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When you run or skip a meal, your body quickly releases a hormone that stops your blood sugar from dropping too low. This hormone, glucagon, tells the liver to break down stored glycogen and release glucose, keeping your muscles active and your brain alert even without new food. In this study guide, you will read …

Muscle fatigue during intense exercise happens when cells run low on oxygen. Pyruvate starts accumulating, and your body shifts to different reactions that allow muscles to keep working. This process produces lactate, which causes the familiar burning sensation and signals that cells rely on anaerobic energy production. This study guide explains how pyruvate forms in …

When you skip meals or exercise for hours, your body runs low on carbohydrates. To keep working, it starts producing glucose from lactate, glycerol, and amino acids. This process takes place mainly in the liver and keeps blood sugar steady during fasting or intense activity. This study guide explains how gluconeogenesis creates glucose from non-carbohydrate …

When you cycle uphill or sprint, your muscles burn glucose and fats to release energy. Molecules inside your cells move electrons to produce ATP, which fuels every movement. One of the most important is Nicotinamide adenine dinucleotide (NAD), which changes form during these reactions and keeps energy production running. This study guide explains how NADH …

When you sprint, your muscles need energy fast. To keep up, your cells rapidly break down glucose into ATP, the fuel that powers movement. This process, called glycolysis, happens in all living cells, whether oxygen is present or not. Even yeast and bacteria rely on glucose breakdown to survive in low-oxygen conditions. This guide explaisystematically …

Every time you walk, eat, or think, your cells extract energy from food. Inside mitochondria, a series of chemical reactions breaks down nutrients, releasing energy for ATP production. This process, known as the Krebs cycle, converts carbon-based molecules into usable energy, keeping cells active and functioning. The Krebs cycle, also called the citric acid cycle, …

DNA replication happens billions of times per second, ensuring every new cell gets an exact copy of genetic material. A single mistake in this process can lead to mutations, yet cells maintain accuracy through specialized enzymes. DNA polymerase builds new DNA strands, proofreads them, and corrects errors to keep genetic information intact. This study guide …

Bacteria develop antibiotic resistance through mutation, allowing them to survive drugs that once killed them. This genetic change spreads, making infections harder to treat. In humans, a mutation in the lactase gene allows some adults to digest milk, unlike most mammals. These genetic changes shape how organisms adapt and survive. A mutation is a permanent …

Your immune system constantly scans for harmful invaders. How does it recognize them? Antigen acts as molecular fingerprints, allowing immune cells to distinguish between harmful and harmless substances. These molecules trigger responses that protect the body from infections and diseases. This guide explains antigens, how they interact with immune cells, and how B cells and …