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Glycolysis

Glycolysis

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Glycolysis is a process that breaks down sugar. It happens in the cytoplasm of a cell. This is where glucose splits into two 3-carbon molecules. These then turn into pyruvate after a series of reactions. Glycolysis is the first step in both aerobic and anaerobic respiration.

What is the equation for glycolysis?

The overall equation for glycolysis is: Sometimes pyruvate is referred to as pyruvic acid, so don’t get confused if you are doing any extra reading! We use the two names interchangeably.

What are the different stages of glycolysis?

Glycolysis is a process that occurs in the cytoplasm where a single glucose molecule is split into two 3-carbon pyruvate molecules. There are ten stages in glycolysis which involve multiple smaller, enzyme-controlled reactions. The process of glycol addition to glucose from two ATP molecules, phosphorylation. Next, glucose split into two 3-carbon triose phosphate molecules, with one hydrogen removed from each molecule. These hydrogen groups are transferred to a hydrogen-carrier molecule, NAD, which forms reduced NAD/NADH. The two triose phosphate molecules are then converted into pyruvate, which also regenerates two ATP molecules per pyruvate molecule. This results in the production of four ATP molecules for every two ATP molecules used up in glycolysis. During each stage of the process, different enzymes are involved.

The investment phase

first phase of glycolysis involves investing two molecules of ATP to split glucose into two 3-carbon molecules. Here's how it works:

  1. Hexokinase catalyses glucose into glucose-6-phosphate, using one molecule of ATP which donates a phosphate group. ATP is converted to ADP. This process, called phosphorylation, makes glucose reactive enough to proceed with subsequent enzymatic reactions.
  2. Phosphoglucose isomerase catalyses glucose-6-phosphate, isomerising it into another 6-carbon phosphorylated sugar called fructose-6-phosphate.
  3. Phosphofructokinase-1 (PFK-1) catalyses fructose-6-phosphate, adding a phosphate from ATP into it. ATP is converted to ADP, and fructose-1,6-bisphosphate is formed. This phosphorylation increases the reactivity of the sugar to allow the molecule to proceed further in the glycolysis process.
  4. Aldolase splits the 6-carbon molecule into two 3-carbon molecules: Glyceraldehyde-3-phosphate (G3P phosphate).
  5. iserase converts DHAP into G3P, as only G3P is used in the next step of glycolysis. Therefore, we now have two molecules of G3P which will both be used in the next step.

The pay-off phase

The final half of glycolysis generates two molecules of pyruvate and four molecules of ATP. Here's how it works:

  1. Glyceraldehyde-3-phosphate (G3P) combines with the enzyme Glyceraldehyde-3-phosphate Dehydrogenase (GAPDH), NAD+, and inorganic phosphate, producing 1,3-biphosphoglycerate (1,3-BPh) and NADH as a by-product.
  2. Phosphoglycerate kinase catalyses the reaction where a phosphate group from 13-blycerate (1,3-BPh) combines with ADP to make ATP, producing 3-phosphoglycerate.
  3. Phosphoglycerate mutase converts 3-phosphoglycerate into 2-phosphoglycerate.
  4. Enolase converts 2-phosphoglycerate into phosphoenolpyruvate with water as a by-product.
  5. Pyruvate kinase converts phosphoenolpyruvate into pyruvate by losing a phosphate group and gaining a hydrogen atom. ADP takes up the lost phosphate group and becomes ATP.

In total, glycolysis produces 2 pyruvate molecules, 2 molecules of ATP, and 2 NADH molecules which go to the electron transport chain. Knowledge of the names of the molecules and enzymes involved, how many ATP molecules are gained/lost, and when NAD/NADH is formed during the process is sufficient for most exams.

Glycolysis and energy yields

The overall yield from a single glucose molecule after glycolysis is two ATP molecules, two NADH molecules, and two pyruvate molecules. Glycolysis occurs in the cytoplasm of the cell and involves a series of enzyme-controlled reactions. It does not require an organelle or membrane to take place and can occur in the absence of oxygen through anaerobic respiration. The enzymes involved in glycolysis are found in the cytoplasm of cells, which suggests that some of the earliest organisms used reactions that resemble glycolysis to gain energy.

Glycolysis

What is glycolysis and its process?

Glycolysis has four stages:Phosphorylation. Two phosphate molecules are added to glucose. We get the two phosphate molecules from splitting two ATP molecules into two ADP molecules and two inorganic phosphate molecules (Pi). This is done via hydrolysis. This then provides the energy needed to activate glucose and lowers the activation energy for the next enzyme-controlled reactions. Creation of triose phosphate. In this stage, each glucose molecule (with the two added Pi groups) is split into two. This forms two molecules of triose phosphate, a 3-carbon molecule. Oxidation. Hydrogen is removed from both triose phosphate molecules. It is then transferred to a hydrogen-carrier molecule, NAD. This forms reduced NAD. ATP production. Both of the triose phosphate molecules, newly oxidised, covert into another 3-carbon molecule known as pyruvate. This process also regenerates two ATP molecules from two molecules of ADP. 

What is the function of glycolysis?

The function of glycolysis is to convert  a 6-carbon glucose molecule into pyruvate through a series of enzyme-controlled reactions. Pyruvate is then used during fermentation (for anaerobic respiration) or the link reaction (for aerobic respiration.)

Where does glycolysis occur?

Glycolysis occurs in the cytoplasm of the cell. A cell’s cytoplasm is a thick liquid in the cell’s membrane that surrounds the cell’s organelles.

Where do the products of glycolysis go?

The products of glycolysis are pyruvate, ATP, NADH, and H+ ions. In aerobic respiration, pyruvate goes into the mitochondrial matrix and converts into acetyl coenzyme A via the link reaction. In anaerobic respiration, pyruvate stays in the cytoplasm of the cell and undergoes fermentation. ATP, NADH, and H+ ions are used in the subsequent reactions in aerobic respiration: the link reaction, the Krebs cycle, and oxidative phosphorylation. 

Does glycolysis require oxygen?

No! Glycolysis takes place during both aerobic and anaerobic respiration. Therefore, it does not need oxygen to occur. The stages of aerobic respiration that require oxygen to occur are the link reaction, the Krebs cycle, and oxidative phosphorylation.

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