CELL MEMBRANE

external image membr1.jpgMembrane Structure

The cell is highly organized with many functional units or organelles. Most of these units are limited by one or more membranes. To perform the function of the organelle, the membrane is specialized in that it contains specific proteins and lipid components that enable it to perform its unique roles for that cell or organelle. In essence membranes are essential for the integrity and function of the cell.
Membrane components may:
external image redball.jpg be protective
external image redball.jpg regulate transport in and out of cell or subcellular domain
external image redball.jpg allow selective receptivity and signal transduction by providing transmembrane that bind signaling molecules
external image redball.jpg allow cell recognition
external image redball.jpg provide anchoring sites for cytoskeletal filaments or components of the extracellular matrix. This allows the cell to maintain its shape and perhaps move to distant sites.
external image redball.jpg help compartmentalize subcellular domains or microdomains
external image redball.jpg provide a stable site for the binding and catalysis of enzymes.
external image redball.jpg regulate the fusion of the membrane with other membranes in the cell via specialized
external image redball.jpg provide a passageway across the membrane for certain molecules.
external image redball.jpg allow directed cell or organelle motility

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CELL MEMBRANE

The semipermeable membrane that encloses the cytoplasm of a cell.

The cell membrane is located around the outside of the cell. It is a protein lipid bilayer. The
hydrophilic heads of the lipids point outwards while the hydrophobic tails occupy the space between the two lipid layers. Several types of proteins are imbedded in the membrane: channel, transport, recognition, receptor, and electron transfer. Channel proteins provide passageways through the membrane for small substances to diffuse through. Transport proteins are involved in the active transport of substances across the membrane. Recognition proteins recognize other cells. Receptor proteins are receptor sites for hormones and other chemicals. Electron transfer proteins are involved in the transfer of electrons in processes like photosynthesis and cellular respiration. Because the proteins constantly shift throughout the cell membrane, it is referred to as a fluid mosaic model. The functions of the cell membrane include: holding cellular material, regulating the movement of materials across the membrane, providing a surface for many chemical reactions, and identifying the cell to the body's immune system.




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Plasma Membrane

All living cells, prokaryotic and eukaryotic, have a plasma membrane that encloses their contents and serves as a semi-porous barrier to the outside environment. The membrane acts as a boundary, holding the cell constituents together and keeping other substances from entering. The plasma membrane is permeable to specific molecules, however, and allows nutrients and other essential elements to enter the cell and waste materials to leave the cell. Small molecules, such as oxygen, carbon dioxide, and water, are able to pass freely across the membrane, but the passage of larger molecules, such as amino acids and sugars, is carefully regulated.


Cell Membrane

While the plant cell has a rigid cell wall, an animal cell membrane is a flexible lipid bilayer. The lipid molecules (mostly phospholipids) that make up the membrane have a polar, hydrophilic head and two hydrophobic hydrocarbon tails. When the lipids are immersed in an aqueous solution the lipids spontaneously bury the tails together and leave the hydrophilic heads exposed. Thus this is a handy membrane to use, because it can automatically fix itself when torn. There are three different major classes of lipid molecules - phospholipids, cholesterol, and glycolipids. Different membranes have different ratios of the three lipids.
What makes the membrane truly special is the presence of different proteins on the surface that are used for various functions such as cell surface receptors, enzymes, surface antigens, and transporters. Many of the membrane-associated proteins have hydrophilic and hydrophobic regions. The hydrophilic regions are used to help anchor the protein inside of the cell membrane. Some proteins extend across the lipid bilayer, others cross the bilayer several times
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Diagram of the cell membrane. The proteins are embedded inside of the cell membrane. The lipid content of the membrane allows the cell membrane to automatically repair itself when it is torn.

The Cell Membrane

external image mbrane1.jpgThe cell membrane is probably the most important organelle in the cell. It holds the cell together and keeps everything intact. It also regulates homeostasis which will be discussed later in this section. Cell membrane is mobile and moves along "grooves" which are set paths the membrane follow. The cell membrane is composed of a phospholipid bilayer, which has hydrophlic heads that are soluble in water and hydrophobic tails which are not soluble in water. The heads are lipid molecules with a phosphate group (PO4) on the end. The top and bottom layers of the membrane have their "stems" facing each other. There are also proteins, some with carbohydrate side chains, some without, in and through the membrane. These proteins can stretch though the top, bottom, or both layers together of the phospholipid bilayer. These proteins have many purposes that are nessesary for the cell to continue life functions. The cell membrane itself has 4 main functions. It holds the cell together, controls which substances go in and out of the cell, and maintains homeostasis.

