis of the test for HIV infection. The HIV test detects the presence of antibodies that recognize viral core protein antigens. Viral core proteins are adsorbed to the bottom of a well. Antibodies from the person being tested are then added to the coated well. Only a person infected with HIV will have antibodies that bind to the antigen. Finally, enzyme-linked antibodies to human antibodies are allowed to react in the well and unbound antibodies are removed by washing. Substrate is then applied. An enzyme reaction yielding a coloured product suggests that the enzyme-linked antibodies were bound to human antibodies, which in turn implies that the patient has antibodies to the viral antigen. Moreover, this assay is quantitative i.e., the rate of the colour-formation reaction is proportional to the amount of antibody originally present.
The sandwich ELISA is used to detect antigen rather than antibody. Antibody to a particular antigen is first adsorbed to the bottom of a well. Next, solution containing the antigen (such as blood or urine, in medical diagnostic tests) is added to the well and binds to the antibody. Finally, a second, different antibody to the antigen is added. In this case, the rate of colour formation is directly proportional to the amount of antigen present. Consequently, it permits
the measurement of small quantities of antigen.
4. (i) Heat loss or gain is directly related to exposed surface area. Shorter extremities are an adaptation to reduce exposed surface area in order to reduce heat loss in cold regions. Therefore, arctic hare (Lepus arcticus) found in tundra region, has smallest external ear size as small size of the ears helps to reduce heat loss in cold conditions.
(ii) Given figure shows the North American hares, in which there is a good correlation between external ear size and average temperature experienced by different species. Here, populations of North American hares from different latitudes follow Allen's rule. Increasing latitude indicates decreasing temperatures. Allen's rule states that "animals of colder areas have shorter extremities (leg tail, ears, feet) as compared to animals of warmer areas".
5.(i) Structure labelled A is b-pleated secondary structure. In b-pleated secondary structure, two or more polypeptide chains get interconnected by hydrogen bonds. A sheet is produced, therefore this structure
is called pleated sheet or b-pleated sheet. Adjacent strands of polypeptides may run in same direction or in opposite directions. The adjacent strands are held together by hydrogen bonds formed between –CO and –NH groups to form sheet.
(ii) The differences between enzyme cellulase and its substrate cellulose are:
Cellulase Cellulose
1. Made up of amino acids.
Made up of b-glucose. 2. Peptide bond between
monomers.
Glycosidic bonds (b1-4) between monomers. 3. Ionic bonds,
hydro-phobic interactions and disulphide bonds are present.
These are not present.
4. No rotation of amino acids.
Alternate b-glucose rotated through 180°. 5. Globular in nature. Fibrous in nature.
nn
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Courtesy : The Times of India
UnIT-VII : GEnETICS AnD EVOLUTIOn
ChApTEr-5 : prInCIpLES OF InhErITAnCE AnD VArIATIOn
Multiple Choice Questions
1. If both parents are carriers for thalassaemia, which is an autosomal recessive disorder, what are the chances of pregnancy resulting in an affected child?
(a) 25% (b) 100% (c) No chance (d) 50%
2. A child’s blood group is ‘O’. His parents’ blood groups cannot be
(a) A and B (b) A and A (c) AB and O (d) B and O.
3. Down’s syndrome in humans is due to (a) three ‘X’ chromosomes
(b) three copies of chromosome 21 (c) monosomy
(d) two ‘Y’ chromosomes.
4. If a cross between two individuals produces offsprings with 50% dominant character (A) and 50% recessive character (a), the genotypes of parents are
(a) Aa × Aa (b) Aa × aa (c) AA × aa (d) AA × Aa.
5. Which one of the following conditions correctly describes the manner of determining the sex? (a) Homozygous sex chromosomes (ZZ) determine
female sex in birds.
(b) XO type of sex chromosomes determine male sex in grasshopper.
(c) XO condition in humans as found in Turner’s syndrome, determines female sex.
(d) Homozygous sex chromosomes (XX) produce male in Drosophila.
6. In a typical Mendelian dihybrid cross, one parent is homozygous for both dominant traits and another parent is homozygous for both recessive traits. In the F2 generation , both parental combinations and recombinations appear. The phenotypic ratio of parental combinations to recombinations is
(a) 10 : 6 (b) 12 : 4 (c) 9 : 7 (d) 15 : 1.
7. The existence of non-beneficial alleles in heterozygous genotype within a population is
(a) genetic load (b) genetic drift (c) genetic flow (d) selection.
8. Due to nondisjunction of chromosomes during spermatogenesis, some sperms carry both sex chromosomes (22A + XY) and some sperms do not carry any sex chromosome (22A + O). If these sperms fertilise normal eggs (22A + X), what types of genetic disorders appear among the offsprings?
(a) Klinefelter’s syndrome and Turner’s syndrome (b) Down’s syndrome and Klinefelter’s syndrome (c) Down’s syndrome and Turner’s syndrome (d) Down’s syndrome and Cri-du-chat syndrome
9. Which of the following genotypes of man shows presence of one Barr body?
(a) XY (b) XXXY
(c) XXY (d) All of these
10. Which of the following is not a X-linked recessive disease?
(a) Haemophilia (b) Colour blindness (c) b-thalassaemia
(d) Glucose-6-phosphate dehydrogenase deficiency
11. Haemophilia is a genetic disorder, in which (a) blood fails to coagulate after an injury (b) there is delayed coagulation of blood (c) blood clots in blood vessels
(d) blood cell count falls.
