The egg is the single- celled structure. The egg of an ostrich is the largest cell in the world and it measures up to 15 to 17 cm.

The cell is the structural unit of life and its size ranges from 0.5 micrometers to 100 micrometers. An egg of ostrich is a single-celled structure and is known as the largest cell. The size of each ostrich egg is 170 mm X 130 mm. Thus, the correct answer is option A.

The human egg (ovum) is the largest cell in the body and can be seen without the aid of a microscope. It is one millimeter in diameter. Whereas, a nerve cell is the longest cell in human body.

Phase contrast microscopy is particularly important in biology. It reveals many cellular structures that are not visible with a simpler bright field microscope. These structures were made visible to earlier microscopists by staining, but this required additional preparation and killed the cells. The phase contrast microscope made it possible for biologists to study living cells and how they proliferate through cell division. So, phase contrast microscope is best to study cell division in functional state compared to EM, SEM and simple microscope.

• The first person to see a living cell under the microscope was Antonie van Leeuwenhoek. 
• He called that cell as animalcule. He is commonly known as "the Father of Microbiology", and considered to be the first microbiologist. 

• The cell is the basic structural and functional unit of life. Schleiden proposed that new cells arise from within the old cells, especially from the nucleus. 
• This was corrected by Rudolf Virchow who proposed, "Theory of cell lineage" it states that all cells arise from pre-existing cells.

• Robert Hooke discovered cell in the year 1665. He observed cork cell in the bark of Spanish oak tree under a simple microscope and was able to see the empty structures surrounded by walls and named it a cell. 
• He elucidated his observation in a book called "Micrographia.

The well-known scientist Robert Brown discovered the nucleus in the year 1831. 

• Robert Hooke discovered cell in the year 1665. He observed cork cell in the bark of Spanish oak tree under a simple microscope and was able to see the empty structures surrounded by walls and named it a cell. 
• He elucidated his observation in a book called "Micrographia". He was unable to see the organelles inside the cell as cork cell which he observed was dead.

The invention of the microscope allowed the first view of cells. English physicist and microscopist Robert Hooke (1635–1702) first described cells in 1665. He made thin slices of cork and likened the boxy partitions he observed to the cells (small rooms) in a monastery. The open spaces Hooke observed were empty, but he and others suggested these spaces might be used for fluid transport in living plants. He did not propose, and gave no indication that he believed, that these structures represented the basic unit of living organisms.

The power of magnification of a microscope is the product of the powers of the occular (eyepiece) and the objective lens. The maximum normal magnifications of the occular and the objective are 10x and 100x respectively, giving a final magnification of 1,000x.

Antony Van Leeuwenhoek first discovered bacteria in pond, which is filled with dirty rain water, with a microscope made by him.

Robert Hooke discovered cell in the year 1665. He observed cork cell in the bark of Spanish oak tree under a simple microscope and was able to see the empty structures surrounded by walls and named it a cell. He elucidated his observation in a book called "Micrographia".

The cell was discovered by Robert Hooke in 1665 when he saw an empty box like compartments in a very thin slice of cork under his microscope. He wrote a book 'Micrographia' and coined the term cellulae which was later changed into the cell.

Antony Van Leeuwenhoek was a tradesman of Holland. He made microscope and discovered bacteria.

Robert Brown discovered an opaque area in many plant cells when observed under the microscope. He called it the nucleus.

The cell was discovered by Robert Hooke in the year 1665 when he saw an empty box like compartments in a very thin slice of cork under his microscope and named it cellulae which later changed into the cell.

Robert Hooke observed section of a cork  below a microscope created or structured by him

Zacharias Janssen was the first scientist who is responsible for the invention of the compound microscope. The scientist worked and manufactured the different type of lenses. He made a cylindrical tube-like structure and arranged many different lenses in different orientations within the tube. This led to the formation of the compound microscope which was also known as the telescope. 

A- A microscope helps in magnifying small objects. Zacharias Jansen and his father Hans created the first microscope in 1595. This microscope was made up of a long brass tube with two lenses. 9X was the maximum magnification of this microscope. Magnification helps in viewing an object by making it larger.

