Consciousness is is a faculty—an ability of some entity—and its operation is a process. It is not composed of physical constituents. In conscious action, there may be non-conscious components (e.g eye movements in seeing something), however the awareness itself cannot be reduced.
Take your reading of these words. You first see a word or phrase, then process it and “hear” the words in your mind, and you understand the meaning of those words. There are indeed, non-conscious components, such as eye movements and the physical page-turnings, but there is no way to reduce the seeing, or the internal “hearing”, or the understanding to one or more physical sub-actions—not without leaving out the essential, conscious aspect. Seeing the words involves physical and physiological processes but is not reducible to just the physical processes.1
One may hold that consciousness is comprised of physical components such as physical processes or matter (see reductive materialism)—this thesis is incorrect. Individual brain processes only add up to a larger-scale brain process, not to a state of awareness. Small physical processes can only add up to a larger-scale physical process.
There are indeed unconscious sub-processes occurring—notably, the brain processes that underlie reading. Neurons fire in this brain centre, then in that one. But what do these individual brain processes add up to? A larger-scale brain process. Not to a state of consciousness, not to the seeing or the understanding.1