1.3.1 31BLipids
Lipids may be broadly defined as hydrophobic or amphiphilic small molecules. In most
membranes, approximately 40%-50% of the mass of the membrane is composed of lipids. Lipids
provide a matrix for protein groups, acts as a barrier for ions & molecules and have the same
structure in all membranes. Lipids are a diverse group of large biological molecules that do not
include polymers and is made up primarily or exclusively of non-polar groups. They are grouped
together and have little or no affinity for water. Due to their non-polar character, lipids typically
dissolve more readily in non-polar solvents such as acetone, ether, and benzene etc. This
solubility characteristic is of extreme importance in cells because lipids act as barriers and form
boundaries between and within cells. The hydrophobic behavior of lipids is based on their
molecular structure. Although they have some polar bonds associated with oxygen, lipids consist
mostly of hydrocarbons. Lipids link covalently with carbohydrates to form glycolipids and with
proteins to form lipoproteins. Biological lipids originate entirely or in part from two distinct
types of biochemical subunits or "building blocks": ketoacyl and isoprene groups [19]. Lipids
can be divided into eight categories: fatty acyls, glycerolipids, phospholipids, sphingolipids,
saccharolipids and polyketides (derived from condensation of ketoacyl subunits); and sterol
lipids and prenol lipids (derived from condensation of isoprene subunits). However the major
part of the cell membrane is constituted of phospholipids.
1.3.2 32BPhospholipids
Phospholipids are a class of lipids and are a major component of all cell membranes. Most
phospholipids contain a diglyceride, a phosphate group, and a simple organic molecule such
as choline. They are similar to fats, but have only two fatty-acids rather than three (see Fig 1-2).
The third hydroxyl group is joined to a phosphate group, which is negative in electrical charge
and is therefore soluble in water. Phospholipids are described as amphipathic (or amphiphilic)
molecules, having both a hydrophobic and a hydrophilic region. The two fatty acid tails which
consist of hydrocarbons are hydrophobic and are excluded from water. Their heads, however
which consist of the phosphate group and its attachments, are hydrophilic and have an affinity
for water. Since the two fatty-acid chains are insoluble in water (hydrophobic), they are thought
to project from the glycerol chain in a direction opposite to that taken by a polar group. When
many phospholipid molecules are placed in water, their hydrophilic heads tend to face water and
the hydrophobic tails are forced to stick together, forming a bilayer (see Fig 1-3)
The phospholipids differ among themselves in the identity of the fatty acids or of the
polar group or both. In phosphoglycerides, a principal class of phospholipids, glycerol forms the
backbone of the molecule, two fatty acid chains are esterified to two of the three hydroxyl groups
in glycerol, and the third hydroxyl group is esterified to phosphate (see Fig 1-5a). The phosphate
group can also be esterified to a hydroxyl group on another hydrophilic compound, such as
serine, ethanolamine, choline, glycerol, and the inositol. The structural formulas of phosphatidyl
choline and the other principal phosphoglycerides—namely, phosphatidyl ethanolamine,
phosphatidyl serine, phosphatidyl inositol, and diphosphatidyl glycerol are shown in Fig 1-4. The
second major class of membrane lipids are that of glycolipids; these are based on the molecule
sphingosine (see Fig 1.5c). Though they possess the basic tuning-fork design of the
phosphoglyceride they differ from them in several ways. The first long chain component is
always a 15:1 hydrocarbon, which moreover, is linked to the base by a simple carbon-carbon
bond rather than the ester bond (-COO-) found in the phosphoglycerides. In addition, a hydroxyl
group is retained. Sphingomyelin, a phospholipid that lacks a glycerol backbone, is found mainly
in plasma membranes (see Fig 1.5b). Instead of a glycerol backbone, it contains sphingosine, an
amino alcohol with a long unsaturated hydrocarbon chain. In sphingomyelin, the hydrophilic
head is similar to that of phosphatidylcholine. In Figure 1.5 the hydrophobic portions of all
molecules are shown in yellow; the hydrophilic, in green. (a) Phosphatidylcholine is a typical
phosphoglyceride. The fatty acyl side chains can be saturated, or they can contain one or more
double bonds. (b) Sphingomyelin is a group of phospholipids that lack a glycerol backbone; a
sphingomyelin may contain a different fatty acyl side chain than oleic acid (shown here).
Linkage of sphingosine (outlined by black dots) to a fatty acid via an amide bond forms a
ceramide. (c) Glucosylcerebroside, one of the simplest glycolipids, consists of the ceramide
formed from sphingosine and oleic acid linked to a single glucose residue. This glycolipid is
abundant in the myelin.
The properties of a phospholipid are characterized by the properties of the fatty acid
chain and the phosphate/amino alcohol. The long hydrocarbon chains of the fatty acids are of
course non-polar. The phosphate group has a negatively charged oxygen and a positively charged
nitrogen to make this group ionic. In addition there are other oxygen of the ester groups, which
make on whole end of the molecule strongly ionic and polar.
