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Why eukaryotes have three different types of cytoskeleton filaments?

Why eukaryotes have three different types of cytoskeleton filaments?


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Ok: IF, microtubules and microfilaments serve different purposes. But whats the biological advantage of having these three types over other organisms? I don't know if I'm being clear.


While eukaryotes contain tubulin-containing microtubles and actin filaments, prokaryotic homologues do exist. For example FitZ protein, the most common bacterial Tubulin homologue[1], is involved in cell division. Genes for this protein are actually present in eukaryotic nuclear DNA and the protein facilitate the division of the mitrochrondria and chloroplast organelles which were created by endosymbiosis[2].

Similarly, MreB, functions as the most common prokaryotic analogue of actin.

Regarding IF filaments, certainly subsets such as nuclear lamins are uniquely found within Eukarya, however a paper published by Bafchi[4] has suggested that IF-like proteins are present within bacteria.

  1. Bi, E., & Lutkenhaus, J. (1991). FtsZ ring structure associated with division in Escherichia coli. Nature, 354(6349), 161-164. doi:10.1038/354161a0
  2. Margolin W. FTSZ AND THE DIVISION OF PROKARYOTIC CELLS AND ORGANELLES. Nature reviews Molecular cell biology. 2005;6(11):862-871. doi:10.1038/nrm1745.
  3. Gunning, P. W., Ghoshdastider, U., Whitaker, S., Popp, D., & Robinson, R. C. (2015). The evolution of compositionally and functionally distinct actin filaments. Journal of Cell Science, 128(11), 2009-2019. doi:10.1242/jcs.165563
  4. Bagchi S, Tomenius H, Belova LM, Ausmees N. Intermediate filament-like proteins in bacteria and a cytoskeletal function in Streptomyces. Molecular Microbiology. 2008;70(4):1037-1050. doi:10.1111/j.1365-2958.2008.06473.x.

3 Main Classes of Protein Fibers in Cytoskeleton | Cell

Microtubules are long thread-like structures measuring about 25 nm in diameter their length varies up to several millimeters.

Microtubules have the following two main properties:

(i) Long and rigid shape, and

(ii) Capacity to generate movement.

Cytoplasmic tubules are similar to microtubules which form the backbone of centrioles, cilia, flagella and mitotic spindle.

The microtubules contain a hollow core of 15 nm diameter their outside diameter is 25 nm. The wall of a microtubule contains 13 filaments which are made up of the protein tubulin. There are two types of tubulins: α-tubulin and β-tubulin. The tubulins make a “dimer” of about 8 nm length and 110,000 Daltons M.W.

The tubulin dimers are arranged length-wise to produce proto-filament. A total of 13 protofilaments construct a microtubule. These protofilaments are arranged in a helical way with respect to the tubulin dimers (Fig. 2.25).

The regions from which the microtubules extend are called microtubule organization centres which are of different types, such as, basal bodies, kinetochores and centrosome. Polymerization of tubulin into microtubule requires the accessory proteins called MAP-1 and MAP-2 (microtubule associated proteins). These proteins (M.W. 300,000 Daltons) are related with the assembly and disassembly of the microtubules.

Protein Fibers in Cytoskeleton: Class # 2. Actin Filaments:

Actin filaments are composed of proteins and are related to the thin filaments of muscles. The monomeric protein is 43,000 Daltons in molecular weight. These actin molecules get polymerized into long filaments. Two such filaments are twisted around each other in a helical manner.

These filaments occur like individual organized filaments, regularly cross-linked filament bundles and less regularly cross-linked filaments. These filaments exert force to move a cell on surface or to change the shape of the cell internally. They interact with ‘myosin’ to generate the force.

Protein Fibers in Cytoskeleton: Class # 3. Intermediate Filaments:

Intermediate filaments are made up of proteins. They are of the following five types each type of filament is found in a particular type of cell:

(i) Neuro-filament, found in neuronal cell.

(ii) Keratin, found in epithelial or skin cells.

(iii) Vimentin, found in mesenchymal cells.

(iv) Desmin, found in myogenic (muscle) cells,

(v) GFAP, found in astroglial (brain) cells.

Each filament has a central region of more than 300 amino acids. It forms a rod with a- helical organisation. The N-terminal and C-terminal regions differ in the particular type of microfilament. Most of these filaments provide rigidity to cell shape.


Biology 171

By the end of this section, you will be able to do the following:

  • Describe the cytoskeleton
  • Compare the roles of microfilaments, intermediate filaments, and microtubules
  • Compare and contrast cilia and flagella
  • Summarize the differences among the components of prokaryotic cells, animal cells, and plant cells

If you were to remove all the organelles from a cell, would the plasma membrane and the cytoplasm be the only components left? No. Within the cytoplasm, there would still be ions and organic molecules, plus a network of protein fibers that help maintain the cell’s shape, secure some organelles in specific positions, allow cytoplasm and vesicles to move within the cell, and enable cells within multicellular organisms to move. Collectively, scientists call this network of protein fibers the cytoskeleton . There are three types of fibers within the cytoskeleton: microfilaments, intermediate filaments, and microtubules ((Figure)). Here, we will examine each.


Microfilaments

Of the three types of protein fibers in the cytoskeleton, microfilaments are the narrowest. They function in cellular movement, have a diameter of about 7 nm, and are comprised of two globular protein intertwined strands, which we call actin ((Figure)). For this reason, we also call microfilaments actin filaments.


ATP powers actin to assemble its filamentous form, which serves as a track for the movement of a motor protein we call myosin. This enables actin to engage in cellular events requiring motion, such as cell division in eukaryotic cells and cytoplasmic streaming, which is the cell cytoplasm’s circular movement in plant cells. Actin and myosin are plentiful in muscle cells. When your actin and myosin filaments slide past each other, your muscles contract.

Microfilaments also provide some rigidity and shape to the cell. They can depolymerize (disassemble) and reform quickly, thus enabling a cell to change its shape and move. White blood cells (your body’s infection-fighting cells) make good use of this ability. They can move to an infection site and phagocytize the pathogen.

To see an example of a white blood cell in action, watch white blood cell chases bacteria a short time-lapse of the cell capturing two bacteria. It engulfs one and then moves on to the other.

