What Are Two Characteristics Of Eukaryotic Cells – Eukaryotic organisms include protozoa, algae, fungi, plants, and animals. Some Eukaryotic Cells are independent, single-celled microorganisms, while others are part of multicellular organisms. The cells of eukaryotic organisms have several distinctive characteristics. First of all, eukaryotic cells are defined by the presence of a nucleus surrounded by a complex nuclear membrane. Eukaryotic cells are also characterized by the presence of membrane-bound organelles in the cytoplasm. Organelles such as mitochondria, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, and peroxisomes are held in place by the cytoskeleton, an internal network that supports the transport of intracellular components and helps maintain cell shape (Figure 1). The genome of a eukaryotic cell is packaged into several rod-shaped chromosomes in contrast to the single round chromosome that characterizes most prokaryotic cells. Table 1 compares the characteristics of eukaryotic cell structures with those of bacteria and archaea.
Figure 1. Click to enlarge. Illustration of a generalized unicellular eukaryotic organism. Note that the cells of eukaryotic organisms vary greatly in structure and function, and an individual cell may not have all of the structures shown here.
What Are Two Characteristics Of Eukaryotic Cells
Eukaryotic cells exhibit a wide range of different cellular morphologies. Possible shapes include spheroidal, ovoid, cuboidal, cylindrical, planar, lenticular, fusiform, discoidal, crescentic, annular, stellate, and polygonal ( Figure 2 ). Some eukaryotic cells are irregular in shape, and some are able to change shape. The shape of a particular type of eukaryotic cell can be influenced by factors such as its primary function, organization of the cytoskeleton, viscosity of the cytoplasm, stiffness of the cell membrane or cell wall (if present), and the physical pressure exerted on it by its environment and/or neighboring cells.
Eukaryotic Cell Characteristics
. (credit a: modification of work by NOAA; credit b, e: modification of work by Centers for Disease Control and Prevention)
Figure 3. Eukaryotic cells have a well-defined nucleus. The nucleus of this mammalian lung cell is the large, dark, oval-shaped structure in the lower half of the image.
Unlike prokaryotic cells, in which DNA is loosely packed within the nucleoid region, eukaryotic cells have a nucleus that is surrounded by a complex nuclear membrane that houses the DNA genome ( Figure 3 ). Containing the cell’s DNA, the nucleus ultimately controls all of the cell’s activities, and plays an important role in reproduction and heredity. Eukaryotic cells usually have DNA organized into multiple linear chromosomes. The DNA in the nucleus is highly organized and condensed to fit inside the nucleus, which is accomplished by wrapping the DNA around proteins called histones.
Although most eukaryotic cells have only one nucleus, there are exceptions. For example, protozoa of the genus Paramecium usually have two complete nuclei: a small nucleus used for reproduction (micronucleus) and a large nucleus that controls cellular metabolism (macronucleus). Also, during sexual reproduction, some fungi temporarily form cells with two nuclei, called heterokaryotic cells. Cells whose nucleus divides and cytoplasm does not divide are called coenocytes.
Endosymbiotic Theory: How Eukaryotic Cells Evolve
Figure 4. In this fluorescence microscope image, all intermediate filaments were stained with a bright green fluorescent stain. The nuclear plate is an intense bright green ring around faint red nuclei.
The nucleus is bound by a complex nuclear membrane, often referred to as the nuclear envelope, which consists of two distinct lipid bilayers adjacent to each other ( Figure 4 ). Despite these connections between the inner and outer membranes, each membrane contains unique lipids and proteins on its inner and outer surfaces. The nuclear envelope contains nuclear pores, which are large rosette-shaped protein complexes that control the movement of materials into and out of the nucleus. The general shape of the nucleus is determined by the nuclear lamina, a network of intermediate filaments that lie just inside the nuclear envelope membranes. Outside the nucleus, additional intermediate filaments form a looser network and serve to anchor the nucleus in place within the cell.
The nucleolus is a dense region in the nucleus where ribosomal RNA (rRNA) biosynthesis occurs. Additionally, the nucleolus is also where ribosome assembly begins. Preribosomal complexes are assembled from rRNA and proteins in the nucleolus; they are then transported to the cytoplasm, where ribosome assembly is completed ( Figure 5 ).
Figure 5. (a) The nucleolus is the dark, dense region inside the nucleus. It is the site of rRNA synthesis and preribosomal assembly. (B) Electron micrograph showing the nucleolus.
Eukaryotic Cells Definition And Examples
Ribosomes found in eukaryotic organelles such as mitochondria or chloroplasts have 70S ribosomes—the same size as prokaryotic ribosomes. However, ribosomes not associated with organelles in eukaryotic cells are composed of 80S ribosomes, which are composed of a 40S small subunit and a 60S large subunit. They differ in size and composition from the ribosomes of prokaryotic cells.
