Every cell, whether a single‑cell bacterium or a complex eukaryotic organism, relies on metabolic processes to produce the energy needed for movement, division, growth, and myriad other functions. Metabolism— the coordinated series of biochemical reactions that convert nutrients into usable energy— is the lifeblood of cellular life.

What Is Cellular Metabolism?

In cell biology, metabolism refers to the enzymatically driven reactions that sustain living organisms. While the term is often used in nutrition to describe how our bodies process food, in molecular biology it specifically denotes the biochemical pathways that generate ATP, the universal energy currency.

Key Metabolic Pathways

Cellular metabolism encompasses several distinct pathways. The most widely studied are cellular respiration and photosynthesis:

• Cellular respiration – the breakdown of glucose to produce ATP, primarily occurring in mitochondria of eukaryotic cells.
• Photosynthesis – the conversion of light energy into chemical energy, performed by chloroplasts in plants, algae, and cyanobacteria.

Cellular Respiration in Eukaryotes

In eukaryotic cells, respiration proceeds through four stages:

• Glycolysis – cytoplasmic conversion of one glucose molecule into two pyruvate molecules, generating 2 ATP and 2 NADH.
• Pyruvate oxidation – mitochondrial entry of pyruvate, producing acetyl‑CoA, 2 CO₂, and 2 NADH per glucose.
• Citric acid (Krebs) cycle – acetyl‑CoA combines with oxaloacetate, yielding 2 CO₂, 3 NADH, 1 FADH₂, and 1 ATP per glucose.
• Oxidative phosphorylation – the electron transport chain harnesses electrons from NADH and FADH₂ to pump protons, driving ATP synthase to produce ~30‑32 ATP per glucose, with water as a final product.

Oxygen serves as the final electron acceptor, making this process aerobic. In the absence of oxygen, cells may rely on anaerobic pathways such as lactic acid fermentation.

Photosynthesis in Plants and Cyanobacteria

Photosynthetic organisms capture light energy in chloroplasts, using two main stages:

• Light‑dependent reactions – occur in thylakoid membranes; chlorophyll absorbs light, producing ATP, NADPH, and splitting water into O₂.
• Calvin cycle (light‑independent reactions) – in the stroma, ATP and NADPH fix CO₂ into glyceraldehyde‑3‑phosphate (G3P), ultimately forming glucose.

Chlorophyll a, the most abundant pigment, absorbs blue and red wavelengths; chlorophyll b extends absorption into the green spectrum, while chlorophyll c is found in dinoflagellates.

Metabolism in Prokaryotes

Prokaryotic organisms exhibit remarkable metabolic diversity, categorized as:

• Heterotrophic – derive carbon from organic compounds.
• Autotrophic – fix CO₂ as their carbon source; many are photosynthetic.
• Phototrophic – use light energy directly.
• Chemotrophic – obtain energy by oxidizing inorganic chemicals.

Oxygen tolerance varies: obligate aerobes require O₂, obligate anaerobes cannot tolerate it, and facultative anaerobes switch between aerobic and anaerobic metabolism depending on conditions. For example, Clostridium botulinum thrives in anaerobic environments and can produce botulism toxin.

Lactic Acid Fermentation

When oxygen is scarce, many organisms—including human muscle cells—use lactic acid fermentation to generate ATP. Glycolysis produces pyruvate, which is reduced to lactic acid by lactate dehydrogenase, regenerating NAD⁺ for continued glycolysis. This pathway is exploited industrially in yogurt production, where Lactobacillus bulgaricus ferments lactose into lactic acid, curdling milk into yogurt.

Anabolic vs. Catabolic Pathways

Metabolic pathways fall into two categories:

• Anabolic – energy‑intensive synthesis of complex molecules from simpler precursors (e.g., photosynthesis).
• Catabolic – energy‑releasing breakdown of complex molecules into simpler ones (e.g., cellular respiration).

Both eukaryotes and prokaryotes depend on a balance of these pathways to maintain cellular function and growth.

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• Gene expression
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