Samacheer Class 11 Bio Botany - Cell The Unit of Life

• Fischer in 1894 & Hardy ( 1899 ) Proposed the Colloidal theory of Protoplasm (the physical basis of life)
• It is a colloidal system with water, many biological import things, glucose, fatty acids, amino acids minerals, vitamins hormones & enzymes are seen.
• Homogenous -These solutes are soluble
• Heterogenous – Solutes are not soluble – This Forms the basis for its colloidal nature.
• Protoplasm occur in 2 states but interconvertible

Prokaryotes and Eukaryotes represent two fundamental types of cells, differing significantly in their structural organization and complexity. Prokaryotic cells, typically 1-5 µm in size, lack a true nucleus; their genetic material is located in a region called the nucleoid, which is not enclosed by a nuclear membrane and does not contain a nucleolus. The DNA in prokaryotes is usually circular and is not associated with histone proteins. RNA and protein synthesis are coupled and occur simultaneously in the cytoplasm. Ribosomes are of the 70S type, composed of 50S and 30S subunits. Membrane-bound organelles are absent. Cell movement, if present, is facilitated by flagella, which differ structurally from eukaryotic flagella. Prokaryotes are typically unicellular and reproduce primarily by binary fission. Examples include bacteria and archaea. In contrast, eukaryotic cells are generally larger, ranging from 10-100 µm. They possess a true nucleus, which is enclosed by a nuclear membrane and contains a nucleolus. Their DNA is usually linear and is complexed with histone proteins to form chromatin. RNA synthesis (transcription) occurs in the nucleus, while protein synthesis (translation) takes place in the cytoplasm. Eukaryotic ribosomes are 80S type, consisting of 60S and 40S subunits. Numerous membrane-bound organelles such as mitochondria, endoplasmic reticulum, Golgi apparatus, and lysosomes are present. Cell movement can involve flagella and cilia, which have a characteristic 9+2 microtubule arrangement. Eukaryotes can be single-celled, colonial, or multicellular, and they divide by mitosis and meiosis. Fungi, plants, and animals are all examples of eukaryotes.

Plant cells and animal cells are both eukaryotic cells but differ in several structural features. Plant cells are typically larger than animal cells. Plant cells possess a cell wall in addition to the plasma membrane, which consists of middle lamellae, primary walls, and secondary walls, whereas animal cells lack a cell wall and have only a plasma membrane. Plasmodesmata are present in plant cells for intercellular communication, but are absent in animal cells. Chloroplasts are found in plant cells for photosynthesis, but are absent in animal cells. Centrioles are absent in most plant cells except in motile cells of lower plants, whereas centrioles are present in animal cells and play a role in spindle fiber formation during cell division. Plant cells contain large, permanent vacuoles surrounded by a tonoplast membrane that occupy up to 90 percent of the cell volume and help maintain turgor pressure, while animal cells have small, temporary vacuoles or lack them entirely. The nucleus in plant cells is typically located along the periphery of the cell due to the large central vacuole, whereas the nucleus in animal cells is centrally located. Lysosomes, which contain digestive enzymes, are rare in plant cells but are abundant and prominent in animal cells, where they perform intracellular digestion functions.

Jonathan Singer & Garth Nicolson (1972) proposed FM model.
* Plasma membrane made up of lipid (phospholipid), protein & little carbohydrate.
I. Phospholipid: Molecule has a hydrophobic tail(repel water) & hydrophilic head (water-loving)
II. Protein of membrane
* Globular in nature intermingles between lipid bipolar most perfect beyond Jt known
as (integral proteins)
Few are superficially attached on either surface of lipid bilayer (peripheral proteins)
* They are involved in transport of molecules across the membrane
* They acts as enzymes
* They acts as receptors or antigens.
III Carbohydrate
* They are short chain of polysaccharides.
(i.e) With protein glycoprotein With lipid glycolipids, glycocalyx
Flip Flapping:
* The movement of membrane lipids from one side of the membrane to the other side by vertical movement called flip-flap movement.
A- hydrolipid tail,
B-hydrophilic head,} lipid
C-intrisic protein
D-extrinsic protein
This movement is very slow than lateral diffusion of lipid molecules.
* Phospholipids can flip flop due to smaller polar regions.
* Proteins cannot do so because of extensive polar regions.

