The compound light microscope passes light rays through specimens and uses glass lenses to view objects. The transmission electron microscope passes electrons through a specimen, focuses with magnetic lenses, and projects images onto a fluorescent screen or photographic film. In the fluid-mosaic model, the plasma membrane is a bilayer of phospholipids with embedded or attached proteins. Glycoproteins and glycolipids serve as identification markers. Embedded proteins may serve as hormone receptors, transport channels, and enzymes in metabolic reactions.
Osmosis is the diffusion of water across a plasma membrane.
Transport by carriers occurs in two manners. In facilitated transport a protein carrier helps a molecule across the membrane, using no energy. Endocytosis occurs as the plasma membrane forms a vesicle around a particle. Exocytosis is the reverse process. The nucleus DNA directs protein synthesis in the cytoplasm.