Why does a eukaryotic cell have extensive internal membrane and many membrane bound organelles?

1.39.4 Concluding Remarks

Eukaryotic cells contain various organelles, and each organelle exerts its own function. As the capacity of each organelle is tightly regulated in accordance with cellular needs, by the signaling between the organelle and the nucleus, mass-producing recombinant proteins in eukaryotic cells requires ample knowledge of the mechanism regulating the capacity of each organelle. In particular, the UPR in the ER and the Golgi stress response is critical for the production of secretory proteins. By revealing the underlying mechanisms regulating the capacity of each organelle, it would become possible to regulate organelle function at will.

Abstract

In eukaryotic cells, genomic DNA is compactly stored in the nucleus by wrapping DNA around histone cores. The nuclear envelope (NE), composed of a double lipid bilayer and a structure called the nuclear lamina, encloses chromatin via the interaction between the nuclear lamina and chromatin.

The nuclear pore complex, a gateway for nucleocytoplasmic transport, is embedded in the NE. All eukaryotic cells contain specific nuclear mechanisms for reading the information contained in DNA compressed in chromatin. At the beginning of mitosis, the nuclear structure drastically changes to allow the precise inheritance of the genome by daughter nuclei.

Abstract

Eukaryotic cells contain membrane-bound compartments endowed with specific functions and characterized by the presence of specialized proteins. Their lipid membranes are generally complex in shape and in composition and further contribute to the identity and properties of each organelle; for example, to make the plasma membrane an efficient barrier or the endoplasmic reticulum a factory for lipid and protein synthesis. An important issue in cell biology is to understand the mechanisms that help to create and maintain the differences in lipid composition between cellular organelles despite the continuous exchange of components through membrane traffic.

Virus Infection

Eukaryotic cells internalize viruses by different routes, including clathrin-mediated endocytosis, caveolin-mediated endocytosis and macropinocytosis (Mercer et al., 2010). Vaccinia virus infects by triggering macropinocytosis (Mercer and Helenius, 2008). Other viruses which infect via macropinocytosis include echovirus 1 (Krieger et al., 2013), adenovirus (Amstutz et al., 2008), Flock House virus (Nakase et al., 2009), Ebola virus (Saeed et al., 2010), and HIV-1 (Liu et al., 2002; Marechal et al., 2001).

4 Eukaryotic Cells Are Subdivided Into Compartments

A eukaryotic cell has its genome inside a separate compartment, the nucleus. In fact, eukaryotic cells have multiple internal cell compartments surrounded by membranes. The nucleus itself is surrounded by a double membrane, the nuclear envelope, which separates the nucleus from the cytoplasm, but allows some communication with the cytoplasm via nuclear pores (Fig. 1.09). The genome of eukaryotes consists of 10,000–50,000 genes carried on several chromosomes. Eukaryotic chromosomes are linear, unlike the circular chromosomes of bacteria. Most eukaryotes are diploid, with two copies of each chromosome. Consequently, they possess at least two copies of each gene. In addition, eukaryotic cells often have multiple copies of certain genes as the result of gene duplication.

Eukaryotes possess a variety of other organelles. These are subcellular structures that carry out specific tasks. Some are separated from the rest of the cell by membranes (so-called membrane-bound organelles) but others (e.g., the ribosome) are not. The endoplasmic reticulum is a membrane system that is continuous with the nuclear envelope and permeates the cytoplasm. The Golgi apparatus is a stack of flattened membrane sacs and associated vesicles that is involved in secretion of proteins, or other materials, to the outside of the cell. Lysosomes are membrane-bound structures containing degradative enzymes and specialized for digestion.

All except a very few eukaryotes contain mitochondria (singular, mitochondrion; Fig. 1.10). These are generally rod-shaped organelles, bounded by a double membrane. They resemble bacteria in their overall size and shape. As will be discussed in more detail (see Chapter 4: Genes, Genomes, and DNA), it is thought that mitochondria are indeed evolved from bacteria that took up residence in the primeval ancestor of eukaryotic cells. Like bacteria, mitochondria each contain a circular molecule of DNA. The mitochondrial genome is similar to a bacterial chromosome, though much smaller. The mitochondrial DNA has some genes needed for mitochondrial function. Mitochondria also have ribosomes that structurally resemble typical prokaryotic ribosomes.

Mitochondria are specialized for generating energy by respiration and are found in all eukaryotes. (A few eukaryotes are known that cannot respire; nonetheless, these retain remnant mitochondrial organelles—discussed later.) In eukaryotes, the enzymes of respiration are located on the inner mitochondrial membrane, which has numerous infoldings to create more membrane area. This contrasts with bacteria, where the respiratory chain is located in the cytoplasmic membrane, as no mitochondria are present.

Chloroplasts are membrane-bound organelles specialized for photosynthesis (Fig. 1.11). They are found only in plants and some single-celled eukaryotes. They are oval- to rod-shaped and contain complex stacks of internal membranes that contain the green, light-absorbing pigment chlorophyll and other components needed for trapping light energy. Like mitochondria, chloroplasts contain a circular DNA molecule and their own ribosomes and are thought to have evolved from a photosynthetic bacterium.

