What Is The Function Of Eukaryotic Cells – Eukaryotic organisms include protozoans, 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 unique characteristics. Most importantly, eukaryotic cells are defined by the presence of a nucleus surrounded by a complex nuclear membrane. Also, eukaryotic cells are characterized by the presence of membrane-bound organelles in the cytoplasm. Organelles such as mitochondria, the endoplasmic reticulum (ER), Golgi apparatus, lysosomes, and peroxisomes are maintained by the cytoskeleton, an internal network that supports the transport of intracellular components and helps maintain cell shape (Figure 1 ). The genome of eukaryotic cells is packaged in multiple rod-shaped chromosomes as opposed to the single circular chromosome that characterizes most prokaryotic cells. Table 1 compares the characteristics of eukaryotic cell structures with those of bacteria and archaea.
Figure 1. Click for larger image. A description of a general, single-celled eukaryotic organism. Remember that the cells of eukaryotic organisms vary greatly in terms of structure and function, and a particular cell may not have all the structures shown in it.
- 1 What Is The Function Of Eukaryotic Cells
- 2 Pdf] An Interactive Exercise To Learn Eukaryotic Cell Structure & Organelle Function
- 3 Comparing Prokaryotic And Eukaryotic Cells
What Is The Function Of Eukaryotic Cells
Eukaryotic cells exhibit a wide variety of cell morphologies. Possible shapes include spheroid, ovoid, cuboidal, cylindrical, flat, lenticular, fusiform, discoidal, crescent, ring stellate, and polygonal (Figure 2). Some eukaryotic cells are irregular in shape, and some have the ability to change shape. The shape of a particular type of eukaryotic cell can be influenced by factors such as its primary function, the organization of its cytoskeleton, the viscosity of its cytoplasm, the stiffness of its cell membrane or cell wall (if it has ), and the physical pressure exerted on it by the surrounding environment and/or neighboring cells.
Difference Between Eukaryotes And Prokaryotes [with Table]
. (credit a: NOAA work modification; credit b, e: Centers for Disease Control and Prevention work modification)
Figure 3. Eukaryotic cells have a well-defined nucleus. The nucleus of this mammalian lung cell is the large, dark, oval structure in the lower part of the image.
Unlike prokaryotic cells, where DNA is loosely contained in the nucleoid region, eukaryotic cells possess a nucleus, surrounded by a complex nuclear membrane that contains the DNA genome (Figure 3). By containing the DNA of the cell, the nucleus ultimately controls all the activities of the cell and also serves an important role in reproduction and heredity. Eukaryotic cells typically have their DNA organized into multiple linear chromosomes. The DNA inside 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, protozoans of the genus Paramecium usually have two complete nuclei: a small nucleus used for reproduction (micronucleus) and a large nucleus that directs cellular metabolism (macronucleus). Additionally, some fungi transiently form cells with two nuclei, called heterokaryotic cells, during sexual reproduction. Cells whose nuclei divide, but the cytoplasm does not, are called coenocytes.
Eukaryotic Cell Organelles
Figure 4. In this fluorescent microscope image, all intermediate filaments are stained with a bright green fluorescent stain. The nuclear lamina is the intense bright green ring around the faint red nuclei.
The nucleus is bound by a complex nuclear membrane, often called the nuclear envelope, which consists of two different lipid bilayers that are in contact with 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 in and out of the nucleus. The overall shape of the nucleus is determined by the nuclear lamina, a meshwork of intermediate filaments found just inside the nuclear envelope membranes. Outside the nucleus, additional intermediate filaments form a looser mesh and serve to anchor the nucleus in position within the cell.
The nucleolus is a dense region within the nucleus where ribosomal RNA (rRNA) biosynthesis occurs. In addition, the nucleolus is also the place where the assembly of ribosomes begins. Preribosomal complexes are assembled from rRNA and proteins in the nucleolus; they are then transported out of the cytoplasm, where ribosome assembly is completed (Figure 5).
Figure 5. (a) The nucleolus is the dark, dense area within the nucleus. It is the site of rRNA synthesis and preribosomal assembly. (b) Electron micrograph showing the nucleolus.
