Current Affairs 11th Class

A most complex tissue in the body, composed of densely packed interconnected nerve cells called neurons (as many as \[{{10}^{10}}\] in the human brain). It specialized in communication between the various parts of the body and in integration of their activities. Nervous tissue is ectodermal (from neural plate) in origin. It forms the nervous system of the body which controls and coordinates the body functions. There is no intercellular matrix between neurons. These have permanently lost the power of division as have no centriole and have minimum power of regeneration. So these cannot be cultured in vitro. Irritability is the main function of nervous tissue. Composition of nervous tissue : Nervous tissue is formed of four types of cells : (1) Neurons (nerve cells)              (2) Neuroglia (3) Ependymal cells                         (4) Neuro-secretory cells Neurons A neuron is a nerve cell with all its branches. Neuron is formed from neuroblast. It is the structural and functional unit of nervous system. It is the longest cell of the body. (1) Cyton : It is also called perikaryon or soma or cell body. Its granular cytoplasm is called neuroplasm which has following structures : (i) A large, spherical, centrally placed nucleus with a single nucleolus. (ii) Numerous fine threads called neurofibrils for the conduction of nerve impulses. (iii) A number of small, basophilic granules called Nissl’s granules formed of rough endoplasmic reticulum with ribosomes and are sites of protein synthesis. (iv) Neuroplasm has large number of mitochondria to provide high energy for impulse conduction. (v) Neuroplasm may have melanophores with melanin pigment and lipochromes with orange or yellow pigment. (vi) A mature neuron has no centriole, so it cannot divide. (vii) A “Barr body” is often seen abutting against the inner surface of nuclear membrane of cytons in females. This has been proved to be a transformed ‘X’ chromosome. (viii) Certain neurons having flask-shaped cytons and called purkinje cells, occur in the cerebellum of the brain. (2) Neuron processes : The processes of neurons, called neurites, extend varying distances from the cyton and are of two types – dendrites or dendrons and an axon or axis cylinder (neuraxon). (i) Dendron : These are several short, tapering much branched processes. The dendrites contain neurofibrils, neurotubules, Nissl’s granules and mitochondria. They conduct nerve impulse towards the cell body. (ii) Axon : This is a single very long, cylindrical process of uniform diameter. It arises from a conical projection, the axon hillock, of the cyton. The axon contains neurofibrils and neurotubules but lacks Nissl’s granules. Axon is usually branched only terminally into slender branches called telodendria. The latter have knobbed ends called endbulbs or axon terminals or buttons or synaptic knobs or end plates. The synaptic knobs contain mitochondria and secretory vesicles.       Types of neurons : Neurons are divided into different categories on different basis. (1) On the basis of functions : Neurons are divided into three categories : Sensory (afferent) more...

It provide support and surface for attachment of muscle. Skeletal connective tissue form the frame work of body. It provide rigidity to body. These protect the various organ and help in locomotion. It is of three types : Cartilage, Bones, Notochord. Cartilage Cartilage is a solid but semi-rigid and flexible connective tissue. Cartilage is a nonvascular connective tissue, consisting of cells embeded in a resilent matrix of chondrin. Chondrin is a protein of cartilage. Regeneration of cartilage can occur from its peri-chondrium. Cartilage is said to be metabolically nearly inactive. In kids the cartilage cells show 2 types of growth. (1) Appositional or Perichondral or Secondary or Exogenous growth : It is due to deposition of matrix and division of chondrogenic cells of periphery. It leads to growth in thickness. (2) Endogenous or Interstitial growth : It is due to deposition of matrix and division in inner cells of cartilage. It leads to growth in size. Types of cartilage : It is of following types – (1) Hyaline cartilage : It is most primitive and glass like cartilage. Its matrix is transparent homogenous and pearly white or bluish green in colour, contain chondrin. It is slightly elastic and also known as articular cartilage because it forms the articular surface of joints. Hyaline cartilage is found in trachea, larynx and bronchi, limb bones (called hyaline cap), sternum, in the hyoid apparatus nasal septum, ribs (sternal parts) larynx (cricoid, thyroid), nasal cartilage (nasal septum).      (2) Fibro cartilage (White fibrous cartilage) : In this cartilage, the small amount of matrix of cartilage is packed with large number of bundles of thick white (collagen) fibres. So it is toughest and less flexible. It is found in intervertebral discs and acts as shock absorber. It is also found in pubic symphysis and helps in parturition (child birth). The intervertebral discs remain contracted when the body is active, but relaxed when the body is at rest. That is why, our body becomes a bit taller during sleep and after death.     (3) Elastic cartilage (Yellow elastic cartilage) : In this cartilage, the matrix is packed with yellow or elastic fibres which run in all directions to form a network. Owing to the presence of yellow fibres, it is very flexible. It gives recoiling power to structures. It is found in mammalian pinna, pharyngotympanic tube, epiglottis, some laryngeal and bronchiolar cartilages.     (4) Calcified cartilage : It is modified hyaline cartilage, It is hard and non elastic due to deposition of calcium salt-hydroxy appetite  in matrix. It is found in pubis of old frog, supra-scapula of frog, quadrate cartilage of frog, shark vertebrae, in man ends of long bone, head of humerus and femur. Calcification may also occur as a regular more...

