Photosynthesis Test - 24

Bisoynthetic phase (Dark or Blackman's reaction) catalyses the assimilation of CO₂ to cabohydrates. These reactions occurs in stroma or matrix of chloroplasts and all the enzyme required for the processes are present in the stroma/matrix of choroplasts. These reactions do not require light, instead assimilatory power (ATP and NADPH) produced during photochemical (light) phase is used in fixation and reduction of CO₂ However, this should not be construed to mean that they occur in darkness or that they are not light dependent.

DCMU  (Dichlorophenyl dimethyl urea) is a herbicide which kills the weeds by inhibiting CO₂ fixation as it is a strong inhibitor of photosystem II (P₆₈₀).DCMU acts by blocking electron flow at the guanine acceptors of PS II, by competing for the binding site of plastoquinone.

Light reaction (or photochemical phase) of photosynthesis mainly occur on the grana thylakoids. Dark reaction (or biosynthetic phase) which involve the synthesis of carbohydrates by CO₂​ fixation occur in the stroma (or matrix) of chloroplasts. The chloroplast matrix of higher plants stores starches temporarily in the form of starch granules. 

Chloroplasts are green plastids which function as the site of photosynthesis in eukaryotic photoautotrophs. A chloroplast consists of 3 parts — envelope, matrix and thylakolds. A chloroplast is covered by an envelope made up of two smooth membranes. The chloroplast matrix or fluid stroma is similar to cytoplasm in its constitution. It contains DNA, RNA, ribosomes, enzymes for CO₂ assimilation, proteins, starch grains and fat droplets or plastoglobuli. In the matrix or stroma are embedded a number of flattened membranous sacs called thylakoids or lamellae. At place: the thylakoids are aggregated to form stacks of discs called grana. A granum may have 20-50 thylakoid discs.

Carboxylation is the most crucial step of Calvin cycle where CO₂ is utilized for the carboxylation of RuBP through the use of ATP and NADPH generated by the light reactions. The reaction is catalysed by enzyme RuBisCO CO₂ combines with RuBP (ribulose-1, 5-biphosphate) to produce a transient intermediate compound called 2-carboxy 3-keto 1,5-biphosphoribotol. The intermediat splits up immediately in the presence of water to form two molecules of 3-phosphoglyceric acid or PGA, which is the first stable product of photosynthesis

Both malic acid and asparic acid are 4-carbon compounds formed in the mesophyll cells during C₄ cycle. PGA (3-phosphoglyceric acid) is a 3-carbon compound, which is the first stable product of Calvin cycle (C₃cycle). RuBP (Ribulose-1,5-biphosphate) is a 5-carbon compound, which is the primary acceptor of CO₂ during C₃ cycle.

1.Carboxylation- RuBP (5 carbon) is carboxylated by CO₂​ in presence of RuBisCO

2.Reduction- of 1,3 bis phosphoglycerate by NADPH in presence of glyceraldehyde ^-3 phosphate dehydrogease, producing Glyceraldehyde ^-3 phosphate (G₃P) and NADP⁺

3.Regeneration-of RuBP, five G₃P molecules produce three RuBP molecules utilizing three ATP

Crassulacean acid metabolism is a mechanism of photosynthesis involving double fixation of CO₂ which occurs in succulents belonging gto Crassulaceae, Cacti, Euphoribas and some other plants of dry habitats where the stomata remain closed during the daytime and open only at night, e.g., Sedum, kalanchoe, Opunita. Sunken stomata are deep seated stomata in which subsidiary cells lie above the guard cells.

Kranz anatomy is shown by C₄ plants such as Sorghum, sugarcane, maize, Cyperus rotundus, etc.

A - Fixation
B - Decarboxylation
C - Regeneration
So, the correct answer (D).

For every CO₂ molecule entering the Calvin cycle, 3 molecules of ATP and 2 molecules of NADPH are required. To make one molecule of glucose, 6 turns of the cycles are required (6 × 3ATP = 18ATP and 6 × 2NADPH = 12NADPH)

In
6 CO₂
18ATP

Out
1 glucose
18ADP 

12NADPH 12NADP, So, the correct answer is ' 3ATP and 2NADPH2'

In C₃ plants, bundle sheath cells usually do not contain chloroplasts and the whole C₃ cycle operates in mesophyll cells of leaves. in C₄ plants showing Hatch-Slack cycle however, intial fixation of  CO₂ occurs in mesophyll cells followed by final fixation in bundle sheath cells.

Kranz anatomy is a characteristic of C₄​ plants. In kranz anatomy, the mesophyll is undifferentiated and its cells occur in concentric layers around vascular bundles. Vascular which are surrounded by large-sized bundle sheath cells which are arranged in a wreath like a manner in one to several layers. The chloroplasts of mesophyll cells are smaller, they have well-developed grana and a peripheral reticulum but no starch. Mesophyll cells possess enzyme PEP case for initial fixation of CO₂​. The chloroplasts of the bundle sheath cells are agranal. They possess a peripheral reticulum and starch grains. Bundle sheath cells possess enzyme RuBisCO.

