The glycolysis pathway requires energy to begin, delivered by chemical fuel: ATP. The primary source of ATP in your cells are mitochondria organelles. 

[Enzyme 1: Hexokinase] The reaction of the first enzyme transfers a phosphate group from ATP to glucose, creating glucose-6- phosphate.

 [Enzyme 2: Phosphoglucose isomerase] The second enzyme rearranges glucose-6-phosphate into its isomer, fructose-6-phosphate. 

[Enzyme 3: Phosphofructokinase] The third enzyme uses a second molecule of ATP, creating fructose 1,6-bisphosphate. The six-carbon sugar is now ready to be broken apart. 

[Enzyme 4: Fructose-bisphosphate aldolase] The fourth enzyme cuts the molecule in half, creating two three-carbon sugars, each with a single phosphate attached. The sugar products are isomers of each other, but only glyceraldehyde-3-phosphate is ready to continue with glycolysis. 

[Enzyme 5: Triosephosphate isomerase] The sugar isomer, dihydroxyacetone phosphate, must first be converted through isomerization before continuing to the sixth enzyme. 

[Enzyme 6: Glyceraldehyde 3-phosphate dehydrogenase] The sixth enzyme adds a second phosphate to glyceraldehyde-3-phosphate, creating 1,3- bisphosphoglycerate, while two electrons are transferred to NAD+ , which is reduced to NADH. 

Halfway through glycolysis, the cell has consumed two ATP molecules breaking down glucose and is now ready to capture energy in return. 

[Enzyme 7: Phosphoglycerate kinase] The seventh enzyme transfers a phosphate group from 1,3-bisphosphoglycerate to ADP, creating chemical fuel: ATP. 

[Enzyme 8: Phosphoglycerate mutase] The eighth enzyme rearranges the phosphate from the end of the molecule to the middle. 

[Enzyme 9: Enolase] The ninth enzyme catalyzes dehydration, increasing the potential energy in preparation for the final glycolysis reaction. 

[Enzyme 10: Pyruvate kinase] The 10th enzyme transfers the remaining phosphate group to ADP, producing ATP and the three-carbon sugar pyruvate. Glycolysis finally produces a net gain in ATP. 

The glycolysis pathway breaks down glucose into two molecules of pyruvate, gaining two ATP and two NADH in the process.

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The product of glycolysis, the pyruvate molecule is the source of carbon and electrons for aerobic respiration reactions inside your mitochondria. 

Pyruvate enters pores on the mitochondria membrane and is transported into the interior. The reactions linking pyruvate with aerobic respiration are performed by a huge protein complex made with multiple copies of three types of enzyme. 

[Enzyme 1: Pyruvate dehydrogenase] The first enzyme catalyzes decarboxylation, transforming three-carbon pyruvate into a two-carbon acetyl group, generating carbon dioxide as waste. 

[Enzyme 2: Dihydrolipoyl transacetylase] The acetyl group is grabbed by the second enzyme using flexible arms to efficiently transfer the product between reaction sites. In the active site of the second enzyme, the acetyl group is attached to coenzyme A, generating acetyl-CoA: fuel for the citric acid cycle. Two electrons from pyruvate oxidation are retained by the second enzyme…

[Enzyme 3: Dihydrolipoyl dehydrogenase] …before passing them to coenzyme NADH, catalyzed by the third enzyme. Coenzyme NADH travels through the matrix, delivering electrons for the electron transport chain. 

In a sequence of reaction steps, the pyruvate dehydrogenase enzyme complex generates fuel for aerobic respiration reactions of the citric acid cycle and the electron transport chain.

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Mitochondria are filled with metabolic enzymes that catalyze a loop of chemical reactions called the citric acid cycle. 

The eight steps of the citric acid cycle gradually break apart two carbon atoms from glycolysis, capturing liberated electrons for the electron transport chain, while generating carbon dioxide as waste. The citric acid cycle starts with the end product of a previous cycle, the 4-carbon molecule oxaloacetate. New carbon enters from glycolysis in the form of acetyl-CoA, a 2-carbon acetyl group attached to coenzyme A. 

Step 1 is the catalytic transfer of a 2-carbon acetyl group from coenzyme A to 4-carbon oxaloacetate, creating 6-carbon citric acid, the molecule that gives the cycle its name. 

Citric acid is used by your cells for the biosynthesis of fatty acids, lipids, and cholesterol, and is also the substrate for Step 2 of the citric acid cycle. The enzyme reaction of Step 2 makes a small change to the citric acid molecule, moving the position of an oxygen atom, converting citrate into isocitrate. 

Step 3 of the cycle removes a carbon atom, forming carbon dioxide as waste, while converting 6- carbon isocitrate to 5-carbon ketoglutarate. During this process, chemical energy is harvested when two electrons are transferred to coenzyme NADH, which delivers them to nearby enzymes of the electron transport chain. 

Step 4 of the citric acid cycle is performed by a huge multienzyme complex, connecting multiple chemical reactions with flexible tethers, efficiently moving reactants between active sites and diverting electrons between pathways. 

The large complex is made with a repeating unit of three core enzymes, working in sequence to remove a carbon atom from ketoglutarate, generating carbon dioxide waste, then attaching 4-carbon succinyl to coenzyme A. The electron from ketoglutarate oxidation is retained by the tethered enzyme, before passing it to coenzyme NADH, which delivers electrons to the electron transport chain. 

