5.1: Characteristics of eukaryotic cells (2023)

learning target

• Identify and describe the unique structures and organelles of eukaryotic cells
• Compare and contrast similar structures found in prokaryotic and eukaryotic cells

Exercise \(\PageIndex{1}\)

Identify two differences between eukaryotic and prokaryotic cells.

Clinical Focus: Part 1

Sarah, 7, came home from school complaining of a large itchy spot on her arm. She kept scratching, which got her parents' attention. Looking closely, they see that it is a red circular dot with a raised red edge (Figure \(\PageIndex{2}\)).

Figure \(\PageIndex{2}\): Ringworm appears as a raised red ring on the skin. (Source: Centers for Disease Control and Prevention)

The next day, Sarah's parents took her to the doctor, who examined the area with a Wood lamp. Ultraviolet light from the Wood lamp made the spot on Sarah's arm fluoresce, confirming what doctors already suspected: Sarah had ringworm. Sarah's mother was ashamed to hear her daughter had "worms". How can this happen?

in the cell:

nuclear

Unlike prokaryotic cells, where DNA is loosely contained within nucleoid regions, eukaryotic cells possess a single nucleus (plural = nucleus) surrounded by a complex nuclear membrane housing the DNA genome (Fig.\(\PageIndex {3}\) ). By containing the cell's DNA, the nucleus ultimately controls all of the cell's activities and also plays an important role in reproduction and heredity. The DNA of eukaryotic cells is usually organized into multiple linear chromosomes. The DNA within the nucleus is highly organized and condensed to fit inside the nucleus by wrapping the DNA around proteins called histones.

Although most eukaryotic cells have only one nucleus, there are exceptions. For example, ProtozoaparameciumThere are usually two complete nuclei: a small nucleus used for reproduction (micronucleus) and a large nucleus (macronucleus) that directs the cell's metabolism. In addition, some fungi briefly form cells with two nuclei called heterokaryotes during sexual reproduction. A cell whose nucleus divides but whose cytoplasm does not divide is called a nuclear cell.

The nucleus is bounded by a complex nuclear envelope, commonly referred to as the nuclear membrane, which consists of two distinct lipid bilayers adjacent to each other (Fig. \(\PageIndex{4}\)). 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, 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, an intermediate silk mesh inside the nuclear envelope. Outside the nucleus, additional intermediate filaments form a looser network that serves to hold the nucleus in place within the cell.

Nucleolus

The nucleolus is a dense region in the nucleus where ribosomal RNA (rRNA) biosynthesis occurs. In addition, the nucleolus is also the site where ribosome assembly begins. How does it do this? Some chromosomes have segments of DNA that encode ribosomal RNA. The pre-ribosome complex is assembled from rRNA and protein into ribosomal subunits in the nucleolus; they are then transported to the cytoplasm where ribosome assembly is complete (Fig. \(\PageIndex{5}\)).

Ribosome

The cytoplasmic ribosome in eukaryotic cells is the 80S ribosome, which consists of a 40S small subunit and a 60S large subunit. This makes them distinct from prokaryotic ribosomes in terms of size and composition. Two types of cytoplasmic eukaryotic ribosomes are defined by their location in the cell: free ribosomes and membrane-bound ribosomes. Free ribosomes float in the cytoplasm and are used to synthesize water-soluble proteins; membrane-bound ribosomes attach to the cytoplasmic side of the rough endoplasmic reticulum and produce proteins that insert into the cell membrane or that are exported from the cell.

In contrast, ribosomes found in eukaryotic organelles such as mitochondria or chloroplasts have 70S ribosomes, the same size as prokaryotic ribosomes. The difference between eukaryotic and prokaryotic ribosomes is clinically relevant because some antibiotic drugs are designed to target one or the other. For example, cycloheximide acts against eukaryotes, while chloramphenicol acts against prokaryote ribosomes.¹Since human cells are eukaryotic, they are generally immune to antibiotics that destroy prokaryotic ribosomes in bacteria. However, negative effects can sometimes occur because the mitochondria in human cells contain prokaryotic ribosomes.

