### **Translation: The Chemical Level of Organization: Atomic and Molecular**

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### **5.1 The Atom**
#### **Definition**  
The atom is the smallest chemical component of our body.  
The 112 atomic elements are represented in the periodic table of elements.  
The atom is a basic unit that cannot be split by natural means.

#### **Structure**  
- **Protons**: Positively charged particles forming the atomic nucleus along with neutrons.  
- **Neutrons**: Neutral particles responsible for isotopes (+ or - neutrons).  
- **Electrons**: Negatively charged particles forming the orbital around the nucleus.  

#### **Body’s Atomic Elements**  
Main elements in the body include: hydrogen, carbon, oxygen, nitrogen, sodium, potassium, phosphorus, calcium, chlorine, sulfur, etc.

#### **Electrons**  
- In living tissues, only electrons are rearranged on electronic shells.  
- Electrons orbit the nucleus in approximate paths called orbitals.  
- The electron determines the atom’s chemical behavior, i.e., its bonding capability.

#### **Atomic Bonds in Living Systems**  
Three main types of bonds:  
1. **Ionic Bonds**: Electrons are transferred between atoms (oxidation-reduction).  
2. **Covalent Bonds**: Two atoms share a pair of electrons. Polar covalent bonds have properties of both covalent and ionic bonds due to unequal electron sharing.  
3. **Hydrogen Bonds**: Common in biological molecules, such as between complementary nucleotides in DNA. These bonds are weak interactions involving hydrogen bonded to oxygen or nitrogen attracted to another electronegative atom.

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### **5.2 The Molecule**  
Molecules are composed of atoms.  
They group together through covalent, ionic, or hydrogen bonds to form the four essential macromolecules for life: proteins, carbohydrates, lipids, and nucleic acids.

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#### **Proteins (Protides)**  
- **Overview**:  
Generic term for nitrogen-containing organic substances such as amino acids, peptides, polypeptides, proteins, enzymes, and cellular receptors.  
Proteins are made up of 20-22 amino acids, each containing a nitrogen atom.  

- **Functions**:  
  - Structural support (collagen, elastin, keratin).  
  - Transport (hemoglobin).  
  - Communication (hormones, neurotransmitters).  
  - Movement (myosin-actin).  
  - Defense (antibodies).  
  - Catalysis (enzymes).  
  - Storage (e.g., ferritin for iron).  

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#### **Carbohydrates**  
- **Overview**:  
Known as hydrates of carbon, including sugars, fruits, and grains.  
Soluble in water and composed of carbon, hydrogen, and oxygen atoms.

- **Structure**:  
  1. **Monosaccharides**: Simple sugars (e.g., glucose, fructose, galactose).  
  2. **Disaccharides**: Combinations of two monosaccharides (e.g., sucrose, lactose, maltose).  
  3. **Polysaccharides**: Long chains of monosaccharides forming macromolecules (e.g., glycogen, starch, cellulose).  

- **Functions**:  
Primary source of readily available energy for cells.  

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#### **Lipids**  
- **Overview**:  
Fats and fat-like substances insoluble in water, including triglycerides, phospholipids, and steroids.  

- **Functions**:  
  - Energy storage.  
  - Cushioning and protecting organs.  
  - Thermal insulation.  

- **Structure**:  
  1. **Triglycerides**: Composed of glycerol and fatty acids.  
  2. **Phospholipids**: Hydrophobic tails (fatty acids) and hydrophilic heads (phosphate group).  
  3. **Steroids**: Include cholesterol, hormones, and vitamin D precursors.  

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#### **Nucleic Acids**  
- **Overview**:  
Store genetic information and participate in cellular energy transfer and messaging (e.g., DNA and RNA).  

- **Structure**:  
Built from nucleotides, which consist of a sugar, a phosphate group, and a nitrogenous base.  

  1. **DNA (Deoxyribonucleic Acid)**:  
     - Double-helix structure with complementary base pairing (A-T, G-C).  
     - Stores genetic instructions for replication and cellular function.  

  2. **RNA (Ribonucleic Acid)**:  
     - Single-stranded and includes uracil instead of thymine (A-U, G-C).  
     - Transcribes and translates DNA instructions for protein synthesis.  

  3. **ATP (Adenosine Triphosphate)**:  
     - Composed of adenine, ribose, and three phosphate groups.  
     - Stores and releases energy for cellular processes.  

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