The outer protective barrier that lacks blood vessels is the epidermis. This layer is avascular, meaning it has no blood supply, which is a key feature of its role as a barrier. The epidermis is made of keratinized cells that form a waterproof shield, primarily in the stratum corneum, helping to prevent water loss and protect the moist tissues beneath from injury and infection. Deeper skin layers, like the dermis and subcutaneous (hypodermis), contain blood vessels and connective tissue, so they are vascular, unlike the epidermis.

UV protection in the skin comes from melanin, the pigment produced by melanocytes in the epidermis. Melanin absorbs ultraviolet rays from the sun and dissipates the energy as harmless heat, reducing DNA damage in the skin’s deeper cells. More melanin means greater protection against UV-induced changes that can lead to skin cancer and photoaging, which is why darker skin has a lower risk for certain UV damages under the same exposure. Keratin is a structural protein that strengthens and forms the outer protective barrier of the epidermis; it isn’t specialized for absorbing UV radiation. Collagen and elastin are proteins in the dermis that provide strength and elasticity; they don’t shield cells from UV light and, in fact, can be degraded by UV exposure over time.

Proteins are made by ribosomes, the cellular machines that translate genetic instructions into amino acid sequences. They can float freely in the cytoplasm or be attached to the rough endoplasmic reticulum, where newly formed proteins are prepared for their specific destinations. Mitochondria mainly generate energy and have their own ribosomes for some internal proteins, but they aren’t the general site of protein synthesis. The nucleus houses DNA and is where transcription occurs, while the Golgi apparatus processes and ships proteins after synthesis. So the actual site of protein synthesis is the ribosome.

Focus on how different surface extensions function. Cilia are numerous, short, hairlike projections that cover the cell surface and beat to move fluid across the surface. This makes them the best fit for moving mucus or other substances along the cell’s exterior, as seen in the respiratory tract. Microvilli are even smaller projections that increase surface area for absorption and secretion and don’t propel fluid. Flagella are longer and fewer, used to propel the entire cell, like a sperm tail. Centrosomes are internal organizers of the cytoskeleton and do not extend outward as surface projections.