جامعه مجازی علوم پزشکی(جمع 🌻)

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📜 متن و سوالات فایل شماره 19

🔘راهنما: ابتدا ویدیو بالا را ببنید و خوب گوش بدهید. سپس متن زیر را بخوانید. مطمئن شوید که معنای همه لغات را می دانید. و در آخر به سوالات تستی طرح شده پاسخ دهید.

🔻 متن و سوالات:

this familiar gelatin dessert actually is a good example of the process of coagulation of proteins into a three dimensional latticework that entraps water molecules to produce a semi-solid gel.

this animation explains the physical chemistry behind the
process.
proteins are synthesized by polymerizing amino acids. the polymerization occurs by repeatedly forming peptide bonds that
link individual amino acids together into a chain.

there are three structural
features that influence the three-dimensional shape of a water s-oluble protein.
primary structure is the peptide bond between individual amino
acids that creates a long chain of connected amino acids.
these long chains of polymerized amino acids have hydrophobic (water repelling) and hydrophilic (water attracting) projections thatare oriented perpendicular to the
chain, as shown in this illustration of a growing protein chain.

secondary structure is the helix that the protein chain curls into, as a result of hydrogen bonds and other weak forces.

tertiary structure is created when the protein molecules fold back on themselves outside of the helical segments, to put the hydrophobic portions to the interior and the hydrophilic
portions to the exterior.

several helical regions can exist in different portions
of the molecule.

when the protein has folded and refolded to reach its most
stable configuration. it will have mostly hydrophilic amino acid residues on the exterior, and mostly hydrophobic residues directed into the interior.

when natural proteins are subjected to physical or chemical treatment, their structure changes, and they become 'un-native' or 'unnatural'. we call that process denaturation.

in this example, heating the proteins in solution im
parts energy to the protein molecules.
this added energy is enough to break the relatively weak forces that hold the protein in its refolded and helical tertiary and secondary configurations.

as the process of denaturation precedes, the protein molecule unfolds more and more
and the internally directed hydrophobic regions now become exposed on the
outside of the molecule.

the peptide bonds are largely
hydrophilic. once these segments are set free from each other, they attract water
molecules. the recruitment of water molecules entraps the water molecules in close proximity to the protein strands.

the hydrophobic portions of the. molecules are also exposed. this situation is unfavored because the
hydrophobic portions of molecules are not stable in an aqueous environment.
hence, upon unfolding, the hydrophobic regions on individual protein molecules
will associate with hydrophobic regions on other protein molecules.

this situation encourages the association of these protein molecules into larger and larger random three-dimensional
structures. the molecules aggregate into very large, water-insoluble collections
that are quite randomly assembled.

as the proteins denature, lattice-work structures
grow amorphously and attract the solvent water molecules into cell-like structures.

the self associated water molecules, in groups, adhere to the surface of hydrophilic regions of the protein while hydrophobic regions of the
protein dissolve into each other and provide the energy to retain the structure.

as this process continues irreversibly, all of the protein
molecules are recruited to this large and soluble mass in a randomly organized structural framework that contains en
trapped water molecules.

one example of the consequences of unfolding And re-associating protein molecules is coagulation of egg white. frying an egg is no more complicated than denaturing
the egg white protein.
the assembly of irreversibly denatured protein molecules results in formation of a solid gel.
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