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  • Cholesterol is Abundant in Cell Membranes

  • Cholesterol is found in every cell of your body. It is especially abundant in the membranes

  • of these cells, where it helps maintain the integrity of these membranes, and plays a

  • role in facilitating cell signaling-- meaning the ability of your cells to communicate with

  • each other so you function as a human, rather than a pile of cells.

  • Molecule for molecule, cholesterol can make up nearly half of the cell membrane.1 Since

  • it is smaller and weighs less than other molecules in the cell membrane, it makes up a lesser

  • proportion of the cell membrane's mass, usually roughly 20 percent.2

  • Cholesterol is also present in membranes of organelles inside the cells, although it usually

  • makes up a smaller proportion of the membrane. For example, the mitochondrion, the so-called

  • "power-house" of the cell, contains only three percent cholesterol by mass, and the endoplasmic

  • reticulum, which is involved in making and modifying proteins, is six percent cholesterol

  • by mass. 3

  • Cholesterol Maintains the Integrity of the Cell Membrane

  • Surrounding each of our cells is a membrane called the plasma membrane. The plasma membrane

  • is a continuous double-layer of phospholipids, interweaved with cholesterol and proteins.

  • Phospholipids are composed of two fatty acids attached to a phosphate compound as a head.

  • The phosphate head is water-soluble, also called "hydrophilic" (water-loving), and the

  • fatty-acids are water-insoluble, or "hydrophobic" (water-fearing). Since outside the cell is

  • a water-containing, or aqueous, environment, and inside the cell is also aqueous, the phosphate

  • heads of the phospholipids face both the cell's inside and the environment outside the cell,

  • while the fatty acids face the inside of the membrane.

  • The membrane is fluid, and the molecules are always moving. It has about the same consistency

  • as olive oil.

  • Cholesterol is an amphipathic molecule, meaning, like phospholipids, it contains a hydrophilic

  • and a hydrophobic portion. Cholesterol's hydroxyl (OH) group aligns with the phosphate heads

  • of the phospholipids. The remaining portion of it tucks into the fatty acid portion of

  • the membrane.

  • Because of the way cholesterol is shaped, part of the steroid ring (the four hydrocarbon

  • rings in between the hydroxyl group and the hydrocarbon "tail") is closely attracted to

  • part of the fatty acid chain on the nearest phospholipid. This helps slightly immobilize

  • the outer surface of the membrane and make it less soluble to very small water-soluble

  • molecules that could otherwise pass through more easily.4

  • Without cholesterol, cell membranes would be too fluid, not firm enough, and too permeable

  • to some molecules. In other words, it keeps the membrane from turning to mush.

  • Cholesterol Helps Maintain the Fluidity of Cell Membranes

  • While cholesterol adds firmness and integrity to the plasma membrane and prevents it from

  • becoming overly fluid, it also helps maintain its fluidity.

  • At the high concentrations it is found in our cell's plasma membranes (close to 50 percent,

  • molecule for molecule) cholesterol helps separate the phospholipids so that the fatty acid chains

  • can't come together and cyrstallize.5

  • Therefore, cholesterol helps prevent extremes-- whether too fluid, or too firm-- in the consistency

  • of the cell membrane.

  • Cholesterol Helps Secure Important Proteins in the Membrane

  • The plasma membrane contains many proteins that perform important functions like channeling

  • or pumping substances into and out of the cell, attaching to other cells, forming borders

  • to keep other proteins in one specific part of the cell, communicating with nearby cells,

  • or responding to endocrine hormones from far-away cells.

  • Because certain proteins' size or shape requires a thicker phospholipid bed to sit in, and

  • because certain proteins need to stick together to function properly, the fluidity of the

  • cell membrane, where the molecules are constantly moving randomly, could pose a problem.

  • Fortunately, the plasma membrane contains many lipid rafts where proteins are secured. A lipid raft contains high concentrations

  • of cholesterol and sphingolipids-- a type of phospholipid-- containing longer and more

  • saturated fatty acid tails.

  • Because the fatty acids are longer and more saturated (straighter), they aggregate more,

  • which cholesterol also helps. That part of the membrane is also thicker, making it ideal

  • for accommodating certain proteins.6

  • Since the fatty acids in lipid rafts are longer, the phospholipids also move in sync with the

  • phospholipids on the other side of the membrane.

  • In the rest of the membrane, the phospholipids on one side of the membrane move independently of those on the

  • other.7

  • By stabilizing certain proteins together in lipid rafts, cholesterol is important to helping

  • these proteins maintain their function.

  • This could range from forming blood clots or

  • thinning blood, to allowing sugar into your cells, to burning fat, to regulating calcium

  • in your blood, and literally includes, in some way, most of the functions in your body,

  • although which proteins exist in lipid rafts and which do not

  • is still being researched.

  • It is the proteins, after all, by which cells communicate with one another. If cells didn't communicate with

  • one another, you and I

  • would be a

  • large pile of unrelated cells rather than

  • the individuals that we are.

Cholesterol is Abundant in Cell Membranes

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B2 membrane cholesterol fatty cell membrane lipid plasma

Cell Membrane Fluidity | Role of cholesterol

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    Amy.Lin posted on 2016/10/06
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function

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UK /'fʌŋkʃn/

  • noun
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molecule

US /ˈmɑlɪˌkjul/

UK /ˈmɒlɪkju:l/

  • noun
  • Two or more atoms chemically combined
integrity

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UK /ɪnˈtegrəti/

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portion

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UK /'pɔ:ʃn/

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  • To divide into portions; distribute.
  • verb
  • To separate something to divide among people
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fluid

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UK /'flu:ɪd/

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mass

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maintain

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communicate

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UK /kə'mju:nɪkeɪt/

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water

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UK /'wɔ:tə(r)/

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cell

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UK /sel/

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