Vacuoles, Endocytosis, and Lysosomes

Cells often contain vacuoles or smaller vesicles that are separated from the cytosol by a single membrane. Their content is often quite acidic. 40 Small vesicles sometimes bud inward from the plasma membrane in a process called endocytosis. In this manner the cell may engulf particles (phagocytosis) or droplets of the external medium (pinocytosis). The resulting endocytotic vesicles or endosomes often fuse with lysosomes, which are small acidified vesicles containing a battery of enzymes powerful enough to digest almost anything in the cell. In cells that engulf bits of food (e.g., ameba) lysosomes provide the digestive enzymes. Lysosomes also take up and digest denatured or damaged proteins and may digest “worn out” or excess cell parts including mitochondria. Lysosomes are vital components of cells, 41 and several serious human diseases result from a lack of specific lysosomal enzymes.

Electron micrograph of a thin section of a young epidermal cell of a sunflower. The tissue was fixed and stained with uranyl acetate and lead citrate. Clearly visible are the nucleus (N), mitochondria (M), chloroplasts (C), a Golgi body dictyosome (G), endoplasmic reticulum, vacuole (V), cell wall, plasmodesmata, and cuticle (upper right, thin dark layer).Micrograph courtesy of H. T. Horner.