Energy Metabolism-II, Biology tutorial

Enzymes and Energy:

Exergonic reactions (example combustion of methane) are spontaneous. Such reactions, although they are spontaneous need the energy input to start. In photosynthesis, energy of activation is provided by sunlight and net gain of energy is trapped in products such a wood.

Though wood has much energy, wood is too stable to burn spontaneously. However it has its potential energy, it won't burn except it is first heated. Heat essential to ignite wood is energy of activation. Once ignited, wood will carry on to burn and release energy. Though heat is usually the effective catalyst, it is generally not effective for cells. Certainly heat required to activate most metabolic reaction would kill most cells. Therefore cells depend on biocatalyst known as enzymes to begin their reactions. Enzymes are globular proteins which speed reactions by decreasing energy of activation of reaction. This they perform by binding substrate so that reaction can happen. For instance, energy of activation to degrade casein (a protein in milk) is 20,600kca1 mol-1 without reactions enzyme, but only 12,600kca1 mol-1 in presence of enzyme. Lessening energy of activation significantly speeds the reaction. For instance, let formulation of H2CO3 form CO2 and H2O, reaction engages in gas exchange and catalyzed by carbonic anhydrase.

CO2 + H2O → (Carbonic anhydrase) H2CO2

Without carbonic anhydrase, only approx 1 molecule forms per second. This is far too slow to be helpful to organisms. Though, in presence of carbonic andydrase, H2CO3 form at the rate of about 600,000 molecules per second, increase of more than 107. This stresses significance of enzymes, they are critical to life as they speed spontaneous reactions to biological useful rate. Higher temperatures usually increase enzymatic activity. Up to point enzymatic activity usually - doubles for every increase of 10°C. Though, beyond 60°C, entropy wins out as protein is denatured and reactions stops. Though few organisms can tolerate high temperatures (like some bacteria thrive at temperature of 70°c), many enzymes work best at lower temperatures. For instance, many enzymes in the bodies function best near body temperature (37oC/98.6oF)

Regulating Metabolism:

Enzymes regulate energy transformations by controlling metabolic reactions. If metabolism is related to series of interconnected roads, then enzymes would function as traffic lights which control flow of energy in cells. How enzymes perform this?

Few hypotheses were postulated as the description for enzymes action. We will look at them before going on to discuss how enzymes really work.

Hypothesis 1 - Enzymes decrease ΔG of a reaction. Identified ΔG to be amount of energy available to form bonds, also called as free energy. This hypothesis is not true. Enzymes don't change ΔG of chemical reaction.

Hypothesis 2 - Enzymes heat calls to increase reaction rates. This second hypothesis is also not true. Enzymes don't heat cells. Heat needed to speed most metabolic reactions to helpful rates would kill cells.

How, Then, Do Enzymes Regulate Metabolic?

Products of pathway frequently affect activity of pathway. For instance, plants use five -step pathway to create isoleucine from threonine when isoleucine collects, it inhibits first enzymes of pathway, thus decreasing production of isoleucine until current supply is utilized. This means of slowing pathway when its products are not required is known as feedback inhibition and is common in plants and animals. Feedback inhibition balances supply and demand in cells, thus averting unnecessary excesses and deficiencies.

One means of feedback inhibition engages allosteric regulation in which product binds weakly to receptor site on enzyme which differs from active site. Allosteric regulation is common in enzymes having more than one subunit. Binding of molecule to allosteric site (generally located where their subunits join) changes enzymes activity, thus affecting cell's metabolism. For instance, isoleucine allesterically inhibits first enzyme of its synthetic (2) pathway, and therefore prevents avoidable buildup of isoleucine. Several enzymes are also inhibited by other molecules which complete for enzyme's active site. These competitive inhibitors mimic substrate and can be overcome by increasing concentration of substrate (that is diluting concentration of inhibitor). Drugs like sulfanilamide competitively inhibit enzymes. Other compounds inactive enzymes by binding to parts of enzymes which are different from active site, thus preventing enzymes from binding substrate at active site. Compounds which perform this, such as lead as nerve gas known as non-competitive inhibitor. Though several non-competitive inhibitors bind reversible to enzyme, several do not. For instance, penicillin is noncompetitive inhibitors. Though several competitive inhibitors which binds irreversibly to enzyme (3) may don't for instance, penicillin in non-competitive inhibitor than binds irreversibly to any enzyme which makes cell walls in bacteria. This blockage of cell wall synthesis eventually kills bacteria, therefore accounting for antibiotic effect or penicillin

The Major Energy Transformations in Plants Photosynthesis and Respiration:

Energy transformations which sustain life happen similarly in all organisms. Most significant energy transformation in plants is photosynthesis and respiration.

Energy-requiring uphill stage of process is cellular respiration. In this process, light energy absorbed by chloroplasts is utilized to release oxygen and decrease dioxide (low energy compound) to carbohydrate (a high energy compound).

Carbon dioxide + water + light → Carbohydrate + oxygen

Carbohydrate fuels activities of plants and other organisms.

Energy-releasing (i.e. exergonic), downhill stage of process is cellular respiration. In this procedure, energy -rich molecules like sugars are oxidized to carbon dioxide and water.

Carbohydrate + oxygen → Carbon dioxide + water + energy

Respiration drives cellular economy of most organisms.

The Flow of Energy:

Study equations for photosynthesis and respiration again. Every process engages oxidation or reduction of carbon by addition or removal of hydrogen. Photosynthesis removes hydrogen from water and adds it to carbon therefore, carbon is reduced in photosynthesis. Cellular respiration removes hydrogen from carbon and includes it to oxygen to form water therefore carbon is oxidized in respiration.

Although one equation is reverse of other, they proceed by different mechanisms that show how photosynthesis and respiration are incorporated in nature. Energy flows through the system and is eventually converted to heat. For instance, sunlight is energy input which sustains life on earth. This is probable only because plants convert sunlight in chemicals energy (e.g. sugars). Animals live by eating plants (or by eating other animals which ate plants) and utilizing their stored energy to power the activities. Such animals are then eaten by another animal to fuel the activities. Eventually, organisms die and are decomposed by bacteria and fungi.

In process, energy stored temporarily by organisms in the so called food chain is released by heat. Organisms may temporarily store different amounts of the energy, but net effect is that it flows by system and is eventually transformed to heat. According to second law of thermodynamics, all energy transformations at every step in food chain are less than 100% efficient. Certainly, there is a 90% loss of usable energy at every stage. This has wonderful implications. For instance, let corn crop, most energy which strikes plants' leaves is reflected or converted to heat. Only small amount of energy is trapped by photosynthesis in sugars. When the sugars are converted to starch, more energy is lost. When this starch is feed to cattle, only approx 10% of usable energy is stored by cow, rest is lost as heat. By the time we consume beef composed from cow, another 90% of useable energy has been lost. Therefore, amount of usable energy in steak is only approx 1 % of that contained in starch of corn.

100 Units energy in corn → 10 units of energy in herbivore → 1 unit of energy in a carnivore

Inefficiency of the transformations, as forecasted by second law of thermodynamics, makes this inefficient process as much of the energy is converted to heat. By inserting extra energy transformation between corn plant and ourselves (that is cow), we lose much of usable energy.

Though energy moves through system in one way flow (that is toward heat), nutrients are cycled. Photosynthesis manufactures sugars and oxygen from carbon dioxide and water in cellular respiration. Therefore nutrients cycles in the ecosystem.

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