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11.12: Growth Responses - Biology

11.12: Growth Responses - Biology


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A plant’s sensory response to external stimuli relies on chemical messengers (hormones). In contrast, animal hormones are produced in specific glands and transported to a distant site for action, and they act alone.

Plant hormones are a group of unrelated chemical substances that affect plant morphogenesis. Five major plant hormones are traditionally described: auxins (particularly IAA), cytokinins, gibberellins, ethylene, and abscisic acid. In addition, other nutrients and environmental conditions can be characterized as growth factors.

Auxins

The term auxin is derived from the Greek word auxein, which means “to grow.” Auxins are the main hormones responsible for cell elongation in phototropism and gravitropism. They also control the differentiation of meristem into vascular tissue, and promote leaf development and arrangement. While many synthetic auxins are used as herbicides, IAA is the only naturally occurring auxin that shows physiological activity. Apical dominance—the inhibition of lateral bud formation—is triggered by auxins produced in the apical meristem. Flowering, fruit setting and ripening, and inhibition of abscission (leaf falling) are other plant responses under the direct or indirect control of auxins. Auxins also act as a relay for the effects of the blue light and red/far-red responses.

Commercial use of auxins is widespread in plant nurseries and for crop production. IAA is used as a rooting hormone to promote growth of adventitious roots on cuttings and detached leaves. Applying synthetic auxins to tomato plants in greenhouses promotes normal fruit development. Outdoor application of auxin promotes synchronization of fruit setting and dropping to coordinate the harvesting season. Fruits such as seedless cucumbers can be induced to set fruit by treating unfertilized plant flowers with auxins.

Cytokinins

The effect of cytokinins was first reported when it was found that adding the liquid endosperm of coconuts to developing plant embryos in culture stimulated their growth. The stimulating growth factor was found to be cytokinin, a hormone that promotes cytokinesis (cell division). Almost 200 naturally occurring or synthetic cytokinins are known to date. Cytokinins are most abundant in growing tissues, such as roots, embryos, and fruits, where cell division is occurring. Cytokinins are known to delay senescence in leaf tissues, promote mitosis, and stimulate differentiation of the meristem in shoots and roots. Many effects on plant development are under the influence of cytokinins, either in conjunction with auxin or another hormone. For example, apical dominance seems to result from a balance between auxins that inhibit lateral buds, and cytokinins that promote bushier growth.

Gibberellins

Gibberellins (GAs) are a group of about 125 closely related plant hormones that stimulate shoot elongation, seed germination, and fruit and flower maturation. GAs are synthesized in the root and stem apical meristems, young leaves, and seed embryos. In urban areas, GA antagonists are sometimes applied to trees under power lines to control growth and reduce the frequency of pruning.

GAs break dormancy (a state of inhibited growth and development) in the seeds of plants that require exposure to cold or light to germinate. Abscisic acid is a strong antagonist of GA action. Other effects of GAs include gender expression, seedless fruit development, and the delay of senescence in leaves and fruit. Seedless grapes are obtained through standard breeding methods and contain inconspicuous seeds that fail to develop. Because GAs are produced by the seeds, and because fruit development and stem elongation are under GA control, these varieties of grapes would normally produce small fruit in compact clusters. Maturing grapes are routinely treated with GA to promote larger fruit size, as well as looser bunches (longer stems), which reduces the instance of mildew infection (Figure 1).

Abscisic Acid

The plant hormone abscisic acid (ABA) was first discovered as the agent that causes the abscission or dropping of cotton bolls. However, more recent studies indicate that ABA plays only a minor role in the abscission process. ABA accumulates as a response to stressful environmental conditions, such as dehydration, cold temperatures, or shortened day lengths. Its activity counters many of the growth-promoting effects of GAs and auxins. ABA inhibits stem elongation and induces dormancy in lateral buds.

ABA induces dormancy in seeds by blocking germination and promoting the synthesis of storage proteins. Plants adapted to temperate climates require a long period of cold temperature before seeds germinate. This mechanism protects young plants from sprouting too early during unseasonably warm weather in winter. As the hormone gradually breaks down over winter, the seed is released from dormancy and germinates when conditions are favorable in spring. Another effect of ABA is to promote the development of winter buds; it mediates the conversion of the apical meristem into a dormant bud. Low soil moisture causes an increase in ABA, which causes stomata to close, reducing water loss in winter buds.

Ethylene

Ethylene is associated with fruit ripening, flower wilting, and leaf fall. Ethylene is unusual because it is a volatile gas (C2H4). Hundreds of years ago, when gas street lamps were installed in city streets, trees that grew close to lamp posts developed twisted, thickened trunks and shed their leaves earlier than expected. These effects were caused by ethylene volatilizing from the lamps.