The Endoplasmic Reticulum (ER) is the largest internal membrane system within eukaryotic cells, first named by K.R. Porter in 1948. It forms an extensive network of interconnected membrane-bound structures that extend throughout the cytoplasm, connecting the outer nuclear membrane to the plasma membrane. The ER consists of three main morphological forms: cisternae, vesicles, and tubules. Cisternae are long, broad, flat, sac-like structures arranged in stacks, forming lamellae, and the space between their membranes is filled with fluid. Vesicles are oval, membrane-bound vascular structures, while tubules are irregular-shaped, branched, and smooth-walled structures that enclose a space. The ER plays a crucial role in various cellular functions. It is associated with both the nuclear membrane and the cell surface membrane, facilitating communication and transport within the cell. The presence or absence of ribosomes on its surface distinguishes two types of ER. When ribosomes are present, it is known as Rough Endoplasmic Reticulum (RER), which is primarily involved in the synthesis and modification of proteins destined for secretion or insertion into membranes. When ribosomes are absent, it is called Smooth Endoplasmic Reticulum (SER), which is involved in lipid synthesis, detoxification of drugs and poisons, and storage of calcium ions.

* 1st observed by A. Kolliker (1880)
* Altmann(1894) – named it as Bio-plasts
* Benda (1897) – named as Mitochondria
Structure
* Ovoid, rod-shaped, pleomorphic structures
* Double membrane
* Outer membrane smooth, & permeable- contain porins
2 compartments
1. outer chamber between 2 membranes
2. Inner chamber filled with matrix
Cristae – Infoldings of inner membrane:
* It contain enzymes for ETS(Electron Transport System)
* Inner membrane has FI particles or exosomes
* Each FI particle has a base, a stem & a rounded head
* Head has ATP synthetase to do oxidative phosphorylation content.
* 73% protein
* 25-30% lipids
* 5-7% RNA, DNA & enzymes(about 60 circular DNA &70’s Ribosomes.
* All enzymes of Kreb’s cycle are found in the matrix except succinate dehydrogenase.
* Mitochondria is a semi-autonomous body
* It’s inheritance is uniparental (i.e) maternal
* It is used to track recent evolutionary time because it mutates 5-10 times faster than DNA in the nucleus.

• A vital organ of green plants.
• Double membrane-bound organelle peripheral space in between the membrane
• Inner chloroplast is filled with gelatinous stroma
• Inside the stroma interconnected sacs called Thylakoids
• Inner space of the thylakoid is the thylakoid lumen
• Thylakoids stacked together like piles of coins known as grana.
• Light is absorbed and converted into chemical energy (carbohydrates) in the granum
• Chloroplast genome encodes for approximately 30 proteins involved in photosystem I & II – cytochrome, b, f, complex and ATP synthase & also one of the subunits of RUBISCO is enclosed by it.
• RUBISCO- is the major protein component of the stroma single most abundant protein on earth
• The thylakoid contain small, rounded photosynthetic units called Quantosomes
• The chloroplast is semi-autonomous, divided by fission.

Ribosomes are dense particles observed in the electron microscope, first by George Palade in 1953. They are not membrane-bound organelles and are crucial for protein synthesis. Each ribosome is made up of two round subunits, one large and one small, which come together to form a complete functional unit. The cohesion of these subunits requires magnesium ions (Mg++). Ribosomes can be formed through denovo synthesis, auto-replication, and originate from the nucleolus. Their primary function is to serve as the sites of protein synthesis, where genetic information from mRNA is translated into polypeptide chains. Ribosomes are composed of approximately 60% ribosomal RNA (rRNA) and 40% protein. In cells actively synthesizing proteins, many ribosomes can attach to a single messenger RNA (mRNA) molecule, forming a structure called a polysome or polyribosome. This arrangement allows for the simultaneous translation of multiple copies of the same polypeptide from a single mRNA template, significantly increasing the efficiency of protein production.

