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Many Organelles In A Eukaryotic Cell Biology Essay

Introduction

The Mitochondria is one of many organelles in a Eukaryotic cell and its function is to break down molecules and take energy from sugars, lipids and protein to produce ATP (Adenosine Triphosphate). ATP consists of one adenosine molecule and three phosphate groups, also known as triphosphate. The Mitochondria is an organelle made up of a double membrane.  They are rather large organelles ranging from 0.5µm to 10µm in length and 1µm in diameter. It produces ATP synthesis in Eukaryotic cells. It is the principle site of aerobic respiration, meaning oxygen must be present for efficient ATP to be generated. The mitochondria provides most (if not all) of the energy for the cell. Often refereed to as the "factory" or "power house" of the cell as they are responsible for energy production, different types of cells have a different number for mitochondrion, according to how much energy they need. For example, muscle cells need lots of energy, so have a lot of mitochondria. Therefore, according to energy requirements, different tissues need different types of respiration and different numbers of mitochondria per cell. 

Mitochondrial DNA

Unlike most other organelles, mitochondria have their own DNA. This DNA is copied and stored within the mitochondria and contains several genes essential to the mitochondria’s role in oxidative metabolism. All mitochondria in your cells have been inherited from your mother. Mitochondrial DNA was discovered in the 1960s by Margit M. K. Nass and Sylvan Nass by electron microscopy using an electron microscope, of which uses a beam of electrons to illuminate a specimen and produce a magnified image. Mitochondrial DNA, also known as mtDNA or mDNA is the DNA (deoxyribonucleic acid) in the Mitochondria. DNA is a type of macromolecule known as a nucleic acid and contains the genetic information necessary for the production of other cell components and for the reproduction of life. In mitochondria, after transcription of mitochondrial DNA into mRNA, the ribosomes translate mRNA into mitochondrial proteins. These proteins are found in the mitochondrial inner and outer membranes and in the matrix.

Outer Mitochondrial Membrane

The outer mitochondrial membrane is made up of a flexible phospholipid bilayer which contains specific proteins which aid in the entry and exit of materials involved in aerobic respiration. The phospholipids molecules that make up the membrane have a polar, hydrophilic head and two hydrophobic hydrocarbon tails. When the lipids are immersed in an aqueous solution the lipids spontaneously bury the tails together and leave the hydrophilic heads exposed. An advantage of this membrane is it can automatically fix itself when torn. There are three different major classes of lipid molecules - phospholipids, cholesterol, and glycolipids.

The main feature of this outer membrane is that it encloses the whole organelle, keeping everything in place and to create a barrier between the inside and outside of the cell so that the surroundings of each side can be different. It is smooth, limits the organelle and regulates the transport of materials into and out of the cell. This is called ‘semi-permeable’ and allows certain things to enter and leave the cell, such as small molecules & ions, but blocks off proteins and other macromolecules. Materials that cannot diffuse through the cell membrane need transport proteins to help get into the cell. Transport proteins help in the diffusion of certain molecules that cannot pass through by themselves due to their charge, size, or polarity. Another function of the membrane is that it provides a stable site for the catalysation of enzymes.

Mitochondrial granules are not very well known but since the granules were discovered in the early fifties, they have been a point of interest for many scientific researchers. Some thought that the granules were a sink of cations (positively charged ions) and that they would eventually regulate ion concentrations in the mitochondria. Others thought that the granules were ‘messengers’ of the mitochondrial inner membrane.

Inner Membrane

The inner membrane of the mitochondria provides attachment sites for enzyme activity. It is fairly impermeable to most molecules and contains proteins with many types of functions, including;

Proteins that carry out the oxidation reactions of the respiratory chain

An enzyme complex called ATP Synthase that produces ATP in the mitochondrial matrix

Specific transport proteins that regulate the passage of molecules in and out of the matrix.

The inner mitochondrial membrane folds inwards to make a series of shelves, also known as cristae, the working surfaces for the reactions that occur in the mitochondria. Cristae increase the surface area for ATP synthesis. It only permits the passage of only certain molecules such as pyruvic acid, ADP and ATP. In aerobic respiration, cristae are the sites of oxidative phosphorylation and electron transport.

Mitochondrial Intermembrane space

Between the outer and inner layer of the mitochondria is the intermembrane space filled with hydrogen ions used for systhesising ATP from ADP. The intermembrane space is not much different to the cytoplasm of the cell because of the channels in the outer membrane of the mitochondria. The channel proteins in the outer membrane are called ‘porins’ and allow free movement of ions and small molecules into the intermembrane space. It tends to have a low pH because of the proton gradient, meaning there is a greater H+ concentration in the intermembrane space then the mitochondrial matrix, which results when protons are pumped from the mitochondrial matrix into the intermembrane space during electron transport.

