Protein

Protein

Introduction to Genetics

General flow: DNA > RNA > Protein

special transfers (RNA > RNA,
RNA > DNA, Protein > Protein)

Genetic code

Transcription
Transcription (Transcription factors,
RNA Polymerase,promoter)
Prokaryotic / Archaeal / Eukaryotic

post-transcriptional modification
(hnRNA,Splicing)

Translation
Translation (Ribosome,tRNA)
Prokaryotic / Archaeal / Eukaryotic

post-translational modification
(functional groups, peptides,
structural changes)
gene regulation

epigenetic regulation
(Genomic imprinting)

transcriptional regulation

post-transcriptional regulation
(sequestration,
alternative splicing,miRNA)

translational regulation

post-translational regulation
(reversible,irreversible)

ask a question , edit

This article is about a class of molecules. For protein as a nutrient, see Protein (nutrient). For other uses, see Protein (disambiguation).


A representation of the 3D structure of myoglobin showing coloured alpha helices. This protein was the first to have its structure solved by X-ray crystallography.
Proteins (also known as polypeptides) are organic compounds made of amino acids arranged in a linear chain and folded into a globular form. The amino acids in a polymer chain are joined together by the peptide bonds between the carboxyl and amino groups of adjacent amino acid residues. The sequence of amino acids in a protein is defined by the sequence of a gene, which is encoded in the genetic code.[1] In general, the genetic code specifies 20 standard amino acids; however, in certain organisms the genetic code can include selenocysteine — and in certain archaea — pyrrolysine. Shortly after or even during synthesis, the residues in a protein are often chemically modified by post-translational modification, which alters the physical and chemical properties, folding, stability, activity, and ultimately, the function of the proteins. Proteins can also work together to achieve a particular function, and they often associate to form stable complexes.[2]
Like other biological macromolecules such as polysaccharides and nucleic acids, proteins are essential parts of organisms and participate in virtually every process within cells. Many proteins are enzymes that catalyze biochemical reactions and are vital to metabolism. Proteins also have structural or mechanical functions, such as actin and myosin in muscle and the proteins in the cytoskeleton, which form a system of scaffolding that maintains cell shape. Other proteins are important in cell signaling, immune responses, cell adhesion, and the cell cycle. Proteins are also necessary in animals' diets, since animals cannot synthesize all the amino acids they need and must obtain essential amino acids from food. Through the process of digestion, animals break down ingested protein into free amino acids that are then used in metabolism.
Proteins were first described by the Dutch chemist Gerhardus Johannes Mulder and named by the Swedish chemist Jöns Jakob Berzelius in 1838. The central role of proteins in living organisms was however not fully appreciated until 1926, when James B. Sumner showed that the enzyme urease was a protein.[3] The first protein to be sequenced was insulin, by Frederick Sanger, who won the Nobel Prize for this achievement in 1958. The first protein structures to be solved were hemoglobin and myoglobin, by Max Perutz and Sir John Cowdery Kendrew, respectively, in 1958.[4][5] The three-dimensional structures of both proteins were first determined by x-ray diffraction analysis; Perutz and Kendrew shared the 1962 Nobel Prize in Chemistry for these discoveries. Proteins may be purified from other cellular components using a variety of techniques such as ultracentrifugation, precipitation, electrophoresis, and chromatography; the advent of genetic engineering has made possible a number of methods to facilitate purification. Methods commonly used to study protein structure and function include immunohistochemistry, site-directed mutagenesis, and mass spectrometry.
Contents
[hide]
• 1 Biochemistry
• 2 Synthesis
o 2.1 Chemical synthesis
• 3 Structure of proteins
o 3.1 Structure determination
• 4 Cellular functions
o 4.1 Enzymes
o 4.2 Cell signaling and ligand binding
o 4.3 Structural proteins
• 5 Methods of study
o 5.1 Protein purification
o 5.2 Cellular localization
o 5.3 Proteomics and bioinformatics
o 5.4 Structure prediction and simulation
• 6 Nutrition
• 7 History and etymology
• 8 See also
• 9 Footnotes
• 10 References
• 11 External links
o 11.1 Databases and projects
o 11.2 Tutorials and educational websites

[edit] Biochemistry
Main articles: Biochemistry, Amino acid, and peptide bond


Resonance structures of the peptide bond that links individual amino acids to form a protein polymer.
Proteins are linear polymers built from series of up to 20 different L-α-amino acids. All amino acids possess common structural features, including an α-carbon to which an amino group, a carboxyl group, and a variable side chain are bonded. Only proline differs from this basic structure as it contains an unusual ring to the N-end amine group, which forces the CO–NH amide moiety into a fixed conformation.[6] The side chains of the standard amino acids, detailed in the list of standard amino acids, have a great variety of chemical structures and properties; it is the combined effect of all of the amino acid side chains in a protein that ultimately determines its three-dimensional structure and its chemical reactivity.[7]



Chemical structure of the peptide bond (left) and a peptide bond between leucine and threonine (right).

The amino acids in a polypeptide chain are linked by peptide bonds. Once linked in the protein chain, an individual amino acid is called a residue, and the linked series of carbon, nitrogen, and oxygen atoms are known as the main chain or protein backbone.[8] The peptide bond has two resonance forms that contribute some double-bond character and inhibit rotation around its axis, so that the alpha carbons are roughly coplanar. The other two dihedral angles in the peptide bond determine the local shape assumed by the protein backbone.[9] The end of the protein with a free carboxyl group is known as the C-terminus or carboxy terminus, whereas the end with a free amino group is known as the N-terminus or amino terminus.
The words protein, polypeptide, and peptide are a little ambiguous and can overlap in meaning. Protein is generally used to refer to the complete biological molecule in a stable conformation, whereas peptide is generally reserved for a short amino acid oligomers often lacking a stable three-dimensional structure. However, the boundary between the two is not well defined and usually lies near 20–30 residues.[10] Polypeptide can refer to any single linear chain of amino acids, usually regardless of length, but often implies an absence of a defined conformation.
[edit] Synthesis
Main article: Protein biosynthesis


