Golgi Complex

The Golgi apparatus or Golgi complex is found in most cells. It is another packaging organelle like the endoplasmic reticulum (ER). It was named after Camillo Golgi, an Italian biologist. It is pronounced GOL-JI in the same way you would say squee-gie, as soft a "G" sound. While layers of membranes may look like the rough ER, they have a very different function. 

Functions of the Golgi apparatus

The Golgi apparatus has many discrete functions, but all are associated with moving molecules from the endoplasmic reticulum to their final destination and modifying certain products along the way. The multiple sacs of the Golgi serve as different chambers for chemical reactions. As the products of the endoplasmic reticulum move through the Golgi apparatus, they are continuously transferred into new environments, and the reactions that can take place are different. In this way, a product can be given modifications, or multiple products can be combined to form large macromolecules. The many sacs and folds of the Golgi apparatus allow for many reactions to take place at the same time, increasing the speed at which an organism can produce products.

Regardless of the product, the vesicles containing the product move from the endoplasmic reticulum and into the cis face of the Golgi apparatus. In layman’s terms, this is the side facing the endoplasmic reticulum. The side furthest from the endoplasmic reticulum is known as the trans face of the Golgi apparatus, and this is where products are headed. After having any modifications or additions to their structure, the products are packaged in vesicles and tagged with markers that indicate where the vesicle needs to end up. These tags can be molecules, such as phosphate groups, or special proteins on the surface of the vesicle. Once tagged, the vesicle is excreted from the Golgi apparatus, on its way to its final destination.

There are many products that are produced by eukaryotes, from proteins that can carry out chemical reactions to lipid molecules that can build new cell membranes. Some products are meant for the endoplasmic reticulum or the Golgi apparatus itself, and travel in the opposite direction of most vesicles. While the endoplasmic reticulum produces most of the products and bases used, it is the Golgi apparatus that is responsible for the final presentation and assembly of products. Often, the environment must be slightly different from that present in the endoplasmic reticulum to obtain certain end products. The many sacs of the Golgi apparatus function to provide many different areas in which reactions can take place in the most favorable of conditions.

The most prevalent theory of how the Golgi apparatus forms is the cisternal maturation model which suggests that the sacs themselves tend to move from the cis face to the trans face of the Golgi apparatus over time, as new sacs are formed closest to the endoplasmic reticulum from vesicles that merge together after being released from the endoplasmic reticulum. These sacs “age” as they move towards the transface of the Golgi apparatus, and their product becomes fully mature.

Although it may seem like there could never be enough lipids to produce the continual flow of cell membrane needed to continually make transport vesicles between the endoplasmic reticulum and the Golgi apparatus, there are constantly segments of cell membrane being produced and recycled by the endoplasmic reticulum, Golgi apparatus, lysosomes, and other organelles in the cell, as well as the outer cell membrane itself. The Golgi apparatus and endoplasmic reticulum work together to produce new cell membrane, as well as recycle the cell membranes of vesicles by merging two membranes when vesicles are absorbed.

One product worthy of mention are lysosomes, sacs containing digestive materials, which are pinched off from the Golgi apparatus and used to process materials which have been phagocytized or to digest organelles which no longer function. The lysosome can then deliver the raw ingredients it has created to the endoplasmic reticulum, to use the materials to create more products.

In secretory cells, or cells which produce large amounts of a substance that your body needs, the Golgi apparatus will be very large. Consider the cells in your stomach that secrete acid. The acid is produced by reactions in the endoplasmic reticulum and is modified as is goes through the Golgi apparatus. Once to the transside of the Golgi apparatus, the acid is packaged in a vesicle and sent towards the cell’s surface. As the vesicle joins with the plasma membrane, the acid is released into the stomach, so it can digest your food.

MITOCHONDRIA

Mitochondria (singular: mitochondrion) are organelles within eukaryotic cells that produce adenosine triphosphate (ATP), the main energy molecule used by the cell. For this reason, the mitochondrion is sometimes referred to as “the powerhouse of the cell”. Mitochondria are found in all eukaryotes, which are all living things that are not bacteria or archaea. It is thought that mitochondria arose from once free-living bacteria that were incorporated into cells.