Eukaryotic cells have extensive intracellular architecture to maintain their shape and move materials and organelles around the cells. The cytoskeleton is a complex network of filaments made of proteins like actin, vinculin, and fibronectin (Fig. 1.12). Besides maintaining cell shape, the cytoskeleton is important for cellular transport. For example, cytoskeletal fibers run through the long axons of neurons, and vesicles filled with neurotransmitters travel up and down the axon to facilitate the communication between the nucleus and the nerve fibers. The cytoskeleton also initiates cellular movements. By increasing the length of fibers on one side of the cell and decreasing their length on the opposite side, the cell can physically move. This is especially true for smaller single-celled eukaryotes and for movements during a multicellular organism’s development. Finally, these cytoskeletal movements are important to processes like cell division, since the very same fibers make up the spindle.

Life is modular. Complex organisms are subdivided into organs. Large and complex cells are divided into organelles.

Eukaryotes have many membrane-bound organelles to perform functions like respiration (mitochondria), enzyme degradation (lysosomes), and protein processing and secretion (Golgi apparatus and endoplasmic reticulum).

Eukaryotic cells have internal structural elements called a cytoskeleton.

10.3 Increase in Antioxidant Enzymes

Eukaryotic cells are constantly exposed to free radicals and have to defend themselves such that no deleterious effect is incurred by the macromolecules. The cells are equipped with the natural antioxidant molecules and the antioxidant enzymes which protect them against the free radical-induced damage. Bael leaf has also been reported to maintain the activities of the antioxidant enzymes, SOD, CAT, and GPx in mice (Singh et al., 2000) and to increase the levels of CAT, glutathione, SOD, and GPx in alloxan-treated diabetic rats (Sabu and Kuttan, 2004) and CCl4- and alcohol-induced hepatotoxicity (Khan and Sultana, 2009; Singanan et al., 2007).

Abstract

The eukaryotic cell comprises of a system of well compartmentalized but intimately communicating organelles and metabolic reaction schemes, derived from prokaryotic antecedents, surviving because of the transcendent complementarity of the functional systems, and bidirectional directives between systems, under the governance of the genome within the nucleus. Central to this is the cell membrane, which has emerged as a dynamic organizational entity involved in signaling and control not only at the surface but throughout the eukaryotic cell in the form of the numerous membrane-bound entities, degradative as well as synthetic. This role was not invented in the eukaryote, but is also well-illustrated in the fundamentally prokaryotic mitochondria. The abundance of the energy made available by the mutualism that led to the establishment of the mitochondria was certainly an important evolutionary step. Given the burgeoning information constantly emerging on cellular biology, we concentrate here upon a few systems of importance in Pharmacognosy.

Abstract

Eukaryotic cells have developed a litany of conserved mechanisms to deal with membrane injuries. The first line of defense consists of homeostatic regulation of membrane tension as a preventative measure against the occurrence of injury. When these measures fail, cells can engage in elaborate signaling mechanisms aimed at quickly restoring integrity. Based on the overall direction of membrane lipid trafficking, these repair mechanisms can be divided into three broad categories: exocytosis, endocytosis, and ectocytosis. For alveolar epithelial cells (AECs), repair of endogenous cell populations is especially important for the prevention of severe lung pathologies. We provide a focus on the pulmonary setting within this chapter while incorporating relevant findings from other cell types. We emphasize the signals and molecular moieties that have demonstrated critical involvement in the repair process within AECs and other cell types that constantly encounter threats to their membrane integrity.

Abstract

Eukaryotic cells are living organisms surrounded by a surface membrane. Inside cells, other membranes define intracellular organelles. The lipid bilayer of all membranes is impermeable to hydrophilic molecules (polar or charged); this is why membranes harbor proteins, called ion channels and carriers, catalyzing the life-requiring exchange of material between a cell and the external space, and between organelles and the cytoplasm. At variance from carriers, ion channels form aqueous pores crossing the lipid bilayer that allow the highly selective transmembrane passage of charged species, namely inorganic ions (e.g., Na+, K+, Ca2+, and Cl–), at high rate (up to tens of millions of ions per second can be transported by a single channel molecule). Ion channels also possess regulatory domains that open and close the pore upon an electric, or chemical, stimulus.

Deubiquitination

Eukaryotic cells also contain DUBs (deubiquitinating enzymes), which are encoded by the UCH (ubiquitin C-terminal hydrolase) and the UBP (ubiquitin-specific processing protease) gene families. UCHs are relatively small proteins (<40 kDa) and only four isoforms have been characterized in the human genome. UCHs mainly hydrolyze small amides and esters at the C-terminus of ubiquitin. In contrast, UBPs are 50–250 kDa proteins and constitute a large family; there are at least 63 distinct human UBPs. UBPs are involved in several biological processes, including the control of growth, differentiation, and genome integrity. In proteasome-dependent proteolysis, the putative major roles of DUBs are to maintain free ubiquitin levels by processing the products of ubiquitin genes, which all encode fusion proteins, and to recycle polyubiquitin degradation signals into free monomers. DUBs also deubiquitinate substrates erroneously tagged for degradation (proofreading), and keep 26S proteasomes free of polyubiquitin chains that can interfere with the binding of another substrate.