In the opinion of some biologists, this network is a continuous structure that begins at the cell membrane, twists through the cytoplasm, and ends at the membrane surrounding the nucleus. Located along the endoplasmic reticulum as well as elsewhere in the cytoplasm are numerous ribosomes. Proteins are manufactured at the ribosomes. The Golgi complex, or Golgi apparatus, is a membranous structure composed of stacks of thin sacs. Newly made proteins move from the endoplasmic reticulum to the Golgi complex, where they are stored for later secretion.
Vacuoles drift through the cytoplasm and usually carry food molecules in solution.
Vacuoles also regulate the water content of certain one-celled animals. When an amoeba, for example, absorbs too much water, it forms a contractile vacuole against the membrane. Lysosomes are structures somewhat similar in appearance to vacuoles but denser. They appear to have a digestive function. Each lysosome is filled with digestive enzymes and encased in a membrane.