Intimal system

Unique to eukaryotic cells, the endomembrane system is a series of membrane tubes, vesicles, and flat disks that synthesize many cellular components and move material within the cell (Fig. \(\PageIndex{7}\)). Due to the large cell size of eukaryotic cells, this system is required to transport substances that cannot be dispersed by diffusion alone. The inner membrane system consists of multiple organelles and the connections between them, including the endoplasmic reticulum, Golgi apparatus, lysosomes, and vesicles.

endoplasmic reticulum

The endoplasmic reticulum (ER) is an interconnected array of tubules and cisterns (flat vesicles) with a single lipid bilayer (Figure \(\PageIndex{7}\)). The space inside the pool is called the ER cavity. 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 different types of molecules. RER is populated with ribosomes bound on the cytoplasmic side of the membrane. These ribosomes produce proteins that are transported to the plasma membrane (Figure \(\PageIndex{8}\)). After synthesis, these proteins are inserted into the RER membrane. RER vesicles containing these newly synthesized proteins then bud and move to the Golgi apparatus for further processing, directly to the plasma membrane, the membrane of another organelle, or outside the cell. SER has no ribosomes and thus appears "smooth". It is involved in the biosynthesis of lipids, the synthesis and degradation of carbohydrates, and the detoxification of drugs and poisons; and the storage of calcium ions.

Vesicles and Vacuoles

Vesicles and vacuoles are single-lipid, bilayer, hollow-internal membranous spheres with storage and transport functions. Aside from the fact that vacuoles are slightly larger than vesicles, there is one very subtle difference between them: the membrane of a vesicle can fuse with the plasma membrane or other membrane systems inside the cell. In addition, some substances, such as enzymes within plant vacuoles, break down macromolecules. The membranes of the vacuoles do not fuse with the membranes of other cellular components, which makes them good storage units.

Transport vesicles leaving the ER fuse with proteins, carbohydrates, and other substances or wastes where they are received by the Golgi apparatus, orcis, Face. Proteins are processed within the Golgi, and then additional transport vesicles containing modified proteins and lipids are pinched from the Golgi on their export, ortrans, Face. These efferent vesicles move to and fuse with the plasma membrane (via exocytosis) or the membrane of other organelles (Fig. \(\PageIndex{9}\)).

(Video) Overview of the Eukaryotic Cell

peroxisome

Christian de Duve is also credited with discovering peroxisomes, membrane-bound organelles not part of the endomembrane system (Fig. \(\PageIndex{10}\)). Peroxisomes form independently in the cytoplasm, synthesis of peroxisomes by free ribosomes and incorporation of these peroxisomes into existing peroxisomes. The growing peroxisomes then divide by a process similar to binary fission.

Peroxisomes were originally named for their ability to produce hydrogen peroxide, a highly reactive molecule that helps break down molecules such as uric acid, amino acids and fatty acids. Peroxisomes also have catalase, which degrades hydrogen peroxide. Along with SER, peroxisomes also play a role in lipid biosynthesis. As with lysosomes, the compartmentalization of these degradative molecules within the organelle helps to protect the cytoplasmic contents from unnecessary damage.

The peroxisomes of certain organisms are specialized for their specific functional needs. For example, glyoxysomes are modified peroxisomes of yeast and plant cells that perform a variety of metabolic functions, including the production of sugar molecules. Likewise, glycosomes are modified peroxisomes produced by certain trypanosomes, the pathogenic protozoa that cause Chagas disease and African sleeping sickness.

Cytoskeleton

Eukaryotic cells have an internal cytoskeleton composed of microfilaments, intermediate filaments and microtubules. This matrix of fibers and tubes provides structural support as well as a network through which materials can be transported within the cell and onto which organelles can be anchored (Fig. \(\PageIndex{11}\)). For example, the process of exocytosis involves the movement of a vesicle through a cytoskeletal network to the plasma membrane, where it can release its contents.

