Specializations of the plasma membrane:

Membrane Junctions?

Tight Junctions

Desmosomes

Gap Junctions

Speicalizations of the plasma membrane

Tight Junction?

Tight Junction- is a membrane junction where  a series of integral protein molecules in the plasma membranes of adjacent cells fuse together, forming an impermeable junction preventing molecules form passing through the extracellular space .

Ex:  tight junctions between epithelial cells linning the digestive track keep digestive enzymes and micro-organisms in the intestine form seeping into the bloodstream.

Specializations of the plasma membrane

Desmosomes?

(des' mo-somz)

Desmosomes- type of membrane junctions, are anchoring junctions on the cytoplasmic face of each plasma membrane are buttonlike thickening called a plaque. Adjacent cells are held together by thin linker protein filaments that extend from the plaques and interdigitate like the teeth of a zipper in the intercellular space. Not only binding neighboring cells together , also contribute to a continuous internal network of strong "guy-wires." This arrangement  distributes tension throughout a cellular sheet and reduces the chance of tearing when it is subjected to pulling forces.

EX: Desmosomes are abundant in tissues subjected to great mechanical stress, such as skin and heart muscle. 

Secializations of the plasma membrane

Gap Junction?

Gap junction- or nexus is a communiating junction between adjacent cells. At gap junctons the adjacent plasma membranes are very close,and the cells are connected by hollow cylinders called connexons. Connexons vary the selectivity of the gap junction channels- Ions, simple sugars, and other small molecules pass through these water-filled channels from one cell to the next.

EX: Gap junctions are present in electrically excitable tissues, such as the heart and smooth muscles, wher ion passage from cell to cell helps synchronize teir electrical activity and contraction.

What are the functions of the plasma membrane?

Passive Processes--Substances cross the membrane without any energy input from the cell.

Active Processes-- The cell provides the metabolic energy (ATP) needed to move substances across the membrane.

functions of the plasma membrane?

Passive Processes?

1.) Diffusion--is the tendency of molecules or ions to scatter evenly throughout the enviornment from areas of high concentration to areas wher their concentration is lower. 

A.) Simple Diffusion-- very small, Non-polar and lipid-soliuble substances diffuse directly through the lipid bilayer. EX: oxygen, carbon dioxiode ,and fat-soluble vitamins.

C.)Facilited diffusion-- certian molecules, notably glucose and other sugars, amino acids, and ions are trasnported passively even though they are unable to pass through the lipid bilayer via Channels, Carriers

i.) Channels-- transmembrane proteins that serve to transport substances, usually ions or water, through aqueous channels from one side of the membrane to the other. Binding  or association sites exist within the channels, and the channels are selective due to pore size and the charges of the amino acids lining the channel. Some channels called leakage channels are always open and simply allow in or water fluzes according to concentration gradients others are controlled by various chemical or electrical signals.

ii.)Carriers --is a transmembrane integral protein that shows specificity for molecules of a certian polar substance or class of substances that are too large to pass through membrane channels, such as sugars and amino acids. The process indicates that the changes in the shape of the carrier allow it to first envelop and then release the transported substance, sheilding it en route from the nonpolar reginons of the membrane 

2.)Osmosis -- The diffusion of a solvent , such a water through a selectively permeable membrane even though water is highly polar it passes through the lipid bilayer's gaps and freely and reversibly through water specific channels called aquaporins 

EX: particulary abundent in red blood cells and in cells involved in water balance  such a the kidney tubule cells.  

3.) Filtration-- is the process that forces water and solutes through a membrane or capillary wall by fluid, or hydrostalic, pressure. The gradient for filtration is a pressure gradient that pushes solute- containing fluid ( filtrate ) from a higher pressure area to a lower- pressure area. Filtration is not selective; only blood cells and protein molecules too large to pass through the membrane pores or the paracellular path (between cells) are held back. EX: hydrostatic pressure exerted by the blood forces fluid out of the capillaries, and this fulid contains solutes that are vital to the tissues it also provides the fluid ultimatley excreted by the kidneys as urine.

functions of the plasma membrane?

Active Process?

1.)Active Transport - active transporters or solute pumps  move solutes , most importantly ions "uphill" or against a concentration gradient using ATP.

A.) Primary acive transport -- the energy to do work comes directly form hydolysis of ATP. EX: sodium-potassium pump

B.) Secondary active transport-- transport is driven indirectly by energy stored in ionic gradients created by operation of primary active transport pumps and are all coupled systems, they move more than oue substance  at a time

a.)Symport system -- is when the two transported substances are moved in the same direction.

b.) Antiport system -- is when the transported substances "wave to each other"  as tey cross the membrane in opposite directions.

2.)Vesicular transport-- Large particles, macromolecules, and fluids are transported across plasma and intracellular membranes. Vesicular transport is the mechanism used for exocytosis (out of the cell), moving substances from the cells interior to extracellular space and endocytosis (within the cell), moving substances across the plasma membrane in the cell from the extracellular enviornment. 

a.)Excytosis -- This mechanism, typically stimulated by a cell-surface signal such  as binding of a hormone to  membrane recptor, accounts for hormone secretion, neurotransmitter release, mucus secretion, and in some cases, ejection of waste. The substance to be removed from the cell is first enclosed  in a membranous sac called a vesicle. in most cases the vesicle migrates to the plasma membrane fuses with it, and then reptures, spilling the sac  contents out of the cell.   

b.)Endocytosis-- 

i.)--phagocytosis --is the type of endocytosis  in which some relatively large or solid material, such as a clump of bacteria or cell debris is englfed by the cell,

ii.)--pinocytosis ,

iii.)--receptor mediated transport