Bio 11

Structure of the plasma membrane plasma membrane:

A living cell is a self-reproducing system of molecules held inside a container, this container is the plasma membrane. Which is a two-ply sheet of lipid molecules, that is a fatty film so thin that it cannot be seen with a high microscope, into which proteins have been inserted. Every cell on Earth uses such a membrane to separate and protect its chemical components from the outside environment. Without membranes there would be no cell, and thus no life.

What are all cell membranes composed of?

All cell membranes are composed of lipids and proteins and share a common general structure.

Lipid bilayer:

Amphipathic molecules, are subject to two conflicting forces: the hydrophilic head is attracted to water, while the hydrophobic tails shun water and seek to aggregate with other hydrophobic molecules(like the fats in animals cells and the oils in plant seeds). This conflict is resolved by the formation of a lipid bilayer-and arrangement that is energetically favorable. With the hydrophilic heads facing water on both surfaces of the bilayer; and the hydrophobic tails being shielded from the water, as they lie next to one another in the interior.

What kind of membrane do eukaryotes have?

Eukaryotic cells have the plasma membrane as well as internal membranes that enclose intracellular compartments. The internal membranes form various organelles, including endoplasmic reticulum, Golgi apparatus, peroxisome, lysosome, transport vesicle, and two of these organelles are enclosed by two membranes, the mitochondria and nucleus.

What does cholesterol do to cell membranes?

In animal cells, membrane fluidity is modulated by the inclusion of the sterol cholesterol. This molecule is present in especially large amounts in the plasma membrane, where is constitutes about 20% of the lipids in the membrane by weight. Cholesterol molecules are short and rigid, so they fill the spaces between neighboring phospholipid molecules left by the kinks in their unsaturated hydrocarbon tails. So, cholesterol tends to stiffen the bilayer, making it less flexible, as well as less permeable.

Membrane Assembly Begins in the ER:

In eukaryotic cells, new phospholipids are manufactured by enzymes bound to the cytosolic surface of the endoplasmic reticulum. Using free fatty acids as substrates, the enzymes deposit the newly make phospholipids exclusively in the cytosolic half of the bilayer, but cell membranes still grow evenly. The transfer of mono lipids from one monolayer to the other is usually done by being catalyzed by enzymes called scramblases, which remove randomly selected phospholipids from one half of the lipid bilayer and insert them in the other. This scrambling causes the newly made phospholipids to be redistributed equally between each monolayer of the ER membrane. Some of this newly assembled membrane will remain in the ER; the rest will be used to supply fresh membrane to other compartments in the cell. Bits of membrane are continually pinching off the ER to form small spherical vesicles that then fuse with other membranes, such as those of the Golgi apparatus.

What does the plasma membrane do?

It serves as a barrier to prevent contents of the cell from escaping and mixing with the surrounding medium. It is penetrated by highly selective channels and transporters(proteins that allow specific, small molecules and ions to be imported and exported), which allow the exchange for nutrients to pass inward across the pasma membrane, and waste products to pass out, all of which enable the cell to grow. Other proteins in the membrane act as sensors, or receptors, that enable the cell to receive information about changes in its environment and respond to them in the correct way. The plasma membrane also grows and changes shape when the cell does, it does this by enlarging its area by adding new membrane without ever loosing its continuity, and it can deform without tearing. If a membrane is pierced, it neither collapses like a ballon nor remains torn; it reseals quickly.

Amphipathic:

Molecules with both hydrophilic and hydrophobic parts are called amphipathic.

What is the most common phospholipid in cell membranes?

Phosphatidylcholine, and it is build from five parts: they hydrophilic head, which consists of a choline linked to a phosphate group; two hydrocarbon chains, which form the hydrophobic tails; and a molecule of glycerol, which links the head to the tails. Each of the hydrophobic tails is a fatty acid-a hydrocarbon chain with a -COOH group at one end-which has been attached to glycerol via this group. A kink in one of the hydrocarbon chains occurs where there is a double bond between two carbon atoms. The "phosphatidyl" part of the name of a phospholipid refers to the phosphate-glycerol-fatty acid portion of the molecule.

Membrane lipids that are amphipathic:

Phospholipids, cholesterol(which is found in animal cell membranes), glycolipids(which have sugarer as part of they hydrophilic head).