Intermediate Filaments

Several strands of fibrous proteins that are wound together comprise intermediate filaments ((Figure)). Cytoskeleton elements get their name from the fact that their diameter, 8 to 10 nm, is between those of microfilaments and microtubules.


Intermediate filaments have no role in cell movement. Their function is purely structural. They bear tension, thus maintaining the cell’s shape, and anchor the nucleus and other organelles in place. (Figure) shows how intermediate filaments create a supportive scaffolding inside the cell.

The intermediate filaments are the most diverse group of cytoskeletal elements. Several fibrous protein types are in the intermediate filaments. You are probably most familiar with keratin, the fibrous protein that strengthens your hair, nails, and the skin’s epidermis.

Microtubules

As their name implies, microtubules are small hollow tubes. Polymerized dimers of α-tubulin and β-tubulin, two globular proteins, comprise the microtubule’s walls ((Figure)). With a diameter of about 25 nm, microtubules are cytoskeletons’ widest components. They help the cell resist compression, provide a track along which vesicles move through the cell, and pull replicated chromosomes to opposite ends of a dividing cell. Like microfilaments, microtubules can disassemble and reform quickly.


Microtubules are also the structural elements of flagella, cilia, and centrioles (the latter are the centrosome’s two perpendicular bodies). In animal cells, the centrosome is the microtubule-organizing center. In eukaryotic cells, flagella and cilia are quite different structurally from their counterparts in prokaryotes, as we discuss below.

Flagella and Cilia

The flagella (singular = flagellum) are long, hair-like structures that extend from the plasma membrane and enable an entire cell to move (for example, sperm, Euglena, and some prokaryotes). When present, the cell has just one flagellum or a few flagella. However, when cilia (singular = cilium) are present, many of them extend along the plasma membrane’s entire surface. They are short, hair-like structures that move entire cells (such as paramecia) or substances along the cell’s outer surface (for example, the cilia of cells lining the Fallopian tubes that move the ovum toward the uterus, or cilia lining the cells of the respiratory tract that trap particulate matter and move it toward your nostrils.)

Despite their differences in length and number, flagella and cilia share a common structural arrangement of microtubules called a “9 + 2 array.” This is an appropriate name because a single flagellum or cilium is made of a ring of nine microtubule doublets, surrounding a single microtubule doublet in the center ((Figure)).


You have now completed a broad survey of prokaryotic and eukaryotic cell components. For a summary of cellular components in prokaryotic and eukaryotic cells, see (Figure).

Components of Prokaryotic and Eukaryotic Cells
Cell Component Function Present in Prokaryotes? Present in Animal Cells? Present in Plant Cells?
Plasma membrane Separates cell from external environment controls passage of organic molecules, ions, water, oxygen, and wastes into and out of cell Yes Yes Yes
Cytoplasm Provides turgor pressure to plant cells as fluid inside the central vacuole site of many metabolic reactions medium in which organelles are found Yes Yes Yes
Nucleolus Darkened area within the nucleus where ribosomal subunits are synthesized. No Yes Yes
Nucleus Cell organelle that houses DNA and directs synthesis of ribosomes and proteins No Yes Yes
Ribosomes Protein synthesis Yes Yes Yes
Mitochondria ATP production/cellular respiration No Yes Yes
Peroxisomes Oxidize and thus break down fatty acids and amino acids, and detoxify poisons No Yes Yes
Vesicles and vacuoles Storage and transport digestive function in plant cells No Yes Yes
Centrosome Unspecified role in cell division in animal cells microtubule source in animal cells No Yes No
Lysosomes Digestion of macromolecules recycling of worn-out organelles No Yes Some
Cell wall Protection, structural support, and maintenance of cell shape Yes, primarily peptidoglycan No Yes, primarily cellulose
Chloroplasts Photosynthesis No No Yes
Endoplasmic reticulum Modifies proteins and synthesizes lipids No Yes Yes
Golgi apparatus Modifies, sorts, tags, packages, and distributes lipids and proteins No Yes Yes
Cytoskeleton Maintains cell’s shape, secures organelles in specific positions, allows cytoplasm and vesicles to move within cell, and enables unicellular organisms to move independently Yes Yes Yes
Flagella Cellular locomotion Some Some No, except for some plant sperm cells
Cilia Cellular locomotion, movement of particles along plasma membrane’s extracellular surface, and filtration Some Some No

Section Summary

The cytoskeleton has three different protein element types. From narrowest to widest, they are the microfilaments (actin filaments), intermediate filaments, and microtubules. Biologists often associate microfilaments with myosin. They provide rigidity and shape to the cell and facilitate cellular movements. Intermediate filaments bear tension and anchor the nucleus and other organelles in place. Microtubules help the cell resist compression, serve as tracks for motor proteins that move vesicles through the cell, and pull replicated chromosomes to opposite ends of a dividing cell. They are also the structural element of centrioles, flagella, and cilia.

Free Response

What are the similarities and differences between the structures of centrioles and flagella?

Centrioles and flagella are alike in that they are made up of microtubules. In centrioles, two rings of nine microtubule “triplets” are arranged at right angles to one another. This arrangement does not occur in flagella.

How do cilia and flagella differ?

Cilia and flagella are alike in that they are made up of microtubules. Cilia are short, hair-like structures that exist in large numbers and usually cover the entire surface of the plasma membrane. Flagella, in contrast, are long, hair-like structures when flagella are present, a cell has just one or two.

Describe how microfilaments and microtubules are involved in the phagocytosis and destruction of a pathogen by a macrophage.

A macrophage engulfs a pathogen by rearranging its actin microfilaments to bend the plasma membrane around the pathogen. Once the pathogen is sealed in an endosome inside the macrophage, the vesicle is walked along microtubules until it combines with a lysosome to digest the pathogen.

Compare and contrast the boundaries that plant, animal, and bacteria cells use to separate themselves from their surrounding environment.

All three cell types have a plasma membrane that borders the cytoplasm on its interior side. In animal cells, the exterior side of the plasma membrane is in contact with the extracellular environment. However, in plant and bacteria cells, a cell wall surrounds the outside of the plasma membrane. In plants, the cell wall is made of cellulose, while in bacteria the cell wall is made of peptidoglycan. Gram-negative bacteria also have an additional capsule made of lipopolysaccharides that surrounds their cell wall.

