What is the fluid portion of the blood called?

A. The cell nucleus contains genetic material (DNA) and is surrounded by a nuclear envelope, which also has a double lipid bilayer. However, the question refers to the general structural component of the cell, and the nucleus is not considered the primary barrier separating the cell from its environment.

B. The cell membrane, also called the plasma membrane, is composed of a phospholipid bilayer. This structure has hydrophilic (water-attracting) heads facing outward and hydrophobic (water-repelling) tails facing inward, creating a semi-permeable barrier that regulates the entry and exit of substances. The cell membrane also contains proteins, cholesterol, and carbohydrates that support signaling, transport, and structural stability.

C. The endoplasmic reticulum (ER) is a membrane-bound organelle involved in protein and lipid synthesis. While it contains lipid membranes, the ER is an internal structure, not the primary boundary of the cell, and its membrane is continuous with the nuclear envelope.

D. Mitochondria are double-membrane organelles responsible for energy production via ATP synthesis. Although they have an inner and outer membrane, the question refers to the cell component composed of a phospholipid bilayer that serves as the main barrier of the cell, which is the plasma membrane.

A. Vision changes in older adults, such as presbyopia, cataracts, or macular degeneration, are caused by age-related alterations in the eyes themselves, not by saliva production. Vision decline is independent of oral secretions.

B. Smell depends on olfactory receptors in the nasal cavity. Although aging can reduce the sensitivity of smell receptors (hyposmia), this is unrelated to saliva levels, so decreased saliva does not impair olfactory function.

C. Hearing loss, often due to presbycusis or damage to inner ear structures, is not influenced by saliva production. The auditory system, not the oral cavity, governs hearing function.

D. Taste is directly affected by saliva production. Saliva is essential for dissolving food particles so that taste buds on the tongue can detect flavors. In older adults, reduced saliva production, called xerostomia or dry mouth, can diminish perception of sweet, salty, sour, and bitter flavors, make chewing and swallowing more difficult, increase the risk of dental caries and oral infections, and reduce appetite, potentially leading to nutritional deficiencies. Because taste and flavor perception depend on adequate saliva, a decrease can significantly affect food enjoyment and nutritional intake. Managing dry mouth with hydration, sugar-free gum, saliva substitutes, and proper oral hygiene helps maintain taste perception and overall nutrition.