The Phospolipid Bilayer

The Phospolipid BIlayer is aqueous environment inside and outside a cell prevents membrane lipids from escaping from the bilayer but nothing stops these molecules from moving about and changing places with one another within the plane of the bilayer. The membrane therefore behaves as a two dimensional fluid which is crucial for membrane function. Membranes consist largely of a lipid bilayer which is a double wall of phospoholipid, cholesterol and glycolipid molecules containing chains of fatty acids. Lipids give cell membranes a fluid character with a consistency approaching that of light oil. The fatty acid chains allow many small, fat soluble molecules such as oxygen to permeate the membrane, but they repel large water soluble molecules, such as sugar and electrically charged ions such as calcium

The top and bottom layers of the membrane have their stems facing each other. Proteins can stretch through the top bottom or both layers of phospolipid bilayer. Embedded in the lipid bilayer are large proteins many of which transport ions and water soluble molecules across the membrane. Some proteins in the plasma membrane form open pores called membrane channels which allow the free diffusion of ions into and out of the cell. Others bind to specific molecules on one side of membrane and in a process that is not clearly understood transport the molecules to the other side. Sometimes one protein simultaneously transports two types of molecules in opposite directions. Most plasma membranes are about 50 percent protein by weight while the membranes of some metabolically active organelles are 75 percent
                     
Phospolipid bilayer has 5 reasons for making it possible for the cell membrane to perform its job. Used to join cells together in cell adhesion Attach the membrane to the cytoskeleton Proteins gather together as enzymes and carry out different steps of metabolic reactions that take place near the cell membrane

Glycoproteins and Glycolipids in Animal and Plant Cells

The Glycolipids are located mainly in the plasma membrane and they are found only in the noncytosolic half of the bilayer. Their sugar groups therefore are exposed on the exterior of the cell where they form part of the protective coat of carbohydrate that surrounds most animal cells. This protective coat is the glycocalyx. Glycolipid molecules acquire their sugar groups in the Golgi apparatus.

Enzymes that add the sugar groups are confined to the inside of the Golgi apparatus so that the sugars are added to lipid molecules in the non cytosolic half of the lipid bilayer. Once a glycolipid molecule has been created in this way it remains trapped in this monolayer. as there are no flippases to transfer the glycolipid to the cytosolic side of the membrane.

There are Two broad types of glycolipids can be distinguished,

1. Glycerol Based

• Fatty acid chains attached to the glycerol molecule.
• A carbohydrate group linked to the 3rd carbon of glycerol with no bridging phosphate group
• Glycerol based glycolipids are the primary form in plants and bacteria

2. Sphingolipid Based

• These are based on the addition of carbohydrate units to the sphingolipid nucleus This type of glycolipid is the main form in animal cell membranes
• Simple glycolipids formed by the addition of a single sugar unit are called cerebrosides
• Addition of straight or branched sugar chains producesgangliosides. Carbohydrates added can be have considerablevariation in structure

Composition and Functions of Cell membranes

The Plasma membranes are made of lipids, proteins and carbohydrates

Lipids
Barrier separating the interior of the cell from its environment. Also act as a barrier between the solutions inside the cell separating contents of an organelle from the cell cytoplasm. For example: nucleus is surrounded by two layers of membranes that are actually extensions of the membrane surrounding the cell.  These nuclear membranes keep the DNA inside of the nucleus

Lipid molecules are called PHOSOPHOLIPIDS that made of fatty acids, glycerol, phosphate and hydrophilic organic derivative. Amphipathic one end of molecule is hydrophobic and the other side is hydrophillic) Fluid with the degree of un saturation of fatty acids determining the fluidity Barrier to polar molecules Basis for the cell signaling system

Proteins
The proteins within the plasma membrane are the functional part of the membrane allowing for transport of materials through the membrane AND sending and receiving signals to and from other cells

Basically these proteins can act as, pumps, gates, receptors, energy transducers and enzymes OR receptors for the endocytosis of material and cell cell signaling. The proteins associated with the outside surface of the lipid by layer are called EXTRINSIC PROTEINS. These can be easily removed