The DNA sequence of a gene encodes the amino acid sequence of a protein.
Proteins are assembled from amino acids using information encoded in genes. Each protein has its own unique amino acid sequence that is specified by the nucleotide sequence of the gene encoding this protein. The genetic code is a set of three-nucleotide sets called codons and each three-nucleotide combination designates an amino acid, for example AUG (adenine-uracil-guanine) is the code for methionine. Because DNA contains four nucleotides, the total number of possible codons is 64; hence, there is some redundancy in the genetic code, with some amino acids specified by more than one codon.[11] Genes encoded in DNA are first transcribed into pre-messenger RNA (mRNA) by proteins such as RNA polymerase. Most organisms then process the pre-mRNA (also known as a primary transcript) using various forms of post-transcriptional modification to form the mature mRNA, which is then used as a template for protein synthesis by the ribosome. In prokaryotes the mRNA may either be used as soon as it is produced, or be bound by a ribosome after having moved away from the nucleoid. In contrast, eukaryotes make mRNA in the cell nucleus and then translocate it across the nuclear membrane into the cytoplasm, where protein synthesis then takes place. The rate of protein synthesis is higher in prokaryotes than eukaryotes and can reach up to 20 amino acids per second.[12]
The process of synthesizing a protein from an mRNA template is known as translation. The mRNA is loaded onto the ribosome and is read three nucleotides at a time by matching each codon to its base pairing anticodon located on a transfer RNA molecule, which carries the amino acid corresponding to the codon it recognizes. The enzyme aminoacyl tRNA synthetase "charges" the tRNA molecules with the correct amino acids. The growing polypeptide is often termed the nascent chain. Proteins are always biosynthesized from N-terminus to C-terminus.[11]
The size of a synthesized protein can be measured by the number of amino acids it contains and by its total molecular mass, which is normally reported in units of daltons (synonymous with atomic mass units), or the derivative unit kilodalton (kDa). Yeast proteins are on average 466 amino acids long and 53 kDa in mass.[10] The largest known proteins are the titins, a component of the muscle sarcomere, with a molecular mass of almost 3,000 kDa and a total length of almost 27,000 amino acids.[13]
[edit] Chemical synthesis
Short proteins can also be synthesized chemically by a family of methods known as peptide synthesis, which rely on organic synthesis techniques such as chemical ligation to produce peptides in high yield.[14] Chemical synthesis allows for the introduction of non-natural amino acids into polypeptide chains, such as attachment of fluorescent probes to amino acid side chains.[15] These methods are useful in laboratory biochemistry and cell biology, though generally not for commercial applications. Chemical synthesis is inefficient for polypeptides longer than about 300 amino acids, and the synthesized proteins may not readily assume their native tertiary structure. Most chemical synthesis methods proceed from C-terminus to N-terminus, opposite the biological reaction.[16]
[edit] Structure of proteins
Main article: Protein structure


Three possible representations of the three-dimensional structure of the protein triose phosphate isomerase. Left: all-atom representation colored by atom type. Middle: Simplified representation illustrating the backbone conformation, colored by secondary structure. Right: Solvent-accessible surface representation colored by residue type (acidic residues red, basic residues blue, polar residues green, nonpolar residues white).
Most proteins fold into unique 3-dimensional structures. The shape into which a protein naturally folds is known as its native conformation.[17] Although many proteins can fold unassisted, simply through the chemical properties of their amino acids, others require the aid of molecular chaperones to fold into their native states.[18] Biochemists often refer to four distinct aspects of a protein's structure:[19]
• Primary structure: the amino acid sequence.
• Secondary structure: regularly repeating local structures stabilized by hydrogen bonds. The most common examples are the alpha helix, beta sheet and turns. Because secondary structures are local, many regions of different secondary structure can be present in the same protein molecule.
• Tertiary structure: the overall shape of a single protein molecule; the spatial relationship of the secondary structures to one another. Tertiary structure is generally stabilized by nonlocal interactions, most commonly the formation of a hydrophobic core, but also through salt bridges, hydrogen bonds, disulfide bonds, and even post-translational modifications. The term "tertiary structure" is often used as synonymous with the term fold. The Tertiary structure is what controls the basic function of the protein.
• Quaternary structure: the structure formed by several protein molecules (polypeptide chains), usually called protein subunits in this context, which function as a single protein complex.
Proteins are not entirely rigid molecules. In addition to these levels of structure, proteins may shift between several related structures while they perform their functions. In the context of these functional rearrangements, these tertiary or quaternary structures are usually referred to as "conformations", and transitions between them are called conformational changes. Such changes are often induced by the binding of a substrate molecule to an enzyme's active site, or the physical region of the protein that participates in chemical catalysis. In solution proteins also undergo variation in structure through thermal vibration and the collision with other molecules.[20]


Molecular surface of several proteins showing their comparative sizes. From left to right are: immunoglobulin G (IgG, an antibody), hemoglobin, insulin (a hormone), adenylate kinase (an enzyme), and glutamine synthetase (an enzyme).
Proteins can be informally divided into three main classes, which correlate with typical tertiary structures: globular proteins, fibrous proteins, and membrane proteins. Almost all globular proteins are soluble and many are enzymes. Fibrous proteins are often structural, such as collagen, the major component of connective tissue, or keratin, the protein component of hair and nails. Membrane proteins often serve as receptors or provide channels for polar or charged molecules to pass through the cell membrane.[21]
A special case of intramolecular hydrogen bonds within proteins, poorly shielded from water attack and hence promoting their own dehydration, are called dehydrons.[22]
[edit] Structure determination
Discovering the tertiary structure of a protein, or the quaternary structure of its complexes, can provide important clues about how the protein performs its function. Common experimental methods of structure determination include X-ray crystallography and NMR spectroscopy, both of which can produce information at atomic resolution. Dual polarisation interferometry is a quantitative analytical method for measuring the overall protein conformation and conformational changes due to interactions or other stimulus. Circular dichroism is another laboratory technique for determining internal beta sheet/ helical composition of proteins. Cryoelectron microscopy is used to produce lower-resolution structural information about very large protein complexes, including assembled viruses;[23] a variant known as electron crystallography can also produce high-resolution information in some cases , especially for two-dimensional crystals of membrane proteins.[24] Solved structures are usually deposited in the Protein Data Bank (PDB), a freely available resource from which structural data about thousands of proteins can be obtained in the form of Cartesian coordinates for each atom in the protein.[25]
Many more gene sequences are known than protein structures. Further, the set of solved structures is biased toward proteins that can be easily subjected to the conditions required in X-ray crystallography, one of the major structure determination methods. In particular, globular proteins are comparatively easy to crystallize in preparation for X-ray crystallography. Membrane proteins, by contrast, are difficult to crystallize and are underrepresented in the PDB.[26] Structural genomics initiatives have attempted to remedy these deficiencies by systematically solving representative structures of major fold classes. Protein structure prediction methods attempt to provide a means of generating a plausible structure for proteins whose structures have not been experimentally determined.
[edit] Cellular functions
Proteins are the chief actors within the cell, said to be carrying out the duties specified by the information encoded in genes.[10] With the exception of certain types of RNA, most other biological molecules are relatively inert elements upon which proteins act. Proteins make up half the dry weight of an Escherichia coli cell, whereas other macromolecules such as DNA and RNA make up only 3% and 20%, respectively.[27] The set of proteins expressed in a particular cell or cell type is known as its proteome.