Aging tissues (especially senescing leaves) and nodes of stems produce ethylene. The best-known effect of the hormone, however, is the promotion of fruit ripening. Ethylene stimulates the conversion of starch and acids to sugars. Some people store unripe fruit, such as avocadoes, in a sealed paper bag to accelerate ripening; the gas released by the first fruit to mature will speed up the maturation of the remaining fruit. Ethylene also triggers leaf and fruit abscission, flower fading and dropping, and promotes germination in some cereals and sprouting of bulbs and potatoes.

Ethylene is widely used in agriculture. Commercial fruit growers control the timing of fruit ripening with application of the gas. Horticulturalists inhibit leaf dropping in ornamental plants by removing ethylene from greenhouses using fans and ventilation.

Nontraditional Hormones

Recent research has discovered a number of compounds that also influence plant development. Their roles are less understood than the effects of the major hormones described so far.

Jasmonates play a major role in defense responses to herbivory. Their levels increase when a plant is wounded by a predator, resulting in an increase in toxic secondary metabolites. They contribute to the production of volatile compounds that attract natural enemies of predators. For example, chewing of tomato plants by caterpillars leads to an increase in jasmonic acid levels, which in turn triggers the release of volatile compounds that attract predators of the pest.

Oligosaccharins also play a role in plant defense against bacterial and fungal infections. They act locally at the site of injury, and can also be transported to other tissues. Strigolactones promote seed germination in some species and inhibit lateral apical development in the absence of auxins. Strigolactones also play a role in the establishment of mycorrhizae, a mutualistic association of plant roots and fungi. Brassinosteroids are important to many developmental and physiological processes. Signals between these compounds and other hormones, notably auxin and GAs, amplifies their physiological effect. Apical dominance, seed germination, gravitropism, and resistance to freezing are all positively influenced by hormones. Root growth and fruit dropping are inhibited by steroids.


Rewiring T-cell responses to soluble factors with chimeric antigen receptors

Chimeric antigen receptor (CAR)-expressing T cells targeting surface-bound tumor antigens have yielded promising clinical outcomes, with two CD19 CAR-T cell therapies recently receiving FDA approval for the treatment of B-cell malignancies. The adoption of CARs for the recognition of soluble ligands, a distinct class of biomarkers in physiology and disease, could considerably broaden the utility of CARs in disease treatment. In this study, we demonstrate that CAR-T cells can be engineered to respond robustly to diverse soluble ligands, including the CD19 ectodomain, GFP variants, and transforming growth factor beta (TGF-β). We additionally show that CAR signaling in response to soluble ligands relies on ligand-mediated CAR dimerization and that CAR responsiveness to soluble ligands can be fine-tuned by adjusting the mechanical coupling between the CAR's ligand-binding and signaling domains. Our results support a role for mechanotransduction in CAR signaling and demonstrate an approach for systematically engineering immune-cell responses to soluble, extracellular ligands.


Single cells continuously experience and react to mechanical challenges in three-dimensional tissues. Spatial constraints in dense tissues, physical activity, and injury all impose changes in cell shape. How cells can measure shape deformations to ensure correct tissue development and homeostasis remains largely unknown (see the Perspective by Shen and Niethammer). Working independently, Venturini et al. and Lomakin et al. now show that the nucleus can act as an intracellular ruler to measure cellular shape variations. The nuclear envelope provides a gauge of cell deformation and activates a mechanotransduction pathway that controls actomyosin contractility and migration plasticity. The cell nucleus thereby allows cells to adapt their behavior to the local tissue microenvironment.

Science, this issue p. eaba2644, p. eaba2894 see also p. 295


Biology Net

Hi there…..I have uploaded the Biology practical examination question from the year 1975! Refer to “Biology Notes” column and you can download the question there with the answers. It is a guide for students and teachers. Bear in mind that the question format is is not the same for the new practical examination which will be starting in 2015. The new format emphasizes on variables, operational definition, inference etc.I will upload the past examination papers gradually.

Check out the Biology Notes column. Here you can get the common apparatus used in the lab.


14.5) Tropic responses

Plants need light and water for photosynthesis. They have developed responses called tropisms to help make sure they grow towards sources of light and water.

Gravi(geo)tropism: is a response in which plant grows towards or away from gravity.

Phototropism: is a response in which a plant grows towards or away from the direction from which light is coming.

There are two main types of tropisms:

  • positive tropisms – the plant grows towards the stimulus
  • negative tropisms – the plant grows away from the stimulus

Seedlings are good material for experiments on sensitivity because their growing roots (radicals) and shoots respond readily to the stimuli of light and gravity.

Advantages of positive phototropism:

  • Leaves exposed to more sunlight and are able to do more photosynthesis,
  • Flowers can be seen by insects for pollination.
  • The plant gets higher for better seed dispersal.