Antonie van Leeuwenhoek used single-lens microscopes, which he made, to make the first observations of bacteria and protozoa. His extensive research on the growth of small animals such as fleas, mussels, and eels helped disprove the theory of spontaneous generation of life.

Oscar Hertwig is a German zoologist and a professor who coined the term cytology. The cytology means the branch of biology which deals with the study of structure and functioning of plant and animal cell.

A- Microscope is a device which is used to view objects that are very small for the naked eye. Light microscopes also known as optical microscopes are the most common type of microscopes. Optical microscopes utilize light and lenses to magnify images. 

• Antonie van Leeuwenhoek was the first to notice living cells under his microscope which he called animalcules through his preliminary microscope.
• M.J. Schleiden co-founded cell theory.
• Kolliker has made major contributions to viewing zoological and histological samples.
• Pallade was awarded the noble for discovering the ribosomes and rough endoplasmic reticulum. Thus, the correct answer is option A.

Robert Hook, an English scientist, discovered a honey-comb like structure in a cork slice using a primitive compound microscope. Since a piece of cork slice is dead tissue so he could only see hollow compartment as the protoplasm was not present in these dead compartments. These compartments were called as 'cell' by him.

The first cell was observed and named by Robert Hooke in 1665. He described that it looked like cellula, small rooms which are inhabited by monks hence derived the name 'cell'. The cell he actually saw was the dead cell walls of the plant (cork) under a microscope. Hooke described the characteristics of a cell he observed in his book 'Micrographia'. 

Phase contrast microscopy is a type of microscopic technique which converts phase shifts in light passing through a transparent specimen to brightness changes in the image. This technique made possible to study living cells without staining.

The fluorescence microscope can localize and quantify specific molecules in the cells. It is the most versatile and powerful technique for localizing proteins within a cell by light microscopy, is fluorescent staining of cells and the observation in the fluorescence microscope.

Robert Hooke discovered cell in the year 1665. He observed cork cell in the bark of Spanish oak tree under a simple microscope and was able to see the empty structures surrounded by walls and named it a cell. He elucidated his observation in a book called "Micrographia".

Fluorescent microscope is a modification of ultraviolet microscope, which was made by Coons in 1945. The instrument uses long wave ultraviolet rays for illumination. It has complementary filters, which allow the viewer to observe directly with the eyes. The microscope is useful in detecting those components which show autofluorescence, e.g., chlorophylls, collagen fibrils, actin filaments, vitamin A, etc. Others can be made fluorescent by coating with fluorochrome dyes like acridine orange and coriphosphine and minerals, eg., proteins, lipids, starch, glycogen, cellulose, etc. The light emitted by them is red, orange, yellow, or green against dark field. It is called as secondary fluorescence. The technique is also useful in diagnosis of viruses, bacteria and protozoans. Immunofluorescent antibody labelling techniques is used in diagnosis of specific molecules like antigens and antibodies. it involves conjugating specific fluorochromes with specific antibodies.

Fluorescent microscope is a modification of ultraviolet microscope which was made by Coons in 1945. The instrument uses long wave ultraviolet rays for illumination. It has complementary filters which allow the viewer to observe directly with the eyes. The microscope is useful in detecting those components which show autofluorescence, e.g., chlorophylls, collagen fibrils, vitamin A, etc. Others can be made fluorescent by coating with fluorochrome dyes like acridine orange and coriphosphine and minerals, eg., proteins, lipids, starch, glycogen, cellulose etc. The light emitted by them is red, orange, yellow, or green against dark field, it is called as secondary fluorescence. The technique is also useful in diagnosis of viruses, bacteria and protozoans. Immunofluorescent antibody labelling techniques is used in diagnosis of specific molecules like antigens and antibodies, it involves conjugating specific fluorochromes with specific antibodies.