12. Given diagram shows a pair of homologous chromosomes during meiosis.
Maximum crossing over will occur between genes (a) A and a, D and d (b) C and d, c and D (c) B and c, b and C (d) A and d, a and D.
13. Wife is PTC non-taster and husband is PTC taster. Their son is taster but daughters are non-tasters. This is not a sex linked trait. Which pedigree is correct?
(a) (b)
(c) (d)
14. The given Punnett’s square represents the pattern of inheritance in a dihybrid cross where yellow (Y) and round (R) seed condition is dominant over white (y) and wrinkled (r) seed condition.
YR Yr yR yr
YR F J N R
Yr G K O S
yR H L P T
yr I M Q U
A plant of type ‘H’ will produce seeds with the genotype identical to seeds produced by the plants of
(a) Type M (b) Type J (c) Type P (d) Type N.
15. A man having the genotype EEFfGgHH can produce P number of genetically different sperms, and a woman of genotype IiLLMmNn can generate Q number of genetically different eggs. Determine the values of P and Q. (a) P = 4, Q = 4 (b) P = 4, Q = 8 (c) P = 8, Q = 4 (d) P = 8, Q = 8 True or False
16. When IA and IB are present together, and both of
them express their own types of proteins, it is called co-dominance.
17. Pedigree analysis provides a strong tool, which is utilized to trace the inheritance of a specific person.
18. Point mutations are commonly observed in cancer cells.
19. Female butterflies have one Z and one W chromosome, whereas the males have a pair of Z chromosomes, besides the autosomes.
20. Deletions and insertions of base pairs of DNA cause frame-shift mutations.
21. Sickle cell anaemia is caused by substitution of valine (Val) by glutamic acid (Glu) at the sixth position of alpha globin chain of haemoglobin.
22. Physical, psychomotor and mental development is retarded in individuals suffering from trisomy of 21st
chromosome.
23. Heterozygotes always express a phenotype which is a blend of both the homozygous parental phenotypes.
24. Punnett square is a graphical representation to calculate probability of all possible genotypes of offspring.
25. Mendel’s approach of using mathematics to explain biological phenomena was totally new and acceptable to most biologists.
Match The Columns
26. Match Column-I with Column-II.
Column-I Column-II
A. Male heterogamety (i) 13-Trisomy B. Patau’s syndrome (ii) Triticale C. Allopolyploidy (iii) 18-Trisomy D. Autoallopolyploidy (iv) Drosophila E. Turner’s syndrome (v) Helianthus F. Female heterogamety (vi) Monosomy G. Edward’s syndrome (vii) ZW-ZZ system
27. Match Column-I with Column-II. (There can be more than one match for items in Column I).
Column-I Column-II
A. Sex linked disorder (i) Sickle cell anaemia B. Translocation (ii) Haemophilia C. Deletion (iii) Turner’s syndrome D. Autosomal disorder (iv) Ancon sheep E. Discontinuous (v) Cystic fibrosis variation
F. Continuous variation (vi) Colour blindness G. Heteroploidy (vii) Skin colour
(viii) Polydactyly
(ix) Klinefelter’s syndrome (x) Length of fingers (xi) Cri-du-chat syndrome (xii) Chronic myeloid
leukemia
(xiii) Philadelphia
chromosome
(xiv) Intrachromosomal
aberration
passage Based Questions
28.(A) Complete the given passage with appropriate words
or phrases.
(a) The male gametes, in humans, are of two types. The (22 + X) sperms are called (i) , while (22 + Y) sperms are called (ii) . As Y chromosome determines male sex of the embryo, it is called (iii).
(b) The (iv) gene on Y chromosome brings about differentiation of embryonic gonads into testes. In its absence, gonads differentiate into ovaries after (v) week of embryonic development, resulting in female sex, which is therefore called (vi) sex.
(B) Read the given passage and correct the errors, wherever present.
Morgan observed during oogenesis in a few insects, a structure called Y body, which was later named Y chromosome. It is an autosome. Presence of one extra copy of it in a male individual leads to Turner’s syndrome.
Assertion & reason
In each of the following questions, a statement of Assertion (A) is given and a corresponding statement of Reason (R) is given just below it. Of the statements, mark the correct answer as :
(a) if both A and R are true and R is the correct explanation of A
(b) if both A and R are true but R is not the correct explanation of A
(c) if A is true but R is false (d) if both A and R are false.
29. Assertion : Environment easily influences the phenotypic expression of polygenic inheritance. Reason : Polygenic inheritance is related to both quantitative and qualitative inheritance.
30. Assertion : In F2 generation of Mendelian crosses, 1/2 the individuals showed genotype of F1 generation. Reason : In such crosses, the F1 individuals incompletely
expressed the dominant trait.
31. Assertion : Trihybrid cross involves inheritance of three unit factors controlling three distinct characters. Reason : The phenotypic trihybrid ratio in F2 generation is 27 : 9 : 9 : 9 : 9 : 3 : 3 : 1.
32. Assertion : The basic principles of inheritance, as formulated by Mendel are collectively called Mendel’s principles of inheritance or Mendelism.
Reason : Principle of dominance and the principle of segregation are Mendel’s first and second laws, respectively.
33. Assertion : The F2 ratio of a Mendelian dihybrid cross shows two types of recombinants.
Reason : Generally, the alleles of the two different characters are free to assort and combine, independent of each other.
34. Assertion : Radiations occur naturally in the form of UV rays, as well as ionic radiations from space. Reason : UV rays produce thiamine dimers, resulting