Plasma Membrane Structure
Plasma Membrane Structure
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Membrane Transport of Small Molecules
Because of the hydrophobic interior of the lipid bilayer, polar molecules cannot enter the cell. However, cells devised means of transferring small polar molecules. Transport proteins, each specialized for a certain molecule, can transport polar molecules across the membrane. There are several types of membrane transport proteins. Uniports simply move solutes from one side to another. Cotransport systems work by simultaneously sending two solutes across the lipid bilayer. There are two types of cotransport systems - symport, in which the solutes are sent in the same direction, or antiport, in which they are sent in opposite directions. These transport proteins work passively, meaning that the cell doesn't have to expend energy sending the solute in or out. This is dependent on the solute moving in its natural direction - i.e. moving from more concentrated solution to less concentrated, or from positive to negative.
Some specific examples of transport membranes are channel proteins, which allow solutes to cross if they are the correct size and charge. Carrier proteins bind to the solute and lead it through the bilayer. These are examples of passive transport. To move a solute against their natural direction - for example higher concentration to lower concentration, energy (ATP) is needed to pump the solute in or out.
An example of active transport is the sodium-potassium pump, which in conjunction with the potassium leak channel, allows the cell the control it's membrane potential. The sodium-potassium-ATPase, which uses the energy of ATP hydrolysis, pump pumps sodium out and potassium in, which creates a high concentration of potassium inside the cell, and a low concentration outside. The reverse applies to the sodium. The potassium leak channel allows the potassium to leak out (so to even out the concentrations), which gives the cell and negative charge on the inside.
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external image total_membrane.gif


A picture of the cell membrane controlling what comes in and out of the cell.
A picture of the cell membrane controlling what comes in and out of the cell.

The cell membrane is a thin, double layer consisting of fatty lipids and protein that separates the cell interior from its surroundings, and controls what goes in and out of the cell. The cell membrane is the outermost layer of animal cells, but an additional covering--called the cell wall--encapsulates organisms such as bacteria, yeasts and plants.

Because maintaining an intact membrane is key to survival, scientists are looking closely at how cells repair outer membranes injured by wounds or infection and how membrane passageways work to balance the amount of vital substances inside and outside the cell.
The plastic bag represents the membrane
The plastic bag represents the membrane
When you think about a membrane, imagine it is like a big plastic bag with some tiny holes. That bag holds all of the cell pieces and fluids inside the cell and keeps any nasty things outside the cell. The holes are there to let some things move in and out of the cell.






Animal cell anatomy
Animal cell anatomy
Parts of a Cell Membrane

The Structure of the Cell Membrane
The cell membrane (or plasma membrane) surrounds all living cells, and is the cell's most important organelle. It controls how substances can move in and out of the cell and is responsible for many other properties of the cell as well. The membranes that surround the nucleus and other organelles are almost identical to the cell membrane. Membranes are composed of phospholipids, proteins and carbohydrates arranged in a fluid mosaic structure, as shown in this diagram.

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Cell Membrane
Structure - same as unit membrane.
Function - acts as a boundary layer to contain the cytoplasm (fluid in cell)
- interlocking surfaces bind cells together
- selectively permeable to select chemicals that pass in and
out of cells


Every cell is enclosed in a membrane, a double layre of phospholipids (lipid bilayer). Teh exposed heads of the bilayer are hydrophillic. However, the hidden tails of the phosopholipids are hydrophobic, so the cell membrane acts as a protective barrier to the uncontrolled flowof water. The membraneis mde more comlex by the presence of numerous proteins that are crucial to cell activity. These proteins include receptors for odors, tastes and hormones, as well as pores responsible for the controlled entry and exit of ions like sodium, potassium, calsium and chloride.
Illustration of an Eukaryotic cell membrane
The cell membrane (also called the plasma membrane, plasmalemma, or "phospholipid bilayer") is a selectively permeable lipid bilayer found in all cells.[1[[http://en.wikipedia.org/wiki/Cell_membrane#cite_note-MBOC-0|]]] It contains a wide variety of biological molecules, primarily proteins and lipids, which are involved in a vast array of cellular processes such as cell adhesion, ion channel

external image cellmembraen.jpgexternal image plantcell.gif