The proteins that are embedded in the membrane are called INTRINSIC PROTEINS. They can only remove with detergents that disrupt the cell membrane. Integral proteins also have a hydrophobic portion that spans the hydrophobic interior of the lipid bi layer. Some of these inner proteins also have INTEGRINS their job is to connect the outside proteins to the cytoskeleton inside the cell

Carbohydrates
Modify the lipid and protein molecules

CELL MEMBRANE (Plasma Membrne)

Cell membrane is possibly the most important organell in the cell. It holds the cell together  keeping everything intact. It is mobile and moves along paths that membranes follow. It is composed of a phosolipid bilayer

Basic Definition cell membrane
The cell membrane is the thin layer that forms the outer boundary of a living cell or of an internal cell compartment The outer boundary is the plasma membrane and compartments enclosed by internal membranes are called organelles. Cell membranes have a dual function
 (1) they both separate important but incompatible processes conducted in the organelles and keep toxic substances out of the cell
(2) they allow specific nutrients, wastes and metabolic products to pass between organelles and between the cell and the outside environment

Main Functions of cell membrane
•    Holds cell together
•    Controls whats goes out of cell
•    Controls what comes into cell
•    Manains homeostasis

The Endomembrane system

The Endomembrane system are consist of,

•    plasma membrane
•    nuclear envelope
•    endoplasmic reticulum
•    Golgi apparatus or Golgi bodies
•    vesicles
•    vaculoles
•    lysosomes

The membrane of all of these is composed of two layers of phospholipids with embedded proteins. Membrane has a consistency of a light oil allowing its membranes to diffuse throughout. Autogenous hypothesis states that the endomembrane system evolved from invagination of the plasma membrane

Double Helix DNA and Pentose Sugars

Double Helix DNA

DNA is double stranded helical model for DNA is shown in the graphic on the left. The easiest way to visualize DNA is as an immensely long rope ladder, twisted into a cork screw shape. Sides of the ladder are alternating sequences of deoxyribose and phosphate while the rungs of the ladder are made in two parts with each part firmly attached to the side of the ladder. Parts in the rung are heterocyclic amines held in position by hydrogen bonding. Then most DNA exists as open ended double helices, some bacterial DNA has been found as a cyclic helix. Some time DNA has also been found as a single strand.

Pentose Sugars in DNA
In nucleic acids two types of pentose sugars can be found. This difference is reflected in their names deoxyribonucleic acid indicates the presence of deoxyribose and ribonucleic acid indicates the presence of ribose

These sugars in the graphic on the left, the structures of both ribose and deoxyribose are shown. Note the red OH on one and the red H on the other are the only differences. The alpha and beta designations are interchangeable and are not a significant difference between these tow types

DNA Forensic and Testing

There are two main types of forensic DNA testing. RFLP and PCR based testing, although these terms are not very descriptive. Generally, RFLP testing requires larger amounts of DNA and the DNA must be underrated. Crime scene evidence that is old or that is present in small amounts is often unsuitable for RFLP testing. Warm moist conditions may accelerate DNA degradation rendering it unsuitable for RFLP in a relatively short period of time

PCR based testing often requires less DNA than RFLP testing and the DNA may be partially degraded, more so than is the case with RFLP. However, PCR still has sample size and degradation limitations that sometimes may be under appreciated. PCR based tests are also extremely sensitive to contaminating DNA at the crime scene and within the test laboratory. During PCR, contaminants may be amplified up to a billion times their original concentration. Contamination can influence PCR results, particularly in the absence of proper handling techniques and proper controls for contamination. PCR is less direct and somewhat more prone to error than RFLP. However, PCR has tended to replace RFLP in forensic testing primarily because PCR based tests are faster and more sensitive.

