Overview of Protists,Protist Morpholog,Encystment and Excystment,Protist Reproductive Cells and Structures,Protist Taxonomy

Most eukaryotes are microbes. 

 It is therefore not surprising that the vast diversity of protists is a function of their capacity to thrive in a wide variety of habitats.

 Their one common require­ ment is moisture because all are susceptible to desiccation.

 Most protists are free living and inhabit freshwater or marine environ­ ments.

 Many terrestrial chemoorganotrophic forms can be found in decaying organic matter and soil. 

Whether terrestrial or aquatic, protists play an important role in nutrient cycling.

Protozoa, or chemoorganoheterotrophic protists, may be saprophytes, securing nutrients from dead organic material by releasing degradative enzymes into the environment. 

 They then absorb the soluble products-a process sometimes called osmotrophy.

 Other protozoa employ holozoic nutrition, in which solid nutrients are acquired by phagocytosis.

 Photo­ autotrophic protists are strict aerobes and, like cyanobacteria, use photosystems I and II to perform oxygenic photosynthesis.

 It is difficult to classify the nutritional strategies of some protists be­ cause they simultaneously use both reduced organic molecules and C02 as carbon sources. 

This strategy is called mixotrophy.

Protist Morphology

Despite their diversity, protists share many common features. 

 In many respects, their morphology and physiology are the same as the cells of multicellular plants and animals.

 However, because many protists are unicellular, all of life's various functions must be performed within a single cell.

 Those that are multicellular lack highly differentiated tissues. Therefore the structural complexity observed in protists arises at the level of specialized organelles.

The protist cell membrane is called the plasmalemma and is identical to that of multicellular organisms.

 In some protists, the cytoplasm immediately under the plasmalemma is divided into an outer gelatinous region called the ectoplasm and an  inner  fluid  region,  the  endoplasm.  

The  ectoplasm  imparts rigidity to the cell body.

 Many protists also have a supportive mechanism called the pellicle.

 The pellicle consists of the plasma­ lemma and a relatively rigid layer just beneath it. The pellicle may

be simple in structure. 

For example, Euglena spp. are protists with a series of overlapping strips with a ridge at the edge of each strip fitting into a groove on the adjacent one, much like how the "tongue-and-groove" boards of a hardwood floor fit together.

 In contrast, the pellicles of ciliate protists are exceptionally complex with two membranes and a variety of associated structures. 

Al­ though pellicles are not as strong and rigid as cell walls, for those that possess them, pellicles impart the characteristic shape asso­ ciated with that particular species.

One or more vacuoles are usually present in the cytoplasm of protozoa.

 These are differentiated into contractile, secretory, and food or phagocytic vacuoles.

 Contractile vacuoles function as osmoregulatory organelles in those protists that live in hypotonic environments, such as freshwater lakes. 

Osmotic balance is maintained by continuous water expulsion.

 Phagocytic vacuoles are conspicuous in protists that ingest food by phagocytosis ( ho­ lozoic protists) and in parasitic species. Phagocytic vacuoles are the sites of food digestion.

 In some organisms, they may occur anywhere on the cell surface, while others have a specialized structure for phagocytosis called the cytostome (cell mouth). 

When digestion commences, the phagocytic vacuole is acidic, and as digestion proceeds, the vacuole membrane forms small blebs.

 These pinch off and carry nutrients throughout the cyto­plasm. 

The undigested contents of the original phagocytic vacu­ ole are expelled from the cell either at a random site on the cell membrane or at a designated position called the cytoproct.

Several energy-conserving organelles are observed in protists.

 Most aerobic chemoorganotrophic protists have mitochondria, while photosynthetic forms have chloroplasts.

 A dense protein­ aceous area, the pyrenoid, which is associated with the synthesis and storage of starch, may be present in chloroplasts. 

The majority of anaerobic chemoorganotrophic protists lack mitochondria; some of these organisms have hydrogenosomes.

