Metabolism: Energy for Life's Work:

Metabolism is the basic property of life occurring from energy transformations in cells. It is totality of all chemical reactions which occur in organism. The reactions don't happen arbitrarily; rather, they happen in step-by-step sequences known as metabolic pathways. In the reactions, product of one reaction turns into another substrate i.e. the starting point for another. Different metabolic pathways in cell are much like roads on map. Every reaction in metabolic pathway reorganizes atoms in new compounds, and each one either absorbs or release energy. Amount of energy needed to break particular bond is equal to amount of energy needed for formation. This amount of energy is known as bond energy. For instance, let energies of given bonds.

Bond        Energy (Kcal Mol^-1)

C - C                 83

C - O                 84

C - H                 99

C = O               174

O = O               118

O - H                111

Therefore C = O bonds are much stronger than C - C bonds. This is significant as metabolism constantly breaks and reforms the bonds to get energy for development, movement and repair. In chemical reactions, net release or uptake of energy equals difference of energy released and energy consumed. For instance, burning mole (16g) of methane releases 160Kcal of energy.

CH₄ + 2O₂ → CO₂ + 2H₂O

The heat of reaction is heat which you feel from stove (that is net amount of energy released by reaction) and is symbolized by ΔH (delta H). It is deduced from total potential energy of molecules, measure known as enthalpy. Thus, heat released into surrounding comes from enthalpy of reacting molecules. In case of burning methane, products (O₂ and H₂O) contain 160Kcal less enthalpy than reactant (CH₄). Heat releasing reactions are known as exothermic reactions and change molecules so that the energy content decreases.

Free Energy:

Potential energy of compound is contained in chemical bonds. When the bonds break, energy which is released used to do work, like form other bonds. Amount of energy available to form other bonds is free energy of molecule and is signified by G (for its discover, Joshua Gibbs). Chemical reactions change amount of free energy accessible for work. This change in free energy is known as DG. (delta DG) and is the most basic property of chemical reaction. It is equal to change in heat content minus change in entropy. Those relationships are symbolized by given formula:

ΔG = ΔH - TΔs

Where

ΔG is change in free energy of reaction and is part of potential energy which can do work. Remaining energy is not available for work due to entropy.

ΔH is change in enthalpy (heat content), that is energy in chemical bonds.

T is temperature estimated on scale of ⁰C above absolute zero

 SΔ is change in entropy or disorder.

Entropy is amplified at higher temperatures as temperatures estimate random molecular motion (that is intensity or potential of heat), that increases disorder. Thus higher temperatures speed reactions and increase disorder. This is also due to water evaporates faster at higher temperatures. Spontaneous reactions change bond energies and release heat. Such reactions generally increase entropy and are known as Exergonic (energy outward) reactions. They have ΔG less than zero and thus form products with less free energy than the reactants. This energy is potentially available for cellular work.

Not all reactions are spontaneous. For instance, let formation of sucrose (table sugar) and water from glucose and fructose.

Glucose + Fructose → Sucrose + H₂O

This reaction has ΔG of + 5.5Kcal, meaning that its products contain 5.5kcal mol^-1 more energy than it reactions. This reaction absorbs energy from surroundings and not spontaneous. These reactions are known as endergonic (energy inward) reactions and won't happen without net input of energy. Free energy of particular reaction finds many reaction's properties. Most significant, ΔG of reaction dictates how much work reaction can do. For instance let oxidation of mole of glucose to carbon dioxide and water

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy

ΔG = 686kcal Mol^-1.

As ΔG of the reaction is less than zero, this reaction is exergonic and spontaneous. Carbon dioxide and water it produces store 686 fewer kcal than does glucose.

Free Energy and Chemical Equilibrium:

When the chemical reaction reaches equilibrium, a ΔG equals zero. Likewise ΔG increase as one moves away from equilibrium. As all of life's process needs work, cells should remain far from equilibrium to live. They achieve this by constantly preventing accumulation of any of reactants of metabolic pathways. For instance, huge dissimilarity in free energy between glucose and oxidation products (carbon dioxide and water) pulls cellular metabolism rapidly in one direction, when reactants form; they are rapidly converted to new compounds by other reactions.

Oxidation, Reduction and Energy Content:

Many energy transformations in organisms engage chemical reactions known as oxidations and reductions.

Oxidation is loss of electrons either alone or with hydrogen, from molecule. This is equal to adding oxygen as oxygen is strongly electronegative and thus attracts electrons from original atom. Oxidation reactions, like breakdown of glucose to carbon dioxide and water degrade molecules in simpler products and are thus examples of catabolism. They are reactions which breakdown compounds to release energy.

Reduction is addition of electrons either alone or with hydrogen to molecule. Reduction changes chemical properties of the molecule, not essentially in the size. Electrons removed from molecule in oxidation decrease another molecule. Reduction reactions like formation of lipids generally engage synthesis of more complex molecules and are thus examples of anabolism. They are reactions which build up compound and need energy input. Oxidation and reduction reactions always happen simultaneously; if something is decreased something else should be oxidized.

Organisms remove energy from energy-rich compounds like sugar and fat through catabolic reactions which perform work example creating cell walls, and replicating genetic information a significant substance by which energy passes in cellular metabolism is adenoisine triposphate, compound more commonly called as ATP.

