Cell Ultrastructure

It is believed that the age of the earth is about 4,500 million years. On the basis of fossil evidence it is also believed that life came into existence on this planet some 3,500 million years ago. The primevial atmosphere of the earth largely had hydrogen and helium with a little amount of nitrogen. In addition there were molecules like H2O, CO, CO2, CH4, NH3 and H2S. When Miller placed H2, CH4, NH3, H2S and H2O in a closed vessel and an electric discharge was passed, formation of organic molecules such as amino acids, aldehydes, organic acids occurred. Thus, a wide range of organic molecules were synthesised abiologically. During the course of biological evolution, aggregates of organic molecules called coacervates were synthesized. Association of RNA and lipids with coacervates created an ancestral organism called progenote. Thus, the most primitive organisms which appeared on this planet were heterotrophs. With the development of ability to utilize CO2 and use light energy for cell metabolism, autotrophs came into existence, and so the phenomenon of photosynthesis.

The organism, according to fossil records, capable of doing photosynthesis existed some 3.000 million years ago. In this early process there was no liberation of oxygen. These organisms gave rise to present day cyanobacteria capable of splitting water and liberate oxygen. In other words, the early organisms were obligate anaerobes. Since ultraviolet light damages nucleic acids and proteins, the early organism lived below the water surface. Due to conversion of atmospheric oxygen into ozone which absorbs ultraviolet light, surface life came into being. The eukaryotes originated some 1,500 million years ago.

Discovery of the 'cell'

Swammerdam, 1658 was perhaps the first person to observe the RBC of frog. Later, in 1665 Robert Hooke examined a piece of cork under a primitive microscope and found a honey comb like structure. He termed each chamber as 'cell'. In fact he observed only the suberised cell walls. Leeuwenhoek, 1676 observed a living cell (Bacterium) having some internal organization, under the microscope. All living beings are made up of cell". "The cell is the basic morphological and functional unit of life." Above written statements essentials of cell theory or cell doctrine. Although the cell theory is credited to Matthias Schleiden (1838) a botanist of Germany and Theodor Schwann (1839) a Zoologist also of Germany, it is actually a result of efforts of many biologists that in 17th century and listed as follows:

  • Works of Robert Hooke and A.V. Leeuwenhoek in 17th century about the establishment of cell.
  • De Mirbel in 1808-09 stated that plants are formed by membranous cellular tissue.
  • J.B. Lamarck in 1809 stated that "nobody have life, if its constituent parts are not cellular tissue".
  • Dutrochet stated in 1827 that "all tissues, all plants and all animal organs are in actual sense variously  modified cellular tissue."
  • Brown in 1831 described nucleus.

Cell Theory

The theory was jointly put forward by Schleiden and Schwann (1839) in their paper "Mi­croscope Investigations on the similarity of structure and growth in animals and plants." Cell theory states that the bodies of all organisms are made up of cells and their products so that cells are units of both structure and function of living organism

  1. Schleiden studied a large variety of plants and their tissues only thus, his observations were based mainly about plants, Schwann on the other hand studied plants and animals both in a comparative manner to give the universal statements of cell theory.
  2. Both of them compared their observations and cell theory was the result of their combined views.

Fundamental Features

Five fundamental observations of the cell theory are:

  1. Organisms are composed of cells and their products.
  2. Each cell is made of a small mass of protoplasm containing a nucleus in its inside and a plasma membrane with or without a cell wall on its outside.
  3. New cells arise from pre-existing cells. 
  4. All cells are basically alike in their chemistry and physiology.
  5. Activities of an organism are the sum total of activities and interactions of its constitu­ent cells.

         The cell theory as stated by Schleiden and Schwann failed to explain the question of origin of cells. Actually Schleiden believed that cells are budded off from the nucleus. A major expansion of cell theory was expressed by Virchow in 1855 in bi; famous statement "Ominis cellula e cellula" (all cells arise from pre existing cells).

         This concept, i.e., cells arise from pre existing cells was the actual idea of Nageli (1846) which later on elaborated by Virchow along with considerable evidences in its support.

         The work of Nageli and Virchow established cell division as the central phenomenon in the continuity of life. Another important discovery was fertilization (Hertwig, 1875), i.e., formation of zygote by the fusion of two nuclei. Discovery c: fertilization led to the discovery of Meiosis (Benden, 1887) thus, constansy of chromosome number from generation to generation, was explained.

         All these discoveries led to the modern version of cell theory which states that:

(1)     Cells are the morphological and physiological units of all living organisms.         

(2)     The properties of a living organism depend on those of its individual cells.                           

(3)     Cells originate only from other cells and continuity is maintained through the genetic material.   

(4)     The cell is the smallest unit of life. Modern Cell Theory

It is also known as cell doctrine or cell principle. Modern cell theory states that

  1. The bodies of all living beings are made up of cells and their products.
  2. Cells are units of structure in the body of living organisms.
  3. Every cell is made up of a small mass of protoplasm having a nucleus, a number of organelles and a covering membrane.
  4. While a cell can survive independently, its organelles cannot do so.
  5. The cells belonging to diverse organisms and different regions of the same organism have a fundamental similarity in their structure, chemical composition and metabolism.
  6. Life exists only in cells because all the activities of life are performed by cells.
  7. Cells are units of function in living organisms, that is, the activities of an organism are the sum total of the activities of its cells.
  8. Depending upon specific requirement, the cells get modified, e.g., elongated in muscle and nerve cells, loss of nucleus in RBCs or cytoplasm in outer skin cells.
  9. Each cell maintains its individuality. It has a specific internal environment and homeostasis.
  10. Growth of an organism involves the growth and multiplication of its cells.
  11. Genetic information is stored and expressed inside cells.
  12. Life passes from one generation to the next in the form of a living cell.
  13. New cells arise from pre-existing cells through division. All new cells contain the same amount and degree of genetic information as contained in the parent cell.
  14. All the present day cells (and hence organisms) have a common ancestry because they are derived from the first cell that evolved on the planet through continuous line of cell genera­tions.
  15. Basically the cells are totipotent (i.e., a single cell can give rise to the whole organism) unless and until they have become extremely specialized.
  16. Though a unit of structure and function, each cell of an organism is able to act inde­pendently in its growth, division, metabolism and even death.
  17. No organism, organ or tissue can have activity that is absent in its cells.