It is a mobile connective tissue derived from mesoderm which consists of fibre-free fluid matrix and specialised living cells that are not formed in situ, can neither divide nor secrete matrix. Vascular tissue regularly circulates in the body, takes part in transport of material and performs such activities as scavenging healing of wounds and defence against pathogens. Vascular tissue is of two types, blood and lymph, Blood In chordates, and in annelids amongst the non chordates, the blood is a red and opaque fluid of salty taste and peculiar smell. It is a little heavier than water. The study of blood is called haematology. It is red coloured liquid connective tissue which originates from the mesoderm. It reaches into the various organs through the blood vessels and transports various chemical substances between different tissues. During embryonic state, the blood is mainly formed in the liver but little blood is also formed in the spleen and ribs. In adults, the blood is formed in the red bone marrow. The blood formation is called as haemopoiesis. Viscosity \[\,4.7,\text{ }{{p}^{H}}7.4\] Specific gravity \[\text{ }10.4\text{ }1.07\] Volume \[\text{ }5-6\text{ }litre/70\text{ }Kg~\,or\text{ }1/{{13}^{th}}\] part of total body weight Plasma It constitutes about 5% of body weight. It represents matrix of blood. Plasma is slightly alkaline and transparent. It forms 55-60% by volume of blood. Plasma contains : Water \[(91-92%),\] Solid \[(8-9%).\]Plasma solid part consists of organic (7%) and inorganic (1%) substances which are as follows : Organic constituents of plasma : Some are its own constituents, while others are those which are transported by it. All these are divisible into following categories : (1) Plasma proteins : Protein constitute about 7% part of plasma and remain in it as colloid particles. These mainly include albumins, globulins, prothrombin and fibrinogen. Globulins are mainly formed by plasma cells in lymphoid organs. Other plasma proteins are mainly formed in liver. These render the plasma viscous, and maintain its osmotic pressure (7.5 atmospheric) and pH. Prothrombin and Fibrinogen are essential for blood clotting. Albumins are mainly responsible for maintaining osmotic pressure in plasma and for osmoregulation in cells and tissue fluids. Globulins help in osmoregulation and transport of proteins and other substances, but most globulins are immunoglobulins, which act as antibodies, destroying harmful bacteria, virus and toxins in blood and tissue fluids. Some proteins, acting as enzymes, also occur in the plasma. (2) Digested nutrients : These include glucose, fats, fatty acids, phospholipids, cholesterol, nucleosides, amino acids, vitamins etc. These are the supplied by the blood to all cells of body. (3) Excretory substances : These chiefly include ammonia collected by blood from body cells and urea, uric acid, creatine, creatinine etc., collected mainly from the liver and transported to kidneys for excretion. (4) Hormones : These are secreted and released in blood by endocrine glands. (5) Dissolved gases : Each 100 ml. of water of blood plasma contains about \[0.29\text{ }ml\]of \[{{O}_{2}},\text{ }5\text{ }ml.\] of \[C{{O}_{2}}\] and 0.5 ml of nitrogen dissolved in it. (6) Defence compounds : more...

