Microscopy and the Discovery of the Cell PDF

**MICROSCOPY AND THE DISCOVERY OF THE CELL** **Early Microscopes** - **Simple Microscopes:** These were early microscopes that used a single lens to magnify objects. They were limited in their magnification capabilities, but they allowed scientists to see things that were previously...

**MICROSCOPY AND THE DISCOVERY OF THE CELL** **Early Microscopes** - **Simple Microscopes:** These were early microscopes that used a single lens to magnify objects. They were limited in their magnification capabilities, but they allowed scientists to see things that were previously invisible to the naked eye. - **Compound Microscopes:** These microscopes used two lenses to magnify objects. The combination of two lenses allowed for much greater magnification than simple microscopes. - **Hans and [[Zacharias Janssen]](coco://sendMessage?ext=%7B%22s%24wiki_link%22%3A%22https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FZacharias_Janssen%22%7D&msg=Zacharias+Janssen):** These father and son are credited with inventing the compound microscope in 1597. They discovered that by combining two lenses in a specific way, they could achieve much greater magnification than with a single lens. **The Importance of the Microscope** - The microscope revolutionized our understanding of the world. It allowed scientists to see things that were too small to be seen with the naked eye, including cells. - The discovery of cells was a major breakthrough in biology. It led to the development of cell theory, which states that all living things are made up of cells. **The 17th Century Microscopes and the Discovery of the Cell** - **[[Robert Hooke]](coco://sendMessage?ext=%7B%22s%24wiki_link%22%3A%22https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FRobert_Hooke%22%7D&msg=Robert+Hooke):** An [[English]](coco://sendMessage?ext=%7B%22s%24wiki_link%22%3A%22https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FEnglish_language%22%7D&msg=English) scientist widely recognized as a pioneer in microscopy. One of his greatest discoveries was the cell which looked like small monastery rooms, hence the name. - **[[Antonie van Leeuwenhoek]](coco://sendMessage?ext=%7B%22s%24wiki_link%22%3A%22https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FAntonie_van_Leeuwenhoek%22%7D&msg=Antonie+van+Leeuwenhoek):** A [[Dutch]](coco://sendMessage?ext=%7B%22s%24wiki_link%22%3A%22https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FDutch_language%22%7D&msg=Dutch) maker of microscopes. He was the first person to see cells in 1665 using a microscope that he himself made. He was able to grind and polish glass, giving his microscopes better quality than the commonly used lenses in his time. His microscopes were capable of giving 270 times magnification which was exceedingly greater than those of the brass plates. - **Early compound microscopes:** magnification was 20 to 30 times. Through his microscopes made pioneering discoveries on red blood cells, capillary systems, and the development of insects. His invention also paved the way for the development of the practical microscope. - Half a century after the invention of the compound microscope, Robert Hooke realized that lenses with very short focal lengths were the key to more magnification. This led to the discovery of using convex or spherical lenses, which improved both resolution and magnification of the compound microscope. Using this microscope, many discoveries were made, especially in the field of microbiology. The two infectious bacteria and cholera bacilli by [[Robert Koch]](coco://sendMessage?ext=%7B%22s%24wiki_link%22%3A%22https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FRobert_Koch%22%7D&msg=Robert+Koch), a German physician and microbiologist, was among the monumental discoveries in microbiology. **Electron Microscopes** - **Modern electron microscopes:** have greater magnifying power and resolution than light microscopes. They can magnify objects up to 500,000 times their actual size. - **Electron microscopes: use beams of accelerated electrons as a source of illumination. Electron microscopes use electrostatic and electromagnetic lenses in forming the image by controlling the electron beam to focus it at a specific plane with reference to the specimen.** **Cell Theory** - **About 150 years passed before scientists were able to organize the observations made by Hooke and van Leeuwenhoek into a unified theory.** - **In 1838, the German botanist, [[Matthias Schleiden]](coco://sendMessage?ext=%7B%22s%24wiki_link%22%3A%22https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FMatthias_Jakob_Schleiden%22%7D&msg=Matthias+Schleiden), discovered that all plants are composed of cells, and a year later, a German zoologist, [[Theodor Schwann]](coco://sendMessage?ext=%7B%22s%24wiki_link%22%3A%22https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FTheodor_Schwann%22%7D&msg=Theodor+Schwann), discovered that all animals are also made up of cells.** - **Several years after the discovery of Schleiden and Schwann, in 1855, [[Rudolph Virchow]](coco://sendMessage?ext=%7B%22s%24wiki_link%22%3A%22https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FRudolf_Virchow%22%7D&msg=Rudolph+Virchow), a German physician, discovered that cells came from pre-existing cells after he worked on how diseases affect living organisms.** - **The observations made by these three scientists together comprised the cell theory which includes the following three tenets:** - **All living things are made up of one or more cells.