The enzyme hexokinase is shown as a simple ball-and-stick molecular model. To scale in the top right-hand corner are two of its substrates, ATP and glucose.
The chief characteristic of proteins that also allows their diverse set of functions is their ability to bind other molecules specifically and tightly. The region of the protein responsible for binding another molecule is known as the binding site and is often a depression or "pocket" on the molecular surface. This binding ability is mediated by the tertiary structure of the protein, which defines the binding site pocket, and by the chemical properties of the surrounding amino acids' side chains. Protein binding can be extraordinarily tight and specific; for example, the ribonuclease inhibitor protein binds to human angiogenin with a sub-femtomolar dissociation constant (<10−15 M) but does not bind at all to its amphibian homolog onconase (>1 M). Extremely minor chemical changes such as the addition of a single methyl group to a binding partner can sometimes suffice to nearly eliminate binding; for example, the aminoacyl tRNA synthetase specific to the amino acid valine discriminates against the very similar side chain of the amino acid isoleucine.[28]
Proteins can bind to other proteins as well as to small-molecule substrates. When proteins bind specifically to other copies of the same molecule, they can oligomerize to form fibrils; this process occurs often in structural proteins that consist of globular monomers that self-associate to form rigid fibers. Protein-protein interactions also regulate enzymatic activity, control progression through the cell cycle, and allow the assembly of large protein complexes that carry out many closely related reactions with a common biological function. Proteins can also bind to, or even be integrated into, cell membranes. The ability of binding partners to induce conformational changes in proteins allows the construction of enormously complex signaling networks.[29] Importantly, as interactions between proteins are reversible, and depend heavily on the availability of different groups of partner proteins to form aggregates that are capable to carry out discrete sets of function, study of the interactions between specific proteins is a key to understand important aspects of cellular function, and ultimately the properties that distinguish particular cell types[30][31].
[edit] Enzymes
Main article: Enzyme
The best-known role of proteins in the cell is as enzymes, which catalyze chemical reactions. Enzymes are usually highly specific and accelerate only one or a few chemical reactions. Enzymes carry out most of the reactions involved in metabolism, as well as manipulating DNA in processes such as DNA replication, DNA repair, and transcription. Some enzymes act on other proteins to add or remove chemical groups in a process known as post-translational modification. About 4,000 reactions are known to be catalyzed by enzymes.[32] The rate acceleration conferred by enzymatic catalysis is often enormous — as much as 1017-fold increase in rate over the uncatalyzed reaction in the case of orotate decarboxylase (78 million years without the enzyme, 18 milliseconds with the enzyme).[33]
The molecules bound and acted upon by enzymes are called substrates. Although enzymes can consist of hundreds of amino acids, it is usually only a small fraction of the residues that come in contact with the substrate, and an even smaller fraction — 3 to 4 residues on average — that are directly involved in catalysis.[34] The region of the enzyme that binds the substrate and contains the catalytic residues is known as the active site.
[edit] Cell signaling and ligand binding


Ribbon diagram of a mouse antibody against cholera that binds a carbohydrate antigen
Many proteins are involved in the process of cell signaling and signal transduction. Some proteins, such as insulin, are extracellular proteins that transmit a signal from the cell in which they were synthesized to other cells in distant tissues. Others are membrane proteins that act as receptors whose main function is to bind a signaling molecule and induce a biochemical response in the cell. Many receptors have a binding site exposed on the cell surface and an effector domain within the cell, which may have enzymatic activity or may undergo a conformational change detected by other proteins within the cell.[35]
Antibodies are protein components of adaptive immune system whose main function is to bind antigens, or foreign substances in the body, and target them for destruction. Antibodies can be secreted into the extracellular environment or anchored in the membranes of specialized B cells known as plasma cells. Whereas enzymes are limited in their binding affinity for their substrates by the necessity of conducting their reaction, antibodies have no such constraints. An antibody's binding affinity to its target is extraordinarily high.[36]
Many ligand transport proteins bind particular small biomolecules and transport them to other locations in the body of a multicellular organism. These proteins must have a high binding affinity when their ligand is present in high concentrations, but must also release the ligand when it is present at low concentrations in the target tissues. The canonical example of a ligand-binding protein is haemoglobin, which transports oxygen from the lungs to other organs and tissues in all vertebrates and has close homologs in every biological kingdom.[37] Lectins are sugar-binding proteins which are highly specific for their sugar moieties. Lectins typically play a role in biological recognition phenomena involving cells and proteins.[38] Receptors and hormones are highly specific binding proteins.
Transmembrane proteins can also serve as ligand transport proteins that alter the permeability of the cell membrane to small molecules and ions. The membrane alone has a hydrophobic core through which polar or charged molecules cannot diffuse. Membrane proteins contain internal channels that allow such molecules to enter and exit the cell. Many ion channel proteins are specialized to select for only a particular ion; for example, potassium and sodium channels often discriminate for only one of the two ions.[39]
[edit] Structural proteins
Structural proteins confer stiffness and rigidity to otherwise-fluid biological components. Most structural proteins are fibrous proteins; for example, actin and tubulin are globular and soluble as monomers, but polymerize to form long, stiff fibers that comprise the cytoskeleton, which allows the cell to maintain its shape and size. Collagen and elastin are critical components of connective tissue such as cartilage, and keratin is found in hard or filamentous structures such as hair, nails, feathers, hooves, and some animal shells.[40]
Other proteins that serve structural functions are motor proteins such as myosin, kinesin, and dynein, which are capable of generating mechanical forces. These proteins are crucial for cellular motility of single celled organisms and the sperm of many multicellular organisms which reproduce sexually. They also generate the forces exerted by contracting muscles.[41]
[edit] Methods of study
Main article: Protein methods
As some of the most commonly studied biological molecules, the activities and structures of proteins are examined both in vitro and in vivo. In vitro studies of purified proteins in controlled environments are useful for learning how a protein carries out its function: for example, enzyme kinetics studies explore the chemical mechanism of an enzyme's catalytic activity and its relative affinity for various possible substrate molecules. By contrast, in vivo experiments on proteins' activities within cells or even within whole organisms can provide complementary information about where a protein functions and how it is regulated.
[edit] Protein purification
Main article: Protein purification
In order to perform in vitro analysis, a protein must be purified away from other cellular components. This process usually begins with cell lysis, in which a cell's membrane is disrupted and its internal contents released into a solution known as a crude lysate. The resulting mixture can be purified using ultracentrifugation, which fractionates the various cellular components into fractions containing soluble proteins; membrane lipids and proteins; cellular organelles, and nucleic acids. Precipitation by a method known as salting out can concentrate the proteins from this lysate. Various types of chromatography are then used to isolate the protein or proteins of interest based on properties such as molecular weight, net charge and binding affinity.[42] The level of purification can be monitored using various types of gel electrophoresis if the desired protein's molecular weight and isoelectric point are known, by spectroscopy if the protein has distinguishable spectroscopic features, or by enzyme assays if the protein has enzymatic activity. Additionally, proteins can be isolated according their charge using electrofocusing.[43]
For natural proteins, a series of purification steps may be necessary to obtain protein sufficiently pure for laboratory applications. To simplify this process, genetic engineering is often used to add chemical features to proteins that make them easier to purify without affecting their structure or activity. Here, a "tag" consisting of a specific amino acid sequence, often a series of histidine residues (a "His-tag"), is attached to one terminus of the protein. As a result, when the lysate is passed over a chromatography column containing nickel, the histidine residues ligate the nickel and attach to the column while the untagged components of the lysate pass unimpeded. A number of different tags have been developed to help researchers purify specific proteins from complex mixtures.[44]
[edit] Cellular localization


