1. Cell membrane
Lecture5 week3

2. Every cell on Earth uses a membrane to separate and protect its chemical components from the outside environment.. Cell membranes act as selective barriers. (a) the plasma membrane separates a cell from the outside and is the only membrane in most bacterial cells. It enables the molecular composition of a cell to differ from that of the cell’s environment. (B) In eukaryotic cells, additional internal membranes enclose individual organelles. In both cases, the membrane prevents molecules on one side from mixing with those on the other

3. The plasma membrane is involved in cell communication, import and export of molecules, and cell growth and motility. (1) receptor proteins in the plasma membrane enable the cell to receive signals from the environment; (2) transport proteins in the membrane enable the import and export of small molecules; (3) the flexibility of the membrane and its capacity for expansion allow cell growth and cell movement

4. The most abundant lipids in cell membranes are the phospholipids, molecules in which the hydrophilic head is linked to the rest of the lipid through a phosphate group. The most common type of phospholipid in most cell membranes is phosphatidylcholine, which has the small molecule choline attached to a phosphate as its hydrophilic head and two long hydrocarbon chains as its hydrophobic tails.

5. All cell membranes are composed of lipids and proteins and share a common general structure. The lipids are arranged in two closely apposed sheets, forming a lipid bilayer. This lipid bilayer gives the membrane its basic structure and serves as a permeability barrier to most water-soluble molecules. The proteins carry out most of the other functions of the membrane and give different membranes their individual characteristics

6. Phospholipids and glycolipids are distributed asymmetrically in the plasma membrane lipid bilayer. Five types of phospholipid molecules are shown in different colours: phosphatidylcholine (red), sphingomyelin (brown), phosphatidylserine (light green), phosphatidylinositol (dark green), and phosphatidylethanolamine (yellow).
the glycolipids are drawn with blue hexagonal head groups to represent sugars. all of the glycolipid molecules are in the external monolayer of the membrane, while cholesterol (grey) is distributed almost equally in both monolayers.
phosphatidylinositol (not shown) is a minor lipid always found in the cytosolic monolayer of the plasma membrane, where it is used in cell signalling. Since its head group is an inositol sugar it is therefore an exception, being localized differently from all other glycolipids.
Essential cell biology, Alberts

7. The fatty parts – the lipids – make up about 50% of the total mass of the membrane.  The fatty acid chains can be either saturated or unsaturated, which accounts for some variety in membrane properties.  The saturated chains have no double bonds (all the carbons’ bonding capacities are used up by hydrogens), so the chains lie straight, pack tightly and interact much with each other.  The unsaturated chains form kinks due to double bonds. 
The kinks in unsaturated fatty acids makes them pack less tightly, and the greater amount of space makes for a more ‘fluid’ membrane.  Here, ‘fluid’ means it is easier for other things (like transmembrane proteins) to move in two dimensions along the plane of the membrane.

8. Although the lipid bilayer provides the basic structure of all cell membranes and serves as a permeability barrier to the molecules on either side of it, most membrane functions are carried out by membrane proteins. In animals, proteins constitute about 50% of the mass of most plasma membranes, the remainder being lipid plus the relatively small amounts of carbohydrate found on glycolipids and glycosylated proteins.

9. Essential cell biology, Alberts

10. Proteins can be associated with the lipid bilayer of a cell membrane in several ways. Many membrane proteins extend through the bilayer, with part of their mass on either side. these transmembrane proteins have both hydrophobic and hydrophilic regions. Their hydrophobic regions lie in the interior of the bilayer, nestled against the hydrophobic tails of the lipid molecules. Their hydrophilic regions are exposed to the aqueous environment on either side of the membrane.
Other membrane proteins are located entirely in the cytosol, associated with the inner leaflet of the lipid bilayer by an amphipathic a helix exposed on the surface of the protein. Some proteins lie entirely outside the bilayer, on one side or the other, attached to the membrane only by one or more covalently attached lipid groups.
Yet other proteins are bound indirectly to one or the other face of the membrane, held in place only by their interactions with other membrane proteins.
Proteins that are directly attached to a lipid bilayer—whether they are transmembrane, monolayer-associated, or lipid-linked—can be removed only by disrupting the bilayer with detergents, as discussed shortly. Such proteins are known as integral membrane proteins. The remaining membrane proteins are known as peripheral membrane proteins; they can be released from the membrane by more gentle extraction procedures that interfere with protein–protein interactions but leave the lipid bilayer intact.

