Cell biology Class 11 Science Microscopy
MICROSCOPY
Micron or micrometre (µ) =
of a meter i.e. 10-6 meter
Angstron (Å) = 10-4 microns or 10-10 meter
Nanometer (nm) = 10-9 meter or 10 Ao
The simple microscope was invented by Jansen and Jansen (1590).
The first compound microscope was made by R. Hooke (1665).
Light microscopy-
In the light microscope the limit of resolution :
depends on the wavelength of the light and the numerical aperture. The limit of resolution is, in general, 0.25 µm.
Phase microscopy –
It is used for the study of living cells which are, in general, transparent to light. The phase contrast principle was worked out by Zernicke (1940).
Interference microscopy-
It provides quantitative information regarding the thickness, dry matter and water content of the object.
Darkfield microscopy or ultramicroscopy-
Objects smaller than the wavelength of light can be detected by this microscopy. Nucleolus, nuclear membrane, mitochondria and lipid droplets appear bright and the background of the cytoplasm appears dark by this microscopy. It is also Called ‘darkfield illumination.
Electron microscopy –
The EM was invented by Knoll and Ruska (1932). The resolution power of it is .3 to .5 nm and the final magnification is 106 times or more. The source of light used in EM is a beam of electrons.
X-ray diffraction –
It is a technique used in molecular biology, especially for the study of nucleic acids and protein structure.
Autoradiography-
It is a method for detecting and localizing radioactive isotopes in cytological preparations or macromolecules by exposure to a photographic emulsion that is sensitive to low energy radiation.
l The human eye is capable of discerning an object not smaller than 0.1 mm.
l Light microscope introduces a 500-fold increase in resolution over the eye (105 nm).
l The electron microscope provides. The 1000-fold increase over the light microscope.
THE CELL
l The cell is a fundamental unit of life.
l It is a microcosm having a definite boundary within which constant chemical activities and flow of energy proceed.
l In plant cells source of stored energy is starch while in bacteria the sugar molecules are directly absorbed from the medium.
l The weight of important components of the cell is expressed in picograms (1 pg = 1012 gm) or in Dalton (1 Dalton = weight of a hydrogen atom).
l The biggest cell is an egg of Ostrich which is 170 ´ 135 mm.
l The smallest cell is the mycoplasma which measures 0.1 – 0.5 µ.
l Plastids and cell-wall are present in plant cells but lack in animal cells.
l Centrioles lack in plant cells (except some algae and primitive fungi)
l Prokaryotic cells do not have a well-organized nucleus with the nuclear membrane.
l A typical cell has a single nucleus (mononucleate) but it may be anucleate i.e without a nucleus (RBCs of man and sieve tubes in plants). binucleate (dikaryotic condition in fungi; Paramecium) and multinucleate (corneocytes).
l Homoeostasis is the process by which the cellular activities are regulated.
Cell-wall
l Cell-wall is characteristically present in plants.
l It is a product of the cytoplasm.
l Primary and secondary cell-walls are chiefly composed of cellulose.
l The principal component of tertiary cell-wall is xylan.
l Cellulose is a linear polymer of D-glucose.
l The enzyme that hydrolyses to D-glucose is cellulase.
l The adjacent cell-walls are cemented together by pectin (calcium pectate).
l Plasmodesmata establish continuity between adjacent plant cells.
Plasma membrane
l It was the electron microscope which made plasma membrane visible.
l It is a trilayered structure and is differentially permeable (semipermeable).
l It is also called unit membrane. The unit membrane concept was given by Robertson (1959).
l Pfeffer (1877) gave the name ‘plasma membrane’.
l Nageli and Cramer (1885) described this membrane as ‘cell membrane’.
l Plowe (1931) coined the term ‘plasmalemma’ for it.
l Plasma membrane consists of lipoproteins.
l One of the most favoured models for the plasma membrane is fluid mosaic model proposed by Singer and Nicholson (1972).
l According to this model the plasma membrane is a continuous lipid-bilayer in which the integral proteins are intercalated.
l The integral proteins pass across the outer and inner surfaces of lipid-bilayer thus providing ‘protein-lipid-lipid protein (P-L-L-P) ‘sequence to the plasma membrane.
l Lipids and many of the proteins of PM are amphipathic molecules i.e. they have water-loving (hydrophilic) and water-hating (hydrophobic) groups within the same molecule.
l There is a potential difference of 60-90 millivolts between inside and outside of the PM, the outside being (+) charged and inside (-) charged. This potential difference is called resting potential’.
l If punctured, plasma membrane is capable of self-repair in presence of calcium ions.
Vacuoles
l In contrast to the animal cells, the majority of the plant cells have vacuoles.
l The vacuole has a single layer membrane called ‘tonoplast’, the term coined by De Vries (1884).
l Organic substances are present in high concentration in vacuoles.
l Red colour of many flowers is due to the presence of pigments in the vacuoles of the petals.
l Vacuoles also serve as waste deposit box because the unwarranted materials are shunted in it.
Plastids
l In 1883, Schimper first used the term ‘plastids’.
l Three main type of plastids are chloroplasts, leucoplasts and chromoplasts.
l Chloroplasts do not arise de nova but from growth and division of other chloroplasts.
l The first example of chloroplast inheritance was discovered by Correns (1909) in Mirabilis jalapa, the four O’ clock plant.
l Leucoplasts are colourless and are found in embryonic cells, germ cells, meristematic cells and in the cells growing in dark.
l Leucoplasts accumulate starch (amyloplasts), protein (proteinoplasts) or fats and essential oils (elaioplasts).
l Starch grain formation in storage cells is the chief function of leucoplasts.
l Chromoplasts possess reduced chlorophyll content and are thus less active photosynthetically.
l The red colour of ripe tomatoes is due to chromoplast containing a red pigment called ‘lycopene’ which is a type of carotenoid.