Cell Biology (Master Stage) - First Semester 2023-2024 PDF

Lectures of cell Biology ( master stage ) : First semester 2023 – 2024 Asst. Prof. Dr. Hayder M. Abbas 2. Ultra-Structure Structure And Organisation Of Cell Organelles All living Organisms are made of cells, and cells have their appropriate structures which facilitate...

Lectures of cell Biology ( master stage ) : First semester 2023 – 2024 Asst. Prof. Dr. Hayder M. Abbas 2. Ultra-Structure Structure And Organisation Of Cell Organelles All living Organisms are made of cells, and cells have their appropriate structures which facilitate them to play their function within living cells. Cells structured in organelles, most of orga organelles of cells are seen by means of electron microscope leads to the term ultra structure. Numbers of organelles are found in cell structure and each one play its role independently and this refers to as division of labor. Figure 4: Structure of animal aand plant cell 2.1 Descriptions of organelles and their function within cells. 2.1.1 The Nucleus This is the largest organelle in a cell. Within cell nucleus, you find a dense structure known as Nucleolus covered and protected by nuclear membrane of envelop envelope. e. This envelope has two membranes and their separation is done by means of a fluid in which nuclear pores that facilitate molecules to pass through are found. Nucleus stores genetic materials. Nucleolus is responsible for Ribonucleic acid production as we well ll as involving in Ribosome protection which later takes journey through nuclear pore to cytoplasm and participate in protein synthesis process.  Lectures of cell Biology ( master stage ) : First semester 2023 – 2024 Asst. Prof. Dr. Hayder M. Abbas Figure 5: Nucleus structure 2.1.2. The Endoplasmic Reticulum (ER) This is an organelle located near and around nucleus and contained cisternae which are sacks that are flat in shape, and are with nuclear envelope. Endoplasmic Reticulum is categorized in two types and those are Rough Endoplasmic reticulum and smooth endoplasmic reticulum. Rough Endoplasmic Reticulum m has many around its outer surfaces but smooth endoplasmic reticulum has no ribosomes. Rough endoplasmic reticulum is responsible in transporting proteins synthesized in ribosomes and smooth endoplasmic reticulum serves in lipids synthesis. Figure 6: Str Structure of Endoplasmic reticulum 2.1.2. Golgi apparatus The Golgi apparatus (GA) are also known as Golgi body. It resides in both plant and animal cells, it is composed by series of five to eight that are cup in shape, cisternae which seems as a stack balloons. oons. In some of flagella protozoan 60 cisternae put together and make Golgi apparatus. The amount of Golgi apparatus varies depending on their functions. Each cell of animals notified to contain 10 and 20. Golgi apparatus are responsible in modifying prot proteins eins brought by ER.  Lectures of cell Biology ( master stage ) : First semester 2023 – 2024 Asst. Prof. Dr. Hayder M. Abbas Figure 7: Structure of Golgi body 2.1.2.Lysosomes Lysosomes are tiny sacs containing fluid in which enzymes are found. These enzymes are responsible in nutrients processing of the cell. Lysosomes are important sites of digestion; they break down heavy molecules in simple molecules that cannot harm the cell. A defining characteristic of lysosomes is that each one is bounded by only a single membrane. Alysosome size is a diameter of approx. 50nm to 1 μm§ , lysosomes possess a single outer membrane containing of a phospholipid bilayer and contain acid hydrolases which are enzymes capable of breaking breaking-down macromolecules. Figure 8:Structure of lysosome  Lectures of cell Biology ( master stage ) : First semester 2023 – 2024 Asst. Prof. Dr. Hayder M. Abbas Action of Lysosomes Figure 9:Structure of action of lysosomes Lysosomes are considered to be a digestion machines, they only work when the cell enters or absorbs or consumes a certain food. When a material located in the cell, lysosomes directly attach and produce their digestive enzymes. These enzymes are responsible in breaking down complex and heavy molecules that can give complex sugars and proteins. But what happens to lysosomes during the absence of food or starvation? The lysosomes continue their activity despite the absence of food in the cell. Here lysosomes can digest cell organelles to produce cell nutrients. 