The catabolism of sucrose breaks it down to monomers of glucose and fructose. The glucose can directly enter the glycolytic pathway while fructose must first be converted to glycogen, which can be broken down to GP and enter the glycolytic pathway as described above.

Search site Search Search. Go back to previous article. Sign in. Learning Objectives Identify the types of sugars involved in glucose metabolism.

Key Points When blood sugar levels drop, glycogen is broken down into glucose phosphate, which is then converted to glucosephosphate and enters glycolysis for ATP production.

In the liver, galactose is converted to glucosephosphate in order to enter the glycolytic pathway. Fructose is converted into glycogen in the liver and then follows the same pathway as glycogen to enter glycolysis. Sucrose is broken down into glucose and fructose; glucose enters the pathway directly while fructose is converted to glycogen.

Key Terms disaccharide : A sugar, such as sucrose, maltose, or lactose, consisting of two monosaccharides combined together. glycogen : A polysaccharide that is the main form of carbohydrate storage in animals; converted to glucose as needed.

Complimentary 1-hour tutoring consultation Schedule Now. Glycolysis is a metabolic pathway which breaks down glucose into two three-carbon compounds and generates energy. Glycolysis is the first step in the breakdown of glucose to extract energy for cellular metabolism.

Glycolysis is the first of the main metabolic pathways of cellular respiration to produce energy in the form of ATP. Through two distinct phases, the six-carbon ring of glucose is cleaved into two three-carbon sugars of pyruvate through a series of enzymatic reactions. The first phase of glycolysis requires energy, while the second phase completes the conversion to pyruvate and produces ATP and NADH for the cell to use for energy.

Glycolysis can be an aerobic or anaerobic reaction. It occurs in the cytosol of cells. In the presence of oxygen, pyruvate is further oxidized to CO 2, and in the absence of oxygen, pyruvate can be fermented to lactate or ethanol.

For every glucose molecule that undergoes glycolysis, there is a net production of 2 ATP molecules, 2 NADH molecules. The enzymes involved in the glycolysis pathway are inhibited by ATP, reducing their activity when the cell has sufficient ATP to meet its energy requirements.

In addition to glucose, many other carbohydrates ultimately enter the pathway to undergo energy-yielding degradation. The most significant are the polysaccharides glycogen and starch. Glycogen and starch enter the glycolytic pathway through the action of two enzymes: glycogen phosphorylase and amalyse.

Glycogen phosphorylase breaks down glycogen into glucose. Starch digestion occurs in reactions catalyzed by enzymes called amylases. The enzymes hydrolyze starch breaking it down into glucose monomers.

Type 2 diabetes mellitus. Hypoglycemia and carbohydrate metabolism. starch: found in plants and digested to release individual glucose molecules in the digestive tract is absorbed and transported to tissues. glycogen: the main storage carbohydrate in animals is mobilized by the liver and muscle to release glucose when hormones indicate energy is required.

glucose : a simple monosaccharide sugar with a molecular formula of C6H12O6; it is a principal source of energy for cellular metabolism. NADH : nicotinamide adenine dinucleotide NAD carrying two electrons and bonded with a hydrogen H ion; the reduced form of NAD.

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Therefore, by the end of this chemical-priming or energy-consuming phase, one glucose molecule is broken down into two glyceraldehydephosphate molecules. The second phase of glycolysis, the energy-yielding phase , creates the energy that is the product of glycolysis.

Glyceraldehydephosphate dehydrogenase converts each three-carbon glyceraldehydephosphate produced during the energy-consuming phase into 1,3-bisphosphoglycerate.

NADH is a high-energy molecule, like ATP, but unlike ATP, it is not used as energy currency by the cell. Because there are two glyceraldehydephosphate molecules, two NADH molecules are synthesized during this step.

Each 1,3-bisphosphoglycerate is subsequently dephosphorylated i. Each phosphate released in this reaction can be added to one molecule of ADP to produce one ATP molecule, resulting in a gain of two ATP molecules.

The enzyme phosphoglycerate mutase then converts the 3-phosphoglycerate molecules into 2-phosphoglycerate. The enolase enzyme then acts upon the 2-phosphoglycerate molecules to convert them into phosphoenolpyruvate molecules. The last step of glycolysis involves the dephosphorylation of the two phosphoenolpyruvate molecules by pyruvate kinase to create two pyruvate molecules and two ATP molecules.