 

Q.1. How a cell is considered as basic unit of life?

         In multicellular organisms there may be different levels of body organisation seen as:

         (1)     A group of similar cells with similar origin that perform specific function constitute a Tissue (e. g., blood, epithelium, muscle, etc.).

         (2)     Several tissue types may join collectively to form an Organ (e. g., Brain, spinal cord, etc.).

         (3)     Two or more organs working together constitute an Organ System (e. g., Nervous system, digestive system, Circulatory system etc.).

                  Thus, the basic origin of all organ systems are cells. The unicellular organisms on the other hand have single cell like body i.e., life here also have cells in its basis.

                  Hence, either unicellular or multicellular life have cell as its basis that's why a cell is considered as the basic unit of life.

 

Cell as Self-Contained Unit

Cells of unicellular organisms carry out all life activities independently. These activities are also shown by each and every cell of a multicellular organism. In the latter case there is some deprivation of structure and function of individual cells but they come to develop division of labour, interdependence and differentiation.

 

  1. Cellular Autonomy:

         All cells (including those of multicellular organisms) are autono­mous because of the following facts :

         Each and every cell converts non-living materials into components of living matter.

         Each cell builds up its own macromolecules from micromolecules.

         All cells obtain or manufacture food.

         A cell is always building new structures to replace the worn out ones.

         Worn out constituents are broken down by another type of cellular components like lysosomes

          All cells require energy. For this, in every cell the food molecules are oxidized in the process of respiration. The energy thus liberated, is temporarily stored in ATP molecules. The energy is used for

         (a)     overcoming entropy

         (b)     physiological activities

         (c)     biosynthetic reactions,

         (d)    The cells exchange gases with their environment,

         (e)     Metabolic wastes are formed and discarded by each cell.

         (f)     A cell regulates its own activities. It, therefore, maintains its own internal physiochemical environment.

         (g)   Cells regulate their activity through

         (h)     flow of energy

         (i)      flow of information.

         (j)      Unless overspecialized, each cell has the ability to divide and form daughters with similar hereditary properties.

         (k)     Cells possess a definite life span,

         (l)      Each cell is capable of independent existence,

         (m)    The whole genetic information is contained in a cell.

  1. Cellular Autonomy in a Unicellular Organism.

         In unicellular organism the cell has complete independent existence. It is not dependent upon any other cell for any function, material or information. The cell depends upon its own internal or intrinsic information. How­ever, it responds to environment with which it is in direct contact. Sexual reproduction requires cooperation with other cells. Otherwise all life activities are carried out by the same cell. The division of labour is absent at the cellular level. It exists at the subcellular level.

  1. Cellular Autonomy in a Multicellular Organism.

         In multicellular organisms all the cells are not in direct contact with external environment. The cells create a new environment around them except those which lie on the surface. The cells are not completely independent. They show interaction and co-operation to one another. There is thus a division of labour. As a result of this division of labour, the cells of a multicellular organism become specialized through differentiation to carry out distinct activities like support in sclerenchyma cell, protec­tion in epidermal cell, secretion in gland cell, contraction in a muscle cell or conduction of stimulus in a nerve cell.

         Due to specialization, the cells of multicellular organisms may lose certain activities essen­tial for autonomy either temporarily or permanently, e.g., erythrocytes do not 'respire'. They have lost the ability to multiply permanently because specialization has led to degeneration of nucleus during the process of maturation. There are other cells which are also highly special­ized, e.g., nerve cells and muscle cells. Mature nerve cells cannot divide. Muscle cells normally do not divide but can be made to do so. All these specialized cells, however, retain the capacity for independent existence. This fact can be demonstrated through cell culture or tissue cul­ture. Tissue culture is the technique of maintaining and growing cells, tissues and organs aseptically on artificial medium in suitable controlled environmental conditions. In this tech­nique, isolated cells not only live and perform all life activities independently, but most of them can also increase their number through repeated multiplication.

Cellular Totipotency

Totipotency (L. totus—all, potens—powerful) or cellular totipotency is the ability of a living somatic nucleated cell to form the complete organism. Theoretically all somatic cells should be totipotent since they carry the full gene complement of the individual. However, during their maturation the cells undergo differentiation and are unable to return to their un-differentiated status. They, however, do so under special circumstances. The phenomenon is called dedifferentiation. The dedifferentiated cells can undergo division and ultimately form the whole individual or a part of it. Totipotency can be easily demonstrated in plant cells. In higher animals it has not yet been experimentally proved. It is because the cells do not undergo independent tissue differentiation. However, nucleus taken from any living somatic cells of frog can be seen to have complete genetic information and hence totipotent. This can be done by implanting it in an egg where the original nucleus has been taken out. The egg develops normally into a new individual similar to the parent which donated the nucleus. The technique was successfully demonstrated by Wilmut and Campbell when they cloned the first mammal, sheep Dolly (February 13, 1997).

Cellular totipotency was first proposed by German botanist Haberlandt in 1902. He thought that since each cell of the organism is derived from fertilized egg and contains the same heredi­tary information, it should be able to regenerate the whole plant. However, Haberlandt's experi­ment to grow isolated green cells failed due to lack of nutrients and perfect asepsis. The two

(a)     Multiplication of rare plants which reproduce through seeds with great difficulty.

(b)     Embryos which fail to reach maturity.

(c)     Multiplication of sterile hybrids.

(d)    Production of virus free plants.

(e)     Multiplication of products of protoplast fusion.

(f)     Shorten the period for development of new varieties,

(g)     Development of resistance to chemicals like weedicides.

(h)     Induction and selection of mutants.

 Q.2.  What is "Totipotency", describe?

         Ans:- A cell that possesses the full genetic potential of an organism is called totipotent and this phenomenon as "Totipotency". The idea of phenomenon is about 100 years old. In 1902 Haberlandt (A German botanist) gave the idea that every living plant cell has the capacity to regenerate a complete plant, but, it was Prof. F.C. Steward who in late 1950s demonstrated that mature cells removed from a carrot and placed in a suitable culture solution develop division power again and provide new carrot plants.

