INTRODUCTION
Cell Biology (formerly Cytology, derived from Greek word “kytos” – meaning “container”)
In broader terms it refers to - study of cells, their structure and organelles, physiological properties, functional significance, molecular organization, interaction with environment, growth, reproduction and their genetics.
Thus it provides idea of how a particular cell type caries out specific function The study could be microscopic and molecular level
It comprises of two broad groups of cells (Prokaryotic – bacteria, archaea and Eukaryotic cells)
Appreciating the similarities, differences and their functioning could lead to importance in several field like molecular biology and biomedical fields (cancer research and developmental biology).
Therefore research in cell biology is closely related to various fields including: Molecular biology (studying biology at molecular level, it includes study of DNA, RNA and protein synthesis and how they are regulated)
Biochemistry (study of chemical process in the living organisms, it deals with structure and function of cellular components including, carbohydrates, proteins, lipids and nucleic acids)
Developmental biology (study of the processes by which organism grow and develop, including genetic control of cell growth, differentiation and morphogenesis)
Cytogenetics (a branch of cell biology deals with cytological and molecular basis of heredity, variation, mutation, phylogeny, morphogeneis and evolution of organisms) Cytochemistry (biochemistry of cells, especially related to macromolecules responsible for cell structure and function)
Cytopathology (branch of pathology that studies and diagnosis diseases on the cellular level e.g. use of Pap smear to detect cervical cancer to detect at early detectable stage) Cytoecology (deals with effects of ecological changes on the chromosome number of the cell)
Microbiology (study of microorganisms).
History of Cell Biology
Aristotle and Paracelsus – Stated that all animals and plants are complicated, are constituted of few elements which are repeated in each of them.
Da Vinci – recommended uses of lenses in viewing small objects.
Conrad Gesner – published results of his studies on the structure of a group of protist called Foraminifera
Galileo Galilei – invented simple microscopes to study compound eyes of insects
Francis Jansen and Zacharia Janssen – invented compound microscopes. Marcello Malpighi – first to use microscope to examine and describe thin slices of animal tissues from organs like brain, liver, kidney and spleen.
Robert Hooke – Coined the term cell, he examined a thin slice of dried cork under the compound microscope, published collection of essays under the title Micrographia, one such essay describe about “Honey comb of chambers or cells”.
Anton Van Leeuwenhoek – improved the art of polished lenses of short focal length and used such lenses to observe
microbes. He is the first to describe in sketches bacilli, cocci and spiral cells of bacteria and also the sperms of various living organisms.
Nehemiah Grew – published details about microscopic examination of sections of flowers, root and stems of plant and
indicated the cellular nature of plant tissues. Mirbel – All plants are composed of cells Schleiden – a botanist, given idea that cells were the units of structure in plants and Schwann – a zoologist, applied the
Schleiden ideas to animals, later formulated as cell theory
Cell is a Basic unit of life The basis for this begins from the points accounted by Aristotle (384-322BC) who has first proposed that a level of complexity is required for life.
All living organisms are composed of fundamental units called cells.
they are the common structure and development of an organism
They reproduce by cell division (binary fission and /mitosis/ meiosis)
They obey “central dogma of life” Cell contents are within a membrane
(plasma membrane) that is made up of lipids and proteins
Everything that exist outside this
biological membrane are non-living e.g. viruses.
Further the findings of Schleiden and Schwann commonly states that cell as the common unit of structure and development and that lead to development of Cell Theory.
Further Xavier Bichat (1771-1802) proposed that organ is composed of different tissues (which are made of fundamental units cells) and several organs grouped together to form the organ system of an organism
Other Scientists (19th century)
Rudolf Virchow – Given the cellular basis of life’s continuity, he stated in Latin that “cells could arise only from the pre-existing cells”, thus he established the
A. Kolliker – Applied the cell theory to embryology and discovered the mitochondria (sarcosomes) in muscles.
C. Nageli and C. Cramer – coined the term “cell membrane”.
Karl August Möbius – first observed the structures that were later called as organelles.
Bengt Lidforss – coined the term “organelles”.
F. Miescher – Isolated nuclei and nucleoprotein from pus cells and spermatozoa. Wilhelm His - Developed microtome for cutting serial section of tissue for cell study.
A. Scheneider- Described chromosome (nuclear filaments) for the first time. E. Strasburger – Described mitosis in plant cells and in 1822 introduced the term cytoplasm and nucleoplasm.
Schleicher- Coined the term karyokinesis.
