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Are red blood cells prokaryotic?

Are red blood cells prokaryotic?



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After searching "do antibiotics impact the immune system" I found out that antibiotics target prokaryotic cells. It all made a lot of sense thinking about all those yogurt recommendations you get after taking antibiotics: the collateral damage is on the prokaryotic cells that live with us, but are not "us" as such.

Except I remembered human red blood cells don't have nuclei, so where's my confusion?

(I'm only a biology enthusiast.)


Are red blood cells prokaryotic?

No!

  1. There are many more differences between procaryotes and eukaryotes than just the presence of a nucleus. See DeNovo's answer for more information.

  2. The terms procaryote vs eukaryote refer, not so much to the physiology of the cell but to a specific evolutionary lineage. Eukaryotes are individuals that belong to the monophyletic group of Eukaryota aka. Eukarya (see here for an intro to phylogeny). As such whether or not a eukaryotic loses its nucleus and start looking exactly like a E. coli won't change anything to the fact that this cell is still in the Eukaryota lineage.


When differences between prokaryotes and eukaryotes are taught in an introductory biology course, a generic prokaryotic cell and a generic eukaryotic cell are typically compared. Cells in a complex multicellular organism, like a human, are quite diverse. Human red blood cells are one example of a highly specialized cell with a mature form that is quite different from the typical eukaryotic cell. Keratinocytes in the epidermis are another example (see Ross Histology, Ch. 15). In both cases, these cells produce large amounts of a single protein, eventually, at their most mature stage, stopping protein synthesis, extruding their nuclei and most other organelles.

The absence of a nucleus or other organelles, however, does not necessarily make either of these cells more susceptible to antibacterial antibiotics. Antibiotics are targeted toward things that bacteria have (positive differences), rather than the absence of typical eukaryotic structures. Almost all antibacterial antibiotics have one of three targets (see Goodman and Gilman Chs. 48, 52-55):

  • The bacterial cell wall or membrane

  • protein synthetic machinery

  • specialized metabolites required by bacteria

There is (almost) no overlap between these structures in bacteria and structures in any of the diverse array of human cells. The one partial exception is a similarity between the mitochondrial and bacterial ribosome, that may, for example, be responsible for some of the toxicity of chloramphenicol (G&G Ch. 55).


No. Prokayotic cells are full organisms with their own DNA, red blood cells are not.


No, they are matured (broken) reticulocytes without the net structure and ribosomal DNA, which themselves are matured (broken) normoblasts that have lost their nucleus.

So basically they're the left over plasma membrane of a once-alive eukaryote cell, now filled with mostly hemoglobin, and little else.

(The above is not necessarily true for all animals, some have nuclei, but it's the case for mammals, and thus humans.)


Are red blood cells prokaryotic? - Biology

Cells can be separated into two major groups&mdashprokaryotes, cells whose DNA is not segregated within a well-defined nucleus surrounded by a membranous nuclear envelope, and eukaryotes eukaryote
, a cell or organism composed of cells that have a membrane-bound nucleus and organelles (mitochondria, chloroplasts see cell, in biology) and genetic material organized in chromosomes in which the DNA is combined with histone proteins.
. Click the link for more information. , those with a membrane-enveloped nucleus. The cyanobacteria and bacteria (kingdom Monera Monera,
taxonomic kingdom that comprises the prokaryotes (bacteria and cyanobacteria). Prokaryotes are single-celled organisms that lack a membrane-bound nucleus and usually lack membrane-bound organelles (mitochondria, chloroplasts see cell, in biology).
. Click the link for more information. ) are prokaryotes. They are smaller in size and simpler in internal structure than eukaryotes and are believed to have evolved much earlier (see evolution evolution,
concept that embodies the belief that existing animals and plants developed by a process of gradual, continuous change from previously existing forms. This theory, also known as descent with modification, constitutes organic evolution.
. Click the link for more information. ). All organisms other than cyanobacteria and bacteria consist of one or more eukaryotic cells.

All cells share a number of common properties they store information in genes gene,
the structural unit of inheritance in living organisms. A gene is, in essence, a segment of DNA that has a particular purpose, i.e., that codes for (contains the chemical information necessary for the creation of) a specific enzyme or other protein.
. Click the link for more information. made of DNA (see nucleic acid nucleic acid,
any of a group of organic substances found in the chromosomes of living cells and viruses that play a central role in the storage and replication of hereditary information and in the expression of this information through protein synthesis.
. Click the link for more information. ) they use proteins protein,
any of the group of highly complex organic compounds found in all living cells and comprising the most abundant class of all biological molecules. Protein comprises approximately 50% of cellular dry weight.
. Click the link for more information. as their main structural material they synthesize proteins in the cell's ribosomes using the information encoded in the DNA and mobilized by means of RNA they use adenosine triphosphate adenosine triphosphate
(ATP) , organic compound composed of adenine, the sugar ribose, and three phosphate groups. ATP serves as the major energy source within the cell to drive a number of biological processes such as photosynthesis, muscle contraction, and the synthesis of
. Click the link for more information. as the means of transferring energy for the cell's internal processes and they are enclosed by a cell membrane, composed of proteins and a double layer of lipid lipids,
a broad class of organic products found in living systems. Most are insoluble in water but soluble in nonpolar solvents. The definition excludes the mineral oils and other petroleum products obtained from fossil material.
. Click the link for more information. molecules, that controls the flow of materials into and out of the cell.

Cell Structure

In a eukaryotic cell's nucleus the DNA, along with certain proteins, is arranged in long, thin threads called chromatin fibers that coil into bodies called chromosomes chromosome
, structural carrier of hereditary characteristics, found in the nucleus of every cell and so named for its readiness to absorb dyes. The term chromosome
. Click the link for more information. during meiosis meiosis
, process of nuclear division in a living cell by which the number of chromosomes is reduced to half the original number. Meiosis occurs only in the process of gametogenesis, i.e., when the gametes, or sex cells (ovum and sperm), are being formed.
. Click the link for more information. . The nucleus also contains one or more nucleoli (sing., nucleolus) that participate in the production on the RNA of ribosomes. The portion of the cell outside the nucleus, called the cytoplasm, contains several additional cell structures (often called organelles). Among the important organelles that may be present are the ribosomes the endoplasmic reticulum, a highly convoluted system of membranes believed to be continuous with the nuclear envelope and responsible for transporting certain newly made proteins the mitochondria, which are present in nearly all eukaryotic cells and extract energy by breaking down the chemical bonds in molecules of complex nutrients during respiration and perform other functions the chloroplasts, which are present only in green plants and convert energy from sunlight by the process of photosynthesis photosynthesis
, process in which green plants, algae, and cyanobacteria utilize the energy of sunlight to manufacture carbohydrates from carbon dioxide and water in the presence of chlorophyll. Some of the plants that lack chlorophyll, e.g.
. Click the link for more information. lysosomes, which contain digestive enzymes peroxisomes, which contain a number of specialized enzymes the centrosomes, which function during cell division the Golgi apparatus, which functions in the synthesis, storage, and secretion of various cellular products filaments and microtubules that form a sort of skeletal system known as a cytoskeleton and also participate in movement of cells and organelles vacuoles containing food in various stages of digestion (see endocytosis endocytosis
, in biology, process by which substances are taken into the cell. When the cell membrane comes into contact with a suitable food, a portion of the cell cytoplasm surges forward to meet and surround the material and a depression forms within the cell wall.
. Click the link for more information. ) and inert granules and crystals. In plant cells there is, in addition to the cell membrane, a thickened cell wall, usually composed chiefly of cellulose cellulose,
chief constituent of the cell walls of plants. Chemically, it is a carbohydrate that is a high molecular weight polysaccharide. Raw cotton is composed of 91% pure cellulose other important natural sources are flax, hemp, jute, straw, and wood.
. Click the link for more information. secreted by the cell.