Thus, totipotency is the capability of a cell to give rise a new individual. The phenomenon is also considered as a tool in organisms is composed of cells. The wing of a butterfly is infact a thin sheet of cells, similarly the tomato you eat is. Of cells and its contents soon become part of your cells. In the previous chapter, we have discussed the cells-about its prokaryotic and eukaryotic nature, etc. Cells are so much our part of life that we can not imagine, an organism that is non cellular in e. In this chapter, we will take a. closer look at the internal structure of cells, firstly, as a brief outline and then in detail.

The shape of the cell may be variable, (i.e., constantly changing), e.g., WBCs, Amoeba, etc., or fixed. The fixed cell maybe:

(1)     Flattened, i.e., scale like cells, e.g., squamous epithelium, endothelium and upper layers of epidermis, etc.

(2)     Cubical, i.e., cube shaped cells, e.g., most of the secretory cells.

(3)     Columnar, i.e., rectangular cells with more length and less width, e.g., cells lining those parts mainly where absorption or r movement is required.

(4)     Spherical, i.e., with spherical appearance, e.g., most of the plant cells of this type and eggs of many animals.

(5)     Spindle shaped, i.e., with spindle appearance, e.g., smooth muscle cell fibres.

(6)     Subglobose or Polyhedral, i.e., a sub type of spherical cells, mainly seen in plants, with equal diameters or slightly it diameters.

(7)     Elongated, i.e., cells which are very long (e.g., nerve cells). Elongated cells are also found in plants.

(8)     Branched cells, i.e., the cells with branched margins, e.g., pigment cells of skin, etc.

As far as size of the cell is concerned, it varies from very small cells of bacteria (0.2- 0.5|i) to the very large cells of ostrich s). Thus, the egg of Ostrich is considered as the largest living cell. In this egg, a considerable part of: is made up of yolk, which is not the protoplasm.

The factors which govern the cell size are:

1)      The nucleo-cytoplasmic index, i.e., the ratio between the volume of nucleus and the cytoplasm.

2)      The ratio of cell surface to the cell volume. The rate of metabolism. initial stages of biological membrane formation because in the presence of aqueous medium such -as have the tendency to clump together to minimize the exposure of their hydrophobic regions for water. This spontaneous (with
energy) clumping produces membrane like surfaces.

Thus, we can say at some point in the chemical evolution prior to the first cell, such amphipathic molecules must have spontaneously formed membranes that enclosed an aqueous solution of complex molecules. This later on leads to that formation of cell like components.

 Surface: Volume Ratio

The factors which set the limit of cell size or volume are :

(i)      Nucleo-cytoplasmic or kern-plasma ratio (ratio of nucleus to cytoplasm) which deter­mines the range of control of metabolic activities by nucleus.

(ii)     Ability of oxygen and other materials to reach every part of the cell,

(iii)    Ability of waste products to pass to the outside.

(iv)    Rate of metabolic activity,

(v)     Ratio of surface area to the volume of the cell.

Metabolically active cells are usually smaller due to higher nucleo-cytoplasmic ratio and higher surface volume ratio. The former will allow the nucleus to have better control of meta­bolic activities while the latter will allow quicker exchange of materials between the cell and its outside environment. Surface volume ratio decreases with the in­crease in cell size or volume.

All active cells are smaller If larger cells are to remain active, they are either cylindrical in shape or possess several extensions of the cell membrane. Microvilli are one of such developments. They are found in-all those cells which are active in absorption. Membrane infoldings also occur in transfer cells found in plants in the region of absorption or secretion of nutrients.

 Types of Cells

A multicellular organism is composed of numerous cells. The cells are of three main types— undifferentiated (stem cells), differentiated (post-mitotic cells) and dedifferentiated.

  • Undifferentiated or Stem Cells. They are unspecialised cells which usually possess the power of division, g., stem apical meristem, root apical meristem, vascular cambium, cork cambium, stratum germinativum of skin, germinal epithelium, bone marrow, etc. Zygote is also an undifferentiated cell.
  • Differentiated or Post-mitotic Cells. The cells are specialized to perform specific functions. Differentiation occurs in shape, size, structure and function through an orderly switching on and off of some particular genes of the cells by means of chemicals named as inducers and repressors. It leads to better organisation, division of labour and higher efficiency. Duplication of work is avoided.
  • Dedifferentiated Cells. They are differentiated cells which revert to undifferentiated state to take over the function of division. The process by which they lose their specialization is called It involves reactivation of certain genes that prevent differentia­tion, allow limited growth and induce division. Cork cambium of plants is always produced through dedifferentiation. Dedifferentiation helps in healing of wounds, regeneration in ani­mals, or vegetative propagation in plants. Cell culture experiments are based on this dedifferentiation of cells.

In common there are six types of cell

  • Prokaryotic cell

         Cell without nuclear membrane & cell organelle

  • Eukaryotic cell

         Cell with nuclear membrane & membrane bond cell organelle

  • Unicellular

         Single cell

  • Multicellular

         Cell more than one

  • Animal cell

         Cell present in animal

  • Plant cell

         Cell present plant

 

Unicellular Vs. Multicellular Organisms

Advantages of Unicellularity

(i)      There is neither loss of any component structure nor of any function.

(ii)     The cells are able to tolerate harsh conditions quite efficiently as they are directly exposed to environment.

(iii)    Ageing and natural death are uncommon. It is because a mature unicellular organism undergoes division at the time of reproduction so that its body becomes part of the daughter or offspring.

(iv)    All types of cell organelles, modes of nutrition and other life processes are evolved in unicellular organisms.

 Disadvantages of Unicellularity

(i)      In unicellular organisms a single cell has to perform all the activities of life,

(ii)   The different cells have little coordination. There cannot be any division of labour.

(iii)    The cell is exposed to external environment. It requires protection which cannot be provided to the cell except at the cost of exchange capability of the surface.

(iv)    The single cell should possess all the organelles in optimum number. As a result it must be large. However, increase in size decreases surface area proportionately. This reduces effi­ciency as it limits the control of nucleus over the cytoplasm and exchange the materials as well as information from the surface.

(v)     Death of the cell will kill the organism,

(vi)    The life span is always short.

(vii)   The organism cannot remain active in adverse environment,

(viii)  A distinct reproductive system is absent.