2.1.2.Mitochondria Mitochondria are rod in shape and are known to be the power generator of the cell as it assist in converting oxygen and nutrient into ATP which is a chemical energy responsible in cell metabolic activities. thee number mitochondria needed by a cell is based on metabolic activities required, and may be one or many depending on this condition. Mitochondria are oblong shaped organelles, and their size is varied in the interval of 1 and 10 micrometer in length and number umber of them depends on metabolic activities that cells wish to accomplish. Different researches done on this organelle shows that it rapidly change the shape and has a constant movement in the cell.  Lectures of cell Biology ( master stage ) : First semester 2023 – 2024 Asst. Prof. Dr. Hayder M. Abbas Figure 10: Mitochondrion structure 2.1.2. Chloroplasts Chloroplasts are useful organelles among plastids as they highly participate in the process of photosynthesis which is a process by which plants synthesize their own food. They are located in outer surface of the cell to receive enough right. Chloroplasts are green colored due to pigment called chlorophyll found in its internal parts. Some of important characteristics of plant is its ability to do photosynthesis as the way they use in making their own food and pass through converting light energy in chemicalal energy. This pearl process take place in all plant kinds in the organelle called chloroplast. All green plants are responsible to have chloroplasts within their structure and in most of plants, chloroplasts are found in the leaves. Figure 11: Chloroplast structure  Lectures of cell Biology ( master stage ) : First semester 2023 – 2024 Asst. Prof. Dr. Hayder M. Abbas 2.1.2. Vacuoles Vacuoles are storage bubbles that reside in cells. They are located in both plant animal cells despite their difference in size. Vacuoles in plant cell are larger than that of animal cells. Vacuoles play important roles in storing ring food and other nutrients essentially for a cell to be healthy. It also sometimes store wastes before sending them out to protect the cell. Vacuole has a very simple structure, only a mass of fluids surrounded by membrane are parts of vacuoles. This fl fluid might contain nutrient or wastes, plant can also profit occasions of storing water by means vacuoles found in plant cells. As discussed early, plant cells have larger vacuoles comparing to that of animal cells. In their growth, plant cell may have one vacuole which is very large in plant cells, and probably occupy the half of the cell volume. Vacuoles are holders of much water in the cell, but also can store plant waste products, and break those wastes in small particles that cannot harm the cell. Vacuo Vacuoles assure for structure of plants as plant uses cell wall in terms providing support and surround. Cell volume may change according to the presence or absence of water within vacuole. Shrinking of plant cells is not a result of cytoplasm amount but depend dependss on amount of materials found in the vacuole Gaining or losing water for the vacuole depends on water amount within plant. Figure 12: Animal and plant vacuole 2.1.2. Ribosomes Cells always need proteins production. Enzymes that facilitate in speeding bi biological process with cells are made of proteins. Other proteins that play important roles in cell function are found in membranes. When a cell enters its way of making proteins it directly search for ribosomes as these organelles known to be proteins synt synthesizers hesizers or builders for the cell. The specialty of Ribosomes is their presence in both prokaryotic and eukaryotic cells. Some organelles like nucleus are found only in eukaryotic cells. In eukaryotic cells Ribosomes are found in different places and are sseen een floating in cytosol. The important role of these proteins floating within the cell is the production of proteins to be used inside the cell. There are other ribosomes located on endoplasmic reticulum, and are responsible in activities inside the cell and nd proteins made for export out of the cell. We need to notify when these ribosomes participate in proteins synthesis. When living cells enter protein making, messenger RNA have to be synthesized in the nucleus.  