Lysosomes are membranous sacs that contain hydrolytic enzymes. Lysosomes digest macromolecules and do it in a variety of circumstances. For example, unicellular organisms such as amoebas and other protists carry out phagocytosis. They engulf smaller organisms and food particles that then are digested by lysosomes. Lysosomes fuse with food vacuoles and lysosome enzymes digest it. Digestive products pass into cytosol where they are used as cell nutrients. Human cells also carry out phagocytosis. Among them are macrophages that are white blood cells that engulf bacteria and other invaders. Lysosomes also take part in such process as autophagy. When damaged organelles in the cell are surrounded by double membrane, lysosomes fuses with outer membrane and dismantles the membrane and the enclosed materials. The small organelle compounds are then released in the cytosol for reuse. Human liver cells, for example, recycle half of its macromolecules each week.