Advantages of positive geotropism:

  • By growing deeply into the soil, the root fixes the plant into the ground firmly,
  • Roots are able to reach more water,
  • Roots have a larger surface area for more diffusion and osmosis.
  • Auxins are a family of plant hormones.
  • They are mostly made in the tips of the growing stems and roots.
  • Diffuse to other parts of the stems or roots.
  • Is unequally distributed in response to light and gravity.

Auxins change the rate of elongation in plant cells, controlling how long they become.


Biology 2 unit

The new Biology syllabus has been developed using the established NSW Education Standards Authority (NESA) syllabus development process. The syllabus includes Australian curriculum content and reflects the new directions of the Stronger HSC Standards reforms.

(Students in ESL English may struggle with the literacy and language requirements of the Biology course.)

Biology can be studied ON ITS OWN or alongside ONE or TWO other Science courses.

Course Description:

Biology is the science of life. It concerns itself with the ways in which living things interact with each other and the environment and how life has developed during the course of the Earth’s history. Biologists study the structure, function, growth, origin, evolution and distribution of living organisms.

This course covers the rich diversity and interconnectedness of life while exploring solutions to health and sustainability issues in a changing world. Areas of focus will include the role of photosynthesis and respiration ecological relationships in coastal habitats the application of bio technologies to induce genetic change the human body’s immune response to infectious disease and the r ange of technologies used to control, prevent and treat non-infectious disease. Students will study biological processes at small and large scales: from cells to rainforests . The coursework involves, research, experimentation, modelling and case studies.

Related careers areas include:

  • health industry: medical / biochemist / pharmaceutical industry
  • quarantine control
  • ecosystem and resource management / planning / design
  • bush regeneration / conservation management
  • quality control in agriculture, food, pharmaceutical, medicine, public health industries
  • marine science / fisheries / aquaculture
  • food technology
  • pathology

The course builds on particular understandings, skills and attitudes that students have acquired during their K-10 Science course.

With sufficient effort and application over two years, students from classes 10A1 and A2 and the top half of all other classes should be able to make a success of their studies in Biology.


Responses of Organisms to Abiotic Factors of Ecology | Biology

Light is the most important and indispensable physicochemical, abiotic ecological factor without which life cannot exist. Organisms get light from the Sun, Moon, stars, lightning, volcanoes and bioluminescent organisms. Among this light energy from the Sun is the most important in nearly all ecosystems. It is the energy that is used by green plants (which contain chlorophyll) during the process of photosynthesis a process during which plants manufacture organic substances by combining inorganic substances.

The energy from the sun comprises of short, high-energy radiations to long, low energy radiations. The amount of energy in the sunrays just before entering the atmosphere is about 2 cal/cm 2 /min. It is called solar constant. As the sunrays travel through the atmosphere a large amount of energy is absorbed.

Electromagnetic Spectrum:

Sunlight is formed of cosmic rays, gamma rays, X rays, Ultraviolet rays, visible light, infrared rays, radio waves, etc. (Fig. 3). Of these the ultraviolet rays, visible light and infrared rays are biologically significant. Ultraviolet rays have a wavelength of 100 nm-390 nm. They are further classified into three categories as shown in Table 1. UV-C and UV-B are absorbed by the ozone layers in the atmosphere.

Visible light is of greatest importance to plants because it is necessary for photo­synthesis. It falls between the ranges of 340 nm-700 nm. This part of the spectrum is also called the photosynthetically active radiation or PAR. Factors such as quality of light, intensity of light and the length of the light period (day length) play an important part in an ecosystem.

Quality of Light (Wavelength or Colour):

Plants absorb blue and red light during photosynthesis. In terrestrial ecosystems, the quality of light does not change much. In aquatic ecosystems, the quality of light can be a limiting factor. Both blue and red light are absorbed and as a result they do not penetrate deeply into the water. To compensate for this, some algae have additional pigments, which are able to absorb other colours as well.

Light Intensity (‘Strength’ of Light):

The intensity of light that reaches the earth varies according to the latitude and season of the year. The southern hemisphere receives less than 12 hours of sunlight during the period between 21st March and 23rd of September, but receives more than 12 hours of sunlight during the following six months.

In frogs and lizards, bright light makes the skin colour light while dim light makes the skin colour darker. Human skin responds to bright light by tanning.

Day Length (Length of the Light Period):

Certain plants flower only during certain times of the year. One of the reasons for this is that these plants are able to ‘measure’ the length of the night (dark periods). However, it was thought earlier that it is the day length (light periods) to which plants reacted and this phenomenon was termed photoperiodism. Photoperiodism can be defined as the relative lengths of daylight and darkness that affect the physiology and behaviour of an organism.