The Study of Cells

Because almost all cells are microscopic, knowledge of the component cell parts increased proportionately to the development of the microscope microscope,
optical instrument used to increase the apparent size of an object. Simple Microscopes

A magnifying glass, an ordinary double convex lens having a short focal length, is a simple microscope. The reading lens and hand lens are instruments of this type.
. Click the link for more information. and other specialized instruments and of allied experimental techniques. Among those who contributed to early knowledge of cells through their use of the microscope were Antony van Leeuwenhoek Leeuwenhoek, Antony van
, 1632�, Dutch student of natural history and maker of microscopes, b. Delft. His use of lenses in examining cloth as a draper's apprentice probably led to his interest in lens making.
. Click the link for more information. , Robert Hooke Hooke, Robert
, 1635�, English physicist, mathematician, and inventor. He became curator of experiments for the Royal Society (1662), professor of geometry at Gresham College (1665), and city surveyor of London after the great 1666 fire.
. Click the link for more information. , and Marcello Malpighi Malpighi, Marcello
, 1628󈟊, Italian anatomist. A pioneer in the use of the microscope, he made many valuable observations on the structure of plants and animals.
. Click the link for more information. . In the 19th cent. Matthias J. Schleiden Schleiden, Matthias Jakob
, 1804󈞽, German botanist. He was professor at the universities of Jena (1839󈞫) and Dorpat (1863󈞬). With Theodor Schwann, he is credited with establishing the foundations of the cell theory.
. Click the link for more information. and Theodor Schwann Schwann, Theodor
, 1810󈞾, German physiologist and histologist. He was a student of J. P. Müller and professor at the universities of Louvain (1838󈞜) and Liège (from 1848).
. Click the link for more information. developed what is now known as the cell theory. The theory was widely promoted after the pronouncement by Rudolf Virchow in 1855 that "omnis cellulae e cellula" [All cells arise from cells]. The study of cell structure came to be called cytology and that of tissues histology. In the 20th cent. appreciation of the biochemistry of the cell flourished, along with a better understanding of its structure cell biology now integrates both chemical and structural information.

See also biochemistry biochemistry,
science concerned chiefly with the chemistry of biological processes it attempts to utilize the tools and concepts of chemistry, particularly organic and physical chemistry, for elucidation of the living system.
. Click the link for more information. .

Bibliography

See L. Thomas, The Lives of a Cell (1974) D. M. Prescott, Cells (1988) B. Alberts et al., Molecular Biology of the Cell (2d ed. 1989) J. M. Lackie and J. A. Dowe, ed., The Dictionary of Cell Biology (1989).


Contents

Cells are of two types: eukaryotic, which contain a nucleus, and prokaryotic, which do not. Prokaryotes are single-celled organisms, while eukaryotes can be either single-celled or multicellular.

Prokaryotic cells

Prokaryotes include bacteria and archaea, two of the three domains of life. Prokaryotic cells were the first form of life on Earth, characterized by having vital biological processes including cell signaling. They are simpler and smaller than eukaryotic cells, and lack a nucleus, and other membrane-bound organelles. The DNA of a prokaryotic cell consists of a single circular chromosome that is in direct contact with the cytoplasm. The nuclear region in the cytoplasm is called the nucleoid. Most prokaryotes are the smallest of all organisms ranging from 0.5 to 2.0 μm in diameter. [13]

A prokaryotic cell has three regions:

  • Enclosing the cell is the cell envelope – generally consisting of a plasma membrane covered by a cell wall which, for some bacteria, may be further covered by a third layer called a capsule. Though most prokaryotes have both a cell membrane and a cell wall, there are exceptions such as Mycoplasma (bacteria) and Thermoplasma (archaea) which only possess the cell membrane layer. The envelope gives rigidity to the cell and separates the interior of the cell from its environment, serving as a protective filter. The cell wall consists of peptidoglycan in bacteria, and acts as an additional barrier against exterior forces. It also prevents the cell from expanding and bursting (cytolysis) from osmotic pressure due to a hypotonic environment. Some eukaryotic cells (plant cells and fungal cells) also have a cell wall.
  • Inside the cell is the cytoplasmic region that contains the genome (DNA), ribosomes and various sorts of inclusions. [4] The genetic material is freely found in the cytoplasm. Prokaryotes can carry extrachromosomal DNA elements called plasmids, which are usually circular. Linear bacterial plasmids have been identified in several species of spirochete bacteria, including members of the genus Borrelia notably Borrelia burgdorferi, which causes Lyme disease. [14] Though not forming a nucleus, the DNA is condensed in a nucleoid. Plasmids encode additional genes, such as antibiotic resistance genes.
  • On the outside, flagella and pili project from the cell's surface. These are structures (not present in all prokaryotes) made of proteins that facilitate movement and communication between cells.

