E.R., Golgi And Vesicles
E.R. And Vesicles
Thousands of structures with membranes crowd the eukaryotic cytoplasm, the most numerous being called the endoplasmic reticulum (ER), a dense network of membranes running through the cytoplasm of every cell and enclosing a network of interconnected tubes (cisternae), which form a connecting channel throughout the cytoplasm. ‘Endoplasmic’ means ‘inside the cytoplasm’, and ‘reticulum’ means ‘network’. The E.R. can make up a significant percentage of the overall volume of the cell. There are two types of ER: rough ER (RER), so-called because its cytoplasmic face is dotted with ribosomes, and smooth ER (SER), which is devoid of ribosomes. The RER is more extensive than SER and acts as a protein producer and storage vessel. Amino acid chains, freshly synthesised by the ribosomes, slide through tiny holes in its membrane into the cisternae, where they remain until they are sent elsewhere in the cytoplasm, or to the cell edge where they are secreted or incorporated into the cell membrane. The membranes of the ER usually form a tightly packed and flattened sac-like structures, that form inter-connected compartments within the cytoplasm. The internal space formed by the membrane sheets is known as the ER lumen. In most cells the ER lumen forms a single internal compartment. The membranes of other organelles are not connected to the ER and appear to form separate distinct compartments within the cytoplasm.
The ER membranes and lumen contain a large number and variety of enzymes which catalyse many different types of chemical reactions. In some cases the membranes serve as a framework for systems of enzymes that carry out sequential biochemical reactions. Other ER enzymes are located within the ER lumen.
If labelled amino acids are introduced into live cells, the radioactivity first appears in the ribosomes. Within a few minutes it is found in the membrane enclosed sacs of the rough ER. The reason for this is that proteins are produced at the ribosomes, threaded through the membrane, and are stored temporarily in the sacs of the rough ER before they are used inside the cell or are secreted to the exterior.
The SER produces steroid hormones and other lipids. The two types of endoplasmic network are not joined, so their cargoes do not get confused. RER connects directly with nuclear pores, through which mRNA molecules for protein manufacture pass. Cells that generate lots of protein, such as stomach cells that secrete digestive enzymes, possess an extensive RER. Likewise, cells in the gonads that produce steroid hormones have a large SER.
Proteins in the RER have a variety of possible destinations.
Many are shuttled to the cell's sorting and packaging factory, the Golgi apparatus, where they are prepared for secretion. As the proteins pass through each layer in the stack, moving progressively closer to the cell margins, they undergo chemical modification, usually involving the addition of sugar to form glycoproteins. Many cell secretions, such as mucus, are glycoproteins. As a result, the endothelial cells that secrete mucus in the lungs and cervix have well-developed Golgi apparatuses.
Proteins travel from the ER through the cisternae of the Golgi apparatus and to the cell edge in a unique manner. A section of endoplasmic tube containing the protein snaps off from the rest of the ER network and closes shut immediately at each end, imprisoning the protein in a vesicle which fuses with a nearby Golgi compartment, thus transferring its contents. The proteins move from one Golgi sac to the next in this way, and eventually protein-filled vesicles are pinched off, ready for secretion. These secretary vesicles often hang around near the cell boundary, waiting to be expelled. The process of secretion, in which the vesicle fuses with the outer membrane and releases its cargo to the outside world, is called exocytosis. Conversely, when the cell membrane swallows up materials from the outside, this is known as endocytosis.
Vesicles are small membrane bound sacs inside the cell which are used for transport between organelles, and for moving substances into and out of the cell. Modified vesicles called lysosomes are used to package digestive enzymes so that they cannot damage the cell interior.
The functions of the ER may be summarised thus:
1) Providing a large surface area for chemical reactions.
2) Providing a pathway for the transport of materials through the cell.
3) Producing proteins, especially enzymes. (Rough ER)
4)Producing lipids and steroids. (Smooth ER)
5)Collecting and storing synthesised material.
6) Providing a structural skeleton to maintain cellular shape. (E.g. The smooth ER of a rod cell from the retina of the eye)
The Golgi Body
The Golgi apparatus was first discovered in brain cells by Camillo Golgi towards the end of the 19th century. The Golgi apparatus is made up of between 6 and 20 saucer-shaped membranous sacs stacked on top of each other (Dictyosome), surrounded by tiny membranous containers or vesicles. In transverse section the Golgi apparatus often appears as closely packed , parallel curved pockets . The pockets are bounded by unit membranes and are called cisternae. Tiny vesicles arise from the edge of the cisternae. Some of these become lysosomes, some fuse with and enlarge the cell membrane, others carry secretions to the cell membrane for release to the exterior. Like the endoplasmic reticulum, the golgi body is well developed in cells whose secretions include glycoproteins. Mucus is a typical glycoprotein secreted by goblet cells which are very common in the respiratory and gastro-intestinal tracts of mammals.
Vesicles containing newly synthesised proteins break off from the rough ER and travel towards the Golgi apparatus where they fuse with its convex face. Here the proteins are ‘finished off’ and packaged before being exported form the cell. They may, for example, have carbohydrates added to them to form glycoproteins. The proteins are concentrated within the Golgi apparatus cisternae.
When the protein is ready, small vesicles break off from the concave face of the Golgi apparatus and move towards the surface of the cell. They fuse with the cell surface membrane and release their contents to the outside. The membranes of the vesicles, originally part of the rough ER membrane, become incorporated in the cell surface membrane.
All proteins pass through the Golgi apparatus in a strict order. First they go through the cis-Golgi network which returns to the ER, any proteins which have been wrongly exported. Then they pass through the stack of cisternae which modify the proteins and lipids undergoing transport and add labels which allow them to be identified and sorted at the next stage, the trans-Golgi network. Here the proteins and lipids are sorted, and then sent to their final destinations.
Cytologists, the biologists who make the cell their special study, have found that the Golgi apparatus sometimes determines the destination of its freight with the help of carbohydrate markers, which act as labels. For example, adding the sugar mannose-6-phosphate to a protein will ensure that it is sent to a particular organelle called a lysosome. Other sugars may direct a Golgi product elsewhere, such as the cell membrane. Most targeting is not, in fact, by sugars but by "signal sequences" in the protein chain. This form of intracellular labelling is called "protein targeting".