BIOL 1140: ATP and Energy Coupling

• BIOL 1140 focuses on harvesting energy, making ATP, and cellular respiration

Adenosine Triphosphate (ATP): Energy Currency of the Cell

• Adenosine triphosphate (ATP) is a nucleotide, but not a nucleic acid nucleotide.
• ATP comprises the five-carbon sugar ribose linked to the nitrogenous base adenine and a chain of three phosphate groups.
• The negative charges of the phosphate groups repel each other, making the bonding arrangement unstable.
• Hydrolysis, or the removal of the third phosphate group, releases large amounts of free energy and is exergonic.

Energy Coupling

• During energy coupling, ATP is hydrolyzed so the terminal phosphate group is transferred to a reactant molecule involved in an endergonic reaction
• It needs energy because ATP releases energy.
• An endergonic reaction needs an exergonic reaction nearby to release energy, with the hydrolysis of ATP being the best exergonic reaction.

ADP is Used to Make ATP (Phosphorylation)

• When ATP releases its third phosphate, it yields adenosine diphosphate (ADP) and a phosphate molecule (P).
• Adding a phosphate to an ADP molecule results in an ATP molecule
• ATP is a "loaded gun" that releases energy by firing off a phosphate "bullet"
• ADP is an "unloaded gun" that will get reloaded with a phosphate "bullet"
• Adding a phosphate group to something is called phosphorylation.
• Cells create ATP by phosphorylating ADP, like loading an unloaded gun.

Regeneration of ATP

• Making ATP by phosphorylating (adding a P to) ADP exemplifies a reaction requires energy.
• ATP synthesis is a series of endergonic reactions that doesn't obtain its energy from ATP directly.
• The continual breaking down and re-synthesis of ATP (“firing a bullet” and “reloading a bullet”) makes ATP a highly renewable energy storage molecule
• Approximately 10 million ATP molecules are broken down and resynthesized each second in a typical cell.

Cellular Respiration (The Overall Picture)

• The main function of cellular respiration is generating ATP for cellular work.
• In cellular respiration, O₂ and glucose are broken down to CO₂ and H₂O.
• In the reaction, the chemical energy in the bonds of glucose are released and stored in the chemical bonds of ATP.
• cellular respiration can produce 32 ATP molecules for each glucose.

The Three Main Stages of Cellular Respiration

• Glycolysis occurs outside the mitochondrion
• The Citric acid (Krebs) cycle occurs inside mitochondria
• Oxidative phosphorylation occurs inside mitochondria

Electrons and Energy

• Cells extract energy from glucose by transferring electrons during chemical reactions.
• In cellular respiration, electrons are transferred from glucose to O₂ through a series of steps
• Biochemists say that electrons “fall” to oxygen to indicate that the electrons move down an energy gradient
• Cellular respiration is a controlled descent of electrons.
• Energy released in small amounts each step from stepping down an energy staircase or moving down an energy gradient can be stored in the chemical bonds of ATP.
• When moving around electrons during cellular respiration, there is movement around energy

Cellular Respiration is a Series of Redox Reactions (Oxidation & Reduction)

• An oxidation-reduction or redox reaction is when electrons move from one molecule to another during a chemical reaction.
• Oxidation is the loss of electrons from one substance.
• Reduction is the gain of electrons from another substance.
• A molecule is oxidized when it loses electrons and reduced when it gains electrons.
• Oxidation and reduction always go together because electron transfer requires both a donor and an acceptor.

Cellular Respiration: NADH (and FADH)

• In cellular respiration, an important element, a coenzyme, is called NAD+.
• NAD+ accepts electrons from Hydrogen atoms (H) and becomes NADH.
• Hydrogen atoms only have one electron and one proton.
• In other words, NAD+ picks up H's (electrons) so it temporarily “stores” that potential energy.
• Note that at times during cellular respiration, a similar electron carrier called FAD is used (FAD is reduced to FADH₂).
• NADH and FADH are electron moving vans.

Cellular Respiration: Glycolysis

• Glycolysis begins with a single molecule of glucose and concludes with two molecules of pyruvate.
• The same six carbons in glucose end up in the two molecules of pyruvate.
• The arrow from glucose to pyruvate symbolizes 9 chemical reactions, each catalyzed by its own enzyme.
• The products formed between glucose and pyruvate are called intermediates.

Cellular Respiration: Glycolysis (Cont.)

• Glycolysis group into two main phases: an energy investment phase and an energy payoff phase.
• Energy is actually consumed to split one glucose molecule into two small sugars in the energy investment phase
• As the glycolysis reaction occurs, two NAD+ are reduced to two molecules of NADH and two *net molecules of ATP are produced (the ATP payoff).

Energy Flow in Glycolysis

• The initial steps of glycolysis require two ATP to hydrolyze
• Glycolysis yields four produced ATP via substrate-level phosphorylation and it carries two electrons and a proton from removing glucose and reducing the electron carrier NAD, for a net gain of two vans

Substrate-Level Phosphorylation

• Each glucose molecule going through glycolysis results in 2 (net) ATP molecules produced and gained by the cell.
• ATP is formed when ADP is phosphorylated.
• In this process, an enzyme transfers a phosphate group from a substrate molecule directly to ADP, forming ATP.
• We can make a lot more throughout the cellular respiration process by employing another phosphorylation method.
• This is not the "best way" to make ATP
• The 3rd / final main stage of cellular respiration is named after new phosphorylation method

Cellular Respiration: Pyruvate Oxidization

• After glycolysis, pyruvate is transported from the cell's cytosplasm into the mitochondrial matrix.
• Pyruvate itself does not enter the citric acid cycle – it is first oxidized into Acetyl CoA.
• This must happen before the citric acid cycle can begin.