Metabolism Basics

Metabolism is the set of chemical processes that occur in living organisms to maintain life. These processes happen through metabolic pathways, which are series of biochemical reactions that convert one substance into another.

There are two main types of metabolic processes. Anabolism builds complex molecules from simple ones, like creating sugar from CO2, and requires energy from ATP. Catabolism breaks down complex molecules into simpler ones, like breaking carbohydrates into glucose, and releases energy in the process.

Enzymes serve as catalysts for these biochemical reactions, speeding them up without being consumed. Some reactions occur spontaneously (naturally without external force), while others are nonspontaneous (requiring some external force to start).

💡 Think of metabolism like your body's power plant: anabolism uses energy to build things (like muscles), while catabolism breaks things down (like food) to release energy!

Energy and Electron Carriers

Free energy in a system determines its capacity to do work—more free energy means less stability. The formula ΔG = ΔH - TΔS helps us understand if reactions happen naturally or need help to get started.

When ΔG is positive, the reaction is nonspontaneous and needs energy input. When ΔG is negative, the reaction is spontaneous and releases energy. These energy principles drive all biochemical reactions in your body.

Electron carriers like NADH and FAD transport electrons between molecules during metabolism. These specialized molecules accept electrons from one molecule and deliver them to another, which is crucial in cellular respiration. They carry the electrons harvested from glucose through the electron transport chain.

Oxidation-reduction reactions occur simultaneously in cells—when one molecule loses electrons (oxidation), another gains them (reduction). This electron movement is what ultimately powers your body!

🔋 Electron carriers are like the delivery trucks of your cellular energy system—they pick up and drop off electrons exactly where they're needed to keep your energy production running smoothly.

Coupled Reactions and Cellular Respiration

Enzymatic coupling happens when an energy-requiring (endergonic) reaction is powered by an energy-releasing (exergonic) reaction. This clever system allows your cells to perform reactions that would otherwise be impossible. All coupled reactions are spontaneous because the energy-releasing reaction "pushes" the energy-requiring one forward.

Cellular respiration is how your body extracts energy from food molecules like glucose. The formula 6O₂ + C₆H₁₂O₆ → 6CO₂ + 6H₂O + ATP shows how oxygen and glucose are converted to carbon dioxide, water, and energy.

This process occurs in three major steps: Glycolysis, the Krebs Cycle, and the Electron Transport Chain. Each step builds on the previous one, creating an efficient energy-extraction system that powers everything your body does.

🔄 Think of coupled reactions like a seesaw: as one side goes down (releasing energy), it pushes the other side up (using that energy)—this teamwork makes impossible reactions possible!

Glycolysis: Breaking Down Glucose

Glycolysis is the first stage of cellular respiration, occurring in the cell's cytoplasm. It breaks down a 6-carbon glucose molecule into two 3-carbon pyruvate molecules, producing energy along the way.

The process requires an initial investment of 2 ATP molecules to get started, but ultimately generates 4 ATP and 2 NADH molecules, giving a net gain of 2 ATP. This energy investment is like priming a pump before getting more energy back.

Glycolysis happens in two main phases. First, glucose is converted into two glyceraldehyde-3-phosphate molecules (using ATP). Then, these molecules are converted into pyruvate while generating ATP. The process involves eight different enzymes working in sequence to catalyze each step.

🔬 Glycolysis is the only part of cellular respiration that doesn't require oxygen, which is why it's the primary way your muscle cells produce energy during intense exercise when oxygen is limited!

Pyruvate Oxidation, Krebs Cycle, and Electron Transport Chain

After glycolysis, pyruvate moves to the mitochondria where it undergoes oxidation. This creates carbon dioxide, NADH, and a molecule called acetyl-CoA, which enters the Krebs Cycle (also called the citric acid cycle).

The Krebs Cycle transforms acetyl-CoA through a series of reactions, generating energy carriers. This cyclical process produces 6 NADH, 2 FADH₂, 2 GTP (similar to ATP), and 4 CO₂ molecules. The NADH and FADH₂ carry electrons to the final stage of cellular respiration.

The Electron Transport Chain consists of proteins embedded in the mitochondria's inner membrane. As electrons move through this chain, their energy helps pump hydrogen ions across the membrane, creating a concentration gradient. When these ions flow back through the ATP synthase enzyme, it spins like a turbine to create ATP. Finally, oxygen accepts electrons to form water, completing the process.

⚡️ The electron transport chain is like a cellular power plant that can produce 32-34 ATP molecules from a single glucose molecule—far more than the earlier steps of respiration!

Fermentation and Photosynthesis

Fermentation is an alternative energy-producing pathway that occurs when oxygen is unavailable. It begins with glycolysis but then takes a different route to recycle NADH back to NAD+, allowing glycolysis to continue. This process only yields 2 ATP per glucose molecule, making it much less efficient than aerobic respiration.

Photosynthesis is essentially cellular respiration in reverse: 6CO₂ + 6H₂O + Sunlight → 6O₂ + C₆H₁₂O₆. Plants and other photosynthetic organisms use this process to convert light energy into chemical energy stored in glucose.

The light-dependent reactions of photosynthesis capture sunlight using chlorophyll in the chloroplast. This energy breaks down water molecules, releasing oxygen as a byproduct, and produces ATP and NADPH. Electrons flow through photosystems to create these energy carriers.

🌱 Photosynthesis and cellular respiration are complementary processes: plants produce oxygen and glucose that animals use for respiration, while animals produce carbon dioxide that plants use for photosynthesis!

Calvin Cycle and Energy Transfer

The Calvin Cycle $also called light-independent or dark reactions$ takes place in the chloroplast's stroma. This cycle uses the ATP and NADPH generated during light-dependent reactions to fix carbon dioxide from the air into simple sugar molecules, which eventually form glucose.

During this process, carbon dioxide is incorporated into existing molecules through carbon fixation, then modified through a series of reactions powered by the energy carriers from the light-dependent reactions. The cycle continuously repeats, building more and more glucose molecules.

Photosynthesis and cellular respiration form a beautiful cycle in nature. Photosynthesis converts carbon dioxide and water into oxygen and glucose using sunlight energy. Cellular respiration does the opposite, using oxygen and glucose to produce carbon dioxide, water, and ATP energy.

🔄 The Calvin Cycle is like a carbon recycling plant—it takes carbon dioxide, a waste product for animals, and transforms it into glucose, the energy source that powers all life!

Membrane Function and ATP-ADP Cycle

Membranes play a crucial role in energy production, especially during photosynthesis. The thylakoid membranes in chloroplasts capture light energy and convert it into chemical energy in the form of ATP and NADPH, while also releasing oxygen as a byproduct.

Proton gradients across these membranes store harvested light energy. As protons flow back across the membrane through chloroplast ATP synthase (similar to what happens in mitochondria), they drive the synthesis of ATP from ADP and phosphate.

The ATP-ADP cycle is fundamental to energy transfer in all cells. ATP (adenosine triphosphate) stores energy in its bonds, particularly the third phosphate group. When this bond breaks, energy is released for cellular work, and ATP converts to ADP (adenosine diphosphate). ADP can then be recharged with energy to become ATP again, creating a continuous energy cycle.

🔋 The ATP-ADP cycle is like a rechargeable battery for your cells—ATP delivers energy where needed, becomes ADP, and then gets recharged again through cellular respiration!