Phospholipids when placed in an aqueous solution, due to its hydrophilic polar head
group and hydrophobic tail its molecules will tend to arrange themselves in such a way that the
hydrophilic heads will remain in contact with the water molecules, while the hydrophobic tails
will orient themselves toward non-polar space, like air, other tails or the container. Most
commonly, the phospholipids molecules will tend to arrange themselves in a double layer
(bilayer sheet) (see Figure 1.6), both of whose surfaces will consist of heads, with their tails
facing each other inside the bilayer membrane
Under certain circumstances such as lipid concentration and solvent quality there is
difficulty in filling all the volume of the interior of a bilayer, while accommodating the area per
head group forced on the molecule by the hydration of the lipid head group leads to the
formation tiny spheroidal micelles (see Figure 1.6), with the hydrophilic head regions in contact
with surrounding solvent, sequestering the hydrophobic single tail regions in the micelle centre.
If amphipathic lipids in high concentration are agitated in an acqueous suspension they form
spherical liposomes (see Figure 1.6).
The basic physics had already been worked out by physicists such as Irving Langmuir
[20], and Evert Gorter et al [21] and showed that bilayer was simply the most efficient (and
therefore the most probable) way for phospholipid molecules to arrange themselves consistent
with minimization of free energy So, in our study we considered phospholipids in the form of a
bilayer and approximated it as a cell membrane.
1.3.1 31BLipidsโครงการอาจทั่วไปกำหนดเป็น hydrophobic หรือ amphiphilic โมเลกุลขนาดเล็กได้ ในที่สุดประกอบด้วยเยื่อหุ้ม ประมาณ 40% - 50% ของมวลของเมมเบรนที่ของโครงการ โครงการมีเมทริกซ์สำหรับกลุ่มโปรตีน ทำหน้าที่เป็นอุปสรรคกันและโมเลกุล และมีโครงสร้างทั้งหมดเข้ากัน โครงการคือ กลุ่มมีความหลากหลายของชีวภาพโมเลกุลขนาดใหญ่ที่ไม่รวมถึงโพลิเมอร์ และขึ้นเป็นหลัก หรือเฉพาะกลุ่มไม่ใช่ขั้วโลก มีการจัดกลุ่มร่วมกัน และมีความสัมพันธ์น้อย หรือไม่มีน้ำ เนื่องจากตนไม่ใช่โพลาร์อักขระ โครงการโดยทั่วไปละลายมากขึ้นหรือสารทำละลายไม่มีขั้วเช่นอะซิโตน อีเทอร์ และเบนซีนเป็นต้น นี้ลักษณะการละลายคือสำคัญมากในเซลล์เนื่องจากโครงการที่ทำหน้าที่เป็นอุปสรรคและแบบฟอร์ม รอยต่อระหว่าง และภาย ในเซลล์ ลักษณะ hydrophobic ของโครงการตามความโครงสร้างโมเลกุล แม้ว่าจะมีพันธบัตรบางขั้วโลกที่เกี่ยวข้องกับออกซิเจน โครงการประกอบด้วยส่วนใหญ่ของสารไฮโดรคาร์บอน เชื่อมโยงโครงการกับคาร์โบไฮเดรต และมีแบบฟอร์ม glycolipids covalentlyวิลเลียม lipoproteins ฟอร์ม โครงการชีวภาพมาทั้งหมด หรือบางส่วนจากทั้งสองแตกต่างกันชนิด subunits ชีวเคมีหรือ "สร้างบล็อก": กลุ่ม ketoacyl และ isoprene [19] โครงการสามารถแบ่งออกเป็น 8 ประเภท: ไขมัน acyls, glycerolipids, phospholipids, sphingolipidssaccharolipids และ polyketides (มาจากสรุป ketoacyl subunits); และสเตอรอลโครงการและโครงการ prenol (มาจากการควบแน่นของ isoprene subunits) อย่างไรก็ตามหลักการส่วนหนึ่งของเยื่อหุ้มเซลล์จะทะลักของ phospholipids1.3.2 32BPhospholipidsPhospholipids มีคลาสของโครงการ และมีส่วนประกอบสำคัญของเยื่อหุ้มเซลล์ทั้งหมด มากที่สุดประกอบด้วย phospholipids diglyceride กับ กลุ่มฟอสเฟต และตัวอย่างอินทรีย์โมเลกุลดังกล่าวเป็น choline พวกเขาเหมือนกับไขมัน แต่มีกรดไขมันเพียงสองมากกว่าสาม (ดูฟิก 1 - 2) กลุ่มไฮดรอกซิล 3 ได้เข้าร่วมกับกลุ่มฟอสเฟต ซึ่งเป็นค่าลบในค่าไฟฟ้าจึงสามารถละลายในน้ำ อธิบาย phospholipids เป็น amphipathic (amphiphilic)โมเลกุล มีทั้งเป็น hydrophobic และภูมิภาค hydrophilic กรดไขมันสอง tails ที่ประกอบด้วยสารไฮโดรคาร์บอนที่เป็น hydrophobic และแยกออกจากน้ำ ศีรษะของพวกเขา อย่างไรก็ตามซึ่งประกอบด้วยกลุ่มฟอสเฟตและสิ่งที่แนบที่เป็น hydrophilic และมีความสัมพันธ์?