Mitochondrial Matrix

The mitochondrial matrix is the primary site of the Krebs cycle. This cycle is also referred to as the TCA cycle or citric acid cycle. It consists of an inner chamber which holds a mixture of hundreds of enzymes that catalyze the oxidation of pyruvate and other small organic molecules. The mitochondrial matrix also contains the mitochondria's DNA, ribosomes and has a pH of about 7.5-7.8.

Metabolic Pathways

Metabolism involves many different types of reactions, including phosphorylation & oxidation-reduction. There are approximately 500 metabolic reactions in a typical cell, including glycolysis and the Krebs cycle. Metabolic processes involve the transfer of electrons during chemical reactions to release energy stored in organic molecules. This released energy is ultimately used to synthesise ATP, therefore, reactions that transfer electrons between reactants are called oxidation-reduction reactions, or redox reactions. In oxidation, a substance loses electrons, or is oxidised. Whereas, in reduction, a substance gains electrons or is reduced - this means the amount of positive charges is reduced.

The β-oxidation pathway occurs in the mitochondrial matrix, as does the krebs cycle. β-oxidation occurs where fatty acids are moved into the mitochondria for energy production. The fatty acid is then degraded into acetyl-CoA and broken down by β-oxidation. β-oxidation involves the removal of acetyl groups from the carboxyl end of the fatty acid chain, eventually each cycle cuts off 2 carbons leaving smaller chains.

Oxidative phosphorylation is a metabolic pathway that uses energy released by the oxidation of nutrients to produce adenosine triphosphate (ATP) and occurs in the inner mitochondrial membrane.

Almost all aerobic organisms carry out oxidative phosphorylation to produce ATP, the molecule that supplies energy to metabolism. This pathway is probably so common because it is a highly efficient way of releasing energy, compared to alternative fermentation processes such as anaerobic glycolysis.

ATP, being the “energy currency” of biological systems means that all living organisms require this energy for the following;

Mechanical work – contraction, for example if there is no ATP in the muscle, it will not work.

Active transport – across membranes, eg: molecules carried from one side of the membrane, to the other.

Synthesizing macromolecules

ATP cannot be stored; instead, glucose is stored in another form until it can be converted to ATP. The turnover of ATP is quite high, for example, a human consumes 40kg of ATP in 24 hours. ATP is also a co-enzyme or cofactor. Co-enzyme A is a universal carrier of acyl groups.

Energy metabolism is highly integrated and extracts the energy from foods, which occurs in the following, three phases:

Breaking down of macromolecules into amino acids, monosaccharides and fatty acids. This hydrolysis reaction produces no energy.

Small molecules degraded to smaller units, but mainly acyl units. Some energy is produced at this level.

The Krebs cycle and electron transport chain, the final metabolic pathway for energy production, in which most of the energy is produced here and is highly efficient because energy is released in smaller steps.

Aerobic Respiration

In the animal cell, aerobic respiration starts in the cell cytoplasm with the process called Glycolysis. This is the first step in the metabolic process, but does not require any oxygen. Glycolysis is the process by which Glucose and Adenosine diphosphate (ADP) is converted into 2 Pyruvate molecules and 2 Adenosine triphosphate molecules (ATP), the energy currency of the cell. The reaction that occurs here is phosphorylation where a phosphate group is transferred to ADP forming ATP. In addition, 4 electrons are harvested as NADH which is used further in the Electron Transport Chain.

Once pyruvate has been created by this process, it has to undergo oxidation to form 2 carbon dioxide molecules and 2 acetyl CoA. Another NADH molecule is transformed and therefore another 4 electrons. The waste product of this reaction is Lactic Acid. When ATP levels are high (eg. When there is excess lipid availability), Acetyl CoA is converted back to Glucose-6-phosphate which is then stored as glycogen.

Oxidation of the pyruvate molecule has to occur in order to provide the substrate for the next step called the Krebs cycle. Once Acetyl CoA has been created, it is actively transported into the mitochondria through the phospholipid bilayer where it settles in the matrix and goes through the Krebs Cycle.

Pyruvate + CoA + NAD+ acetyl – CoA + CO 2 + NADH

Krebs cycle

The Krebs cycle also called the citric acid cycle or TCA (tricarboxylic acid cycle) is another metabolic pathway involving eight enzymes that are essential for energy production through aerobic respiration. This pathway is also an important source used in the building blocks of gluconeogenesis, amino acid synthesis, and fatty acid synthesis. The Krebs cycle takes place in mitochondria where a 4 carbon compound is combined with the acetyl (2 carbon) group to form a 6 carbon tricarboxylate compound known as citrate. The citrate is then oxidised, releasing carbon dioxide. 

