Final answer:
Ribosomes, essential for protein synthesis, appear as little knobs on the rough endoplasmic reticulum when viewed through an electron microscope, and are also free-floating in the cytoplasm.
Step-by-step explanation:
Ribosomes appear as little knobs on the endoplasmic reticulum. Ribosomes are cellular structures essential for protein synthesis. They can be observed through an electron microscope either as clusters are known as polyribosomes or as individual tiny dots in the cytoplasm.
Often, they are attached to the cytoplasmic side of the rough endoplasmic reticulum (RER), which gives the RER a 'rough' appearance. These ribosomes may also attach to the cytoplasmic side of the plasma membrane or the outer membrane of the nuclear envelope.
Subunits of ribosomes, comprised of protein and ribosomal RNA (rRNA), are responsible for translating messenger RNA (mRNA) into a sequence of amino acids to form proteins.