Semen contains protein, which is found in sperm cells and the fluid that carries them, known as seminal plasma. To understand its role, it is important to distinguish between sperm and semen. Sperm are the microscopic reproductive cells, whereas semen is the complete fluid ejaculated, which is composed of sperm and secretions from several male reproductive glands.
The Composition of Semen
Semen is a complex biological fluid, with sperm cells constituting a small fraction, typically less than five percent, of its total volume. The vast majority of semen is seminal plasma, a fluid mixture produced by accessory glands. This plasma is primarily water but is rich with substances designed to support sperm. It contains fructose, a sugar that provides energy for the sperm cells’ journey, and a variety of minerals, including zinc and calcium. The fluid also carries a diverse array of enzymes and proteins that are involved in the reproductive process.
The seminal vesicles produce about 65% of the fluid, which is rich in fructose and the proteins responsible for coagulation. The prostate gland contributes around 25% of the volume, providing enzymes like prostate-specific antigen (PSA), citrate, and zinc. Smaller contributions come from the bulbourethral glands and the epididymis, which add mucus for lubrication and other factors that help mature the sperm.
Protein Quantity and Nutritional Information
The concentration of protein in seminal plasma ranges from 35 to 55 milligrams per milliliter (mg/mL). Considering that a typical ejaculate volume is between 1.5 and 5 milliliters, the total amount of protein is quite small, averaging between 200 and 500 milligrams per ejaculation. For context, this is significantly less protein than what is found in a single almond or a small spoonful of yogurt.
Due to this low quantity, consuming semen provides no meaningful nutritional benefits. The caloric content is minimal, and its trace nutrients are too low to contribute to daily dietary needs. Semen’s composition is tailored for reproduction, not nutrition.
Biological Functions of Seminal Proteins
The proteins in semen have specific jobs related to fertilization. Immediately following ejaculation, proteins secreted by the seminal vesicles, primarily semenogelin I and semenogelin II, cause the semen to thicken and form a gel-like coagulum. This coagulation helps the semen remain within the female reproductive tract, preventing immediate backflow and keeping the sperm near the cervix.
This gel state is not permanent. Within about 15 to 30 minutes, the coagulum begins to break down in a process called liquefaction. This is driven by a protein-degrading enzyme from the prostate gland known as prostate-specific antigen (PSA). PSA cleaves the semenogelin proteins, causing the gel to become watery and allowing the once-trapped sperm to become motile and swim freely. This process also benefits from seminal plasma’s alkaline pH, which helps neutralize the acidic environment of the vagina.