Various Metabolic reactions

Metabolism refers to all the chemical reactions that occur within living organisms to maintain life. These reactions are broadly classified into two categories: catabolic reactions (breaking down molecules to release energy) and anabolic reactions (building complex molecules from simpler ones). Below are some key metabolic reactions with examples:

1. Glycolysis (Catabolic)

Glycolysis is the process where glucose (a six-carbon molecule) is broken down into two molecules of pyruvate (a three-carbon molecule). This occurs in the cytoplasm and produces ATP (energy) and NADH (an electron carrier).

• Example: During high-intensity exercise, muscle cells break down glucose via glycolysis to rapidly produce ATP.

Steps involved:

• Glucose → Glucose-6-phosphate (via hexokinase enzyme)
• Several steps later, Phosphoenolpyruvate (PEP) is converted into pyruvate by the enzyme pyruvate kinase.

Products: 2 ATP (net), 2 NADH, and 2 pyruvate molecules.

2. Citric Acid Cycle (Krebs Cycle) (Catabolic)

The citric acid cycle takes place in the mitochondrial matrix and is a central part of cellular respiration. Here, acetyl-CoA (derived from pyruvate) is oxidized to CO₂, and electrons are transferred to NAD⁺ and FAD, forming NADH and FADH₂.

• Example: After glycolysis, pyruvate enters the mitochondria, is converted to acetyl-CoA, and then enters the citric acid cycle.

Steps involved:

• Acetyl-CoA reacts with oxaloacetate to form citrate.
• The cycle generates 3 NADH, 1 FADH₂, and 1 ATP (or GTP) per acetyl-CoA.

Products: NADH, FADH₂, ATP, and CO₂.

3. Oxidative Phosphorylation (Catabolic)

Oxidative phosphorylation occurs in the inner mitochondrial membrane and is responsible for the bulk of ATP production. This process involves the electron transport chain (ETC) and chemiosmosis.

• Example: After the citric acid cycle, NADH and FADH₂ donate electrons to the electron transport chain, driving the production of ATP.

Steps involved:

• Electrons are passed along complexes I to IV in the electron transport chain, generating a proton gradient across the membrane.
• ATP is synthesized by ATP synthase as protons flow back into the mitochondrial matrix.

Products: 32-34 ATP molecules per glucose molecule.

4. Gluconeogenesis (Anabolic)

Gluconeogenesis is the process of synthesizing glucose from non-carbohydrate sources, such as amino acids, glycerol, and lactate. It mainly occurs in the liver and kidneys during periods of fasting or intense exercise.

• Example: After prolonged fasting, the body converts amino acids from muscle tissue into glucose for energy.

Steps involved:

• Pyruvate is converted back to phosphoenolpyruvate (PEP) by pyruvate carboxylase and PEP carboxykinase.
• Glucose-6-phosphate is finally converted into glucose.

Products: Glucose (energy source).

5. Fatty Acid Oxidation (Beta-Oxidation) (Catabolic)

Fatty acid oxidation takes place in the mitochondria and is the process by which fatty acids are broken down into acetyl-CoA, which can then enter the citric acid cycle.

• Example: During prolonged exercise, when glycogen stores are depleted, the body relies on fat stores, and fatty acids are oxidized for energy.

Steps involved:

• Fatty acids are activated in the cytoplasm and transported into the mitochondria by the carnitine shuttle.
• In the mitochondria, they undergo successive cycles of oxidation, producing acetyl-CoA, NADH, and FADH₂.

Products: Acetyl-CoA, NADH, FADH₂.

6. Photosynthesis (Anabolic)

Photosynthesis occurs in chloroplasts of plants and some bacteria, where light energy is used to convert carbon dioxide and water into glucose and oxygen.

• Example: Plants convert sunlight into energy during the day to produce glucose, which is used for their growth.

Steps involved:

• Light-dependent reactions: Light energy is absorbed by chlorophyll, which powers the production of ATP and NADPH.
• Calvin Cycle (Light-independent reactions): ATP and NADPH are used to fix CO₂ into glucose.

Products: Glucose and O₂.

7. Protein Synthesis (Anabolic)

This process involves the creation of proteins from amino acids in cells. It is an anabolic process that takes place in the ribosomes.

• Example: After exercise, muscle cells increase protein synthesis to repair and build muscle tissue.

Steps involved:

• Transcription: DNA is transcribed into mRNA in the nucleus.
• Translation: mRNA is translated into a polypeptide chain by ribosomes with the help of tRNA bringing in amino acids.

Products: Proteins.

8. Pentose Phosphate Pathway (PPP) (Anabolic)

This pathway occurs in the cytoplasm and serves as an alternative to glycolysis. It produces NADPH (for biosynthesis) and ribose-5-phosphate (for nucleotide synthesis).

• Example: The PPP is especially active in cells involved in fatty acid synthesis, such as liver and adipose tissues.

Steps involved:

• Glucose-6-phosphate is converted into ribulose-5-phosphate.
• NADPH is produced, which is crucial for reductive biosynthesis and maintaining reduced glutathione in red blood cells.

Products: NADPH, ribose-5-phosphate.

9. Urea Cycle (Catabolic/Detoxifying)

The urea cycle occurs in the liver and is responsible for converting toxic ammonia (from amino acid breakdown) into urea for excretion.

• Example: After protein digestion, excess nitrogen in the form of ammonia is detoxified via the urea cycle.

Steps involved:

• Ammonia combines with CO₂ to form carbamoyl phosphate.
• Urea is ultimately produced and excreted via the kidneys.

Products: Urea (waste product).

These reactions are interconnected, and the balance between catabolism and anabolism ensures cells function properly, manage energy needs, and maintain homeostasis.