Eukaryotic cells

Plants, animals, fungi, slime moulds, protozoa, and algae are all eukaryotic. These cells are about fifteen times wider than a typical prokaryote and can be as much as a thousand times greater in volume. The main distinguishing feature of eukaryotes as compared to prokaryotes is compartmentalization: the presence of membrane-bound organelles (compartments) in which specific activities take place. Most important among these is a cell nucleus, [4] an organelle that houses the cell's DNA. This nucleus gives the eukaryote its name, which means "true kernel (nucleus)". Other differences include:

  • The plasma membrane resembles that of prokaryotes in function, with minor differences in the setup. Cell walls may or may not be present.
  • The eukaryotic DNA is organized in one or more linear molecules, called chromosomes, which are associated with histone proteins. All chromosomal DNA is stored in the cell nucleus, separated from the cytoplasm by a membrane. [4] Some eukaryotic organelles such as mitochondria also contain some DNA.
  • Many eukaryotic cells are ciliated with primary cilia. Primary cilia play important roles in chemosensation, mechanosensation, and thermosensation. Each cilium may thus be "viewed as a sensory cellular antennae that coordinates a large number of cellular signaling pathways, sometimes coupling the signaling to ciliary motility or alternatively to cell division and differentiation." [15]
  • Motile eukaryotes can move using motile cilia or flagella. Motile cells are absent in conifers and flowering plants. [16] Eukaryotic flagella are more complex than those of prokaryotes. [17]

All cells, whether prokaryotic or eukaryotic, have a membrane that envelops the cell, regulates what moves in and out (selectively permeable), and maintains the electric potential of the cell. Inside the membrane, the cytoplasm takes up most of the cell's volume. All cells (except red blood cells which lack a cell nucleus and most organelles to accommodate maximum space for hemoglobin) possess DNA, the hereditary material of genes, and RNA, containing the information necessary to build various proteins such as enzymes, the cell's primary machinery. There are also other kinds of biomolecules in cells. This article lists these primary cellular components, then briefly describes their function.

Membrane

The cell membrane, or plasma membrane, is a biological membrane that surrounds the cytoplasm of a cell. In animals, the plasma membrane is the outer boundary of the cell, while in plants and prokaryotes it is usually covered by a cell wall. This membrane serves to separate and protect a cell from its surrounding environment and is made mostly from a double layer of phospholipids, which are amphiphilic (partly hydrophobic and partly hydrophilic). Hence, the layer is called a phospholipid bilayer, or sometimes a fluid mosaic membrane. Embedded within this membrane is a macromolecular structure called the porosome the universal secretory portal in cells and a variety of protein molecules that act as channels and pumps that move different molecules into and out of the cell. [4] The membrane is semi-permeable, and selectively permeable, in that it can either let a substance (molecule or ion) pass through freely, pass through to a limited extent or not pass through at all. Cell surface membranes also contain receptor proteins that allow cells to detect external signaling molecules such as hormones.

Cytoskeleton

The cytoskeleton acts to organize and maintain the cell's shape anchors organelles in place helps during endocytosis, the uptake of external materials by a cell, and cytokinesis, the separation of daughter cells after cell division and moves parts of the cell in processes of growth and mobility. The eukaryotic cytoskeleton is composed of microtubules, intermediate filaments and microfilaments. In the cytoskeleton of a neuron the intermediate filaments are known as neurofilaments. There are a great number of proteins associated with them, each controlling a cell's structure by directing, bundling, and aligning filaments. [4] The prokaryotic cytoskeleton is less well-studied but is involved in the maintenance of cell shape, polarity and cytokinesis. [19] The subunit protein of microfilaments is a small, monomeric protein called actin. The subunit of microtubules is a dimeric molecule called tubulin. Intermediate filaments are heteropolymers whose subunits vary among the cell types in different tissues. But some of the subunit protein of intermediate filaments include vimentin, desmin, lamin (lamins A, B and C), keratin (multiple acidic and basic keratins), neurofilament proteins (NF–L, NF–M).

Genetic material

Two different kinds of genetic material exist: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Cells use DNA for their long-term information storage. The biological information contained in an organism is encoded in its DNA sequence. [4] RNA is used for information transport (e.g., mRNA) and enzymatic functions (e.g., ribosomal RNA). Transfer RNA (tRNA) molecules are used to add amino acids during protein translation.

Prokaryotic genetic material is organized in a simple circular bacterial chromosome in the nucleoid region of the cytoplasm. Eukaryotic genetic material is divided into different, [4] linear molecules called chromosomes inside a discrete nucleus, usually with additional genetic material in some organelles like mitochondria and chloroplasts (see endosymbiotic theory).

A human cell has genetic material contained in the cell nucleus (the nuclear genome) and in the mitochondria (the mitochondrial genome). In humans the nuclear genome is divided into 46 linear DNA molecules called chromosomes, including 22 homologous chromosome pairs and a pair of sex chromosomes. The mitochondrial genome is a circular DNA molecule distinct from the nuclear DNA. Although the mitochondrial DNA is very small compared to nuclear chromosomes, [4] it codes for 13 proteins involved in mitochondrial energy production and specific tRNAs.

Foreign genetic material (most commonly DNA) can also be artificially introduced into the cell by a process called transfection. This can be transient, if the DNA is not inserted into the cell's genome, or stable, if it is. Certain viruses also insert their genetic material into the genome.

Organelles

Organelles are parts of the cell which are adapted and/or specialized for carrying out one or more vital functions, analogous to the organs of the human body (such as the heart, lung, and kidney, with each organ performing a different function). [4] Both eukaryotic and prokaryotic cells have organelles, but prokaryotic organelles are generally simpler and are not membrane-bound.

There are several types of organelles in a cell. Some (such as the nucleus and golgi apparatus) are typically solitary, while others (such as mitochondria, chloroplasts, peroxisomes and lysosomes) can be numerous (hundreds to thousands). The cytosol is the gelatinous fluid that fills the cell and surrounds the organelles.

Eukaryotic

  • Cell nucleus: A cell's information center, the cell nucleus is the most conspicuous organelle found in a eukaryotic cell. It houses the cell's chromosomes, and is the place where almost all DNA replication and RNA synthesis (transcription) occur. The nucleus is spherical and separated from the cytoplasm by a double membrane called the nuclear envelope. The nuclear envelope isolates and protects a cell's DNA from various molecules that could accidentally damage its structure or interfere with its processing. During processing, DNA is transcribed, or copied into a special RNA, called messenger RNA (mRNA). This mRNA is then transported out of the nucleus, where it is translated into a specific protein molecule. The nucleolus is a specialized region within the nucleus where ribosome subunits are assembled. In prokaryotes, DNA processing takes place in the cytoplasm. [4]
  • Mitochondria and chloroplasts: generate energy for the cell. Mitochondria are self-replicating organelles that occur in various numbers, shapes, and sizes in the cytoplasm of all eukaryotic cells. [4]Respiration occurs in the cell mitochondria, which generate the cell's energy by oxidative phosphorylation, using oxygen to release energy stored in cellular nutrients (typically pertaining to glucose) to generate ATP. Mitochondria multiply by binary fission, like prokaryotes. Chloroplasts can only be found in plants and algae, and they capture the sun's energy to make carbohydrates through photosynthesis.
  • Endoplasmic reticulum: The endoplasmic reticulum (ER) is a transport network for molecules targeted for certain modifications and specific destinations, as compared to molecules that float freely in the cytoplasm. The ER has two forms: the rough ER, which has ribosomes on its surface that secrete proteins into the ER, and the smooth ER, which lacks ribosomes. [4] The smooth ER plays a role in calcium sequestration and release.
  • Golgi apparatus: The primary function of the Golgi apparatus is to process and package the macromolecules such as proteins and lipids that are synthesized by the cell.
  • Lysosomes and peroxisomes: Lysosomes contain digestive enzymes (acid hydrolases). They digest excess or worn-out organelles, food particles, and engulfed viruses or bacteria. Peroxisomes have enzymes that rid the cell of toxic peroxides. The cell could not house these destructive enzymes if they were not contained in a membrane-bound system. [4]
  • Centrosome: the cytoskeleton organiser: The centrosome produces the microtubules of a cell – a key component of the cytoskeleton. It directs the transport through the ER and the Golgi apparatus. Centrosomes are composed of two centrioles, which separate during cell division and help in the formation of the mitotic spindle. A single centrosome is present in the animal cells. They are also found in some fungi and algae cells.
  • Vacuoles: Vacuoles sequester waste products and in plant cells store water. They are often described as liquid filled space and are surrounded by a membrane. Some cells, most notably Amoeba, have contractile vacuoles, which can pump water out of the cell if there is too much water. The vacuoles of plant cells and fungal cells are usually larger than those of animal cells.