Proteins in different cellular compartments and structures tagged with green fluorescent protein (here, white).
The study of proteins in vivo is often concerned with the synthesis and localization of the protein within the cell. Although many intracellular proteins are synthesized in the cytoplasm and membrane-bound or secreted proteins in the endoplasmic reticulum, the specifics of how proteins are targeted to specific organelles or cellular structures is often unclear. A useful technique for assessing cellular localization uses genetic engineering to express in a cell a fusion protein or chimera consisting of the natural protein of interest linked to a "reporter" such as green fluorescent protein (GFP).[45] The fused protein's position within the cell can be cleanly and efficiently visualized using microscopy,[46] as shown in the figure opposite.
Other methods for elucidating the cellular location of proteins requires the use of known compartmental markers for regions such as the ER, the Golgi, lysosomes/vacuoles, mitochondria, chloroplasts, plasma membrane, etc. With the use of fluorescently-tagged versions of these markers or of antibodies to known markers, it becomes much simpler to identify the localization of a protein of interest. For example, indirect immunofluorescence will allow for fluorescence colocalization and demonstration of location. Fluorescent dyes are used to label cellular compartments for a similar purpose.[47]
Other possibilities exist, as well. For example, immunohistochemistry usually utilizes an antibody to one or more proteins of interest that are conjugated to enzymes yielding either luminescent or chromogenic signals that can be compared between samples, allowing for localization information. Another applicable technique is cofractionation in sucrose (or other material) gradients using isopycnic centrifugation.[48] While this technique does not prove colocalization of a compartment of known density and the protein of interest, it does increase the likelihood, and is more amenable to large-scale studies.
Finally, the gold-standard method of cellular localization is immunoelectron microscopy. This technique also uses an antibody to the protein of interest, along with classical electron microscopy techniques. The sample is prepared for normal electron microscopic examination, and then treated with an antibody to the protein of interest that is conjugated to an extremely electro-dense material, usually gold. This allows for the localization of both ultrastructural details as well as the protein of interest.[49]
Through another genetic engineering application known as site-directed mutagenesis, researchers can alter the protein sequence and hence its structure, cellular localization, and susceptibility to regulation. This technique even allows the incorporation on unnatural amino acids into proteins, using modified tRNAs,[50] and may allow the rational design of new proteins with novel properties.[51]
[edit] Proteomics and bioinformatics
Main articles: Proteomics and Bioinformatics
The total complement of proteins present at a time in a cell or cell type is known as its proteome, and the study of such large-scale data sets defines the field of proteomics, named by analogy to the related field of genomics. Key experimental techniques in proteomics include 2D electrophoresis,[52] which allows the separation of a large number of proteins, mass spectrometry,[53] which allows rapid high-throughput identification of proteins and sequencing of peptides (most often after in-gel digestion), protein microarrays,[54] which allow the detection of the relative levels of a large number of proteins present in a cell, and two-hybrid screening, which allows the systematic exploration of protein-protein interactions.[55] The total complement of biologically possible such interactions is known as the interactome.[56] A systematic attempt to determine the structures of proteins representing every possible fold is known as structural genomics.[57]
The large amount of genomic and proteomic data available for a variety of organisms, including the human genome, allows researchers to efficiently identify homologous proteins in distantly related organisms by sequence alignment. Sequence profiling tools can perform more specific sequence manipulations such as restriction enzyme maps, open reading frame analyses for nucleotide sequences, and secondary structure prediction. From this data phylogenetic trees can be constructed and evolutionary hypotheses developed using special software like ClustalW regarding the ancestry of modern organisms and the genes they express. The field of bioinformatics seeks to assemble, annotate, and analyze genomic and proteomic data, applying computational techniques to biological problems such as gene finding and cladistics.
[edit] Structure prediction and simulation
Main articles: protein structure prediction and List of protein structure prediction software
Complementary to the field of structural genomics, protein structure prediction seeks to develop efficient ways to provide plausible models for proteins whose structures have not yet been determined experimentally [58]. The most successful type of structure prediction, known as homology modeling, relies on the existence of a "template" structure with sequence similarity to the protein being modeled; structural genomics' goal is to provide sufficient representation in solved structures to model most of those that remain.[59] Although producing accurate models remains a challenge when only distantly related template structures are available, it has been suggested that sequence alignment is the bottleneck in this process, as quite accurate models can be produced if a "perfect" sequence alignment is known.[60] Many structure prediction methods have served to inform the emerging field of protein engineering, in which novel protein folds have already been designed.[61] A more complex computational problem is the prediction of intermolecular interactions, such as in molecular docking and protein-protein interaction prediction.[62]
The processes of protein folding and binding can be simulated using such technique as molecular mechanics, in particular, molecular dynamics and Monte Carlo, which increasingly take advantage of parallel and distributed computing (Folding@Home project[63]; molecular modeling on GPU). The folding of small alpha-helical protein domains such as the villin headpiece[64] and the HIV accessory protein[65] have been successfully simulated in silico, and hybrid methods that combine standard molecular dynamics with quantum mechanics calculations have allowed exploration of the electronic states of rhodopsins.[66]
[edit] Nutrition
Further information: Protein in nutrition
Most microorganisms and plants can biosynthesize all 20 standard amino acids, while animals (including humans) must obtain some of the amino acids from the diet.[27] The amino acids that an organism cannot synthesize on its own are referred to as essential amino acids. Key enzymes that synthesize certain amino acids are not present in animals — such as aspartokinase, which catalyzes the first step in the synthesis of lysine, methionine, and threonine from aspartate. If amino acids are present in the environment, microorganisms can conserve energy by taking up the amino acids from their surroundings and downregulating their biosynthetic pathways.
In animals, amino acids are obtained through the consumption of foods containing protein. Ingested proteins are broken down through digestion, which typically involves denaturation of the protein through exposure to acid and hydrolysis by enzymes called proteases. Some ingested amino acids are used for protein biosynthesis, while others are converted to glucose through gluconeogenesis, or fed into the citric acid cycle. This use of protein as a fuel is particularly important under starvation conditions as it allows the body's own proteins to be used to support life, particularly those found in muscle.[67] Amino acids are also an important dietary source of nitrogen.[citation needed]
[edit] History and etymology
Further information: History of molecular biology
Proteins were recognized as a distinct class of biological molecules in the eighteenth century by Antoine Fourcroy and others, distinguished by the molecules' ability to coagulate or flocculate under treatments with heat or acid. Noted examples at the time included albumin from egg whites, blood serum albumin, fibrin, and wheat gluten. Dutch chemist Gerhardus Johannes Mulder carried out elemental analysis of common proteins and found that nearly all proteins had the same empirical formula, C400H620N100O120P1S1.[68] He came to the erroneous conclusion that they might be composed of a single type of (very large) molecule. The term "protein" to describe these molecules was proposed in 1838 by Mulder's associate Jöns Jakob Berzelius; protein is derived from the Greek word πρωτεῖος (proteios), meaning "primary"[69], "in the lead", or "standing in front".[70] Mulder went on to identify the products of protein degradation such as the amino acid leucine for which he found a (nearly correct) molecular weight of 131 Da.[68]
The difficulty in purifying proteins in large quantities made them very difficult for early protein biochemists to study. Hence, early studies focused on proteins that could be purified in large quantities, e.g., those of blood, egg white, various toxins, and digestive/metabolic enzymes obtained from slaughterhouses. In the 1950s, the Armour Hot Dog Co. purified 1 kg of pure bovine pancreatic ribonuclease A and made it freely available to scientists; this gesture helped ribonuclease A become a major target for biochemical study for the following decades.[68]
Linus Pauling is credited with the successful prediction of regular protein secondary structures based on hydrogen bonding, an idea first put forth by William Astbury in 1933.[71] Later work by Walter Kauzmann on denaturation,[72][73] based partly on previous studies by Kaj Linderstrøm-Lang,[74] contributed an understanding of protein folding and structure mediated by hydrophobic interactions. In 1949 Fred Sanger correctly determined the amino acid sequence of insulin, thus conclusively demonstrating that proteins consisted of linear polymers of amino acids rather than branched chains, colloids, or cyclols.[75] The first atomic-resolution structures of proteins were solved by X-ray crystallography in the 1960s and by NMR in the 1980s. As of 2009, the Protein Data Bank has over 55,000 atomic-resolution structures of proteins.[76] In more recent times, cryo-electron microscopy of large macromolecular assemblies[77] and computational protein structure prediction of small protein domains[78] are two methods approaching atomic resolution.
[edit] See also
• Expression cloning
• Intein
• List of proteins
• List of recombinant proteins
• Prion
• Protein design
• Protein dynamics
• Protein structure prediction software
• Proteopathy
• Proteopedia
• Cdx protein family
[edit] Footnotes
1. ^ Ridley, M. (2006). Genome. New York, NY: Harper Perennial. ISBN 0-06-019497-9
2. ^ Maton A, Hopkins J, McLaughlin CW, Johnson S, Warner MQ, LaHart D, Wright JD. (1993). Human Biology and Health. Englewood Cliffs, New Jersey, USA: Prentice Hall. ISBN 0-13-981176-1. OCLC 32308337.
3. ^ Sumner, JB. (1926). "The isolation and crystallization of the enzyme urease. Preliminary paper". Journal of Biological Chemistry 69: 435–41. http://www.jbc.org/cgi/reprint/69/2/435.pdf?ijkey=028d5e540dab50accbf86e01be08db51ef49008f.
4. ^ Muirhead H, Perutz M. (1963). "Structure of hemoglobin. A three-dimensional fourier synthesis of reduced human hemoglobin at 5.5 Å resolution". Nature 199 (4894): 633–38. doi:10.1038/199633a0. PMID 14074546.
5. ^ Kendrew J, Bodo G, Dintzis H, Parrish R, Wyckoff H, Phillips D. (1958). "A three-dimensional model of the myoglobin molecule obtained by x-ray analysis". Nature 181 (4610): 662–66. doi:10.1038/181662a0. PMID 13517261.
6. ^ Nelson DL, Cox MM. (2005). Lehninger's Principles of Biochemistry, 4th Edition. W. H. Freeman and Company, New York.
7. ^ Gutteridge A, Thornton JM. (2005). "Understanding nature's catalytic toolkit". Trends in Biochemical Sciences 30 (11): 622–29. doi:10.1016/j.tibs.2005.09.006. PMID 16214343.
8. ^ Murray et al., p. 19.
9. ^ Murray et al., p. 31.
10. ^ a b c Lodish H, Berk A, Matsudaira P, Kaiser CA, Krieger M, Scott MP, Zipurksy SL, Darnell J. (2004). Molecular Cell Biology 5th ed. WH Freeman and Company: New York, NY.
11. ^ a b van Holde and Mathews, pp. 1002–42.
12. ^ Dobson CM. (2000). "The nature and significance of protein folding". in Pain RH. (ed.). Mechanisms of Protein Folding. Oxford, Oxfordshire: Oxford University Press. ISBN 0-19-963789-X.
13. ^ Fulton A, Isaacs W. (1991). "Titin, a huge, elastic sarcomeric protein with a probable role in morphogenesis". Bioessays 13 (4): 157–61. doi:10.1002/bies.950130403. PMID 1859393.
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[edit] References
• Branden C, Tooze J. (1999). Introduction to Protein Structure. New York: Garland Pub. ISBN 0-8153-2305-0.
• Murray RF, Harper HW, Granner DK, Mayes PA, Rodwell VW. (2006). Harper's Illustrated Biochemistry. New York: Lange Medical Books/McGraw-Hill. ISBN 0-07-146197-3.
• Van Holde KE, Mathews CK. (1996). Biochemistry. Menlo Park, Calif: Benjamin/Cummings Pub. Co., Inc. ISBN 0-8053-3931-0.
• Jörg von Hagen, VCH-Wiley 2008 Proteomics Sample Preparation. ISBN 978-3-527-31796-7