Many protists feature cilia or flagella at some point in their life cycle. Their formation is associated with a basal bodylike or­ ganelle called the kinetosome. In addition to aiding in motility, these organelles may be used to generate water currents for feed­ ing and respiration.

Encystment and Excystment

Many protists are capable of encystment. During encystment, the organism becomes simpler in morphology and develops into a resting stage called a cyst. The cyst is a dormant form marked by the presence of a cell wall and very low metabolic activity. Cyst formation is particularly common among aquatic, free-living protists and parasitic forms. Cysts serve three major functions:

(1) they protect against adverse changes in the environment, such as nutrient deficiency, desiccation, adverse pH, and low levels of 02

 (2) they are sites for nuclear reorganization and cell division (reproductive cysts) and

 (3) they serve as a means of transfer between hosts in parasitic species (i.e., they are the infective stage).

 Protists escape from cysts by a process called excystment . Although the exact stimulus for excystment is un­ known for most protists, it is generally triggered by a return to favorable environmental conditions. For example, cysts of para­ sitic species excyst after ingestion by the host.

Most protists have both asexual and sexual reproductive phases in their life cycles. The most common method of asexual repro­ duction is binary fission. During this process, the nucleus first undergoes mitosis and then the cytoplasm divides by cytokinesis to form two identical individuals . Multiple fission is also common, as is budding. Some filamentous, photosynthetic protists undergo fragmentation so that each piece of the broken filament grows independently.

Sexual reproduction involves the formation of gametes. Protist cells that produce gametes are termed gamonts. The fusion of haploid gametes is called syngamy. Among protists, syngamy can involve the fusion of two morphologically simi­ lar gametes (isogamy) or two morphologically different types . Meiosis may occur before the formation and union of gametes, as in most animals, or just after fertilization, as is the case with lower plants. Furthermore, the exchange of nuclear material may occur in the familiar fashion-between two different individuals (conjugation)-or by the develop­ ment of a genetically distinct nucleus within a single individ­ ual (autogamy).

With this level of reproductive complexity, perhaps it is not surprising that the nuclei among protists show considerable di­ versity. Most commonly, a vesicular nucleus is present. This is 1 to  10  1-1m in diameter,  spherical,  and has a distinct nucleolus and uncondensed chromosomes . Ovular nuclei are up to 10 times this size and possess many peripheral nucleoli. Still others have chromosomal nuclei, in which the chromo­ somes remain condensed throughout the cell cycle. Finally, many ciliated forms have two types of nuclei: a large macronu­ cleus with distinct nucleoli and condensed chromatin, and a smaller, diploid micronucleus with dispersed chromatin but lacking nucleoli .

Protist Reproductive Cells and Structures

Most protists have both asexual and sexual reproductive phases in their life cycles. The most common method of asexual repro­ duction is binary fission. During this process, the nucleus first undergoes mitosis and then the cytoplasm divides by cytokinesis to form two identical individuals . Multiple fission is also common, as is budding. Some filamentous, photosynthetic protists undergo fragmentation so that each piece of the broken filament grows independently.

Protist Taxonomy

Ever since Antony van Leeuwenhoek described the first proto­ zoan "animalcule" in 1674, the taxonomic classification of the protists has remained in flux. 

 During the twentieth century, classification schemes were based on morphology rather than evolutionary relationships.

 Protists were often classified into four major groups based on their means of locomotion: flagel­ lates (Mastigophora), ciliates (Infusoria or Ciliophora), amoebae (Sarcodina), and stationary forms (Sporozoa).

 Although these terms may still be encountered, they are without evolutionary context and should be avoided.

 While it is now agreed that the old classification system is best abandoned, little agreement exists on what should take its place. Here we use the higher-level classification system for the eukaryotes based on morphologi­ cal, biochemical, and phylogenetic analyses proposed by the International Society of Protistologists in 2005. This scheme does not use formal hierarchical rank designations such as class and order, reflecting the fact that protist taxonomy remains an area of active research. 