Chromoplasts and leucoplasts are two types of plastids that differ fundamentally in their pigmentation and functions. Chromoplasts are colored plastids that contain various pigments and are involved in photosynthesis or pigment storage. Chloroplasts, a type of chromoplast, occur in green algae and higher plants and contain chlorophyll a and b pigments for photosynthesis. Phaeoplasts occur in brown algae and dinoflagellates and contain the pigment fucoxanthin. Rhodoplasts occur in red algae and contain the pigment phycoerythrin and store protein. Leucoplasts, in contrast, are colorless plastids that lack pigments and are primarily involved in storage of various compounds. Amyloplasts are leucoplasts that store starch and occur in storage organs such as tapioca roots and potato tubers. Elaioplasts are leucoplasts that store lipids and oils, occurring in seeds such as groundnut and sunflower. Aleuroplasts or proteinoplasts are leucoplasts that store proteins and are found in seeds such as moong dal and other legumes. The fundamental distinction is that chromoplasts are pigmented and involved in light-dependent functions, while leucoplasts are non-pigmented and serve primarily as storage organelles for carbohydrates, lipids, and proteins.

Lysosomes are membrane-bound organelles containing hydrolytic enzymes and perform several critical functions in the cell. The first major function is intracellular digestion, where lysosomes digest various macromolecules including carbohydrates, proteins, and lipids present in the cytoplasm. They break down these molecules into simpler components that can be reused by the cell or eliminated. The second function is autophagy, which occurs during adverse conditions or starvation when the cell needs energy. During autophagy, lysosomes digest and break down the cell's own organelles such as mitochondria, endoplasmic reticulum, and ribosomes, recycling their components for cellular use. The third function is autolysis or programmed cell death, where lysosomes release their enzymes into the cytoplasm, causing self-destruction of the cell. This occurs when the cell is damaged, diseased, or aged beyond its useful lifespan. Autolytic enzymes disrupt intracellular molecules and structures, leading to cell death. These functions make lysosomes essential for cellular homeostasis, nutrient recycling, and the elimination of damaged or senescent cells from tissues.

• Central hub, surrounded by nine triplet peripheral fibrils (tubulin) connected to the tubules by radial spokes (9 + 0) pattern Cilia or Flagella Spindle fibres
• Centriole is the basal body of Flagella, Lilia or, Spindle fibers.
• It is a nonmembranous organelle

Prokaryotes and eukaryotes differ significantly in the types and nature of cellular inclusions they contain. In prokaryotes, reserve materials include phosphate granules and cyanophycean granules that store nutrients. Organic inclusions in prokaryotes consist of poly-3-hydroxybutyrate granules, sulfur granules, and carboxysomes that serve metabolic functions. Gas vacuoles are present in prokaryotes to provide buoyancy. Inorganic inclusions in prokaryotes include metachromatic granules such as polyphosphate granules called volutin granules and sulfur granules. In eukaryotes, reserve materials include starch grains and glycogen granules for carbohydrate storage. Organic inclusions in eukaryotes comprise aleurone grains and fat droplets, along with various secretory products such as essential oils, resins, gums, latex, and tannins. Inorganic inclusions in eukaryotes are more diverse and include calcium carbonate crystals, calcium oxalate crystals, and silica crystals. Specific examples include cystoliths, which are calcium carbonate deposits found in hypodermal cells of Ficus bengalensis; raphides, which are needle-shaped calcium oxalate crystals found in Eichhornia; and prismatic crystals of calcium oxalate found in dry scales of Allium cepa. These differences reflect the greater complexity and specialization of eukaryotic cells compared to prokaryotic cells.

Autosomes and allosomes are two categories of chromosomes that differ in their number, function, and role in inheritance. Autosomes are non-sex chromosomes that control somatic or body characteristics of an organism. In humans, there are 44 autosomes out of the total 46 chromosomes in a diploid cell. Autosomes are present in pairs, with one chromosome inherited from each parent, and they carry genes for various traits such as height, eye color, blood type, and other physical and biochemical characteristics. Allosomes, also called sex chromosomes, are chromosomes involved in sex determination and sexual characteristics. In humans, there are only 2 allosomes out of 46 total chromosomes. Males have XY allosomes while females have XX allosomes. The Y chromosome carries the male-determining factor and is much smaller than the X chromosome. Allosomes also carry genes for sex-linked traits that show different inheritance patterns in males and females. The fundamental distinction is that autosomes control general body characteristics and are the same in both sexes, while allosomes determine biological sex and carry sex-linked genes with different expression patterns between males and females.

• Yes. It connects plasma membrane & nuclear membrane, giving support to the Cytosol so we can call it the endoskeleton of the cell.
• It also helps in the exchange of substances in and out of the cell.

