**Assertion: All immunoglobulin molecules have a basic structure composed of four polypeptide chains.**

**Reason: The polypeptide chains consist of two identical light and heavy chains connected by disulfide bonds.**

Immunoglobulins, also known as antibodies, are proteins produced by the immune system to recognize and neutralize foreign substances, such as pathogens or toxins. These molecules play a crucial role in the immune response and provide targeted defense against specific antigens.

The structure of immunoglobulins is fundamental to their function. They have a basic structure composed of four polypeptide chains, which are held together by disulfide bonds. The Assertion states that all immunoglobulin molecules have this basic structure, while the Reason explains that the polypeptide chains consist of two identical light and heavy chains connected by disulfide bonds.

Now, let's delve into the details to understand the structure of immunoglobulin molecules and how the polypeptide chains are connected.

**Structure of Immunoglobulin Molecules:**

1. **Four Polypeptide Chains:** Immunoglobulin molecules are composed of four polypeptide chains arranged in a Y-shaped structure. These chains are categorized into two types: heavy chains (H) and light chains (L). Each immunoglobulin molecule consists of two identical heavy chains and two identical light chains.

2. **Heavy Chains:** The heavy chains are larger and more complex than the light chains. They are further classified into different types, such as IgG, IgA, IgM, IgD, and IgE, based on their structural and functional characteristics. The heavy chains provide the antibody with its unique properties and determine its class or isotype.

3. **Light Chains:** The light chains are smaller and simpler compared to the heavy chains. They are categorized into two types: kappa (κ) and lambda (λ) chains. The light chains are identical within a specific immunoglobulin molecule, meaning both light chains are either kappa or lambda chains.

**Connection of Polypeptide Chains:**

1. **Disulfide Bonds:** The polypeptide chains of immunoglobulin molecules are connected by disulfide bonds, which are formed between the cysteine residues present in the chains. Disulfide bonds are strong covalent bonds that contribute to the stability and integrity of the antibody structure.

2. **Hinge Region:** In addition to the disulfide bonds, immunoglobulin molecules also possess a flexible hinge region. This hinge region allows flexibility and movement between the two arms of the Y-shaped structure. The hinge region is important for the antibody's ability to bind to different antigens and facilitate immune responses effectively.

In conclusion, the basic structure of immunoglobulin molecules is composed of four polypeptide chains, including two identical heavy chains and two identical light chains. These chains are connected by disulfide bonds, providing stability to the antibody structure. Understanding the structure of immunoglobulin molecules is essential for comprehending their functions in the immune system and their role in defending against infections and diseases.

Both are true but reason does not explain it properly.