(ix)    Reproduction involves disappearance of the parent and formation of the reproductive structures.

 Advantages of Multicellularity

  1. Dual Function, In multicellular organisms, cells perform dual function, one for them­selves (for their sustenance) and the other for the whole organism.
  2. Reduction in Workload. Despite performing dual function, the cells of a multicellular organism have lesser workload due to reduction in certain activities like protection, contractibility, movement, etc.

Flow of Information

Cells require information for regulating their activities. There are two types of information — intrinsic and extrinsic. Intrinsic Information. It is contained in each and every cell as genetic information. Genetic information is coded inside DNA molecules. The code consists of sequence of nitrogen bases in triplets. The triplet base sequence is also called genetic code. One base triplet determines one amino acid of a polypeptide chain. Depending upon specific triggers, DNA molecules open at specific points and transcribe the information to mRNA molecules in the form of complementary base triplets. mRNA mol­ecules pass into the cytoplasm and get associated with ribosomes. Here, they translate the information by directing the amino acids to join in a particular sequence according to base triplets. Thus, a specific polypeptide is synthesised.

The flow of intrinsic information from DNA to mRNA and then to polypeptide is often called central dogma of molecular biology or cell biology.

            transcription                          translation
DNA ———————> mRNA ———————» Polypeptide

Polypeptides get transformed into particular proteins and enzymes. Proteins build up the cellular machinery while enzymes regulate specific metabolic reactions.

Extrinsic Information. It comes from outside. It is brought to the cells in the form of specific stimuli and information molecules. Stimuli are commonly brought by nerves. The nerves se­crete specific chemicals that stimu­late particular systems of the cells. Information molecules are com­monly hormones.

Hormones are of two types.

(i)      Within the Cell. Some of the hormones are able to pass into the cells. Here, they combine with specific intracellular recep­tors. The latter carry the hormones to the nucleus. Inside the nucleus, the hormone functions as an inducer. It removes the repressor of particular genes. The activated genes transcription specific mRNAs for synthesis of particular proteins and enzymes inside the cytoplasm.

(ii)   Cell Surface. In this category the hormones do not enter the cells. Instead, they combine with membrane recep­tors. The hormone-receptor complex can function in two ways. It may change the membrane permeability e.g., opening of calcium channels. On entering a cell, calcium combines with protein calmodulin. The activated protein has multiple effects. The second method is through the formation of second chemical messenger like cyclic or cAMP. cAMP activates an enzyme system already present in the cell.

                      Differences in Flow of Intrinsic (genetic) and Extrinsic Information

 

Intrinsic Information

 

Extrinsic Information

 

1.

2.

 

Information resides within the cell. It is not influenced by external environment.

 

1.

2.

 

Information is brought from outside. It is component of external environment of the cell.

 

3.

 

The trigger is genetic clock or some cyto-plasmic ingredient.

 

3.

 

The trigger is in the external environment of the cell that causes a nerve or endocrine gland to send the extrinsic information.

4.

 

It is present in genetic material.

 

4.

 

It is present in the nervous system or endo crine system

 

5..

 

A receptor is not involved.

The information is transferred to mRNA that helps in synthesis of specific proteins  for cellular structure or enzymes for metabolism.

 

5.

 

 A   receptor   is   commonly   required   for receiving extrinsic information. The information passes into the cell for either activation of certain parts of DNA or an already present enzyme system.

 

 3. Viruses, the small acellular organisms are considered as exception of cell theory. Why, the viruses are not considered as living organisms solely in strict sense?

         Ans, Organisms are distinguished by their ability, to grow, to show metabolism, reproduce independently, evolve in their environments and display cellular level of body organisation. Viruses do not show any of these characteristics. They are infact fragments of DNA or RNA enclosed in protein. The protein units here are called Capsomeres    which    are    sometimes surrounded by a membrane like envelope also. As nucleic acid present in them, these have the capability to produce their own copies but due to the absence of supporting machinery  (ribosome,  t-RNA,  etc)  they require living cells for this purpose. So viruses donot have any living characteristic except replication and replication happens only when living cells are available to assist them. That's why we  onsider them acellular   particles    with    some    living characteristics, i.e., a connecting link between living and non living.

Characteristics of Procaryotic Cells

  1. Nuclear Material. DNA is naked and lies variously coiled in the cytoplasm. It is often called gonophore, nuclear body or nucleoid. It is equivalent to a single naked chromosome and is, therefore, also called prochromosome. Many prokaryotes also have additional small cir­cular DNA entities called plasmids. Plasmids carry additional specific factors like nitrogen fixation, resistance, fertility, etc.
  2. Nuclear Components. Nuclear envelope, nucleoplasm, nucleolus and histone cover­ing of chromatin are absent. In eukaryots (=eukaryotic) cells, a typical nucleus is found.
  3. Types. Prokaryote contains three main types of organisms—blue-green algae (BGA = cyanobacteria, e.g., Oscillatoria, Nostoc), bacteria (e.g., Pneumococcus, Corynebacterium diphtheriae) and pleuropneumonia-like organisms or PPLO (e.g., Mycoplasma gallisepticum, M. laidlawii). PPLOs are the smallest free living organisms.
  4. Cell Wall.

         It is present in bacteria and cyanobacteria. A cell wall is absent in mycoplasma or PPLO.