Accordingly plants are classified as follows:

These plants flower only if they experience nights, which are longer than a certain critical length. Chrysanthemums (Chrysanthemum sp.), the Poinsettias (Euphorbia pulcherrima) and thorn apple (Datura stramonium) are examples of short- day plants.

These plants flower if they experience nights, which are shorter than a certain critical length, Spinach, wheat, barley and radish are examples of long-day plants.

The flowering of day- neutral plants is not influenced by night length. The tomato and the maize plant (Zea mays) are examples of day-neutral plants.

Light requirements of plants differ and as a result distinct layers, or stratification, can be observed in an ecosystem. Plants which grow well in bright sunlight are called heliophytes (Greek helios, Sun) and plants which grow well in shady conditions are known as sciophytes (Greek skia, shade).

Zonation of Light in Aquatic Ecosystems (Fig. 4):

About 10% of light falling on water is reflected back to the atmosphere. The remaining 90% penetrate into the water body. On the basis of penetration of light, the water column of the ocean is divided into three zones supper euphotic zone, a middle disphotic zone and a lower aphotic zone.

The zonation is also categorised as follows:

This zone includes the shallow coastal line where light is available up to the bottom. Rooted vegetation occurs along this zone.

This zone includes the water body to a depth to which light can penetrate. This includes the euphotic zone where abundant light penetration occurs and the disphotic zone where light is received but not sufficient for photosynthesis.

This zone includes the region where there is no light penetration and no producers are present in this region.

The benthic zone includes the bottom of the ocean.

Biological Effects of Light:

High intensity of light increases metabolic activity in animals by increasing enzyme activity.

Light induces photo­chemical reactions in the formation of colour pigments called melanophores. Animals living in cave, bottom of the ocean do not possess colour.

c. Protective Coloration:

Animals develop colour patterns to conceal themselves from predators to bend with the surroundings. For example, the leaf insect, Phyllium is green in colour.

d. Colour Change in Animals:

The Chamaeleon is able to change its colour according to its background. This happens because of the distribution of the melanophores depending on the light entering the eye.

Light enables organisms to see objects in the environment where it is found. Animals possess specific organs to ‘see’ like the eyespots in protozoa, compound eyes in insects and crustaceans, eyes in vertebrates etc. Animals that live in habitats where there is dim light have large eyes that are powerful as in the owls and loris. In animals that live in habitat where there is no light, the eyes are reduced.

Animals are classified into the following categories according to the influence of light on reproduction:

This group of animals are sexually active when the days are long, e.g. birds.

This group are sexually active when the days are short, e.g. sheep, deer, goats.

iii. Day-Neutral Animals:

In this group reproduction is influenced by light, e.g. man, cow.

In the oceans, planktons move to the surface in the early morning and evenings and move to the deeper parts of the ocean when there is high intensity. This movement is called diurnal migration.

The daily rhythm in synchrony with the rotation of the earth is called circadian rhythm. This is endogenous, i.e. initiated by internal factors and is due to a biological clock present in organism. For example, many plants show rhythm of their leaves for sleep. They close or droop during night time and open at daytime. Sleeping and waking in man follow circadian rhythm.

Adaptations of Plants to Changing Light Conditions:

Light requirements of plants differ and as a result distinct layers or stratification can be observed in an ecosystem. Plants which grow well in bright sunlight are called heliophytes (Greek helios, sun) and plants which grow well in shady conditions are known as sciophytes (Greek skia, shade).

Heliophytes have a high rate of respiration and are adapted to high light intensities, while sciophytes have low rate of photosynthesis, respiration, metabolism and growth. The morphological features of heliophytes and sciophytes are summarised in Table 2.

2. Temperature:

Temperature is an ecological abiotic factor. It is a form of energy and is called the thermal energy. It penetrates into each and every region of the biosphere and affects all forms of life. It influences the various stages of life activities such as growth, metabolism, reproduction, movement, distribution, behaviour, death, etc.

Temperature is usually measured in Fahrenheit or Centigrade. The biosphere obtains its thermal energy from the Sun in the form of solar radiation. It is a variable factor. It varies from place-to-place and time-to-time. It is high in the day and at night it is low. It is high at the sea level and low at high altitudes. It is high at the equator and low in the Polar Regions. It is more in the terrestrial habitat and low in the aquatic habitat. The maximum temperature recorded on land is 85°C as in the desert and the lowest temperature is about – 70°C as in Siberia.

Temperature Fluctuations:

The temperature is high during daytime and low at night. This is called diurnal variation. The temperature on land is high at the sea level, but low at high altitudes. Approximately, an increase in altitude of 150 m results in a decrease in 1°C temperature. On land, maximum temperature is found along the equator. It gradually decreases towards the poles. Temperature varies according to the season. The temperature reaches its maximum during summer, while it is minimal during winter.

Temperature fluctuation in aquatic habitat is less than that of terrestrial habitat.

