THE BRIGHTEST HUB FOR HEALTH AND WELLNESS

The discovery of mitochondria as the “power plant” of the cell is among the greatest breakthroughs in the history of cell biology. With more than a century of mitochondria research, we now know that the mitochondria functions in the conversion of food (glucose, fatty acid, amino acid) into chemical energy in the form of Adenosine Triphosphate (ATP). ATP drives most of the biochemical processes in any living cell, which include protein synthesis, DNA synthesis, cell division, and among others.

Below is the list of mitochondrial functions.

[ad#co-1]

The Functions of Mitochondria

1. Production of ATP in two series of reactions: Citric Acid Cycle or Kreb’s Cycle and the Electron Transport Chain.
2. Production of heat
3. Storage of calcium ions
4. Regulation of membrane potential, cell proliferation, and cell metabolism
5. Initiation of apoptosis or programmed cell death
6. Calcium signaling
7. Biosynthesis of heme and steroids
8. Metabolic detoxification

[ad#afterpost]

Ribosomes are subcellular structures where protein synthesis occurs. Their primary function is to translate genes into proteins. They read the genetic instruction provided by the messenger RNA (mRNA) to assemble amino acids into long polypeptides (proteins). They are suspended in the cytosol, or attached in the rough endoplasmic reticulum (its rough appearance is due to the attached ribosomes) and in the nuclear envelope.

Ribosomes are made up of 35% proteins (also called ribonucleoproteins) and 65% RNA (also called ribosomal RNA) that forms a complex with each other. A ribosome measures 20 nanometer or 200 Angstroms; it is very small that a sophisticated microscope such as electron microscope is needed to view it. One ribosome is made up of two subunits that work together to synthesize protein using the genetic instruction in the messenger RNA.

In this article let us tackle a little bit of the history of ribosomes: who discovered ribosomes and how did the ribosome gets its name.

[ad#co-1]

Discovery of the Ribosome by George Palade with the Collaboration of Brilliant Scientists

The invention of the electron microscope has paved the way for the discovery of the ribosome. The Romanian cell biologist George Palade was the first to observe the ribosomes as dense particles or granule in the cytoplasm using the electron microscope in 1955. He published his discovery in the Journal of Biophysics, Biochemistry, and Cytology. (Take note that the term “ribosomes” came out later) Palade also refined the differential centrifugation procedure to isolate ribosomes from lysed cells t0 study them better.

With research collaboration with Philip Siekevitz, Palade found out the molecules attached to the rough endoplasmic reticulum are nothing but ribosomes. Further collaboration with other scientists led to the discovery of the function of ribosomes as the sites of protein synthesis.

Palade has focused his career to the study of the structure and functions of ribosomes. His contribution to the study of ribosomes was recognized when he shared the Nobel Prize for Physiology and Medicine with his co-researchers in 1974. In addition to the Nobel Prize, Palade also received numerous recognitions from different scientific societies, foundations, and universities during his lifetime.

How Did The Ribosome Gets Its Name

The term “ribosome” was proposed by scientist Richard B. Roberts during a scientific symposium on microsomal particles and protein synthesis in 1958. The term has arisen when there was disagreement among scientists in the symposium on how to call or designate the subcellular structure (the ribosome). Scientists at the symposium accepted the term because it is satisfactory and sounds good. Since then, the term “ribosome” was widely used to describe the subcellular structure involved in protein synthesis.[ad#afterpost]

The mitochondrion (sing.: mitochondria) is a eukaryotic membrane-enclosed cell organelle that generates chemical energy (in the form of ATP) needed for the metabolic activities of the cell. The rod-shaped organelle measures from 0.5 to 10 micrometers and located in the cytosol. It is considered the “power plant” or “power house” of the cell because it produces most of the ATP needed to carry out all the biochemical processes within the cell. It also has its own DNA separate from the nuclear DNA.

The discovery of mitochondria has a long history. Many scientists studied the functions and structures of mitochondria since the 1850s.

The mitochondrion has a complex structure and biochemistry. In terms of structure, the mitochondrion is consists of the outer mitochondrial membrane, the inner mitochondrial membrane, the intermembrane space, the cristae, and the matrix.

The general function of the mitochondrion is energy conversion (food to energy). Its other functions include calcium storage, heme and steroid synthesis, calcium signaling, apoptosis, and regulation of membrane potential, cell proliferation, and cell metabolism. There are mitochondria that have specialized functions, like the liver mitochondria which contain enzymes for metabolic detoxification.

Structure of Mitochondria

[ad#co-1]

Outer Mitochondrial Membrane

This structure is composed of phospholipids and proteins (1:1 mol. weight ratio) just like eukaryotic cell membrane. It encloses the entire organelle and protects all structures found within the mitochondrion.

The outer membrane contains a lot of integral proteins called porins, which form channels where small molecules pass through. The porins are very selective; they only allow the entry of small molecules with molecular weight of 5000 daltons and below. Larger molecules (e.g. protein) are only allowed to pass the outer membrane through the aid of translocases.

There is a structure that connects the outer mitochondrial membrane to the endoplasmic reticulum (ER). This structure is called the mitochondria-associated ER-membrane (MAM) and involved in the transfer of lipids from the ER into the intermembrane space (the space between the outer membrane and the inner mitochondrial membrane).  The structure is also needed for calcium signaling.