Like green plants, some purple and green sulphur bacteria are capable of synthesizing their organic food in presence of light and in absence of \[{{O}_{2}},\]which is known as bacterial photosynthesis. Van Niel was the first to point out these similarities. Oxygen is not liberated in bacteria during process of photosynthesis. Their photosynthesis is non-oxygenic. Because bacteria use \[{{H}_{2}}S\]in place of water \[({{H}_{2}}O)\] as hydrogen donor. Photosynthetic bacteria are anaerobic. Only one type of pigment system (PSI) is found in bacteria except cyanobacteria which possess both PSI and PSII. Bacteria has two type of photosynthetic pigments. Bacteriochlorophyll and Bacterioviridin. The photosynthetic bacteria fall under three categories (1) Green sulphur bacteria : It contains chlorobium chlorophyll, which absorb 720-750nm (far red light) of wavelength of light. e.g., Chlorobium. (2) Purple sulphur bacteria : e.g., Chromatium. (3) Purple non-sulphur bacteria : e.g., Rhodospirillum, Rhodopseudomonas. Characteristics of bacterial photosynthesis are : (1) No definite chloroplasts but contain simple structures having pigments called chromatophores (term coined by Schmitz). (2) Contain chlorobium chlorophyll or bacterio-chlorophyll. (3) Use longer wavelengths of light \[(720-950nm).\] (4) No utilization of \[{{H}_{2}}O\](but use \[{{H}_{2}}S\]or other reduced organic and inorganic substances). (5) No evolution of \[{{O}_{2}}.\] (6) Photoreductant is \[NAD{{H}_{2}}(Not\,\,NADP{{H}_{2}}).\] (7) Only one photoact and hence one pigment system and thus one reaction centre, i.e., \[{{P}_{890}}.\] (8) Cyclic photophosphorylation is dominant. (9) It occurs in presence of light and in absence of \[{{O}_{2}}.\]

Some forms of bacteria obtain energy by chemosynthesis. This process of carbohydrate formation in which organisms use chemical reactions to obtain energy from inorganic compounds is called chemosynthesis. Such chemoautotrophic bacteria do not require light and synthesize all organic cell requirements from \[C{{O}_{2}}\] and \[{{H}_{2}}O\] and salts at the expense of oxidation of inorganic substances like (\[{{H}_{2}},N{{O}_{3}}^{},S{{O}_{4}}\]or carbonate). Some examples of chemosynthesis are : (1) Nitrifying bacteria : e.g., Nitrosomonas, Nitrosococcus, Nitrobacter etc. (2) Sulphur bacteria : e.g., Beggiatoa, Thiothrix and Thiobacillus. (3) Iron bacteria : e.g., Ferrobacillus, Leptothrix and Cladothrix. (4) Hydrogen bacteria : e.g., Bacillus pentotrophus (5) Carbon bacteria : e.g., Carboxydomonas, Bacillus oligocarbophilus.

Blackmann's law of limiting factors F.F. Blackmann (1905) proposed the law of limiting factors according to which 'when process is conditioned to its rapidity by a number of factors, the rate of process is limited by the pace of the slowest factor'.  is usually a limiting factor in photosynthesis under field conditions particularly on clear summer days under adequate water supply. Blackmann's law of limiting factor is modification of Liebig's law of minimum, which states that rate of process controlled by several factors is only as rapid as the slowest factor permits. Theory of three cardinal points was given by Sachs in 1860. According to this concept, there is minimum, optimum and maximum for each factor. For every factor, there is a minimum value when photosynthesis starts, an optimum value showing highest rate and a maximum value, above which photosynthesis fails to take place. Factors : The rate of photosynthetic process is affected by several external (Environmental) and internal factors. External factors (1) Light : The ultimate source of light for photosynthesis in green plants is solar radiation, which moves in the form of electromagnetic waves. Out of the total solar energy reaching to the earth about 2% is used in photosynthesis and about 10% is used in other metabolic activities. Light varies in intensity, quality (Wavelength) and duration. The effect of light on photosynthesis can be studied under these three headings. (i) Light intensity : The total light perceived by a plant depends on its general form (viz., height, size of leaves, etc.) and arrangement of leaves. Of the total light falling on a leaf, about 80% is absorbed, 10% is reflected and 10% is transmitted. In general, rate of photosynthesis is more in intense light than diffused light. (Upto 10% light is utilized in sugarcane, i.e., Most efficient converter). Another photosynthetic superstar of field growing plants is Oenothera claviformis (Winter evening-primrose), which utilizes about 8% light. However, this light intensity varies from plant to plant, e.g., more in heliophytes (sun loving plants) and less in sciophytes (shade loving plants). For a complete plant, rate of photosynthesis increases with increase in light intensity, except very high light intensity where 'Solarization' phenomenon occurs, i.e., photo-oxidation of different cellular components including chlorophyll occurs. It also affects the opening and closing of stomata thereby affecting the gaseous exchange. The value of light saturation at which further increase is not accompanied by an increase in uptake is called light saturation point. (ii) Light quality : Photosynthetic pigments absorb visible part of the radiation i.e.,  to For example, chlorophyll absorbs blue and red light. Usually plants show high rate of photosynthesis in the blue and red light. Maximum photosynthesis has been observed in red light than in blue light. The green light has minimum effect. On the other hand, red algae shows maximum photosynthesis in green light and brown algae in blue light. (iii) Duration of light : Longer duration of light period favours photosynthesis. Generally, if the plants get 10 to 12hrs light per more...