  1. Flagella and Fimbriae. Flagella are present in some bacteria only . The bacterial flagella are single-stranded as com­pared to 11-stranded flagella of eukaryotes. In some bacteria, non motile appendages called pili or fimbrae also occur. They take part in attachment (e.g., Neisseria gonorrhoeae) and conjugation (e.g., Escherichia coif).
  2. Photosynthetic Thylakoids. Blue-green algae and some bacteria are photo-autotrophic. Their photosynthetic thylakoids lie freely in the cytoplasm. They are not organised into chloroplasts.
  3. Membrane-lined Cell Organelles. The prokaryotic cells lack mitochondria, endoplasmic reticulum,   golgi   apparatus,   lysosomes, microtubules, microfilaments and centrioles.
  4. Vacuoles. Typical vacuoles are doubtful. In­stead complex gas vacuoles are found.
  5. Ribosomes. Ribosomes are 70S as com­pared to 8OS. Similar 70S ribosomes occur inside chloroplasts and mitochondria of eucaryotes.
  6. One-Envelope System. In prokaryotic cells, membrane bound cell organelles are absent so that there is a single membrane that surrounds the cell. Hence, prokaryotes have a single membrane or one-envelope system. In eukaryotes many organelles are surrounded by their own covering membranes in addition to the cell membrane that covers the whole cell. Therefore, eukaryotes have a double membrane or two-envelope system of organisation.
  7. Cyclosis. Cytoplasm of prokaryotes does not show streaming movements or cyclosis.
  8. Spindle. Mitotic spindle is not formed.
  9. Sexual Reproduction. It is absent. There­fore, meiosis and gamete formation are unknown.
  10. DNA Content. It is low.
  11. Transcription and Translation. Both transcription and translation occur in the cytoplasm.
  12. Respiratory Enzymes. They usually lie in contact with cell membrane.
  13. Endocytosis and Exocytosis. They seen to be absent in procaryotes.
  14. Nitrogen Fixation. It occurs only in some procaryotes, bacteria and cyanobacteri

4. What are smallest living organisms? Describe their notable features?

         Ans.    Among living organisms Mycoplasma may represent the smallest self replicating organisms that are capable of a cell free existence. The Mycoplasma are highly pleomorphic (i. e., with irregular morphology) because they lack a cell wall. These can produce filaments that resemble fungi that's why called Mycoplasma (mykes = fungus; plasma = formed). The cells of mycoplasma are very small (ranging from 0.1 to 0.25um) thus, they correspond to the size of some larger viruses. Their size and plasticity allowed them to pass through filters which retained bacteria, that's why these were originally considered as viruses. Studies of their DNA suggest that these are genetically related to the gram positive bacteria (Bacillus, Streptococcus and Lactobacillus) which gradually lost their genetic material through degenerative evolution. Mycoplasma can be grown on artificial media that provide them nutritional or physical requirements especially sterols (if necessary). Colonies are less than 1 mm in diameter and have a characteristic "fried egg appearance." The outer covering of Mycoplasma is 3 layered made up of proteins and sterols. Internal structure is similar to a typical prokaryotic cell with double stranded DNA as the genetic material. Mesosomes are absent but ribosomes, soluble proteins, amino acids and other metabolites are present in cytoplasm.

  1. RNA is also found in Mycoplasma cells.
  2. The most common human pathogen among the mycoplasma is M. pneumoniae which is the cause of a common form of M. pneumonia.
  3. Other genera Spiroplasma contains cells with a tight corkscrew morphology, and the members are serious plant cathogens and common parasites of plant feeding insects. Some common diseases of this group are "Little leaf diseases" of brinjal, "Citrus greening" of citrus, "Grassy shoot disease" of sugarcane, "Yellow dwarf disease" of rice etc.
  4. Another genus Ureaplasma, so named because these can enzymatically split the urea in urine and are occasionally associated with urinary tract infections (Mycoplasmal urethritis).

         Mycoplasma was first discovered by E. Nocard and E, R. Roux (1898) of France and called PPLO (Pleuropneumonia like organisms). These were later on given the name Asterococcus mycoides by Borrel et a/., (1910). Later on Nowak (1929) put Asterococcus under the genus Mycoplasma. |JL Mycoplasmas are resistant to antibiotics like Penicillin (which act on cell wall) but are inhibited by tetracyclin and other antibiotics which act on metabolic pathways. , As mycoplasma lack cell wall, they give negative (-) response to Gram staining.

Protoplasm

The term 'protoplasm' was coined by Punkinje, 1840 for what is contained in side a cell. The protoplasm is divided into nucleoplasm (nuclear protoplasm) and cytoplasm (extranuclear protoplasm). The pH of the protoplasm is around 6.8. It contains 90% water and constitutes 95% weight of an organism. The main constituents of protoplasm are oxygen (62%), carbon (20%), hydrogen (10%) and nitrogen (3%). Besides, it also contains Ca, P, Cl, S, K, Na, Mg, I and Fe.

Several theories have been proposed to explain the organization of protoplasm. According to Heitzmann's Reticular Hypothesis, the protoplasm possesses a delicate reticular structure. Flemming, in his Fibrillar Hypothesis regarded it to be a filamentous structure. The Alveolar Hypothesis of Butschli, however, regards protoplasm to be a liquid showing alveolar or foamy structure. Altmann's Granular Hypothesis differentiates protoplasm into a homogenous ground substance called 'hyaloplasm' containing granules which he termed as 'bioplasts'. The bioplasts were later recognised as mitochondria.

Wilson, 1925 proposed the Colloidal Hypothesis to explain the nature and organization of protoplasm. According to him, the watery dispersion medium constitutes the continuous phase where protein molecules are present as disperse phase. The proteins of protoplasm are thus present in a colloidal suspension. The colloidal particles show brownian movement, solation and gelation, tyndall effect, imbibition and possess electric charge. They (proteins) can be separated from crystalloid by dialysis. Besides, the protoplasm is, of course, a living substance showing properties like nutrition, metabolism and irritability,

Q.5.  Describe the events related to protoplasm in chronological order.                             

Ans:-The events related to protoplasm are as follows :

(1)     F. Dujardin in 1835 described it as Sarcode in Protozoa and considered it to be as living matter.

(2)     J.E. Purkinje in 1839-40 and Hugo Von Mohl in 1846 coined the term Protoplasm and described its relationship to the cell.

(3)     Huxley in 1868 stated it as the physical basis of life.

(4)     Strasburger in 1882 differentiate it into cytoplasm and nucleoplasm.

(5)     Wilson in 1925 proposed the colloidal theory for its organisation.

Q.6. Why Protoplasm is considered as living while Deutoplasm is nonliving? Describe the examples of deutoplasm also.

         Ans:- Protoplasm is considered as living because it metabolises and self prepetuates, i.e., it has the power of internal growth. Deutoplasm on the otherhand is the name given to non-living substances produced by the protoplasm itself, for example : yolk bodies, lipid droplets, secretory granules, pigments etc. These substances don't have the power of internal growth that's why we consider them as non living substances. Alongwith, protoplasm has all the biological properties of living organisms, i.e., nutrition, respiration, excretion, metabolism, reproduction and off course Irritability (i.e., response to external stimuli), Conductivity, etc. Deutoplasmic material does not show any of the above written properties. In protoplasm protein molecules are present in dispersed phase while water serves for dispersion medium.

