Membrane-Bound Organelles and Defining Characteristics of Eukaryotic Cells

moeOne major difference we can make between most living organisms is whether they are composed of prokaryotic or eukaryotic cells. Eukaryotic organisms can be unicellular or multicellular. Whereas eukaryotic cells contain a true nucleus enclosed in a membrane, prokaryotic cells do not contain any such structure. Each cell is composed of a cell membrane that encloses the cytosol (which is a semi-solid) in which the organelles are suspended. In eukaryotic cells, membrane bound organelles allow for compartmentalization of the cells various functions. Eukaryotes have membrane-bound organelles while prokaryotes don’t. Eukaryotes divide by mitosis while prokaryotes usually undergo binary fission. As the control center of the cell, the nucleus contains all of the genetic material necessary for replication of the cell.2 The nucleus is surrounded by the nuclear membrane or envelope, a double membrane that maintains a nuclear environment separate and distinct from the cytoplasm. Nuclear pores are small openings in the nuclear membrane that allow the nucleus to selectively choose which materials are allowed to be taken up from the cytoplasm into the nucleus and vice versa.2 Two distinct environments exist within the cell due to separation of the nucleus from the cytoplasm by the nuclear membrane. This allows for compartmentalization of transcription and translation processes using DNA. Mitochondria are often called the power plants of the cell, in reference to their important metabolic functions. The mitochondrion contains two layers: the outer and inner membranes. The outer membrane serves as a barrier between the cytosol and the inner environment of the mitochondrion. The inner membrane has numerous foldings known as cristae. The cristae contains the molecules and enzymes necessary for the electron transport chain to function effectively. The cristae are highly folded structures that increase the surface area available for electron transport chain enzymes to function. The intermembrane space is the space between the inner and outer membranes of the mitochondria and the space inside the inner membrane is known as the mitochondrial matrix. The pumping of protons from the mitochondrial matrix to the intermembrane space sets up a proton-motive force and, ultimately, these protons flow through ATP synthase to generate ATP during oxidative phosphorylation. The mitochondria can also kill the cell, in addition to keeping it alive, by the release of certain enzymes from the electron transport chain. This release initiates a process known as apoptosis, or programmed cell death.

mitochondria
Diagram of Mitochondria

 

Lysosomes are membrane-bound structures containing hydrolytic enzymes that are capable of breaking down many different substrates, including substances ingested by endocytosis and cellular waste products. The lysosomal membrane sequesters these enzymes to prevent damage to the cell. However, release of these enzymes can occur in a process known as autolysis. Like mitochondria, when lysosomes release their hydrolytic enzymes, it results in apoptosis. In this case, the released enzymes directly lead to the degradation of cellular components. The endoplasmic reticulum (ER) is a series of interconnected membranes that are actually continuous with the nuclear envelope. The single membrane of the endoplasmic reticulum contains complex structures formed from the numerous invaginations, with a central lumen. There is two smooth ER and rough ER. The rough ER (RER) contains ribosomes, which allow the translation of proteins destined for secretion directly into its lumen. The smooth ER (SER), however, lacks ribosomes and is primarily used for lipid synthesis and tagging. It can also detoxify certain drugs and poisons. The SER also transports proteins from the RER to the Golgi apparatus. The Golgi apparatus consists of stacked membrane-bound sacs. Materials from the ER are transferred to the Golgi apparatus in vesicles. Once in the Golgi apparatus, these cellular products may be modified by the addition of various groups, including carbohydrates, phosphates, and sulfates. The Golgi apparatus can also modify products from the cell through the release of signal sequences, which directs the delivery of the product to a specific cellular location. Cellular products are repackaged in vesicles, after modification and sorting in the Golgi apparatus, which are subsequently transferred to the appropriate cellular location. If the product is destined for secretion, then the secretory vesicle merges with the cell membrane and its contents are released via exocytosisPeroxisomes contain hydrogen peroxide (H2O2) and one of its primary functions is to breakdown long chain fatty acids through β-oxidation. Peroxisomes assist in the synthesis of phospholipids and holds some of the enzymes utilised in the pentose phosphate pathway. They also contain enzymes that are capable of breaking down hydrogen peroxide into oxygen and water. They are common in the liver and kidney cells of animals.

 

References

1) B Alberts, A. J. (2002). Molecular Biology of the Cell (4th ed.). New York: Garland Science.

2) O’Sullivan JM, P. D. (2013). The nucleolus: a raft adrift in the nuclear sea or the keystone in nuclear structure? Biomolecular Concepts, 277-86.

3) Hardin, J., Bertoni, G., & Kleinsmith, L. J. (2015). Becker’s World of the Cell (8th ed.). New York: Pearson.

4) Eurell, J. A., & al, e. (2006). Dellmann’s textbook of veterinary histology. Wiley-Blackwell.

5) Dorland, W. A. (2012). Dorland’s Illustrated Medical Dictionary (32 ed.). Elsevier.

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