Before seventeenth century it was considered that plants take their food from the soil.

• Van Helmont (1648) concluded that all food of the plant is derived from water and not from soil.
• Stephen Hales (Father of Plant Physiology) (1727) reported that plants obtain a part of their nutrition from air and light may also play a role in this process.
• Joseph Priestley (1772) demonstrated that green plants (mint plant) purify the foul air (i.e., Phlogiston), produced by burning of candle, and convert it into pure air (i.e., Dephlogiston).
• Jan Ingen-Housz (1779) concluded by his experiment that purification of air was done by green parts of plant only and that too in the presence of sunlight. Green leaves and stalks liberate dephlogisticated air (Having \[{{O}_{2}}\]) during sunlight and phlogisticated air (Having \[C{{O}_{2}}\]) during dark.
• Jean Senebier (1782) proved that plants absorb \[C{{O}_{2}}\]and release \[{{O}_{2}}\] in presence of light. He also showed that the rate of \[{{O}_{2}}\] evolution depends upon the rate of \[C{{O}_{2}}\]consumption.
• Nicolus de Saussure (1804) showed the importance of water in the process of photosynthesis. He further showed that the amount of \[C{{O}_{2}}\]absorbed is equal to the amount of \[{{O}_{2}}\] released.
• Julius Robert Mayer (1845) proposed that light has radiant energy and this radiant energy is converted to chemical energy by plants, which serves to maintain life of the plants and also animals.
• Liebig (1845) indicated that main source of carbon in plants is \[C{{O}_{2}}.\]
• Bousingault (1860) reported that the volume of \[C{{O}_{2}}\] absorbed is equal to volume of \[{{O}_{2}}\] evolved and that \[C{{O}_{2}}\] absorption and \[{{O}_{2}}\] evolution get start immediately after the plant was exposed to sunlight.
• Julius Von Sachs (1862) demonstrated that first visible product of photosynthesis is starch. He also showed that chlorophyll is confined to the chloroplasts.
• Melvin Calvin (1954) traced the path of carbon in photosynthesis (Associated with dark reactions) and gave the \[{{C}_{3}}\] cycle (Now named Calvin cycle). He was awarded Nobel prize in 1961 for the technique to trace metabolic pathway by using radioactive isotope.
• Huber, Michel and Deisenhofer (1985) crystallised the photosynthetic reaction center from the purple photosynthetic bacterium, Rhodopseudomonas viridis. They analysed its structure by X-ray diffraction technique. In 1988 they were awarded Nobel prize in chemistry for this work.

