Respiration is the breaking of the C-C bonds of complex compounds through oxidation within the cells, leading to the release of a considerable amount of energy. The compounds that are oxidised during this process are known as respiratory substrates. Usually, carbohydrates are oxidised to release energy, but proteins, fats and even organic acids can be used. The energy released is not used directly but is used to synthesise ATP, which is broken down whenever energy is needed. Hence, ATP acts as the energy currency of the cell.

Do Plants Breathe? Plants require O2 for respiration and release CO2. Unlike animals, plants have no specialized organs for gaseous exchange but have stomata and lenticels.

• Reasons for absence of respiratory organs:

  1. Each plant part takes care of its own gas-exchange needs. There is very little transport of gases from one plant part to another.

  2. Plants do not present great demands for gas exchange. Roots, stems and leaves respire at rates far lower than animals do.

  3. The distance that gases must diffuse even in large, bulky plants is not great. In thick woody stems, living cells are organised in thin layers inside and beneath the bark; the interior contains dead cells (pith) that provide mechanical support.

Glycolysis (EMP Pathway) The term glycolysis has originated from the Greek words, glycos for sugar, and lysis for splitting. The scheme of glycolysis was given by Gustav Embden, Otto Meyerhof, and J. Parnas, and is often referred to as the EMP pathway.

• In anaerobic organisms, it is the only process in respiration.

• It occurs in the cytoplasm of the cell and is present in all living organisms.

• In this process, glucose undergoes partial oxidation to form two molecules of Pyruvic acid (3-carbon).

• Steps:

  • Sucrose is converted into glucose and fructose by the enzyme invertase.

  • Glucose is phosphorylated to Glucose-6-phosphate by Hexokinase (uses 1 ATP).

  • Key Step: Fructose-6-phosphate is converted to Fructose-1,6-bisphosphate (uses 2nd ATP).

  • Splitting occurs: Fructose-1,6-bisphosphate splits into PGAL (3-phosphoglyceraldehyde) and DHAP (Dihydroxy Acetone Phosphate).

  • Energy Yield: There are two steps where ATP is generated directly (Substrate Level Phosphorylation): conversion of 1,3-bisphosphoglyceric acid (BPGA) to 3-phosphoglyceric acid (PGA) and Phosphoenolpyruvate (PEP) to Pyruvic acid.

  • NAD+ is reduced to NADH + H+ during the conversion of PGAL to BPGA.

• Net Products: 2 Pyruvic Acid + 2 ATP + 2 NADH.

Fermentation (Anaerobic Respiration) Occurs in many prokaryotes and unicellular eukaryotes.

• Alcoholic Fermentation: Yeast performs this. Pyruvic acid is converted to CO2 and Ethanol. Enzymes involved: Pyruvic acid decarboxylase and Alcohol dehydrogenase.

• Lactic Acid Fermentation: In some bacteria and animal muscles (during exercise when oxygen is inadequate), pyruvic acid is reduced to Lactic acid by Lactate dehydrogenase.

• Drawbacks: Hazardous products (acid/alcohol) are formed. Energy production is low (less than 7% of energy in glucose is released). Yeasts poison themselves to death when the concentration of alcohol reaches about 13%.

   

Aerobic Respiration For aerobic respiration to take place, pyruvate enters the mitochondria. The crucial events are:

1. Link Reaction: Pyruvate is transported into the mitochondria and undergoes oxidative decarboxylation by the enzyme complex Pyruvate dehydrogenase.

    

   • Pyruvic acid + CoA + NAD+ -> Acetyl CoA + CO2 + NADH + H+

   • Acetyl CoA then enters the Krebs Cycle.

2. Tricarboxylic Acid Cycle (TCA / Krebs Cycle):

   • Starts with the condensation of Acetyl CoA with Oxaloacetic acid (OAA) and water to yield Citric acid. Enzyme: Citrate synthase.

   • Citrate is isomerised to Isocitrate.

   • Two successive steps of decarboxylation lead to the formation of alpha-ketoglutaric acid (5C) and then Succinyl-CoA (4C).

   • GTP synthesis: Conversion of Succinyl-CoA to Succinic acid produces GTP (Substrate level phosphorylation).

   • FADH2 production: Conversion of Succinate to Fumarate reduces FAD+ to FADH2.

   • Summary: For every 2 molecules of Acetyl CoA (from 1 Glucose): 4 CO2, 6 NADH, 2 FADH2, and 2 ATP/GTP are produced.

3. Electron Transport System (ETS) and Oxidative Phosphorylation:

   • Located in the inner mitochondrial membrane.

   • Electrons from NADH (Complex I) and FADH2 (Complex II) are passed through carriers to generate a proton gradient.

   • Complex I: NADH dehydrogenase.

   • Complex II: FADH2 transfers electrons to Ubiquinone.

   • Complex III: Cytochrome bc1 complex.

   • Complex IV: Cytochrome c oxidase (contains Cytochromes a and a3, and two copper centres).

   • Cytochrome c: A small protein attached to the outer surface of the inner membrane, acts as a mobile carrier between Complex III and IV.

   • Final Acceptor: Oxygen acts as the final hydrogen acceptor interacting with protons to form metabolic water.

   • Complex V (ATP Synthase): Protons return from inter-membrane space to the matrix through F0, driving F1 to synthesize ATP.

   • Ratio: Oxidation of 1 NADH produces 3 ATP; 1 FADH2 produces 2 ATP.

The Respiratory Balance Sheet Calculations are based on assumptions (sequential pathway, NADH transferred to mitochondria, no intermediates withdrawn).

• Net Gain: 38 ATP molecules for one glucose molecule (in aerobes). Fermentation yields only 2 ATP.

Amphibolic Pathway Respiration is traditionally a catabolic process (breakdown). However, intermediates of the pathway are withdrawn to synthesize other substrates (e.g., Acetyl CoA used to synthesize fatty acids). Since it involves both breakdown (catabolism) and synthesis (anabolism), the respiratory pathway is better termed an Amphibolic Pathway.

• Fatty acids enter as Acetyl CoA.

• Proteins enter as Pyruvate or Acetyl CoA (after deamination).

Respiratory Quotient (RQ) The ratio of the volume of CO2 evolved to the volume of O2 consumed. RQ = CO2 evolved / O2 consumed.

• Carbohydrates: RQ = 1.0.

• Fats: RQ is less than 1. For Tripalmitin, RQ = 0.7.

• Proteins: RQ = 0.9.

• Organic Acids: RQ > 1.