Protoplasm shows following properties of colloids:

  1. Brownian movement
  2. Tyndall effect
  3. Solation and Gelation
  4. Electrophoresis
  5. Viscosity
  6. Hydration (Imbibition)
  7. Dialysis etc.

         Hence, according to this theory protoplasm is considered as colloidal solution with different constituents.

  1. 7-.Living beings are made up of same atoms and chemical energy and obey same laws of Physics and Chemistry, as non living beings. However, life has many properties which differentiate it from non livings. Describe unique properties of livings.

         Ans:- Unique properties of livings are :

         (1)     More complex organisation with more controlled and efficient use of energy.

         (2)     Body with 1 or more cells which are infact assemblages of large, fragile molecules enclosed within a semipermeable membrane.

         (3)     The chemistry of life is governed largely by enzymes, which in turn are controlled by genes.

         (4)     Utilisation of energy for growth and reproduction by livings.

         (5)     The ability of homeostasis, i.e., maintenance of internal environment of the body.

         (6)     The property of irritability (i.e., response to an external stimulus) development (i.e., series of changes which are characteristic of their kind and governed by the instructions within their genes.)

                  The chemical composition of protoplasm can be seen as :

Q.8. Why the cell walls are not seen in animal cells?

Ans :- Of all the multicellular organisms, only animals do not have rigid walls around their cells. Actually an animal is adapted move its entire body from place to place, an action made possible by specialised nerve and muscle cells. Muscles operate:rapidly changing their length, i. e., in the case of animals, we normally saw a very frequent change in sizes and shapes. Sue: pronounced changes in cell sizes and shapes are not possible if a rigid cell wall is seen around the cell. Rest of the living beings restricted for such movements or move very little, do not show such frequent changes, thus, have a rigid cell wall around them.

         Similarly, the growth in an animal includes increase in organ size and tissues while plants grow by adding new tissues and 1 organs to them without reorganizing the previously existing ones. Thus, the presence of such a cell wall hampers the| growth of an animal.

Q.9.  What stops cytoplasm from leaking out across the plasma membrane ?

         Ans:- It is the selectively permeable nature of plasma membrane which stops cytoplasm from leaking across the plasma membrane. Although, the semi fluid nature of cytoplasm also plays its own role but it is also there that the ionic content c: cytoplasm has the tendency to move readily (i.e., leak) across the plasma membrane through diffusion but larger organ;: molecules do not have such tendency.

Q.10. Outline the advantages of internal membranes on eukaryotic cells.

            Ans:- With internal membranes and consequent development of membrane-bounded organelles with specialised functions eukaryotic cells become longer and more complex than prokaryotic cells. This evolution allowed the eukaryotic cells:: exploit external environment resources not available to prokaryotic cells and to become the most diverse of all organism It eventually led to the development of multicellular organisms

Differences between Prokaryotic cell & Eukaryotic cell

                         Prokayotic cell

                         Eukaryotic cell

1.The cell size is usually small (0.1— 5.0 um).

1.The cell size is comparatively larger (5 — 100 µm).

 

2.A procaryotic cell has one envelope organ­isation.

2.A eucaryotic cell has two envelope organisation.

 

3.The flagella, if present, are single stranded, 4 — 5 ^m (length) x 12 nm (diameter) and without differentiation of axoneme and sheath

3.The flagella, if present, are 11-stranded, 150 — 200nm (length) x 200 nm (diameter). They show differentiation of axoneme and sheath.

 

4.An organised nucleus is absent. Instead a nucleoid is found.

4.An organised nucleus is found. It is diff­erentiated into nuclear envelope, chromatin, one or more nucleoli, and nucleoplasm.

5.Cell wall, if present, possesses muramic acid.

5.Cell wall, if present, does not contain muramic acid.

6.DNA is naked, that is, without an association with hi stones.

6.Nuclear DNA is associated with histone proteins.

7.DNA is usually circular.

7.Nuclear DNA is linear. Extra nuclear DNA is commonly circular.

8.The amount of DNA is comparatively low.

8.The amount of DNA is comparatively very high.

 

9.DNA lies freely in the cytoplasm. It is not associated with any organelle.

 

9.Most of the cell DNA lies in the nucleus. A small quantity is also found in the plastids and mitochondria.'

10.The  amount  of DNA  remains  the  same throughout the life cycle

10.The amount of DNA shows a regular alter­nation between diploid and haploid stages.

11.Transcription and translation occur in the cytoplasm.

11.Transcription occurs in the nucleus while translation takes place in the cytoplasm.

12.Protein synthesis occurs only in cytoplasm.

12.Protein synthesis takes place in cytoplasm, mitochondria and plastids.

13.Respiratory   enzymes   are  associated   with plasma membrane.

13.Respiratory enzymes are present in both cytoplasm as well as mitochondria.

14.Endocytosis and exocytosis are rare.

14.They are quite common.

15.Endoplasmic reticulum is absent.

 

15.Cytoplasm is differentiated into cytoplasmic matrix and endoplasmic reticulum.

16.Cytoplasm does not show streaming move­ments

16.Cytoplasm usually shows streaming move­ments.

 17.Ribosomes are of 70S type.

 

17.Ribosomes are of 8OS type. 70S ribosomes however, occur in plastids and mitochondria

18.Mitochondria are absent.

18.Mitochondria are usually present

19.Golgi apparatus is absent.

19.Golgi apparatus is present.

20.Centrioles (centrosome, central apparatus) are
absent

20.Centrioles are usually present in cells of organisms in which motile stage is present at one or the other time of the life cycle.

21.Lysosomes and other microbodies are absent.

21.Microbodies including lysosomes or their equivalent are present.

22.True or sap vacuoles are usually absent. In­stead, gas vacuoles, may be found.

22.True or sap vacuoles are commonly found.

 

23.Microtubules and microfilaments are com­monly absent.

23.Microtubules and microfilaments are impor­tant constituents of eucaryotic cells.