One source of the acetyl-CoA that enters the Krebs cycle is the conversion of pyruvate from glycolysis to acetyl-CoA by pyruvate dehydrogenase. Acetyl-CoA is a key metabolic joint, gathered not only from glycolysis but also from the oxidation of fatty acids. As the cycle proceeds, the Krebs cycle components are oxidized, transferring their energy to create reduced NADH and FADH2. The oxidation of these components releases two carbon dioxide molecules for each acetyl-CoA that enters the cycle, leaving the amount of carbons the same with each run of the cycle. This carbon dioxide, along with more released by pyruvate dehydrogenase, is the source of CO2 released into the atmosphere when you breathe. The Krebs cycle, like other metabolic pathways, is regulated to efficiently meet the needs of the cell. Towards the end of the cycle, citrate is finally oxidized to oxaloacetate and the cycle repeats.  

The Krebs cycle generates two ATP molecules per molecule of glucose, the same number generated by glycolysis. More importantly, the Krebs cycle and the oxidation of pyruvate harvest many energised electrons, which can be directed to the electron transport chain to drive the synthesis of much more ATP.

Electron Transport Chain

The final step in the process is the Electron Transport Chain. It is made up of a series of protein complexes embedded in the inner mitochondrial membrane and is the site of oxidative phosphorylation in eukaryotes. During oxidative phosphorylation, electrons move down the chain and are transferred from electron donors to electron acceptors such as oxygen in redox reactions, transferring energy as they move. During this transfer, hydrogen ions are pumped. This pumping generates the gradient used by the ATP synthase complex to synthesize ATP. Oxidation phosphorylation occurs in the inner mitochondrial membrane across from the Krebs cycle which is located in the mitochondrial matrix. The NADH in the Krebs cycle is oxidized, releasing energy to power the ATP synthase.

The oxidation of reduced cofactors will yield:

NADH 3 ATP

FADH2

2 ATP

The NADH and FADH2 that are formed by glycolysis and the Krebs cycle carry 2 electrons and transport them to the site of the Electron Transport Chain in the inner mitochondrial membrane. This builds an electrochemical gradient and is important in attracting electrons to the site of the chain reaction. The basic concept of the electron transport chain involves electrons stepping down from one energy level to another. Rather than releasing a single explosive burst of energy, it releases stored energy with each fall as the move down to the lower electron receptor.

The following protein complexes are found in the transport chain; 

NADH Dehydrogenase 

Cytochrome b-C1 

Cytochrome oxidase 

And the complex that makes ATP, ATP synthase. 

 

In addition to these complexes, two carriers are also involved. They are;

Ubiquinone

and

Cytochrome c

Other key components in this process are NADH and the electrons from it, hydrogen ions, oxygen, water, ADP and Pi (inorganic phosphate), which combine to form ATP. At the start of the electron transport chain, two electrons are passed from NADH into the NADH dehydrogenase complex. Along with this transfer is the pumping of one hydrogen ion for each electron. 

Next, the two electrons are transferred to Ubiquinone. Ubiquinone is called a mobile transfer molecule because it moves the electrons to the Cytochrome b-C1 complex. Each electron is then passed from the Cytochrome b-C1 complex to Cytochrome c. Cytochrome c accepts electrons one at a time. One hydrogen ion is pumped through the complex as each electron is transferred. The next major step occurs in the Cytochrome oxidase complex. This step requires 4 electrons. These 4 electrons react with an oxygen molecule and 8 hydrogen ions. The 4 electrons and oxygen are used to form 2 water molecules that power 4 hydrogen ions are pumped across the membrane. This series of hydrogen pumping steps creates the hydrogen ion gradient, also known as “chemiosmotic” gradient, named by its discover Peter Mitchell. The potential energy in this gradient is used by the molecular motor in the ATP synthase complex to produce from ADP and inorganic phosphate. 

ATP synthase complex

As hydrogen ions flow down their gradient, they enter a half tunnel in a stator, which is anchored in the inner mitochondrial membrane. Without a stator, rotation would not be able to perform the work necessary for ATP synthesis. Hydrogen ions then enter a slot within a rotor, changing the subunit so that the rotor spins within the inner membrane. Each hydrogen ion has to make one complete turn before leaving the rotor, before then passing through the second half of the tunnel in the stator and into the mitochondrial matrix. During the spinning of the rotor, it causes an internal rod to also spin. This rod extends into the catalytic knob below it, which is held in a fixed position by another part of the stator. To accomplish the production of ATP from ADP and Pi, turning of the rod activates catalytic sites in the knob. This completes the creation of ATP.

In conclusion …………………

Simply put, the mitochondrion is an energy transformer. CO2, the end product of cellular respiration is diffused out, leaving the cell plasma membrane. 

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