THE EUKARYOTIC CELL

THE EUKARYOTIC CELL

As mentioned earlier, eukaryotic organisms include algae,protozoa, fungi, plants, and animals.

The eukaryotic cell is typically larger and structurally more complex than the prokaryotic cell By comparing the structure of the prokaryotic cell in with that of the eukaryotic cell, the differences between the two types of Cells become apparent.

The following discussion of Eukaryotic cells will parallel our discussion of prokaryotic cells by starting with structures that extend to the outside of the cell.

FLAGELLA AND CILIA

Many types of eukaryotic cells have projections that are used for cellular locomotion or for moving substances
along the surface of the cell.

These projections contain cytoplasm and are enclosed by the plasma membrane.

If the projections are few and are long in relation to the size 0 the cell,they are called flagella.

If the projections are numerous and short, they are called cilia (singular: cilium) Algae of the genus Euglena (ū-glē'na) use a flagellum for locomotion, whereas protozoa, such as Tetrahymena.(tet-rä-hï'me-nä), use cilia for locomotion.  

Both flagella and cilia are anchored to the plasma membrane by a basal body, and both consist of nine pairs of microtubules (doublets) arranged in a ring, plus another two microtubules in the center of the ring, an arrangement called a 9 + 2 array. 

Microtubules are long, hollow tubes made up of a protein called tubulin a prokaryotic flagellum rotates,but a eukaryotic flagellum moves in a wavelike manner.

To help keep foreign material out of the lungs,ciliated cells of the human respiratory system move the material along the surface of the cells in the bronchial tubes and trachea toward the throat and mouth.

THE CELL WALL AND GLYCOCALYX

LEARNING OBJECTIVE

Compare and contrast prokaryotic and eukaryotic cell walls and glycocalyxes Most eukaryotic cells have cell walls, although they are generally much simpler than those of prokaryotic cell:s.

Many algae have cell walls consisting of the polysaccharide cellulose (as do all plants); other chemicals may be present as well. 

Cell walls of some fungi also contain cellulose, but in most fungi the principal structural component of the cell wall is the polysaccharide chitin, a polymer of Nacetylglucosamine (NAG) units. 

(Chitin is also the main structural component of the exoskeleton of crustaceansand insects.) The cell walls of yeasts contain the polysaccharides glucan and mannan. 

In eukaryotes that lack a cellwall, the plasma membrane may be the outer covering however, cells that have direct contact with the environment may have coatings outside the plasma membrane Protozoa do not have a typical cell wall; instead, they have a flexible outer protein covering called a pellicle.

THE PLASMA (CYTOPLASMIC) MEMBRANE

The plasma (cytoplasmic) membrane of eukaryotic and prokaryotic cells is very similar in function and basic There are, however, differences in the types of proteins found in the membranes.

Eukaryotic membranes also contain carbohydrates, which serve as attachment sites for bacteria and as receptor sites that assume a role in such functions as cell recognition.

Eukaryotic plasma Structure.

THE PLASMA (CYTOPLASMIC) MEMBRANE

Compare and contrast prokaryotic and eukaryotic plasma membranes.

The plasma (cytoplasmic) membrane of eukaryotic and prokaryotic cells is very similar in function and basic.

There are, however, differences in the types of proteins found in the membranes.Eukaryotic membranes also contain carbohydrates, which serve as attachment sites for bacteria and as receptor sites that assume a role in such functions as cell-cell recognition. 

Eukaryotic plasma membranes also contain sterols, complex lipids not found in prokaryotic plasma membranes (with the exception of Mycoplasma cells). 

Sterols seem to be associated with the ability of the membranes to resist lysis resulting from in creased osmotic pressure Substances can cross eukaryotic and prokaryotic plasma membranes by simple diffusion, facilitated diffusion, osmotic active transport. 

Group translocation does not occur in eukaryotic cells. However, eukaryotic cells can use a mechsis, oranism called endocytosis 1s occurs when a segment o the plasma membrane surrounds a particle or large molecule,encloses it, and brings it into the cell.

Two very important types of endocytosis are phagocytosis and pinocytosis. 

During phagocytosis, cellular projections called pseudopods engulf particles and bring into the cell. 