Eukaryotic and prokaryotic

  • Ribosomes: The ribosome is a large complex of RNA and protein molecules. [4] They each consist of two subunits, and act as an assembly line where RNA from the nucleus is used to synthesise proteins from amino acids. Ribosomes can be found either floating freely or bound to a membrane (the rough endoplasmatic reticulum in eukaryotes, or the cell membrane in prokaryotes). [20]

Many cells also have structures which exist wholly or partially outside the cell membrane. These structures are notable because they are not protected from the external environment by the semipermeable cell membrane. In order to assemble these structures, their components must be carried across the cell membrane by export processes.

Cell wall

Many types of prokaryotic and eukaryotic cells have a cell wall. The cell wall acts to protect the cell mechanically and chemically from its environment, and is an additional layer of protection to the cell membrane. Different types of cell have cell walls made up of different materials plant cell walls are primarily made up of cellulose, fungi cell walls are made up of chitin and bacteria cell walls are made up of peptidoglycan.

Prokaryotic

Capsule

A gelatinous capsule is present in some bacteria outside the cell membrane and cell wall. The capsule may be polysaccharide as in pneumococci, meningococci or polypeptide as Bacillus anthracis or hyaluronic acid as in streptococci. Capsules are not marked by normal staining protocols and can be detected by India ink or methyl blue which allows for higher contrast between the cells for observation. [21] : 87

Flagella

Flagella are organelles for cellular mobility. The bacterial flagellum stretches from cytoplasm through the cell membrane(s) and extrudes through the cell wall. They are long and thick thread-like appendages, protein in nature. A different type of flagellum is found in archaea and a different type is found in eukaryotes.

Fimbriae

A fimbria (plural fimbriae also known as a pilus, plural pili) is a short, thin, hair-like filament found on the surface of bacteria. Fimbriae are formed of a protein called pilin (antigenic) and are responsible for the attachment of bacteria to specific receptors on human cells (cell adhesion). There are special types of pili involved in bacterial conjugation.

Replication

Cell division involves a single cell (called a mother cell) dividing into two daughter cells. This leads to growth in multicellular organisms (the growth of tissue) and to procreation (vegetative reproduction) in unicellular organisms. Prokaryotic cells divide by binary fission, while eukaryotic cells usually undergo a process of nuclear division, called mitosis, followed by division of the cell, called cytokinesis. A diploid cell may also undergo meiosis to produce haploid cells, usually four. Haploid cells serve as gametes in multicellular organisms, fusing to form new diploid cells.

DNA replication, or the process of duplicating a cell's genome, [4] always happens when a cell divides through mitosis or binary fission. This occurs during the S phase of the cell cycle.

In meiosis, the DNA is replicated only once, while the cell divides twice. DNA replication only occurs before meiosis I. DNA replication does not occur when the cells divide the second time, in meiosis II. [22] Replication, like all cellular activities, requires specialized proteins for carrying out the job. [4]

DNA repair

In general, cells of all organisms contain enzyme systems that scan their DNA for damages and carry out repair processes when damages are detected. [23] Diverse repair processes have evolved in organisms ranging from bacteria to humans. The widespread prevalence of these repair processes indicates the importance of maintaining cellular DNA in an undamaged state in order to avoid cell death or errors of replication due to damages that could lead to mutation. E. coli bacteria are a well-studied example of a cellular organism with diverse well-defined DNA repair processes. These include: (1) nucleotide excision repair, (2) DNA mismatch repair, (3) non-homologous end joining of double-strand breaks, (4) recombinational repair and (5) light-dependent repair (photoreactivation).

Growth and metabolism

Between successive cell divisions, cells grow through the functioning of cellular metabolism. Cell metabolism is the process by which individual cells process nutrient molecules. Metabolism has two distinct divisions: catabolism, in which the cell breaks down complex molecules to produce energy and reducing power, and anabolism, in which the cell uses energy and reducing power to construct complex molecules and perform other biological functions. Complex sugars consumed by the organism can be broken down into simpler sugar molecules called monosaccharides such as glucose. Once inside the cell, glucose is broken down to make adenosine triphosphate (ATP), [4] a molecule that possesses readily available energy, through two different pathways.

Protein synthesis

Cells are capable of synthesizing new proteins, which are essential for the modulation and maintenance of cellular activities. This process involves the formation of new protein molecules from amino acid building blocks based on information encoded in DNA/RNA. Protein synthesis generally consists of two major steps: transcription and translation.

Transcription is the process where genetic information in DNA is used to produce a complementary RNA strand. This RNA strand is then processed to give messenger RNA (mRNA), which is free to migrate through the cell. mRNA molecules bind to protein-RNA complexes called ribosomes located in the cytosol, where they are translated into polypeptide sequences. The ribosome mediates the formation of a polypeptide sequence based on the mRNA sequence. The mRNA sequence directly relates to the polypeptide sequence by binding to transfer RNA (tRNA) adapter molecules in binding pockets within the ribosome. The new polypeptide then folds into a functional three-dimensional protein molecule.

Motility

Unicellular organisms can move in order to find food or escape predators. Common mechanisms of motion include flagella and cilia.

