2.3 Chemical Reactions

Energy and Chemical Reactions

Chemical reactions power every biological process in your body. Whether you're digesting food, contracting a muscle, or firing a nerve impulse, energy is being transferred or transformed. Understanding how these reactions work at the chemical level is foundational to anatomy and physiology.

Kinetic vs. Potential Energy

Kinetic energy is the energy of motion. Anything that's moving has it: molecules vibrating, blood flowing, your legs walking.

Potential energy is stored energy, available but not currently in use. In your body, the most relevant form is chemical energy stored in the bonds between atoms. When those bonds break, energy can be released.

These two types of energy connect directly to how reactions behave:

• Exergonic reactions release energy. The products end up with less potential energy than the reactants, and the difference is released (often as heat). These reactions are thermodynamically favorable, meaning they can occur spontaneously. Examples: cellular respiration, hydrolysis of ATP.
• Endergonic reactions absorb energy from their surroundings. The products have more potential energy than the reactants, so energy input is required. Examples: protein synthesis, ATP synthesis.

Chemical equilibrium is the state where the forward and reverse reactions proceed at equal rates. Reactant and product concentrations stop changing, but the reactions haven't stopped. Both directions are still occurring, just at the same speed.

Energy Forms in the Body

Your body converts energy between several forms to carry out its functions:

• Chemical energy is potential energy stored in molecular bonds. Molecules like ATP, glucose, and fats serve as energy carriers or fuel sources. Breaking these bonds (exergonic) releases energy; forming new bonds (endergonic) stores it.
• Mechanical energy is the kinetic energy of physical movement. Muscle contraction, blood pumping through vessels, and air moving in and out of your lungs all involve mechanical energy.
• Electrical energy comes from the movement of charged particles (ions and electrons). This form is critical for nerve impulse transmission, the heartbeat, and signaling muscle contraction.
• Thermal energy (heat) is the kinetic energy of randomly moving atoms and molecules. Many metabolic reactions generate heat as a byproduct. This heat helps maintain body temperature at roughly 37°C, which keeps enzymatic reactions running at proper rates.

Chemical Reactions in Biological Systems

Types of Biological Chemical Reactions

Synthesis (anabolic) reactions combine smaller molecules into larger, more complex ones. These require energy input (endergonic). Your body uses them to build proteins from amino acids, assemble glycogen from glucose molecules, and construct lipids for cell membranes.

Decomposition (catabolic) reactions break larger molecules into smaller, simpler ones and release energy (exergonic). Digestion is a clear example: enzymes break down proteins, carbohydrates, and fats into absorbable units. Glycogen breakdown and beta-oxidation of fatty acids also fall here.

Exchange reactions involve atoms or functional groups swapping between molecules. Parts of the reactants are rearranged to form new products. These can be either exergonic or endergonic depending on the specific reaction. Transamination (transferring an amino group between molecules) and phosphorylation (transferring a phosphate group) are common examples.

Redox (oxidation-reduction) reactions involve the transfer of electrons between molecules. One molecule loses electrons (oxidation) while another gains them (reduction). These reactions are central to energy metabolism. The electron transport chain in cellular respiration, for instance, uses a series of redox reactions to generate ATP.

Factors Affecting Reaction Speed

Several factors determine how fast a chemical reaction proceeds in the body:

• Temperature — Higher temperatures increase molecular kinetic energy, causing more frequent and forceful collisions between reactants. Enzymes in the human body work best near 37°C. Extreme temperatures denature enzymes by disrupting their three-dimensional shape, which halts the reaction.
• Reactant concentration — More reactant molecules in a given space means more collisions and a faster reaction rate. In biological systems, this applies to enzyme-substrate interactions: increasing substrate concentration speeds up the reaction until the enzyme becomes saturated.
• Catalysts (enzymes) — Enzymes lower the activation energy required for a reaction, making it proceed much faster. They are not consumed or permanently changed in the process, and each enzyme is highly specific to its substrate.
• pH — Each enzyme has an optimal pH range where it functions best. Pepsin in the stomach works best around pH 2, while pancreatic enzymes prefer around pH 8. Deviations from the optimal pH alter the enzyme's shape and reduce or eliminate its activity.
• Surface area — Greater surface area means more sites available for reactions to occur. This is why mechanical digestion (chewing, churning) matters: breaking food into smaller pieces exposes more surface area for enzymes to act on.

Reaction Kinetics

Reaction rate describes how quickly reactants are converted into products. All five factors above influence this rate.

Activation energy is the minimum energy needed to start a reaction. Think of it as the initial push required to get the reaction going. Even exergonic reactions need this initial push. Enzymes work by lowering this energy barrier, which is why they're so essential to biological chemistry.

The law of mass action states that the rate of a reaction is proportional to the concentration of the reactants. As reactant concentrations increase, the reaction speeds up. As products accumulate and reactants are used up, the reaction slows until it reaches equilibrium.