When cells require energy, they hydrolyse Adenoisine triphosphate or ATP. ATP is nucleoside triphosphate made of adenine (nitrogen containing base), ribose (a five-carbon sugar) and three phosphate(HPO₄²) groups. ATP molecules have much energy which is released when terminal phosphate group (symbolized by squiggly line,) is cleaved from molecule. As breakdown of ATP links energy exchanges in cells, ATP is energy currency of cells. When cells require energy to perform something, they spend ATP by converting it to adenosine diphosphate (ATP), inorganic phosphate (Pi), and energy.

Δ G = -7.3 kcal mol^-l

Many properties of ATP make it preferably suited as energy currency of cells.

• Amount of energy released by converting ATP to ADP + Pi (7.3 kcal mol^-l) is approx twice as much as is required to drive most cellular reactions. Rest of energy is dissipated as heat.
• Much of its energy is instantly available to cells. Though fat and starch also store large amounts of energy, the energy should first be converted to ATP before it can be utilized. ATP signifies eagerly available energy cash of cell; fats and starch are analogous to energy stocks and bonds.
• Unlike covalent bonds connecting carbon and hydrogen in molecule like methane and glucose, terminal phosphate bond of ATP is unstable, due to this it breaks so simply.

ATP is common energy currency - Every cell of all organisms employs ATP for energy transformation. Like all different appliances which can plug in electrical outlet and perform various things, so too can different chemical reaction employs cell's ATP to perform various types of work.

Organisms employ ATP for nearly all the work comprising making new cells and macromolecules, pumping materials and moving materials by cells and throughout organism. Achieving all of this work needs huge amounts of ATP. For instance, typical adult utilizes equivalent of approx 200kg (4401b) of ATP per day, but has only few grams of ATP on hand at any one time. ATP is thus recycled at very furious pace, turning over complete supply every minute or so.

ADP + Pi + energy     ATP ΔG = + 7.3 kcal mol^-1

Since ΔG for the reaction is positive, every reaction needs the input of energy that comes from molecules which are broken down in other reactions which are coupled to synthesis of ATP. Coupled Reactions just as cells couple breakdown of food to production of ATP, so too do they. Couple the breakdown of ATP to other reactions which happen at same time and place in cell. The coupled reactions drive other reactions which perform work or make other molecules.

For example, let formation of sucrose (table sugar) and water from glucose and fructose.

Glucose + fructose → sucrose + H₂0

ΔG = + 5.5 Kcal mol^-l

As ΔG for reaction exceeds zero, reaction is not spontaneous and won't happen without net input of energy. This input of energy is given by 2 moles of ATP, each of which gives 7.3 Kcal mol-l of energy.

ATP ADP + Pi + energy

ΔG = -7.3 Kcal mol^-1

This changes equation for sucrose synthesis to:

Glucoses + fructose + 2ATP Sucrose + H₂0 + 2ADP = 2Pi + energy and makes ΔG = 5.5 - 14.6 = -9.1 Kcal mol^-l. Reaction proceeds as its overall ΔG is negative. In the reaction, breakdown of ATP is coupled to formation of sucrose.

ATP achieves much of its work by transferring phosphate group to another molecule in process known as phosphorylation. Phosphorylation energizes molecules receiving phosphorylate group so that they can be utilized in later reactions.

Other Compounds Involved in Energy Metabolism:

Many other compounds beside ATP influence energy transformations in plant cells. Cofactors are frequently ions, for instance, Mg²+ is cofactor needed to transfer phosphate groups between molecules. Organic cofactors are known as co-enzymes and generally carry protons or electrons. These are frequently nucleotides, unlike ATP, their energy content relies on the oxidation state, not on presence or absence of particular phosphate bond. Co-enzymes are vitamins which happen in all cells. Humans and other animals should get vitamins from food, plants produce their own vitamins.

NAD⁺ Nicotinamide Adenine Dinucleotide

NAD⁺ is similar to ATP in that it is composed of adenine, ribose and phosphate groups.

Though, active part of NAD+ is nitrogen having ring, known as nicotinamide that is derivative of nicotinic acid (niacin, or vitamin B3, one of compounds added to products like cornflakes to make them "vitamin fortified"). NAD⁺ is decreased when it accepts two electrons and a proton from active site of enzyme or from substrate.

NAD⁺ + 2h⁺ 2^e- NADH + H⁺

ΔG = -52.6 Kcal mol^-l

NADH+ H⁺ is completely reduced and is thus energy rich. It is utilized to make ATP land to decrease other compounds in cells.

NADP⁺: Nicotinamide Adenine Dinucleotide Phosphate

NAD⁺ has structure like NAD+ with added phosphate group

NADPH supplies hydrogen which reduces CO₂ to carbohydrate in photosynthesis. NADPH also supplies hydrogen utilized to decrease nitrate to ammonia.

FAD: Flavin Adenine Dinucleotide

FAD is one of coenzyme forms of rigo-flavin (vitamin B2) FAD, similar to NAD, carries 2 electrons, though 'FAD accepts both protons to become F ADH2 functions in cellular respiration.

Other Nucleoside Triphosphates:

Seven other nucleoside triphosphate function in cellular metabolism. For instance uridine triphosphate (UTP) is concerned in making cell walls, guanosine triphosphate (GTP) is concerned in protein synthesis, and cytidine triphosphate (CTP) is concerned in membrane production. Cytochromes like chlorophyll and haemoglobin, cytochromes are the group of metal- containing molecules which participate in metabolism by transferring electrons.

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