Jantung
Dari Wikipedia Bahasa Melayu, ensiklopedia bebas.
Lompat ke: pandu arah, gelintar
Jantung


Jantung manusia


Kedudukan jantung dan paru-paru
Bahasa Latin
Cor
Sistem organ
Kardiovaskular
Arteri
Aorta, arteri pulmonari, arteri koronari
Vena
Vena kava superior, vena kava inferior, vena pulmonari, vena koronari, venae cordis minimae
Saraf
Simpatetik, vagus
Jantung atau dalam bahasa Inggeris dikenali sebagai heart (Latin, cor) merupakan organ berongga yang berfungsi mengepam darah melalui saluran darah dengan denyutan yang sekata yang berulang-ulang. Istilah kardium bermaksud berkaitan dengan jantung, berasal dari perkataan Greek kardia untuk "jantung".
Isi kandungan
[sorok]
• 1 Struktur
• 2 Kitar kardiak
• 3 Aturan kitar kardiak
• 4 Penyakit dan rawatan
• 5 Pertolongan cemas
• 6 Penyakit yang berkaitan dengan jantung
• 7 Pautan luar

[sunting] Struktur
Dalam tubuh manusia, jantung terletak sebelah kiri sedikit dari tengah dada, dan di belakang tulang dada (sternum). Ia diselaputi oleh kantung yang dikenali sebagai perikardium dan dikelilingi oleh peparu. Secara purata, jantung orang dewasa mempunyai berat sekitar 300-350 g. Ia terdiri dari empat ruang, dua atrium di atas dan dua ventrikel di bawah.
Dinding otot yang tebal (septum) membahagikan atrium dan ventrikel kanan dari atrium dan ventrikel kiri. Ia memisahkan darah beroksigen dan terdeoksigen dari bercampur. Injap antara atrium dan ventrikel hanya membenarkan aliran darah secara satu hala dari atrium ke ventrikel.
Ventrikel adalah bahagian jantung yang mengepam darah ke seluruh tubuh termasuk paru-paru. Dinding ventrikel adalah lebih tebal berbanding atrium, dan pengecutan dinding ventrikel adalah lebih penting bagi memastikan darah mengalir.
Darah terdeoksigen dari tubuh memasuki atrium kanan melalui 2 salur, vena kava superior (superior vena cava) dan vena kava inferior (inferior vena cava). Darah kemudian mengalir ke ventrikel kanan. Ventrikel kanan mengepam darah terdeoksigen ini ke peparu melalui arteri pulmonari. Selepas darah kehilangan karbon dioksida dan menyerap oksigen dari peparu, ia mengalir melalui vena pulmonari ke atrium kiri. Dari atrium kiri, darah beroksigen dipam ke ventrikel kiri. Ventrikel kiri merupakan pam utama yang membekalkan darah melalui aorta ke seluruh tubuh kecuali peparu.
Ventrikel kiri adalah lebih tebal berbanding kanan. Ini disebabkan oleh keperluan untuk mengenakan tekanan yang tinggi bagi mengatasi rintangan yang dikenakan oleh tubuh. Ventrikel kanan hanya perlu mengepam darah ke peparu, jadi ia tidak memerlukan otot dinding yang kuat. Ini juga diperlukan kerana dua sebab lain: 1) kapilari peparu adalah lemah; tekanan tinggi akan merosakkan kapilari tersebut dan 2) aliran darah yang perlahan adalah diperlukan bagi memberi masa untuk pertukaran gas antara darah dan peparu.
Dinding jantung terdiri daripada tiga lapisan. Lapisan terluar dikenali sebagai perikardium (pericardium), lapisan tengah dipanggil myokardium (myocardium), dan lapisan terdalam dipanggil endokardium (endocardium). Perikardium boleh dibahagikan lagi kepada dua lapisan iaitu fibrous pericardium (luar) dan serous pericardium (dalam). Myokardium adalah lapisan yang paling tebal dan terdiri daripada otot jantung. Ia membentuk majoriti keseluruhan dinding jantung. Endokardium merupakan lapisan terdalam yang terdiri daripada sel epitelium leper dan tisu penyambung.
Bekalan darah yang banyak diperlukan untuk membekalkan nutrien, terutama oksigen, kepada jantung. Darah ini dibekalkan oleh arteri koronari kiri dan kanan, yang bercabang keluar dari aorta. Bekalan darah kepada jantung dipanggil kitaran darah koronari
[sunting] Kitar kardiak


