Which organelle contains enzymes for cellular respiration?

Cellular respiration is a complex process by which cells convert nutrients, primarily glucose, into energy in the form of adenosine triphosphate (ATP). This process occurs in both the cytoplasm and mitochondria of cells and involves three main stages: glycolysis, the Krebs cycle (citric acid cycle), and the electron transport chain (ETC).

Note: Cellular respiration is typically described as a three-stage process, although some explanations break it down into four stages for clarity. Four stages:
  1. glycolysis
  2. Pyruvate Oxidation
  3. Krebs cycle (citric acid cycle)
  4. The electron transport chain (ETC). 

01. Glycolysis

Location: Cytoplasm

Process: 
  • Glycolysis is the first step in cellular respiration and occurs in the cytoplasm. It involves the breakdown of one molecule of glucose (a six-carbon sugar) into two molecules of pyruvate (each containing three carbons).
  • This process consists of ten enzymatic steps that convert glucose into pyruvate, producing a net gain of energy-rich molecules.
Key Enzymes:
  • Hexokinase
  • Phosphoglucose Isomerase (Glucose-6-phosphate isomerase)
  • Phosphofructokinase-1 (PFK-1)
  • Aldolase
  • Triose Phosphate Isomerase
  • Glyceraldehyde-3-phosphate Dehydrogenase
  • Phosphoglycerate Kinase
  • Phosphoglycerate Mutase
  • Enolase
  • Pyruvate Kinase
Products: 
  • 2 ATP (net gain)
  • 2 NADH (nicotinamide adenine dinucleotide, reduced form)
  • 2 Pyruvate molecules
Note: Glycolysis does not require oxygen and is the initial stage before entering the mitochondrion.

02. Pyruvate Oxidation

Location: Mitochondrial matrix

Process: 
  • Each pyruvate molecule is transported into the mitochondrion where it is converted into acetyl-CoA. This conversion is catalyzed by the pyruvate dehydrogenase complex.
  • During this process, each pyruvate molecule loses one carbon in the form of carbon dioxide (CO₂) and generates one NADH.
Key Enzymes:
  • Pyruvate Dehydrogenase (E1)
  • Dihydrolipoyl Transacetylase (E2)
  • Dihydrolipoyl Dehydrogenase (E3)
Products: 
  • 2 Acetyl-CoA (from 2 pyruvate molecules)
  • 2 NADH
  • 2 CO₂

03. Krebs Cycle (Citric Acid Cycle)

Location: Mitochondrial matrix

Process: 
  • Acetyl-CoA enters the Krebs cycle, where it combines with oxaloacetate to form citrate.
  • This cycle consists of a series of reactions that regenerate oxaloacetate, allowing the cycle to continue.
  • For each acetyl-CoA that enters the cycle, energy is captured in the form of ATP (or GTP), NADH, and FADH₂.
Key Enzymes:
  • Citrate synthase
  • Aconitase
  • Isocitrate dehydrogenase
  • Alpha-ketoglutarate dehydrogenase
  • Succinyl-CoA synthetase (also known as succinate thiokinase)
  • Succinate dehydrogenase
  • Fumarase
  • Malate dehydrogenase
Products: 
  • 2 ATP
  • 6 NADH
  • 2 FADH₂ (flavin adenine dinucleotide, reduced form)
  • 4 CO₂ (per glucose molecule)

04. Electron Transport Chain (ETC) and Oxidative Phosphorylation

Location: Inner mitochondrial membrane

Process: 
  • NADH and FADH₂ donate electrons to the ETC, a series of protein complexes and other molecules embedded in the inner mitochondrial membrane.
  • As electrons pass through the chain, they release energy used to pump protons (H⁺) from the mitochondrial matrix to the intermembrane space, creating a proton gradient.
  • Oxygen serves as the final electron acceptor, combining with electrons and protons to form water (H₂O).
  • ATP Synthesis: The proton gradient drives protons back into the mitochondrial matrix through ATP synthase, a process known as chemiosmosis. This flow of protons through ATP synthase catalyzes the conversion of ADP and inorganic phosphate (Pi) into ATP.
Key Enzymes:
  • NADH dehydrogenase (Complex I)
  • Succinate dehydrogenase (Complex II)
  • Cytochrome bc1 complex (Complex III)
  • Cytochrome c oxidase (Complex IV)
  • ATP synthase (Complex V)
  • Additionally, there are mobile electron carriers involved: Ubiquinone (Coenzyme Q) and Cytochrome c
Products:
  • Approximately 34 ATP
  • Water (H₂O)
  • Regenerated NAD⁺ and FAD


In total, cellular respiration can produces a total of up to 38 ATP molecules per glucose molecule (2 from glycolysis, 2 from the Krebs cycle, and about 34 from the ETC), along with NADH, FADH₂, CO₂, and water.


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SAQ 1

  1. What is a cell? What are the essential characteristics of cells?
  2. Explain the fluid mosaic model of the plasma membrane
  3. Which organelles are involved in photosynthesis?
  4. Why the mitochondria is called the powerhouse of the cell?
  5. Which organelle contains enzymes for cellular respiration?
  6. Why mitochondria and chloroplast are called semi-autonomous?
  7. Mention any two advantages of the extensive network of the endoplasmic reticulum
  8. What is the function of peroxisomes in plant cells?
  9. Explain the following terms: (a) chromatin network (b) chromosomes (c) Nucleosome (d) Solenoid Model
  10. What is the function of the nucleolus in the cell?

SAQ 2



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