Phagocytosis is used by white blood them cells to destroy bacteria and foreign substances pinocytosis, the plasma membrane folds inward, bringing extracellular fluid into the cell, along with whatever substances are dissolved in the fluid.

Pinocytosis is oneof the ways viruses can enter animal cells.

CYTOPLASM

The cytoplasm of eukaryotic cells encompasses the substance inside the plasma membrane and outside the nucleus.

The cytoplasm is the substance in which various cellular components are found. 

(The term cytosol refers to the fluid portion of cytoplasm.) A majordifference between eukaryotic and prokaryotic cytoplasm is that eukaryotic cytoplasm has a  complex internal structure, consisting of exceedingly small rods (microfilaments and intermediate filaments) and cylinders (microtubules)Together,they form the cytoskeleton. 

The cytoskeletorn provides support and shape and assists in transporting substances through the cell (and even in moving the entire cell, as in phagocytosis).

The movement of eukaryotic cytoplasm from one part of the cell to another, which helps distribute nutrients and move the cell over a surface, is called cytoplasmic streaming.

Another difference between prokaryotic and eukaryotic cytoplasm is that many of the important enzymes found in the cytoplasmic fluid of prokaryotes are sequestered in the organelles of eukaryotes.

Ribosomes

Attached to the outer surface of rough endoplasmic reticulum are ribosomes which are also found free in the cytoplasm. 

As in prokaryotes, ribosomes are the sites of protein synthesis in the cell.

The ribosomes of eukaryotic endoplasmic reticulum and cytoplasm are somewhat larger and denser than those of prokaryotic cells.

These eukaryotic ribosomes are 80S ribosomes, each of which consists of a large 60S subunit containing three molecules of rRNA and a smaller 40S subunit with one molecule of rRNA. 

The subunits are made separately in the nucleolus and, once produced, exit the nucleus and join together in the cytosol.

Chloroplasts mitochondria contain 70S ribosomes, which Some ribosomes, called free ribosomes, are unattached to any structure in the cytoplasm. 

Primarily, free ribosomes synthesize proteins used inside the cell.

Other ribosomes, called membrane-bound ribosomes, attach to the nuclear membrane and the endoplasmic reticulum. 

These ribosomes synthesize proteins destined for insertion in the plasma membrane or for export from the cell.

Ribosomes located within mitochondria synthesize mitochondri proteins. 

Sometimes 10 to 20 ribosomes join together in a stringlike arrangement called a polyribosome.

ORGANELLES

Organelles are structures with specific shapes and specialized functions and are characteristic of eukaryotic cells. 

They include the nucleus,endoplasmic reticulum,Golgi complex,lysosomes,vacuoles,mitochondria,chloroplasts,peroxisomes, and centrosomes. 

Not all of the organelles described are found in all cells. Certain cells have theirown type and distribution of organelles based on speciaization, age, and level of activity.

THE NUCLEUS

most characteristic eukaryotic organelle is the nucleus.

The nucleus is usually herical or oval, is frequently the largest structure in the cell, and contains almost all of the cell's hereditary information (DNA). 

Some DNA is also found in mitochondria and in the chloroplasts of photosynthetic organisms The nucleus is surrounded by a double membrane called the nuclear envelope.

Both membranes resemble the plasma membrane in structure.

Tiny channels in the membrane called nuclear pores allow the nucleus to communicate with the cytoplasm  Nuclear pores control the movement of substances between the nucleus and cytoplasm. 

Within the nuclear envelope are one or more spherical bodies called nucleoli (singular: nucleolus) Nucleoli are actually condensed regions of chromosomes where ribosomal RNA is being synthesized. 

Ribosomal RNA is an essential component of ribosomes.

ENDOPLASMIC RETICULUM

Within the cytoplasm of eukaryotic cells is the endoplasmic reticulum,or ER,an extensive network of flattened membranous sacs or tubules called cisterns. 

The ER network is continuous with the nuclear envelope.  

What is the difference between roush ER and smooth ER?

Most eukaryotic cells contain two distinct, but interrelated, forms of ER that differ in structure and function.

The membrane of rough ER is continuous with the nuclea membrane and usually unfolds into a series of flattened sacs. 

