In a plant cell these 'void' areas are spaces of the central vacuole, delimitted by tonoplast and contaning the cell sap.
found in the cytoplasmic matrix of plant cells only.
Complementary
The endoplasmic reticulum (ER) is thought to be an extension of the nuclear membrane. This structure is seen to connect the nuclear membrane to the cell membrane. When viewed under high magnification, this organelle is seen to encompass the entire cytoplasmic space between the nucleus and cytoplasm.
Chloroplasts
Electron microscope
Homologous structures are any structures that have similarity and come from the same lineage. This can be seen in the forelimbs of mammals. Analogous structures are very similar but come from different ancestors. This can be seen in the North American flying squirrel vs the Australian sugar glider. They both have similar features but come from 2 different locations. In fact, the Australian sugar glider is more related to the kangaroo than the flying squirrel
Large cell structures such as the nucleus and mitochondria can be seen under the light microscope if they are properly stained. In addition, chromosomes and centromeres can be seen during cell division.
If it is an optical microscope then the structures that can be see are those that are too small to be seen by the naked eye and larger than 1/2 a wavelength of light.
Probably in tertiary structures as primary and secondary structures are those structures seen during the formation of proteins and tertiary proteins are mature proteins.
Midbrain?
The Great Wall of China is one.
The cell structures within a leaf cannot be seen with a stereo microscope. A microscope that is equipped with transmitted light and also stronger magnification is needed in order to do so.