Two types of eukaryotic ribosomes not associated with organelles are defined by their location in the cell: free ribosomes and membrane-bound ribosomes. Free ribosomes are in the cytoplasm and serve for the synthesis of water-soluble proteins; membrane-bound ribosomes are attached to the rough endoplasmic reticulum and produce proteins for insertion into the cell membrane or proteins destined for export from the cell.
The differences between eukaryotic and prokaryotic ribosomes are clinically important because some antibiotics are designed for one or the other. For example, cycloheximide targets eukaryotic action, while chloramphenicol targets prokaryotic ribosomes. Because human cells are eukaryotic, antibiotics that destroy prokaryotic ribosomes in bacteria usually do not harm them. However, negative side effects can sometimes occur because the mitochondria in human cells contain prokaryotic ribosomes.
The endomembrane system, unique to eukaryotic cells, is a series of membrane-bound tubules, vesicles, and flattened discs that synthesize many cellular components and move materials within the cell ( Figure 6 ). Because of their larger cell size, eukaryotic cells require this system to transport materials that cannot be dispersed by diffusion alone. The endomembrane system includes several organelles and connections between them, including the endoplasmic reticulum, Golgi apparatus, lysosomes, and vesicles.
Eukaryotic Cells [short]
Figure 6. The endomembrane system consists of a series of membrane-bound intracellular structures that facilitate the movement of materials across the cell and toward the cell membrane.
The endoplasmic reticulum (ER) is a connected series of tubules and cisternae (flattened sacs) with a single lipid bilayer ( Figure 7 ). The spaces inside the tanks are called the ambulance lumen. There are two types of ER, rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER). These two different types of ER are sites for the synthesis of distinctly different types of molecules. The RER is dotted with ribosomes bound to the cytoplasmic side of the membrane. These ribosomes make proteins destined for the plasma membrane (Figure 7). After synthesis, these proteins are inserted into the RER membrane. Small RER sacs containing these newly synthesized proteins then grow as transport vesicles and move either to the Golgi apparatus for further processing, directly to the plasma membrane, to the membrane of another organelle, or out of the cell. Transport vesicles are single-lipid, bilayered, membrane-bound spheres with a hollow interior that carry molecules. SER has no ribosomes and thus appears “smooth”. It is involved in the biosynthesis of lipids, carbohydrate metabolism and detoxification of toxic compounds in the cell.
Figure 7. The rough endoplasmic reticulum is studded with ribosomes for the synthesis of membrane proteins (which give it its rough appearance).
The Golgi apparatus was discovered in the endomembrane system in 1898 by the Italian scientist Camillo Golgi (1843–1926), who developed a new staining technique that showed folded membrane structures inside cells
Prokaryotic Vs. Eukaryotic Cells
, the causative agent of malaria. The Golgi apparatus consists of a series of membrane disks called dictyosomes, each with a single lipid bilayer, that are stacked together ( Figure 8 ).
Figure 8. Transmission electron micrograph (left) of the Golgi apparatus in a leukocyte. The illustration (right) shows cube-shaped, stacked discs and several transport vesicles. The Golgi apparatus modifies lipids and proteins, producing glycolipids and glycoproteins, respectively, which are normally inserted into the plasma membrane.
Enzymes in the Golgi apparatus modify lipids and proteins transported from the ER to the Golgi, often adding carbohydrate components to them, producing glycolipids, glycoproteins, or proteoglycans. Glycolipids and glycoproteins are often inserted into the plasma membrane and are important for signal recognition by other cells or infectious particles. Different cell types can be distinguished from each other by the structure and arrangement of glycolipids and glycoproteins contained in their plasma membranes. These glycolipids and glycoproteins usually also serve as cell surface receptors.
, face Proteins are processed in the Golgi apparatus, and then additional transport vesicles containing the modified proteins and lipids are pushed away from the Golgi apparatus at the exit or
Prokaryotic Vs Eukaryotic Cell: 9 Differences & Examples
, face These exit vesicles move to and fuse with the plasma membrane or the membrane of other organelles.
Exocytosis is the process by which secretory vesicles (spherical membranous sacs) release their contents to the outside of the cell (Figure 8). All cells have constitutive secretory pathways in which secretory vesicles transport soluble proteins that are constantly (constitutively) released from the cell. Some specialized cells also have regulated secretory pathways that are used to store soluble proteins in secretory vesicles. Regulated secretion involves substances that are released only in response to specific events or signals. For example, some cells of the human immune system (such as mast cells) release histamine
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