Microfilaments consist of two intertwined actin chains, each composed of actin monomers, forming filamentous cables with a diameter of 6 nm²(Figure \(\PageIndex{11}\)). Actin filaments work together with motor proteins such as myosin to affect muscle contraction in animals or amoeba movement in some eukaryotic microorganisms. In amoeba organisms, actin exists in two forms: a stiffer polymerized gel form and a more fluid unpolymerized soluble form. The gel-form actin creates stability in the cytoplasm, the gel-like region that resides within the plasma membrane of the amoeba protozoa. The forward flow of pseudopodia through soluble actin filaments, which then undergo a gel-sol cycle, creates temporary extensions of the plasma membrane of the cell called pseudopodia (meaning "false feet"), which result in cell motility. Once the cytoplasm extends outward, forming pseudopodia, the remaining cytoplasm flows upward to join the leading edge, creating forward motion. In addition to amoeboid motility, microfilaments are involved in a variety of other processes in eukaryotic cells, including cytoplasmic flux (the movement or circulation of cytoplasm within a cell), cleavage furrow formation during cell division, and muscle movement in animals (Fig. \(\page index{12}\)). These functions are a consequence of the dynamic nature of actin filaments, which can polymerize and depolymerize with relative ease in response to cellular signals, and their interactions with molecular motors in different types of eukaryotic cells.

Intermediate filaments (Fig. \(\PageIndex{13}\)) are a diverse group of cytoskeletal filaments that act as electrical cables within the cell. They are called "intermediates" because their 10 nm diameter is thicker than actin but thinner than microtubules.³They are composed of multiple strands of polymeric subunits, which in turn are composed of various monomers. Intermediate filaments tend to be more persistent in cells and maintain the position of the nucleus. They also form the nuclear lamina (lining or layer) inside the nuclear envelope. In addition, intermediate filaments play a role in anchoring cells together in animal tissues. The intermediate filament protein desmin is found in desmosomes, protein structures that link muscle cells together and help them resist external physical forces. The intermediate filament protein keratin is a structural protein found in hair, skin and nails.

Microtubules (Figure \(\PageIndex{14}\)) are the third type of cytoskeletal fibers composed of tubulin dimers (α-tubulin and β-tubulin). They form hollow tubes with a diameter of 23 nm that serve as beams within the cytoskeleton.⁴Like microfilaments, microtubules are dynamic, with the ability to assemble and disassemble rapidly. Microtubules also work with motor proteins such as dynein and kinesin to move organelles and vesicles within the cytoplasm. In addition, microtubules are the main components of eukaryotic flagella and cilia, constituting the fibril and matrix components (Fig. \(\PageIndex{21}\)). Microtubules are also involved in cell division, forming the mitotic spindle, which is used to separate chromosomes during mitosis and meiosis. The mitotic spindle arises from two centrosomes located at the ends of the cell, which are essentially microtubule organizing centers.

endosymbiotic theory

While scientists are making progress in understanding how cells function in plant and animal tissues, others are studying the structure of the cells themselves. In 1831, Scottish botanist Robert Brown (1773-1858) was the first to describe the nuclei he observed in plant cells. Then, in the early 1880s, German botanist Andreas Schimper (1856-1901) first described the chloroplasts of plant cells, identified their role in starch formation during photosynthesis, and showed that they divide independently of the nucleus.

Based on the ability of chloroplasts to reproduce independently, Russian botanist Konstantin Mereschkowski (1855-1921) proposed in 1905 that chloroplasts may have originated from ancestral photosynthetic bacteria that lived symbiotically in eukaryotic cells. He proposed a similar origin for plant cell nuclei. This is the first formulation of the endosymbiotic hypothesis and will explain how eukaryotic cells evolved from ancestral bacteria.

Mereskowski's endosymbiotic hypothesis was further developed by the American anatomist Ivan Wallin (1883-1969), who began to examine experimentally the similarities between mitochondria, chloroplasts, and bacteria, In other words, to test the endosymbiosis hypothesis through objective investigation. Wallin published a series of papers in support of the endosymbiosis hypothesis in the 1920s, including a 1926 publication with Mereschkowski. Wolin claimed that he could grow mitochondria outside eukaryotic host cells. Many scientists believe his mitochondrial cultures were caused by bacterial contamination. Dissenting scientists were supported by modern genome-sequencing work showing that much of the mitochondrial genome had been transferred to the nucleus of the host cell, preventing the mitochondria from surviving on their own.^{6 7}

Walling's ideas for the endosymbiotic hypothesis were largely ignored for the next 50 years because scientists didn't know that these organelles contained their own DNA. However, the endosymbiosis hypothesis was revived with the discovery of mitochondrial and chloroplast DNA in the 1960s. American geneticist Lynn Margulis (1938-2011) published his views on the endosymbiotic hypothesis of the origin of mitochondria and chloroplasts in 1967.⁸In the decade before her paper, advances in microscopy allowed scientists to distinguish prokaryotic cells from eukaryotic cells. Margulis reviews the literature in her publication and argues that eukaryotic organelles such as mitochondria and chloroplasts are of prokaryotic origin. She presents a growing body of microscopic, genetic, molecular biological, fossil and geological data to support her claims.