Glossary


Eukaryotes

Eukaryotes are organisms made up of one or more cells that possess a membrane-bound nucleus (that houses DNA) as well as membrane-bound organelles. In contrast to prokaryotes, DNA is organized into linear chromosomes. Eukaryotic organisms may be multicellular or single-celled organisms. Unicellular eukaryotes are known as protists (which include amoeba, algae and dinoflagellates). Protists tend to be much larger (10-1000x) than prokaryotes, and like all eukaryotes have a true nucleus and organelles. Within Eukarya, multicellularity arose at least three times giving rise to modern day plants, animals, fungi.

Origin of the endomembrane hypothesis

Eukaryotes are thought to have emerged from an ancestral archaean, due to their close proximity suggested by Carl Woese's phylogeny (Fig. 1). While all prokaryotes have circular DNA, all eukaryotes have linear DNA. Therefore, the cell that is ancestral to all eukaryotes must have had linear DNA. At sometime in the past, the ancestral archaean, circular DNA was cut and made linear. This actually posed a significant issue in DNA replication, due to issues of chromosome shortening. This was resolved with the emergence of telomerase, of a protein/RNA hybrid, that allowed the ends of the chromosomes to be fully replicated.

The predominant nuclear origination hypothesis suggests infoldings of the cellular membrane (also called the plasma membrane) of an ancestral archaean (Fig. 3a). Imagine a group of brown paper bags. If you began to compress them together, the paper would begin to fold in on itself. This is a good metaphor for the infolding hypothesis. Evidence to supports this comes from the presence of such infoldings in prokaryotic species that multiply in constrained areas (Fig. 3b). Unlike a paper bag, the cellular membrane is composed of phospholipids, which are capable of detaching from the outer membrane. The nuclear origin hypothesis suggests that the detached infoldings condensed and encased the DNA as the phospholipids reattached to each other (Figure 3), forming nuclear membrane and the endoplasmic reticulum, which form a contiguous system, with the these organelles physically connected to each other. The nucleus and the endoplasmic reticulum are a part of the endomembrane system, within eukaryotic cells, which also include the Golgi apparatus, vesicles and lysosomes (Fig. 4). These organelles are thought to have emerged in a similar fashion (possibly synonymously) as the nucleus and endoplasmic reticulum, but are separate from the nucleus and endoplasmic reticulum.

Figure 3. Origin of the endomembrane hypothesis. a) Eukaryotes emerged from an ancestral archaean cell. DNA is some prokaryotes are concentrated within the cell, in an area known as a nucleoid. b) The cell membrane of the ancestral archaean cell began to infold on itself forming plasma membrane within the cell. We see this in certain species of bacteria, and in some cases these infolding isolate the nucleiod.. c) The first true nucleus formed with the infoldings separated from the cell membrane, and encased the chromosomes, creating a nuclear envelope. The endoplasmic reticulum is physically connected to the nuclear envelope, yet reaches out into the cytoplasm of the cell. Additional organelles of the endomembrane system include the Golgi apparatus, vesicles and lysosomes.

Figure 4. Endomembrane system of a eukaryotic cell. Linear DNA, known as chromosomes, housed within the nucleus generates RNA molecules. Processed RNA molecules exit the nuclear envelope via nuclear pores and enter into the rough endoplasmic reticulum (or the cytoplasm) and attach to a ribosome (synthesized in the nucleolus of the nucleus), where a protein is synthesized. Attached to the rough endoplasmic reticulum is the smooth reticulum, which lacks ribosomes and is responsible for lipid synthesis. Once a protein is synthesized, it may travel to the Golgi apparatus along the cytoskeleton network. At the Golgi apparatus, the protein can be modified and packaged for use within the cell or excreted. Lysosomes are specialized vesicles that are responsible for breaking down a variety of biochemicals.

Unlike prokaryotes, eukaryotes have a true nucleus (Figure 4), defined as DNA housed inside of a nuclear envelope, also called the nuclear membrane. The nuclear envelope is a double-membrane composed predominately of phospholipid bilayers, and is punctuated by nuclear pores. These openings in the nuclear envelope allow the movement of specific molecules into and out of the nucleus. Namely, mRNA leaves the nucleus through the nuclear pores and nucleotides enter into the nucleus in order to create new DNA and RNA. Inside the nucleus several strands of linear DNA, known as chromosomes, are housed. When you look at the nucleus under the microscope, you will see a dark spot within the nucleus, the nucleolus, which is the ribosome-manufacturing facility.

It is thought that the primary advantage the nucleus provides is gene regulation , by separating transcription and translation. Transcription is the process in which a segment of DNA (known as a gene ) codes for a messenger RNA ( mRNA ). Following transcription, eukaryotes undergo post-transcriptional modification in which segments ( introns ) of the unprocessed mRNA (or primary transcript) are removed prior to leaving the nucleus, and the remaining segments (exons) are reattached, forming a mature mRNA. The mature mRNA leaves the nucleus through a nuclear pore and travel to a ribosome . The ribosome reads the mature mRNA and with the help of transfer RNA ( tRNA ) links amino acids in a specific sequence generating a protein. Prokaryotes (which don’t have a nucleus) go through transcription and translation simultaneously, and therefore are incapable of gene regulation via post-transcriptional modification.

The endoplasmic reticulum is an endmembrane organelle contiguous with the nuclear envelope that consists of a series of flattened, interconnected membranes. Rough endoplasmic reticulum houses ribosomes and is an important location for the synthesis of proteins, vesicles and lysosomes . Smooth endoplasmic reticulum lacks ribosomes, and therefore is incapable of protein synthesis, but primarily functions as the production center for lipids, namely: phospholipids, lipids and steroids.

In review, within the nucleus DNA synthesizes RNA molecules via transcription, in which a precise segment of DNA (known as a gene ) opens up and synthesizes a specific strand of mRNA (refer to Figure 6). After the mRNA is synthesized, post-transcriptional modification occurs in which segments (introns) of the mRNA and the remaining mRNA segments ( exons ) are reattached. This mature mRNA (composed of reattached exons) leaves the nucleus through the nuclear pore and either enters into the cytoplasm or the rough endoplasmic reticulum (r ough ER ) . Either in the cytoplasm or the rough ER, the mature mRNA attaches to a ribosome. At the ribosome, the process of translation begins in which the mRNA codes for the synthesis of a protein. Once the protein is completely synthesized at the ribosome, the protein leaves the rough ER through a transport vesicle.

