CBSE Questions for Class 11 Medical Biology Photosynthesis In Higher Plants Quiz 2 - MCQExams.com

• Blackman's reactions or dark reactions take place in the stroma of the chloroplast. In dark reactions, sugars are synthesized from carbon dioxide by using $$ATP$$ and $$NADPH_2$$​, which are end products of Hill's reaction or light reaction.

• The Calvin Cycle is a common method of carbon fixation in all photosynthetic plants be C$$_3$$, C$$_4$$ and CAM plants.
• C$$_3$$ plant use the Calvin Cycle for the initial steps that incorporate CO$$_2$$ into organic matter, forming a 3-carbon compound as the 1st stable intermediate. 
• C$$_4$$ plant prefaces the Calvin Cycle with reactions that incorporate CO$$_2$$ into 4-carbon compound. This pathway is found mostly in hot regions with intense sunlight. 
• CAM plants use Crassulacean acid metabolism as an adaptation for arid conditions. CO$$_2$$ entering the stomata during the night is converted into organic acids, which release CO$$_2$$ for the Calvin Cycle during the day, when the stomata is closed. 
• This process is not directly dependent on sunlight but fueled by ATP and NADPH produced in the light reactions. 
• The Calvin cycle take place in the stroma (the inner space of chloroplasts).

• The dimorphic chloroplast is a characteristic feature of C$$_4$$ plants.
• Among the options, sugarcane is a C$$_4$$ plant, which possesses dimorphic chloroplast. 
• They have a special type of anatomy known as Kranz anatomy. 
• They have two types of chloroplast-containing cells- bundle sheath cells and mesophyll cells. 
• Bundle sheath cells contain chloroplast lacking grana while mesophyll cells contain chloroplast with abundant grana.

• PGA (Phosphoglyceric acid) is the first stable 3 carbon compound formed in $$C_3$$ cycle. 
• It is formed by the reaction of $$RUBP$$ and $$CO_2$$, to produces a six-carbon intermediate that splits into 2 molecules of the 3-carbon compound. 

• Emerson effect is also known as the red drop effect. 
• In this effect, a sharp reduction in the rate of photosynthesis is found when monochromatic light of 680 nm is used. 
• It occurs mainly due to the presence of two different pigment systems, i.e., PS I and PSII.

• The first step in the pathway is the conversion of pyruvate to phosphoenolpyruvate (PEP), by the enzyme pyruvate orthophosphate dikinase. 
• This reaction requires inorganic phosphate and ATP plus pyruvate, producing phosphoenolpyruvate, AMP and inorganic pyrophosphate (PPi). 
• The next step is the fixation of $$CO_2$$ ( step 'A') into oxaloacetate by the enzyme PEP carboxylase. Both of these steps occur in the mesophyll cells.
• The product is usually converted to malate, a simple organic compound, which is transported to the bundle-sheath cells. 
• Here, it is decarboxylated to produce $$CO_2$$ and pyruvate (step 'B'). 
• The $$CO_2$$ now enters the Calvin cycle and the pyruvate is transported back to the mesophyll cell (step 'C'). So, it is called regeneration. 

• Noncyclic photophosphorylation results in the production of $$NADPH_2$$ and involves both photosystems I and II.
• These two photosystems work in series, first PS II and the PS I. The two photosystems are connected through an electron transport chain. Both $$ATP$$ and $$NADPH_2$$ are synthesized by this kind of electron flow.
•  First, in PS II, the P$$_680$$ molecule absorbs 680 nm wavelength of red light causing electrons to become excited and jump into an orbit that is farther from the atomic nucleus.
• These electrons are picked up by an electron acceptor which passes them to an electron transport system of cytochromes. This movement of electrons is downhill on the redox potential scale. 
• The electrons are then passed onto the pigments of PS I, without being used as they pass through the electron transport chain. Simultaneously, electrons in the reaction center of PS I (P $$_700$$) are excited when they receive light of wavelength 700 nm, and these electrons are transferred to another acceptor molecule that has a greater redox potential. These electrons are then moved downhill again to a molecule of NADP $$^+$$. The addition of these electrons reduces the NADP $$^+$$ to NADPH$$_2$$.

• Dark reaction is the second phase of photosynthesis. It is purely enzymatic and slower than the photochemical reaction (light reaction). 
• It takes place in the stroma region of the chloroplast and is independent of light. It can occur either in the presence or in the absence of light provided that the assimilatory power is available. 
• During the dark reaction, ATP and NADPH (products of light reaction) are utilized to fix CO$$_2$$, which is further used to produce carbohydrates.

• Green plants and algae use two types of photosystems, PS-I with chlorophyll $$P_{700}$$ in its reaction center and PS-II with $$P_{680}$$ in its reaction center. 
• The two photosystems are linked by a chain of electron carrires. Light excites $$P_{680}$$.
• So, the correct answer is option A.