Fluorescent microscope is a modification of ultraviolet microscope which was made by Coons in 1945. The instrument uses long wave ultraviolet rays for illumination. It has complementary filters which allow the viewer to observe directly with the eyes. The microscope is useful in detecting those components which show autofluorescence eg. Chlorophylls, collagen fibrils, vitamin A etc.

X-ray crystallography is a technique to study the structural details of crystallizable proteins and nucleic acids. X-ray microscope developed by Kirkpatrick is useful in studying detailed structures of chemicals present in solid state, e.g., haemoglobin, insulin, DNA, RNA etc. due to diffraction of X rays by different atoms of the substance. 

Phase contrast microscope has a phase plate and an annular diaphragm. They bring about changes in light rays passing through the specimen producing differences in light intensity. Denser parts alter path of light more than the thinner parts. This produces varying contrast for different regions. 

Fluorescent microscope is useful in detecting those components which show autofluorescence, e.g., chlorophylls, collagen fibrils, vitamin A. Others can be made fluorescent by coating with fluorochrome dyes like acridine orange and coriphosphine. This is called as secondary fluorescence and is useful in detecting specific biomolecules.

The most powerful lens of the light microscope is the 100X oil immersion objective because light is refracted every time it passes through a medium with a different refractive index, the quality of the image is reduced with each passage. Thus, by reducing the number of such passages to a minimum, the clarity, brilliance and resolving power is preserved. Immersion oil has been formulated so that it has a refractive index identical to that of glass. Thus, two changes in refractive index can be eliminated by placing a drop of immersion oil on the specimen, and immersing the 100X oil immersion objective directly into the drop. 

Nucleus, Golgi bodies and chloroplasts are the organelles of the cell and hence can easily be visible under the compound microscope. The nuclear membrane is the membrane which separates the nucleus with the cytoplasm. It is a double lipid bilayer membrane which can only be visible under the electron microscope.
Thus, the correct answer is option (D), 'membrane separating the nucleus from the cytoplasm'.

Stethoscope is a medical instrument used to hear internal sounds of an animal body. Generally used to hear lung and heart sounds.
Microscope is an instrument which is used for observing very small objects. It uses lens or combination of lenses to produce a magnified image of tiny objects which are not visible to the naked eye. Microorganisms are not visible to naked eyes. Hence, microscope is used to observe them. 

Robert Hooke discovered cell in the year 1665. He observed cork cell in the bark of Spanish oak tree under a simple microscope and was able to see the empty structures surrounded by walls and named it a cell. He elucidated his observation in a book called "Micrographia".

Average diameter of microorganisms, like bacteria, is 0.2 to 2 micrometer. They cannot be seen with the naked eye. Microscope is used to observe such small microorganisms. It uses lens or combination of lenses to produce a magnified image of tiny objects which are not visible to the naked eye.

• Robert Hooke observed “honeycomb-like structures” in cork cells under the self-designed microscope. He called the hollow shells. A German biologist Rudolph Virchow observed that all living cells arise from pre-existing cells ('omnis-cellula e cellula') which were added to the cell theory as a doctrine. The term “cell nucleus” was used the first time by Robert Brown. Leeuwenhoek formed a single lensed microscope to observe the tiny organisms which were then named as microorganisms. Thus, the correct answer is option A.

A microscope is an instrument used to see objects that are too small to see by naked eyes. It consists the lens which magnifies the image of cells. 

In the beginning of the 19th century, Theodor Schwann (1839) established cell theory and postulated that cell is the basic unit of life. 

The cell was first discovered by Robert Hooke in 1665. He gave 60 'observations' in detail of various objects under a coarse, compound microscope. One observation was from very thin slices of bottle cork. Hooke discovered a multitude of tiny pores that he named "cells". He described a honeycomb like network of cellulae (latin for storage rooms) in cork slices.

Resolving power is the ability to distinguish two close objects as distinct. It is calculated by the formula 0.61 wavelength/ numerical aperture, where  numerical aperture is the light collecting ability of lens. It is clear that resolving power will be more if wavelength is less and numerical aperture is more. Thus, the correct answer is option A.