• In 1882- observed by Flemming in Oocytes of animal Salamander &Giant nucleus of unicellular Algae Acetabularia
• The highly condensed chromosomes form a chromosomal axis, from which lateral loops of DNA formed as a result of intense RNA synthesis

Light Microscope
Electron Microscope
Another name = compound microscope
1st introduced by Ernest Ruska & developed by G.Binnin & H. Roher (1981)
Principle
Principle
The transmission of visible light from the source of eye through a sample
It uses a beam of accelerated electrons as source of illumination.
Resolving power – Lesser
Resolving power – Higher
Magnification – Less
Magnification-1,00,000 times than the light
Purpose – studying in schools & college
Purpose Microscope Research purpose -can be seen in scientific laboratories
Pattern of working:
The microscope transmits visible light from eye through sample where
interaction occur and magnified image is visible.
The specimen to be viewed under EM should be dehydrated and impregnated with election opaque chemicals like gold, palladium for withstanding electrons & also for contrast.
Types:1 Only one
Types: 2 types TEM, SEM

The cell wall performs several critical functions essential for plant cell survival and function. First, it provides definite shape and rigidity to the cell, maintaining the structural integrity and form of the plant tissue. Second, it acts as a barrier that prevents the entry of several large molecules and harmful substances into the cell, providing selective permeability. Third, it offers protection to the internal protoplasm against mechanical injury and physical damage from external forces. Fourth, it maintains osmotic pressure within the cell, preventing the cell from bursting when water enters through osmosis by providing a rigid framework that resists excessive turgor pressure. Fifth, the cell wall serves as a defensive device, acting as the first line of defense against pathogenic microorganisms and providing structural support that helps the plant resist environmental stresses. Additionally, the cell wall facilitates cell-to-cell communication through plasmodesmata and provides attachment points for various cellular processes.

Cell transport is the main function
* PM act as a channel of transport for molecules
* PM is selectively permeable to molecules
It transported by
* Energy-dependent processes,
* Energy independent processes Membrane proteins involved processes
* Endocytosis & Exocytosis large quantity of solids and liquids are transported into a cell or out of cells.
I. Endocytosis 2 types
a) Phagocytosis particle is engulfed by membrance which fold around it forming vesicles, enzymes digest and products are absorbed.
b) Pinocytosis Fluid droplets are engulfed by forming vesicles.
II. Exocytosis -Vesicles fuse with the plasma membrane and eject contents.
-This may be a secretion in the case of digestive enzymes hormones or mucus.

The cell wall is a rigid, protective layer found outside the plasma membrane in plant cells, fungi, algae, and most bacteria. It plays a crucial role in maintaining the structural integrity and overall health of the cell. Firstly, it offers a definite shape and provides rigidity to the cell, preventing it from collapsing. Secondly, it serves as a selective barrier, regulating the entry of various molecules into the cells and thus protecting the internal protoplasm from harmful substances. Thirdly, the cell wall provides robust protection against mechanical injury and stress, acting as a physical shield. Fourthly, it is vital in preventing the bursting of cells by maintaining turgor pressure, especially in hypotonic environments where water tends to rush into the cell. This turgor pressure is essential for plant support and growth. Lastly, the cell wall acts as a primary mechanism of defense for the cells against pathogens and environmental stresses, containing enzymes and structural components that resist microbial invasion.

• They are organs of Photosynthesis.
• Light reactions & dark reactions take place in the granum & stroma respectively.
• Chloroplast also play important role in the Photorespiration or C 2 cycle.

Jonathan Singer and Garth Nicolson (1972) proposed fluid model: It is made up of lipids and proteins together with a little amount of carbohydrate. The lipid membrane is made up of phospholipid. The phospholipid molecule has a hydrophobic tail and hydrophilic head. The hydrophobic tail repels water and water-loving polar molecule are called hydrophilic molecule. They have polar phosphate group responsible for attracting water. Water-hating non – polar molecule are called as a hydrophobic molecules. They have fatty acid which is non – polar which cannot attract water.
Hydrophilic head attracts water. The proteins of the membrane are globular proteins which are found intermingled between the lipid bilayer most of which are projecting beyond the lipid bilayer. These proteins are called as integral proteins. Few are superficially attached on either surface of the lipid bilayer which are called as peripheral proteins. The proteins are involved in the transport of molecules across the membranes and also act as enzymes, receptors or antigens.

