1. What is the use of sugar in addition to energy supply?
Sugar, particularly glucose, serves more than just as an energy source. It plays a crucial role in the structure and function of various biological molecules. For instance, it is a key component in nucleic acids, proteoglycans, glycoproteins, and glycolipids. These molecules are essential for cell signaling, structural support, and cellular recognition. Additionally, sugar is involved in the formation of complex carbohydrates that contribute to the integrity of cell membranes and extracellular matrices.
2. How does glucose change to lactic acid under anoxic conditions? What is the point?
In the absence of oxygen, glucose undergoes anaerobic glycolysis, where it is converted into lactic acid. During this process, NADH produced in glycolysis is reoxidized to NAD+ by reducing pyruvate to lactic acid. This regeneration of NAD+ is vital because it allows glycolysis to continue without the need for oxygen. In muscle cells during intense exercise, this pathway provides a rapid source of ATP when oxygen levels are insufficient. The accumulation of lactic acid can be transported to the liver, where it is converted back into glucose through the Cori cycle.
3. Describe the composition and catalysis of the pyruvate dehydrogenase complex and what factors are regulated?
The pyruvate dehydrogenase complex is a multi-enzyme system composed of three main enzymes: pyruvate dehydrogenase (E1), dihydrolipoamide transacetylase (E2), and dihydrolipoamide dehydrogenase (E3). It also requires coenzymes such as thiamine pyrophosphate (TPP), flavin adenine dinucleotide (FAD), lipoic acid, nicotinamide adenine dinucleotide (NAD+), and coenzyme A (CoA). This complex catalyzes the irreversible conversion of pyruvate to acetyl-CoA, which enters the citric acid cycle. The activity of the complex is regulated by allosteric effectors and post-translational modifications, such as phosphorylation.
4. There are several regulatory enzymes in TAC? What substances are they regulated by each? What reactions do they catalyze?
The tricarboxylic acid (TCA) cycle includes several regulatory enzymes, including citrate synthase, isocitrate dehydrogenase, and α-ketoglutarate dehydrogenase. Citrate synthase is inhibited by NADH, succinyl-CoA, and ATP, while it is activated by ADP. Isocitrate dehydrogenase is inhibited by ATP and NADH but activated by Ca²⺠and ADP. α-Ketoglutarate dehydrogenase is inhibited by succinyl-CoA and NADH, and activated by Ca²âº. These enzymes help regulate the flow of metabolites through the TCA cycle based on the cell's energy needs.
5. What is the pentose phosphate pathway? How does it react? What is the physiological significance?
The pentose phosphate pathway is an alternative metabolic route for glucose, mainly occurring in the cytoplasm. It generates ribose-5-phosphate, which is used in nucleotide synthesis, and NADPH, which is essential for antioxidant defense and biosynthesis. The pathway involves the oxidation of glucose-6-phosphate to 6-phosphogluconolactone, followed by hydrolysis to 6-phosphogluconate, and then further oxidation to ribulose-5-phosphate and COâ‚‚. The overall reaction produces one molecule of ribose-5-phosphate, two molecules of NADPH, and one molecule of COâ‚‚ per glucose-6-phosphate consumed.
6. How does the liver synthesize glycogen and how to break down glycogen? What factors are regulated?
Glycogen synthesis, or glycogenesis, occurs when glucose is stored in the liver and muscles as a readily available energy source. The process begins with the conversion of glucose to glucose-6-phosphate, which is then transformed into UDP-glucose. Glycogen synthase adds glucose units to the growing glycogen chain. When glycogen is broken down, glycogen phosphorylase cleaves glucose-1-phosphate from the glycogen chain, which is then converted to glucose-6-phosphate. Regulation of these processes involves hormonal signals like insulin and glucagon, as well as intracellular second messengers such as cAMP.
7. How do non-sugar substances turn into sugar? Which enzymes are most noteworthy?
Gluconeogenesis is the process by which non-carbohydrate precursors, such as lactate, glycerol, and certain amino acids, are converted into glucose. Key enzymes include pyruvate carboxylase, phosphoenolpyruvate carboxykinase, fructose-1,6-bisphosphatase, and glucose-6-phosphatase. These enzymes bypass the irreversible steps of glycolysis, allowing the liver to maintain blood glucose levels during fasting or prolonged exercise.
8. How do ATP, AMP, and NAD+ influence the metabolism of sugar?
ATP and NAD+ act as indicators of the cell's energy status. High levels of ATP inhibit glycolysis and promote gluconeogenesis, while low ATP and high AMP activate glycolysis. NAD+ promotes glycolysis, whereas NADH favors gluconeogenesis. Hormones like insulin and glucagon also play a critical role in regulating these pathways, ensuring that the body maintains stable blood glucose levels.
9. What are the sources of blood sugar and which ones are there? What hormones have an important effect on maintaining blood sugar levels? How do they regulate blood sugar levels?
Blood glucose comes from dietary intake, glycogen breakdown, and gluconeogenesis. Insulin lowers blood sugar by promoting glucose uptake and storage, while glucagon raises it by stimulating glycogen breakdown and gluconeogenesis. Adrenaline increases glucose availability during stress, and cortisol helps maintain glucose levels during fasting by promoting protein breakdown and gluconeogenesis. These hormones work together to ensure a steady supply of glucose for vital organs like the brain and red blood cells.
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