Structure and Replication of DNA

Bio Coach Module of DNA is designed to help you understand DNA structure and replication. As to solve problems, this will be reviewing the chemical structure of DNA and the process of DNA replication. Animations and interactive activities will enrich your review experience in a dynamic way. This module is designed to be a supplement to, but not a replacement for, your textbook and classroom notes. You can test your understanding of DNA structure and replication by using the Self Quiz at the end of the module

Importance of Protein to Cells

Proteins are organic compounds made of amino acids arranged in a linear chain and folded into a globular form. The amino acids in a polymer are joined together by the peptide bonds between the carboxyl and amino groups of adjacent amino acid residues. The sequence of amino acids in a protein is defined by the sequence of a gene, which is encoded in the genetic code. In general, the genetic code specifies 20 standard amino acids, however, in certain organisms the genetic code can include selenocysteine and in certain archaic pyrrolysine. Shortly after or even during synthesis, the residues in a protein are often chemically modified by post-translational modification, which alters the physical and chemical properties, folding, stability, activity, and ultimately, the function of the proteins. Proteins can also work together to achieve a particular function and they often associate to form stable complexes. Of the most distinguishing features of polypeptides is their ability to fold into a global state, or "structure". The extent to which proteins fold into a defined structure varies widely. Data supports that some protein structures fold into a highly rigid structure with small fluctuations and are therefore considered to be single structure. Other proteins have been shown to undergo large rearrangements from one conformation to another. This conformational change is often associated with a signaling event. Thus, the structure of a protein serves a medium through which to regulate either the function of a protein or activity of an enzyme. Not all proteins requiring a folding process in order to function as some function in an unfolded state

Biological macromolecules such as polysaccharides and nucleic acids, proteins are essential parts of organisms and participate in virtually every process within cells. Many proteins are enzymes that catalyze biochemical reactions and are vital to metabolism. Proteins also have structural or mechanical functions, such as acting and myosin in muscle and the proteins in the cytoskeleton, which form a system of scaffolding that maintains cell shape. Other proteins are important in cell signaling, immune responses, cell adhesion, and the cell cycle. Proteins are also necessary in animals' diets, since animals cannot synthesize all the amino acids they need and must obtain essential amino acids from food

Proteins may be purified from other cellular components using a variety of techniques such as ultracentrifugation, precipitation, electrophoresis, and chromatography; the advent of genetic engineering has made possible a number of methods to facilitate purification. Methods commonly used to study protein structure and function includes immunohistochemistry, site directed mutagenesis, nuclear magnetic resonance and mass spectrometry

Biosynthesis of Protein and Method Protein Biosynthesis

The Protein synthesis is the process in which cells build proteins. Term is sometimes used to refer only to protein translation but more often it refers to a multi step process, beginning with amino acid synthesis and transcription of nuclear DNA into messenger RNA, which is then used as input to translation.

Cistron DNA is transcribed into a variety of RNA intermediates. Last version is used as a template in synthesis of a polypeptide chain. Proteins can often be synthesized directly from genes by translating mRNA. When a protein needs to be available on short notice or in large quantities, a protein precursor is produced. Proportion is an inactive protein containing one or more inhibitory peptides that can be activated when the inhibitory sequence is removed by proteolysis during posttranslational modification.

A proportion is a form that contains a signal sequence that specifies its insertion into or through membranes. Signal peptide is cleaved off in the endoplasmic reticulum. Preproproteins have both sequences still present. Synthesis of protein, a succession of tRNA molecules charged with appropriate amino acids have to be brought together with an mRNA molecule and matched up by base pairing through their anti cordons with each of its successive cordons. Amino acids then have to be linked together to extend the growing protein chain, tRNAs, relieved of their burdens, have to be released. Whole complex of processes is carried out by a giant multi molecular machine, Ribosome, formed of two main chains of RNA, called ribosomal RNA (rRNA), and more than 50 different proteins. This molecular juggernaut latches onto the end of an mRNA molecule and then trundles along it, capturing loaded tRNA molecules and stitching together the amino acids they carry to form a new protein chain. Protein biosynthesis, although very similar, is different for prokaryotes and eukaryotes