Thermal Stratification:

In lakes and ponds a gradual decrease in temperature from the surface to the bottom is seen. This leads to different layers of water with different temperatures. The arrangement of different layers based on temperature differences is called thermal stratification.

Two types of stratification are observed:

a. Summer stratification and

a. Summer Stratification:

During summer there are three distinct layers as shown in Fig. 5.

i. Upper Layer or Epilimnion:

The epilimnion is warm and the temperature fluctuates with the temperature of the atmosphere.

ii. Lower Layer or Hypolimnion:

The bottom layer is the hypolimnion. The temperature is about 5-7°C.

iii. Middle Layer or Thermocline or Metalimnion:

The thermocline is characterised by a gradation of temperature from top (at about 21°C) to bottom (at about 7°C). This zone is also called the transition zone.

b. Winter Stratification:

During winter only two layers are seen, an upper layer of ice and a lower layer of water column which is at 4°C. (Fig. 5).

Biological Effects of Temperature:

a. Eurythermal and Stenothermal Organisms:

Organisms that can tolerate wide range of temperature fluctuations are called eurythermal organisms, e.g. man, lizard, amphibians. Those that cannot tolerate wide range of temperature fluctuations are called stenothermal organisms, e.g. corals, snails

b. Poikilothermic and Homeothermic Animals:

Animals in which the body temperature changes according to the fluctuations in the environmental temperature are called poikilothermic or cold-blooded animals or ectotherms. During cold, the body temperature also drops. For example, all animals except birds and mammals.

In birds and mammals, the body temperature remains constant and is not dependent on environment temperature. These animals are called homeotherms or warm- blooded or endotherms. When the environment temperature drops the animal maintains its temperature by metabolic activities.

Hibernation, aestivation, migration are some behavioural adaptations of animals.

Effect of Temperature on Growth and Development:

Temperature affects growth and development of animals. For example, the oyster, Ostraea virginica grows to 1.4 mm when it is reared at 10°C, but when reared at 20°C it grows to 10.3 mm. Similarly, the eggs of the mackerel fish does not develop below 8°C and above 25°C. Low temperature prevents metamorphosis in salamanders and makes the animal neotenous.

Effect of Temperature on Morphology:

The morphological characters of organisms are altered by temperature. Temperature influences the size of animals and the relative proportions of the parts of the body. Three rules have been put forth to understand how the temperature influences various characteristic features.

The mammals in colder areas are larger is size than in warmer climates. This is called the Bergman’s rule. For example, the penguins found in Antarctica attain a body length of 100- 200cm, whereas the penguins of equatorial Galapagos Islands are about 49cm long.

According to Allen’s rule, extremities of the mammals, like the tail, snout, ears and legs are relatively shorter in colder regions than in warmer regions. In the Arctic rabbit the ears are shorter, while in the Californian rabbit, the ears are longer.

The explanation in both the cases is that endothermic organisms in colder climates should have smaller surface area relative to volume across which they lose heat. Allen’s rule has widespread applicability when compared to Bergman’s rule because of number of factors that affect body size, though it is true at an intra-specific level.

According to Gloger’s rule the animals in the tropic are darker and heavily pigmented than their counterparts of the colder and dry regions.

Effect of Temperature on Distribution:

Temperature is a limiting factor on the distribution of animals. The distribution of warm-blooded animals is not affected by temperature. But cold-blooded animals are abundant in tropical and temperate regions, and their number rapidly diminishes towards the poles.

Effect of Temperature on Plants:

a. The opening of the flowers of various plants during the day and night is often due to temperature difference between the day and night.

b. The seed of some plants (biennials) normally germinate in the spring or summer. These seeds require a cold treatment of winter. This is called vernalisation. Vernalisation can be induced in seeds artificially. This adaptation ensures that seeds do not germinate during autumn, but only after winter, when the seedlings have better chances to survive.

c. Deciduous trees lose their leaves in winter and enter into a state of dormancy, where the buds are covered for protection against the cold.

d. In the desert due to great temperature variation between day and night organisms exhibit distinct periods of activity, e.g. many cacti flower at night are pollinated by nocturnal insects. Cactus is among the most drought-resistant plants on the planet.

i. Leaves are modified into spines. These spines protect the plant from animals, shade it from the Sun and also collect moisture. This also reduces transpiration.

ii. Extensive shallow root systems that are spread out just below the surface to allow the plant to absorb water immediately as it rains.

iii. Succulent stems have the ability to store water. This enables the cacti to survive in dry climate and can survive years of drought on the water collected from a single rainfall.

iv. Waxy skin to seal in moisture.

v. Cacti depend on chlorophyll in the outer tissue of stems to conduct photosynthesis for the manufacture of food.

vi. Cacti close their stomata during the day and open them at night to reduce transpiration. These plants exhibit the CAM pathway of photosynthesis.