สำหรับน้ำ เนื่องจากโซ่กรดไขมันสองละลายในน้ำ (hydrophobic), พวกเขาจะคิดว่าไปยังโครงการจากโซ่กลีเซอรในทิศทางตรงข้ามกับที่นำโดยขั้วโลก เมื่อหลายโมเลกุลฟอสโฟลิพิดอยู่ในน้ำ หัว hydrophilic ของพวกเขามักจะ เผชิญกับน้ำ และหาง hydrophobic ถูกบังคับให้ติดกัน ขึ้นรูปเป็น bilayer (ดูฟิก 1-3)Phospholipids แตกต่างกันในตัวเองในข้อมูลประจำตัว ของกรดไขมัน หรือกลุ่มขั้วโลกหรือทั้งสองอย่าง ใน phosphoglycerides คลาของ phospholipids กลีเซอรฟอร์มหลักbackbone of the molecule, two fatty acid chains are esterified to two of the three hydroxyl groupsin glycerol, and the third hydroxyl group is esterified to phosphate (see Fig 1-5a). The phosphategroup can also be esterified to a hydroxyl group on another hydrophilic compound, such asserine, ethanolamine, choline, glycerol, and the inositol. The structural formulas of phosphatidylcholine and the other principal phosphoglycerides—namely, phosphatidyl ethanolamine,phosphatidyl serine, phosphatidyl inositol, and diphosphatidyl glycerol are shown in Fig 1-4. Thesecond major class of membrane lipids are that of glycolipids; these are based on the moleculesphingosine (see Fig 1.5c). Though they possess the basic tuning-fork design of thephosphoglyceride they differ from them in several ways. The first long chain component isalways a 15:1 hydrocarbon, which moreover, is linked to the base by a simple carbon-carbonbond rather than the ester bond (-COO-) found in the phosphoglycerides. In addition, a hydroxylgroup is retained. Sphingomyelin, a phospholipid that lacks a glycerol backbone, is found mainlyin plasma membranes (see Fig 1.5b). Instead of a glycerol backbone, it contains sphingosine, anamino alcohol with a long unsaturated hydrocarbon chain. In sphingomyelin, the hydrophilichead is similar to that of phosphatidylcholine. In Figure 1.5 the hydrophobic portions of all molecules are shown in yellow; the hydrophilic, in green. (a) Phosphatidylcholine is a typicalphosphoglyceride. The fatty acyl side chains can be saturated, or they can contain one or moredouble bonds. (b) Sphingomyelin is a group of phospholipids that lack a glycerol backbone; asphingomyelin may contain a different fatty acyl side chain than oleic acid (shown here).Linkage of sphingosine (outlined by black dots) to a fatty acid via an amide bond forms aceramide. (c) Glucosylcerebroside, one of the simplest glycolipids, consists of the ceramideformed from sphingosine and oleic acid linked to a single glucose residue. This glycolipid isabundant in the myelin.The properties of a phospholipid are characterized by the properties of the fatty acidchain and the phosphate/amino alcohol. The long hydrocarbon chains of the fatty acids are of course non-polar. The phosphate group has a negatively charged oxygen and a positively chargednitrogen to make this group ionic. In addition there are other oxygen of the ester groups, whichmake on whole end of the molecule strongly ionic and polar.Phospholipids when placed in an aqueous solution, due to its hydrophilic polar headgroup and hydrophobic tail its molecules will tend to arrange themselves in such a way that thehydrophilic heads will remain in contact with the water molecules, while the hydrophobic tailswill orient themselves toward non-polar space, like air, other tails or the container. Mostcommonly, the phospholipids molecules will tend to arrange themselves in a double layer(bilayer sheet) (see Figure 1.6), both of whose surfaces will consist of heads, with their tails
facing each other inside the bilayer membrane
Under certain circumstances such as lipid concentration and solvent quality there is
difficulty in filling all the volume of the interior of a bilayer, while accommodating the area per
head group forced on the molecule by the hydration of the lipid head group leads to the
formation tiny spheroidal micelles (see Figure 1.6), with the hydrophilic head regions in contact
with surrounding solvent, sequestering the hydrophobic single tail regions in the micelle centre.
If amphipathic lipids in high concentration are agitated in an acqueous suspension they form
spherical liposomes (see Figure 1.6).
The basic physics had already been worked out by physicists such as Irving Langmuir
[20], and Evert Gorter et al [21] and showed that bilayer was simply the most efficient (and
therefore the most probable) way for phospholipid molecules to arrange themselves consistent
with minimization of free energy So, in our study we considered phospholipids in the form of a
bilayer and approximated it as a cell membrane.
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