Chapter 3

Non-Membrane-Bound Cell Organelles

James F. Thompson, Ph.D.

The Organelles of Typical Cells• Cytoplasmic organelles you should know– Mitochondria– Ribosomes*– Rough endoplasmic reticulum (Rough ER)– Smooth endoplasmic reticulum (Smooth ER)– Golgi apparatus– Lysosomes & Peroxisomes– Cytoskeleton*– Nucleus– Nucleoli*

• Inclusion bodies: substancesnot enclosed in membrane

– Centrioles*– Cilia– Flagella

[* not membrane-bound]

Ribosomes

Ribosomes self assemble from rRNA molecules and proteins.

They form two subunits which come together to form the functional whole, the ribosome, which is where translation, the synthesis of protein molecules occur.

Ribosomes

Ribosomes are located in the general cytoplasm or may shift to become attached to the cytoplasmic side of the rough endoplasmic reticulum (RER); another set of ribosomes are located inside mitochondria.

Ribosomes

Ribosome surface features ensure that the mRNA and tRNAsalign correctly for the translation of proteins.

The Cytoskeleton

The three main cytoskeletal elements are the fibrous proteins microtubules, intermediate filaments and microfilaments.

Microtubules• Large diameter, hollow tubes made of coiled tubulin subunits.• They assemble or disassemble as needed, using ATP energy.• They anchor and move organelles in the cytoplasm.• They form the spindle fibers seen in nuclear divisions (mitosis and

meiosis) which move the chromosomes to the cell poles.

Intermediate Filaments• Four families of intermediate filaments are known.• Some give shape to cells like a scaffolding.• Some bind to cytoskeletal anchors and to cell junctions

such as desmosomes.• They are less involved in cell movements.

Microfilaments• The smallest filament are composed

of actin• They are involved in cell motility

and cell shape• They are responsible for muscle

contraction when actin interacts with myosin

Microfilament- and Microtubule- Dependent Motility

The Nucleolus / Nucleoli

They are dark staining, oval/spherical bodies within the nucleus of cells which are synthesizing large quantities of proteins

hepatocytes

The Nucleolus / Nucleoli• They are composed of

clusters of DNA, RNA, and proteins

• They are the sites of ribosomal RNA synthesis and ribosomal subunit assembly

• They disappear during nuclear divisions and reappear later

Centrosome & Centromeres• Centrosome– Microtubule organizing

center– Contains centrioles

• Centrioles– Involved in mitosis and

orient the spindle fibers to the poles of the dividing cell

– Give rise to cilia and flagella

Centrosome & Centromeres

Note: Cilia and flagella are membrane bound

Inclusion Bodies

Sporadic inclusion body myositis (IBM) is the most common muscle disease in old people.

Reticulocytes are immature red blood cells which do not have a cell nucleus. They are called reticulocytes because of a reticular (mesh-like) network of ribosomal RNA that becomes visible with certain special stains.

End Chapter 3

Non-Membrane-Bound Cell Organelles