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Topologically equivalent spaces are shown in red. In principle, cycles of vesicle budding and fusion permit any lumen to communicate with any other and with the cell exterior. The blue arrows indicate the outward direction of vesicle traffic from the ER to Golgi apparatus to plasma membrane (or lysosomes), and the black dots represent protein molecules that are secreted by the cell. Some organelles, most notably mitochondria and (in plant cells) chloroplasts, however, do not take part in this vesicular communication and so are isolated from the traffic between organelles shown here.