The Classification of the Protists  as proposed by the International Society of Protistologists is presented in a table on the text website.

 

 

Comparison of Bacterial, Archaeal, and Eukaryotic Cells

 A comparison of the bacterial and eukaryotic cells in  demonstrates that there are many fundamental differences be­tween these cells. 

These differences are also observed between archaeal and eukaryotic cells, because archaea are similar to bacteria at the gross structural level.

 Eukaryotic cells have  a membrane-enclosed nucleus.

 In contrast, bacterial and archaeal cells lack a true, membrane-delimited nucleus.

 Most bacteria and archaea are smaller than eukaryotic cells, often about the size of eukaryotic mitochondria and chloroplasts.

Many other major distinctions between these groups exist.

 that bacterial and archaeal cells are much simpler structurally.

 In particular, an extensive and diverse col­ lection of membrane-delimited organelles is missing.

 Further­ more, bacterial and archaeal cells are functionally simpler in several ways.

 They lack mitosis and meiosis, and have a simpler genetic organization.

 Many complex eukaryotic processes are ab­ sent in bacteria and archaea: endocytosis, intracellular digestion, directed cytoplasmic streaming, and ameboid movement are just a few.

Despite the many significant differences, all cells are re­ markably similar on the biochemical level, as we discuss in succeeding chapters. 

With a few exceptions, the genetic code is the same in all, as is the way in which the genetic informa­ tion in DNA is expressed. 

The principles underlying meta­ bolic processes and many important metabolic pathways are identical.

 Thus beneath the profound structural and func­ tional differences between bacterial, archaeal, and eukaryotic cells, there is an even more fundamental unity: a molecular unity that is basic to all known life processes.

Retrieve, Infer, Apply

1. Outline the major differences between bacterial, archaeal, and eukaryotic cells. How are they similar?

2. What characteristics make members of Archaea more like eukaryotes? What features make them more like bacteria?

 

Chloroplasts

 Chloroplasts

Plastids are cytoplasmic organelles of photosynthetic protists and plants.

 They often possess pigments such as chlorophylls and carotenoids, and are the sites of synthesis and storage of food reserves. 

The most important type of plastid is the chloro­ plast.

 Chloroplasts contain chlorophyll and use light energy to convert C02 and water to carbohydrates and 0 that is, they are the site of photosynthesis.

 Two major types of chloroplasts have been identified: those that evolved from a primary endosymbi­otic event and those that evolved from a secondary or tertiary event .

 The chloroplasts of plants and some photosynthetic protists are primary plastids and are the focus of this discussion.

Chloroplasts are quite variable in size and shape, but they share many structural features.

 Most are oval with dimen­ sions of 2 to 4 flm by 5 to 10 flm, but some photosynthetic protists possess one huge chloroplast that fills much of  the cell.

Like mitochondria, chloroplasts are encompassed by two membranes . 

A matrix called the stroma is en­ closed by the inner membrane.

 The stroma contains DNA, ribosomes, lipid droplets, starch granules, and a complex in­ternal membrane system whose most prominent components are flattened, membrane  delimited  sacs  called  thylakoids.

Mitochondria,the powerhouse of the cell

Found in most eukaryotic cells, mitochondria frequently are called the "powerhouses" of the cell .

 Metabolic processes such as the tricarboxylic acid cycle and the generation of ATP, the major energy cur­ rency of all life forms, take place here.

 When viewed with a transmission electron microscope, many mitochondria are cy­ lindrical structures and measure approximately 0.3 to 1.0 fliD by 5 to 10 11m.

 (In other words, they are about the same size as bacterial cells.)

 Some cells possess 1,000 or more mitochon­  have a single, giant, tubular mitochondrion twisted into a continuous network permeating the cytoplasm.