Glyoxysomes, peroxisomes, and sphaerosomes are all single membrane-bound subcellular organelles with distinct structures and functions. Glyoxysomes are specialized organelles found primarily in germinating seeds, particularly in oilseeds like castor seeds, and contain enzymes of the glyoxylate pathway. In glyoxysomes, beta-oxidation of fatty acids occurs, breaking down stored lipids into smaller molecules that can be used for energy and biosynthesis during seed germination. Peroxisomes are found in all green plants and contain enzymes involved in the C2 cycle and photorespiration, which is the process of oxidative photosynthetic carbon loss. These organelles play an important role in protecting cells from the toxic effects of hydrogen peroxide by containing the enzyme catalase, which breaks down H2O2 into water and oxygen. Sphaerosomes are specialized organelles found in endosperm cells of oilseeds like coconut and function primarily in the storage of fats and lipids. While all three organelles are membrane-bound and involved in metabolic processes, they differ in their enzymatic composition, location, and specific metabolic roles within the plant cell.

The Golgi apparatus, also known as the Golgi complex or Golgi body, is a vital organelle in eukaryotic cells, primarily involved in modifying, sorting, and packaging proteins and lipids synthesized in the endoplasmic reticulum for secretion or delivery to other organelles. Its functions are diverse and crucial for cellular operation. Firstly, it is the primary site for the synthesis of glycoproteins and glycolipids, which are essential components of cell membranes and play roles in cell recognition and signaling. Secondly, the Golgi apparatus is responsible for transporting and storing lipids, processing them for various cellular uses. Thirdly, it plays a critical role in the formation of lysosomes, which are membrane-bound organelles containing digestive enzymes. Fourthly, it is involved in the production and packaging of various digestive enzymes. Fifthly, in plant cells, the Golgi apparatus is instrumental in the formation of the cell plate during cell division and contributes to the synthesis of cell wall components. Furthermore, it secretes carbohydrates necessary for the formation of plant cell walls and insect cuticles. Lastly, the Golgi apparatus is involved in the synthesis and packaging of zymogen granules, which are vesicles containing proenzymes or precursors of enzymes, particularly in secretory cells.

Resolution and magnification are two distinct but related concepts crucial in microscopy for observing biological specimens. Magnification refers to the ability of a microscope to enlarge the apparent size of an object. It is quantitatively expressed by the formula: Magnification = (Size of image seen with microscope) / (Size of image seen with normal eyes). Essentially, it tells us how many times larger the image appears compared to its actual size. For instance, a 100x magnification means the object appears 100 times larger. On the other hand, resolution, also known as resolving power, is the ability of a lens or optical system to distinguish between two closely spaced objects as separate entities. It is a measure of the clarity and detail of an image. A higher resolution means finer details can be observed. The formula for resolution (R) in light microscopy is given by: R = (0.61 * λ) / NA, where λ (lambda) is the wavelength of light used, and NA is the numerical aperture of the objective lens. A smaller value of R indicates better resolution. Therefore, while magnification simply makes an object appear larger, resolution determines whether two points on that enlarged object can be seen as distinct, separate points rather than a single blurred entity.

• Short cellular-numerous microtubule bound projections of plasma membrane.
• Each Cilium has membrane-bound structures, basal body,rootlets, basal plate shaft
• Shaft (axoneme) consists of nine pairs of microtubule doublets, arranged in a
• circle along the periphery with a two central tubules (9+2) arrangement of microtubules is present.
• Microtubules – made up of tubulin.
• Motor protein dynein – connects the outer microtubules pair & links them to the central pair.
• Nexin – links the peripheral doublets of microtubules.

Prokaryotes and mesokaryotes differ in several fundamental characteristics. First, regarding nuclear organization, prokaryotes lack a true nucleus and possess only a nucleoid region containing DNA, whereas mesokaryotes have a true nucleus enclosed by a nuclear membrane. Second, in terms of size, prokaryotes are typically 1-5 micrometers in diameter, while mesokaryotes are larger, ranging from 5-10 micrometers in diameter. Third, concerning DNA structure, prokaryotes have DNA that is usually circular and lacks histone proteins, whereas mesokaryotes possess linear DNA that is not associated with histone proteins. Fourth, regarding ribosomes, prokaryotes have 70S ribosomes composed of 50S and 30S subunits, while mesokaryotes have 80S ribosomes composed of 60S and 40S subunits. Additionally, prokaryotes lack membrane-bound organelles, whereas mesokaryotes possess various organelles such as mitochondria and chloroplasts. Examples of prokaryotes include bacteria and archaea, while mesokaryotes include dinoflagellates and certain protozoans.