Pandangan anterior keratan rentas jantung. Anak panah putih menunjukkan arah darah bergerak
Setiap degupan jantung melibatkan turutan yang dikenali sebagai "kitar kardiak". Ia terbahagi kepada tiga bahagian: "sistol atrium" (atrial systole), "sistol ventrikel" (ventricular systole) dan "diastol kardiak sepenuhnya" (complete cardiac diastole). Sistol atrium adalah pengecutan kedua-dua atrium, sistol ventrikel adalah pengecutan kedua-dua ventrikel, manakala diastol kardiak pula merupakan pengenduran keseluruhan otot-otot jantung.
Apabila sistol atrium berlaku, injap atrioventrikular (atrioventricular valves) akan terbuka. Darah dipam masuk ke dalam ventrikel. Apabila sistol atrium berakhir, sistol ventrikel pula bermula. Tekanan tinggi dalam ventrikel menyebabkan injap atrioventrikular tertutup, dan injap sabit (semilunar valves) terbuka. Ini menyebabkan darah hanya dipam ke dalam aorta dan arteri pulmonari tetapi tidak ke dalam atrium.
Diastol kardiak berlaku setelah darah dipam keluar dari jantung. Pada masa ini, darah di dalam aorta akan mengalir balik ke dalam jantung, tetapi ini tidak berlaku kerana penutupan injap sabit.
Bunyi jantung yang kita dengari adalah disebabkan oleh penutupan injap atrioventrikular (bunyi pertama) dan penutupan injap sabit (bunyi kedua).
[sunting] Aturan kitar kardiak
Otot kardiak adalah myogenik. Ini bererti bahawa berbeza dengan otot rangka yang memerlukan rangsangan (sama ada sedar atau reflex), rangsangan otot jantung adalah secara automatik. Pengecutan berirama berlaku sendiri, walaupun frekuensi boleh berubah disebabkan keresahan, kesan hormon, senaman atau berasa terancam.
Irama pengecutan diselaraskan oleh node sinoatrial dan node atrioventrikular. Node sinoatrial, sering dikenali sebagai perentak jantung, terletak di bahagian atas dinding atrium kanan dan bertanggungjawab menghasilkan impuls eletrik yang memulakan pengecutan atrium. Apabila impuls ini tiba di node atrioventrikular yang terletak di dinding antara ruang ventrikel, ia akan dilambatkan sedikit. Ini bertujuan memastikan atrium telah mengecut sepenuhnya. Selepas itu, impuls ini dialirkan melalui berkas His (bundle of His) di dalam septum dan dialirkan ke dalam dinding-dinding ventrikel. Impuls ini menyebabkan pengecutan ventrikel berlaku.
Jantung mampu terus berdegup walaupun setelah dikeluarkan dari tubuh manusia yang hidup. Perkara ini terus menakjubkan manusia sepanjang zaman. Malah kaum Aztec yang tinggal di Amerika Selatan telah begitu kagum dengan keupayaan jantung berdegup di luar tubuh ini, sehinggakan mereka mengamalkan pengorbanan manusia dengan meragut keluar jantung dari mangsa pengorbanan hidup-hidup sebagai bahan persembahan kepada dewa matahari.
[sunting] Penyakit dan rawatan
Bidang kajian mengenai jantung dikenali sebagai kardiologi. Penyakit penting bagi jantung termasuk:
• Penyakit jantung koronari merupakan kekurangan bekalan oksigen kepada otot jantung Ia menyebabkan sakit dada teruk dan ketidak selesaan yang dikenali sebagai angina.
• Myocardial infarction atau umumnya dikenali sebagai sakit jantung berlaku disebabkan kematian sel jantung akibat kitaran darah kepada jantung terganggu. Penyakit jantung koronari yang berterusan akan menyebabkan sakit jantung.
• Congestive heart failure atau kegagalan jantung adalah kehilangan daya pam jantung.
• Endokarditis, myokarditis dan perikarditis adalah keradangan jantung.
• Arrhythmia kardiak (cardiac arrhythmia) adalah ketidaktentuan degupan jantung.
Sekiranya arteri koronari tersumbat, tempat masalah boleh dipintas dengan pembedahan pintas arteri koronari (coronary artery bypass surgery) atau ia boleh diluaskan melalui kaedah angioplasty.
Beta blocker adalah dadah yang merendahkan kadar degupan jantung dan tekanan darah dan mengurangkan keperluan jantung untuk oksigen. Nitroglycerin dan bahan lain yang membekalkan nitrik oksida (NO) digunakan untuk merawat sakit jantung kerana ia menyebabkan pengembangan arteri koronari.
Perentak jantung buatan (artificial pacemaker) diimplan ke dalam jantung pesakit yang mengidap arrhythmia.
Pemindahan jantung dilakukan terhadap pesakit yang mengidap penyakit jantung koronari atau kegagalan jantung tahap akhir.
Lihat juga: Prosedur dan ujian diagnostik kardiologi
[sunting] Pertolongan cemas
Sekiranya seseorang mengalami jantung terhenti (cardiac arrest) (tiada degupan jantung), pemulihan kardio-pulmonari perlu dimulakan.
[sunting] Penyakit yang berkaitan dengan jantung
• Penyakit jantung koronari
• Sakit jantung
[sunting] Pautan luar
• Lebih lanjut mengenai jantung.
• http://www.isdam.org/isdam/anatomi.html

Sistem pencernaan makanan pada manusia
Diarsipkan di bawah: Sistem Pencernaan — gurungeblog @ 5:34 am
Tags: Anus, Esofagus, Lambung, manusia, Rektum, Rongga Mulut, Sistem pencernaan makanan, Usus Besar, Usus Halus

sistem-pencernaan
Sistem pencernaan makanan pada manusia terdiri dari beberapa organ, berturut-turut dimulai dari
1. Rongga Mulut,
2. Esofagus
3. Lambung
4. Usus Halus
5. Usus Besar
6. Rektum
7. Anus.