On the basis of discovery of Nicolas de Saussure that "The amount of \[{{O}_{2}}\] released from plants is equal to the amount of \[C{{O}_{2}}\] absorbed by plants", it was considered that \[{{O}_{2}}\] released in photosynthesis comes from \[C{{O}_{2}},\] but Ruben proved that this concept is wrong. In 1930, C.B. Van Niel proved that, sulphur bacteria use \[{{H}_{2}}S\](in place of water) and \[C{{O}_{2}}\] to synthesize carbohydrates as follows: \[6C{{O}_{2}}+12{{H}_{2}}S\xrightarrow{\,\,\,\,}{{C}_{6}}{{H}_{12}}{{O}_{6}}+6{{H}_{2}}O+12S\] This led Van Niel to the postulation that in green plants, water \[({{H}_{2}}O)\] is utilized in place of \[{{H}_{2}}S\] and \[{{O}_{2}}\] is evolved in place of sulphur (S). He indicated that water is electron donar in photosynthesis. \[6C{{O}_{2}}+12{{H}_{2}}O\xrightarrow{\,\,\,\,}{{C}_{6}}{{H}_{12}}{{O}_{6}}+6{{H}_{2}}O+6{{O}_{2}}\] This was confirmed by Ruben and Kamen in 1941 using Chlorella a green alga. They used isotopes of oxygen in water, i.e., \[{{H}_{2}}^{18}O\] instead of \[{{H}_{2}}O\] (normal) and noticed that liberated oxygen contains \[^{18}O\] of water and not of \[C{{O}_{2}}.\] The overall reaction can be given as under : \[6C{{O}_{2}}+12{{H}_{2}}^{18}O\underset{\text{Chlorophyll}}{\mathop{\xrightarrow{\text{Light}}}}\,{{C}_{6}}{{H}_{12}}{{O}_{6}}+{{6}^{18}}{{O}_{2}}+6{{H}_{2}}O\]

Photosynthesis is an oxidation reduction process in which water is oxidised to release O2 and CO2 is reduced to form starch and sugars. Scientists have shown that photosynthesis is completed in two phases. (1) Light phase or Photochemical reactions or Light dependent reactions or Hill's reactions : During this stage energy from sunlight is absorbed and converted to chemical energy which is stored in ATP and \[NADPH+{{H}^{+}}.\] (2) Dark phase or Chemical dark reactions or Light independent reactions or Blackman reaction or Biosynthetic phase : During this stage carbohydrates are synthesized from carbon dioxide using the energy stored in the ATP and NADPH formed in the light dependent reactions. Evidence for light and dark reactions in photosynthesis : (1) Physical separation of chloroplast into grana and stroma fractions : It is now possible to separate grana and stroma fractions of chloroplast. If light is given to grana fraction in presence of suitable H-acceptor and in complete absence of \[C{{O}_{2}},\]then ATP and \[NADP{{H}_{2}}\]are produced (i.e., assimilatory powers). If these assimilatory powers (ATP and\[NADP{{H}_{2}}\]) are given to stroma fraction in presence of \[C{{O}_{2}}\]and absence of light, then carbohydrates are formed. (2) Experiments with intermittent light or Discontinuous light : Rate of photosynthesis is faster in intermittent light (Alternate light and dark periods) than in continuous light. It is because light reaction is much faster than dark reaction, so in continuous light, there is accumulation of ATP and \[NADP{{H}_{2}}\]and hence reduction in rate of photosynthesis but in discontinuous light, ATP and \[NADP{{H}_{2}}\]formed in light are fully consumed during dark in reduction of \[C{{O}_{2}}\] to carbohydrates. Accumulation of \[NADP{{H}_{2}}\] and ATP is prevented because they are not produced during dark periods. (3) Temperature coefficient studies : Blackman found that \[{{Q}_{10}}\] was greater than 2 in experiment when photosynthesis was rapid and that \[{{Q}_{10}}\] dropped from 2 often reaching unity, i.e., 1 when the rate of photosynthesis was low. These results show that in photosynthesis there is a dark reaction (\[{{Q}_{10}}\] more than 2) and a photochemical or light reaction (with \[{{Q}_{10}}\] being unity). \[{{Q}_{10}}=\frac{\text{Reaction}\,\text{rate}\,\text{of}\,(t+10){}^\circ C}{\text{Reaction}\,\text{at}\,t{}^\circ C}\] Light reaction (Photochemical reactions) : Light reaction occurs in grana fraction of chloroplast and in this reaction are included those activities, which are dependent on light. Assimilatory powers (ATP and\[NADP{{H}_{2}}\]) are mainly produced in this light reaction. Robin Hill (1939) first of all showed that if chloroplasts extracted from leaves of Stellaria media and Lamium album are suspended in a test tube containing suitable electron acceptors, e.g., Potassium ferroxalate (Some plants require only this chemical) and potassium ferricyanide, oxygen is released due to photochemical splitting of water. Under these conditions, no \[C{{O}_{2}}\]was consumed and no carbohydrate was produced, but light-driven reduction of the electron acceptors was accompained, by \[{{O}_{2}}\] evolution.    \[\underset{\begin{smallmatrix} \text{Electron} \\\text{acceptor} \end{smallmatrix}}{\mathop{4F{{e}^{3+}}}}\,+\underset{\begin{smallmatrix} \text{Electron} \\\,\,\text{donor} \end{smallmatrix}}{\mathop{2{{H}_{2}}O}}\,\overset{\,\,\,\,\,\,\,}{\longleftrightarrow}\underset{\begin{smallmatrix} \text{Reduced} \\\,\text{Product}\end{smallmatrix}}{\mathop{4F{{e}^{2+}}}}\,+4{{H}^{+}}+{{O}_{2}}\uparrow \]   The splitting of water during photosynthesis is called photolysis. This reaction on the name of its discoverer is known as Hill reaction. Hill reaction proves that (1) In photosynthesis oxygen is released from water. (2) Electrons more...