• Phosphoenolpyruvate carboxylase, an enzyme found in all multicellular plants, catalyzes the formation of oxaloacetate from phosphoenolpyruvate (PEP) and a hydrocarbonate ion. 
• This reaction is harnessed by C$${_4}$$ plants to capture and concentrate carbon dioxide into the photosynthetic bundle sheath cells. 

• 'Z-scheme' shows the transfer of electrons involving photosystem I and II. The photosynthesis process in chloroplasts begins when an electron of P680 of PSII attains a higher energy level. 
• This energy is used to reduce a chain of electron acceptors that have subsequently lowered redox potentials. This chain of electron acceptors is known as an electron transport chain. When this chain reaches PS I, an electron is again excited, creating a high redox-potential. 
• The electron transport chain of photosynthesis is often put in a diagram called the z-scheme because the redox diagram from P680 to P700 resembles the letter 'z'.

• Robert Hill (1939) first of all showed that if chloroplast extracted from leaves of Stellaria media and Lamium album are suspended in a test tube containing suitable electron acceptor e.g., potassium ferrioxalate and potassium ferricyanide, $$O_{2}$$ is released due to the photochemical splitting of water. 

In PS I, the reaction center chlorophyll a has an absorption peak at 700 nm, hence is called P$${_7}$$$${_0}$$$${_0}$$, while in PS II it has absorption maxima at 680 nm and is called P$${_7}$$$${_0}$$$${_0}$$. C$${_4}$$ cycle or Hatch and Slack pathway operates in C$${_4}$$ plants only. The primary acceptor is Phosphoenol pyruvic acid. The first stable compound formed is Oxaloacetic acid which is 4-carbon compound.

• Bundle sheath cells are specialized cells found in $$C_4$$ plants, that lie below a layer of mesophyll cells. 
• The bundle sheath cells are the sites within $$C_4$$ plants, where the Calvin cycle enzymes are located. 
• Bundle sheath cells have dense chloroplasts while mesophyll chloroplasts are scattered.

• Photorespiration is a process in plant metabolism, which attempts to reduce the consequences of a wasteful oxygenation reaction by the enzyme RuBisCO. 
• The desired reaction is the addition of carbon dioxide to RuBP (carboxylation), a key step in the Calvin–Benson cycle, however approximately 25% of reactions by RuBisCO instead add oxygen to RuBP (oxygenation), producing a product that cannot be used within the Calvin–Benson cycle. This process reduces the efficiency of photosynthesis, potentially reducing photosynthetic output by 25% in $$C_3$$ plants. 
• Photorespiration involves a complex network of enzyme reactions, that exchange metabolites between chloroplasts, leaf peroxisomes, and mitochondria.
  

• Peroxisomes lie between chloroplasts and mitochondria in the plant cell and serve to pass the 2-carbon products of oxygenation on for further metabolism.
• In the chloroplast, the phosphoglycolate is dephosphorylated. Glycolate is transported to the peroxisome, where molecular oxygen further oxidizes it into glyoxylate. 
• The product is hydrogen peroxide, which is rapidly broken down by catalase to water and oxygen. The glyoxylate is amidated to the amino acid glycine in the peroxisome.

• The process of photorespiration occurs in plants when the concentration of the oxygen increases and the Ribulose bisphosphate Carboxylase enzyme combines with oxygen, instead of carbon dioxide and functions as an oxygenase enzyme. 
• This process is considered wasteful for plants because it deviates the C$$_{3}$$ cycle. This process needs three organelles, chloroplast, peroxisomes, and mitochondria to be completed. 
• It occurs in the chloroplast where oxygen is utilized by combining with RuBiSCO and glycolate is formed. This glycolate then moves to the peroxisomes and glycolate is converted to glyoxylate using oxygen.
• Glyoxylate then moves to the mitochondria and glycine is formed. From glycine, serine is formed in mitochondria which goes to peroxisomes again where the hydroxypyruvate and glycerate are formed and the glycerate finally arrives back to the chloroplast to take part in glucose formation. So, photorespiration is a deviation of  C$$_{3}$$ cycle. 
• The oxygen-consuming reactions have occurred in the stroma of chloroplast and peroxisomes.

• Photorespiration occurs when the $$CO_2$$ levels inside a leaf become low.
• This happens on hot dry days when a plant is forced to close its stomata to prevent excess water loss. 
• If the plant continues to attempt to fix $$CO_2$$, when its stomata are closed, the $$CO_2$$ will get used up and the $$O_2$$ ratio in the leaf will increase relative to $$CO_2$$ concentrations. 
• Photorespiration is absent in $$C_4$$ plants and CAM plants. Thus they are more efficient than $$C_3$$ plants.

So, the correct answer is option B.