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Cell Membrane


Cell membrane: is the outer boundary of the cell that is made up of a bilayer of phospholpids. It allows molecules to go in or out because of selective permeability.http://cellbio.utmb.edu/cellbio/membrane.htm
Question: What is the function of the cell membrane?
http://www.newton.dep.anl.gov/askasci/bio99/bio99393.htm
Answer: Many, many specific functions, but they can best be summarized this way:
the cell membrane forms a barrier between the inside of the cell and the outside,
so that the chemical environments on the two sides can be different. The cell controls
those differences so as to optimize the workings of the chemical machinery inside the
cytoplasm. The cell also causes brief changes in the internal environment by moving
stuff across the membrane in a controlled way. These changes are the ways in which
cell respond to events in the outside world.

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.

The cell membrane is a selectively permeable lipid bilayer found in all cells.
It contains a wide variety of molecules (proteins and lipids) which are involved in cell adhesion.
Membrane 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 [[http://cellbio.utmb.edu/cellbio/membran2.htm#Membrane Receptors|receptors]] 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 [[http://cellbio.utmb.edu/cellbio/membrane3.htm#tight junctions|junctions ]])
external image redball.jpg provide a passageway across the membrane for certain molecules, such as in [[http://cellbio.utmb.edu/cellbio/membrane3.htm#Gap Junctions|gap junctions]].
external image redball.jpg allow directed cell or organelle motility
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 conductance and cell signaling. The plasma membrane also serves as the attachment point for both the intracellular cytoskeleton and, if present, the extracellular cell wall.
The cell membrane is probably the most important organelle in the cell. It holds the cell together and keeps everything intact
http://www.stemnet.nf.ca/~dpower/cell/mbrane.htm

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.

We have been talking about cells being a unit of organization in biology. Let's look at the cell membrane and see how that membrane keeps all of the pieces inside. 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.
http://www.biology4kids.com/files/cell_membrane.html
Cell membrane is like a plastic bag
Cell membrane is like a plastic bag



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 [[http://cellbio.utmb.edu/cellbio/membran2.htm#Membrane Receptors|receptors]] 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 [[http://cellbio.utmb.edu/cellbio/membrane3.htm#tight junctions|junctions ]])
external image redball.jpg provide a passageway across the membrane for certain molecules, such as in [[http://cellbio.utmb.edu/cellbio/membrane3.htm#Gap Junctions|gap junctions]].
external image redball.jpg allow directed cell or organelle motility
http://cellbio.utmb.edu/cellbio/membrane.htm
cell membrane - the thin layer of protein and fat that surrounds the cell, but is inside the cell wall. The cell membrane is semipermeable, allowing some substances to pass into the cell and blocking others.
cell membrane: Every cell is enclosed in a membrane, a double layer of phospholipids (lipid bilayer). The exposed heads of the bilayer are "hydrophilic" (water loving), meaning that they are compatible with water both within the cytosol and outside of the cell. However, the hidden tails of the phosopholipids are "hydrophobic" (water fearing), so the cell membrane acts as a protective barrier to the uncontrolled flow of water. The membrane is made more complex 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 (Na+) potassium (K+), calcium (Ca++) and chloride (Cl-).
external image CellMembrane-full;init:.jpgexternal image CellMembrane.jpg


Function

The cell membrane surrounds the cytoplasm of a cell and, in animal cells, physically separates the intracellular components from the extracellular environment, thereby serving a function similar to that of skin. In fungi, some bacteria, and plants, an additional cell wall forms the outermost boundary; however, the cell wall plays mostly a mechanical support role rather than a role as a selective boundary. The cell membrane also plays a role in anchoring the cytoskeleton to provide shape to the cell, and in attaching to the extracellular matrix to help group cells together in the formation of tissues. The barrier is selectively permeable and able to regulate what enters and exits the cell, thus facilitating the transport of materials needed for survival. The movement of substances across the membrane can be either passive, occurring without the input of cellular energy, or active, requiring the cell to expend energy in moving it. The membrane also maintains the cell potential.
Specific proteins embedded in the cell membrane can act as molecular signals that allow cells to communicate with each other. Protein receptors are found ubiquitously and function to receive signals from both the environment and other cells. These signals are transduced and passed in a different form into the cell. For example, a hormone binding to a receptor could open an ion channel in the receptor and allow calcium ions to flow into the cell. Other proteins on the surface of the cell membrane serve as "markers" that identify a cell to other cells. The interaction of these markers with their respective receptors forms the basis of cell-cell interaction in the immune system.

Structure


Lipid bilayer

Diagram of the arrangement of amphipathic lipid molecules to form a lipid bilayer. The yellow polar head groups separate the grey hydrophobic tails from the aqueous cytosolic and extracellular environments.
The cell membrane consists primarily of a thin layer of amphipathic phospholipids which spontaneously arrange so that the hydrophobic "tail" regions are shielded from the surrounding polar fluid, causing the more hydrophilic "head" regions to associate with the cytosolic and extracellular faces of the resulting bilayer. This forms a continuous, spherical lipid bilayer.
The arrangement of hydrophilic heads and hydrophobic tails of the lipid bilayer prevent polar solutes (e.g. amino acids, nucleic acids, carbohydrates, proteins, and ions) from diffusing across the membrane, but generally allows for the passive diffusion of hydrophobic molecules. This affords the cell the ability to control the movement of these substances via transmembrane protein complexes such as pores and gates.
Flippases and Scramblases concentrate phosphatidyl serine, which carries a negative charge, on the inner membrane. Along with NANA, this creates an extra barrier to charged moities moving through the membrane.
Membranes serve diverse functions in eukaryotic and prokaryotic cells. One important role is to regulate the movement of materials into and out of cells. The phospholipid bilayer structure (fluid mosaic model) with specific membrane proteins accounts for the selective permeability of the membrane and passive and active transport mechanisms. In addition, membranes in prokaryotes and in the mitochondria and chloroplasts of eukaryotes facilitate the synthesis of ATP through chemiosmosis.

Illustration of an Eukaryotic cell membrane
Cell Membrane
: Every cell is enclosed in a membrane. The membrane is a double layer of lipids (lipid bilayer) but is made quite complex by the presence of numerous proteins that are important to cell activity. These proteins include receptors, pores, and enzymes. The membrane is responsible for the controlled entry and exit of ions like sodium (Na) potassium (K), calcium (Ca++). See the description of ion channels.

external image CellWall.jpg