The mitochondrion is bounded by two membranes: an outer mitochondrial membrane separated from an inner mitochon­ drial membrane by a 6 to 8 nm intermembrane space . 

The outer mitochondrial membrane contains porins and thus is similar to the outer membrane of Gram-negative bacteria.

 The inner membrane has infoldings called cristae (s., crista), which greatly increase its surface area.

 The shape of cristae differs in mitochondria from various species. 

P latelike (laminar) cristae, cristae shaped like disks, tubular cristae, and cristae in the shape of vesicles have all been observed.

 The inner membrane encloses the mitochondrial matrix, a dense material containing ribo­ somes, DNA, and often large calcium phosphate granules.

 In many organisms, mitochondrial DNA is a closed circle, like most bacterial DNA. However, in some protists, mitochondrial DNA is linear.

Each mitochondrial compartment has a characteristic chemical and enzymatic composition. For example, the outer and inner mitochondrial membranes possess different lipids.

 Enzymes and electron carriers involved in electron transport and oxidative phosphorylation are located only in the inner membrane.

 Enzymes of the tricarboxylic acid cycle and those involved with the catabo­ lism (breaking down) of fatty acids are located in the matrix.

The mitochondrion uses its DNA and ribosomes to synthe­ size some of its own proteins.

 In fact, mutations in mitochondrial DNA often lead to serious diseases in humans.

 However, most mitochondrial proteins are manufactured under the direction of the nucleus and must the nucleus and must be transported into the mitochondrion.

 Mitochondria reproduce by binary fission, a reproductive pro- cess  used  by  many   bacteria.

 

Nucleus , nucleus function , nucleus definition , cell nucleus , nucleus diagram

The nucleus is by far the most visually prominent organelle in eukaryotic cells. 

It was discovered early in the study of cell struc­ ture and was shown by Robert Brown in 1831 to be a constant feature of eukaryotic cells.

 The nucleus is the repository for the cell's genetic information.

Nuclei are membrane-delimited spherical bodies about 5 to 7 11m in diameter . 

They contain more than one chromosome; the exact number depends on the organism, cell type, and stage in the life cycle.

 Each eukaryotic chromosome is composed of chromatin. 

Chromatin is a complex of DNA and proteins, including histones.

 Histones are small basic proteins rich in the amino acids lysine, arginine, or both.

 There are five types of histones in most eukaryotic cells: HI, H2A, H2B, H3, and H4.

 Eight histone molecules form an ellipsoid about 11 nm long and 6.5 to 7 nm in diameter around which the DNA wraps to form a "beads-on-a-string" formation . 

Each bead is called a nucleosome.

Chromosomes are very dynamic and vary in terms of their degree of compaction.

 When the cell is not dividing, there is less compaction.

 The highest degree of compaction occurs dur­ ing cell division.

 Compaction is brought about in part byhistones and other proteins, including condensins.

 Recall that condensins are also used by bacterial and archaeal cells to compact their chromosomes during cell division.

 Tethering of one region of the chromosome to another and tethering of the chromosome to nuclear structures also contribute to chromo­ some folding and compaction.

a complex structure consisting of inner and outer membranes separated by a perinuclear space.

 The nuclear envelope is contin­ uous with the ER at several points, and its outer membrane is covered with ribosomes.

 A network of intermediate filaments, called the nuclear lamina, is observed in animal cells.

 It lies against the inner surface of the nuclear envelope and supports it.

 Many nuclear pores penetrate the envelope, and each pore is formed by about 30 proteins; each pore plus the associated pro­ teins is called a nuclear pore complex .

 Pores are about 70 nm in diameter and collectively occupy about 10 to 25% of the nuclear surface. 

The nuclear pore complexes serve as trans­ port routes between the nucleus and surrounding cytoplasm.

 Small molecules move through the nuclear pore complex un­ aided.

 However, large molecules are transported through the nuclear pore complex. Some nuclear pore complex proteins are involved in these transport processes.

