Respiration in Plants : Aerobic Respiration / Glycolysis

Respiration in plants : Glycolysis

Respiration is the process by which living cells break down complex high-energy food molecules into simple low energy molecules, releasing the energy trapped within the chemical bonds.

Synthesis of energy-rich compounds like carbohydrates, fats, proteins etc., also need certain amount energy and thus, are endergonic (energy absorbing) whereas respiration is an exergonic (energy releasing) reaction.

In respiration, many types of high-energy compounds are oxidised.  These are termed as respiratory substrates and may include carbohydrates, fats, and proteins.  Of these, carbohydrates such as glucose, fructose (hexoses), sucrose etc., are the main substrates.  Hexoses are the first energy-rich compounds to be oxidised during respiration.

Types of Respiration:

Depending on the use of molecular oxygen in oxidation, respiration can be classified into two types.  Following are the two types of respiration:

  1. Aerobic respiration: Most of the organism use molecular oxygen for oxidation of their foods.  It is termed as aerobic respiration or oxyrespiration.
  2. Anaerobic respiration: When oxygen is not available for the oxidation of carbohydrates, then carbon dioxide and ethyl alcohol or lactic acid are formed.  This is termed as anaerobic respiration or an oxyrespiration.  It results in incomplete oxidation of the substrate, hence, the energy released is much less than in aerobic respiration.

Mechanism of Aerobic Respiration:

The mechanism of respiration involves following two processes.

  1. Glycolysis: The sequence of reactions which converts glucose into pyruvic acid with the production of ATP is termed as glycolysis. It is a series of reactions in which 6 carbon glucose molecules converts into two 3 carbon molecules of pyruvic acid. It also produces 4 molecules of ATP.  Of these, two molecules of ATP are used up during degradation of glucose into pyruvic acid.  Hence, the net gain is 2 ATP molecules.  All reactions of glycolysis occur in the cytoplasm.  Two molecules of NADH2 are also produced during glycolysis.
  2. Further break down of pyruvic acid molecules by aerobic or anaerobic methods. Complete oxidation of pyruvic acid occurs in the presence of oxygen.  The end products are carbon dioxide and water.  This oxidation is carried out by a cyclic series of reactions termed as tricarboxylic acid cycle or citric cyclic or Krebs’ cycle.  All reactions of Krebs cycle occur within mitochondria.

In the absence of oxygen, pyruvic acid first converts into acetaldehyde and finally into ethyl alcohol.  Under certain conditions, degradation of pyruvic acid into lactic acid occurs.  These reactions take place in the cytoplasm.  Under anaerobic conditions, one molecule of glucose produces 2 ATP (from glycolysis).

Glycolysis:

Glycolysis involves a series of reactions given by three scientists, Embden–Meyerhoff–Parnas.  Hence, it is termed as EMP pathway.  Following are the steps of glycolysis:

Phosphorylation of sugar:

In the first step of glycolysis, phosphorylation of glucose, a stable 6 carbon sugar occurs into fructose 6 phosphate and then to fructose 1, 6 biphosphate . Hence this process, requires two molecules of ATP  .  Following are the steps of phosphorylation of sugar:

  1. First, phosphorylation: ATP donates a phosphate group to the 6th carbon atom of the glucose molecule. The reaction takes place in the presence of enzyme, hexokinase to form glucose 6 phosphate.

Glucose+ATP  →  Glucose-6-phosphate+ADP

  1. Isomerization: It involves isomerization of glucose 6 phosphate molecule to form a 6 carbon compound, fructose 6 phosphate. The reaction takes place in the presence of enzyme, phosphoglucoisomerase.

Glucose-6-phosphate  → Fructose-6-phosphate

  1. Second phosphorylation: Phosphorylation of fructose 6 phosphate takes place with the help of ATP to form fructose 1, 6 biphosphate .  Thus, the phosphorylation of glucose into fructose 1, 6 biphosphate requires two molecules of ATP.

Fructose-6-phosphate+ATP  →  Fructose 1:6 biphosphate+ADP

Splitting of sugar:

 In this step, fructose 1, 6 biphosphate , a 6 carbon molecule splits into two 3 carbon molecules, each with phosphate group attached to one end.

Production of phosphoglyceraldehyde:

Fructose 1, 6 biphosphate splits into two 3 carbon molecules – phosphoglyceraldehyde (PGAL) and dihydroxyacetone phosphate (DHAP).  Of these two, oxidation of only three phosphoglyceraldehyde occurs.  Dihydroxyacetone phosphate enzymatically converts into phosphoglyceraldehyde.

Fructose 1:6 biphosphate  ⇌ Glyceraldehyde-3- phosphate or phosphoglyceraldehyde (PGAL) + Dihydroxyacetone-3-phosphate

Formation of pyruvic acid:

  1. Production of 1, 3 diphosphoglyceric acid:   3 phosphoglyceraldehyde loses hydrogen to NAD (nicotinamide adenine dinucleotide) to form NADH2  and two molecules of 1, 3 diphosphoglyceric acid.  The reaction takes place in  presence of enzyme, 3 phosphoglyceraldehyde dehydrogenase.

Glyceraldehyde-3- phosphate+H3PO4+ NAD+   ⇌  1:3 diphosphoglyceric acid+NADH+H+

  1. Formation of 3 phosphoglyceric acid and ATP: In this reaction, one high-energy phosphate group from each of the two diphosphoglyceric acid molecules transfers to two ADP molecules, thus, forming two ATP molecules. The reaction also requires enzyme, phosphoglycerokinase.  Thus, 1, 3 diphosphoglyceric acid  converts to 3 phosphoglyceric acid.  The direct synthesis of ATP from metabolites is termed as substrate level phosphorylation.

1:3 diphosphoglyceric acid+ADP  ⇌   3 phosphoglyceric acid+ATP

  1. Isomerisation (Production of 2 phosphoglyceric acid): Two molecules of 3 phosphoglyceric acid gives rise to two molecules of 2 phosphoglyceric acid with the help of enzyme, phosphoglyceromutase.

3 phosphoglyceric acid  ⇌   2 phosphoglyceric acid

  1. Dehydration (Formation of phosphoenolpyruvic acid): Removal of a molecule of water  from each molecule of 2 phosphoenolpyruvic acid leads to the formation of two molecules of phosphoenolpyruvic acid. The reaction occurs with the help of enzyme, enolase.

2 phosphoglyceric acid  ⇌   Phosphoenolpyruvic acid+H2O

  1. Production of pyruvic acid and formation of ATP:  During the formation of  Phosphoenolpyruvic acid, the phosphate radical picks up energy. Thus it helps in the production of ATP by substrate level phosphorylation.  The reaction occurs with the help of enzyme, pyruvate kinase.  As a result,  pyruvate or pyruvic acid is formed.

Phosphoenolpyruvic acid+ADP  →  Pyruvic acid+ATP

Respiration in Plants : Schematic representation of Glycolysis or EMP pathway

Fig: Respiration in Plants (Schematic representation of Glycolysis or EMP pathway)

Net production of glycolysis:

  1. Each molecule of glucose produces two molecules of pyruvic acid at the end of glycolysis.
  2. There is a net gain of 2 molecules of ATP in glycolysis.
  3. There is a net gain of two molecules of NADH2 .
  4. During aerobic respiration, each NADH2 forms 3 ATP and H2O through electron transport system of mitochondria. During aerobic respiration there is an additional gain of 6 ATP molecules in glycolysis, resulting in a total net gain of 8 ATP.
  5. Reactions of glycolysis do not require oxygen.
  6. There is no evolution of carbon dioxide in glycolysis.