Cell Organelles

This review will emphasize mammalian cells but also make reference to organelles found in plant cells.

It is useful to view the cell as a functional unit consisting of distinct compartments separated by membranes.

I. The plasma membrane separates the cell from its environment. It is a lipid bilayer consisting of phospholipids (phosphatidyserine, phosphatidylethanolamine, phosphatidylinositol, etc.) and proteins – integral ones embedded in the lipid bilayer as well as peripheral proteins attached by carbohydrate or lipid linkages.

II. Cytosol – This consists of water, electrolytes, and free floating (cytosolic) proteins. Strictly speaking, the cytosol includes all cellular components besides the plasma membrane and organelles.

III. Nucleus – contains DNA, organized as chromosomes in eukaryotic cells. Prokaryotes lack a nuclear membrane; their DNA is often found in a “nucleosome”.

A. Nuclear envelope – a double membrane surrounding the contents of the nucleus

B. Nuclear pores – protein channels that allow materials to enter or exit the nucleus

C. Nucleolus – area of the nucleus specialized for the synthesis of ribosomal components, esp. rRNA

IV. Endoplasmic Reticulum – the cell’s internal membrane system

A. Rough ER (rER) – is studded with ribosomes. Proteins synthesized here include membrane proteins (surface receptors, ion channels) or proteins destined for secretion from the cell (e.g. insulin, antibodies).

B. Smooth ER (sER) has no ribosomes associated with it. Steroid hormone synthesis occurs here, esp. in the adrenal cortex.

V. Golgi apparatus – Protein packaging occurs here in the form of glycosylation. Sugars are added to proteins and then trimmed. Some carbohydrate tags seem to serve as address labels; other sugars may make some proteins more resistant to degradation.

VI. Ribosomes – these complexes are composed of 2/3 protein and 1/3 ribosomal RNA (rRNA). Protein synthesis occurs here. Free ribosomes are located in the cytosol. Membrane bound ribosomes are located along the rER.

VII. Mitochondria – the power houses of the cell. ATP is produced here by aerobic respiration. Mitochondria also  contain their own DNA and ribosomes, supporting the theory that they are the descendants of free living bacteria captured by primitive eukaryotic cells over 1 billion years ago.

VIII. Chloroplasts – These organelles are found in plant cells and protozoa. Photosynthetic cyanobacteria are essentially free living chloroplasts. Since chloroplasts contain their own DNA and ribosomes, scientists believe that protozoans and primitive plant cells captured the free living ancestors of chloroplasts (as well as mitochondria) over 1 billion years ago.

IX. Centrioles – These are a pair of cylindrical structures that act as a microtubule organizing center. The centrioles are located in a region called the centrosome.  During mitosis, the mitotic spindle seems to polymerize around the centrioles. For no apparent reason, most plant cells lack centrioles. An ill defined region of proteins serves as an MTOC in plant cells.

X. Cytoskeleton – three groups of protein provide cellular shape and support as well as function in internal transport and cell mobility.

A. Microfilaments – made of actin. Actin is one of the most abundant cellular proteins. It can exist in the globular form (G-actin) or polymerize into the filamentous form (F-actin). The brush border of intestinal enterocytes is also made of actin.

B. Microtubules – made of tubulin. These are essential in the formation of the mitotic spindle as well as the Internal transport of large structures, including secretory vesicles and even organelles the size of mitochondria.

C. Intermediate filaments – apparently serve a structural support role. The most well known example is GFAP (glial fibrillary acidic protein) in astrocytes.

XI. Lysosomes – these sacs contain degradative enzymes, including proteases, amylases, and lipases. A defect in one or more of these enzymes (or in transporting the appropriate enzymes to the lysosomes) results in lysosomal storage diseases (Tay-Sach’s, Pompei’s, I-cell disease). These diseases tend to be fatal in childhood.

XII. Peroxisomes – esp. abundant in the liver, they contain enzymes to break down free radicals. The key enzymes here are catalase / peroxidase and SOD (superoxide dismutase).

XIII. Vacuoles – these are more prominent in plant cells. They can be used to store water, starch, or insoluble waste. XIV. Motor proteins – are involved in cellular locomotion as well as waste removal. Examples in human cells include cilia on bronchial epithelial cells and flagella in the tails of spermatocytes. Internal motor proteins include various forms of myosin (muscle and myoepithelial cells), dynein, and kinesin.