Unit: Cellular energetics
Chapter: Cellular energy
Reference: Cellular energy, Anabolic pathways, Catabolic pathways, Systems and Surroundings, Isolated System, Closed System, Open System, The First Law of Thermodynamics, Entropy and second law of thermodynamics, ATP Hydrolysis of ATP, ATP-Coupled Reaction Process, Exergonic reaction, Endergonic reaction
Learning objectives
- To understand about system and surroundings
- To learn about the generation of energy in a cell
Cellular energy
Our bodies contain trillions of cells. Inside each of them are huge numbers of tiny, energy-producing power plants called “mitochondria”. Mitochondria convert the food we eat and the air we breathe into “ATP”, a special type of fuel that powers our cells, and in turn, us.
For example, the monosaccharide glucose, (the most basic form of carbohydrate) can be combined with oxygen. The high-energy electrons that are found in the glucose are transferred to the oxygen and potential energy is released. The energy is stored in the form of ATP.
The energy is used to do work by the cell, usually by the released phosphate binding to another molecule, activating it. For example, in the mechanical work of muscle contraction, ATP supplies the energy to move the contractile muscle proteins.
A set of chemical reactions occurring in a living organism is termed as Metabolism.
Biomolecules are termed as Metabolites. These Metabolites are converted into each other in a series of linked reactions called metabolic pathways. There are 2 types of metabolic pathways:
Anabolic pathways
- Formation of complex structures from simpler ones
- Biosynthetic pathways
- Consume energy
Catabolic pathways
- Formation of simpler structures from complex ones
- Degradation pathways
- Release energy
Systems and Surroundings:
Thermodynamics in biology refers to the study of energy transfers that occur in molecules or collections of molecules. When we are discussing thermodynamics, the particular matter or collection of items that we're interested in is called the system. In contrast, everything that's not included in the system we've defined is called the surroundings.
For instance, consider heating a pot of water on the stove. Here, the system might include the stove, pot, and water, while the surroundings would be everything else: the kitchen, house, etc.
There are three kinds of systems:
Isolated System – An isolated system is incapable of exchanging both energy and mass with its surroundings. The universe is seen as a self-contained system.
Closed System – Across the border of a closed system, energy is transferred but the mass is not transferred. Closed systems include refrigerators and the compression of gas in piston-cylinder assemblies.
Open System – In an open system, both mass and energy can be transmitted between the system and its surroundings. An open system is exemplified by a steam turbine.
The First Law of Thermodynamics- also known as the law of conservation. of energy, states that energy can neither be created nor destroyed. It may change from one form to another, but the energy in a closed system remains constant.
Entropy and second law of thermodynamics
Second law of thermodynamics tells about the degradation of energy caused due to entropy generation during heat transfer between system & surrounding through a finite temperature difference.
The entropy is a property of thermodynamic system which is measure of randomness in a system. The change in entropy is caused due to internal as well as external dissipative effects. The change in entropy of the universe is always non-negative
ATP
Adenosine triphosphate or ATP is the energy currency of the cell. ATP hydrolysis releases the energy present in the high-energy terminal phosphate bonds, which is utilised to carry out various cellular reactions, such as muscle contraction, carbon fixation, etc.
Hydrolysis of ATP
The terminal phosphoanhydride bonds are known as high-energy bonds. They release a large amount of energy on hydrolysis to power the energy-requiring cellular processes.
The reaction of ATP hydrolysis is as follows:
ATP + H2O ⇋ ADP + Pi + Energy
This reaction is reversible. The reversible reaction requires energy to produce ATP. ADP and Pi regenerate ATP. Thus, energy gets stored in the form of ATP and when energy is required, ATP is hydrolysed. The reverse reaction of ATP hydrolysis or the reaction for ATP synthesis is as follows:
ADP + Pi + Energy ⇋ ATP + H2O
The enzyme ATP synthase catalyses the synthesis of ATP
ATP-Coupled Reaction Process:
Energy is either absorbed or released when a reaction occurs. This depends on the relative abilities to break bonds and forming bonds. Two important reactions occur in ATP coupling, namely, exergonic and endergonic reactions.
Exergonic reaction: Energy is emitted into the surroundings in an exergonic reaction. The bonds being generated are stronger than the broken bonds.
Endergonic reaction: Energy is extracted from the atmosphere in an endergonic reaction. The bonds being produced are weaker than the breaks of the bonds.
This is a typical feature of biological systems where two coupled half-reactions, one spontaneous and the other non-spontaneous, are interpretable as any enzyme-catalysed reactions. ATP (adenosine triphosphate) is often hydrolysed by organisms to create ADP (adenosine diphosphate) as a spontaneous coupling reaction.
The high-energy phosphate bonds formed between two phosphate groups of ATP by ejecting water exhibit a wide negative hydrolysis ΔG and are called high-energy bonds. As for all bonds, however, energy is required to break such bonds, but when considering thermodynamics of phosphate ions, the thermodynamic Gibbs energy difference is strong "energy releasing"; ΔG is – 31 kJ/mol for this reaction.
ATP+H2O↔ADP+Pi+energy
ATP is the main metabolism-generated 'energy' molecule, which is the 'energy source' in the cell. ATP is being sent to wherever a non-spontaneous reaction needs to occur so that the two reactions are combined to thermodynamically benefit the overall reaction.
When reaction coupling includes ATP, a phosphorylated molecule (addition of the phosphate group to the molecule from ATP) is often the shared intermediate.
Solved examples
Example 1. ATP hydrolysis is _________.
a) Exergonic b) Endergonic c) Anabolic d) A condensation reaction
Solution 1: a. ATP hydrolysis is exergonic reaction.
Example 2. Exothermic reactions are ones that….
a) Release energy to the surroundings
b) Absorb energy from the surroundings
c) Remain at a constant temperature
d) Decrease in temperature
Solution 2: a. Exothermic reactions are ones that release energy to the surroundings.
Summary
- ATP is generally referred to as the "energy currency" of the cell.
- The energy released by hydrolysis (breakdown) of ATP is used to power many energy-requiring cellular reactions.
- Sugars like glucose are made by plants through a process called photosynthesis.
- Metabolic pathways can be divided into two categories- catabolic and anabolic based on their effects.
- The first law of thermodynamics states that energy cannot be created or destroyed.
- The second law of thermodynamics states that every energy transfer that takes place will increase the entropy of the universe and reduce the amount of usable energy available to do work.