Manufacturing of margarine:

The fats produced by plants are generally unsaturated and therefore liquid at room temperature, unlike animal fats such as butter or lard, which are generally saturated and therefore solid at room temperature. Margarine is made of hydrogenated vegetable oils their double bonds have been removed by the addition of hydrogen, so that they are more solid and buttermilk at room temperature.

The fluidity of a cell membrane:

The fluidity of a cell membrane is the ease with which its lipid molecules move within the plane of the bilayer, this important function has to be maintained within certain limits.

What does the fluidity of a lipid bilayer depend on?

The fluidity of a lipid bilayer depends on its phospholipid composition, and the nature of the hydrocarbon tails: the loser and more regular the packing of the tails. the more viscous and less fluid the bilayer will be. Two major properties of hydrocarbon tails affect how tightly they pack in the bilayer: their length and the number of double bonds they contain. A shorter chain length reduces the tendency of the hydrocarbon tails to interact with one another and therefore increases the fluidity of the bilayer. The hydrocarbon tails of membrane phospholipids vary in length between 14 and 24 carbon atoms, with 18-20 atoms being the most usual. Most phospholipids contain one hydrocarbon tails that has one or more double bonds between adjacent C atoms, and a second tail with single bonds. The chain that harbors a double bond does not contain the maximum number of H atoms that could be attached to its C backbone; and thus is said to be unsaturated with respect to H. The hydrocarbon tail with no double bonds has a full complement of H atoms and is said to be saturated. Each double bond in an unsaturated tail creates a small kink in the tail, which makes it more difficult for the tails to pack against one another, and therefore the lipid b players that contain a large proportion of unsaturated hydrocarbon tails are more fluid that those with lower proportions. In bacterial and yeast cells, which have to adapt to varying temperatures, both the lengths and the unsaturation of the hydrocarbon tails in the bilayer are constantly adjusted to maintain the membrane at a relatively constant fluidity: at higher temperatures, the cell makes membrane lipids with tails that are longer and that contain fewer double bonds.

Structure of lipids in a cell membrane:

The lipids in a cell membrane are arranged in two closely apposed sheets, forming a lipid bilayer. Which serves as a permeability barrier to most water-soluble molecules. Each single lipid molecule has a hydrophilic (water-loving) head and a hydrophobic (water-fearing) tail.

What are the most abundant lipids in cell membranes?

The most abundant lipids in cell membranes are the phospholipids, which has a phosphate-containing, hydrophilic head linked to a pair of hydrophobic tail. EX-phosphatidylcholine has the small molecule choline attached to a phosphate group as its hydrophilic head.

Lipid bilayers are self-sealing:

The same forces that drive the amphipathic molecules to form a bilayer help to make the bilayer self-sealing. Any tear in the sheet will create a free edge that is exposed to water, and this situation is energetically unfavorable, so the molecules of the bilayer will spontaneously rearrange to eliminate the free edge. If the tear is small, this spontaneous rearrangement will exclude the water molecules and lead to repair of the bilayer, restoring a single continuous sheet. If the tear is large, the sheet may begin to fold in on itself and break into separate closed vesicles, either way, the free edges are quickly eliminated. The only way a finite amphipathic sheet can avoid having free edges is to bend and seal, forming a boundary around a closed space, so phospholipids necessarily assemble into self-sealing containers that define close compartments.

What kind of membrane does the simplest bacteria have?

The simplest bacteria have only a single membrane, the plasma membrane.

Most cell membranes are asymmetrical:

The two halves of the bilayer often include very different sets of phospholipids. This asymmetry begins in the Golgi apparatus, this membrane contains another family of phospholipid-handling enzyme, called flippases. These enzymes remove specific phospholipids from the side of the bilayer facing the exterior space and flip them into the monolayer that faces the cytosol. The action of these flippases and similar enzymes in the plasma membrane initiates and maintains the asymmetric arrangement of phospholipids of the membranes in animal cells. This asymmetry is preserved as membranes bud from one organelle and fuse with another-or with the plasma membrane. This means that all cell membranes have distinct "inside" and "outside" faces: the cytosolic monolayer always faces the cytosol, while the noncytosolic monolayer is exposed to either the cell exterior-in the case of the plasma membrane- or to the interior space(lumen) of an organelle. This conversion applies not only to the phospholipids that make up the membrane, but to any proteins that might be inserted in the membrane, the positioning of proteins is crucial for their function.