W.Flemming – introduced the term chromatin and described the longitudinal splitting of chromosome
W. Roux – proposed that chromosome contain the units of heredity. E. Metchnikof – observed and named phagocytosis process
Other Scientists (20th century)
E. Fischer – Got Nobel Prize for his pioneering studies on proteins. E. Buchner – Awarded Nobel Prize for discovery of enzymes.
A.V. Hill and O. Meyerhof – Awarded Nobel Prize for their studies on the metabolism of muscle tissue and for relationship between muscle metabolism and lactic acid.
T. Svedberg – Awarded Noble Prize for his studies on colloids and for development of analytical ultracentrifugation.
K. Landsteiner – Awarded Nobel Prize for discovery of human blood groups. M. Knoll and E. Ruska – Built the transmission electron microscope.
W.H. Lewis – Discovered pinocytosis.
F. Zernike – Invented Phase Contrast microscope and for this he was awarded Nobel Prize.
J. Danielli and H. Davson – Proposed protein-lipid-protein structure (sandwich model) of plasma membrane.
Hans Adolf Krebs – Discovered citric acid cycle.
Edmund Beecher Wilson – known as America’s first cellular biologist, discovered the sex chromosomes and their arrangement in humans.
Konstantin Mereschkowski – described the theory of endosymbiosis.
P. Mitchell – awarded Nobel Prize for his discovery of chemiosmotic mechanism of ATP synthesis.
Current Scientists
Lynn Margulis – American biologist best known for the theory of origin of eukaryotic organelles, which is supporting evidence to endosymbiotic theory.
Gunter Blobel –Nobel Prize was awarded for discovering the protein targeting in cells.
Peter Agre – Nobel Prize winner for his discovery about porins. Ira Mellan – cell biologist discovered endosomes.
Martin Chalf, Osamu Shimomura and Roger Y. Tsien – awarded Nobel Prize (2008) for discovering GFP, which was extensively used in cell biology.
Yoshinori Ohsumi – awarded Nobel Prize (2016) for his discoveries of mechanism for autophagy.
Cell Theory
Finding of Schleiden and Schwann were later resulted in the formulation of Cell theory, their proposal is “cell the basic unit of the living tissue”. It was amended by Rudolph Virchow (1858) as “cells come from pre existing cells”. Thus cell theory in nut shell is
All living things are composed of one or more cells
Cell is the basic unit of structure and function of living organisms
New cells are produced from pre existing cells. Modern interpretation of cell theory is
Cell is the fundamental unit of structure and function of a living organism
All known living things are made up of cells
Cells are the smallest living things, anything smaller than its size are not considered alive/living
All cells comes from pre existing cells by division
Energy flow (metabolism and biochemical reactions) occur within cells
Cell contains hereditary information (DNA) which is passed from cell to cell during division.
All cells are basically same in chemical composition
Some organism are unicellular (e.g. Amoeba) and others are multicellular (made up of many cells).
The activity of the organism depends upon the total activity of the independent cells.
All kinds of true cells share the following three basic characteristics
1. A set of gene which constitute the blueprints for regulating cellular activities and making new cells.
2. A limiting plasma membrane that permits controlled exchange of matter and energy with the external world.
3. Metabolic machinery for sustaining life activities such as growth reproduction and repair of parts.
Exceptions to the Cell theory
However a few cells do not obey these basic rules for cells, those exceptions are:
Viruses – acellular entities, considered as alive by some, yet they are not made up of cells (lacks plasma membrane and metabolic machinery for energy production and for synthesis of proteins), but have features of life (genetically determined macromolecular organization, a genetic or hereditary material in the form of DNA or RNA, capacity of auto reproduction). But as per the cell theory points they are not alive, however recently they were elevated to the fourth domain of life.
The first formed cell of the world does not originate from pre existing cells.
Prions – infectious protein entities that cause ailments like Alzheimer’s disease
Mitochondria and chloroplasts have their own genetic material and thus reproduce independently from the rest of the cell.
Protoplasm theory
After the middle of the 19th century, cell biologist has started giving greater importance to the inner content of the cell.
Hugo von Mohl (1846) first used the term protoplasm to denote “tough, slimy, granular, semi fluid substance within the plant cells”.
Max Schultze (1861) established similarity between sarcode and protoplasm i.e similarity between the protoplasm of animal and plant cell. This led to stating a theory (protoplasmic theory) by O. Hertwig. It states that cell is a accumulation of living substances or protoplasm definitely limited in space and possessing a nucleus and cell membrane.
