Glycolysis: The First Step in Cellular Respiration
- Glycolysis breaks down one molecule of glucose (6C) into two molecules of pyruvate (3C).
- It occurs in the cytoplasm and does not require oxygen, making it an anaerobic process.
- It produces a small amount of ATP and NADH, which are used in later stages of cellular respiration.
Glycolysis
The metabolic pathway that breaks down one molecule of glucose (6C) into two molecules of pyruvate (3C), producing a net gain of 2 ATP and 2 NADH. It occurs in the cytoplasm and does not require oxygen.
Each step in glycolysis is catalyzed by a specific enzyme, ensuring the process is efficient and tightly regulated.
Step-by-Step of Glycolysis
Glycolysis involves a series of enzyme-catalyzed reactions that can be divided into four key stages.
Stage 1: Phosphorylation (Energy Investment)
Phosphorylation
Phosphorylation refers to the addition of a phosphate group to a molecule
- Glucose is phosphorylated, meaning a phosphate group is added to it.
- The enzyme hexokinase adds the first phosphate, forming glucose-6-phosphate.
- A second phosphate is added in a subsequent step, forming fructose-1,6-bisphosphate.
- This requires 2 ATP to be used (energy investment).
- Adding phosphate groups makes glucose more reactive and traps it inside the cell (phosphorylated glucose can't cross the membrane).
- Think of it as "priming" the molecule for breakdown.
Stage 2: Lysis (Splitting the Sugar)
- The 6-carbon sugar (fructose-1,6-bisphosphate) is split into two 3-carbon molecules called triose phosphates.
- Each triose phosphate is converted into glyceraldehyde-3-phosphate (G3P).
- Imagine cutting a log into smaller pieces before burning it for energy.
- Lysis is like chopping the glucose molecule into manageable parts.
From this point forward, every reaction happens twice per glucose, once for each G3P molecule.
Stage 3: Oxidation
- Each G3P is oxidized, hydrogen atoms (with their electrons) are removed.
- The hydrogens are transferred to NAD⁺, reducing it to NADH.
- Simultaneously, an inorganic phosphate (Pi) is added to the molecule.
- Don’t confuse NAD+ with NADH.
- NAD+ is the oxidized form, while NADH is the reduced form that carries electrons.
Stage 4: ATP Formation (Energy Payoff)
- The remaining reactions convert the 3-carbon molecules into pyruvate (3C).
- During these steps, phosphate groups are transferred directly from intermediates to ADP, forming ATP.
- This is called substrate-level phosphorylation (direct transfer of phosphate, not using the ETC).
If you invest USD 2 in a project and earn USD 4 in return, your net profit is $2. Same logic here.
Products of Glycolysis (Per Glucose)
- Starting with one glucose molecule, glycolysis produces:
- 2 pyruvate (3C) (one from each G3P)
- 2 ATP (net) (4 produced − 2 used)
- 2 NADH (one from each G3P oxidation)
- Glycolysis is the universal first step of cellular respiration, it occurs in nearly all living organisms.
- It works without oxygen, so cells can extract some energy even in anaerobic conditions.
- The pyruvate, ATP, and NADH produced are essential for the later stages of aerobic respiration, which produce the bulk of cellular ATP.
These products are the starting materials for the next stages of cellular respiration (link reaction and Krebs cycle in aerobic conditions, or fermentation in anaerobic conditions).
Self review- Where does glycolysis occur in the cell?
- Does glycolysis require oxygen?
- What is phosphorylation and why is it necessary in glycolysis?
- How many ATP are used in the phosphorylation stage?
- What does "lysis" mean in the context of glycolysis?
- How many ATP are produced in total during glycolysis?
- What is the net ATP yield per glucose and why?
- How many NADH are produced per glucose?
- What are the three end products of glycolysis per glucose molecule?