Often the most noticeable structure within the nucleus is the nucleolus .

 A nucleus may contain from one to many nucleoli.

 Although the nucleolus is not membrane-enclosed, it is a complex organelle with separate granular and fibrillar regions.

 It is present in nondividing cells but frequently disappears during mi­ tosis. After mitosis, the nucleolus reforms around the nucleolar organizer, a particular part of a specific chromosome.

The nucleolus plays a major role in ribosome synthesis.

 The DNA of the nucleolar organizer directs the production of ribo­ somal RNA (rRNA).

 This RNA is synthesized in a single long piece that is cut to form the final rRNA molecules.

 The processed rRNAs combine with ribosomal proteins (which have been syn­ thesized in the cytoplasm) to form partially completed ribosomal subunits.

 The granules seen in the nucleolus are probably these subunits.

 Immature ribosomal subunits then leave the nucleus, presumably by way of the nuclear pore complexes, and mature in the cytoplasm.

 

Endoplasmic Reticulum

• 
• 
  
• The endoplasmic reticulum (ER) is an irregular network of branching and fusing membranous tubules, around 40 to 70 nm in diameter, and many flattened sacs called cisternae (s., cisterna). 
• The nature of the ER varies with the functional and physiological status of the cell.    
• In cells synthesizing a great deal of protein to be secreted, a large part of the ER is studded on its outer surface with ribosomes and is called rough endoplasmic reticulum . 
• Other cells, such as those producing large quantities of lipids, have ER that lacks ribosomes. This is smooth endoplasmic reticu­ lum (SER).
•  The endoplasmic reticulum has many important func­ tions.
•  Not only does it transport proteins, lipids, and other materials through the cell, it is also involved in the synthesis of many of the materials it transports.
•  Lipids and proteins are synthesized by ER-associated enzymes and ribosomes.
•  Poly­ peptide chains synthesized on RER-bound ribosomes may be inserted either into the ER membrane or into its lumen for transport elsewhere. 
• The ER is also a major site of cell mem­ brane synthesis.

immunohematology,Blood group genteics,Blood Group Antibodies,Autoantibodies,Main Blood group system,

 Introduction:

• Each species of animals , including humans has certain antigens on the surface of the red cells which are unique for that species.

• These are known as isoantigens. Similarly certain antigens , the alloantigens are common to some, but not all members of that particular species.

• Blood group serology involves the detection of these antigens & their antibodies.

Blood group genteics:

• All the antigenic substances present on RBC of indiviual are inherited. The unit of inheritance is a gene & each antigen is controlled by gene for that antigen.

• The position of each gene on a chromosome is called its locus.

• Some terminology:

• Allele or allelomorph: Two or more genes responsible for same chracteristics occupying the same position on the chromosomes.

• Homozygous: A person who has inherited same allelic genes from both parents eg: having both blue eyed colour genes.

• Hetrogygous: A person who has inherited different allelic genes for trait. eg: having a blue & brown eye colour genes.

Dominat gene: Certain alleles are stronger than other alleles. eg: brown eyes genes mask the presence of blue eye genes, & are therefore dominant over blue eye genes. A dominat gene expresses itself in both homozygous & hetrozygous states.

• Recessive gene: They are expressed only when dominant allele is absent eg: in Homogygous state. Colour of eye is blue ,if both alleles are for blue eyes.

• Co-dominat gene: In this ,various alleles for the same character are equally dominant eg: alleles for a particular blood group system.

• Genotype: The genetic make up of an individual or cell is called its genotype.

• Phenotype: It is the observable characteristics of individual or a cell eg: an individual of group Bo genotype would be expressed as group B because O gene is rcessive.

Blood Group Antigens:

• The blood group antigens include substances on the RBC & on other consitutents such as leucocytes, platelets & plasma.

• The blood consitutent antigens which are governed by single gene or a group of genes are inherited as groups.