Inner Mitochondrial Membrane

This membrane is also made up of proteins and phospholipids but the amount of proteins is three times higher than the amount of phospholipids by molecular weight. There are five types of proteins in the inner membrane in terms of function: proteins involved in redox reactions of oxidative phosphorylation; proteins that act as ATP synthases which generate ATP in the matrix; proteins involved in protein import machinery; proteins that regulate the passage of metabolites into and out of the matrix; and lastly, the mitochondria fusion and fission protein.

An unusual plasma membrane in the inner membrane is the cardiolipin which primary function is to make the membrane more impermeable to molecules.

Unlike the outer membrane, there are no porins in the inner membrane and almost all molecules need transporters (translocases) to pass through the membrane and reach the matrix.

Moreover, it is in the inner membrane where the electron transport chain happens. Proteins in the inner membrane create a membrane potential, which when released, generates ATP.

Intermembrane Space

The intermembrane space is the space between the outer and inner mitochondrial membranes. Small molecules from the cytosol  such as ions and sugars are transported here through the porins.  Proteins with small molecular weight are also transported here through the action of translocases. One important protein that is located in the space is the cytochrome c.

Cristae

The cristae (sing.: crista) are compartments produced through the invagination or folding of the inner membrane. The cristae increase the surface area of the inner membrane to enhance energy production. The cristae have high density of proteins involved in ATP generation. Cells that have greater demand for ATP such as the muscles (for contraction), have more cristae than ordinary cells with low ATP demand.

Matrix

This is the space enclosed by the inner membrane and highly important for ATP production through the aid of ATP synthases located in the inner membrane. The matrix contains a high concentration of enzymes involved in the citric acid cycle as well as in the oxidation of pyruvate and fatty acids.

The mitochondrial genome is located in the matrix.  It contains the complete instruction for the biosynthesis of mitochondrial protein.

The matrix contains the materials needed for mitochondrial protein synthesis which include ribosomes and RNA (tRNA and rRNA).

Click this to read the article on mitochondrial functions.

We cannot give the credit of “who discovered the mitochondria (singular: mitochondrion)” to just a single person because the discovery of the organelle is through a gradual process (spanning more than a century and a half) and involves the contributions of many scientists. There were those who studied its structure and there were also those who studied its function. Let us meet the people who discovered mitochondria in this article.

Early Discovery of Mitochondria (19th Century)

In 1857, the Swiss physiologist and anatomist, Albert von Kolliker described granule-like structures present in muscle cells. Other scientists during von Kolliker’s time found the granule-like structures in other cell types. Note that these scientists used simple microscopes to view the granule-like structures in the cells. These scientists also don’t know the function of these structures.

To better study the granule-like structures described by Kolliker, Richard Altman employed a dye technique to put color to the structures so that they would become more visible under a microscope. The dye technique helped Kolliker to easily identify the structures and distinguished them from other structures inside the cell. He later named the structures as the bioblasts and proposed a hypothesis saying that the bioblasts are the basic units of cell activity.

The “bioblasts” name for the granular structures was changed to mitochondria by Carl Benda in 1898. He coined the name from two greek words: “mitos” for thread and “chondros” for granule.

Von Kolliker, Altman, and Benda just started the long history of mitochondria discovery. Scientists after them elucidated the functions of mitochondria that we know today.

[ad#co-1]

Discovery of Mitochondria Functions (20th Century and Beyond)

German biochemist Otto Heinrich Warburg proposed in 1912 that an enzyme located inside the cell facilitates oxygen processing (also called respiration). He demonstrated that cyanide inhibits cell respiration at the cellular level.

In 1923, entomologist David Keilin observed in an experiment that the oxidation state of a certain hemoprotein called cytochrome is changed during respiration. Cytochrome is involved in the electron transport- a pathway involved in the production of the energy-rich compound adenosine triphosphate (ATP). Keilin later discovered that cytochrome is found in the inner membrane of the mitochondrion.

It was 1929 when scientists C. H. Fiske and Y. Subbarow isolated ATP, the compound produced in the mitochondria that carries the chemical energy. The use of ATP in cellular respiration was demonstrated by H. M. Kalckar and V. A. Belitser in a process called oxidative phosphorylation: the release of phosphate from ATP to activate a protein.

Peter D. Mitchell was awarded the Nobel Prize for proposing the theory of chemiosmotics in 1978. The theory explains how the movement of ions across mitochondrial membranes is related to the production of ATP during cellular respiration.

Like Mitchell, another scientist studying the mitochondria was awarded the Nobel Prize. His name is Paul Boyer who discovered the role of the mitochondria in converting ADP to ATP.

In addition to the scientists mentioned above, there are other scientists who also contributed in the elucidation of functions of the mitochondria but they are too many to mention in this article. Research on mitochondrial function and structure continues today. Scientists are discovering more mitochondrial functions. They found out that mitochondria has a role in cell signaling, cellular differentiation, cell death, cell cycle, cell growth, and cell aging.

So Who Discovered Mitochondria?

The best answer would be THE SCIENTISTS (past and present) who in one way or the other contributed in the study of mitochondria: the powerhouse of the cell.