What are glycoproteins, and what functions do proteins, including glycoproteins, have in the plasma membrane? (pages \(48-49)\)

Glycoproteins are proteins with covalently attached carbohydrates, usually in the form of oligosaccharides. These conjugated molecules can be found in various cell types and play significant roles in cellular functions.

Proteins have various functions in the plasma membrane, including: 1. Transport: Proteins can act as channels or carriers to facilitate the movement of molecules across the membrane. 2. Enzymatic activity: Some proteins can carry out chemical reactions that are important for cell function. 3. Signal transduction: Proteins can act as receptors for recognizing signaling molecules, such as hormones, neurotransmitters, or growth factors. This helps in transmitting information from the outside to the inside of the cell. 4. Cell adhesion: Proteins can help cells adhere to one another or to the extracellular matrix, providing structural support and maintaining tissue integrity. 5. Cell recognition: Proteins can act as markers for cell recognition, allowing cells to identify and interact with one another.

Glycoproteins play several important roles in the plasma membrane, including: 1. Cell-cell recognition and adhesion: The carbohydrate portion of glycoproteins can act as a marker for cell recognition, allowing cells to identify and adhere to other cells or to the extracellular matrix. 2. Signal transduction: Some glycoproteins can act as receptors for signaling molecules, transmitting information from the outside of the cell to the inside. This helps in cellular communication and coordination. 3. Protection: The carbohydrate portion of glycoproteins can protect the protein from degradation by proteolytic enzymes. Additionally, glycoproteins can form a protective barrier on the cell surface, shielding it from the external environment. 4. Modulation of protein function: The attached carbohydrates can influence the function of the protein part, by altering its stability, folding, or activity. In summary, glycoproteins are proteins with attached carbohydrates, playing diverse roles in the plasma membrane, including cell-cell recognition, signal transduction, and protection. They are essential for maintaining cellular functions and integrity.