Again, this hypothesis was initially unpopular, but thanks to the advent of DNA sequencing, growing genetic evidence supports the endosymbiotic theory, which is now defined as the theory that mitochondria and chloroplasts are The result of establishing a symbiotic relationship within. Host (Figure \(\PageIndex{15}\)). As Margulis' original theory of endosymbiosis gained wide acceptance, she expanded on it in her 1981 book Symbiosis in Cellular Evolution. In it, she explains how endosymbiosis is a major driver of the evolution of organisms. Recent gene sequencing and phylogenetic analyzes have shown that mitochondrial and chloroplast DNA are highly related to their bacterial counterparts in DNA sequence and chromosome structure. However, mitochondrial DNA and chloroplast DNA are reduced compared to nuclear DNA because many genes have been transferred from the organelle to the nucleus of the host cell. Furthermore, mitochondrial and chloroplast ribosomes are structurally similar to bacterial ribosomes rather than to the eukaryotic ribosomes of their hosts. Finally, the binary fission of these organelles is very similar to that of bacteria in contrast to the mitosis that eukaryotic cells undergo. Since Margulis' original proposal, scientists have observed several examples of bacterial endosymbionts in modern eukaryotic cells. Examples include endosymbiotic bacteria found in the guts of certain insects, such as cockroaches,9 and photosynthetic bacteria-like organelles found in protists.¹⁰

mitochondria

Mitochondria (singular = mitochondria) are often referred to as the "powerhouses" or "energy factories" of the cell, as they are responsible for making the cell's main energy-carrying molecule, adenosine triphosphate (ATP). ATP represents the short-term stored energy of cells. Cellular respiration is the process of using the chemical energy in glucose and other nutrients to make ATP. In mitochondria, this process uses oxygen and produces carbon dioxide as a waste product. In fact, the carbon dioxide you exhale with each breath comes from a cellular reaction that produces carbon dioxide as a by-product (Figure \(\PageIndex{16}\)). The term "mitochondrion" was first coined by German microbiologist Carl Benda in 1898, and was later associated with the respiratory process by Otto Warburg in 1913.

Each mitochondria has two lipid membranes. The outer membrane is a remnant of the original host cell membrane structure. The inner membrane is derived from the bacterial plasma membrane. The electron transport chain of aerobic respiration uses integral proteins embedded in the inner membrane. The mitochondrial matrix, corresponding to the location of the protobacterial cytoplasm, is the current location of many metabolic enzymes. It also contains mitochondrial DNA and 70S ribosomes. Invasions of the inner membrane, called cristae, evolved to increase the surface area at which biochemical reactions are located. Different types of eukaryotic cell cristae have different folding patterns, which are used to distinguish different eukaryotes.

Figure\(\PageIndex{16}\): Each mitochondria is surrounded by two membranes, and the inside of the membrane is extensively folded into cristae, where the intermembrane space is located. The mitochondrial matrix contains mitochondrial DNA, ribosomes, and metabolic enzymes. Transmission electron micrograph of mitochondria on the right shows two membranes, including cristae and mitochondrial matrix. (Credit "Photomicrographs": Modified from the work of Matthew Britton; scale data via Matt Russell)

Chloroplast

Plant cells and algal cells contain chloroplasts, organelles where photosynthesis occurs (Figure \(\PageIndex{17}\)). Photosynthesis is a series of reactions that uses carbon dioxide, water and light energy to make glucose and oxygen. All chloroplasts have at least three membrane systems: outer membrane, inner membrane and thylakoid membrane system. Inside the outer and inner membranes is the chloroplast stroma, a gelatinous fluid that makes up most of the volume of the chloroplast, in which the thylakoid system floats. The thylakoid system is a highly dynamic collection of folded membrane vesicles. It is where the green photosynthetic pigment chlorophyll is found and the light reaction of photosynthesis occurs. In most plant chloroplasts, the thylakoids are arranged in piles called "grana" (singular: granum), while in some algal chloroplasts the thylakoids are free-floating.