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Ribosomes found in eukaryotic organelles such as mitochondria or chloroplasts have 70S ribosomes—the same size as prokaryotic ribosomes. However, the nonorganelle-associated ribosomes in eukaryotic cells are 80S ribosomes, which consist of a 40S small subunit and a 60S large subunit. In terms of size and composition, it differs from the ribosomes of prokaryotic cells.
The two types of nonorganelle-associated eukaryotic ribosomes are determined by their location in the cell: free ribosomes and membrane-bound ribosomes. Free ribosomes are found in the cytoplasm and serve to synthesize water-soluble proteins; Membrane-bound ribosomes are found 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 relevant because some antibiotic drugs are designed to target one or the other. For example, cycloheximide targets eukaryotic action, whereas chloramphenicol targets prokaryotic ribosomes. Because human cells are eukaryotic, they are generally not harmed by antibiotics that destroy prokaryotic ribosomes in bacteria. However, sometimes negative effects can occur because the mitochondria in human cells contain prokaryotic ribosomes.
The endomembrane system, unique to eukaryotic cells, is a series of membranous tubules, sacs, and flattened disks that synthesize many cell components and transport materials within the cell (Figure 6). Because of their larger cell size, eukaryotic cells need this system to transport materials that cannot be spread by diffusion alone. The endomembrane system consists of several organelles and connections between them, including the endoplasmic reticulum, Golgi apparatus, lysosomes, and vesicles.
What Is An Animal Cell?
Figure 6. The endomembrane system consists of a series of membranous intracellular structures that facilitate the movement of materials throughout the cell and across the cell membrane.
The endoplasmic reticulum (ER) is an interconnected array of tubules and cisternae (flattened sac) with a single lipid bilayer (Figure 7). The spaces within the cisternae are called the lumen of the ER. 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 filled with ribosomes bound to the cytoplasmic side of the membrane. These ribosomes produce proteins destined for the plasma membrane (Figure 7). Following synthesis, these proteins are inserted into the RER membrane. Small RER sacs containing these newly synthesized proteins bud as transport vesicles and move 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, bilayer, membranous spheres with a hollow interior that transport molecules. SER has no ribosomes and, therefore, appears “smooth.” It is involved in the biosynthesis of lipids, carbohydrate metabolism, and detoxification of toxic compounds within the cell.
Figure 7. The rough endoplasmic reticulum is equipped with ribosomes for the synthesis of membrane proteins (which gives it its rough appearance).
The Golgi apparatus was discovered within the endomembrane system in 1898 by the Italian scientist Camillo Golgi (1843–1926), who developed a novel staining technique that showed stacked membrane structures within cells of
Pdf] An Interactive Exercise To Learn Eukaryotic Cell Structure & Organelle Function
, the causative agent of malaria. The Golgi apparatus consists of a series of membranous disks called dictyosomes, each with a single lipid bilayer, which are packed together (Figure 8).
Figure 8. A transmission electron micrograph (left) of a Golgi apparatus in a white blood cell. The illustration (right) shows cup-shaped, stacked disks and several transport vesicles. The Golgi apparatus transforms 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 moieties 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 types of cells can be distinguished from each other by the structure and arrangement of glycolipids and glycoproteins contained in their plasma membranes. These glycolipids and glycoproteins also typically serve as receptors on the cell surface.
, face. Proteins are processed within the Golgi apparatus, and then additional transport vesicles containing modified proteins and lipids are pinched off from the Golgi apparatus on its way out, or
Comparing Prokaryotic And Eukaryotic Cells
, face. These outgoing vesicles migrate and fuse with the plasma membrane or the membrane of other organelles.
Exocytosis is the process by which secretory vesicles (spherical membranous sac) release their contents outside the cell (Figure 8). All cells have a constitutive secretory pathway in which secretory vesicles carry soluble proteins that are continuously released from the cell (constitutively). Some specialized cells also have regulated secretory pathways, which are used to store soluble proteins in secretory vesicles. Controlled secretion involves substances that are released only in response to certain events or signals. For example, some cells of the human immune system (eg, mast cells) release histamine
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