Many other desert trees and shrubs have also adapted by eliminating leaves – replacing them with thorns, not spines, or by greatly reducing leaf size to eliminate transpiration. Many xerophytes may accumulate proline in the cells of its leaves to maintain osmotic and water potential. Chaperonins, the heat shock proteins provide physiological adaptations to plants to high temperatures. These proteins maintain the structures and avoid denaturation of other proteins.

e. Plants living in cold climates can tolerate frost conditions. When the temperature drops the plant becomes dormant and exhibits slow rate of photosynthesis and respiration. Antifreeze proteins are found in some plants which avoid chilling and frost damage by increasing their sugars and alcohols to lower the freezing point of cell fluids. This causes super cooling of the cell sap for short periods of time without causing freezing.

Structural Adaptations in a Camel

In hot deserts, temperature is very high. To escape from the heat desert animals have the different adaptations for resistance to heat. This can be understood from the adaptations of a camel: All desert dwellers have adapted to conserve water, food and energy. The camel is one of the best survivors in the desert and it is rightly called the ‘ship of the desert’ because it is adapted very well to the conditions of the desert.

The adaptations in the camel are briefly described below:

a. The camel can store fat in their hump, which is used as energy source whenever food is scarce.

b. Camels have long legs to keep the heat away from the body.

c. Camels have long eye lashes and small ears with lots of hair. They can also close their nostrils. These adaptive features keep them from getting sand in their eyes, ears and nose during sandstorms.

d. The body temperature of the camel ranges from 34°C (93°F) at night up to 41°C (106°F) during the day. Only above this threshold they start to sweat. This allows them to preserve about 5 litres of water a day.

e. A camel can manage for up to 2 weeks without water, and can drink 200 litres at the same time.

f. They have flat padded feet, which are perfect for walking on loose, hot sand.

g. The red blood cells of the camel are oval in shape, unlike those of other animals, which are circular. This facilitates their movement in a dehydrated state.

h. The thick coat of the camel reflects sunlight. It also insulates them from the intense heat that radiates from hot desert sand. The long legs also help by keeping the body further away from the sand. Thick fur and under wool provide warmth during cold desert nights and some insulation against daytime heat.

i. In camels, the body temperature is labile. During day the body temperature rises to 40.6°C, while at night it drops to 33.8°C.

Regular change in temperature at specific intervals of time is called thermoperiodicity.

a. Diurnal Thermoperiodicity:

The change in temperature in a 24-hour period is called diurnal thermoperiodicity. Animal’s active during the day is called diurnal and those active during the night is called nocturnal.

b. Seasonal Thermoperiodicity:

The variation in temperature in the different seasons of the year is called seasonal thermoperiodicity. It controls important events such as seed germination, flowering, fruiting, leaf shedd­ing, etc. in plants. It also affects growth, development, morphology and coloration in animals.

3. Water:

Water covers 70% of the earth’s surface and is found as vapour in the atmosphere and in the soil as soil water. 97% of the water is found in the oceans and 3% is found as freshwater. Approximately 70% of freshwater is found as ice caps and glaciers, 20% as underground water while the remaining is found in lakes, streams and rivers. Water is essential for life and all organisms depend on it to survive in especially desert areas.

The Water Cycle in Nature:

Water cycles through the biosphere and is constantly exchanged between the physical and the biotic environment. The circulation of water that does not involve living organism is the global water cycle and that which involves living systems is the biological water cycle.

The water or hydrologic cycle is depicted in Fig. 6. Water evaporates from the oceans and bodies of freshwater when the sun’s rays falls on it. Vapourised freshwater rises into the atmosphere, forms clouds, cools and falls as rain over the oceans and the land.

When it rains, some of the water sinks or percolates into the ground and saturates the Earth to a certain level. The top of the saturation level is called the ground water table or simply the water table. Ground water is also sometimes located in a porous layer, called an aquifer, which lies between two sloping layers of impervious rock.

Ground water comes back to the surface naturally as springs or mechanically by pumps or making wells. Water also evaporates from these places to the atmosphere. This completes the global water cycle.

Organisms also use water and become part of the water cycle. Plants absorb water from the soil and return it back to the atmosphere by respiration and transpiration. Animals drink water from water bodies and by eating plants and return water back to the environment by respiration and excretion.

Death and decay of organisms also add water to the physical environment. Water returned to the physical environment forms clouds that come down as rain for being utilised by the organisms. This comprises the biological water cycle.

Adaptations of Plants in Water:

Water constitutes the hydrosphere and includes both fresh and seawater. Aquatic plants are called hydrophytes. These plants possess specialised parenchyma called aerenchyma that possesses air filled spaces in the leaves and stem. This enables the plants to float.

