Cellular Respiration and Enzymes

Nicotinamide adenine dinucleotide phosphate, abbreviated NADP is a cofactor used in anabolic reactions, such as lipid and nucleic acid synthesis, which require NADPH as a reducing agent.

The end product of glycolysis is pyruvic acid. The next step is the formation of acetyl coenzyme A (acetyl CoA). Acetyl CoA is the initiator of the citric acid cycle. In carbohydrate metabolism, acetyl CoA is the link between glycolysis and the citric acid cycle. The initiating step of the citric acid cycle occurs when a four-carbon compound (oxaloacetic acid) condenses with acetyl CoA (2 carbons) to form a citric acid (6 carbons).

NAD+ and FAD are co-enzymes required for the completion of many biochemical pathways. However, there is no need for an enormous amount of synthesis of either of these co-enzymes as there as a mechanism of recycling the co-enzymes after completion of one step in the metabolic pathway. These co-enzymes are predominantly required for the synthesis of ATP (energy currency of the cell) and will be used in the electron transport chain for the production of the same. So, they are not required to be synthesized in huge quantities but they play a crucial role in cell energetics.

Any chemical reaction includes the formation of transition state between reactant and product. The energy required for formation of the transition state is termed as activation energy. Enzymes enhance the rate of reaction by lowering down the activation energy of transition state. Thus, the correct answer is option C.

The transition state is the state of the activated complex which is highly unstable and readily changes into the product.  At the transition state, the enzyme attaches to the substrate to form an enzyme-substrate complex by forming a weak chemical bond. Binding energy is the free energy that is released by the formation of weak interactions between a complementary substrate and enzyme.

Enzymes are biological catalysts. A catalyst is a chemical that increases the rate of a chemical reaction without itself being changed by the reaction. Without a catalyst, the amount of energy required for the reaction is large. Enzymes and catalysts both affect the rate of a reaction. The lower the activation energy for a reaction, the faster the rate of chemical reaction. Thus enzymes speed up reactions by lowering activation energy.

Destructive metabolism is the breakdown of more complex substances into simpler ones with release of energy.

Metabolism refers to series of reactions wherein product of one reaction becomes reactant for the next one. It includes catabolism (degradative pathways) and anabolism (synthetic pathways). Catabolic reactions are the reactions in which complex molecules are broken down into smaller simpler molecules. The catabolic reactions are exergonic reactions( energy releasing reactions). While photosynthesis is an anabolic reaction which requires energy for the formation of complex molecules. Hence option B is the correct answer.

The metabolism is a chemical processes that occur within a living organism in order to maintain life and basic unit of life is cell, hence all metabolic reactions occur  in cell. So, the correct option is 'Cellular level'

Both RUBISCO and PEP carboxylase are important enzymes of dark reaction of photosynthesis. RUBISCO catalyzes carboxylation in C3 plants. In the presence of magnesium activated RUBISCO, carbon dioxide combines with ribulose bis phosphate to form phosphoglyceric acid. In C4 plants, mesophyll cells fix carbon dioxide with the help of phospho enol pyruvate in the presence of magnesium activated phospho-enol pyruvate carboxylase.

Enzymes are proteins which accelerate the rate of the reaction. The substrate and product have different energy state. The energy gap between the substrate and the product is known as activation energy which is the minimum energy required to initiate a reaction. The enzymes lower the activation energy and lead to the faster conversion of substrate to the product.

A modulator is a type of molecule which either stimulates or inhibits enzyme activity. Those which stimulate enzyme activity are known as inducers while those which inhibit enzyme activity are known as inhibitors. The inhibitors can be competitive, non-competitive or allosteric inhibitors. 

Allosteric means in a different steric conformation or occurrence of more than one shapes. Allosteric modulators bind to a site different from the active site and bring changes in the shape or conformation of the enzyme molecule. The change in shape can activate the enzyme or inhibit the activity of enzyme. An allosteric inhibitor will bind to allosteric site and bring in the conformational changes to reduce the enzyme activity.

Feedback inhibition is the inhibition mechanism in metabolic pathways where the end product of one reaction are the substrates for next reaction. Such feedback inhibition is usually directed at the first or first few reactions in the pathway to help conserve precious cellular resources, thus in case the end product is not required the metabolic pathway is blocked at the initial stages itself. The feed back regulation is mostly brought about by the allosteric modulators. Usually the first steps in metabolic pathways are catalysed by allosteric enzymes whose catalysis rate can be increased with the help of positive modulators and decreased with the help of negative modulators.