Antonie van Leeuwenhoek used single-lens microscopes, which he made, to make the first observations of bacteria and protozoa. His extensive research on the growth of small animals such as fleas, mussels, and eels helped disprove the theory of spontaneous generation of life.

• Cell biology is the branch of biology that studies the structure and function of the cell, which is the basic unit of the life. 
• Cell biology is also concerned with the physiological properties, metabolic processes, signalling pathways, life cycle, chemical composition and interaction of the cell with their environment. Hence, the study of structure, physiology, biochemistry, development, evolution, genetics of a cell is called cell biology.
  
• So, the correct answer is 'cell biology'. 
  

• The first man to observe living cells was Anton Van Leeuwenhoek. He with his improved microscope, discovered free-living cells of algae Spirogyra in 1674 while examining his pond water. 

• Cytology is the study of the cells, especially their appearance and structure. Cells are the small parts that make up all living things, and their effects on each other and their environment. There are two types of cells. Prokaryotic cells do not have a clear and easy-to-see nucleus and do not have a membrane, or wall, around them. Eukaryotic cells have an easy-to-see nucleus where all of the cell's functions take place and have a membrane around them. Hence, the study related to the structure of the cells is the cytology
• So, the correct answer is 'cytology'.
  

• The discovery of cells was first acknowledged by Robert Hook in 1665 and he named the individual unit as cells. Around similar times Marcello Malpighi, an Italian scientist, and doctor observed cell structure of plant leaves and named each unit as utricles. This makes Malpighi the first scientist to observe living cells.

White was the first one to successfully attempt the technique of tissue culture. He cultured tomato roots in a nutrient media composed of vitamins. He developed the continuously growing callus from the root which had the property of totipotency.

The Leeuwenhoek microscope was a simple single lens device but it had greater clarity and magnification than compound microscopes of its time. Designed by a Dutchman, Antony van Leeuwenhoek, the microscope was completely handmade including the screws and rivets.

Cell biology is the study of structure and functions of a cell which also revolves around the concept that cell is the basic and fundamental unit of life. It also explores a detailed understanding of tissues and organisms composed of cells. It involves the study of unicellular as well as multicellular organisms, their generalised properties and highly intricate function specialised to a group of cells. So, the correct answer is option C.

Science had recently emerged as a profession and yet it was an amateur who became a scientific celebrity throughout Europe. It was Anton van Leeuwenhoek, a Dutch merchant without university studies, who discovered microscopic life at the end of the 17th century.

Cell structure consists of the whole cell with the cell membrane, nucleus, ribosomes, Golgi bodies which are dispersed in the cytoplasm of a cell. So basically these are cytoplasmic structures which are so tiny that they can only be observed under the microscope. And study of these cells (both animal and plant) structures, functions and their chemistry under the microscope is called as cytology.

In 1830, J.J. Lister (the father of Joseph Lister) invented the darkfield microscope, in which the standard brightfield (Abbe) condenser is replaced with a single or double-reflecting dark field condenser.

A stop is an opaque object that blocks the central light when placed underneath the stage condenser. This also causes light to scatter in all azimuths, resulting in a cone of light that allows for dark field observation.

Phase Contrast Microscope was developed by Zernike. The microscope has a phase plate and an annular diaphragm. They bring about changes in light rays passing through the specimen producing differences in light intensity. Denser parts alter path of light more than the thinner parts. This produces varying contrast for different regions. Hence, staining is not required and the difference between denser and lighter parts of the specimen is revealed by the phase difference. The microscope is useful in the study of small living organisms, living cells, constituents of living cells, effect of various factors over living structures, and study of various cellular events like behaviour of chromosomes during mitosis and meiosis, spindle formation, cytokinesis, pinocytosis, phagocytosis, cyclosis, glandular secretions, spermatogenesis, oogenesis etc.

Phase contrast microscopy is a type of microscopic technique which converts phase shifts in light passing through a transparent specimen to brightness changes in the image. This technique made possible to study living cells without staining.

