Revise Energy for Cambridge 9700 (12.1)

Life is an active process that requires a continuous input of energy to counteract entropy and maintain a highly organised state. This energy powers all essential life processes, which can be broadly categorised as:

• Metabolism: This includes both anabolic reactions (synthesis of complex molecules like proteins and DNA from simpler ones) and catabolic reactions (breakdown of molecules to release energy). Energy is required for synthesis.
• Active Transport: Moving ions and molecules against their concentration gradient across cell membranes. This is vital for nutrient absorption, waste removal, and nerve impulse transmission.
• Mechanical Work: This involves movement at various levels, including muscle contraction for locomotion, the beating of cilia and flagella, and the movement of chromosomes during cell division.
• Maintenance and Repair: Organisms expend energy to maintain body temperature (in endotherms), repair damaged tissues, and maintain a stable internal environment (homeostasis).
• Specialised Functions: Some organisms use energy for unique processes like bioluminescence (producing light) or generating electrical discharges.

While large molecules like glucose store significant amounts of chemical energy, this energy is not directly usable by cells. It must be converted into a readily accessible form: adenosine triphosphate (ATP). ATP is a nucleotide derivative that acts as the immediate energy source for cellular reactions.

ATP is composed of three parts:

1. Adenine: A nitrogen-containing organic base.
2. Ribose: A five-carbon sugar.
3. Three Phosphate Groups: A chain of three phosphate groups linked to the ribose sugar. The bonds between these phosphate groups are relatively unstable and are often called 'high-energy' bonds because their hydrolysis releases a significant amount of energy.

ATP is superior to glucose as an immediate energy source because its hydrolysis releases energy in small, manageable packets, preventing waste and cellular damage that would result from the massive energy release from glucose oxidation.

ATP functions in a continuous cycle of synthesis and breakdown, acting as a rechargeable battery for the cell.

ATP Hydrolysis (Energy Release): When a cell needs energy, the terminal phosphate bond of an ATP molecule is broken by hydrolysis (reaction with water). This is an exergonic reaction catalysed by the enzyme ATPase. The products are adenosine diphosphate (ADP), an inorganic phosphate ion (Pᵢ), and a release of usable energy.

ATP Synthesis (Phosphorylation): To regenerate ATP, an inorganic phosphate group is added back to ADP. This is an endergonic reaction called phosphorylation, which requires an input of energy. This energy is primarily derived from the catabolism of glucose during cellular respiration or from light energy during photosynthesis. This process is catalysed by the enzyme ATP synthase.

ATP + H₂O ⇌ ADP + Pᵢ + Energy (~30.5 kJ mol⁻¹) This reversible reaction shows ATP hydrolysis (forward reaction) releasing energy, and ATP synthesis (reverse reaction) requiring energy.

This cycle allows for energy coupling, where the energy released from ATP hydrolysis is directly used to drive other energy-requiring (endergonic) reactions in the cell, such as active transport or the synthesis of a protein.

The majority of ATP is synthesised via a process called chemiosmosis. This process relies on the enzyme ATP synthase, a large protein complex embedded in the inner mitochondrial membrane (in respiration) and the thylakoid membrane (in photosynthesis). During the electron transport chain, protons (H⁺ ions) are pumped across the membrane, creating a steep electrochemical gradient (also known as a proton-motive force). These protons then flow back down their concentration gradient through a channel in the ATP synthase enzyme. This flow of protons causes a part of the enzyme to rotate, driving the catalytic activity that joins ADP and Pᵢ to form ATP. It functions like a tiny molecular turbine.

Coenzymes are non-protein organic molecules that are essential for the function of certain enzymes. In the context of energy metabolism, several coenzymes act as carriers for electrons, hydrogen ions, or chemical groups.

• NAD (Nicotinamide adenine dinucleotide): A key coenzyme in respiration. It acts as a hydrogen carrier by accepting hydrogen atoms (becoming reduced NAD or NADH) during glycolysis and the Krebs cycle. It then donates these hydrogens to the electron transport chain, where their energy is used to generate ATP.
• FAD (Flavin adenine dinucleotide): Another hydrogen carrier used in respiration, specifically in the Krebs cycle. It accepts hydrogens to become reduced FAD (FADH₂) and, like NAD, delivers them to the electron transport chain.
• Coenzyme A (CoA): This coenzyme is responsible for carrying acetyl groups. In the link reaction, it combines with a two-carbon fragment from pyruvate to form acetyl CoA, which then enters the Krebs cycle.