Rongga Mulut

rongga-mulut
Mulut merupakan saluran pertama yang dilalui makanan. Pada rongga mulut, dilengkapi alat pencernaan dan kelenjar pencernaan untuk membantu pencernaan makanan. Pada Mulut terdapat :
a.Gigi
Memiliki fungsi memotong, mengoyak dan menggiling makanan menjadi partikel yang kecil-kecil. Perhatikan gambar disamping.
b..Lidah
Memiliki peran mengatur letak makanan di dalam mulut serta mengecap rasa makanan.
c..Kelenjar Ludah
Ada 3 kelenjar ludah pada rongga mulut. Ketiga kelenjar ludah tersebut menghasilkan ludah setiap harinya sekitar 1 sampai 2,5 liter ludah. Kandungan ludah pada manusia adalah : air, mucus, enzim amilase, zat antibakteri, dll. Fungsi ludah adalah melumasi rongga mulut serta mencerna karbohidrat menjadi disakarida.
Esofagus (Kerongkongan)
Merupakan saluran yang menghubungkan antara rongga mulut dengan lambung. Pada ujung saluran esophagus setelah mulut terdapat daerah yang disebut faring. Pada faring terdapat klep, yaitu epiglotis yang mengatur makanan agar tidak masuk ke trakea (tenggorokan). Fungsi esophagus adalah menyalurkan makanan ke lambung. Agar makanan dapat berjalan sepanjang esophagus, terdapat gerakan peristaltik sehingga makanan dapat berjalan menuju lambung
Lambung

lambung
Lambung adalah kelanjutan dari esophagus, berbentuk seperti kantung. Lambung dapat menampung makanan 1 liter hingga mencapai 2 liter. Dinding lambung disusun oleh otot-otot polos yang berfungsi menggerus makanan secara mekanik melalui kontraksi otot-otot tersebut. Ada 3 jenis otot polos yang menyusun lambung, yaitu otot memanjang, otot melingkar, dan otot menyerong.
Selain pencernaan mekanik, pada lambung terjadi pencernaan kimiawi dengan bantuan senyawa kimia yang dihasilkan lambung. Senyawa kimiawi yang dihasilkan lambung adalah :
• Asam HCl ,Mengaktifkan pepsinogen menjadi pepsin. Sebagai disinfektan, serta merangsang pengeluaran hormon sekretin dan kolesistokinin pada usus halus
• Lipase , Memecah lemak menjadi asam lemak dan gliserol. Namun lipase yang dihasilkan sangat sedikit
• Renin , Mengendapkan protein pada susu (kasein) dari air susu (ASI). Hanya dimiliki oleh bayi.
• Mukus , Melindungi dinding lambung dari kerusakan akibat asam HCl.
Hasil penggerusan makanan di lambung secara mekanik dan kimiawi akan menjadikan makanan menjadi bubur yang disebut bubur kim.
Fungsi HCI Lambung :
1. Merangsang keluamya sekretin
2. Mengaktifkan Pepsinogen menjadi Pepsin untuk memecah protein.
3. Desinfektan
4. Merangsang keluarnya hormon Kolesistokinin yang berfungsi merangsang empdu mengeluarkan getahnya.
Usus Halus

usus-halus
Usus halus merupakan kelanjutan dari lambung. Usus halus memiliki panjang sekitar 6-8 meter. Usus halus terbagi menjadi 3 bagian yaitu duodenum (± 25 cm), jejunum (± 2,5 m), serta ileum (± 3,6 m). Pada usus halus hanya terjadi pencernaan secara kimiawi saja, dengan bantuan senyawa kimia yang dihasilkan oleh usus halus serta senyawa kimia dari kelenjar pankreas yang dilepaskan ke usus halus.
Senyawa yang dihasilkan oleh usus halus adalah :
• Disakaridase Menguraikan disakarida menjadi monosakarida
• Erepsinogen Erepsin yang belum aktif yang akan diubah menjadi erepsin. Erepsin mengubah pepton menjadi asam amino.
• Hormon Sekretin Merangsang kelenjar pancreas mengeluarkan senyawa kimia yang dihasilkan ke usus halus
• Hormon CCK (Kolesistokinin) Merangsang hati untuk mengeluarkan cairan empedu ke dalam usus halus.
Selain itu, senyawa kimia yang dihasilkan kelenjar pankreas adalah :
• Bikarbonat Menetralkan suasana asam dari makanan yang berasal dari lambung
• Enterokinase Mengaktifkan erepsinogen menjadi erepsin serta mengaktifkan tripsinogen menjadi tripsin. Tripsin mengubah pepton menjadi asam amino.
• Amilase Mengubah amilum menjadi disakarida
• Lipase Mencerna lemak menjadi asam lemak dan gliserol
• Tripsinogen Tripsin yang belum aktif.
• Kimotripsin Mengubah peptone menjadi asam amino
• Nuklease Menguraikan nukleotida menjadi nukleosida dan gugus pospat
• Hormon Insulin Menurunkan kadar gula dalam darah sampai menjadi kadar normal
• Hormon Glukagon Menaikkan kadar gula darah sampai menjadi kadar normal
PROSES PENCERNAAN MAKANAN
Pencernaan makanan secara kimiawi pada usus halus terjadi pada suasana basa. Prosesnya sebagai berikut :
a. Makanan yang berasal dari lambung dan bersuasana asam akan dinetralkan oleh bikarbonat dari pancreas.
b. Makanan yang kini berada di usus halus kemudian dicerna sesuai kandungan zatnya. Makanan dari kelompok karbohidrat akan dicerna oleh amylase pancreas menjadi disakarida. Disakarida kemudian diuraikan oleh disakaridase menjadi monosakarida, yaitu glukosa. Glukaosa hasil pencernaan kemudian diserap usus halus, dan diedarkan ke seluruh tubuh oleh peredaran darah.
c. Makanan dari kelompok protein setelah dilambung dicerna menjadi pepton, maka pepton akan diuraikan oleh enzim tripsin, kimotripsin, dan erepsin menjadi asam amino. Asam amino kemudian diserap usus dan diedarkan ke seluruh tubuh oleh peredaran darah.
d. Makanan dari kelompok lemak, pertama-tama akan dilarutkan (diemulsifikasi) oleh cairan empedu yang dihasilkan hati menjadi butiran-butiran lemak (droplet lemak). Droplet lemak kemudian diuraikan oleh enzim lipase menjadi asam lemak dan gliserol. Asam lemak dan gliserol kemudian diserap usus dan diedarkan menuju jantung oleh pembuluh limfe.
Usus Besar (Kolon)

