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  #121  
Old Thursday, June 18, 2009
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Default Cell Biology: Transcription

Transcription of DNA to RNA


Transcription is the name given to the chemical synthesis of RNA from a DNA template. In other words, DNA is transcribed in order to make RNA, which is then decoded to produce proteins.



Transcription is the first stage of the expression of genes into proteins. In transcription, a mRNA (messenger RNA) intermediate is transcribed from one of the strands of the DNA molecule. The RNA is called messenger RNA because it carries the 'message' or genetic information from the DNA to the ribosomes, where the information is used to make proteins. RNA and DNA use complementary coding, where base pairs match up, similar to how the strands of DNA bind to form a double helix. One difference between DNA and RNA is that RNA uses uracil in place of the thymine used in DNA. RNA polymerase mediates the manufacture of an RNA strand that complements the DNA strand. RNA is synthesized in the 5' -> 3' direction (as seen from the growing RNA transcript). There are some proofreading mechanisms for transcription, but not as many as for DNA replication. Sometimes coding errors occur.

Comparison of Transcription in Prokaryotes Versus Eukaryotes
There are significant differences in the process of transcription in prokaryotes versus eukaryotes.
In prokaryotes (bacteria), transcription occurs in the cytoplasm. Translation of the mRNA into proteins also occurs in the cytoplasm. In eukaryotes, transcription occurs in the cell's nucleus. mRNA then moves to the cytoplasm for translation.
DNA in prokaryotes is much more accessible to RNA polymerase than DNA in eukaryotes. Eukaryotic DNA is wrapped around proteins called histones to form structures called nucleosomes. Eukaryotic DNA is packed to form chromatin. While RNA polymerase interacts directly with prokaryotic DNA, other proteins mediate the interation between RNA polymerase and DNA in eukaryotes.
mRNA produced as a result of transcription is not modified in prokaryotic cells. Eukaryotic cells modify mRNA by RNA splicing, 5' end capping, and addition of a polyA tail.


Steps of Transcription
Transcription may be broken into five stages: pre-initiation, initiation, promoter clearance, elongation and termination

Transcription - Pre-Initiation
The first step of transcription is called pre-initiation. RNA polymerase and cofactors bind to DNA and unwind it, creating an initiation bubble. This is a space that grants RNA polymerase access to a single strand of the DNA molecule.

Transcription - Initiation
The initiation of transcription in bacteria begins with the binding of RNA polymerase to the promoter in DNA. Transcription initiation is more complex in eukaryotes, where a group of proteins called transcription factors mediate the binding of RNA polymerase and the initiation of transcription.

Transcription - Promoter Clearance
RNA polymerase must clear the promoter once the first bond has been synthesized. Approximately 23 nucleotides must be synthesized before RNA polymerase loses its tendency to slip away and prematurely release the RNA transcript

Transcription - Elongation
One strand of DNA serves as the template for RNA synthesis, but multiple rounds of transcription may occur so that many copies of a gene may be produced.

Transcription - Termination
Termination is the final step of transcription. Termination results in the release of the newly synthesized mRNA from the elongation complex.





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  #122  
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Default Cell Biology : Translation

Translation

Translation the process in which amino acids are linked together in a specific order according to the rules specified by the genetic code .


The first step in protein synthesis is the transcription of mRNA from a DNA gene in the nucleus. At some other prior time, the various other types of RNA have been synthesized using the appropriate DNA. The RNAs migrate from the nucleus into the cytoplasm.

Initiation:
In the cytoplasm, protein synthesis is actually initiated by the AUG codon on mRNA. The AUG codon signals both the interaction of the ribosome with m-RNA and also the tRNA with the anticodons (UAC). The tRNA which initiates the protein synthesis has N-formyl-methionine attached. The formyl group is really formic acid converted to an amide using the -NH2 group on methionine
The next step is for a second tRNA to approach the mRNA (codon - CCG). This is the code for proline. The anticodon of the proline tRNA which reads this is GGC. The final process is to start growing peptide chain by having amine of proline to bond to the carboxyl acid group of methinone (met) in order to elongate the peptide.

Elongation:
Elongation of the peptide begins as various tRNA's read the next codon. In the example on the left the next tRNA to read the mRNA is tyrosine. When the correct match with the anticodons of a tRNA has been found, the tyrosine forms a peptide bond with the growing peptide chain .
The proline is now hydrolyzed from the tRNA. The proline tRNA now moves away from the ribosome and back into the cytoplasm to reattach another proline amino acid.