** - **The cell is the basic unit of structure and function in living organisms.** - **All cells come from pre-existing cells.** **Cell Shape and Size** - **Diversity in Cell Shapes:** Cells come in a variety of shapes, from simple blobs to more complex structures. This diversity is reflected in the different tissues and organisms. - **Examples of Cell Shapes:** Red blood cells, adipose (fat) cells, muscle cells, root cells, leaf cells, the rods and cones in the retina, the cerebral Purkinje cells, and the unicellular flagellate, Euglena, are examples of cells with different shapes. - **Diversity in Cell Sizes:** Cells vary in size, ranging from 0.2 µm (micrometers) for bacteria to 2 m long for some animal nerve cells. Most plant and animal cells range from 10 to 50 µm in diameter and are visible under the microscope. - **The Ratio of Volume to Surface Area:** As a cell grows, its volume increases faster than its surface area. Volume increases by the cube, while surface area increases by the square. **Internal Organization** - **Prokaryotic Cells:** Lack a nuclear envelope and membrane-bound organelles. An example of which is the bacterial cell. Bacterial cells are small, about 0.2 to 5 µm, less complex, usually exist in unicellular forms, and have limited capabilities compared with eukaryotic cells. - **Eukaryotic Cells:** Are generally larger and have very distinct nuclei that are clearly surrounded by nuclear membranes. They also have numerous membrane-bound organelles found in the cytoplasm. [Protists], plants, fungi, and animals have eukaryotic cells. - **Eukaryotic organisms:** Like animals, are made up of highly integrated aggregations of large cells measuring approximately 10 to 50 µm in diameter. Some organisms, like protists, consist of a single eukaryotic cell. These cells are comparatively large (5 to 200 µm or more) with membrane-bound nucleus and organelles. All cells come from pre-existing cells. Because of the intricate organization of the DNA in eukaryotic cells, cell division is also comparatively different from that of prokaryotic cells. Mitosis is the most common type of cell division in eukaryotes, compared to binary fission in prokaryotes. A. **Prokaryotic Cells** **Prokaryotic cells lack a membrane-bound nucleus and membrane-bound organelles.** This is a key difference between prokaryotic and eukaryotic cells. - **The following structures are found in prokaryotic cells:** - **Capsule:** A sticky outer layer that provides protection to the cell. - **Cell wall:** A structure that confers rigidity and shape to the cell. - **Plasma membrane:** A structure that serves as a permeability barrier. - **Plasmid:** A genetic material. - **Nucleoid:** A DNA-containing region within the cytoplasm - **Cytoplasm:** the region where chromosomes (DNA), ribosomes, and various inclusions are found - **Ribosome:** the site where protein synthesized - **Pilus(plural,pill):** a hair-like appendage that functions in adhesion - **Flagellum:** facilitates movement of bacteria B. **Eukaryotic Cells** - **Eukaryotic cells have three main parts: the cell membrane, the cytoplasm, and the nucleus.** The nucleus contains the cell\'s genetic material (DNA) and is surrounded by a nuclear membrane. The cytoplasm is the gel-like substance that fills the cell and contains various organelles. - **The cell membrane, sometimes called the plasma membrane, separates the cell from its external environment.** It is the outermost covering of animal cells while that of plant cells is called cell wall. The cell membrane functions as a selectively permeable membrane that regulates the entrance and exit of substances into the cell. It provides shape and flexibility to the cell. **Cell Membrane** - **Knowledge of the structure of the cell membrane stems from century-long studies of scientists, which began when lipids and proteins were recognized as components of the cell membrane.** - **In 1925, [[Evert Gorter]](coco://sendMessage?ext=%7B%22s%24wiki_link%22%3A%22https%3A%2F%2Fnl.wikipedia.org%2Fwiki%2FEvert_Gorter%22%7D&msg=Evert+Gorter) and François Grendel hypothesized that the cell membrane is composed of a lipid bilayer.** They studied the lipids from erythrocytes (red blood cells) and found out that the plasma membrane has a layer facing inward, away from the aqueous environment that surrounds the membrane, and is hydrophobic (water-fearing); the other layer facing outward, toward the outside environment of the cell, is hydrophilic (water-loving). **The [Davson-Danielli Model]** - **In 1935, English physiologist [[Hugh Davson]](coco://sendMessage?ext=%7B%22s%24wiki_link%22%3A%22https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FHugh_Davson%22%7D&msg=Hugh+Davson) and English biologist [[James Danielli]](coco://sendMessage?ext=%7B%22s%24wiki_link%22%3A%22https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FJames_Danielli%22%7D&msg=James+Danielli) proposed a \"sandwich-like model\" of the cell membrane.** This model, which came to be known as the **Davson-Danielli model**, describes the cell membrane as a phospholipid bilayer sandwiched by a coat of protein on either surface. - **However, the Davson-Danielli model was eventually discredited as scientists came to understand better the proteins found in the membrane.** In place of the Davson-Danielli model, the model proposed by Jonathan Singer and Garth Nicolson is now the widely-accepted model. **The Singer-Nicolson Model** - The Singer-Nicolson model also known as the **fluid mosaic model** describes the plasma membrane **Cytoplasm** - **All the materials in the cell that surround the nucleus make up the cytoplasm.