Function of Mitochondria

Mitochondria produce ATP through process of cellular respiration—specifically, aerobic respiration, which requires oxygen. The citric acid cycle, or Krebs cycle, takes place in the mitochondria. This cycle involves the oxidation of pyruvate, which comes from glucose, to form the molecule acetyl-CoA. Acetyl-CoA is in turn oxidized and ATP is produced.

The citric acid cycle reduces nicotinamide adenine dinucleotide (NAD+) to NADH. NADH is then used in the process of oxidative phosphorylation, which also takes place in the mitochondria. Electrons from NADH travel through protein complexes that are embedded in the inner membrane of the mitochondria. This set of proteins is called an electron transport chain. Energy from the electron transport chain is then used to transport proteins back across the membrane, which power ATP synthase to form ATP.

The amount of mitochondria in a cell depends on how much energy that cell needs to produce. Muscle cells, for example, have many mitochondria because they need to produce energy to move the body. Red blood cells, which carry oxygen to other cells, have none; they do not need to produce energy. Mitochondria are analogous to a furnace or a powerhouse in the cell because, like furnaces and powerhouses, mitochondria produce energy from basic components (in this case, molecules that have been broken down so that they can be used).

Mitochondria have many other functions as well. They can store calcium, which maintains homeostasis of calcium levels in the cell. They also regulate the cell’s metabolism and have roles in apoptosis (controlled cell death), cell signaling, and thermogenesis (heat production).

Structure of Mitochondria

Mitochondria have two membranes, an outer membrane and an inner membrane. These membranes are made of phospholipid layers, just like the cell’s outer membrane. The outer membrane covers the surface of the mitochondrion, while the inner membrane is located within and has many folds called cristae. The folds increase surface area of the membrane, which is important because the inner membrane holds the proteins involved in the electron transport chain. It is also where many other chemical reactions take place to carry out the mitochondria’s many functions. An increased surface area creates more space for more reactions to occur, and increases the mitochondria’s output. The space between the outer and inner membranes is called the intermembrane space, and the space inside the inner membrane is called the matrix.

This diagram shows the structure of a mitochondrion.

Evolution of Mitochondria

Mitochondria are thought to have evolved from free-living bacteria that developed into a symbiotic relationship with a prokaryotic cell, providing it energy in return for a safe place to live. It eventually became an organelle, a specialized structure within the cell, the presence of which are used to distinguish eukaryotic cells from prokaryotic cells. This occurred over a long process of millions of years, and now the mitochondria inside the cell cannot live separately from it. The idea that mitochondria evolved this way is called endosymbiotic theory.

Endosymbiotic theory has multiple forms of evidence. For example, mitochondria have their own DNA that is separate from the DNA in the cell’s nucleus. It is called mitochondrial DNA or mtDNA, and it is only passed down through females because sperm do not have mitochondria. You received your mtDNA from your mother, and you can only pass it on if you are a female who has a child. It is also circular, like bacterial DNA. Another form of evidence is the way new mitochondria are created in the cell. New mitochondria only arise from binary fission, or splitting, which is the same way that bacteria asexually reproduce. If all of the mitochondria are removed from a cell, it can’t make new ones because there are no existing mitochondria there to split. Also, the genome of mitochondria and Rickettsia bacteria (bacteria that can cause spotted fever and typhus) have been compared, and the sequence is so similar that it suggests that mitochondria are closely related to Rickettsia.

Chloroplasts, the organelles in plants where photosynthesis occurs, are also thought to have evolved from endosymbiotic bacteria for similar reasons: they have separate, circular DNA, a double membrane structure, and split through binary fission.

Related Biology Terms

• Adenosine triphosphate – The main energy molecule used to power cellular activities; it is produced by mitochondria.
• Endosymbiotic theory – The idea that mitochondria and chloroplasts (in plants) evolved from bacteria that was once free-living being incorporated into the cell.
• Electron transport chain – A step in oxidative phosphorylation and the production of ATP where electrons travel through a series of protein complexes.
• Citric acid cycle – A series of chemical reactions in the matrix of the mitochondria that releases energy through the oxidation of acetyl-CoA.

Quiz

1. Which is a function of mitochondria?
A. Regulating metabolism
B. Producing ATP
C. Storing calcium
D. All of the above