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This illustration shows a phospholipid bilayer with proteins and cholesterol embedded in it. Integral membrane proteins span the entire membrane. Protein channels are integral membrane proteins with a central pore through which molecules can pass. Peripheral proteins are associated with the phospholipid head groups on one side of the membrane only. A glycoprotein is shown with the protein portion of the molecule embedded in the membrane and the carbohydrate portion jutting out from the membrane. A glycolipid is also shown with the lipid portion embedded in the membrane and the carbohydrate portion jutting out of the membrane.
The fluid mosaic model of the plasma membrane describes the plasma membrane as a fluid combination of phospholipids, cholesterol, and proteins. Carbohydrates attached to lipids (glycolipids) and to proteins (glycoproteins) extend from the outward-facing surface of the membrane.

The principal components of a plasma membrane are lipids (phospholipids and cholesterol), proteins, and carbohydrates attached to some of the lipids and some of the proteins. A phospholipid is a molecule consisting of glycerol, two fatty acids, and a phosphate-linked head group. Cholesterol, another lipid composed of four fused carbon rings, is found alongside the phospholipids in the core of the membrane. The proportions of proteins, lipids, and carbohydrates in the plasma membrane vary with cell type, but for a typical human cell, protein accounts for about 50 percent of the composition by mass, lipids (of all types) account for about 40 percent of the composition by mass, with the remaining 10 percent of the composition by mass being carbohydrates. However, the concentration of proteins and lipids varies with different cell membranes. For example, myelin, an outgrowth of the membrane of specialized cells that insulates the axons of the peripheral nerves, contains only 18 percent protein and 76 percent lipid. The mitochondrial inner membrane contains 76 percent protein and only 24 percent lipid. The plasma membrane of human red blood cells is 30 percent lipid. Carbohydrates are present only on the exterior surface of the plasma membrane and are attached to proteins, forming glycoproteins , or attached to lipids, forming glycolipids .

Phospholipids

The main fabric of the membrane is composed of amphiphilic, phospholipid molecules. The hydrophilic    or “water-loving” areas of these molecules (which look like a collection of balls in an artist’s rendition of the model) ( [link] ) are in contact with the aqueous fluid both inside and outside the cell. Hydrophobic , or water-hating molecules, tend to be non-polar. They interact with other non-polar molecules in chemical reactions, but generally do not interact with polar molecules. When placed in water, hydrophobic molecules tend to form a ball or cluster. The hydrophilic regions of the phospholipids tend to form hydrogen bonds with water and other polar molecules on both the exterior and interior of the cell. Thus, the membrane surfaces that face the interior and exterior of the cell are hydrophilic. In contrast, the interior of the cell membrane is hydrophobic and will not interact with water. Therefore, phospholipids form an excellent two-layer cell membrane that separates fluid within the cell from the fluid outside of the cell.

A phospholipid molecule ( [link] ) consists of a three-carbon glycerol backbone with two fatty acid molecules attached to carbons 1 and 2, and a phosphate-containing group attached to the third carbon. This arrangement gives the overall molecule an area described as its head (the phosphate-containing group), which has a polar character or negative charge, and an area called the tail (the fatty acids), which has no charge. The head can form hydrogen bonds, but the tail cannot. A molecule with this arrangement of a positively or negatively charged area and an uncharged, or non-polar, area is referred to as amphiphilic    or “dual-loving.”

Questions & Answers

a person with blood group A can not donate blood to a person with blood group B
KHUNGA Reply
differentiate between cytoplasm and protoplasm
Annet Reply
cell as a unit of life
Bhrah Reply
differentiate between cytoplasm and protoplasm
Annet
forms which cell exist
Zainab Reply
Wats a molecules in terms of biology?
Ayomide Reply
Important of kreb's cycle
yeboah
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is agurment btn two or more people
Okello
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KINEI Reply
the scientific study of the life and structure of plants and animals.
Boame
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Guor
After finishing school
Hellen
study of living things n non living things
Benjamin
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Okello
study of living organsm
Hellen Reply
which element is the largest
Haja Reply
francium
Izzati
francium
Paul
how many groups do we have in the periodic table
Theola Reply
seven groups
Moses
the one that makes eight is the noble gases
Moses
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Theola
we have eight groups in the periodic table
Ramsey
there are actually 18 groups or numbered columns
Izzati
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Izzati
8 groups
Boame
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Smart Reply
nearly a hundred million
Izzati
one
Moses
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two times
Theola
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Okello
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Martin
it repair worn out tissue
Moses
when is the right time for playing sex to prevent pregnancy?
Guor
at anytime
Okello
what is biology
Darkoa Reply
it's study of all living organisms and their characteristics
Ice
by what proces do ameaba reproduce?
Okello Reply
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Janet
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Okello
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Mathew
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amoeba reproduce asexually
Chidi
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Rachel
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Izzati
what is active transport
Amona Reply
is the movement's of water & mineral salts from the reason of highly concentrated to the reason of low concentration across semi permeable membrane & it take place in plant
Okello

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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