Main stages of metabolism
The global metabolic process can be divided into three sequential stages that ensure the connection of the organism with the external environment:
- Ingestion of substances. Begins with respiration (oxygen supply) and nutrition. In the digestive tract, polymers (proteins, fats, and carbohydrates) undergo hydrolysis to monomers. The resulting simple compounds are absorbed, easily enter the bloodstream, and are delivered to the tissues.
- Intermediary metabolism (cellular metabolism). Direct transformation of metabolites inside cells. At this stage, pathways of breakdown and synthesis closely intertwine, forming a single network connected by energy flows.
- Excretion. The final stage in which the organism gets rid of the resulting end products: water, carbon dioxide, and nitrogenous waste (specifically, urea).
Two sides of metabolism: catabolism and anabolism
Intracellular metabolism is strictly divided into two opposite, but interdependent directions:
- Catabolism (dissimilation) is the breakdown of complex organic structures into simple end products (water, urea, carbon dioxide). The reactions are exergonic in nature — they proceed with the release of energy.
- Anabolism (assimilation) is the totality of biosynthetic reactions. The organism assembles its own structural and functional polymers from simple molecules. These processes are endergonic, meaning they require the obligatory consumption of energy obtained during the catabolism stage.
ATP-ADP cycle: the universal energy currency
A central role in energy metabolism is played by adenosine triphosphate (ATP). The energy released during the oxidation of nutrients is not dissipated in vain, but is accumulated in the special high-energy bonds of this molecule.
Cellular functioning is ensured by a continuous turnover of molecules, known as the ATP-ADP cycle:
- ATP synthesis. Due to the oxidation of fats, carbohydrates, and proteins, inorganic phosphate is attached to ADP (adenosine diphosphate).
- ATP hydrolysis. The molecule is split back into ADP and phosphate, releasing energy to perform functional activity.
The energy of hydrolysis is spent on ensuring all types of work: biosynthesis of new macromolecules, muscle contractions, active membrane transport of substances, as well as osmotic, electrical work, and heat production.
Features of ATP metabolism
The energy balance requires colossal amounts of ATP, but its turnover has strict limitations. The cell is not capable of storing ATP in reserve (absence of storage).
Any formed molecule is consumed almost immediately — in just one minute. This dictates the necessity of absolute continuity of the process: the number of spent molecules must be immediately replenished through their de novo synthesis.
Briefly: answers to topical questions
What enzymes are involved in ATP synthesis in the respiratory chain?
Sources for the respiratory chain and oxidative phosphorylation indicate:
- Complex I (NADH dehydrogenase) — an enzyme complex of the respiratory chain; inhibited by barbiturates and rotenone.
- Complex III ($QH_2$ dehydrogenase) — an enzyme complex of the respiratory chain; inhibited by antimycin A.
- Complex IV (cytochrome c oxidase) — an enzyme complex of the respiratory chain; inhibited by CO, $H_2S$, and cyanide.
- ATP synthase — uses the energy of the proton gradient ($\Delta\mu H^+$) for ATP synthesis according to the scheme: ADP + $P_i$ + energy of $\Delta\mu H^+$ → ATP + $H_2O$.
Electrons from reduced coenzymes ($NADH$, $FADH_2$) are transferred along the electron transport chain to oxygen, and the transfer energy is used to create a proton gradient.
How does active membrane transport occur at the expense of ATP?
Active membrane transport using ATP energy is carried out by the mechanisms of primary active and secondary active transport.
- Primary active transport occurs with the direct expenditure of ATP energy with the participation of transport ATPases: $Na^+$,$K^+$-ATPase, $H^+$-ATPase, $Ca^{2+}$-ATPase.
- Secondary active transport occurs due to the concentration gradient of another substance, most often $Na^+$, which was previously created with the expenditure of ATP, for example, by the work of $Na^+$,$K^+$-ATPase. ATP energy is not directly spent on the transport of this substance. When a substance binds to the active site of the carrier protein, the protein conformation changes. Types of secondary active transport: active symport and active antiport.
What pathways of carbohydrate breakdown provide ATP synthesis?
ATP synthesis during carbohydrate breakdown is provided by anaerobic and aerobic glucose breakdown.
- Anaerobic glycolysis proceeds without the participation of oxygen. ATP is formed exclusively through substrate-level phosphorylation; 2 mol of ATP and 2 mol of lactate are formed from 1 mol of glucose.
- Aerobic glucose breakdown includes aerobic glycolysis (glucose → 2 pyruvate) and the oxidation of pyruvate in the common catabolic pathway. During aerobic glycolysis, ATP synthesis proceeds via two pathways: substrate-level phosphorylation and oxidative phosphorylation.
- According to sources, complete aerobic catabolism of 1 mol of glucose yields 38 mol of ATP at classical P/O ratios; a yield of 30 or 32 mol of ATP is also indicated depending on the shuttle mechanism of NADH transport from the cytosol to the mitochondria.
How not to confuse the terms: Catabolism —
Class and exam questions
What are the end products of catabolism?
As a result of dissimilation reactions, complex substances are broken down into simplest compounds. The main ones are carbon dioxide, water, and nitrogenous waste products such as urea.
Why is ATP called the main energy accumulator?
All energy released during exergonic catabolic reactions is stored in the high-energy bonds of ATP. During the subsequent hydrolysis of this molecule, the accumulated energy is released to support cellular work.
What types of cellular work are supported by ATP energy?
Energy is expended on mechanical, osmotic, and electrical work. In practice, these are muscle contractions, active transport of substances across membranes, heat production, and macromolecule biosynthesis.
Can the organism create a reserve of ATP for the future?
No, adenosine triphosphate is not stored in tissues. Molecules are synthesized and consumed within one minute, so the ATP-ADP cycle must function continuously.
What else to study on this topic
- Processes of polymer hydrolysis in the digestive tract
- Mechanisms of monomer absorption into the systemic circulation
- Biochemical pathways of urea and nitrogenous waste excretion
- Mechanisms of active membrane transport
- Types of cellular work: osmotic, mechanical, and electrical
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