Elongation and Termination:
When the stop signal on mRNA is reached, the protein synthesis is terminated. The last amino acid is hydrolyzed from its t-RNA.
The peptide chain leaves the ribosome. The N-formyl-methionine that was used to initiate the protein synthesis is also hydrolyzed from the completed peptide at this time.
The ribosome is now ready to repeat the synthesis several more times.

Eukaryotic Protein Synthesis Differs from Prokaryotic Protein Synthesis Primarily in Translation Initiation

The basic plan of protein synthesis in eukaryotes and archaea is similar to that in bacteria. The major structural and mechanistic themes recur in all domains of life. However, eukaryotic protein synthesis entails more protein components than does prokaryotic protein synthesis, and some steps are more intricate. Some noteworthy similarities and differences are as follows:

1. Ribosomes. Eukaryotic ribosomes are larger. They consist of a 60S large subunit and a 40S small subunit, which come together to form an 80S particle having a mass of 4200 kd, compared with 2700 kd for the prokaryotic 70S ribosome. The 40S subunit contains an 18S RNA that is homologous to the prokaryotic 16S RNA. The 60S subunit contains three RNAs: the 5S and 28S RNAs are the counterparts of the prokaryotic 5S and 23S molecules; its 5.8S RNA is unique to eukaryotes.

2. Initiator tRNA. In eukaryotes, the initiating amino acid is methionine rather than N-formylmethionine. However, as in prokaryotes, a special tRNA participates in initiation. This aminoacyl-tRNA is called Met-tRNAi or Met-tRNAf (the subscript "i" stands for initiation, and "f" indicates that it can be formylated in vitro).

3. Initiation. The initiating codon in eukaryotes is always AUG. Eukaryotes, in contrast with prokaryotes, do not use a specific purine-rich sequence on the 5′ side to distinguish initiator AUGs from internal ones. Instead, the AUG nearest the 5′ end of mRNA is usually selected as the start site. A 40S ribosome attaches to the cap at the 5′ end of eukaryotic mRNA and searches for an AUG codon by moving step-by-step in the 3′ direction . This scanning process in eukaryotic protein synthesis is powered by helicases that hydrolyze ATP. Pairing of the anticodon of Met-tRNAi with the AUG codon of mRNA signals that the target has been found. In almost all cases, eukaryotic mRNA has only one start site and hence is the template for a single protein. In contrast, a prokaryotic mRNA can have multiple Shine-Dalgarno sequences and, hence, start sites, and it can serve as a template for the synthesis of several proteins. Eukaryotes utilize many more initiation factors than do prokaryotes, and their interplay is much more intricate. The prefix eIF denotes a eukaryotic initiation factor. For example, eIF-4E is a protein that binds directly to the 7-methylguanosine cap , whereas eIF-4A is a helicase. The difference in initiation mechanism between prokaryotes and eukaryotes is, in part, a consequence of the difference in RNA processing. The 5′ end of mRNA is readily available to ribosomes immediately after transcription in prokaryotes. In contrast, pre-mRNA must be processed and transported to the cytoplasm in eukaryotes before translation is initiated. Thus, there is ample opportunity for the formation of complex secondary structures that must be removed to expose signals in the mature mRNA. The 5′ cap provides an easily recognizable starting point. In addition, the complexity of eukaryotic translation initiation provides another mechanism for gene expression .

4. Elongation and termination. Eukaryotic elongation factors EF1α and EF1βγ are the counterparts of prokaryotic EF-Tu and EF-Ts. The GTP form of EF1α delivers aminoacyl-tRNA to the A site of the ribosome, and EF1βγ catalyzes the exchange of GTP for bound GDP. Eukaryotic EF2 mediates GTP-driven translocation in much the same way as does prokaryotic EF-G. Termination in eukaryotes is carried out by a single release factor, eRF1, compared with two in prokaryotes. Finally, eIF3, like its prokaryotic counterpart IF3, prevents the reassociation of ribosomal subunits in the absence of an initiation complex.
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  #123  
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Default Difference b/w Mitosis and Meiosis

Difference b/w Mitosis and Meiosis
DNA duplication occurs in both mitosis and meiosis. This duplication occurs during S-phase of mitosis as well as S-phase of meiosis I. The difference between mitosis and meiosis can only be understood if we have a brief idea of what these two cell division processes are:

Mitosis

Mitosis is the cell division process in which a eukaryotic cell divides the chromosomes into two identical sets of two daughter nuclei in its cell nucleus. It is followed by cytokinesis, which equally divides the nuclei, cytoplasm, organelles and cell membrane into two daughter cells. Mitosis and cytokinesis together form the mitotic (M) phase of the cell cycle. The sequence of events are divided into different stages named as prophase, prometaphase, metaphase, anaphase and telophase. Mitosis occurs in different ways in different species. For example, animals undergo an open mitosis process in which the nuclear envelope breaks down before the chromosomes separate, while fungi and yeast undergo a closed mitosis in which the chromosomes divide within an intact cell nucleus.

Meiosis

Meiosis is a process of reductional division in which the number of chromosomes per cell is halved. Before it begins, the DNA in the original cell is duplicated during S-phase of the cell cycle. Meiosis separates the replicated chromosomes into four haploid gametes or spores. If it produces gametes, these cells should fuse during fertilization to create a new diploid cell or zygote. In plants, meiosis produces spores which results in the formation of haploid cells that can divide vegetatively without undergoing fertilization. The different stages involved in meiosis are meiosis I, prophase I, metaphase I, anaphase I, telophase I and meiosis II. Meiosis is necessary for sexual reproduction and therefore occurs in all eukaryotes that reproduce sexually. Meiosis does not occur in archaea or bacteria as they reproduce asexually through binary fission process.

Difference Between Mitosis and Meiosis

The differences between mitosis and meiosis are as follows:


1.Mitosis takes place within somatic cells (cells that make up the body).
Meiosis takes place within gamete cells (sex cells).

2.One single division of the mother cell results in two daughter cells.
Two divisions of the mother cell result in four meiotic products or haploid gametes.

3.A mitotic mother cell can either be haploid or diploid.
A meiotic mother cell is always diploid

4.The number of chromosomes per nucleus remains the same after division.
The meiotic products contain a haploid (n) number of chromosomes in contrast to the (2n) number of chromosomes in mother cell.

5.It is preceded by a S-phase in which the amount of DNA is duplicated.
In meiosis, only meiosis I is preceded by a S-phase

6.In mitosis, there is no pairing of homologous chromosomes.
During prophase I, complete pairing of all homologous chromosomes takes place.

7.There is no exchange of DNA (crossing-over) between chromosomes.
There is at least one crossing-over or DNA exchange per homologous pair of chromosomes.

8.The centromeres split during anaphase.
The centromeres do separate during anaphase II, but not during anaphase I.

9.The genotype of the daughter cells is identical to that of the mother cells.
Meiotic products differ in their genotype from the mother cell.

10.After mitosis, each daughter cell has exactly same DNA strands.
After meiosis, each daughter cell has only half of the DNA strands.

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  #124  
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Default

Quote:
Originally Posted by AFRMS
Respiration in Insects

All insects are aerobic organisms -- they must obtain oxygen (O2) from their environment in order to survive. They use the same metabolic reactions as other animals (glycolysis, Kreb's cycle, and the electron transport system) to convert nutrients (e.g. sugars) into the chemical bond energy of ATP. During the final step of this process, oxygen atoms react with hydrogen ions to produce water, releasing energy that is captured in a phosphate bond of ATP.

The respiratory system is responsible for delivering sufficient oxygen to all cells of the body and for removing carbon dioxide (CO2) that is produced as a waste product of cellular respiration. The respiratory system of insects (and many other arthropods) is separate from the circulatory system. It is a complex network of tubes (called a tracheal system) that delivers oxygen-containing air to every cell of the body.
Air enters the insect's body through valve-like openings in the exoskeleton. These openings (called spiracles) are located laterally along the thorax and abdomen of most insects -- usually one pair of spiracles per body segment. Air flow is regulated by small muscles that operate one or two flap-like valves within each spiracle -- contracting to close the spiracle, or relaxing to open it.

After passing through a spiracle, air enters a longitudinal tracheal trunk, eventually diffusing throughout a complex, branching network of tracheal tubes that subdivides into smaller and smaller diameters and reaches every part of the body. At the end of each tracheal branch, a special cell (the tracheole) provides a thin, moist interface for the exchange of gasses between atmospheric air and a living cell. Oxygen in the tracheal tube first dissolves in the liquid of the tracheole and then diffuses into the cytoplasm of an adjacent cell. At the same time, carbon dioxide, produced as a waste product of cellular respiration, diffuses out of the cell and, eventually, out of the body through the tracheal system.