There are three types of centromere in Eukaryotes. They are as follows:
* Point Centromere: The type of centromere in which the kinetochore is assembled as a result of protein recognition of specific DNA sequences. Kinetochores assembled on point centromere bind a single microtubule. It is also called a localized centromere. It occurs in budding yeasts.
* Regional Centromere: In regional centromere where the kinetochore is assembled on a variable array of repeated DNA sequences. Kinetochore assembled on regional centromeres bind multiple microtubules. It occurs in fission yeast cell, humans and so on.
* Holocentromere: The microtubules bind all along the mitotic chromosome. Example: Caenorhabditis Elegans (transparent nematode) and many insects.

The cell wall performs multiple essential functions in plant cells. First, it provides definite shape and rigidity to the cell, maintaining the structural form and preventing the cell from collapsing. This rigidity is crucial for maintaining the overall architecture of plant tissues and organs. Second, the cell wall acts as a barrier that prevents the entry of several large molecules and harmful substances into the cell, thereby providing selective permeability and protecting the cell from unwanted external materials. Third, it protects the internal protoplasm against mechanical injury and physical damage from external forces, acting as a protective layer that shields the delicate cellular contents. Fourth, the cell wall maintains osmotic pressure within the cell by providing a rigid framework that resists excessive turgor pressure, thereby preventing the cell from bursting when water enters through osmosis. Additionally, the cell wall plays a major role as a defensive device, providing the first line of defense against pathogenic microorganisms and environmental stresses. The cell wall also facilitates cell-to-cell communication through plasmodesmata and provides structural support for the entire plant body.

The primary wall and secondary wall of plant cells differ significantly in their formation, composition, structure, and function. The primary wall is formed first during cell growth and is thin, elastic, and extensible, allowing the cell to expand and increase in size. Its matrix is composed of hemicellulose that binds microfibrils together, pectin as a filling material, and glycoproteins that control the orientation of microfibrils. The primary wall contains relatively more water and is composed of cellulose and pectin arranged in a somewhat loose manner. In contrast, the secondary wall is formed later after the cell has stopped growing and is thick, inelastic, and rigid, providing strength and support to the cell. The secondary wall has a more compact structure with cellulose and other polysaccharides arranged in different orientations, giving it a laminated structure that provides considerable mechanical strength. The secondary wall consists of three distinct sublayers designated as S1, S2, and S3, each with different orientations of cellulose microfibrils. While the primary wall does not determine the final shape of the cell, the secondary wall plays a crucial role in determining the shape and providing structural rigidity to the mature cell. The primary wall is present in all plant cells, whereas the secondary wall is found only in certain specialized cells such as those in wood and fibers.

Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM) are two powerful electron microscopy techniques used to visualize biological samples at very high magnifications and resolutions, but they differ significantly in their principles, applications, and the type of image produced. TEM has a very high resolving power, typically in the range of 2-10 Å (angstroms), allowing for the visualization of internal cellular structures and even molecular details. SEM, in contrast, has a comparatively lower resolving power, usually between 5-20 nm (nanometers), which is still much better than light microscopy but less than TEM. TEM is most commonly used when detailed internal ultrastructure of a specimen is required, as it provides a two-dimensional, cross-sectional image of the sample. The electron beam passes through a very thin section of the specimen. SEM, however, is used to study the surface topography and three-dimensional (3D) structure of a specimen, as the electron beam scans the surface and detects secondary electrons emitted from the sample. Consequently, TEM provides a 2-dimensional image, while SEM provides a 3D image. In terms of magnification, TEM typically achieves magnifications ranging from 100,000 to 300,000 times, and sometimes even higher, while SEM usually offers magnifications up to 200,000 times. TEM requires ultra-thin sections of samples, which are often stained with heavy metals, whereas SEM typically uses samples coated with a thin layer of conductive material like gold.