In multicellular organisms, cells can move during processes such as wound healing, the immune response and cancer metastasis. For example, in wound healing in animals, white blood cells move to the wound site to kill the microorganisms that cause infection. Cell motility involves many receptors, crosslinking, bundling, binding, adhesion, motor and other proteins. [24] The process is divided into three steps – protrusion of the leading edge of the cell, adhesion of the leading edge and de-adhesion at the cell body and rear, and cytoskeletal contraction to pull the cell forward. Each step is driven by physical forces generated by unique segments of the cytoskeleton. [25] [26]

Navigation, control and communication

In August 2020, scientists described one way cells – in particular cells of a slime mold and mouse pancreatic cancer–derived cells – are able to navigate efficiently through a body and identify the best routes through complex mazes: generating gradients after breaking down diffused chemoattractants which enable them to sense upcoming maze junctions before reaching them, including around corners. [27] [28] [29]


22.1 Prokaryotic Diversity

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

  • Describe the evolutionary history of prokaryotes
  • Discuss the distinguishing features of extremophiles
  • Explain why it is difficult to culture prokaryotes

Prokaryotes are ubiquitous. They cover every imaginable surface where there is sufficient moisture, and they also live on and inside virtually all other living things. In the typical human body, prokaryotic cells outnumber human body cells by about ten to one. They comprise the majority of living things in all ecosystems. Some prokaryotes thrive in environments that are inhospitable for most living things. Prokaryotes recycle nutrients —essential substances (such as carbon and nitrogen)—and they drive the evolution of new ecosystems, some of which are natural and others man-made. Prokaryotes have been on Earth since long before multicellular life appeared. Indeed, eukaryotic cells are thought to be the descendants of ancient prokaryotic communities.

Prokaryotes, the First Inhabitants of Earth

When and where did cellular life begin? What were the conditions on Earth when life began? We now know that prokaryotes were likely the first forms of cellular life on Earth, and they existed for billions of years before plants and animals appeared. The Earth and its moon are dated at about 4.54 billion years in age. This estimate is based on evidence from radiometric dating of meteorite material together with other substrate material from Earth and the moon. Early Earth had a very different atmosphere (contained less molecular oxygen) than it does today and was subjected to strong solar radiation thus, the first organisms probably would have flourished where they were more protected, such as in the deep ocean or far beneath the surface of the Earth. Strong volcanic activity was common on Earth at this time, so it is likely that these first organisms—the first prokaryotes—were adapted to very high temperatures. Because early Earth was prone to geological upheaval and volcanic eruption, and was subject to bombardment by mutagenic radiation from the sun, the first organisms were prokaryotes that must have withstood these harsh conditions.

Microbial Mats

Microbial mats or large biofilms may represent the earliest forms of prokaryotic life on Earth there is fossil evidence of their presence starting about 3.5 billion years ago. It is remarkable that cellular life appeared on Earth only a billion years after the Earth itself formed, suggesting that pre-cellular “life” that could replicate itself had evolved much earlier. A microbial mat is a multi-layered sheet of prokaryotes (Figure 22.2) that includes mostly bacteria, but also archaeans. Microbial mats are only a few centimeters thick, and they typically grow where different types of materials interface, mostly on moist surfaces. The various types of prokaryotes that comprise them carry out different metabolic pathways, and that is the reason for their various colors. Prokaryotes in a microbial mat are held together by a glue-like sticky substance that they secrete called extracellular matrix.

The first microbial mats likely obtained their energy from chemicals found near hydrothermal vents. A hydrothermal vent is a breakage or fissure in the Earth’s surface that releases geothermally heated water. With the evolution of photosynthesis about three billion years ago, some prokaryotes in microbial mats came to use a more widely available energy source—sunlight—whereas others were still dependent on chemicals from hydrothermal vents for energy and food.

Stromatolites

Fossilized microbial mats represent the earliest record of life on Earth. A stromatolite is a sedimentary structure formed when minerals are precipitated out of water by prokaryotes in a microbial mat (Figure 22.3). Stromatolites form layered rocks made of carbonate or silicate. Although most stromatolites are artifacts from the past, there are places on Earth where stromatolites are still forming. For example, growing stromatolites have been found in the Anza-Borrego Desert State Park in San Diego County, California.

The Ancient Atmosphere

Evidence indicates that during the first two billion years of Earth’s existence, the atmosphere was anoxic , meaning that there was no molecular oxygen. Therefore, only those organisms that can grow without oxygen—anaerobic organisms—were able to live. Autotrophic organisms that convert solar energy into chemical energy are called phototrophs , and they appeared within one billion years of the formation of Earth. Then, cyanobacteria , also known as “blue-green algae,” evolved from these simple phototrophs at least one billion years later. It was the ancestral cyanobacteria (Figure 22.4) that began the “oxygenation” of the atmosphere: Increased atmospheric oxygen allowed the evolution of more efficient O2-utilizing catabolic pathways. It also opened up the land to increased colonization, because some O2 is converted into O3 (ozone) and ozone effectively absorbs the ultraviolet light that could have otherwise caused lethal mutations in DNA. The current evidence suggests that the increase in O2 concentrations allowed the evolution of other life forms.

Microbes Are Adaptable: Life in Moderate and Extreme Environments

Some organisms have developed strategies that allow them to survive harsh conditions. Almost all prokaryotes have a cell wall, a protective structure that allows them to survive in both hypertonic and hypotonic aqueous conditions. Some soil bacteria are able to form endospores that resist heat and drought, thereby allowing the organism to survive until favorable conditions recur. These adaptations, along with others, allow bacteria to remain the most abundant life form in all terrestrial and aquatic ecosystems.

Prokaryotes thrive in a vast array of environments: Some grow in conditions that would seem very normal to us, whereas others are able to thrive and grow under conditions that would kill a plant or an animal. Bacteria and archaea that are adapted to grow under extreme conditions are called extremophiles , meaning “lovers of extremes.” Extremophiles have been found in all kinds of environments: the depths of the oceans, hot springs, the Arctic and the Antarctic, in very dry places, deep inside Earth, in harsh chemical environments, and in high radiation environments (Figure 22.5), just to mention a few. Because they have specialized adaptations that allow them to live in extreme conditions, many extremophiles cannot survive in moderate environments. There are many different groups of extremophiles: They are identified based on the conditions in which they grow best, and several habitats are extreme in multiple ways. For example, a soda lake is both salty and alkaline, so organisms that live in a soda lake must be both alkaliphiles and halophiles (Table 22.1). Other extremophiles, like radioresistant organisms, do not prefer an extreme environment (in this case, one with high levels of radiation), but have adapted to survive in it (Figure 22.5). Organisms like these give us a better understanding of prokaryotic diversity and open up the possibility of finding new prokaryotic species that may lead to the discovery of new therapeutic drugs or have industrial applications.