Each tracheal tube develops as an invagination of the ectoderm during embryonic development. To prevent its collapse under pressure, a thin, reinforcing "wire" of cuticle (the taenidia) winds spirally through the membranous wall. This design (similar in structure to a heater hose on an automobile or an exhaust duct on a clothes dryer) gives tracheal tubes the ability to flex and stretch without developing kinks that might restrict air flow.

The absence of taenidia in certain parts of the tracheal system allows the formation of collapsible air sacs, balloon-like structures that may store a reserve of air. In dry terrestrial environments, this temporary air supply allows an insect to conserve water by closing its spiracles during periods of high evaporative stress. Aquatic insects consume the stored air while under water or use it to regulate buoyancy. During a molt, air sacs fill and enlarge as the insect breaks free of the old exoskeleton and expands a new one. Between molts, the air sacs provide room for new growth -- shrinking in volume as they are compressed by expansion of internal organs.

Small insects rely almost exclusively on passive diffusion and physical activity for the movement of gasses within the tracheal system. However, larger insects may require active ventilation of the tracheal system (especially when active or under heat stress). They accomplish this by opening some spiracles and closing others while using abdominal muscles to alternately expand and contract body volume. Although these pulsating movements flush air from one end of the body to the other through the longitudinal tracheal trunks, diffusion is still important for distributing oxygen to individual cells through the network of smaller tracheal tubes. In fact, the rate of gas diffusion is regarded as one of the main limiting factors (along with weight of the exoskeleton) that prevents real insects from growing as large as the ones we see in horror movies!












AFRMS pls post a short note on Book Lungs too
also a short note on Transcription Bubble
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  #125  
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Default @ Afrms

or i think the title is Translation Bubble ??????
a short note which is often appearing in the paper
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Default @ Aj khan

Quote:
Originally Posted by aj khan
or i think the title is Translation Bubble ??????
a short note which is often appearing in the paper
Ok i will post them ,currently i am focusing on physiology,
if you can go through this thread and point out the missing topics i would be grateful.
I know there are some shorts notes that appear every now and then that are difficult to find in books.
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Default Book Lungs in arachnid arthropods

Book Lungs in arachnid arthropods


A saccular respiratory organ found in some arachnids, such as scorpions and spiders, consisting of several parallel membranous folds arranged like the pages in a book.



Book lung is form of respiratory organ found in certain air-breathing arachnid arthropods (scorpions and some spiders).
Each book lung consists of a series of thin plates that are highly vascular (i.e., richly supplied with blood) and are arranged in relation to each other like the pages of a book.
These plates extend into an internal pouch formed by the external skeleton that opens to the exterior by a small slit. This provides an extensive surface for the exchange of oxygen and carbon dioxide with the surrounding air.
There are four pairs in scorpions and up to two in spiders.

book lung, terrestrial respiratory organ characteristic of arachnids such as scorpions and primitive spiders. Each book lung consists of hollow flat plates. Air bathes the outer surface of the plates and blood circulates within them, facilitating the exchange of gases. In most species, adequate gas exchange occurs without any muscular movement to ventilate the lung.


Book lung is a type of respiration organ used for atmospheric gas exchange and is found in arachnids, such as scorpions and spiders. Each of these organs is found inside a ventral abdominal cavity and connects with the surroundings through a small opening. Book lungs are not related to the lungs of modern land-dwelling vertebrates. Their name describes their structure. Stacks of alternating air pockets and hemolymph-filled tissue gives them an appearance similar to a "folded" book. Their number varies from just one pair in most spiders to four pairs in scorpions. Sometimes the book lungs can be absent and the gas exchange is performed by the thin walls inside the cavity instead, with its surface area increased by branching into the body as thin tubes called tracheae. It is possible that the tracheae have evolved directly from the book lungs, because in some spiders the tracheae have a small number of greatly elongated chambers. Many arachnids, like mites and harvestmen (Opiliones), have no traces of book lungs and breathe through tracheae or through their body surface only.
The unfolded "pages" (plates) of the book lung are filled with hemolymph (the arthropod blood). The folds maximize the surface exposed to air, and thereby maximize the amount of gas exchanged with the environment. In most species, no motion of the plates is required to facilitate this kind of respiration.
The oldest book lungs have been recovered from extinct trigonotarbid arachnids preserved in the 410 million year old Rhynie chert of Scotland. These Devonian fossil lungs are almost indistinguishable from the lungs of modern arachnids.
The absence or presence of book lungs divides the Arachnida into two main groups, but says nothing about the relationships between them: the pulmonate arachnids (book lungs present; scorpions, whip scorpions, Schizomida, Amblypygi, and spiders), and the apulmonate arachnids (book lungs absent; microwhip scorpions, harvestmen, Acarina, pseudoscorpions, Ricinulei and sunspiders). One of the long-running controversies in arachnid evolution is whether the book lung evolved once in the arachnid common ancestor, or whether it evolved in multiple groups of arachnids in parallel as they came onto land.


