GLYCOLYSIS
Introduction :
★ Glycolysis, also known as the EMP pathway (Embden–Meyerhof–Parnas), is a metabolic
process in which one molecule of glucose is partially broken down/ oxidised to produce two
molecules of pyruvate, releasing energy in the process.
★ It is a ten-step process composed of enzyme-catalyzed reactions. It takes place in the
cytosol of the cell.
★ Almost all cellular organisms perform glycolysis. It is estimated to have evolved over 3.5
billion years ago, suggesting that it is one of the ancient metabolic pathways and has been
well conserved throughout. This also explains why glycolysis is an oxygen independent
process as the primitive environment of Earth was deprived of oxygen.
★ Glucose has relatively high potential energy, and thus it is a good fuel. It is not only an
excellent fuel, but also a remarkably versatile precursor, capable of supplying a huge array
of metabolic intermediates for biosynthetic reactions.
★ As glycolysis is an oxygen independent process it can be performed in aerobic as well as
anaerobic conditions.
★ Under aerobic conditions, the pyruvate produced undergoes further oxidation in the
mitochondria via the Citric Acid Cycle and oxidative phosphorylation to yield a net total of
around 30–32 ATP molecules per glucose
★ Under anaerobic conditions, the pyruvate is converted to lactate or ethanol, yielding no
additional energy. In this state, the cell relies solely on the net 2 ATP molecules generated
during glycolysis.
★ Glycolysis consists of two phases: Investment phase and the Payoff phase
★ In the investment phase (steps 1-5), 2 ATP molecules are consumed.
★ In the payoff phase (steps 6-10), 4 molecules of ATP and 2 molecules of NADH are
produced, which helps in recovering the investment and also leads to the net gain of 2 ATP
and 2 NADH molecules.
★ Overall equation is as follows:
, Glucose + 2 NAD⁺ + 2 ADP + 2 Pᵢ → 2 Pyruvate + 2 NADH + 2 H⁺ + 2 ATP + 2 H₂O
★ Apart from energy production glycolysis also helps in catabolism and anabolism by providing
carbon skeletons which are used to synthesize:
● Amino acids: From pyruvate and 3-phosphoglycerate.
● Nucleotides: Linked via Glucose-6-phosphate entering the Pentose Phosphate Pathway.
● Lipids/Glycerol: From Dihydroxyacetone phosphate (DHAP).
Hence in this way glycolysis connects carbohydrates metabolism to other biosynthetic
pathways in the cell.
Steps In Glycolysis
Step 1
Phosphorylation of glucose
★ Glucose enters the cell with the help of specialized transporters like GLUT (Glucose
Transporter) and SGLT (Sodium–Glucose Linked Transporter ).
★ SGLTs are secondary active symporters (Na+ cotransporter) and move glucose into the cell
against its gradient.
★ GLUTs are facilitative (bidirectional) transporters ,i.e. GLUT can move glucose both in and
out of the cell. To prevent the efflux (moving out) of glucose from the cell, the glucose
molecule is phosphorylated.
★ Here the glucose molecule gets a negative charge due to the addition of the phosphate
group and gets trapped inside the cell, as glucose‑6‑phosphate cannot pass through GLUTs.
★ The addition of a phosphate group on one side of glucose increases its chemical reactivity
and commits it to metabolic pathways.
★ As glucose is inherently a stable molecule and it needs to be activated. This is a crucial step
to break the molecule apart, this is called chemical priming or energy investment.
Introduction :
★ Glycolysis, also known as the EMP pathway (Embden–Meyerhof–Parnas), is a metabolic
process in which one molecule of glucose is partially broken down/ oxidised to produce two
molecules of pyruvate, releasing energy in the process.
★ It is a ten-step process composed of enzyme-catalyzed reactions. It takes place in the
cytosol of the cell.
★ Almost all cellular organisms perform glycolysis. It is estimated to have evolved over 3.5
billion years ago, suggesting that it is one of the ancient metabolic pathways and has been
well conserved throughout. This also explains why glycolysis is an oxygen independent
process as the primitive environment of Earth was deprived of oxygen.
★ Glucose has relatively high potential energy, and thus it is a good fuel. It is not only an
excellent fuel, but also a remarkably versatile precursor, capable of supplying a huge array
of metabolic intermediates for biosynthetic reactions.
★ As glycolysis is an oxygen independent process it can be performed in aerobic as well as
anaerobic conditions.
★ Under aerobic conditions, the pyruvate produced undergoes further oxidation in the
mitochondria via the Citric Acid Cycle and oxidative phosphorylation to yield a net total of
around 30–32 ATP molecules per glucose
★ Under anaerobic conditions, the pyruvate is converted to lactate or ethanol, yielding no
additional energy. In this state, the cell relies solely on the net 2 ATP molecules generated
during glycolysis.
★ Glycolysis consists of two phases: Investment phase and the Payoff phase
★ In the investment phase (steps 1-5), 2 ATP molecules are consumed.
★ In the payoff phase (steps 6-10), 4 molecules of ATP and 2 molecules of NADH are
produced, which helps in recovering the investment and also leads to the net gain of 2 ATP
and 2 NADH molecules.
★ Overall equation is as follows:
, Glucose + 2 NAD⁺ + 2 ADP + 2 Pᵢ → 2 Pyruvate + 2 NADH + 2 H⁺ + 2 ATP + 2 H₂O
★ Apart from energy production glycolysis also helps in catabolism and anabolism by providing
carbon skeletons which are used to synthesize:
● Amino acids: From pyruvate and 3-phosphoglycerate.
● Nucleotides: Linked via Glucose-6-phosphate entering the Pentose Phosphate Pathway.
● Lipids/Glycerol: From Dihydroxyacetone phosphate (DHAP).
Hence in this way glycolysis connects carbohydrates metabolism to other biosynthetic
pathways in the cell.
Steps In Glycolysis
Step 1
Phosphorylation of glucose
★ Glucose enters the cell with the help of specialized transporters like GLUT (Glucose
Transporter) and SGLT (Sodium–Glucose Linked Transporter ).
★ SGLTs are secondary active symporters (Na+ cotransporter) and move glucose into the cell
against its gradient.
★ GLUTs are facilitative (bidirectional) transporters ,i.e. GLUT can move glucose both in and
out of the cell. To prevent the efflux (moving out) of glucose from the cell, the glucose
molecule is phosphorylated.
★ Here the glucose molecule gets a negative charge due to the addition of the phosphate
group and gets trapped inside the cell, as glucose‑6‑phosphate cannot pass through GLUTs.
★ The addition of a phosphate group on one side of glucose increases its chemical reactivity
and commits it to metabolic pathways.
★ As glucose is inherently a stable molecule and it needs to be activated. This is a crucial step
to break the molecule apart, this is called chemical priming or energy investment.
