Energy, Chemical Reactions, and Cellular Respiration

Overview of Energy in Living Organisms

All living organisms require energy to perform essential life processes. This energy is necessary for powering muscles, pumping blood, absorbing nutrients, exchanging respiratory gases, synthesizing new molecules, and establishing cellular ion concentrations. The primary molecule that stores and transfers energy in cells is ATP (adenosine triphosphate).

• ATP: Known as the "energy currency" of the cell, ATP stores energy in its high-energy phosphate bonds.

• Energy is released from ATP when these bonds are broken, fueling cellular activities.

Forms of Energy

Energy exists in various forms within biological systems, each with specific roles in physiology.

• Chemical Energy: A form of potential energy stored in the chemical bonds of molecules. It is released when these bonds are broken. Used for movement, molecule synthesis, and maintaining concentration gradients. Key molecules for chemical energy storage include triglycerides, glucose, and ATP.

• Electrical Energy: The movement of charged particles, such as ions across the plasma membrane of neurons.

• Mechanical Energy: Energy exhibited by objects in motion due to applied force, such as muscle contraction during walking.

• Sound Energy: Produced by the compression of molecules, such as sound waves vibrating the eardrum.

• Radiant Energy: Energy carried by electromagnetic waves, such as visible light striking the retina.

• Heat: Kinetic energy from the movement of atoms, ions, or molecules. It is usually not available to do work and is measured as the temperature of a substance.

Chemical Equations and Metabolism

Metabolism encompasses all biochemical reactions occurring in living organisms. These reactions involve breaking and forming chemical bonds, and are represented as chemical equations:

• Reactants: The starting substances in a chemical reaction.

• Products: The substances formed as a result of the reaction.

Classification of Chemical Reactions

Chemical reactions in the body can be classified based on the changes occurring to the molecules involved.

Decomposition Reactions

• An initial large molecule is broken down into smaller structures.

• General equation:

• Example: Hydrolysis of sucrose into glucose and fructose.

• In the body, decomposition reactions are called catabolism or catabolic reactions.

Synthesis Reactions

• Two or more structures are combined to form a larger structure.

• General equation:

• Example: Dehydration synthesis forming a dipeptide from two amino acids.

• All synthesis reactions in the body are collectively called anabolism or anabolic reactions.

Exchange Reactions

• Groups are exchanged between two chemical structures, involving both decomposition and synthesis components.

• General equation:

• Example: Production of ATP in muscle tissue.

Enzymes: Biological Catalysts

Enzymes are proteins that act as biological catalysts, accelerating chemical reactions by lowering the activation energy required. They are highly specific, typically catalyzing only one type of reaction or acting on a specific substrate.

• Structure: Enzymes have an active site that binds to a specific substrate, forming an enzyme-substrate complex.

• Function: Enzymes facilitate reactions that would occur naturally, but at a much faster rate.

• Naming: Enzyme names are often based on their substrate or product, with the suffix "-ase" (e.g., lactase, DNA polymerase).

Enzyme Location and Examples

• Some enzymes remain within cells (e.g., DNA polymerase).

• Some are embedded in plasma membranes (e.g., lactase in the small intestine).

• Some are secreted from cells (e.g., pancreatic amylase).

Mechanism of Enzyme Action

1. Substrate enters the enzyme's active site, forming an enzyme-substrate complex.

2. The enzyme changes shape slightly to fit the substrate more closely (induced fit model).

3. This change stresses chemical bonds, allowing new bonds to form or old bonds to break.

4. Products are released, and the enzyme is free to catalyze another reaction.

Cofactors and Coenzymes

• Cofactors: Nonprotein molecules or ions required for enzyme activity. Inorganic cofactors are often minerals.

• Coenzymes: Organic cofactors, usually derived from vitamins.

Clinical View: Lactose Intolerance

• Caused by a deficiency or abnormality of the enzyme lactase, which is required to break down lactose into glucose and galactose.

• Common symptoms: abdominal upset, nausea, diarrhea, bloating, and gas.

• Management: lactase enzyme supplements, avoidance of milk, or consumption of lactose-free milk.

Cellular Respiration

Cellular respiration is the process by which organic molecules are oxidized and disassembled by a series of enzymes, releasing energy stored in chemical bonds to synthesize ATP. Oxygen is required for maximum ATP production.

• Overall chemical reaction for glucose oxidation:

Stages of Cellular Respiration

1. Glycolysis: Occurs in the cytosol, does not require oxygen (anaerobic). Glucose is converted to pyruvate, producing 2 ATP and 2 NADH per glucose molecule.

2. Intermediate Stage: Pyruvate is converted to acetyl CoA by pyruvate dehydrogenase. This occurs twice per glucose molecule, producing NADH.

3. Citric Acid Cycle (Krebs Cycle): Occurs in the mitochondrial matrix, requires oxygen. Acetyl CoA is oxidized, producing 1 ATP, 3 NADH, and 1 FADH2 per cycle. Two cycles occur per glucose molecule.

4. Electron Transport System (ETS): Located in the inner mitochondrial membrane. Electrons from NADH and FADH2 are transferred through a series of carriers, generating a proton gradient used to produce ATP.

Electron Transport Chain (ETC) Details

• Electrons from NADH enter at Complex I, passing through three H+ pumps, generating 3 ATP per NADH.

• Electrons from FADH2 enter at Complex II, passing through two H+ pumps, generating 2 ATP per FADH2.

• Actual net ATP yield per glucose is lower due to energy costs and inefficiencies.

Clinical View: Cyanide Poisoning

• Cyanide binds to a specific electron carrier in the electron transport system, inhibiting electron transport and ATP production.

• Prevents electrons from reaching oxygen, leading to cellular energy failure.

• Symptoms include memory loss, nausea, vomiting, rapid breathing, arrhythmias, respiratory failure, hypotension, and neurological symptoms.