Decker and Tio (1959) reported that light induces oxidation of photosynthetic intermediates with the help of oxygen in tobacco. It is called as photorespiration. The photorespiration is defined by Krotkov (1963) as an extra input of \[{{O}_{2}}\] and extra release of \[C{{O}_{2}}\] by green plants is light. Photorespiration is the uptake of \[{{O}_{2}}\] and release of \[C{{O}_{2}}\] in light and results from the biosynthesis of glycolate in chloroplasts and subsequent metabolism of glycolate acid in the same leaf cell. Biochemical mechanism for photorespiration is also called glycolate metabolism. Loss of energy occurs during this process. The process of photorespiration involves the involvement of chloroplasts, peroxisomes and mitochondria. RuBP carboxylase also catalyses another reaction which interferes with the successful functioning of Calvin cycle.     Biochemical mechanism (1) Ribulose-1, 5-biphosphate \[\xrightarrow{{{O}_{2}}}\] 2 Phoshoglycolic acid + 3 Phoshoglyceric acid   (2) 2 Phosphoglycolic acid \[+{{H}_{2}}O\xrightarrow{Phosphatase}\] Glycolic acid + Phosphoric acid. (3) Glycolic acid \[+{{O}_{2}}\underset{\text{Oxidase}}{\mathop{\xrightarrow{\text{Glycolic}\,\text{acid}}}}\,\] Glyoxylic acid\[+{{H}_{2}}{{O}_{2}}\] \[2{{H}_{2}}{{O}_{2}}\xrightarrow{\text{Catalase}}2{{H}_{2}}O+{{O}_{2}}\] (4) Glyoxylic acid + Glutamic acid \[\underset{\text{transaminase}}{\mathop{\xrightarrow{\text{Glutamate}-\text{glyoxylate}}}}\,\] Glycine \[+\,\,\alpha -\]keto glutaric acid (5) 2 Glycine \[+{{H}_{2}}O+NA{{D}^{+}}\xrightarrow{{}}\]Serine\[+C{{O}_{2}}+N{{H}_{3}}+NADH\] (6) Serine + Glyoxylic acid \[\xrightarrow{{}}\] Hydroxypyruvic acid + Glycine Hydroxypyruvic acid \[\xrightarrow{{}}\] Glyceric acid (7) Glyceric acid + ATP ® 3 phosphoglyceric acid + ADP + phosphate Importance of photorespiration : Photorespiration is quite different from respiration as no ATP or NADH are produced. Moreover, the process is harmful to plants because as much as half the photosynthetically fixed carbon dioxide (in the form of RuBP) may be lost into the atmosphere through this process. Any increase in \[{{O}_{2}}\] concentration would favour the uptake of \[{{O}_{2}}\] rather than \[C{{O}_{2}}\] and thus, inhibit photosynthesis for this rubisco functions as RuBP oxygenase. Photorespiration is closely related to \[C{{O}_{2}}\] compensation point and occurs only in those plants which have high \[C{{O}_{2}}\] compensation point such as \[{{C}_{3}}\] plants. Photorespiration generally occurs in temperate plants. Few photorespiring plants are : Rice, bean, wheat, barley etc. Inhibitors of glycolic acid oxidase such as hydroxy sulphonates inhibit the process of photorespiration. Unlike usual mitochondria respiration neither reduced coenzymes are generated in photorespiration nor the oxidation of glycolate is coupled with the formation of ATP molecules. Photorespiration (\[{{C}_{2}}\] cycle) is enhanced by bright light, high temperature, high oxygen and low \[C{{O}_{2}}\]concentration.