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  #128  
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Default Transcription Bubble

Transcription Bubble

A transcription bubble is a molecular structure that occurs during the transcription or replication of DNA when DNA helicase and DNA topoisomerase "unzip" the DNA double strand. DNA polymerase or RNA polymerase may then bind to the exposed DNA and begin synthesizing a new strand of DNA or RNA. As DNA and/or RNA polymerase progresses down the DNA strand in the 3' to 5' direction, more of the DNA double strand is unwound, creating a replication or transcription bubble in the process that may be seen with specialized staining techniques and microscopy.

During the elongation step in transcription, the transcription complex consisting of RNA polymerase plus various elongation factors moves along the double-stranded DNA copying the template strand to produce a single-stranded RNA. In the example shown below the RNA product is mRNA and the enzyme would be RNA polymerase II in eukaryotes.

The transcription bubble spans about 20 nucleotides of DNA. This corresponds to the opening of two turns of the double helix. During transcription, a transient DNA:RNA double helix forms and this is sufficient to form one turn of the hybrid helix. As the complex moves down the gene from left to right, ribonucleotides are added one at a time to the growing 3′ end of the RNA. This is positioned in the active site of RNA polymerase.








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  #129  
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Default Cell Biology Chromosomes

Chromosomes


A chromosome is an organized structure of DNA and protein that is found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions. The word chromosome comes from the Greek χρῶμα (chroma, color) and σῶμα (soma, body) due to their property of being very strongly stained by particular dyes. Chromosomes vary widely between different organisms. The DNA molecule may be circular or linear, and can be composed of 10,000 to 1,000,000,000[1] nucleotides in a long chain. Typically eukaryotic cells (cells with nuclei) have large linear chromosomes and prokaryotic cells (cells without defined nuclei) have smaller circular chromosomes, although there are many exceptions to this rule. Furthermore, cells may contain more than one type of chromosome; for example, mitochondria in most eukaryotes and chloroplasts in plants have their own small chromosomes.

In eukaryotes, nuclear chromosomes are packaged by proteins into a condensed structure called chromatin. This allows the very long DNA molecules to fit into the cell nucleus. The structure of chromosomes and chromatin varies through the cell cycle. Chromosomes are the essential unit for cellular division and must be replicated, divided, and passed successfully to their daughter cells so as to ensure the genetic diversity and survival of their progeny. Chromosomes may exist as either duplicated or unduplicated—unduplicated chromosomes are single linear strands, whereas duplicated chromosomes (copied during synthesis phase) contain two copies joined by a centromere. Compaction of the duplicated chromosomes during mitosis and meiosis results in the classic four-arm structure (pictured to the right). Chromosomal recombination plays a vital role in genetic diversity. If these structures are manipulated incorrectly, through processes known as chromosomal instability and translocation, the cell may undergo mitotic catastrophe and die, or it may aberrantly evade apoptosis leading to the progression of cancer.