Figure 5. Endocytosis, pinocytosis and exocytosis. In endocytosis, larger particles can enter the cell in pockets of the cell membrane. These pockets pinch together forming vesicles. Lysosomes fuse with these vesicles and break down smal biological molecules (pinocytosis) or food particles (phagocytosis). Lysosomes can also fuse with the cell's own defective organelles and recycle in their material for reuse, in a process known as autophagy. Materials destined for excretion are packaged in secretory vesicles, which fuse with the cell membrane, releasing the waste material into the intermembrane space.

Vesicles are organelles within a cell, consisting of fluid enclosed by a phospholipid bilayer. Vesicles perform many different functions within the cell: the movement of substances into and out of a cell, as well as movement of substances within a cell, and as storage units. Certain large biomolecules, such as proteins are incapable of passing across the cell membrane via passive transport mechanisms, such as diffusion. In endocytosis, the cell membrane engulfs larger molecules via active transport (Figure 5). These molecules collect in pockets of the phospholipid bilayer, which grow inward, using energy (ATP). As the pocket of the cell membrane grows inward, the phospholipid bilayer begins to pinch together toward the exterior of the cell. Eventually, the pocket detaches from the cell membrane and becomes a vesicle. Since vesicles are made up phospholipids, they are capable of merging with (and emerging from) other membranes of organelles, as well as the cell membrane. In exocytosis, a form of active transport, a secretory vesicle fuses with the cell membrane releasing waste material (Figure 5). Transport vesicles move biological molecules from one organelle to another, using energy (ATP). For example, certain proteins are transported within a transport vesicle from the rough endoplasmic reticulum to the Golgi apparatus for further processing.

Vesicles do more than move substances into, within and out of the cell. Since vesicles are separated from the cytosol, the main body of fluid within the cell, vesicles can have a different biochemical environment. Vacuoles are specialized vesicles that hold predominately water and can be found in many eukaryotes, including plants and animals. In animal cells vacuoles are very small, while in plant cells the vacuole is typically the largest organelle of the cell. In most plant cells, the vacuole allows the cell to maintain osmotic balance and nutrient storage. Vacuoles also store pigments in the cells of colorful plant organs, such as flower petals, providing them color. Vacuoles within plant seeds can store proteins and fats, which serve as a rich energy source for developing seeds.

Figure 6. Protein synthesis, modification and shipment. DNA codes for the production of mRNA within the nucleus. The mRNA exits the nuclear envelope through a nuclear pore and enters into the rough endoplasmic reticulum, where it attaches to a ribosome and a protein is synthesized. The protein leaves the rough ER through a transport vesicle and enters the cis-face of the Golgi apparatus, where it is modified and identified within the cisternae of the Golgi. The modified protein exits the Golgi through the trans-face via a secretory vesicle, where (depending on the protein) it either attaches to another organelle or cell membrane, or it gets excreted from the cell via exocytosis.

Lysosomes are specialized vesicles involved in cellular digestion. They contain a variety of enzymes that allow them to break down a wide array of biological molecules engulfed by the cell. In a process known as phagocytosis (Figure 5), vesicles containing food enter into the cell via endocytosis fuse with lysosomes, which release their digestive enzymes into the merged organelle (Figure 5), breaking down biomolecules such as carbohydrates, lipids, nucleic acids and proteins into their fundamental monomers. These monomers can be used by the cell's mechanisms to formulate its own unique biomolecules (i.e. DNA and proteins). In addition to digesting food particles, lysosomes assist the cell by recycling its own defective organelles and biomolecules, in a process known as autophagy. Once these structures are broken down, the cell can reassemble the monomers to form new biomolecules and organelles. Lysosomes are also thought to be involved in pre-programmed cell death in multicellular species. As organisms develop from a single cell into an adult, they take on many different forms. It is thought that this change occurs by the predetermined death of certain cells during specific times of development. For example, at a very early stage of human development, the embryos have tails and webbed hands and feet. As the fetus develops, the tail begins to shrink and the webbing between the fingers and toes disappears. This happens because those cells are killed by their very own lysosomes, but aids in the overall development of that organism.

Some synthesized proteins require further processing and are transported to the Golgi apparatus to be modified. A transport vesicle from the rough endoplasmic reticulum with the unmodified protein travels to the cis-face of the Golgi apparatus along the cell’s cytoskeleton network. The Golgi apparatus is a collection of flattened membranes known as cisternae. Once the transport vesicle containing proteins enters into the Golgi apparatus, some of the the peptide bonds are broken and rearranged, creating an altered protein. In addition, identification tags are put on the protein that allow it to be placed exactly where it is designed to be. The modified protein leaves the Golgi apparatus through the trans-face and connects to the cytoskeleton network, traveling to its identified location. Proteins packaged in transport vesicles are either destined for secretion via exocytosis or to be used by the cell. The Golgi apparatus can be thought of as a post office, in which proteins are labeled, packaged and shipped to the appropriate destination.

Figure 7. Cell movement in unicellular organism by cilia and flagella. One function of the cytoskeleton allows unicellular organisms to move. Cilia are multiple, small protuberances of filaments that emerge beyond the cell membrane, which beat in a wave-like fashion allowing cell movement and move nutrient-rich water across the surface. Larger filaments, known as flagella, extending from the cytoskeleton whip in a back and forth motion allowing for a highly efficient form of mobility.

The cytoskeleton is an interconnected network of protein filaments that exists in all cells, including prokaryotes and eukaryotes. The cytoskeleton serves many different functions within the cell. Its primary function is to provide resistance against compression allowing the cell to maintain its overall shape (akin to an animal's skeleton). This allows unicellular growing next to each other resistance to mechanical deformation. In multicellular organisms, this resistance allows tissue stabilization and structural integrity.