The different types of hydrophytes are summarised in Table 3:

Adaptations of Animals in Aquatic Habitat:

A number of animals live in the aquatic medium, i.e. water. There are animals that are found exclusively in the fresh water, while there are some that are found living in the marine environment there are some that are capable of living in both fresh and marine water. A few examples of animals that are aquatic are vertebrates like fish, mammals (whales, dolphins, seals, sea lions, etc.), invertebrates like starfish, prawns, lobsters, octopus, oysters, etc.

Adaptations in animals living in water are called aquatic adaptations and organisms living in water are called aquatic organisms. Aquatic organisms found on the surface of the water are called pelagic organisms for which they possess special adaptations. Similarly organisms living in the deep sea, called benthic animals are adapted to live in such conditions.

Adaptations of fish are briefly discussed below:

The body of fish is streamlined or boat shaped and therefore it offers little resistance to swimming.

The fins are outgrowths of the body. There are different types of fins such as the pectoral fins, pelvic fins, dorsal fins, anal fins and the caudal fins. These help in locomotion in water. The pectoral, pelvic and dorsal fins act as balancers while the caudal fins act as rudder to change directions.

Fish possess gills that enable exchange of gases between the blood and the surrounding water.

d. Lateral Line Sense Organs:

Lateral line sense organs are canals that extend the entire length of the body. These are filled with mucous and water and contain specialised organs to detect temperature, pressure and mechanical disturbances in water. They can help in echolocation of objects like food and prey.

The swim or air bladder in some fish helps in swimming or serves as a hydrostatic organ, or as a sense organ or even as a sound producing organ. The fish can fill or empty the bladder and the fish can float or sink lower in water.

f. Scales and Mucus Glands:

Scales protect the body of the fish. The mucous glands secrete mucous and prevent the diffusion of water through the skin.

4. Air:

Air or atmosphere is the gaseous envelope that surrounds the lithosphere and hydrosphere. The atmosphere is a mixture of gases. Nitrogen makes up four-fifths of it and oxygen makes up a little more than one-fifth. Small quantities of other gases like argon, neon, helium, krypton, xenon, carbon dioxide, hydrogen and ozone are also found.

The most important gases used by plants and animals are oxygen, carbon dioxide and nitrogen.

Oxygen is used by all living organisms during respiration.

Carbon dioxide is used by green plants during photo-synthesis.

Nitrogen is made available to the plants by certain bacteria and through the action of lightning.

Layers of the Atmosphere:

The atmosphere is made of five or more dis­tinct layers that differ in density, temperature, composition and properties (Fig. 7).

d. Thermosphere – 70-400 kms

e. Exosphere – 400 kms and beyond

a. Troposphere (0-10 kms):

The troposphere is the layer with which organisms have intimate contact and it is the seat of weather and climate. It is the densest part of the atmosphere and air pressure drops with increasing altitude. It contains more water vapour and carbon dioxide than any other layer. These two gases affect the heat balance of the Earth.

The temperature at the ground is around 25°C and it drops to about 5°C every km of altitude gained, until it reaches a low of around -60°C at about 10-11 kms. The upper limit of the troposphere is known as tropopause. The branch of the atmospheric science that deals with the characteristics of the troposphere is called ‘micrometeorology’.

b. Stratosphere (10-40 kms):

It is less dense than the troposphere. It contains much the same gases except that there is less water vapour. At about 25 kms is a concentrated layer of ozone. This zone is known as the ozonosphere. This layer absorbs most of the ultraviolet radiation of the Sun. From a low of 60°C at 10 kms, the temperature slowly rises to about the base of the overlying mesosphere.

c. Mesosphere (40-70 kms):

The composition of gases are the same but less dense than the stratosphere. The mesosphere has a layer of ionised or electrified air at 50- 70 kms above the Earth. It is caused by the action of the solar ultraviolet radiation on the air molecules and is charged with electrons. Ozone is also found by the action of UV and X rays on oxygen. The temperature drops to about -90°C at about 80 kms above the surface of the earth.

d. Thermosphere (70-400 kms):

The thermosphere is radically different from the other atmospheric layers. Ozone, carbon di­oxide and water are virtually absent. The density is very low, but is dense enough to burn up fast moving meteors. Most of the gas atoms in this layer are electrically charged by the radiation of the Sun.

Three distinct ionised regions are found in this layer, E, F1 and F2 layers. The E layer is found at an altitude of 90-120 kms and is made of nitrogen and oxygen. The F1 has oxygen atoms and in the F2 layer nitrogen ions are predominantly found.