usus-besar
Merupakan usus yang memiliki diameter lebih besar dari usus halus. Memiliki panjang 1,5 meter, dan berbentuk seperti huruf U terbalik. Usus besar dibagi menjadi 3 daerah, yaitu : Kolon asenden, Kolon Transversum, dan Kolon desenden. Fungsi kolon adalah :
a. Menyerap air selama proses pencernaan.
b. Tempat dihasilkannya vitamin K, dan vitamin H (Biotin) sebagai hasil simbiosis dengan bakteri usus, misalnya E.coli.
c. Membentuk massa feses
d. Mendorong sisa makanan hasil pencernaan (feses) keluar dari tubuh. Pengeluaran feses dari tubuh ddefekasi.
Rektum dan Anus
Merupakan lubang tempat pembuangan feses dari tubuh. Sebelum dibuang lewat anus, feses ditampung terlebih dahulu pada bagian rectum. Apabila feses sudah siap dibuang maka otot spinkter rectum mengatur pembukaan dan penutupan anus. Otot spinkter yang menyusun rektum ada 2, yaitu otot polos dan otot lurik.
Gangguan Sistem Pencernaan
• Apendikitis-Radang usus buntu.
• Diare- Feses yang sangat cair akibat peristaltik yang terlalu cepat.
• Kontipasi -Kesukaran dalam proses Defekasi (buang air besar)
• Maldigesti-Terlalu banyak makan atau makan suatu zat yang merangsang lambung.
• Parotitis-Infeksi pada kelenjar parotis disebut juga Gondong
• Tukak Lambung/Maag-”Radang” pada dinding lambung, umumnya diakibatkan infeksi Helicobacter pylori
• Xerostomia-Produksi air liur yang sangat sedikit
Gangguan pada sistem pencernaan makanan dapat disebabkan oleh pola makan yang salah, infeksi bakteri, dan kelainan alat pencernaan. Di antara gangguan-gangguan ini adalah diare, sembelit, tukak lambung, peritonitis, kolik, sampai pada infeksi usus buntu (apendisitis).
Diare
Apabila kim dari perut mengalir ke usus terlalu cepat maka defekasi menjadi lebih sering dengan feses yang mengandung banyak air. Keadaan seperti ini disebut diare. Penyebab diare antara lain ansietas (stres), makanan tertentu, atau organisme perusak yang melukai dinding usus. Diare dalam waktu lama menyebabkan hilangnya air dan garam-garam mineral, sehingga terjadi dehidrasi.
Konstipasi (Sembelit)
Sembelit terjadi jika kim masuk ke usus dengan sangat lambat. Akibatnya, air terlalu banyak diserap usus, maka feses menjadi keras dan kering. Sembelit ini disebabkan karena kurang mengkonsumsi makanan yang berupa tumbuhan berserat dan banyak mengkonsumsi daging.
Tukak Lambung (Ulkus)
Dinding lambung diselubungi mukus yang di dalamnya juga terkandung enzim. Jika pertahanan mukus rusak, enzim pencernaan akan memakan bagian-bagian kecil dari lapisan permukaan lambung. Hasil dari kegiatan ini adalah terjadinya tukak lambung. Tukak lambung menyebabkan berlubangnya dinding lambung sehingga isi lambung jatuh di rongga perut. Sebagian besar tukak lambung ini disebabkan oleh infeksi bakteri jenis tertentu.
Beberapa gangguan lain pada sistem pencernaan antara lain sebagai berikut: Peritonitis; merupakan peradangan pada selaput perut (peritonium).
Gangguan lain adalah salah cerna akibat makan makanan yang merangsang lambung, seperti alkohol dan cabe yang mengakibatkan rasa nyeri yang disebut kolik. Sedangkan produksi HCl yang berlebihan dapat menyebabkan terjadinya gesekan pada dinding lambung dan usus halus, sehingga timbul rasa nyeri yang disebut tukak lambung. Gesekan akan lebih parah kalau lambung dalam keadaan kosong akibat makan tidak teratur yang pada akhirnya akan mengakibatkan pendarahan pada lambung.
Gangguan lain pada lambung adalah gastritis atau peradangan pada lambung. Dapat pula apendiks terinfeksi sehingga terjadi peradangan yang disebut apendisitis.
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Sistem Pernafasan Manusia



Pengetahuan Umum - Manusia dan Kehidupannya

Sebagai makhluk hidup kita masih hidup sampai saat ini karena setiap saat kita selalu bernafas menghirup udara. Makhluk hidup, di dunia ini, baik itu hewan maupun manusia akan mati (wafat) jika sudah tidak dapat bernafas lagi. Sebenarnya bagaimana sih sistem pernafasan yang terdapat dalam tubuh kita ? Untuk lebih jelasnya kamu dapat membaca keseluruhan tulisan ini.
Sistem pernafasan secara garis besarnya terdiri dari paru-paru dan susunan saluran yang menghubungkan paru-paru dengan yang lainnya, yaitu hidung, tekak, pangkal tenggorok, tenggorok, cabang tenggorok.
Pada awalnya kita menghirup udara melalui rongga hidung yang kemudian melewati tekak dan pangkal tenggorok kemudian terus ke tenggorokan. Tenggorok bentuknya seperti pipa yang kuat, terletak di depan kerongkongan, melalui leher sampai mencapai rongga dada sebelah atas. Dinding tenggorok diperkuat oleh beberapa cincin rawan yang pada bagian belakangnya terbuka. Dalam rongga dada, tenggorok bercabang dua yaitu tenggorok kanan dan kiri yang masing-masing cabang memasuki paru-paru kanan dan paru-paru kiri.
Kedua cabang tenggorok tersebut mempunyai ranting-ranting seperti pada pohon. Pada ranting-rantingnya yang terakhir terdapat gelembung-gelembung paru-paru yang amat kecil dan amat tipis dindingnya. Gelembung-gelembung itu hanya dapat dilihat dengan mikroskop. Dalam dindingnya mengalir darah melalui pembuluh-pembuluh kapiler, sehingga mudah terjadi pertukaran gas dari darah ke udara yang terdapat dalam gelembung paru-paru dan sebaliknya. Darah tersebut mengambil zat pembakar (oksigen) dan mengeluarkan karbondioksida.
Rongga dada terbagi atas 3 bagian. Di depan dan di tengah agak ke kiri terletak kandung jantung yang menyelubungi seluruh jantung. Di belakang kandung jantung terdapat beberapa alat yaitu tenggorok, kerongkongan dan aorta. Organ pernafasan tersebut terpendam dalam susunan jaringan ikat yang tebal. Bersama kandung jantung organ tadi merupakan suatu sekat yang membagi rongga dada di tengahnya. Sekat itu dinamakan "Sekat Dada". Disebelah kanan dan kirinya terdapat rongga yang dilapisi oleh selaput paru-paru parietal yaitu rongga selaput paru-paru. Rongga ini seluruhnya ditempati oleh paru-paru.
Antara permukaan paru-paru yang juga dilapisi oleh selaput paru-paru visceral dan dinding rongga selaput paru-paru terdapat celah yang sempit yang berisikan sedikit cairan. Sekat dada khususnya jantung tidak terletak tepat ditengah-tengah rongga dada, tetapi agak ke kiri, sehingga menyebabkan paru-paru kiri lebih kecil dari paru-paru kanan. Isi rongga dada dapat diperbesar berkat pengaruh otot-otot pengangkatan iga-iga, kontraksi sekat rongga badan yang melengkung ke atas. Paru-paru mengikuti perluasan rongga dada maka terhisaplah udara melalui saluran pernapasan yang telah diuraikan di atas. Bila tenaga-tenaga yang melapangkan dada berhenti bekerja, maka kekenyalan dinding dada dan paru-paru menyebabkan penyempitan rongga dada kembali. Pada waktu tersebut iga-iga menurun kembali, sekat rongga badan melengkung lagi ke atas, sehingga kelebihan udara didesak keluar dari paru-paru. Proses tersebut terjadi bila kita menghembuskan nafas (mengeluarkan nafas).
Dari penjabaran di atas dapat ditarik kesimpulan-kesimpulan yaitu pertama, fungsi utama dari adanya sistem pernafasan kita adalah untuk memberikan darah gas oksigen yang nantinya disalurkan keseluruh tubuh. Kedua, ketika kita bernafas maka yang kita hirup adalah gas oksigen (lambang kimianya O2 ) sedangkan gas yang dilepaskan diesbut gas karbondioksida dengan lambang kimianya CO2.