Enzymes of Step 5 separate succinyl from coenzyme A by breaking an energetic bond, providing enough energy to create GTP. GTP is a building block of RNA and an energy source for protein synthesis. In the most direct path to ATP from the citric acid cycle, other enzymes in the matrix can use GTP to make the chemical fuel ATP. 

Step 6 is performed by an enzyme embedded in the inner mitochondrial membrane. This enzyme is also known as Complex II of the electron transport chain. The enzyme catalyzes the oxidation of succinate to create fumarate, releasing two electrons that hop through the interior to coenzyme Q, which is reduced. Coenzyme Q travels in the membrane, carrying the electrons to nearby enzymes of the electron transport chain. 

Step 7 takes fumarate generated by Step 6 and reacts it with water to create 4-carbon malate. 

The final step of the citric acid cycle uses malate to regenerate oxaloacetate, transferring electrons to coenzyme NADH, which supplies the electron transport chain. Oxaloacetate can return to Step 1 for another loop of the cycle and is also an essential building block for making amino acids and the genetic code of DNA and RNA. 

At the center of cellular metabolism, the citric acid cycle generates both chemical energy and material for biosynthesis of the entire diversity of molecules found inside a living cell.

source: https://www.biointeractive.org/

The center of activity that brings your cells to life is found inside the dynamic mitochondria organelle. The inner mitochondrial membrane is coated with enzymes that catalyze the chemical reactions of respiration, working in sequence to generate the electron transport chain. 

The first step in the electron transport chain is performed by Enzyme Complex I. Complex I receives electrons from coenzyme NADH, a substrate produced by the citric acid cycle. The catalytic mechanism of Enzyme Complex I connects two different kinds of reaction. 

Coenzyme NADH is oxidized at one end of the enzyme, releasing two electrons that hop through the interior to coenzyme Q, which is reduced. Traveling amongst membrane lipids, coenzyme Q carries electrons to the next step in the electron transport chain. The movement of charged electrons through Complex I makes it bend in shape, transmitting energy for pumping four protons across the membrane. 

The second step of the electron transport chain is performed by Enzyme Complex III. The mechanism of Complex III separates electrons from coenzyme Q, passing one electron to cytochrome C, which is reduced. 

A complete reaction cycle of Enzyme Complex III transports four protons across the membrane. Traveling within the intermembrane space, reduced cytochrome C carries the electron to the final step in the electron transport chain. 

The destination for electron transport is a molecule of oxygen, held inside Enzyme Complex IV. Reduced cytochrome C delivers electrons that transfer to the reaction center of Enzyme Complex IV. A molecule of oxygen from the air you breathe is captured, split, and reduced. The separated oxygen atoms accept electrons and pick up protons, creating two molecules of water. 

The substrate providing electrons to Enzyme Complex I is coenzyme NADH, a product of the citric acid cycle. A second supply of electrons for the electron transport chain is from Step 6 of the citric acid cycle, performed by Enzyme Complex II. Enzyme Complex II catalyzes oxidation of succinate, releasing two electrons, which hop through the enzyme to coenzyme Q, which is reduced. 

Traveling through the membrane, coenzyme Q carries the electrons to enzyme Complex III of the electron transport chain. 

source: https://www.biointeractive.org/

All life on earth depends on this tiny energetic molecule: adenosine triphosphate, or ATP. ATP drives biochemical activity inside your living cells and is a key building block of DNA and RNA. Generating ATP for your living cells are mitochondria, electrochemical batteries that convert energy from the food you eat, and oxygen from the air you breathe, into ATP. 

So where does your ATP come from? Deep inside your mitochondria, rows of molecular motors generate ATP, the molecule essential to all life on earth. 

Enzymes bring together reactants to form a chemical bond, converting mechanical energy into chemical energy. A ring of enzymes works in step, creating three molecules of ATP with each cycle. 

Inside the molecular motor, a rotating axle powers the sequence. The axle is attached to a rotary molecular motor, moved by the force of protons pushing from the other side of the membrane. A difference in proton concentration propels the molecular mechanism. 

Synthesis of ATP:

ATP drives biochemical activity inside your living cells and is a key building block of DNA and RNA 

source: https://www.biointeractive.org/

Adenosine triphosphate, or ATP, is the molecule that powers all life on Earth. Energy from ATP is required for essential activity, including pumping ions across membranes and pulling apart the DNA helix. 

The ATP molecule is made of three main structures: an adenine nucleobase, a ribose sugar, and three phosphates. Energy is released when the bond holding the end phosphate group is broken, converting ATP to ADP through a water-consuming hydrolysis reaction. 

To stay alive, your cells depend on enzyme pumps to maintain differences in ion concentration across membranes. The flow of calcium ions controls muscle contraction, nerve transmission, gene regulation, and cell death. Because of enzyme pumps, the concentration of calcium ions inside your living cells is 10,000 times lower than outside. 

The ATP hydrolysis reaction breaks the end phosphate bond, releasing energy driving atoms to rearrange inside the enzyme, transforming its shape. With each transformation, channels inside the enzyme open and close, sending ions in opposite directions: hydrogen in and calcium out of the cell. 

DNA helicase is another type of ATP-powered enzyme. This mechanism is so critical for life that 1% of your genetic code is for helicase enzymes. 

Helicase enzymes are motor proteins that move along the DNA double helix, mechanically separating strands powered by energy from ATP. The ring-shaped enzyme uses ATP hydrolysis to provide energy for the mechanical separation of strands of the DNA helix. With each step of the enzyme, two pairs of DNA bases are separated, requiring energy from two ATP molecules around the ring.

source: https://www.biointeractive.org/