Membrane-bound organelles

Microtubules Determine the Intracellular Location, Shapes, and Dynamics of Membrane-Bound Organelles... 

Figure 3. Electron micrographs of myelinated axons of Xenopus laevis. Upper figure Cross section of axon showing microtubules in groups in association with membrane-bound organelles. Lower figure Longitudinal section of axon showing neurofilaments and microtubules in close proximity to membrane-bound organelles. (Courtesy of Dr. R. Smith.)...

Hepatic peroxisome proliferation, characterized by liver enlargement due to hyperplasia and hypertrophy, has been proposed as a basis for differences in species susceptibility to trichloroethylene carcinogenicity. Peroxisomes are membrane-bound organelles which contain enzymes generally involved in lipid metabolism. 

MTs serve multiple roles in neurons. Besides acting as the substrate for the transport of membrane-bounded organelles, MTs are necessary for the extension of neurites during development they provide the structural basis for maintaining neurites after extension and they also help maintain the definition and integrity of intracellular compartments. The diversity of these functions is reflected in differences in the biochemistry and metabolic stability of different MTs. 

Eukaryotic cells have evolved a complex, intracellular membrane organization. This organization is partially achieved by compartmentalization of cellular processes within specialized membrane-bounded organelles. Each organelle has a unique protein and lipid composition. This internal membrane system allows cells to perform two essential functions to sort and deliver fully processed membrane proteins, lipids and carbohydrates to specific intracellular compartments, the plasma membrane and the cell exterior, and to uptake macromolecules from the cell exterior (reviewed in [1,2]). Both processes are highly developed in cells of the nervous system, playing critical roles in the function and even survival of neurons and glia. 

Newly synthesized membrane and secretory proteins destined for the axon travel by fast anterograde transport. However, not all membrane-bounded organelles (MBOs) are destined for the axon. As a result, the first stage of transport must be synthesis, sorting and packaging of organelles (see Ch. 9). Once assembled, the organelle must then be committed to the transport... 

The most abundant organelles within the cytoplasm are the granules, which are membrane-bound organelles containing an array of antimicrobial proteins. As discussed above ( 2.4), three major types have been identified to date azurophilic, specific and gelatinase-containing granules. Additionally, newly-described structures called secretory vesicles have been identified. 

Lysosomes are membrane-bound organelles that contain hydrolytic enzymes to break down macromolecules and other organelles taken up by the lysosomes. The pH within this organelle is very low (about 5.0) and the catalytic activities of the enzymes, within it, are highest at this pH. The pH in the cytosol is about 7.1, so that any enzymes released from the lysosome are not catalyticaUy active in the cytosol. 

Enzymatically active, partially purified (washed) rubber particles can be isolated such that, when provided with an appropriate APP primer, magnesium ion cofactor, and IPP monomer, rubber is produced in vitro [253-255]. Fresh latex can be separated by centrifugation into three phases. The bottom fraction (20% of the latex) contains membrane-bound organelles. The middle fraction is called the C-serum. The top fraction phase contains the rubber particles. Biochemical smdies have established that latex in this fractionated form is unstable. These smdies also suggest that the bottom fraction is required for initiation of polymer synthesis. 

Topical eukaryotic cells (Fig. 1-7) are much larger than prokaryotic cells—commonly 5 to 100 pm in diameter, with cell volumes a thousand to a million times larger than those of bacteria. The distinguishing characteristics of eukaryotes are the nucleus and a variety of membrane-bounded organelles with specific functions mitochondria, endoplasmic reticulum, Golgi complexes, and lysosomes. Plant cells also contain vacuoles and chloroplasts (Fig. 1-7). Also present in the cytoplasm of many cells are granules or droplets containing stored nutrients such as starch and fat. 

Membrane-bounded organelles Absent Mitochondria, chloroplasts (in plants, some algae), endoplasmic reticulum, Golgi complexes, lysosomes (in animals), etc. 

For studies of membrane composition, the first task is to isolate a selected membrane. When eukaryotic cells are subjected to mechanical shear, their plasma membranes are torn and fragmented, releasing cytoplasmic components and membrane-bounded organelles such as mitochondria, chloroplasts, lysosomes, and nuclei. Plasma membrane fragments and intact organelles can be isolated by centrifugal techniques described in Chapter 1 (see Fig. 1-8). 

Plants must be especially versatile in their handling of carbohydrates, for several reasons. First, plants are autotrophs, able to convert inorganic carbon (as C02) into organic compounds. Second, biosynthesis occurs primarily in plastids, membrane-bounded organelles unique to plants, and the movement of intermediates between cellular compartments is an important aspect of metabolism. Third, plants are not motile they cannot move to find better supplies of water, sunlight, or nutrients. They must have sufficient metabolic flexibility to allow them to adapt to changing conditions in the place where they are rooted. Finally, plants have thick cell walls made of carbohydrate polymers, which must be assembled outside the plasma membrane and which constitute a significant proportion of the cell s carbohydrate. 

Much of the chemistry of the cell is common to all living systems and is directed towards ensuring growth and cell multiplication, or at least the survival of the cell. Organisms also share various structural characteristics. They all contain genetic material (DNA), membranes (the boundary material between the cell and the environment), cytoplasm (small particulate materials, ribosomes and enzyme complexes), and cell walls or surfaces (complex structures external to the membrane). In addition, there are various distinct membrane-bound organelles in eukaryotic organisms which have specialised functions within the cell (Tables S.4, 5.5 and 5.6)(8-, 7). 

Both the light and dark reactions occur in the chloroplast, a specialised membrane-bound organelle. For further details of this process the reader is referred to a general biochemistry textbook such as that by Leninger07 . 

As we will see, the evolutionary tree is bisected into a lower prokaryotic domain and an upper eukaryotic domain. The terms prokaryote and eukaryote refer to the most basic division between cell types. The fundamental difference is that eukaryotic cells contain a membrane-bounded nucleus, whereas prokaryotes do not. The cells of prokaryotes usually lack most of the other membrane-bounded organelles as well. Plants, fungi, and animals are eukaryotes, and bacteria are prokaryotes. The biochemical functions associated with organelles are frequently present in bacteria, but they are usually located on the inner plasma membrane. 

Prokaryote. A unicellular organism that contains a single chromosome, no nucleus, no membrane-bound organelles, and has characteristic ribosomes and biochemistry. 

Potentized homeopathic drugs are capable of producing effects on both prokaryotic and eukaryotic cells. Prokaryotic cells are usually smaller in size (1 - 10 pm) than eukaryotic ones (5 - 100 pm). Membrane-bound organelles like mitochondria, endoplasmic reticulum, Golgi complexes etc. are present in eukaryotic cells but absent in prokaryotic ones. While eukaryotic cells have nucleus containing DNA with histone and non-histone proteins in chromosoms, prokaryotic cells have no nucleus and their DNA with non-histone proteins lies in nucleoid without any membranous envelope. However, both types of cells are covered by plasma membrane with some common features. 

Eukaryotic cell Advanced cell type with a nuclear membrane surrounding genetic material and numerous membrane-bound organelles dispersed in a complex cellular structure see Eukaryotes. Volume 1(8). 

Figure 1.2 The cell is the basic unit of life. Above is a diagram of a eukaryote cell, the more complex of the two categories of cells. Unlike the more primitive prokaryotes, eukaryotes have nuclear membranes and membrane-bound organelles.

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