11. Membrane proteins can associate with the lipid bilayer in several different ways. (a) transmembrane proteins can extend across the bilayer as a single a helix, as multiple a helices, or as a rolled-up b sheet (called a b barrel). (B) Some membrane proteins are anchored to the cytosolic surface by an amphipathic a helix. (C) Others are attached to either side of the bilayer solely by a covalent attachment to a lipid molecule (red zigzag lines). (D) Finally, many proteins are attached to the membrane only by relatively weak, noncovalent interactions with other membrane proteins.

12. Membrane Pores
When protein spans the membrane several times usually form pores that allow molecules to move back and forth through the membrane
Multiple  helix span membrane
Hydrophilic on the inside of the channel
Hydrophobic on the outer surface of the channel

13. Membrane Fluidity
There are multiple factors that lead to membrane fluidity. First, the mosaic characteristic of the membrane helps the plasma membrane remain fluid. The integral proteins and lipids exist in the membrane as separate but loosely-attached molecules.
The second factor that leads to fluidity is the nature of the phospholipids themselves:
The fatty parts – the lipids – make up about 50% of the total mass of the membrane.  The fatty acid chains can be either saturated or unsaturated, which accounts for some variety in membrane properties.  The saturated chains have no double bonds (all the carbons’ bonding capacities are used up by hydrogens), so the chains lie straight, pack tightly and interact much with each other.  The unsaturated chains form kinks due to double bonds. 
The kinks in unsaturated fatty acids makes them pack less tightly, and the greater amount of space makes for a more ‘fluid’ membrane.  Here, ‘fluid’ means it is easier for other things (like transmembrane proteins) to move in two dimensions along the plane of the membrane.

14. Membrane Fluidity
In animals, the third factor that keeps the membrane fluid is cholesterol. It lies alongside the phospholipids in the membrane and tends to dampen the effects of temperature on the membrane. Thus, cholesterol functions as a buffer, preventing lower temperatures from inhibiting fluidity and preventing higher temperatures from increasing fluidity too much. Cholesterol extends in both directions the range of temperature in which the membrane is appropriately fluid and, consequently, functional. Cholesterol also serves other functions, such as organizing clusters of transmembrane proteins into lipid rafts.

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16. Membrane fluidity
For all cells, membrane fluidity is important for many reasons:
It enables membrane proteins to diffuse rapidly in the plane of the bilayer and to interact with one another, as is crucial, for example, in cell signalling.
It permits membrane lipids and proteins to diffuse from sites where they are inserted into the bilayer after their synthesis to other regions of the cell.
It allows membranes to fuse with one another and mix their molecules, and it ensures that membrane molecules are distributed evenly between daughter cells when a cell divides.
If biological membranes were not fluid, it is hard to imagine how cells could live, grow, and reproduce.

17. In summary:
The cell membrane must be a dynamic structure if the cell is to grow and respond to environmental changes. To keep the membrane fluid at physiological temperatures the cell alters the composition of the phospholipids. The right ratio of saturated to unsaturated fatty acids keeps the membrane fluid at any temperature conducive to life.
Cell membranes consist of a continuous double layer—a bilayer—of lipid molecules in which proteins are embedded.
The lipid bilayer provides the basic structure and barrier function of all cell membranes.
Membrane lipid molecules have both hydrophobic and hydrophilic regions. They assemble spontaneously into bilayers when placed in water, forming closed compartments that reseal if torn.
There are three major classes of membrane lipid molecules: phospholipids, sterols, and glycolipids.
The lipid bilayer is fluid, and individual lipid molecules are able to diffuse within their own monolayer; they do not, however, spontaneously flip from one monolayer to the other.
The two layers of the plasma membrane have different lipid compositions, reflecting the different functions of the two faces of a cell membrane.
In animal cells cholesterol helps to prevent the packing of fatty acid tails and thus lowers the requirement of unsaturated fatty acids. This helps maintain the fluid nature of the cell membrane without it becoming too liquid at body temperature.

18. Some cells adjust their membrane fluidity by modifying the lipid composition of their membranes.
Membrane proteins are responsible for most of the functions of a membrane, such as the transport of small water-soluble molecules across the lipid bilayer.
Transmembrane proteins extend across the lipid bilayer, usually as one or more a helices but sometimes as a b sheet curved into the form of a barrel.
Other membrane proteins do not extend across the lipid bilayer but are attached to one or the other side of the membrane, either by noncovalent association with other membrane proteins or by covalent attachment to lipids.
Although many membrane proteins can diffuse rapidly in the plane of the membrane, cells have ways of confining proteins to specific membrane domains and of immobilizing particular proteins by attaching them to intracellular or extracellular macromolecules.
Many of the proteins and some of the lipids exposed on the surface of cells have attached sugars, which help protect and lubricate the cell surface and are involved in cell–cell recognition.