The cytoskeleton is also involved in cell movement. The proteins of the cytoskeleton are capable of contracting and releasing, which alter the shape of the cell allowing the cell to move. For example, in muscle cells actin filaments contract to shorten the cell. In some cells, the cytoskeleton extends beyond the cell membrane, and either form cilia or flagella (Figure 7). Cilia are protuberances that project beyond the body of the cell, and are common in many different types of cells. Certain prokaryotic and eukaryotic unicellular species have cilia that beat in coordinated waves allowing them to maneuver through water or to move water over the cell surface as a feeding mechanism. This is an example of motile cilia. Motile cilia also exist in multicellular species. For example, cell inside the trachea of vertebrates beat continuously to remove mucus and dirt particles out of the body. Motile cilia in the Fallopian tubes in humans are responsible for carrying the egg from the ovary to the uterus. Flagella are typically much longer, thicker but less numerous than cilia. These extreme extension of the cytoskeleton whip back and forth creating an effiencent locomotion mechanism.

In eukaryotes, the cytoskeleton network is also responsible for the movement of vesicles within the cell. Once a protein is created by a ribosome in the rough endoplasmic reticulum, it leaves as the ER pinches off, forming a transport vesicle. This transport vesicle moves along the cytoskeleton filaments towards the Golgi apparatus. Once processed, the protein leaves the Golgi apparatus in a vesicle to be transported with or out of the cell via exocytosis. The cytoskeleton also plays a role in endocytosis and organelle transport.

The cytoskeleton is critical in cell division. Prokaryotes divide in a process known as binary fission. Following the replication of DNA, the filaments of the cytoskeleton constrict migrating the DNA strands to opposite poles of the cell. Another filament constricts the middle of the cell, pinching together the cell membrane and cell wall, eventually producing two cells. During mitosis and meiosis in eukaryotes, filaments emerge from centrioles and attach to the chromosomes. These filaments constrict, eventually separating the paired chromosomes and pulling them to opposite poles. Once this occurs, a different filament constricts the cytoplasm, in a process known as cytokinesis, dividing the mother cell into two daughter cells.

Figure 8. Structures of a mitochondrion. The mitochondrion is a double-membraned organelle within eukaryotes involved in aerobic respiration. The outer membrane have proteins, called porins, that allow up to medium sized molecules in and out of the mitochondrion, generating an aqueous solution in the intermembrane space similar to the cytosol. However, large proteins made by the mitochondrion remain. The inner membrane is highly folded into structures known as cristae and is primarily responsible for oxidative phosphorylation in the electron transport chain, with the protein, ATP synthase, ultimately generating most of the ATP produced during cellular respiration. The environment inside the inner membrane, known as the matrix, contains a variety of enzymes (most notably those responsible for the citric acid cycle) and residual mitochondrial DNA capable of synthesizing its own RNA and proteins.

The mitochondrion is the organelle responsible for cellular respiration in nearly all eukaryotes. Mitochondria are organelles that have been described as the powerhouse of the cell because they synthesize most of eukaryotes' Adenosine Triphosphate (ATP), the chemical source of energy used by all organisms. Mitochondria are composed of several compartments (Figure 8) that serve specific functions. Mitochondria have two membranes (an inner and outer membrane) and an intermembrane space between. While the outer membrane is spherical to oblong shaped, the inner membrane is folded in on itself forming several invaginations, known as cristae. The outer membrane has protein complexes known as porins, that allow small to medium sized molecules to freely diffuse into and out of the organelle. Therefore the intermembrane space is quite similar to the cytosol, with the exception of large molecules (specifically proteins) that are incapable of moving across the outer membrane. The foldings, or cristae, of the inner membrane increase the surface area, which is important as this is where much of the biochemistry of the electron transport chain of cellular respiration occurs, including: the redox reactions of oxidative phosphorylation and the synthesis of ATP. The space inside of the inner membrane, known as the matrix, is where many biochemical reactions occur, including the citric acid cycle. Also housed within the matrix is DNA and ribosomes. But how?

Ancestral eukaryotes generated ATP by performing glycolysis within cytosol within the cell. All present day eukaryotes also go through glycolysis. However, the net production from glycolysis is 2 ATP. Eukaryotes with a mitochondrion can net up to 36 ATP, an energy efficiency of 16 times greater.

There are two hypotheses about the origin of mitochondria: endosymbiotic and autogenous. The endosymbiotic (when one organism likes inside another mutually benefiting both) hypothesis (Figure 9) suggests the mitochondrion was originally a prokaryotic cell engulfed by an early eukaryotic cell over 2 billion years ago. Whereas the autogenous hypothesis suggests that DNA from an ancestral eukaryote split and exited the nucleus, which was entrapped by a phospholipid bilayer, further engulfed by an additional bilayer, giving you two membranes. This must have occurred prior to the original divergence of the Bacteria from Archaea/Eukarya, and is retained as a remnant in eukaryotes. However, mitochondria have many similarities with bacteria causing most scientists to discredit the autogenous hypothesis in support of the endosymbiotic hypothesis.

Figure 9. Origin of the mitochondrion. According to the endosymbiosis hypothesis, an ancestral eukaryote (a) engulfed, but did not digest, a protobacterium (b). In an increasingly oxygen rich environment, this bacterium was capable of extracting much more energy (ATP) from biological molecules than the eukaryote alone. The protobacteium benefited from a supply of undigested food particles, and eventually became an endosymbiotic mitochondrion (c), living inside the eukaryote.

The endosymbiotic hypothesis posits a single unicellular eukaryotic cell (see Figure 3c) engulfed a bacterium (now known as the mitochondrion), approximately between 1.7 and 2.2 billion years ago. It is unclear whether this engulfment of the mitochondrion occurred before, during or after the origin of the nucleus. Instead of digesting the bacterium for food, the eukaryotic cell retained it. This proved to be one of the most important interactions for the evolution of life on Earth. Why? This eukaryotic cell could take one molecule of glucose and net 32 ATP, rather than 2 ATP prior to the engulfment. It is thought that the mitochondrion also benefited from this relationship in two ways. First, it got protection. Being housed in a much larger cell (100-1000x larger than itself), it was no longer preyed upon by bacteria-eating microorganisms. Second, the the eukaryote alone is incapable of digesting many biological molecules. This first mitochondrion flourished inside its host due to an abundance of half-eaten molecules. Perhaps the best evidence that the bacteria benefited from this relationship is that bacteria resembling the mitochondrion are extinct, but mitochondria are not. What is extraordinary to ponder is that this happened once….in a single cell. Following the emergence of the first ancestral eukaryote, this new organism was so successful that it multiplied and diversified at such a tremendous rate that scientists are still trying to understand how eukaryotes are related just beyond the root of Eukarya. Whatever happened, this single cell was so successful that it eventually gave rise to all protists, plants, fungi and animals on earth.