In practice "chromosome" is a rather loosely defined term. In prokaryotes and viruses, the term genophore is more appropriate when no chromatin is present. However, a large body of work uses the term chromosome regardless of chromatin content. In prokaryotes DNA is usually arranged as a circle which is tightly coiled in on itself, sometimes accompanied by one or more smaller circular DNA molecule called a plasmid. These small circular genomes are also found in mitochondria and chloroplasts, reflecting their bacterial origins. The simplest genophores are found in viruses: these DNA or RNA molecules are short linear or circular genophores that often lack structural proteins.


http://www.youtube.com/v/er8dAhfM9pA









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Default Physiology;Gaseous Exchange

Gaseous Exchange
Breathing consists of two phases, inspiration and expiration. During inspiration, the diaphragm and the intercostal muscles contract. The diaphragm moves downwards increasing the volume of the thoracic (chest) cavity, and the intercostal muscles pull the ribs up expanding the rib cage and further increasing this volume. This increase of volume lowers the air pressure in the alveoli to below atmospheric pressure. Because air always flows from a region of high pressure to a region of lower pressure, it rushes in through the respiratory tract and into the alveoli. This is called negative pressure breathing, changing the pressure inside the lungs relative to the pressure of the outside atmosphere. In contrast to inspiration, during expiration the diaphragm and intercostal muscles relax. This returns the thoracic cavity to it's original volume, increasing the air pressure in the lungs, and forcing the air out.

External Respiration

When a breath is taken, air passes in through the nostrils, through the nasal passages, into the pharynx, through the larynx, down the trachea, into one of the main bronchi, then into smaller broncial tubules, through even smaller bronchioles, and into a microscopic air sac called an alveolus. It is here that external respiration occurs. Simply put, it is the exchange of oxygen and carbon dioxide between the air and the blood in the lungs. Blood enters the lungs via the pulmonary arteries. It then proceeds through arterioles and into the alveolar capillaries. Oxygen and carbon dioxide are exchanged between blood and the air. This blood then flows out of the alveolar capillaries, through vaneuoles, and back to the heart via the pulmonary veins. For an explanation as to why gasses are exchanged here, see partial pressure.

Gas Transport

If 100mL of plasma is exposed to an atmosphere with a pO2 of 100mm Hg, only 0.3mL of oxygen would be absorbed. However, if 100mL of blood is exposed to the same atmosphere, about 19mL of oxygen would be absorbed. This is due to the presence of hemoglobin, the main means of oxygen transport in the body. The respiratory pigment hemoglobin is made up of an iron-containing porphyron, haem, combined with the protein globin. Each iron atom in haem is attached to four pyrole groups by covalent bonds. A fifth covalent bond of the iron is attached to the globin part of the molecule and the sixth covalent bond is available for combination with oxygen. There are four iron atoms in each heamoglobin molecule and therefore four heam groups.

Oxygen Transport

In the loading and unloading of oxygen, there is a cooperation between these four haem groups. When oxygen binds to one of the groups, the others change shape slighty and their attraction to oxygen increases. The loading of the first oxygen, results in the rapid loading of the next three (forming oxyhaemoglobin). At the other end, when one group unloads it's oxygen, the other three rapidly unload as their groups change shape again having less attraction for oxygen. This method of cooperative binding and release can be seen in the dissociation curve for hemoglobin. Over the range of oxygen concentrations where the curve has a steep slope, the slightest change in concentration will cause hemoglobin to load or unload a substantial amount of oxygen. Notice that the steep part of the curve corresponds to the range of oxygen concentrations found in the tissues. When the cells in a particular location begin to work harder, e.g. during exercise, oxygen concentration dips in that location, as the oxygen is used in cellular respiration. Because of the cooperation between the haem groups, this slight change in concentration is enough to cause a large increase in the amount of oxygen unloaded.

As with all proteins, hemoglobin’s shape shift is sensitive to a variety of environmental conditions. A drop in pH lowers the attraction of hemoglobin to oxygen, an effect known as the Bohr shift. Because carbon dioxide reacts with water to produce carbonic acid, an active tissue will lower the pH of it's surroundings and encourage hemoglobin to give up extra oxygen, to be used in cellular respiration. Hemoglobin a notable molecule for it's ability to transport oxygen’s from regions of supply to regions of demand.
Carbon Dioxide Transport - Out of the carbon dioxide released from respiring cells, 7% dissolves into the plasma, 23% binds to the multiple amino groups of hemoglobin (Caroxyhaemoglobin), and 70% is carried as bicarbonate ions. Carbon dioxide created by respiring cells diffuses into the blood plasma and then into the red blood cells, where most of it is converted to bicarbonate ions. It first reacts with water forming carbonic acid, which then breaks down into H+ and CO3-. Most of the hydrogen ions that are produced attach to hemoglobin or other proteins. In this
Internal Respiration
The body tissues need the oxygen and have to get rid of the carbon dioxide, so the blood carried throughout the body exchanges oxygen and carbon dioxide with the body's tissues. Internal respiration is basically the exchange of gasses between the blood in the capillaries and the body's cells.