The thermospheric layers are important for communication. They reflect radio waves back to the Earth. There is a wide range of temperature in the thermosphere from a low of about -90°C at 80 kms altitude to several 1000°C at about 500 kms and higher. The motions of ionised gas generate electricity, which in turn causes variations in the Earth’s magnetic field.

e. Exosphere (400 kms and Higher):

This layer extends into space. The gases in this layer are extremely thin. Hydrogen is the chief constituent of this layer.

5. Wind:

Differential solar radiation in different regions of the Earth as well as rotation of the Earth causes air to move and form wind. Depending upon the velocity of the wind it is called breeze, gale, storm or hurricane. Dust storms and squall are also modified forms of wind the former carries dust particles while the latter carries rain or snow.

Winds or air currents arise on a world-wide scale as a result of a complex interaction between hot air expanding and rising (convection) in the mid-latitudes. This has various effects on the rotation of the Earth and results in a centrifugal force which tends to lift the air at the equator. This force is known as the Coriolis force and tends to deflect winds to the left of the southern hemisphere and to the right in the northern hemisphere. Winds carry water vapour, which may condense and fall in the form of rain, snow or hail.

Wind plays a role in pollination and seed dispersal of some plants, as well as the dispersal of some animals, such as insects. Wind erosion can remove and redistribute topsoil, especially where vegetation has been reduced. Warm berg wind results in desiccation, which creates a fire hazard. If plants are exposed to strong prevailing winds are they usually smaller than those in less windy conditions.


Availability of data and materials

Data generated and analyzed during this study are included in the published article and its supplementary information files. Additional files 3 and 4 contain all the raw data and the corresponding calculations for the primary screen of light sensitivity for the 4686 deletion mutants and the confirmation assay #1, respectively. Additional file 5 provides individual values for figures where the number of independent replicates is less than 6 (n < 6).


Disengagement of light responses in Arabidopsis by localized developmental factors

Plant development and growth are profoundly influenced by environmental cues such as light intensity and composition. In particular, changes in red (600 nm to 700 nm) and far-red (700 nm to 750 nm) light inform about the threat of competing plants nearby, which deplete red light and generate an unfavorable shade environment enriched in far-red light ( 1 ). Plants detect red and far-red light using an evolutionarily conserved family of photoreceptors named phytochromes, which are activated by red light and inactivated by far-red light ( 2 ). Phytochromes play an important role in shaping plant architecture, in part, by restricting the rate of stem growth ( 3 ). When plants encounter a canopy of neighboring plants, the far-red light-enriched shade environment inactivates phytochromes to promote stem growth, thereby allowing the plants to escape shade via the so-called shade avoidance response ( 4 , 5 ).

In the plant model species Arabidopsis thaliana (Arabidopsis), phytochromes are ubiquitously expressed in all tissue/organ types throughout the life cycle ( 6 ). This expression pattern enables all tissues/organs to continuously monitor and respond to changes in the local light environment. However, intriguingly, shade does not elicit growth in all stem tissues: While the embryonic stem (hypocotyl) and the leaf petiole are exquisitely sensitive to shade, the internodes—which are located at the boundary region connecting the hypocotyl, the bases of the leaves, and the shoot apical meristem—are almost completely unresponsive (Fig. 1A) ( 7 ). The lack of …


P LANT C ELLULAR AND M OLECULAR R ESPONSES TO H IGH S ALINITY

AbstractPlant responses to salinity stress are reviewed with emphasis on molecular mechanisms of signal transduction and on the physiological consequences of altered gene expression that affect biochemical reactions downstream of stress sensing. We make extensive use of comparisons with model organisms, halophytic plants, and yeast, which provide a paradigm for many responses to salinity exhibited by stress-sensitive plants. Among biochemical responses, we emphasize osmolyte biosynthesis and function, water flux control, and membrane transport of ions for maintenance and re-establishment of homeostasis. The advances in understanding the effectiveness of stress responses, and distinctions between pathology and adaptive advantage, are increasingly based on transgenic plant and mutant analyses, in particular the analysis of Arabidopsis mutants defective in elements of stress signal transduction pathways. We summarize evidence for plant stress signaling systems, some of which have components analogous to those that regulate osmotic stress responses of yeast. There is evidence also of signaling cascades that are not known to exist in the unicellular eukaryote, some that presumably function in intercellular coordination or regulation of effector genes in a cell-/tissue-specific context required for tolerance of plants. A complex set of stress-responsive transcription factors is emerging. The imminent availability of genomic DNA sequences and global and cell-specific transcript expression data, combined with determinant identification based on gain- and loss-of-function molecular genetics, will provide the infrastructure for functional physiological dissection of salt tolerance determinants in an organismal context. Furthermore, protein interaction analysis and evaluation of allelism, additivity, and epistasis allow determination of ordered relationships between stress signaling components. Finally, genetic activation and suppression screens will lead inevitably to an understanding of the interrelationships of the multiple signaling systems that control stress-adaptive responses in plants.


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