A significant body of evidence exists to support the endosymbiotic hypothesis. 1) Mitochondria are the same size as the average bacterium. 2) The strongest evidence is that mitochondria have their own DNA, and can manufacture some of their own, RNA, ribosomes and proteins. And in fact, if you sequence the DNA of the mitochondrion and compare it with taxa from the entire tree of life, the DNA sequence is most closely related to a bacteria (not a eukaryote). In addition, mitochondrial DNA is composed of multiple copies of the same, circular chromosomes. Eukaryotes, in contrast, have DNA that is organized as many different, linear chromosomes. 3) Mitochondria are double-membraned, most organelles are single-membraned. It is thought that when the original eukaryote engulfed the mitochondrion, the membrane of the original eukaryote (phagocytic vacuole) wrapped up the mitochondrion, like a bag within bag). 4) Interestingly, the internal membrane of the mitochondrion has a lipid composition more similar to bacteria, whereas the external membrane has a lipid composition more similar to eukaryotes. Only one other eukaryotic organelle has two membranes, and it is thought that this organelle was consumed by a descendent of this first aerobic eukaryote. That organelle was the chloroplast, which helped give rise to photosynthetic eukaryotes and eventually plants.

Figure 10. Comparison of a chloroplast and cyanobacterium. Chloroplasts are thought to have originated from a singular, endosymbiotic event in which a eukaryote engulfed, but did not digest, a cyanobacterium. In both structures, chlorophyll (the site of the light reactions) is housed on membranes of internal structures known as thylakoids. Membranes of thylakoids in cyanobacteria run parallel to the cell membrane. Whereas thylakoids in chloroplasts stack generating structures called grana, enhancing the internal surface area allowing for more chlorophyll and thus, greater efficiency. Both cyanobacteria and chloroplast have nucleoids, containing circular DNA capable of producing RNA and proteins. They also both have two membranes, likely a remnant of an endosymbiotic event. The fluid between the thylakoids and inner membrane, known as the stroma, is the location of the Calvin cycle.

Chloroplasts are photosynthetic organelles found in plant and algal cells. The photosynthetic pigment, chlorophyll, captures light energy and converts it into chemical energy. Chlorophyll is housed on the membrane of pancake-shaped structures, known as thylakoids, within the body of the chloroplast. Thylakoids tend to stack on top of each other forming stacks, known as grana. Adjacent grana can be connected together by membranous bridges, called lumen. Light reactions of photosynthesis occur on the membrane of the thylakoid, splitting water (H2O). Energy is generated during this anabolic reaction, which is used to synthesize ATP. Single oxygen atoms bind to form oxygen gas (O2), a waste product. Hydrogens are captured by NADP to form NADPH. ATP and NADPH enter into the stroma, or fluid between the thylakoids and cell membrane. In the stroma, ATP is used to drive the Calvin cycle, in a series of catabolic reactions occurs where hydrogens are stripped from NADPH and rearranged with the atoms of carbon dioxide (CO2) to form the sugar, glucose (C6H12O6).

Approximately a billion years ago, a cyanobacterium entered into eukayrotic cell. It either entered as an internal parasite, or was engulfed similar to how the mitochondrion. However the cyanobacterium entered the cell, modern day chloroplasts do not have a eukaryotic phagocytic vacuole, which was either lost, in the case of engulfment, or never occurred, in the case of parasitism. Cyanobacteria (and primitive chloroplasts, i.e. glaucophytes) represent a type of bacteria known as Gram-negative, which have a double membrane with a cell wall sandwiched between, made of peptidoglycan, a polysaccharide absent in eukaryotes but present in bacteria (but not archaea). Thus it is posited the double membrane of the chloroplast represent the inner and outer membranes of the original cyanobacterium. However unlike the mitochondrion, there is no presence of a eukaryotic membrane, in contemporary plant cells.

Figure 11. Origin of the chloroplast. A heterotrophic, mitochondrion-containing eukaryote (a) was either parasitized by or engulfed a cyanobacterium (b), producing an autotrophic, photosynthetic eukaryotic cell (c).

This new structure gave the eukaryote the significant advantage of becoming autotrophic, being able to generate chemical energy from sunlight via photosynthesis. Prior to this eukaryotes were heterotrophic, in which they had to extract food from their environment either through filter feeding or predation. The eukaryotic host must have had mitochondria prior to the engulfment of the cyanobacterium, as all photosynthetic eukaryotes presently have mitochondria. Over time, most of the cyanobacterial DNA was either lost or assimilated into the nuclear chromosomes of the eukaryotic host. Most proteins needed by modern day chloroplasts are synthesized by the eukaryotic transcription and translation, and imported to the chloroplast. Some bacterial DNA (plastid DNA) persists within modern-day chloroplasts. At approximately 100 genes, the plastid DNA is capable of producing some of its own proteins, similar to (but far fewer than) the mitochondrion.

Most scientists concur that all chloroplasts within eukaryotes have been traced back to a singular endosymbiotic event, with one exception, even though many eukaryotes with chloroplasts are distantly related to each other. This suggests that the chloroplast endosymbiosis event occurred early in eukaryote evolution, but has been being lost several times over throughout evolutionary history. There are three chloroplast lineages (most primitive to most recent): glaucophyte (blue-green algae), rhodophyte (red algae) and chloroplastidan (green algae and land plants).

Plant cells vs. animal cells

Plants and animals are eukaryotic organisms. So they share a similar internal structure of organelles and other cellular structures (Figure 12-13). Plant cells differ in three main ways from animal cells. (1) Plant cells have photosynthetic organelles, chloroplasts, that appear green under a compound microscope. (2) While animal cells have vacuoles, a plant cell’s vacuoles are very large in comparison. The main function of plant vacuoles is to store water between precipitation events. (3) Plant cells have thick primary cell walls made of the polysaccharide, cellulose. Plant cells involved in structural support also have a secondary cell wall made of a very dense polysaccharide, lignin. This gives plants a roughly polygonal shape, whereas animal cells (which lack a cell wall) have a more amorphous shape.