Additional readings.

Gaseous Exchange In The Body

The exchange of Oxygen (O2) and Carbon Dioxide (CO2) between alveolar air and pulmonary blood occurs via passive diffusion. This is governed by the behavior of gases as described by Dalton's Law and Henry's Law.
Gaseous exchange in the body occurs in two places:
1. Between the air in the alveoli of the lungs and the blood in pulmonary capillaries (External respiration).
2. Between the systemic capillaries and tissue cells (Internal respiration).

External Respiration

This process results in the conversion of deoxygenated blood from the right side of the heart to oxygenated blood returning to the left side of the heart. Gases are exchanged by diffusion according to the differences in their partial pressures.
The now deoxygenated blood returns to the heart and is pumped to the lungs where the process of external respiration begins again.
The partial pressure of O2 in alveolar air is 105mmHg, while the resting partial pressure of O2 in deoxygenated blood is ~40mmHg. Due to this difference O2 diffuses down its concentration gradient from the alveolar air into the deoxygenated blood until equilibrium is reached, the result being that the blood becomes oxygenated.
The partial pressure of CO2 in alveolar air is 40mmHg, while in deoxygenated blood it is 45 mmHg. CO2 therefore diffuses in the opposite direction to O2, again down its concentration gradient. The result being that CO2 is removed from the blood and exhaled.
The now deoxygenated blood returns to the heart and is pumped to the lungs where the process of external respiration begins again.
The rate of gas exchange during external respiration depends on several factors:
·Partial pressure difference of the gases
·Surface area available for gas exchange
·Diffusion distance
·Solubility and molecular weight of the gases

Internal Respiration

The process of internal respiration is much the same as external respiration, only the site in which it occurs is different. Internal respiration results in the conversion of oxygenated blood (from the capillaries) to deoxygenated blood. Once again the gases are exchanged in accordance with their partial pressures.
The partial pressure of O2 in capillary blood is ~100mmHg, whilst in the tissues it is ~ 40mmHg. Due to this difference O2 diffuses from the blood, through the interstitial fluid into the tissue cells until the partial pressure of O2 in the blood decreases to ~40mmHg.
CO2 again diffuses in the opposite direction. The partial pressure of CO2 in the tissue cells is 45mmHg, whilst in the blood it is 40mmHg. Therefore, CO2 diffuses from the tissue cells through the interstitial fluid into the blood until its partial pressure in the blood reaches ~45mmHg.
The deoxygenated blood returns to the heart from where it is pumped to the lungs and the process of external respiration begins again.

Gaseous Exchange

Erythrocyte (red blood cells) in the mammalian body are filled with a globular protein called Haemoglobin. This molecule consists of 4 polypeptide chains each with an Iron Haem group in the centre. This Haem group is very important for gaseous exchange.
Once the Erythrocyte, laden with Oxygen molecules, reaches some tissues which have an Oxygen deficit, the following reaction occurs:

CO2 + H2O <> H2CO3 <> HCO3- + H +
(<> = a reversible reaction)

This equation is catalysed by the highly efficient enzyme called Carbonic Anhydrase. The CO2 is absorbed into the red blood cell then in this equation it is reacted with water to make firstly to Hydrogen Carbonate and then to Hydrogen Carbonate- ions and H+ ions. The HCO3- ions are pumped out of the cell and are replaced with Cl- ions to keep the charges balanced.
The next stage in this process is as follows:

H+ + HbO8 <> HHb+ + 4O2

Here I have used Hb for Haemoglobin for simplicity because Hemoglobin is actually many thousands of molecules long.

This stage of the reaction involves the Hydrogen ions bonding with HbO8 (Oxygen bonded with Haemoglobin). The Hydrogen ions displace the Oxygen molecules from the Haem groups and allow the Oxygen molecules to come into solution in the red blood cell. These then diffuse through the phospholipid bilayer membrane of the red blood cell and move into the surrounding tissues where Oxygen is needed leaving the Hydrogen bonded with the Haemoglobin acting as a spectator ion, as it is not actually used for anything.

Once this reaction has occurred there will be Oxygen in the tissues and the Carbon Dioxide which was in the tissues will now be carried back to the lungs to be exchanged for more Oxygen.
This reaction is entirely reversible and the opposite reaction happens at the lung to absorb new Oxygen.


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