SCB203 - Lecture 4 Chemical Reactions

Lecture 4: Chemical Reactions

Overview

• Focus on the following key aspects of chemical reactions:

  • Definition and importance of chemical reactions.

  • Energy dynamics in chemical reactions.

  • Forms of energy present in the human body.

  • Different types of chemical reactions and their characteristics.

  • Factors affecting reaction rates, including the role of enzymes.

Definition of Chemical Reactions

• Chemical Reactions: Process involving the formation, breaking, or rearranging of chemical bonds between atoms or molecules.

• Electrons can be transferred between reactants, leading to new substances.

Chemical Notation

• Chemical Notation: Symbols and abbreviations used to represent a chemical reaction.

• Consists of:

  • Reactants: Substances that undergo a change.

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

• Example:

  CO2 + H2O ↔ H2CO3

  • Reactants: Carbon Dioxide (CO2), Water (H2O)

  • Product: Carbonic Acid (H2CO3)

  • Reversible Reaction: Denoted by a double arrow.

Energy in Chemical Reactions

Definition of Energy

• Energy: The capacity to perform work.

Types of Energy

1. Potential Energy:

   • Stored energy capable of doing work when released.

2. Kinetic Energy:

   • Energy present in motion; all atoms possess kinetic energy.

Forms of Energy in the Human Body

• Chemical Energy: Drives cellular processes, found in chemical bonds in food (used for ATP synthesis).

• Electrical Energy: Produced by ion flow, crucial for nerve impulses and muscle contractions.

• Mechanical Energy: Energy transferred directly through physical means, such as muscle contraction.

Types of Chemical Reactions

1. Catabolic Reactions

• Larger substances are broken down into smaller components (decomposition).

• Example: Complex carbohydrates into simple sugars.

2. Exchange Reactions

• Atoms from reactants swap places.

• Example: HCl + NaOH → H2O + NaCl.

• Oxidation-reduction reactions: A type of exchange reaction where electrons are exchanged.

3. Anabolic Reactions

• Formation of complex molecules from simpler ones (synthesis).

• Example: Glucose + Glucose → Glycogen.

Reaction Rates and Enzymes

Overview of Reaction Rates

• Time required for a chemical reaction varies widely depending on different factors.

Activation Energy (Ea)

• Activation Energy: Energy needed to initiate a reaction by overcoming electron repulsion.

• Analogy: Rolling a ball uphill requires energy—once over the hill, it can move downhill freely.

Role of Enzymes

• Enzymes lower activation energy, enabling reactants to reach transition states faster and convert to products.

Factors Affecting Reaction Rates

1. Concentration

• Higher concentrations of reactants increase the likelihood of collisions and reactions.

2. Temperature

• Increased temperature raises kinetic energy, improving reaction rates. However, excessive heat can harm biological molecules.

3. Reactant Properties

• Size and physical state (solid, liquid, gas) matter; smaller, gaseous reactants typically react faster due to higher kinetic energy.

4. Presence of a Catalyst

• Catalyst: Substance that accelerates reactions by reducing activation energy without being consumed in the process. Biological catalysts are known as enzymes.

Characteristics of Enzymes

• Lowers activation energy and speeds up reactions.

• Highly specific to substrates and reactions; interact with unique active sites.

• Enzymes are proteins, and many end with the suffix -ase.

• Not permanently altered in the reaction, thus can be reused multiple times.

Mechanism of Enzyme Action

1. Substrates bind to the enzyme's active site.

2. Enzyme-substrate complex undergoes an 'induced fit', changing shape slightly to enhance the reaction.

3. This increases reaction rates by reducing activation energy.

4. Products are formed and release back to the environment, returning the enzyme to its original state.

Enzyme Deficiencies

1. Tay Sachs Disease: Hexosaminidase A deficiency leads to nerve cell damage and early death.

2. Severe Combined Immunodeficiency Syndrome: Often caused by adenosine deaminase deficiency, severely impacts immune function.

3. Phenylketonuria (PKU): Phenylalanine hydroxylase deficiency that leads to toxic accumulation and requires dietary restrictions to manage symptoms.

Summary Questions (Review)

• What is NaCl in the reaction NaOH + HCl → NaCl + H2O? (C) Product

• The digestion of food represents: (B) Catabolic reaction.

• Identify the exchange reaction: (C) AB + CD → AD + BC.

• Which lowers activation energy? (B) Enzyme.

• Best description of enzyme function: "Enzymes speed chemical reactions by lowering the activation energy."

Study Guide Overview

• Understand the definitions and importance of chemical reactions and energy.

• Recognize types of chemical reactions and mechanisms, especially related to enzymes.

• Be able to explain factors affecting reaction rates, particularly in biological contexts.

Enzymes

Definition and Function

• Enzymes: Biological catalysts that accelerate chemical reactions by lowering activation energy. They are typically proteins and are highly specific to their substrates, interacting with unique active sites.

Mechanism of Action

1. Substrate Binding: Substrates bind to the enzyme's active site.

2. Induced Fit: The enzyme undergoes a slight shape change to enhance the reaction, forming an enzyme-substrate complex.

3. Reaction and Release: Products are formed and released back into the environment, with the enzyme returning to its original state for reuse.

Characteristics

• Specificity: Enzymes are specific to substrates and reactions based on their unique active sites.

• Reusability: Enzymes are not permanently altered during the reaction and can be reused multiple times.

• Suffix: Many enzymes end with the suffix -ase (e.g., lactase, amylase).

Role in Biological Systems

• Enzymes play a crucial role in metabolic pathways and are essential for various cellular processes, including digestion, energy production, and DNA replication.

Enzyme Deficiencies

• Certain enzyme deficiencies can lead to metabolic disorders, such as:

  • Tay Sachs Disease: Caused by hexosaminidase A deficiency; leads to nerve cell damage and early death.

  • Severe Combined Immunodeficiency Syndrome: Often due to adenosine deaminase deficiency; severely impacts immune function.

  • Phenylketonuria (PKU): Caused by phenylalanine hydroxylase deficiency; leads to toxic accumulation requiring dietary restrictions to manage symptoms.

Summary

Enzymes are vital biological catalysts, essential for accelerating chemical reactions, and understanding their mechanisms and functions is critical for studying biochemical processes and associated disorders.