The outer surface of rough ER is studded with ribosomes, the sites of protein synthesis. 

Proteins synthesized by ribosomes that are attached to rough ER enter cisterns within the ER for processing and sorting. In some cases, enzymes withir the cisterns attach the proteins to carbohydrates to form gly coproteins.

GOLGI COMPLEX

Most of the proteins synthesized by ribosomes attached to rough ER are ultimately transported to other regions of the cell.

The first step in the transport pathway is through an organelle called the Golgi complex.It consists of 3 to20 cisterns that resemble a stack of pita bread. 

The cisterns are often curved, giving the Golgi complex acuplike shape Proteins synthesized by ribosomes on the rough ER are surrounded by a portion of the ER membrane, which eventually buds from the membrane surface to form a transport vesicle.

The transport vesicle fuses with a cistern of the.

Golgi complex, releasing proteins into the cistern.

The proteins are modified and move from one cistern to another via transfer vesicles that bud from the cisterns' edges.

Enzymes in the cisterns modify the proteins to form glycoproteins, glycolipids, and lipoproteins.

Some of the processed proteins leave the cisterns in secretory vesicles, which detach from the cistern and deliver the proteins to the plasma membrane,where they are discharged by exocytosis.

Other processed proteins leave the cisterns in vesicles that deliver their contents to the plasma membrane for in corporation into the membrane.

Finally,some processed proteins leave the cisterns in vesicles that are called storage vesicles.

The major storage vesicle is a lysosome, whose structure and functions are discussed next.

LYSOSOMES

Lysosomes are formed from Golgi complexes and loo
like membrane-enclosed spheres. 

Unlike mitochondria,lysosomes have only a single membrane and lack internal structure But they contain as many as different kinds of powerful digestive enzymes capable of down various molecules.

Moreover,these enalso digest bacteria that enter the cell.

Human the blood cells,which use phagocytosis to ingest bacteria breaking ezymes canria,contain large numbers of lysosomes.

VACUOLES

vacuole  is a space or cavity in the cytoplasm of a cell that is enclosed by a membrane called a tonolasm Plast.

In plant cells, vacuoles may occupy 5 to 90% of the cell volume, depending on the type of cell.

Vacuoles are derived from the Golgi complex and have several diverse functions Some vacuoles serve as temporary storage organelles for substances such as proteins,sugars,organic acids,and inorganic ions.Other vacuoles form during endocytosis to help bring food into the cell. Many plant cells also store metabolic wastes and poisons that would otherwise be injurious if they accumulated in the cytoplasm. 

Finally, vacuoles may take up water, enabling plant cells to increase in size and also providing rigidity to leaves and stems

MITOCHONDRIA

Spherical or rod-shaped organelles called mitochondria
(singular: mitochondrion) appear throughout the cytoplasm of most eukaryotic cells.. 

The number of mitochondria per cell varies greatly among different types of cells.

For example, the protozoan Giardia has no mitochondria, whereas liver cells contain 1000 to 2000 per cell. 

A mitochondrion consists of a double membrane simar in structure to the plasma membrane (Figure 4.27).outer mitochondrial membrane is smooth,but the inner mitochondrial membrane is arranged in a series of folds called cristae (singular: crista). 

The center of the mitochondrion is a semifluid substance called the matrix Because of the nature and arrangement of the cristae, the inner membrane provides an enormous surface area on which chemical reactions can occur. 

Some proteins thatfunction in cellular respiration, including the enzyme that makes ATP, are located on the cristae of the inner mito- chondrial membrane, and many of the metabolic steps involved in cellular respiration are concentrated in the matrixMitochondria are often called the "powerhouses of the cell" because of their central role in ATP production Mitochondria contain 70S ribosomes and some DNA of their own,as well as the machinery necessary to replicate,transcribe,and translate the information encoded by their DNA.

CHLOROPLASTS

Algae and green plants contain a unique organelle called a chloroplast a membrane-enclosed structure that contains both the pigment chlorophyll and the enzymes required for the light-gathering phases of photosynthesis.

The chlorophyll is contained in flattened membrane sacs called thylakoids; stacks of thylakoids are called grana (singular: granum).