Extremophile Conditions for Optimal Growth
Acidophiles pH 3 or below
Alkaliphiles pH 9 or above
Thermophiles Temperature 60–80 °C (140–176 °F)
Hyperthermophiles Temperature 80–122 °C (176–250 °F)
Psychrophiles Temperature of -15-10 °C (5-50 °F) or lower
Halophiles Salt concentration of at least 0.2 M
Osmophiles High sugar concentration

Prokaryotes in the Dead Sea

One example of a very harsh environment is the Dead Sea, a hypersaline basin that is located between Jordan and Israel. Hypersaline environments are essentially concentrated seawater. In the Dead Sea, the sodium concentration is 10 times higher than that of seawater, and the water contains high levels of magnesium (about 40 times higher than in seawater) that would be toxic to most living things. Iron, calcium, and magnesium, elements that form divalent ions (Fe 2+ , Ca 2+ , and Mg 2+ ), produce what is commonly referred to as “hard” water. Taken together, the high concentration of divalent cations, the acidic pH (6.0), and the intense solar radiation flux make the Dead Sea a unique, and uniquely hostile, ecosystem 1 (Figure 22.6).

What sort of prokaryotes do we find in the Dead Sea? The extremely salt-tolerant bacterial mats include Halobacterium, Haloferax volcanii (which is found in other locations, not only the Dead Sea), Halorubrum sodomense, and Halobaculum gomorrense, and the archaean Haloarcula marismortui, among others.

Unculturable Prokaryotes and the Viable-but-Non-Culturable State

The process of culturing bacteria is complex and is one of the greatest discoveries of modern science. German physician Robert Koch is credited with discovering the techniques for pure culture, including staining and using growth media. Microbiologists typically grow prokaryotes in the laboratory using an appropriate culture medium containing all the nutrients needed by the target organism. The medium can be liquid, broth, or solid. After an incubation time at the right temperature, there should be evidence of microbial growth (Figure 22.7). Koch's assistant Julius Petri invented the Petri dish, whose use persists in today’s laboratories. Koch worked primarily with the Mycobacterium tuberculosis bacterium that causes tuberculosis and developed guidelines, called Koch's postulates , to identify the organisms responsible for specific diseases. Koch's postulates continue to be widely used in the medical community. Koch’s postulates include that an organism can be identified as the cause of disease when it is present in all infected samples and absent in all healthy samples, and it is able to reproduce the infection after being cultured multiple times. Today, cultures remain a primary diagnostic tool in medicine and other areas of molecular biology.

Koch's postulates can be fully applied only to organisms that can be isolated and cultured. Some prokaryotes, however, cannot grow in a laboratory setting. In fact, over 99 percent of bacteria and archaea are unculturable. For the most part, this is due to a lack of knowledge as to what to feed these organisms and how to grow them they may have special requirements for growth that remain unknown to scientists, such as needing specific micronutrients, pH, temperature, pressure, co-factors, or co-metabolites. Some bacteria cannot be cultured because they are obligate intracellular parasites and cannot be grown outside a host cell.

In other cases, culturable organisms become unculturable under stressful conditions, even though the same organism could be cultured previously. Those organisms that cannot be cultured but are not dead are in a viable-but-non-culturable (VBNC) state. The VBNC state occurs when prokaryotes respond to environmental stressors by entering a dormant state that allows their survival. The criteria for entering into the VBNC state are not completely understood. In a process called resuscitation , the prokaryote can go back to “normal” life when environmental conditions improve.

Is the VBNC state an unusual way of living for prokaryotes? In fact, most of the prokaryotes living in the soil or in oceanic waters are non-culturable. It has been said that only a small fraction, perhaps one percent, of prokaryotes can be cultured under laboratory conditions. If these organisms are non-culturable, then how is it known whether they are present and alive? Microbiologists use molecular techniques, such as the polymerase chain reaction (PCR), to amplify selected portions of DNA of prokaryotes, e.g., 16S rRNA genes, demonstrating their existence. (Recall that PCR can make billions of copies of a DNA segment in a process called amplification.)

The Ecology of Biofilms

Some prokaryotes may be unculturable because they require the presence of other prokaryotic species. Until a couple of decades ago, microbiologists used to think of prokaryotes as isolated entities living apart. This model, however, does not reflect the true ecology of prokaryotes, most of which prefer to live in communities where they can interact. As we have seen, a biofilm is a microbial community (Figure 22.8) held together in a gummy-textured matrix that consists primarily of polysaccharides secreted by the organisms, together with some proteins and nucleic acids. Biofilms typically grow attached to surfaces. Some of the best-studied biofilms are composed of prokaryotes, although fungal biofilms have also been described, as well as some composed of a mixture of fungi and bacteria.

Biofilms are present almost everywhere: they can cause the clogging of pipes and readily colonize surfaces in industrial settings. In recent, large-scale outbreaks of bacterial contamination of food, biofilms have played a major role. They also colonize household surfaces, such as kitchen counters, cutting boards, sinks, and toilets, as well as places on the human body, such as the surfaces of our teeth.

Interactions among the organisms that populate a biofilm, together with their protective exopolysaccharidic (EPS) environment, make these communities more robust than free-living, or planktonic, prokaryotes. The sticky substance that holds bacteria together also excludes most antibiotics and disinfectants, making biofilm bacteria hardier than their planktonic counterparts. Overall, biofilms are very difficult to destroy because they are resistant to many common forms of sterilization.

Visual Connection

Compared to free-floating bacteria, bacteria in biofilms often show increased resistance to antibiotics and detergents. Why do you think this might be the case?


Details Are Red blood cells prokaryotic or eukaryotic ?

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Biology Blog

11 comments:

I think that this group has done a good job, however, it could still be improved. I personallay think that they should not use words that are too sophisicated as some of us might not understand. If it is a scientific tearm, i hope that the group could futher elaborate.

In this report, I learnt that haemoglobin is found in the cytoplasm of red blood cells. I also learnt more about the structure of haemoglobin based on the diagrams.

From this report, I learnt that haemoglobin is found in the cytoplasm in the red blood cells and it helps to bind the oxygen and release them throughout the body.
However, there were some sophisticated words that I did not understand. They should have used less sophisticated words so that the readers would be able to understand.

through this report, i have learnt that haemoglobin is present in red blood cells,and that it helps in binding oxygen to release throughout the body, though i must say that there some complex words here. it would be best if we could understand everything.

Interesting fact learnt: The haemogoblin in the cytoplasm helps to bind the oxygen and release them throughout the body. Due to this, we would not have any difficulty breathing or suffocate to death.

Short and simple but i feel that you could elaborate more and give more details

I do understand everything they've written as I've read about them before. But as my classmates do not understand some of the complex organelles, I thhink it's also better that they try to use more simplified terms.

I I learnt that the heamoglobin is present in the red blood cell.
However, i also feel that you could elaborate more on the details.

Can't find any information on functions of organelles in a premature red blood cell?


Prokaryotes Examples

A cell is the smallest biological unit of life with most having a nucleus in its center. However, there are cells without a nucleus, which are called prokaryotes. They are a group of organisms which lack a cell nucleus. The organisms with this type of cell are called prokaryotic organisms or prokaryotes. These organisms were the first to be found in the planet Earth.

