cellular respiration is the enzymatic breakdown of glucose in the presence of oxygen to produce cellular energy. the cytoskeleton is responsible for cell shape motility movement of the cell as a whole and motility of organelles within a cell microtubules are tube-like structural proteins that help chromosomes move throughout the cell. they are modular proteins because microtubules can add or subtract protein units to become longer or shorter respectively while continuing to provide the cell with structural support during cell division. kinetochores are the protein structures located at the intersection of the two sister chromatids that serves as an attachment site for microtubules and spindle fibers during cell division to help pull the two chromatids apart. histones are the proteins that help chromatin to condense. they serve as spools around which dna winds and play a role in gene regulation. without histones the unwound dna in chromosomes would be very long and unable to fit into the nucleus of a cell.