Figure 12. A typical animal cell. Plant cells have many of same internal cellular structures and organelles as animals cells, with a few exceptions. While both animal and plant contain vacuoles useful for water and nutrient storage, a plant cell's vacuoles is enormous by comparison. This is a result of an immotile life style, and being dependent on rain. Animals in contrast are motile, and either live in water or are capable of moving to search for water.

Figure 13. A typical plant cell. In addition to large vacuoles, plant cells contain chloroplasts, whereas animal cells do not. Plant cells also have a cell wall made of the polysaccharide, cellulose that helps the cell maintain its rigid structure as plants grow.


Myosins on the Move.

In recent years, the number of identified myosin superfamily members has increased exponentially. It has become clear that virtually all eukaryotic cells, not only muscle cells, express a multitude of different myosin molecules (14). The myosins share a conserved motor domain that consists of separate head and neck regions. The head region contains the ATP- and actin-binding sites and exhibits actin-activated ATPase activity. The neck region consists of an extended α-helix of variable length that binds between one and six light polypeptide chains of calmodulin or calmodulin-related proteins. This neck region is proposed to serve as a lever arm for force production (15). Physiological modification of the light chains (i.e., by phosphorylation) contributes to the regulation of motor function. During its mechanochemical cycle, the myosin responsible for muscle contraction does not move processively along the actin filament. Instead, it holds on for only a short period and then spends considerable time detached from the filament (16). Therefore, to achieve continuous movement along actin filaments, a high density of myosin motors is required. Accordingly, muscle myosin self-assembles into filaments. The multitude of newly discovered myosin molecules are believed not to form filaments, and the mechanochemical properties for most remain to be investigated. In addition to the motor domain, the different myosin molecules contain diverse tail domains that are postulated to specify function, perhaps by determining the target of force generation. These tail domains frequently contain amino acid sequence motifs that are also found in other proteins (17). The elucidation of the targets to which the different myosins bind represents an important challenge for the future. The tail domain of a myosin identified from rat tissue, myr5, serves to negatively regulate signal transduction by the small Ras-related G protein Rho (18). Rho regulates the organization of the actin cytoskeleton and various other cellular processes (19, 20). This finding suggests the fascinating possibility that initiation and flow of information might be coupled to directed force production along actin filaments.


Flagella and Cilia

Flagella (singular = flagellum) are long, hair-like structures that extend from the plasma membrane and are used to move an entire cell, (for example, sperm, Euglena). When present, the cell has just one flagellum or a few flagella. When cilia (singular = cilium) are present, however, they are many in number and extend along the entire surface of the plasma membrane. They are short, hair-like structures that are used to move entire cells (such as paramecium) or move substances along the outer surface of the cell (for example, the cilia of cells lining the fallopian tubes that move the ovum toward the uterus, or cilia lining the cells of the respiratory tract that move particulate matter toward the throat that mucus has trapped).


Microtubules

As their name implies, microtubules are small hollow tubes. The walls of the microtubule are made of polymerized dimers of α-tubulin and β-tubulin, two globular proteins (Figure). With a diameter of about 25 nm, microtubules are the widest components of the cytoskeleton. They help the cell resist compression, provide a track along which vesicles move through the cell, and pull replicated chromosomes to opposite ends of a dividing cell. Like microfilaments, microtubules can dissolve and reform quickly.

Microtubules are hollow. Their walls consist of 13 polymerized dimers of α-tubulin and β-tubulin (right image). The left image shows the molecular structure of the tube.

Microtubules are also the structural elements of flagella, cilia, and centrioles (the latter are the two perpendicular bodies of the centrosome). In fact, in animal cells, the centrosome is the microtubule-organizing center. In eukaryotic cells, flagella and cilia are quite different structurally from their counterparts in prokaryotes, as discussed below.


Summary

  • The cytoplasm consists of everything inside the plasma membrane of the cell.
  • The cytoskeleton is a cellular “skeleton” that crisscrosses the cytoplasm. Three main cytoskeleton fibers are microtubules, intermediate filaments, and microfilaments.
  • Microtubules are the thickest of the cytoskeleton structures and are most commonly made of filaments which are polymers of alpha and beta tubulin.
  • Microfilament are the thinnest of the cytoskeleton structures and are made of two thin actin chains that are twisted around one another.

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Why eukaryotes have three different types of cytoskeleton filaments? - Biology

By the end of this section, you will be able to do the following:

  • Describe the cytoskeleton
  • Compare the roles of microfilaments, intermediate filaments, and microtubules
  • Compare and contrast cilia and flagella
  • Summarize the differences among the components of prokaryotic cells, animal cells, and plant cells

If you were to remove all the organelles from a cell, would the plasma membrane and the cytoplasm be the only components left? No. Within the cytoplasm, there would still be ions and organic molecules, plus a network of protein fibers that help maintain the cell’s shape, secure some organelles in specific positions, allow cytoplasm and vesicles to move within the cell, and enable cells within multicellular organisms to move. Collectively, scientists call this network of protein fibers the cytoskeleton. There are three types of fibers within the cytoskeleton: microfilaments, intermediate filaments, and microtubules ((Figure)). Here, we will examine each.

Figure 1. Microfilaments thicken the cortex around the cell’s inner edge. Like rubber bands, they resist tension. There are microtubules in the cell’s interior where they maintain their shape by resisting compressive forces. There are intermediate filaments throughout the cell that hold organelles in place.

Microfilaments

Of the three types of protein fibers in the cytoskeleton, microfilaments are the narrowest. They function in cellular movement, have a diameter of about 7 nm, and are comprised of two globular protein intertwined strands, which we call actin ((Figure)). For this reason, we also call microfilaments actin filaments.

Figure 2. Two intertwined actin strands comprise microfilaments.

ATP powers actin to assemble its filamentous form, which serves as a track for the movement of a motor protein we call myosin. This enables actin to engage in cellular events requiring motion, such as cell division in eukaryotic cells and cytoplasmic streaming, which is the cell cytoplasm’s circular movement in plant cells. Actin and myosin are plentiful in muscle cells. When your actin and myosin filaments slide past each other, your muscles contract.