Figure \(\PageIndex{17}\): Photosynthesis occurs in chloroplasts, which have outer and inner membranes. Stacks of thylakoids called grana form the third membranous layer.

Other organelles similar to mitochondria occur in other types of eukaryotes, but they function differently. Hydrogenosomes exist in some anaerobic eukaryotes and are the site of anaerobic hydrogen production. Protonosomes usually lack their own DNA and ribosomes. The kinetoplast is a variant of the mitochondria found in some eukaryotic pathogens. In these organisms, each cell has a single, long, branched mitochondria in which kinetoplasmic DNA is organized as multiple circular DNA fragments concentrated at one pole of the cell.

Boundary Layer: Cell Membrane

Extracellular

extracellular matrix

The cells of animals and some protozoa do not have cell walls to help maintain shape and provide structural stability. Instead, many of these types of eukaryotic cells produce an extracellular matrix for this purpose. They secrete a sticky substance of carbohydrates and proteins into the space between adjacent cells (Fig. \(\PageIndex{18}\)). Some protein components assemble into the basement membrane, and the rest of the extracellular matrix components adhere to the basement membrane. Proteoglycans generally form the bulk of the extracellular matrix, while fibrous proteins such as collagen provide strength. Both proteoglycans and collagen are attached to fibronectin, which in turn is attached to integrins. These integrin proteins interact with transmembrane proteins in the plasma membrane of eukaryotic cells that lack cell walls. This makes them both similar and different from the glycocalyx found in prokaryotic cells.

In animal cells, the extracellular matrix enables cells within tissues to withstand external stress and transmits signals from the outside of the cell to the inside. The amount of extracellular matrix is ​​quite widespread in the various types of connective tissue, and variations in the extracellular matrix can give different types of tissue their different properties. In addition, the extracellular matrix of host cells is often the site for microbial pathogens to attach to establish infection. For example,Streptococcus pyogenes,The bacteria that cause strep throat and various other infections bind to fibronectin in the extracellular matrix of the oropharynx (upper throat region).

flagella and cilia

Some eukaryotic cells use flagella for locomotion; however, eukaryotic flagella are structurally different from those found in prokaryotic cells. The prokaryotic flagellum is a rigid, rotating structure, while the eukaryotic flagellum is more like a flexible whip, consisting of nine pairs of parallel microtubules surrounded by a central pair of microtubules. This arrangement is called a 9+2 array (Figure \(\PageIndex{19}\)). The parallel microtubules move relative to each other using the dynein motor protein, causing the flagella to bend.

Cilia (singular: cilium) are similar external structures found in some eukaryotic cells. Cilia, characteristic of eukaryotes, are shorter than flagella and typically cover the entire surface of the cell. However, they are structurally similar to flagella (9+2 microtubule arrays) and use the same motility mechanism. At the base of each cilium and flagella is a structure called the basal body. The basal body helps connect the cilium, or flagella, to the cell and consists of a series of triplet microtubules that resemble centrioles but are embedded in the plasma membrane. Due to their short length, the cilia perform fast, flexible undulating movements. Cilia may have other functions besides locomotion, such as sweeping particles past or into cells. For example, ciliated protozoa use the sweeping of cilia to move food particles into their mouthparts, while ciliated cells in the airways of mammals beat in sync to sweep mucus and debris out of the lungs (Fig. \(\PageIndex{19}\ )).