A prokaryotic cell is mostly composed of a plasma membrane, cell wall, cytoplasm, genetic material in the nucleoid and ribosome. They are single-celled and are much smaller compared to eukaryotic cells. They exist in different shapes including spherical, rod, flat, coccus, spirochete, and some are also shapeless, not having a consistent shape.

Only a few of them can move, swim, spin, or rotate with the help of a helical shaped membrane called flagella. The ways prokaryotes receive nutrients include synthesizing their own food by using light energy from the atmosphere, prepare its own food by the process of chemosynthesis, or depend on other substances for nutrition when they cannot synthesize their own food.

1. Escherichia Coli Bacterium (E. coli)

It is a rod-shaped bacterium commonly found in the lower intestine of warm-blooded organisms. Most E. coli strains are harmless, but some can cause food poisoning, and are occasionally responsible for food recalls. Harmless E. coli can be beneficial by producing vitamin K2 and preventing the intestine in becoming colonized with pathogenic bacteria.

2. Streptococcus Bacterium

This prokaryote is responsible for strep throat. It is an infection of the back of the throat which includes the tonsils. Symptoms include fever, red tonsils, sore throat and enlarged lymph nodes in the neck. Cell division in this bacterium occurs along a single axis and they grow in chains or pairs.

3. Streptomyces Soil Bacteria

Over 500 of this type of bacteria have been described. They are predominantly found in soil and in decaying vegetation, with most producing spores. They have a distinct earthy odor resulting from the production of a volatile metabolite, geosmin.

The subclass of archaea are prokaryotes and are able to survive in very harsh environments. An example of archaea can be found in geothermally active areas and live in extremely acidic mud pots, which is called sulfolobus acidocaldarius archeobacterium.


Umuc Biology 102 103 Lab 3 Cell Structure And Function Answer Key

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This contains 100% correct material for UMUC Biology 103 LAB03. However, this is an Answer Key, which means, you should put it in your own words. Here is a sample for the Pre lab questions answered:

1. Identify the major similarities and differences between prokaryotic and eukaryotic cells. (2 pts)

Prokaryotes tend to be less complex than eukaryotic cells, with fewer organelles and (generally) fewer requirements for survival. Eukaryotes have a nucleus, while prokaryotes do not. Both eukaryotes and prokaryotes have DNA, a cell membrane, and cytoplasm.

2. Where is the DNA housed in a prokaryotic cell? Where is it housed in a eukaryotic cell? (2 pts)

DNA is housed in the nucleus in eukaryotic cells. Prokaryotic cells do not have a nucleus, and thus DNA exists freely in the cytoplasm.

3. Identify three structures which provide support and protection in a eukaryotic cell. (2 pts)

The cell membrane, the cytoplasm, and the cytoskeleton (microtubules, microfilaments, etc.).

The rest of the questions are answered as well:

Experiment 1: Cell Structure and Function

1. Label each of the arrows in the following slide image:

2. What is the difference between the rough and smooth endoplasmic reticulum?

3. Would an animal cell be able to survive without a mitochondria? Why or why not?

4. What could you determine about a specimen if you observed a slide image showing the specimen with a cell wall, but no nucleus or mitochondria?

5. Hypothesize why parts of a plant, such as the leaves, are green, but other parts, such as the roots, are not. Use scientific reasoning to support your hypothesis.


Animal-Like Protists

There are four separate phyla of protists with animal characteristics. In early classification schemes, they were clumped together and called protozoa to separate them from the more plantlike protists:

Bionote

Malaria is still a problem in developing tropical countries where millions of people become infected annually.

Sporozoa

Sporozoa are among the best known protists because they are all parasites, including human parasites. They usually live in a host organism and reproduce by spores, which are dormant cells enclosed in a protective membrane. Whenever the spores land on an appropriate host, they are able to enter by various means and then grow to maturity as parasites. The parasitic cells have specialized organelles for penetrating host cell membranes. More exotic sporozoa have life cycles that involve two hosts.

The plasmodium is the parasite that causes malaria by entering human red blood cells and digesting their nutritional contents until the red blood cells become nonfunctional. The plasmodium then grows, reproduces, and infects neighboring red blood cells. Occasionally a female anopheles mosquito will withdraw some plasmodium-infected blood as part of her normal dietary requirement and then transfer the plasmodium to another unsuspecting victim.

Sarcodina

The phylum Sarcodina is best known for their bloblike structures, called pseudopodia, that provide a means of locomotion. Pseudopods are temporary membrane-bound cytoplasm projections that direct the motion of the sarcodines. This innate flexibility allows the sarcodines to assume virtually any shape. Amoebas are typical sarcodines that use pseudopods to locate, surround, and engulf food sources. Other interesting examples include the foraminifera, which are aquatic protists mostly known by the calcium carbonate shells they secrete, which sometimes accumulate in large deposits when they die, such as the famous White Cliffs of Dover, England. Because foraminifers only inhabit warm waters, whenever a geologist discovers a strata containing their fossilized shells, the climate for that aquatic environment at that time can be estimated fairly accurately.

Ciliata

The ciliates (phylum Ciliophora) exhibit several advancements not associated with the previous protists. They exist as free-living, nonparasitic, fresh- or saltwater, unicellular or colonial organisms. They also have developed short, hairlike structures, called cilia that move in rhythm for locomotion. Cilia are often described as functioning similar to oars for movement of a ship, which is accurate except cilia sometimes surround the entire organism. They allow directed movement toward a food source and away from inhospitable territories. Paramecia are a common example of a ciliate and exhibit another interesting phenomenon?they have two nuclei. A large macronucleus controls the everyday activities of the cell, and a smaller micronucleus (often more than one) functions during gamete exchange. Under normal conditions, paramecia reproduce asexually by binary fission (refer to Cell Theory, Form, and Function) during periods of stress, however, they conjugate, meaning they exchange haploid micronuclei with another paramecium. Refer to the illustration Paramecium conjugation for a pictorial representation.

Because no offspring or fertilized eggs are produced, technically, sexual reproduction did not occur, but gametes were exchanged by mature adults, resulting in a new genetic complement for both paramecia! Paramecia also contain most organelles that more advanced life-forms utilize. For instance, in addition to the mitochondria and nucleus, they also use food vacuoles containing digestive enzymes, an anal pore for waste removal, and contractile or water vacuoles for water transportation.

Zoomastigina

Zoomastigina, also known as flagellates, are known for their specialized flagella, which are whiplike structures that propel the flagellates through their aquatic environment. Typically flagellates have only one flagella, but may have up to four working in sync. Although most flagellates are harmless, simply surrounding and engulfing their food, others are human parasites. One of the most interesting parasites is the trypanosome, which causes African sleeping sickness. The symptoms are well known: fever, chills, and skin rash. Affected individuals become very weak, unconscious, and may fall into a fatal coma.