Fluorescent microscope is a modification of ultraviolet microscope, which was made by Coons. The instrument uses long wave ultra violet rays for illumination. The microscope is useful in detecting those components which show autofluorescence, e.g., chlorophyll, collagen fibrils, vitamin A.

A. Histology is the branch of biology that deals with the study of animal and plant tissues.

• A microscope is an instrument, which is used for observing very small objects. 
• It uses lens or combination of lenses to produce a magnified image of tiny objects, which are not visible to the naked eyes. 
• Cells are also very small, which cannot be seen by the naked eyes. Hence, a microscope is used to observe cells. So option B is correct.

The deposition of callus occurs around the Microspore Mother Cell (MMC) prior to the cell undergoing meiotic division. The division results in the formation of a cluster of 4 haploid cells the microspores. These cells are held together by the callose to form a microspore tetrad. The callose deposition allows the development of the microspores into pollen grains by preventing them from sticking to each other. After complete development of the pollen the callose wall is disintegrated.

• Robert Brown discovered the nucleus of the cell. Robert Hooke discovered the cell. 

Most of the "animalcules" are now referred to as unicellular organisms, although Leeuwenhoek observed multicellular organisms in pond water. He was also the first to document microscopic observations of muscle fibers, bacteria, spermatozoa, red blood cells, crystals in gouty tophi, and blood flow in capillaries.

Frits Zernike invented phase contrast microscopy which was proved to be an advancement in microscopy. He was awarded Nobel prize in Physics in 1953.
Phase contrast microscopy is a optical microscopy technique that converts phase shifts in light passing through a transparent specimen to brightness changes in the image.

Scanning electron microscopes do provide 3D images but not naturally. However, 3D images can be obtained by different methods such as:
Photogrammetry where 2 or 3 images are there from tilted specimen.
Photometric stereo where there is use of 4 images from detector.

Fluorescence is the emission of light by a substance that has absorbed light. Here, the emitted light has a longer wavelength and, therefore, lower energy than the absorbed radiation.

The detailed structural views of protein-DNA interfaces have been obtained through X-ray crystallography and NMR spectroscopy.

Phase contrast microscopy is particularly important in biology. It reveals many cellular structures that are not visible with a simpler bright field microscope. These structures were made visible to earlier microscopists by staining, but this required additional preparation and killed the cells. The phase contrast microscope made it possible for biologists to study living cells and how they proliferate through cell division. After its invention in the early 1930s, phase contrast microscopy proved to be such an advancement in microscopy, that its inventor Frits Zernike was awarded the Nobel prize in 1953.

A fluorescence microscope is an optical microscope that uses fluorescence and phosphorescence instead of or in addition to, reflection and absorption to study properties of organic or inorganic substances. The "fluorescence microscope" refers to any microscope that uses fluorescence to generate an image, whether it is a more simple set up like an epifluorescence microscope or a more complicated design such as a confocal microscope, which uses optical sectioning to get better resolution of the fluorescent image. So, fluorescent microscope is used to localise cell structure stained with fluorochrome dyes and not for increasing resolving power, for obtaining X-ray diffraction pattern of DNA and to study cell and their components in a living state. 

The microscope is the instrument used to magnify the image of an object that we cannot see by our naked eyes. It is of two types on the basis of a number of lenses, simple microscope, and compound microscope. A light microscope is a compound microscope that uses focused light and two lenses to magnify a specimen.

The compound microscope is the instrument used to magnify the image of the object that we cannot see by naked eyes. It uses two lenses or lens systems, one is the objective lens which enlarged the image of the object and the second is an ocular lens which magnifies the image formed by the first. The total magnification is the product of the magnifications of two lens systems. Hence, the microscope which has a 10X objective lens and 10X ocular lens, it would magnify the object by 10 × 10 = 100 times. 

Antonie Philips van Leeuwenhoek was a Dutch businessman and scientist. He belonged to Holland. He discovered protozoa, a single-celled organisms. He named it as animalcules. He also had contributed in the improvement of the microscope and laid foundation for microbiology.