Understanding the Importance of Proteins in Living Cells

______ are essential for the functioning of living cells and are the physical basis of life. This is evident from various examples: 1. Motion and locomotion rely on contractile proteins. 2. Biochemical reactions are facilitated by enzymes, which are predominantly composed of proteins. 3. Cells' structural integrity and the extracellular matrix that surrounds them are primarily made up of collagens, which are the most abundant proteins in the human body. 4. The transport of materials in body fluids depends on various proteins, including transferrin and transmembrane receptors for hormones. Transcription factors, necessary for gene transcription, are also proteins. 5. ______ play a crucial role in the immune system as they form antibodies, a major component of immune responses.

Molecular Size ______ are large macromolecules composed of one or more unbranched chains of amino acids. They can vary in size, with typical proteins containing 200–300 amino acids. However, some proteins are smaller (peptides), while others are much larger, like titin found in muscles, with molecular masses ranging from approximately 6,000 for insulin to several M for certain structural proteins.

Most plasma proteins are synthesized in the liver and secreted by hepatocytes, except for immunoglobulins produced in plasma cells. Genes encode the unique amino acid sequence of proteins through the DNA's nucleotide sequence, which is translated into mRNA in the nucleus.