24.Thylakoids, if present, lie freely in cytoplasm.

24.Thylakoids, if present, are grouped inside the chloroplasts.

25.Gametes   are   not   formed,   since   sexual reproduction is absent.

 

25.Gametes ,are formed either directly or through meiosis, as sexual reproduction is found in the life cycle.

26.A spindle apparatus is not formed during division.

26.A spindle apparatus is produced during nuclear division

27.Cell membr.ane may be infolded to form a complex structure called mesosome.

27.Mesosome-like structures are absent

28.Cell   membrane  takes   part   in   separating replication products.

28. Cell membrane does not have any role in separating replication products. This is done by spindle apparatus.

29.DNA does not have superfluous parts. There­fore, RNA does not require any processing.

29.RNA requires processing as DNA possesses superfluous parts.

30.Nucleoid is equivalent to a single chromo­some or prochromosome.

30.Nucleus contains more than one chromosome.

 

31.A distinction of interphase and M-phase is absent.

31.There is a distinction of I-phase and M-phase.

 

32.Additional small circular DNA segments or plasmids may occur.

32.Plasmids are usually absent.

 

 Differences between Plant cell & Animal cell

                               Plant cell

                       Animal cell

1.A plant cell has a rigid wall on the outside

1.A cell wall is absent, through other structures occur in some acellular organisms formerly included amongst animals.

2.It has a definite form.

2.A definite form is less common.

3.It is usually larger in size.

3.An animal cell is comparatively smaller in size.

4.It cannot change its shape.

4.An animal cell can often change its shape.

5.It cannot change its pisition or more about.

5.Many animal cells can change position or move about.

6.Plastids are found in plant cells.

6.Plastids are usually absent.

7.Plant cells exposed to sunlight possess chlorophyll containing plastids called chloroplasts.

7.Chlorophyll is absent.

8.A mature plant cell contains a large central vacuole.

8.An animal cell often possesses many small vacuoles.

9.Nucleus lies on one side in the peripheral cytoplasm.

9.Nucleus usually lies in the centre.

10.Nucleus is elliptical.

10.Nucleus is rounded.

11.Mitochondria are comparatively fewer.

12.Mitochondria are generally numerous.

13.Cristae are tubular in plant mitochondria

13.Cristae are plate-like in animal mito­chondria.

14.Plant cells do not burst if placed in hypotonic solution due to the presence of cell wall.

14.Animal cells usually burst if placed in hypotonic solution unless and until they possess contractile vacuoles.

15.Centrioles are usually absent except in lower plants.

15.Centrioles are found in animal cells.

16.Spindle formed during nuclear division is anastral.

16.Spindle formed during nuclear division is amphiastral.

17.Golgi apparatus consists of a number of distinct or unconnected units called dictyosomes.

17.Golgi apparatus is either localised or consists of a well connected single complex.

18.The cell cannot take part in phagocytosis.

18.It can ingest materials through phagocytosis.

19.Lysosomes are rare. Their activity is performed by specialised vacuoles.

19.Typical Lysosomes occurs in animal cell

20.Glyoxysomes may be present.

20.They are absent.         

21.A plant cell produces all the materials needed by it.

21.An animal cell cannot synthesise certain aminoacids, fattyacids, vitamins and coenzymes needed by it.

22.Crystals of inorganic substances may occur inside the cells.

22.Crystals usually do not occur in animal cells.

 

23.Reserve food is generally starch and fat.

23.Reserve food is usually glycogen and fat.

24.A tissue fluid does not bathe the individual cells.

24.A tissue fluid containing NaCl bathes the cells.

25.Adjacent cells may be connected through plasmodesmata.

25.Adjacent cells are connected through a number of junctions.

26.Adjacent cells are connected by middle lamella.

26.Adjacent cells are joined by various types of junctions.

27.Cytokinesis occurs by cell plate.

27.Cytokinesis takes place by cleavage.

 

 

 

 

Important notes by AKB sir:-
  1. RBC of frog was first cell described by Swammerdam (1658). Grew, the Father of Plant Anatomy (1671), gave cell concept stating that cell is the basic unit of structure and found in all organisms.
  2. Schleiden proposed that each cell in multicellular organisms leads a double life- one as an individual and other as part of community.
  3. Protoplast:- a term coined by Hanstein (1880) include all the living constituents of the protoplasm viz. vacuoles, nucleus, cytoplasm and membranes. A naked cell without cell wall as obtained by lysozymes, is also called protoplast.
  4. Smallest prokaryotic cell (Mycoplasma hominis— PPLO) is 0.1-0.3 µm in size; Largest Prokaryote cell:- Spirulina (13-15 urn). Mycoplasmas are bacteria without cell wall. Bacteria are smallest living cell with cell wall.
  5. Unicellular eukaryote is 1-1000 µm in size.
  6. Ageing is absent in unicells.
  7. Cells of Multicellular eukaryotes 5-100 µm in size.
  8. Our body has 100 million cells of 200 to 260 types. The number of cells in body is (wt. in kg X 1015) e.g., a person of 50 kg wt. has 50 x 1015 cells.
  9. Egg of Whale Shark (Rincodon) is largest and 30 cm in diameter.
  10. In human beings, nerve cell (Longest animal cell) is 90 cm. Human sperm 60 µm; human egg cell (100 µ) in diameter; kidney cells 30 µm and RBC is 6-8 µm in size (smallest).
  11. Longest plant cell is Fibre of ramie (Bcehmeria nivea) which is 55 cm in size, Jute is 30-90 cm and Hemp fibre is about 100 cm long.
  12. Young dividing cells like cancer are killed by X-rays easily.
  13. Largest unicellular eukaryotic plant is Acetabularia (Umbrella plant) is 10 cm and animal is Amoeba (1mm). Smallest unicellular eukaryotic plant is Chlamydomonas and animal is Plasmodium (2 µm).
  14. Viruses do not have a cellular structure and cytoplasm and hence called acellular. They are smallest (0.02 to 0.3 µm) acellular structures and have a cell volume of (7 X 10-7 µm3).
  15. Ostrich egg is not considered as true cell as it stores a large amount of reserve food for developing foetus.
  16. In human beings RBCs (6 to 8u in size) are smallest and nerve fibre is longest (90 cm) cell. Human sperm is 60 µm in length and egg 100 µm in diameter.
  17. Term acellular is used for those organisms which do not have cellular organisation e.g., viruses. Term unicellular is used for organisms having cellular organisation e.g., protists / Amoeba / Chlamydomonas / prokaryotes, yeast. One celled organisms are better described as acellular or noncellular as their body is not divided into cells and this single cell is a complete organisms doing all functions.
  18. Acaryotic cell is that which does not have nucleus e.g.. RBC, sieve cell.
  19. Cell organelle (organoids) are highly organized subcellular protoplasmic structures having a definite shape and function.
  20. Smallest organelle : Ribosome; Largest organellemitochondria in animal cell and chloroplast in plant cells.
  21. Smallest component of cell — an Microfilaments ; Largest component of cell is Nucleus.
  22. Nucleus is not an organelle, it is extracytoplasmic component of cell.
  23. The specialized cells lose some of their autonomous activities, e.g., muscle and nerve cells do not divide and RBC do not respire aerobically.
  24. Position, cell wall, age, viscosity of cytoplasm, skeleton and function of the cell control the shape of cell e.g., RBC is biconvex/biconcave to increase surface area ; Nerve cells are large as they are to conduct impulses.
  25. Cells regulate their activities by flow of energy and flow of extrinsic (Hormonal) and intrinsic (genetic) information.
  26. Intracellular (sub cellular) compartments in cell help in efficient functioning of cell. These compartments are found in eukaryotic cells of plants, animals, fungi and protists. These are absent in Monerans (Prokaryotic cells). The membrane
  27. Bounded compartments in the cells are called organelles (organoids) e.g., nucleus, plastids, ER etc. Each organelle has a specific structure and function. Compartmentalisation is essential for cellular life.
  28. Plants contain two organ system only like Shoot system & Root system
  29. Animal cells burst in H2O as they don’t have cell wall to resist turgour pressure due to endosmosis
  30. Thixotropism is reversible sol  reversible to gel system.