Key Concepts and Summary

• A eukaryotic cell is defined by the presence ofnuclearContains a DNA genome and consists ofnuclear envelope(ornuclear envelope) consists of two lipid bilayers and regulates the transport of substances into and out of the nucleus through nuclear pores.
• Eukaryotic cell morphology varies widely and may be maintained by a variety of structures, including the cytoskeleton, cell membrane, and/or cell wall
• thisNucleolusLocated in the nucleus of eukaryotic cells, it is the site of ribosome synthesis and the first stage of ribosome assembly.
• Eukaryotic cells contain80S ribosomeIn the rough endoplasmic reticulum (membrane bound ribosome) and cytoplasmic (free ribosome). They contain 70s ribosomes in mitochondria and chloroplasts.
• Eukaryotic cells have evolvedIntimasystem, comprising membrane-bound organelles involved in trafficking. These include vesicles, endoplasmic reticulum, and Golgi apparatus.
• thissmooth endoplasmic reticulumPlays a role in lipid biosynthesis, carbohydrate metabolism and detoxification of toxic compounds. thisrough endoplasmic reticulumContains membrane-bound 80S ribosomes that synthesize proteins that are transported to the cell membrane
• thisgolgi apparatusProteins and lipids are processed, usually by adding sugar molecules, to produce glycoproteins or glycolipids, which are components of plasma membranes used for intercellular communication.
• molten bodyContains digestive enzymes to break down small particles ingestedendocytosis, ingested large particles or cellsPhagocytosis, and damage intracellular components.
• thisCytoskeleton, Composed by amicrofilament,intermediate filament, andmicrotubules, provide structural support for eukaryotic cells, and serve as a network for intracellular material transport.
• The central bodyIs an important microtubule organizing center in the formation of the mitotic spindle in mitosis.
• endosymbiotic theoryPoint out that mitochondria and chloroplasts (organelles found in many types of organisms) originated in bacteria. Significant structural and genetic information supports this theory.
• mitochondriaIs the site of cellular respiration. They have two membranes: an outer membrane and an inner membrane with cristae. The mitochondrial matrix is ​​located within the inner membrane and contains mitochondrial DNA, 70S ribosomes, and metabolic enzymes.
• ChloroplastPlants and some protists use them to help harvest solar energy and convert it into sugars for food, structural parts, or other uses.
• The plasma membrane of eukaryotic cells is structurally similar to prokaryotic cells, and membrane components move according to a fluid mosaic model. However, eukaryotic cell membranes contain sterols, which alter membrane fluidity, and glycoproteins and glycolipids, which help cells recognize other cells and infectious particles
• The cells of fungi, algae, plants, and some protists havecell wall,while the cells of animals and some protozoa are stickyextracellular matrixProvides structural support and mediates cell signaling.
• Eukaryotic flagella are structurally different from prokaryotic flagella but serve a similar purpose (motility).ciliaStructurally similar to eukaryotic flagella, but shorter; they are used for motility, feeding, or locomotion of extracellular particles.

footnote

1. A.E. Barnhill, M.T. Brewer, S.A. Carlson. "Antimicrobials exert adverse effects through predictable or specific inhibition of host mitochondrial components."Antimicrobials and Chemotherapy56 No. 8 (2012): 4046–4051.
2. Fox E, Cleveland DW. "Structural scaffolding of intermediate filaments in health and disease."science279 No. 5350 (1998): 514–519.
3. E. Fox, D.W. Cleveland. "Structural scaffolding of intermediate filaments in health and disease."science279 No. 5350 (1998): 514–519.
4. E. Fox, D.W. Cleveland. "Structural scaffolding of intermediate filaments in health and disease."science279 No. 5350 (1998): 514–519.
5. N. Allright, J.H.P. Hackstein. "Hydrogen bodies: one organelle, many origins."Biology55 No. 8 (2005): 657–658.
6. M. Duzik. "Protists." OpenStax CNX. November 27, 2013.http://cnx.org/contents/f7048bb6-e46...ef291cf7049c@1
7. J.M. Jaynes, L.P. Vernon. "Cyanelle'sparadox cyanobacteria: Almost cyanobacterial chloroplasts. "Trends in Biochemical Sciences7 no. 1 (1982): 22-24.

contributor

• Nina Parker (Shenandoah University), Mark Schneegurt (Wichita State University), Anh-Hue Thi Tu (Georgia Southwestern State University), Philip Lister (Central New Mexico Community College) and Brian M. Forster (Saint Joseph University) Wait for the contributing author. Original content from Openstax (CC BY 4.0; free access:https://openstax.org/books/microbiology/pages/1-introduction）