Figure 1 Rough endoplasmic reticulum in a mammalian cell
ER bound to ribosomes is called rough ER because the ribosomes appear as black dots on the ER in electron microscope photos, giving the ER a rough texture. These organelles are quite small, made up of 50 proteins and several long RNAs intricately bound together. Ribosomes have no membrane. Ribosomes disassemble into two subunits when not actively synthesizing protein.
As well as rough endoplasmic reticulum, there exists in a cell smooth endoplasmic reticulum, which does not have ribosomes bound to the outer membrane. The smooth ER is the location where synthesis of fatty acids and phospholipids takes place. There are far fewer smooth ER organelles in most mammalian cells than rough ER.
Both smooth and rough endoplasmic reticulum are made up of cisternae.
Proteins synthesised on the rough endoplasmic reticulum are secreted in small membrane bounded transport vesicles and bud to move to a structure called the Golgi complex. The Golgi processes and sorts proteins and lipids that come to it from vesicles that have bound to it. Of the three defined regions in the golgi, vesicles coming in from the rough ER fuse with the cis region.
Figure 2 Golgi complex
The Golgi itself is made up with a series of flattened membrane vesicles called cisternae which are surrounded by other spherical, membrane bound vesicles.
Proteins are sorted in the golgi for secretion or other functions such as membrane proteins and are then transported out of the organelle by a set of vesicles seeming to bud from the trans side of the complex.
Another very important cell organelle is the mitochondrion. Mitochondria are the sites of aerobic respiration, and generally are the major energy production centre in eukaryotes. Mitochondria have two membranes, an inner and an outer, clearly visible in this electron microscope photo of a mitochondrion. There are many reticulations, or many infoldings, of the inner membrane. This serves to increase the surface area of membrane on which membrane-bound reactions can take place. The existence of this double membrane has led many biologists to theorize that mitochondria are the descendants of some bacteria that was endocytosed by a larger cell billions of years ago, but not digested. This fascinating theory of symbiosis, which might lend an explanation to the development of eukaryotic cells, has additional supporting evidence. Mitochondria have their own DNA and their own ribosomes; and those ribosomes are more similar to bacterial ribosomes than to eukaryotic ribosomes.
Mitochondria “occupy up to 25% of the volume of the cytoplasm.”(Reference 1- Lodish et al, see references).They are the main sites of ATP production, the energy currency of cells.
Figure 3 an electron microscope view of a mitochondrion.
The mitochondrion, the second largest organelle with unique genetic structure, performs many functions, including the recycling of product of the cell’s metabolism, the formation of urea, transportation of proteins, and the synthesis of ATP.
Respiration of glucose occurs here, with 28 molecules of ATP being produced per glucose molecule in mitochondria.
Mitochondria are mostly individual organelles that operate on their own without much interaction with other organelles in the cytoplasm. It is extremely important in a cell that energy be extracted by the mitochondria, but the organelle doesn’t have to cooperate with others to do so.
The outer membrane is composed of about equal amounts of lipid and protein and contains porins, allowing large molecules in and out (up to 10000 molecular mass). Mitochondrial porin is a pore-forming protein also called VDAC which stands for voltage-dependent anion channel, and plays an important role in regulating flow of metabolites such as phosphate, organic anions, chloride and the adenine nucleotides through the membrane.
The inner membrane is much less permeable and has a higher proportion of protein than lipid, and is folded into cristae, increasing the surface area greatly. The larger surface area enables more ATP formation per mitochondrion, increasing the efficiency of the cell. There are two areas in the mitochondrion- the intermembrane space and the matrix, an area bounded by the inner membrane. “Oxidative phosphorylation takes place in the inner mitochondrial membrane, in contrast with most of the reactions of the citric acid cycle and fatty acid oxidation, which take place in the matrix”.(Reference 2- Stryer et al, see references)
As well as these larger, more important organelles described above, there are many smaller organelles with which the cell would function either much less efficiently or not at all.
Lysosomes are single membrane bound organelles which are the site of digestion. They contain digestive enzymes and are acidic compared to the cell cytoplasm. Their function in mammalian and other types of eukaryotic cell is to degrade unneeded cellular components along with some outside items.
Endosomes internalise plasma membrane proteins and soluble materials from the extracellular medium, and sort them back into membranes or to lysosomes for degradation.
Peroxisomes contain enzymes that oxidise compounds in the cell without producing ATP. The products of this are used in various biosynthetic reactions.
As well as structural compartmentation, the cytoplasm that contains them is fundamental to cellular functions. It acts as a solvent and transport medium for all the substances used in a cell, holds all of the organelles in place as well as enzymes and ribosomes, and is used for communication between different parts of the cell. In many cases it holds the shape of the cell to due pressure of water against the outside.
All of the membranes that hold the organelles, as well as the cell itself are comprised of proteins and phospholipids in different ratios in the fluid mosaic model seen below.
Figure 4 the fluid mosaic model of membranes in the cell
The entire cell is enclosed by a single cell membrane comprised of a fluid mosaic mix of proteins and phospholipids with carbohydrates that interact with the environment on the outer side of the membrane. The cell membrane provides a semi-permeable barrier and an alternative environment to the cytoplasm for organelles to be situated, providing anchorage sites. It can also detect changes in the extracellular environment.
Every mammalian cell serves a function for the structure or organ around it, and different cell functions require different amounts of each organelle. For example, a very metabolically active cell such as those in the muscles would require a large amount of mitochondria, and a pancreatic cell whose main function involves secretions would have many rough endoplasmic reticulum organelles. Some mammalian cells do not have certain organelles at all, but these are very specialised cells such as red blood cells and are not common, as most cells have basically the same organelle structures.
References:
Figure 1.
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Figure 2.
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Figure 3.
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Figure 4.
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Reference 1- Lodish et al in Molecular Cell Biology 5th ed, Freeman, 2003, p.171
Reference 1- Stryer et al in Biochemistry 5th ed, Freeman, 2003, p.493