** It is the ground substance and the **biggest part of the cell where organelles and cellular inclusions are found.** - **The following are organelles, tiny structures that perform specific functions, and other cellular inclusions found in the cytoplasm.** **CYTOPLASMIC ORGANELLES\ Endoplasmic Reticulum (ER)** - **The endoplasmic reticulum (ER) is a network of interconnected channels and tubules in the cytoplasm.** It is enclosed by a membrane. - **There are two types of endoplasmic reticulum: the rough endoplasmic reticulum (RER) and the smooth endoplasmic reticulum (SER).** - **The RER is studded with ribosomes, which are responsible for protein synthesis.** The ribosomes attached to the RER synthesize proteins that are destined for secretion from the cell. - **The [[SER]](coco://sendMessage?ext=%7B%22s%24wiki_link%22%3A%22https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FEndoplasmic_reticulum%23Smooth_endoplasmic_reticulum%22%7D&msg=SER) is a network of interconnected tubules that lack ribosomes.** It plays a role in the manufacture and metabolism of lipids. It also plays a role in the biosynthesis of steroid hormones in the endocrine cells. **Golgi Apparatus** - **The Golgi apparatus is composed of sets of cisternae and numerous vesicles filled with fluid and suspended substances.** It is responsible for the processing, packaging, and sorting of secretory materials for use within and outside the cell. For example, after protein is synthesized by the ribosome, it passes into the channels and moves into the fluid-filled sacs of the Golgi apparatus - a membranous pouch that buds off from the smooth ER then migrates and fuses with the plasma membrane. **Mitochondria** - **The mitochondrion (plural, mitochondria) is sometimes referred to as the powerhouse of the cell.** It contains enzymes that help in the chemical oxidation of food molecules and produces energy in the form of ATP. It varies in size, shape, and number depending on the degree of cellular activity. Some of the most prominent parts of the mitochondria are the cristae (plural, cristae) and the matrix. The [**[cristae]**](coco://sendMessage?ext=%7B%22s%24wiki_link%22%3A%22https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FCrista%22%7D&msg=cristae) are numerous folds of the inner membrane. The **matrix** is the space inside the inner membrane. Studies have shown that cells that are active most of the time like the liver cells, have more mitochondria compared to those that are less active like the skin cells. A single liver cell may contain as many as 2500 mitochondria while a skin cell will have only a few hundreds of them. **Take note that mitochondria have their own DNA.** which means that new mitochondria arise only when existing ones divide. **Lysosomes** - **Lysosomes are small, spherical, membrane-bound organelles which contain a number of enzymes for intracellular digestion.** A lysosome is bounded by a single layered membrane. It contains around 40 kinds of enzymes that help digest food,disease-causing bacteria engulfed by white blood cells, and worn-out and broken parts of the cell **Lipid Droplets** - **Lipid droplets store fatty acids and sterols.** They take up much space and volume in adipocytes (fat cells). They appear as black spherical bodies of varying sizes when stained with osmium tetroxide. A. **Ribosomes** - **Ribosomes are the most numerous of all cytoplasmic structures.** They are spherical structures and measure about 15 to 20 nanometers (nm) in diameter. They are the sites where proteins are made. The distribution of ribosomes inside the cell varies, depending on how the proteins will be used. They could be found attached to the endoplasmic reticulum or free in the cytoplasm. **Proteins that are needed by the cell itself are produced by those ribosomes that float freely in the cytoplasm, while proteins that will be exported outside of the cell are produced by those attached to the ER.** B. **Centrioles** - **The centriole has a central role in cell division where it participates in the formation of the mitotic spindle.** It is usually adjacent to the nucleus. Its wall is composed of nine groups of parallel subunits arranged in a longitudinal fashion. Each of the nine groups consists of three microtubules aligned and fused together so that they appear as 3 circles in a row. C. **Microtubules** - **Microtubules are long, slender, protein tubes.** Together with the **microfilaments**, they form the **cytoskeleton** of the cell. They are composed of linear polymers of tubulin. - A network of microtubules forms the spindle apparatus that appears during cell division. These also form the cores of the cilia and flagella of sperm cells and play a role in maintaining cell shape. D. **Microfilaments** - **Microfilaments support the cell to maintain its structure and shape as it provides resiliency against forces that can alter the shape.** Spindle fibers are examples of microfilaments that aid in the movement of chromosomes during cell division. They are also important in cytoplasmic streaming or cyclosis. E. **Glycogen Granules** - **Glycogen granules, which are abundant in liver cells, play an important role in the glucose cycle.** F. **Pigments** - **Pigments are substances that do not require staining by dyes because they already possess their own color.** These are especially abundant in plant cells.