In summary, one glucose molecule breaks down into two pyruvate molecules, and creates two net ATP molecules by substrate-level phosphorylation and two NADH molecules by glycolysis.

Therefore, glycolysis generates energy for the cell and creates pyruvate molecules that can be processed further through the aerobic Krebs cycle also called the citric acid cycle or tricarboxylic acid cycle ; converted into lactic acid or alcohol in yeast by fermentation; or used later for the synthesis of glucose through gluconeogenesis.

When oxygen O 2 is limited or absent, pyruvate enters an alternate, anaerobic pathway. In these reactions, pyruvate can be converted into lactic acid. In this reaction, lactic acid replaces oxygen as the final electron acceptor.

This lactic acid fermentation occurs in most cells of the body when oxygen is limited or mitochondria are absent or nonfunctional. For example, because erythrocytes red blood cells lack mitochondria, they must produce their ATP using this same fermentation pathway.

This is an effective pathway of ATP production for short periods of time, ranging from seconds to a few minutes. The lactic acid produced diffuses into the plasma and is carried to the liver, where it is converted back into pyruvate or glucose via the Cori cycle.

Similarly, when a person exercises, muscles use ATP faster than oxygen can be delivered to them. They depend on glycolysis and lactic acid production for rapid ATP production. The NADH and FADH 2 pass electrons on to the electron transport chain, which uses the transferred energy to produce ATP by oxidative phosphorylation.

As the terminal step in the electron transport chain, oxygen is the terminal electron acceptor, combining with electrons and hydrogen ions to produce water inside the mitochondria. The pyruvate molecules generated during glycolysis are transported across the mitochondrial membrane into the inner mitochondrial matrix, where they are metabolized by enzymes in a pathway called the Krebs cycle Figure 4.

The Krebs cycle is also commonly called the citric acid cycle or the tricarboxylic acid TCA cycle. During the Krebs cycle, high-energy molecules, including ATP, NADH, and FADH 2 , are created. NADH and FADH 2 then pass electrons through the electron transport chain in the mitochondria to generate more ATP molecules.

The three-carbon pyruvate molecule generated during glycolysis moves from the cytoplasm into the mitochondrial matrix, where it is converted by the enzyme pyruvate dehydrogenase into a two-carbon acetyl coenzyme A acetyl CoA molecule.

This reaction is an oxidative decarboxylation reaction. Acetyl CoA enters the Krebs cycle by combining with a four-carbon molecule, oxaloacetate, to form the six-carbon molecule citrate, or citric acid, at the same time releasing the coenzyme A molecule. The six-carbon citrate molecule is systematically converted to a five-carbon molecule and then a four-carbon molecule, ending with oxaloacetate, the beginning of the cycle.

Along the way, each citrate molecule will produce one ATP, one FADH 2 , and three NADH. The FADH 2 and NADH will enter the oxidative phosphorylation system located in the inner mitochondrial membrane.

In addition, the Krebs cycle supplies the starting materials to process and break down proteins and fats. To start the Krebs cycle, citrate synthase combines acetyl CoA and oxaloacetate to form a six-carbon citrate molecule; CoA is subsequently released and can combine with another pyruvate molecule to begin the cycle again.

The aconitase enzyme converts citrate into isocitrate. In two successive steps of oxidative decarboxylation, two molecules of CO 2 and two NADH molecules are produced when isocitrate dehydrogenase converts isocitrate into the five-carbon α-ketoglutarate, which is then catalyzed and converted into the four-carbon succinyl CoA by α-ketoglutarate dehydrogenase.

The enzyme succinyl CoA dehydrogenase then converts succinyl CoA into succinate and forms the high-energy molecule GTP, which transfers its energy to ADP to produce ATP by substrate-level phosphorylation.

Succinate dehydrogenase then converts succinate into fumarate, forming a molecule of FADH 2. Oxaloacetate is then ready to combine with the next acetyl CoA to start the Krebs cycle again see Figure 4. For each turn of the cycle, three NADH, one ATP through GTP , and one FADH 2 are created.

Each carbon of pyruvate is converted into CO 2 , which is released as a byproduct of oxidative aerobic respiration. The electron transport chain ETC uses the NADH and FADH 2 produced by the Krebs cycle to generate ATP. Electrons from NADH and FADH 2 are transferred through protein complexes embedded in the inner mitochondrial membrane by a series of enzymatic reactions.

In the presence of oxygen, energy is passed, stepwise, through the electron carriers to collect gradually the energy needed to attach a phosphate to ADP and produce ATP.