 

Practice Question

  1. The primitive atmosphere of the earth had no
    (a) Hydrogen (b) Helium                      (c) Nitrogen               (d) Oxygen
  2. Eukaryotes probably originated

         (a)   10 million years ago                                          (b)  150 million years ago

         (c)  1500 million years ago                                       (d)  15 million years ago

  1. The term progenote refers to

         (a) A symbiont                                                         (b) An ancestral organism

         (c) A viroid                                                              (d) A coacervate

  1. Schleiden was basically a

         (a) Zoologist                      (b) Botanist                    (c) Thinker                      (d) Chemist

  1. Who proposed the theory that 'cells arise only from the pre-existing cells' ?

         (a) Mohe                            (b) Virchow                    (c) Haechel                     (d) Brown

  1. The smallest bacterium measures

         (a) 0.5n                               (b) 0.05n                        (c) 1.5n-                          (d) 0.15n,

  1. A prokaryotic cell lacks

         (a) E.R.                              (b) Golgi complex ;        (c) Both of these           

         (d) E.R., golgi and lysosomes

  1. The early organisms that appeared on the earth were

         (a) Facultative aerobes                                             (b) Facultative anaerobes    

         (c) Obligate anaerobes                                             (d) Obligate aerobes

  1. The early organisms lived

         (a) On water surface         (b) Under water surface (c) On the soil                (d) Under the soil

  1. The surface life came into being due to the conversion of

         (a) COintoCO2                        (b) O2intoO3                         (c) N2intoNO3                     (d) H2 and O2 into H2O

  1. During evolution, large organic molecules synthesised were

         (a) Nucleic acids                                                      (b) Protein macromolecules 

         (b) Autotrophs                                                         (d) Coacervates

  1. The R.B.C. of frog were first observed by

         (a) Swammerdam              (b) Leeuwenhoek           (c) Dutrochet                  (d) Robert Hooke

  1. Animal cells differ from plant cells in having
  2. a) Centrosomes               (b) Golgi bodies             c) Mitochondria             (d) Vacuoles
  3. The cell organelles are found in

         (a) Bacterial cells               (b) Cyanobacterial cells  (c) Procaryotic cells        d)  Eucaryotic cells       

  1. The pH of protoplasm is around
  2. a) 2.8                            (b) 6.8                             (c) 8.2                             (d) 8.6   
  3. The cell was observed for the first time by

         (a) Swammerdam              (b) Leeuwenhoek           (c) Robert Hooke          

         (d)  Schleiden and Schwann

  1. The main constituents of protoplasm are

         (a) C, H, O, Ca                  (b) C, H, O, N                (c) C,H,Ca, Fe          (d) C, H, P, Ca, Fe

  1. The percentage of oxygen in protoplasm is around

         (a) 60                                (b) 20                              (c) 10                              (d) 5

  1. The percentage of carbon in protoplasm is about

         (a)  5                                  (b) 20                              (c) 10                              (d) 12

  1. What Robert Hooke saw in the year 1665 under the microscope as a cell was really

         (a) The nuclear membrane                                       (b) The cell wall

         (c) The plasmalemma                                               (d)                                  The ectoplasm

  1. The reticular theory about the organization of protoplasm was proposed by

         (a)  Altmann                      (b) Virchow                    (c) Dutrochet             (d) Heitzmann                 

  1. The alveolar organization of protoplasm was suggested by

         (a) Flemming                      (b) Altmann                   (c) Butschli                (d) Wilson          

  1. The dispersed phase of protoplasm, according to colloidal hypothesis, is constituted by

         (a) Proteins                        (b) Bioplast                    (c) Organelles                 (d) Lipids

  1. The proteins of the protoplasm are present as

         (a) An emulsion                 (b) Colloidal suspension (c) Supernatant               (d) Colloidal emulsion

  1. Point out the most correct statement

(a)     The cell theory postulated by Schleiden and Schwann was new and correct.

(b)     The cell theory postulated by Schleiden and Schwann was not new as well as not wrong.

(c)     The cell theory postulated by Schleiden and Schwann was new but wrong.

(d)    The cell throry postulated by Schleiden and Schwann was not new as well as wrong but the credit should be given to the proposers of the theory for being able to attract the attention of the biologists of their time.

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