29.1 An Overview of Metabolism and Biochemical Energy

Metabolism and Biochemical Energy

Metabolism is the complete set of chemical reactions that keep cells alive, and understanding it requires knowing how energy flows through biological systems. This topic connects the organic chemistry you've learned (functional groups, reaction mechanisms, thermodynamics) to the real chemistry happening inside living organisms.

ATP as Cellular Energy Currency

Adenosine triphosphate (ATP) is the primary energy carrier in cells. It's composed of an adenosine group (adenine base + ribose sugar) bonded to three phosphate groups. The phosphoanhydride bonds between those phosphate groups are often called "high-energy bonds," though what that really means is that hydrolyzing them is thermodynamically favorable.

When ATP loses its terminal phosphate, it releases about $-30.5 \text{ kJ/mol}$ of free energy under standard biochemical conditions:

$ATP + H_2O \rightarrow ADP + P_i + \text{Energy}$

That released energy drives cellular work: muscle contraction, nerve impulse transmission, active transport across membranes, and biosynthesis of new molecules.

ATP is constantly recycled. Catabolic pathways oxidize nutrients like glucose, and the energy released phosphorylates ADP back to ATP:

$ADP + P_i + \text{Energy} \rightarrow ATP$

Your body turns over roughly its own weight in ATP every day, so this recycling is essential.

Stages of Catabolic Breakdown

The breakdown of food into usable energy happens in four stages, each progressively extracting more chemical energy from nutrient molecules.

Stage 1: Digestion Enzymes in the gastrointestinal tract hydrolyze macromolecules into their monomers:

• Carbohydrates → monosaccharides (glucose, fructose)
• Proteins → amino acids (glycine, alanine, etc.)
• Lipids → fatty acids and glycerol

Stage 2: Absorption Intestinal epithelial cells absorb these monomers, and the bloodstream transports them to the liver and other tissues for further processing.

Stage 3: Cellular Catabolism Inside cells, monomers are broken down through specific pathways:

• Glycolysis converts glucose (6 carbons) into two molecules of pyruvate (3 carbons each)
• The citric acid cycle (Krebs cycle) oxidizes acetyl-CoA derived from pyruvate, generating $CO_2$, $NADH$, and $FADH_2$
• Beta-oxidation cleaves fatty acids two carbons at a time, producing acetyl-CoA
• Amino acids are deaminated (their $-NH_2$ group is removed), and the carbon skeletons enter the citric acid cycle at various points

Stage 4: Oxidative Phosphorylation This is where the bulk of ATP is produced. The electron carriers $NADH$ and $FADH_2$ from Stage 3 donate electrons to the electron transport chain in the inner mitochondrial membrane. As electrons pass through the chain, a proton gradient builds up across the membrane. ATP synthase then uses that gradient to drive the phosphorylation of ADP to ATP. A single molecule of glucose can yield approximately 30–32 ATP through this entire process.

Energetics of Catabolism vs. Anabolism

These two categories of metabolic pathways are thermodynamic opposites, and they depend on each other.

Catabolic pathways break down complex molecules into simpler ones. These are exergonic reactions, meaning they release free energy (negative $\Delta G$). Examples include the oxidation of glucose, fatty acids, and amino acids. The energy released is captured as ATP.

Anabolic pathways build complex molecules from simpler precursors. These are endergonic reactions, meaning they require an input of free energy (positive $\Delta G$). Synthesizing proteins from amino acids, assembling lipid membranes, and building polysaccharides like glycogen all fall into this category. ATP provides the energy to drive these reactions forward.

Biochemical Reactions and Energy Transfer

A few key concepts tie all of this together:

• Free energy ($\Delta G$): The thermodynamic quantity that determines whether a reaction is spontaneous. Negative $\Delta G$ means the reaction releases energy; positive $\Delta G$ means it requires energy input.
• Redox reactions: Electron transfer reactions are central to energy production. In catabolism, nutrients are oxidized (they lose electrons), and coenzymes like $NAD^+$ are reduced (they gain electrons to become $NADH$). Those electrons ultimately drive ATP synthesis.
• Enzymes: Biological catalysts that lower the activation energy of metabolic reactions without being consumed. They don't change the thermodynamics of a reaction, but they make it proceed fast enough to sustain life.
• Coenzymes: Non-protein organic molecules (often derived from vitamins) that assist enzymes. $NAD^+$, $FAD$, and coenzyme A are among the most important in metabolic pathways, serving as electron carriers or acyl-group carriers.