The enzyme RuBisCO carries out photosynthetic carboxylation of RuBP The enzyme was previously called as carboxydismutase. RuBisCO is the most abundant protein of the biological world. RuBisCO is located int the stroma on the outer surface of thylakoid membranes. As it is capable of carrying out both carboxylation and oxygenation and oxygenation reaction in C₃ and C₄ plants respectively, thus it is correctly named as RuBP carboxylase-oxygenase. 

The active site of RuBisCO for carboxylation and oxygenation is the same. The enzyme has more affinity for CO₂​ as compared to oxygen. Active oxygenation occurs only when CO_2​:O₂​ ratio favours it. At hight temperature, concentration of dissolved CO₂ in equilibrium with air decreases more than concentration of O₂, which favours oxygenation, High temperature occurs in tropical areas. Therefore, tropical plants are the major sufferers. At high temperature, RuBisCo functions as oxygenase and instead of fixing CO, oxidizes RuBP to produce a 3-C phosphoglyceric acid and a 2-C phosphoglycolate. This is the first reaction of photorespiration or C₂ cycle.

The Optimum temperature Is 10^∘C−25^∘C for C₃, plants and 30°C-45°C for C₄ plants.

In C₄ plants, the first stable photosynthetic product is a 4-carbon compound, i.e., oxaloacetic acid (OAA), which is formed by initial fixation of CO₂ by the carboxylation of phosphoenol pyruvate in the mesophyll cells. So, when radioactive ¹⁴CO₂ is introduced into the reaction, it is first incorporated into the OAA.

In a CAM plant, although the first product formed is OAA, but here the initial CO₂ fixation occurs at night.

Photorespiration (Photosynthetic carbon oxidative or PCO cycle) is the light dependent process of oxygenation of ribulose-1,5-biphosphate (RuBP) and release of CO₂ by the photosynthetic organs of a plant. Under normal conditions, in Calvin cycle, carboxylation of RuBP takes place whereas during photorespiration instead of carboxylation, oxygenation of RuBP takes place. This is due to abnormal behaviour of enzyme RuBisCO, which at high temperature (more than 35^∘C), functions as oxygenase (instead of carboxylase). Instead of fixing CO₂, it performs oxygenation of RuBP to produce a 3-carbon phosphoglyceric acid (PGA) and a 2-carbon phosphoglycolate. It is the first reaction of photorespiration and can be represented as:

In this way, photorespiration interferes with the successful functioning of Calvin cycle by causing oxygenation of RuBP instead of carboxylation.

Photorespiratory pathway (or C₂​ pathway) results in the release of CO_2​ with the utilization of ATP. In the photorespiratory pathway, three is no synthesis of ATP or NADPH.

The site for photorespiration is chloroplast peroxisome and mitochondrion. It is also called photosynthetic carbon oxidative or PCO cycle.

A limiting factor is defined as a factor which is deficient to such an extent that increase in its magnitude directly increaes the rate of the process. In the given graph, rate of photosynthesis initially increaes with an increase in light intesntiy (region A) but soon it levelled off. Thus. initially light intensity was limiting the rate of photosynthesis. Hoever, when light intensity was present in sufficient amounts (region C), rate of photosynthesis did not increase further. This is due to the fact that in region C, some other factor (e.g. CO₂ concentration) became the limiting factor. At this region, the rate of photosynthesis could be further enhanced only by the increase in availability of other limiting factor (e.g. CO₂)

Blackman's (1905) law of limiting factors states that a chemical process is affected by more than one factor, then its rate will be determined by the factor which is nearest to its minimal value: it is the factor which directly affects the process its quantity is changed.

CO₂​ compensation point or threshold value is that concentration of CO₂​ at which illuminated plant parts stop absorbing carbon dioxide from their environment, At this value, CO₂​ fixed in photosynthesis is equal to CO_2​ evolved in respiration and photorespiration. The value is 25-100 ppm in C₃​ plants and 0-10 ppm in C₄​ plants .The reason for low compensation value for C₄​ plants is the greater efficiency of CO_2​ fixation through PEP-carboxylase. The optimum CO_2​ concentration for C₄​ plants is 360 ppm and C₃​ plants, it is 450 ppm.  

The rate of photosynthesis usually increases with an increase in temperature up to 40^oC. Above this temperature, the rate of photosynthesis starts decreasing due to the inactivation of enzymes. The minimum temperature at which most plants start photosynthesis is 0^o−5^oC but it can be as low as - 20^oC for lichens and - 35^oC for some gymnosperms.The maximum temperature at which photosynthesis can occur is 55^oC in some desert plants and 75^o for hot spring algae. When the temperature is increased from minimum to optimum, the rate of photosynthesis doubles for every 10^oC rise in temperature. 

The C_4​ plants respond to higher temperatures and show higher rate of photosynthesis while C₃​ plants have a much lower temperature optimum. The temperature optimum for photosynthesis of different plants depends on the habitat that they are adapted to. Tropical plants have a higher temperature optimum than the plants adapted to temperate climates as tropical areas have higher temperature as compared to temperate areas.