• Blood groups controlled by set of genes are called a blood group system.

• Most blood group antigens follow the law of inheritance. Chemically blood group antigens are usually glycoproteins, lipoproteins or glycolipids in nature.

Blood Group Antibodies:

• Specific blood antibodies develop in response to antigenic stimulation by particular blood group antigen.

• The level of antibody activity depends on the immunogenicity of corresponding antigen.

• Alloantidodies:

• Are antibodies which are present in some members of species but not all. eg: anti-A antibodies are present in all

humans beigns belonging to blood B & O.

• Due to the presence of alloantibodies, it is necessary to select a blood donor who is negative for corresponding antigen,i.e if recipient has anti-A antibodies, he cannot recieve blood which contain group A antigen.

• They are of three type:

1. Naturally occurring: The antigenic stimulus is unknown. ABO antibodies belong to this type.

2. As a result of immunization through transfusion, some red cell antigens which are absent in the recipent may be introduced & induce antibody production.

 

3. Induced by exposure to foetal erythrocytes either during pregneancy or during delivery. These antibodies are similar to those produced by blood transfusion. Development of Rh antibodies in a Rh negative mother by a Rh positive foetus is good example.

Autoantibodies:

• An autoantibody is the antibody which is induced by an antigen in same individual. It also reacts with same antigen if it present in other individual.

• In some cases the reaction of antigen & its autoantibody may not show any demonstratable clinical symptoms, whereas sometimes such a reaction may lead to haemolytic anaemia, leucopenia or thrombocytopenia.

• It can be divided into two general catagories depending on their optimum reacting Temperature.

1. Warm autoantibodies: These consitutue about 85% of autoantibodies. Their optimal temperature is 370C. They are generally IgG in nature & can be detected with direct antibody test.

2. Cold auto antibodies: About 15% are of this type. they are generally IgM in nature & agglutinate RBC strongly at 40C weakly at 240C & not at all 370C. Most of are present in normal individual, very few are associated with diseases. It is important to be aware of cold antibodies, when the patients body temperature needs to be lowered for procedures such as cardiopulmonary bypass.

: Main Blood group system:

• Approximately 600 blood group antigens have been describes so far.

• The konwoledge of blood group antigens is important during blood transfusion.

• Among these ABO system is most important blood group system in blood transfusion & in organ transplantation because of two unique features:

1. Strongly reactive antibodies are present in sera of individual who lack the corresponding antigens:

2. A & B antigens are present on many tissues cells in addition to red cells.

• The ABO system consist of four blood groups or phenotypes : A, B, AB & O. The two antigen A & B are responsible for these four groups.

• If A antigen is present on red cell individual is said to belong to A group ,If B antigen is present on red cell individual is said to belong to B group, & If AB antigen is present on red cell individual is said to belong to AB group.

• While O groups have neither A or B antigen on their red cell.

• Three allelic genes A, B & O,can be inherited in ABO system. The following combination of alleles is

possible: AA, AO, AB, BB, BO, OO resulting in A, A, AB, B, B & O group individuals respectively.

• This so because A & B genes are dominat & O gene is recessive.

Law of Inheritance of ABO Groups:

• According to Bernstein's theory two laws of inheritance have been proved.

1. It is not possible that an offspring can possess antigen A, B or both unless it is inherited from one or both parents.

2. A parent of blood group AB cannot produce an offspring of group O. Similarly a parent of blood group O cannot produce AB offspring. The reason for this law is that group AB is hetrozygygous & therefore A & B genes must come from two parents.

Group O individual have antigen called H antigen on the surface of red cells , which is precusssor of A & B antigen.

Subgroups of ABO:

• Both groups A & B can be further subdivided . The most important of these subgroups are A1, A2. Both A1 & A2 cells react with anti A antiserum.

• Approximately 20% of group A & AB persons belong to subgroup A2 & 80% to A1. Several subgroups of B have been identified but they are very rare.