Glycolisis ATP-ADP

Q1 Gen Bio QRT2

What is the ATP to ADP cycle?

The ATP to ADP cycle is the fundamental process by which cells manage and transfer energy. Think of ATP (adenosine triphosphate) as a fully charged battery and ADP (adenosine diphosphate) as a partially charged one. The cycle consists of two main parts.

What happens during ATP Hydrolysis (Energy Release)?

When a cell needs energy to perform work (like muscle contraction or active transport), an enzyme called ATPase facilitates the breakdown of ATP. A water molecule is used to cleave the bond connecting the third phosphate group to the rest of the molecule. This reaction, known as hydrolysis, breaks the high-energy bond, releasing a significant amount of free energy. The products of this reaction are ADP, a free inorganic phosphate group (Pi), and the released energy. The simplified chemical equation is: ATP + H₂O → ADP + Pi + energy.

What happens during ADP Phosphorylation (Energy Storage)?

To "recharge the battery," the cell must convert ADP back into ATP. This process requires an input of energy. Energy from the breakdown of food molecules (like glucose) is used to add a phosphate group back onto the ADP molecule. This is known as phosphorylation, and it can occur through several metabolic pathways, most notably cellular respiration (oxidative phosphorylation) and glycolysis (substrate-level phosphorylation). The energy released from food is essentially "stored" in the new high-energy phosphate bond. The simplified chemical equation for this is: ADP + Pi + energy → ATP + H₂O.

Why is the ATP to ADP cycle important?

This continuous cycle of breaking down and rebuilding ATP allows the cell to have a constant supply of readily available energy to power all of its life-sustaining activities.

What is glycolysis?

Glycolysis is a series of 10 enzyme-catalyzed reactions that break down a single molecule of glucose into two molecules of pyruvate. This process occurs in the cytoplasm of a cell and is the first stage of cellular respiration. It can be divided into two main phases: the energy-requiring phase and the energy-releasing (or "payoff") phase.

What happens during Phase 1 of glycolysis (Energy-Requiring Phase)?

In this phase, the cell invests energy in the form of ATP to modify the glucose molecule.

What happens in Step 1 of glycolysis (Phosphorylation of Glucose)?

An ATP molecule is used to transfer a phosphate group to glucose, forming glucose-6-phosphate. This reaction is catalyzed by the enzyme hexokinase. The negative charge of the phosphate group traps the glucose inside the cell, as it cannot easily cross the cell membrane.

What happens in Step 2 of glycolysis (Isomerization)?

Glucose-6-phosphate is rearranged into its isomer, fructose-6-phosphate. This is a reversible reaction catalyzed by the enzyme phosphoglucose isomerase.

What happens in Step 3 of glycolysis (Second Phosphorylation)?

Another ATP molecule is used to transfer a phosphate group to fructose-6-phosphate, creating fructose-1,6-bisphosphate. This is a crucial regulatory step, catalyzed by the enzyme phosphofructokinase. This step is often referred to as the "committed step" of glycolysis.

What happens in Step 4 of glycolysis (Cleavage)?

The six-carbon fructose-1,6-bisphosphate is split into two three-carbon sugar isomers: glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP). This reaction is catalyzed by the enzyme aldolase.

What happens in Step 5 of glycolysis (Isomerization of DHAP)?

DHAP is converted into Glyceraldehyde-3-phosphate (G3P). This ensures that both of the three-carbon molecules can proceed through the second half of the pathway. The enzyme for this step is triose phosphate isomerase. At this point, one molecule of glucose has been converted into two molecules of G3P, and two molecules of ATP have been consumed.

What happens during Phase 2 of glycolysis (Energy-Releasing Phase)?

In this phase, the two molecules of G3P are converted to pyruvate, generating ATP and NADH.

What happens in Step 6 of glycolysis (Oxidation and Phosphorylation)?

Each of the two G3P molecules is oxidized (loses electrons) and simultaneously receives a phosphate group from the cytoplasm, not from an ATP molecule. This reaction forms 1,3-bisphosphoglycerate and reduces two molecules of NAD+ to NADH. This is catalyzed by the enzyme glyceraldehyde-3-phosphate dehydrogenase.

What happens in Step 7 of glycolysis (First ATP Production)?

A phosphate group is removed from each of the two 1,3-bisphosphoglycerate molecules and transferred to ADP, creating two molecules of ATP and two molecules of 3-phosphoglycerate. This is the first example of substrate-level phosphorylation in glycolysis. The enzyme is phosphoglycerate kinase.

What happens in Step 8 of glycolysis (Phosphate Relocation)?

The phosphate group on each of the two 3-phosphoglycerate molecules is moved from the third carbon to the second carbon, forming two molecules of 2-phosphoglycerate. The enzyme is phosphoglycerate mutase.

What happens in Step 9 of glycolysis (Dehydration)?

A water molecule is removed from each of the two 2-phosphoglycerate molecules, creating a double bond and forming two molecules of phosphoenolpyruvate (PEP). This reaction is catalyzed by the enzyme enolase.

What happens in Step 10 of glycolysis (Second ATP Production)?

The final step is an irreversible reaction where a phosphate group is transferred from each of the two PEP molecules to ADP, producing two molecules of ATP and two molecules of pyruvate. This is the second example of substrate-level phosphorylation and is catalyzed by the enzyme pyruvate kinase.

What is the summary of glycolysis?

Input: 1 Glucose molecule, 2 ATP, 2 NAD+, 4 ADP + 4 Pi. Output: 2 Pyruvate molecules, 4 ATP (gross), 2 NADH, 2 H+ and 2 H₂O. Net Gain: 2 ATP, 2 NADH, 2 Pyruvate.

What happens to pyruvate after glycolysis if oxygen is available?

The two pyruvate molecules can proceed to the Krebs cycle for further energy production (aerobic respiration).

What happens to pyruvate after glycolysis if oxygen is not available?

If oxygen is not available (anaerobic conditions), the pyruvate will undergo fermentation.