Later, Huxley and Hertwig referred protoplasm as physical basis of life. Protoplasm as complex colloidal system, having many inorganic and organic compounds in such conditions that ability to perform life processes.
However, Protoplasmic theory has not gained much importance, since with the development of advance techniques, cell can be fractionated, large molecules (macromolecules) seems to be important cellular constituents.
Organismal Theory
Early dissatisfaction over the concepts of cell theory and protoplasmic theory led to the development of organismal theory, which emphasis that “living mass as a whole is important for life rather a single cell”.
“Original cell expands into whole organism” Thus it is opposite to the cell theory which states that one cell divides into many cell with original cell equivalent to many. Thus as per this theory, the multicellular organism is a highly differentiated protoplasmic individual, differentiation involves separation of the protoplasm into subordinate semi-independent entities, the so called cells. Even the embryological development of a small single protoplasmic individual includes only growth and progressive internal differentiation of a small single protoplasmic individual (egg). Some discoveries shaped this theory they are: Reichert – organism has a structured plan
Strasburger – Cells are connected in an organism by cytoplasmic bridges SIMILARITIES AND DIFFERENCES BETWEEN PROKARYOTES AND
EUKARYOTES Sl.
No. Features Prokaryote Eukaryote
Similarities
Proposed by Stanier and Van Niel (1962)
Cell contents bounded by a plasma membrane Genetic information encoded on DNA
Ribosomes act as site of protein synthesis. However, rRNA of prokaryote (50S & 30S) is different from eukaryotes (60S & 40S).
Diferences 1. Cell size & volume 1-10 microns
average diameter 0.5-5µm High surface area to volume ratio
10-100 microns Upto 40 µm diameter Low surface area to high volume ratio
murein (bacteria) and
pseudomuerin (archaea) in plants, algae (cellulose), fungi (chitin). Genetic features
1. Genome DNA is present freely in the cytoplasm i.e. NOT bounded by nuclear membrane
(Exception: Pirellula and
Gemmata)
DNA is contained within a nuclear membrane thus it has a bound nucleus 2. Chromosome Usually single circular or as
nucleoid. (exception:
Borrelia burgdorferi – linear; Burkholderia cepacia – 3 chromosomes)
Multiple, linear chromosome generally in pairs
3. Protein synthesis The transcription coupled
translation are common The transcription and translation are temporally and spatially separated 4. Histones Are absent in bacteria but
present in archaea DNA are complexed with histone proteins 5. Sensitivity to
diphtheria toxin Bacteria is resistant, however, archaea are susceptible
Internal features
Membrane bound organelles absent (but has unique structures like chlorosomes, Carboxysomes - made of
non-unit membrane)
Unit membrane bound
organelles present,
Mitochondria & Chloroplast (double membrane)
1. Cell membrane Steroids rare but hopanoids (pentacyclic ringed compounds, analogues to cholesterol, commonly occur in bacteria & play role in membrane integrity and pH homeostasis) are common
Cell membrane contains respiratory chain (for oxidative phosphorylation) and photosynthetic
machinery (for
photophosphorylation)
Steroids rather universal In cell membrane
respiratory chain is absent (However, oxidative
phosphorylation-in cell membrane of Mitochondria) and photosynthetic
machinery (for
photophosphorylation-in cell membrane of chloroplast)
2. Flagella Simple, 20 nm in diameter, made of flagellin,
Extracellular
Complex “9+2” arrangement of
microtubules, made of tubulin
200 nm in diameter, Intracellular
3. Cytoskeletal elements
Tubulin and actin like proteins present
Both microtubules and microfilaments are present 4. Endo / exocytosis
(bulk transport) Absent Present
5. Cellular
differentiation Usually absent (exception cyanobacteria, Caulobacter sp & Hyphomicrobium sp.)
Cells differentiate to form tissues, organs, organ systems and the entire organism
6. Cytoplasmic
streaming absent present
7. Adjacent cell connections
Absent, since single celled forms
Present, Plasmodesmata (Plants)
Tight, Adherens junctions & desmosomes (Animals) 8. Cell-cell
communication By quorum sensing By signal transduction 9. Programmed cell
death (apoptosis) Absent Present
Organelles
1. Endoplasmic
reticulum Absent Present
2. Golgi apparatus Absent Present
3. Mitochondria Absent Present (in animals)
4. Chloroplasts Absent Present (in plants)
5. Centrosome Absent Present
6. Lysosome Absent Present
8. Vacuoles Present Present 9. PHB,
carboxysomes,
chlorosomes
magnetosomes
Present (bounded by non-unit membrane)