Like mitochondria, chloroplasts contain 70S ribosomes,DNA, and enzymes involved in protein synthesis.

They are capable of multiplying on their own within the cell.

The How are mitochondria similar to prokaryolic cells?way both chloroplasts and mitochondria multiply-by increasing in size and then dividing in two-is strikingly reminiscent of bacterial multiplication.

PEROXISOMES

Organelles similar in structure to lysosomes, but smaller,
are called peroxisomes.

Although peroxisomes were once thought to form by budding off the ER, it is now generally agreed that they form by the division of preexisting peroxisomes.

Peroxisomes contain one or more enzymes that can oxidize various organic substances.

For example, substances such as amino acids and fatty acids are oxidized in peroxisomes as part of normal metabolism.

In addition, enzymes in peroxisomes oxidize toxic substances, such as alcohol.

A by-product of the oxidation reactions is hydrogen peroxide (H2O2), a potentially toxic compound.

However, peroxisomes also contain the enzyme catalase, which decomposes H202 Because the generation and degradation of H2O2 occurs within the same organelle, peroxisomes protect other parts of the cell from the toxic effects of H202.

CENTROSOME

The centrosome, located near the nucleus, consists of two components: the pericentriolar area and centrioles.

The pericentriolar material is a region of the cytosol composed of a dense network of small protein fibers.

This area is the organizing center for the mitotic spindle,which plays a critical role in cell division, and for buleformation in nondividing cells.

Within the pericentriolar material is a pair of cylindrical structures called centrioles, each of which is composed of nine clusters of three microtubules (triplets) arranged in a circular pattern, an arrangement called a 9 + 0 array.

The refers to the nine clusters of microtubules, and the 0 refers to the absence of microtubules in the center.

THE EVOLUTION OF EUKARYOTES

Biologists generally believe that life arose on Earth in the
of very simple organisms, similar to prokaryotic cells, hout 3.5 to 4 billion years ago.

About 2.5 billion years ago,the first eukaryotic cells evolved from prokaryotic cells.

Recal that prokaryotes and eukaryotes differ mainly in that eukaryotes contain highly specialized organelles.

The the explaining the origin of eukaryotes from prokaryotes,theory pioneered by Lynn Margulis,is the endosymbiotic theory.

According to this theory,larger bacterial cells lost their cell walls and engulfed smaller bacterial cells.

This relationship, in which one organism lives within another, is called endosymbiosis (symbiosis lving together) According to the endosymbiotic theory, the an cestral eukaryote developed a rudimentary nucleus when the plasma membrane folded around the chromosome.

This cell,called a nucleoplasm,

may have ingested aerobic bacteria.Some ingested bacteria lived inside the host nucleoplasm.

This arrangement evolved into a symbiotic relationship in which the host' nucleoplasm supplied nutrients and the endosymbiotic bacterium produced energy that could be used by the nucleoplasm.

Similarly, chloroplasts may be descendants ofphotosynthetic prokaryotes ingested by this early nucleoplasm.

Eukaryotic flagella and cilia are believed to have originated from symbiotic associations between the plasma membrane of early eukaryotes and motile spiral bacteria called spirochetes.

A living example that suggests how flagella developed and eukaryotic cells provide evidence for the endosymbiotic theory.

For example

both mitochondria and chloroplasts resemble bacteria in size and shape.

Further,theseorganelles contain circular DNA,which is typical of prokaryotes, and the organelles can reproduce independently of their host cell.

Moreover,mitochondrial and chloroplast ribosomes resemble those of prokaryotes,and their mechanism of protein synthesis is more similar to that found in bacteria than eukaryotes. 

Also,the same antibiotics that inhibit protein synthesis on ribosomes in bacteria also inhibit protein synthesis on ribosomes in mitochondria and chloroplasts.

 

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Milan Tomic

Hi. I’m Designer of Blog Magic. I’m CEO/Founder of ThemeXpose. I’m Creative Art Director, Web Designer, UI/UX Designer, Interaction Designer, Industrial Designer, Web Developer, Business Enthusiast, StartUp Enthusiast, Speaker, Writer and Photographer. Inspired to make things looks better.

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