CELL MEMBRANE

CELL MEMEBRANE
The cell membrane is a semi periable membrane covering the cell and acting as a boundary.
THE STRUCTURE AND FUNTIONS;
It is made up a bilayer of phospholipid sandwiched between two layers of protein
PHOSPHOLIPID; This is a structure made up of a hydrophilic head facing the outside of the memebrane cover and a hydrophobic tail facing  the inside of the membrane. The hydrophilic head allows certain micromolecules such as urea, glucose and ions in, while the hydrophobic tail allows carbon(iv)oxide, oxygen and alchohol in.
PROTEIN;This is incorporated between the phospholipid mesh. Its of two types; those that cut through the phospholipid (integral protein) and those that just hang on to another protein (peripheral protein). The integral protein serves as structural channels for hydrophilic (water soluble) substances to pass through the membrane, as carriers for water soluble substances, as enzymes and as receptors for water soluble chemicals e.g peptide hormone. The peripheral protein mainly serve as enzymes and control the pathway of substances.
CARBOHYDRATES; They form short chain polysaccharides and incorporate with the proteins on the surface of the membrane to form glycoproteins- these help in cell recognition phenomenom, intercellular adhesion and attraction of substances to the surface of the membrane. They could also attach with the surface lipids forming glycolipids (glycans)-provide chemical and mechanical protection to the membrane
CHOLESTEROL; This is incorporated in the hydrophobic tail of the phospholipid to moderate the fluidity of the membrane
METHOD OF TRANSPORT ACROSS THE SURFACE OF THE CELL MEMBRANE
          The two major methods are Active and Passive transport
PASSIVE TRANSPORT; This includes diffusion, osmosis, fascilitated transport. Some of the features are
They depend on a concentration gradient
FOR FASCILITATED PASSIVE-
They also require concentration gradient
They involve larger hydrophilic substances e.g glucose, urea,ions
They require carriers
CARRIERS; these are substances to which molecule bind to specifically bur reversibly in  the manner of enzyme to substrate
ACTIVE TRANSPORT; this is a type of transport method that does not require a concentration gradient, infact they move against high concentration gradients. They make use of ATP, they may make use of carriers, occurs through dynamic protein pores
BULK TRANSPORT; This  involve the engulfing of large molecule or small particles by cytoplasmic extensions thus forming membrane-bound vacuoles inside the cytoplasm.
EQUILIBRIUM ACROSS MEMBRANE
          In the cell there are certain substances that cannot get out of the cell due to their sizes ( mainly proteins) nevertheless, their presence constitute an inbalance in the net charge of the cell. The proteins being negatively charged gives the cell an overall negative charge on the inside thereby repelling other negative charges around and attracting positive charges. There are also certain diffusible ions in and around the cell ( especially K+AND Cl-). Though most cells have their membranes being impermeable to Na+ very few still escape out by the help of ATP.
          As the negative net charge is established in the cell, it attracts positive ions (cations) which will be K+ into the cell to balance the charges but as expected that there should also be an efflux of the protein molecules, there is no efflux of the protein molecules because this pull out of the cell is overcome by the inward pull of the charge on the protein until a point of dynamic equilibrium is reached where the rate of attraction is exactly equal to the rate of diffusion. The K+ is being pulled in by the protein’s charge but forced out by its concentration inside, while the Cl- is being pushed out by the protein’s charged and forced in by its concentration outside. The Na+ is being transported outside by the aid of ATP.

EXTRACELLULAR AND INTRACELLULAR FLUID; DIFFERENCE
Extracellular fluid contains more Na+ ions than K+ , while intracellular fluid contains more K+ ions than Na+
Extracellular fluid contains more Cl- compared to the intracellular fluid
Extracellular fluid has more cations than anions because they have just monovalent ions(cations being more), while intracellular fluid has more anions especially abundant in polyvalent anaions ( proteins and DNA)