Microfilaments also provide some rigidity and shape to the cell. They can depolymerize (disassemble) and reform quickly, thus enabling a cell to change its shape and move. White blood cells (your body’s infection-fighting cells) make good use of this ability. They can move to an infection site and phagocytize the pathogen.

Link to Learning

To see an example of a white blood cell in action, watch a short time-lapse video of the cell capturing two bacteria. It engulfs one and then moves on to the other.

Intermediate Filaments

Several strands of fibrous proteins that are wound together comprise intermediate filaments ((Figure)). Cytoskeleton elements get their name from the fact that their diameter, 8 to 10 nm, is between those of microfilaments and microtubules.

Figure 3. Intermediate filaments consist of several intertwined strands of fibrous proteins.

Intermediate filaments have no role in cell movement. Their function is purely structural. They bear tension, thus maintaining the cell’s shape, and anchor the nucleus and other organelles in place. (Figure) shows how intermediate filaments create a supportive scaffolding inside the cell.

The intermediate filaments are the most diverse group of cytoskeletal elements. Several fibrous protein types are in the intermediate filaments. You are probably most familiar with keratin, the fibrous protein that strengthens your hair, nails, and the skin’s epidermis.

Microtubules

As their name implies, microtubules are small hollow tubes. Polymerized dimers of α-tubulin and β-tubulin, two globular proteins, comprise the microtubule’s walls ((Figure)). With a diameter of about 25 nm, microtubules are cytoskeletons’ widest components. They help the cell resist compression, provide a track along which vesicles move through the cell, and pull replicated chromosomes to opposite ends of a dividing cell. Like microfilaments, microtubules can disassemble and reform quickly.

Figure 4. Microtubules are hollow. Their walls consist of 13 polymerized dimers of α-tubulin and β-tubulin (right image). The left image shows the tube’s molecular structure.

Microtubules are also the structural elements of flagella, cilia, and centrioles (the latter are the centrosome’s two perpendicular bodies). In animal cells, the centrosome is the microtubule-organizing center. In eukaryotic cells, flagella and cilia are quite different structurally from their counterparts in prokaryotes, as we discuss below.

Flagella and Cilia

The flagella (singular = flagellum) are long, hair-like structures that extend from the plasma membrane and enable an entire cell to move (for example, sperm, Euglena, and some prokaryotes). When present, the cell has just one flagellum or a few flagella. However, when cilia (singular = cilium) are present, many of them extend along the plasma membrane’s entire surface. They are short, hair-like structures that move entire cells (such as paramecia) or substances along the cell’s outer surface (for example, the cilia of cells lining the Fallopian tubes that move the ovum toward the uterus, or cilia lining the cells of the respiratory tract that trap particulate matter and move it toward your nostrils.)

Despite their differences in length and number, flagella and cilia share a common structural arrangement of microtubules called a “9 + 2 array.” This is an appropriate name because a single flagellum or cilium is made of a ring of nine microtubule doublets, surrounding a single microtubule doublet in the center ((Figure)).

Figure 5. This transmission electron micrograph of two flagella shows the microtubules’ 9 + 2 array: nine microtubule doublets surround a single microtubule doublet. (credit: modification of work by Dartmouth Electron Microscope Facility, Dartmouth College scale-bar data from Matt Russell)

You have now completed a broad survey of prokaryotic and eukaryotic cell components. For a summary of cellular components in prokaryotic and eukaryotic cells, see (Figure).

Components of Prokaryotic and Eukaryotic Cells
Cell Component Function Present in Prokaryotes? Present in Animal Cells? Present in Plant Cells?
Plasma membrane Separates cell from external environment controls passage of organic molecules, ions, water, oxygen, and wastes into and out of cell Yes Yes Yes
Cytoplasm Provides turgor pressure to plant cells as fluid inside the central vacuole site of many metabolic reactions medium in which organelles are found Yes Yes Yes
Nucleolus Darkened area within the nucleus where ribosomal subunits are synthesized. No Yes Yes
Nucleus Cell organelle that houses DNA and directs synthesis of ribosomes and proteins No Yes Yes
Ribosomes Protein synthesis Yes Yes Yes
Mitochondria ATP production/cellular respiration No Yes Yes
Peroxisomes Oxidize and thus break down fatty acids and amino acids, and detoxify poisons No Yes Yes
Vesicles and vacuoles Storage and transport digestive function in plant cells No Yes Yes
Centrosome Unspecified role in cell division in animal cells microtubule source in animal cells No Yes No
Lysosomes Digestion of macromolecules recycling of worn-out organelles No Yes Some
Cell wall Protection, structural support, and maintenance of cell shape Yes, primarily peptidoglycan No Yes, primarily cellulose
Chloroplasts Photosynthesis No No Yes
Endoplasmic reticulum Modifies proteins and synthesizes lipids No Yes Yes
Golgi apparatus Modifies, sorts, tags, packages, and distributes lipids and proteins No Yes Yes
Cytoskeleton Maintains cell’s shape, secures organelles in specific positions, allows cytoplasm and vesicles to move within cell, and enables unicellular organisms to move independently Yes Yes Yes
Flagella Cellular locomotion Some Some No, except for some plant sperm cells
Cilia Cellular locomotion, movement of particles along plasma membrane’s extracellular surface, and filtration Some Some No

Section Summary

The cytoskeleton has three different protein element types. From narrowest to widest, they are the microfilaments (actin filaments), intermediate filaments, and microtubules. Biologists often associate microfilaments with myosin. They provide rigidity and shape to the cell and facilitate cellular movements. Intermediate filaments bear tension and anchor the nucleus and other organelles in place. Microtubules help the cell resist compression, serve as tracks for motor proteins that move vesicles through the cell, and pull replicated chromosomes to opposite ends of a dividing cell. They are also the structural element of centrioles, flagella, and cilia.

Review Questions

Which of the following have the ability to disassemble and reform quickly?


Conclusion

Thus, we examined the structure and functions of the cytoskeleton. It plays an exceptionally important role in the life of the cell, providing its most important processes.

All cytoskeletal components interact. This is confirmed by the existence of direct contacts of microfilaments, intermediate filaments and microtubules.

According to modern ideas, the most important link that unites various cellular parts and carries out data transfer is the cytoskeleton.


Watch the video: Prokaryotic Vs. Eukaryotic Cells (December 2022).