Metabolic Mechanisms of cells

Cellular metabolism is the set of chemical reactions in cells that maintain life, converting food into energy and building blocks, and eliminating waste. It is a highly coordinated process of two main parts: catabolism, which breaks down molecules to release energy, and anabolism, which uses energy to build complex molecules. Catabolism comprises respiration and nutrient consumption to produce energy. Anabolism encompasses building components for cellular replication and protein production.

1) Replication mechanisms of the cells.

Asexual Replication

• Definition: Involves a single parent cell producing an offspring cell without gametes.

• Outcome: Offspring cells are genetically identical clones of the parent cell.

• Mechanisms:

• Mitosis:

• Cell divides into two daughter cells. Mitosis is common in prokaryotic cells.

• Each daughter cell receives a complete set of chromosomes.

• Used for growth, repair, and maintenance in multicellular organisms.

• Budding:

  -A new cell develops from an outgrowth (bud) of the parent cell (commonly seen in yeast cells).

  -The bud eventually detaches from parent cell, forming a genetically identical offspring cell.

Sexual Replication

• Definition: Requires two parent cells, with gametes, contributing genetic material (i.e mature haploid male cell and female germ cell (totipotent cells), which are able to unite to form a zygote cell (a pluripotent cell).

• Outcome: Offspring cell is genetically unique, combining traits from both parent cells.

• Mechanism:

  -Meiosis:

• Specialized cell division that reduces chromosome number by half.

• Produces haploid gametes (i.e. sperm and egg).

• Meiosis is more common in eukaryotic cells and in yeast-cell sporulation process.

2) Respiration mechanisms of the cells.

Cellular respiration is the process by which cells convert nutrients into usable energy (ATP).

Aerobic respiration uses oxygen as the final electron acceptor to yield ATP. Regular by-products of cellular biomass generation are CO2 and H2O.

Anaerobic respiration occurs without oxygen, using alternative electron acceptors, like sulfate or nitrate. Denitrification is a specific type of anaerobic respiration where nitrate is reduced to N2 gas as a by-product, completing the nitrogen cycle.

3) Nutrient Consumption mechanisms of the cells.

Autotrophic cells

• Generate their own organic molecules (sugars, amino acids, lipids) from inorganic sources.

• Photosynthetic organisms (plant cells, algae cells) use CO2 as a carbon source, light as energy, and water as an electron donor.

• Chemolithoautotrophs oxidize inorganic compounds (e.g., ammonia, sulfide) to fix carbon.

Heterotrophic cells

• Depend on external organic molecules (sugars, amino acids, lipids) as both carbon sources and energy.

• Most bacteria, fungi, and animal cells fall into this category.

Core Metabolic Pathways

• Glycolysis

• Universal pathway breaking down glucose (From sugars) into pyruvate.

• Produces ATP and NADH, serving as an entry point for carbon metabolism in heterotrophs.

• TCA Cycle (Tricarboxylic Acid Cycle / Krebs Cycle)

• Oxidizes acetyl-CoA derived from pyruvate and other substrates.

• Generates NADH, FADH2, and precursors for biosynthesis.

• Links carbon metabolism to energy production.

• Electron Transport Chain (ETC)

  -Uses NADH and FADH2 to drive ATP synthesis via oxidative phosphorylation.

  -Final electron acceptors vary: oxygen in aerobic organisms, alternative molecules (nitrate, sulfate) in anaerobes.

Carbon and Nitrogen Utilization (Carbon source and Nitrogen source)

Carbon and nitrogen are essential for cells for catabolism (to produce energy), and for anabolism (as building blocks to build cellular components)

• Carbon source:

• Autotrophs: CO2

• Heterotrophs: Organic molecules (sugars, glucose, fatty acids (ketoacidosis), amino acids).

• Determines whether cells build biomass from inorganic or organic carbon.

• Nitrogen source:

  -Essential for the synthesis of amino acids, nucleotides, and cofactors.

  -Can be assimilated as ammonium, nitrate, or organic nitrogen.

  -Specialized microbial processes include:

  —-> Nitrification: Oxidation of ammonium (NH₄⁺) to nitrite (NO₂⁻) and nitrate (NO₃⁻).

  —-> Provides energy for nitrifying bacteria and contributes to nitrogen cycling in ecosystems.

4) The “Central Dogma of Molecular Biology.”

The Central Dogma of Molecular Biology describes the directional flow of genetic information: DNA → RNA → Protein. It explains how genetic instructions stored in DNA are transcribed into RNA and then translated into proteins, which carry out most cellular functions.

Detailed Explanation (Click HERE for more information)

• Origin: The concept was first articulated by Francis Crick in 1957 and published in 1958. Crick emphasized that once information is transferred into protein, it cannot flow back to nucleic acids.

• Core Principle:

• Replication: DNA can duplicate itself, ensuring genetic continuity.

• Transcription: DNA sequences are transcribed into messenger RNA (mRNA).

• Translation: mRNA is decoded by ribosomes to synthesize proteins, which perform structural, enzymatic, and regulatory roles.

• Irreversibility: The dogma states that information cannot flow backward from protein to nucleic acid. In other words, proteins do not serve as templates for RNA or DNA.

• Exceptions: While the central dogma provides a foundational framework, biology has revealed exceptions:

• Reverse transcription: Retroviruses (like HIV) use reverse transcriptase to copy RNA back into DNA.

• RNA viruses: Some viruses bypass DNA entirely, using RNA directly as genetic material.

• Prions: Infectious proteins that replicate without nucleic acids, challenging the strict DNA → RNA → Protein pathway.

• Biological Significance:

  -Provides a conceptual framework for molecular biology.

  -Explains how genotype (DNA sequence) leads to phenotype (protein function).

  -Underpins biotechnology applications such as recombinant protein production, gene therapy, and CRISPR-based editing.

Central Dogma of Molecular Biology- source: MDPI.

Click HERE (DNA to RNA)and HERE (RNA to Protein) to see 3D-model videos that explain the Central Dogma of Molecular Biology.