There are different types of mounting based on the portion of a specimen to be observed.
* Whole-mount: The whole organism or smaller structure is mounted over a slide and observed.
* Squash: This is a preparation where the material to be observed is crushed/squashed onto a slide so as to reveal its contents. Example: Pollen grains, mitosis, and meiosis in root tips and flower buds to observe chromosomes.
* Smears: Here the specimen is in the fluid (blood and microbial cultures etc) are scraped, brushed, or aspirated from the surface of the organ. Example: Epithelial cells.
* Sections: Freehand sections from a specimen and thin sections are selected, stained, and mounted on a slide. Example: Leaf and stem of plants.

Signal transduction is a fundamental biological process by which a cell receives external information or stimuli and converts it into a specific cellular response. It involves a series of molecular events that relay the signal from the cell's exterior to its interior, ultimately leading to changes in cell behavior, gene expression, or metabolic activity. This intricate process typically begins with a signaling molecule, often called a ligand, binding to a specific receptor protein located on the cell membrane or within the cytoplasm. The cell membrane serves as a crucial site for the initial chemical interaction of signal transduction, where many receptors are embedded. Upon ligand binding, the receptor undergoes a conformational change, initiating a cascade of intracellular events involving various signaling molecules, enzymes, and second messengers. These events amplify and diversify the signal, eventually leading to the activation or inhibition of specific cellular pathways. Nitric oxide is an example of a small, diffusible signaling molecule that can readily cross cell membranes and act as a key mediator in various signal transduction pathways, influencing processes like vasodilation and neurotransmission. The entire process ensures that cells can effectively communicate with their environment and coordinate their activities within a multicellular organism.

Histochemical stains are used to identify and visualize different cellular and tissue components under the microscope. Eosin is a stain that produces a pink or red color and has affinity for cytoplasm and cellulose, making it useful for visualizing the cytoplasmic contents and cell wall structures. Methylene blue is a stain that produces a blue color and has affinity for the nucleus, allowing for clear visualization of nuclear material and chromatin. Saffranine is a stain that produces a red color and has affinity for the cell wall, particularly the lignin component, making it valuable for identifying lignified tissues such as xylem vessels and fibers. Janus green is a stain that produces a greenish-blue color and has affinity for mitochondria, enabling the visualization of these energy-producing organelles within the cell. These stains are essential tools in histochemistry for differentiating various cellular structures and components based on their chemical composition and staining properties, allowing researchers and students to better understand cellular organization and tissue structure.

Holocentric chromosomes are chromosomes that have centromere activity distributed along the entire length of the chromosome rather than being localized at a single point. In holocentric chromosomes, during mitosis, microtubules attach along the whole surface of the mitotic chromosome, rather than being concentrated at a single kinetochore region. This diffuse distribution of centromeric activity means that the chromosome can segregate properly even if it breaks into fragments, as each fragment retains centromeric activity. Holocentric chromosomes are found in certain organisms such as Caenorhabditis elegans, a transparent nematode commonly used in genetic research, and in many insects including grasshoppers, crickets, and some beetles. The presence of holocentric chromosomes represents an alternative chromosomal organization strategy compared to monocentric chromosomes found in most higher organisms, where the centromere is localized to a specific region. This unique chromosomal structure has important implications for chromosome segregation, genetic recombination, and the evolution of these organisms.

Point centromeres and regional centromeres differ in their structure, organization, and function. Point centromeres are localized centromeres where the kinetochore is assembled as a result of protein recognition of specific DNA sequences. In point centromeres, the kinetochore is assembled on a small, defined region of the chromosome and binds a single microtubule during cell division. This type of centromere is found in budding yeasts such as Saccharomyces cerevisiae, where the centromeric DNA sequences are relatively short and well-defined. In contrast, regional centromeres are large centromeres where the kinetochore is assembled on a variable array of repeated DNA sequences that span a larger chromosomal region. The kinetochores assembled on regional centromeres bind multiple microtubules, providing stronger attachment and more stable chromosome segregation during mitosis. Regional centromeres are found in fission yeast cells such as Schizosaccharomyces pombe and in human cells, where the centromeric regions contain repetitive DNA sequences and span several hundred thousand base pairs. The structural differences between point and regional centromeres reflect different evolutionary strategies for ensuring accurate chromosome segregation during cell division.