The trypanosome is transmitted by the tsetse fly and lives in the bloodstream, and continually change their surface molecular structure to gain invisibility to the host's immune system and remain undetected in attacks on the host. The disease attacks the nervous system of infected individuals.


Are red blood cells prokaryotic? - Biology

The astounding ultrastructural change of developing erythroblasts

Some of our CFU-E cells managed to survive and produced erythroblasts, our first immature red blood cells. These cells are big, possess a large nucleus and a plasmalemma with abundant short filopodia. They basically look like some kind of leukocyte instead of the small biconcave red blood cells we are used to see (see Figure 1, left). These big cells will have to undergo an extensive metamorphosis in order to become mature erythrocytes, their nuclei will become smaller and its chromatin will become condensed, they're going to get rid of all their organelles (including and especially the nucleus) and finally change their plasma membrane and cortex structure into a rigid yet elastic envelope for the cell.


The ghost of apoptosis haunts the erythroid cell line during all its development. Some of the cell death mechanisms we discussed in the previous section are activated again in the erythroblast line after and independently of the rescue of their CFU-E progenitor cells. We can see in Figure 3, left how genes of the apoptosis cascade like Nix, PUMA, BIM and MCL1 are induced in the erythroblast cell line. The cocnentrations of some of this factors increases with time while the concentration of some others decreases. But why activating apoptosis? What's the common link between apoptosis and erythroblast metamorphosis? Most processes occuring under apoptosis also happen during this metamorphosis: dramatic chromatin condensation and autophagy of the cell's organelles are the main ones. Mature mammalian erythrocytes don't have any nucleus or organelles. Although apoptosis ends up in the entire breakdown of cell integrity, this doesn't happen in erythroblasts. Erythroblasts seem to use a partial version of apoptosis, they even activate the core proteins of apoptosis, the caspases. Caspase-3 and Caspase-9 are activated in erythroblasts, (Yael Zermati et al. 2001) these activated cysteine proteases will end up destroying the cell in a controlled way. The first and most visible change in erythroblast maturation is the progressive condensation of nuclear chromatin and decrease in nuclear size. Early normoblasts(nucleated erythroblasts) have a big nucleus with relatively low-condensed chromatin (click here to see their ultrastructure). Chromatin is mainly condensed in vivo by specific enzymes: histone deacetylases (HDACs) that will remove the acetyl groups attached to the histones that bind to the DNA, and histone methyltransferases (HMTs) that will cover the histone tails with methyl groups and finally condense the DNA (see Figure 3, right). Acetylated DNA is loose, overmethylated DNA is condensed, these enzymes just turn loose DNA into condensed DNA. This will just inactivate the genes, compact condensation of nuclear DNA also requires other factors to proceed. Nucleoplasmin is a protein that decompacts the sperm's nuclear DNA after entering the oocyte. Deactivating this protein by dephosphorilation (removing a phosphate group) seems to play a crucial role in DNA compaction during apoptosis (Zhigang Lu et al. 2004), this process is mediated by a protein phosphatase probably activated during apoptosis. Subsequently, DNA is finally destroyed by Caspase Activated DNAses (CAD) and related proteins, leaving a ravaged nucleus that will soon be expelled out of the cell.

Getting rid of the nucleus

Mammalian red blood cells are small enucleated cells, which means that they had to get rid of the condensed nucleus at some point during their development. It is important to note that, unlike what happens during normal apoptosis, the nucleus retains its structural integrity and does not break down. This breaking down is mediated during normal apoptosis(and cell division) by the phosphorylation of the lamin intermediate filaments that form a network under the nuclear envelope and maintain its structure. This doesn't happen in erythroblasts, nuclei are expelled out of the cell in a single unit. This process has deeply fascinated cytologists for a long time and several hypothesis emerged to explain how it happened. Some argued a kind of unequal cytokinesis that would move the nuclei out of the cell by using the force of the mytotic spindle, others argued a spindle-independent mechanism. Recent findings seem to confirm the latter (Ganesan Keerthivasan et al, 2010), as big vacuoles form in the boundary between the nucleus and the cell cytoplasm and seem to finally separate the nucleus from the cell by the controlled fusion of these vesicles (see Fig. 4):

These nuclei and the membrane around them will be recognized and endocytosed by the BM stromal macrophages, click here to see how.

Devouring your own organelles

Having no nucleus means no protein turnover and the unstoppable senescence and breakdown of your organelles as no one is producing replacement proteins for the ones that inevitably will break down. This is outright dangerous for the cell seeing how mitochondria regulate cell death and will start cell destruction pathways if broken or impaired or how breaking down lysosomes may impair the cell, so the cell organelles need to be destroyed too. Instead of just kicking them out of the cell like the nucleus, the cell starts a process called autophagy in which it surrounds its own organelles into digestive compartments. This is also a usual phenomenon during apoptosis and cell starvation and is in fact partly mediated by one of the apoptosis proteins we talked about in the previous section, our good old friend Nix ( Rachel L. Schweers et al, 2007).

As we already commented, proerythroblasts start as relatively rounded cells many times larger than mature erythrocytes. How do they manage to modify their shape into the biconcave one we all know? Erythroblasts become smaller and smaller every time they undergo cell division until their decaying nuclei can no longer divide and are expelled out of the cell. The cell then undergoes a programmed process of huge endocytosis of plasmalemma, but what does this process accomplish?

There is an organized framework of filaments under the plasmalemma called the cell cortex. In the immature erythrobalst this layer is highly dynamic and allows the formation of membrane protrusions and gives the cell an overall rounded shape. Mature erythrocytes (and reticulocytes) have a rigid cortex consisting of crosslinked cytoskeletal filaments like spectrin.
which are mechanically coupled to the plasmalemma. The point of such a change in cortex organization is that the crosslinked spectrin network provides the mature red blood cell an increased capacity to survive the shear forces it will encounter in blood vessels. It may seem a minor problem at first sight, but shear forces are a great mortality factor in, for instance, metastatic cancer cells that lack a rigid cortex and enter the blood circulation. These cells will usually break up and burst, but some cancer cells have developed the capacity to bind platelets and shield themselves. By engulfing the plasmalemma and the old cell cortex attached to it, the cell may be able to rapidly "update" the cell cortex into the new organization. Despite its rigid nature, the red blood cell cortex is very elastic allowing a great deformability to resist both the shear force and the passge through the incredibly narrow blood capillaries.

But what is the relationship with cell shape? The spectrin network exerts a tensional force on the membrane through its attachments to it (mediated by ERM family proteins) and the biconcave shape is the one resulting from an equilibrium between the tensional forced imposed by the cortex and the less strained shape, a biconcave disc.