Lectures of cell Biology ( master stage ) : First semester 2023 – 2024 Asst. Prof. Dr. Hayder M. Abbas This messenger RNA gets out from nucleus to cytoplasm to meet ribosomes, where two subunits of ribosomes combine with messenger RNA and begin the process of synthesizing proteins. Simply protein synthesis needs amino acids. Transfer RNA is also located in the cell and simple gets bonding with amino acids floating around the cell. Due to instruction from messenger RNA, ribosomes connect to transfer RNA and break down the bonding structure between transfer RNA and amino acids, so they pull off amino acids. Transfer RNA also liberated to go back and connects with amino acids. Ribosomes construct a chain of amino acid or polypeptide that will be broken in simple proteins. Protein synthesis process is found below Figure 14: Amino acid chain in protein synthesis 2.1.2. Cell Wall Cell wall is found only on plant cells. This is a non living part of the cell, and is known to be extra cytoplasmic product. Cell wall is more sized than plasma membranes. Its responsibility is to give a shape of the plant and manage plant cell growth. It protects the cell against the entry of unnecessary molecules and invading germs. Cell walls have different layers. It has three basic layers, intracellular layer or middle lamella, primary and secondary layer. The middle lamella plays a role of cementing ttogether the primary  Lectures of cell Biology ( master stage ) : First semester 2023 – 2024 Asst. Prof. Dr. Hayder M. Abbas walls of two contiguous cells and the secondary wall is laid over the primary. The middle lamella is mainly made of a pectic compound appearing to be calcium pectate. The primary wall is largely made of cellulose and the secondary wall may be of cellulose or cellulose whose inside found other substances. Figure 15: Cell wall structure 2.1.9.1 Primary cell wall Cellulose is known to be the main chemical components of the primary cell wall, and made of organized microfibrils. Microfibril Microfibrilss is made of carbohydrate which is a cellulose component linked together. Cellulose has the bulk of material that cell walls are made in. 2.1.9.2 Secondary cell wall The secondary cell wall is deposited inside the primary cell, and show cell maturity. Thi This secondary part of cell wall can sometimes have the same components like that of primary cell wall. The specialty is that this part contains lignin. Lignin is aromatic alcohol group that build secondary cell wall. This important part helps in xylem format formation, ion, and gives strength and rigidity of the cell. In mature tissues this party is found. 2.1.9.3 Middle lamella This is important part of a cell wall which is rich in pectins. It assures that two neighboring cell cemented together. The position of middle llamella amella facilitates neighbor cells to share their contents by means of special conduits. plasmodesmata, are small passages that penetrate and enter middle lamella and both primary and secondary cell wall, and support the exchange of transporting cytoplasmic contents from one cell to another. 2.1.10 Plasma Membrane Plasma membrane is found in both prokaryotic and eukaryotic cells. It is cover that binds cell contents and known to semi-porous porous barrier to the external environment. It plays a role of boundary and holds the cell components together, without neglecting keeping other molecules from entering. Here it accepts for a substance to enter or not. Most of substance allowed by plasma membrane to enter include: oxygen, carbon dioxide, and water but also adding essential nutrients of the cell. However, waste materials are permitted to get out of the cell. Based on accepted principle known as  Lectures of cell Biology ( master stage ) : First semester 2023 – 2024 Asst. Prof. Dr. Hayder M. Abbas fluid mosaic model, plasma membrane is made of two layers (bilayer) of lipids, oils and all found in all cells. 2.1.11 Centrioles The centrioles are cell organelles that are cylinders in shape. They can be found in most of cells having the real nucleus. Centrioles are composed of grouped microtubules and are responsible in organizing and fixing microtubules during cell divisi division on in animal cells. During replication, centrioles replicate in interphase of mitosis and meiosis. Figure 16: Structure of centrioles and spindle fibers Centrioles in Plant and animal Cell You better know that Plant cells do not possess centrioles. Its po polele structure is different from that of animal cells. This difference in cell pole structure is due to absence of cellular organelles assisting in being focal point. Due to this issue, some spindles have no localization. In animal cells we found centrosomes containing two that are barrel in shape and are are called centrioles. The centrioles assist in organizing the mitotic spindle and in the completion stage of cytokinesis. The centrioles are essential in the formation of the mitotic spindle. These centriol centrioles es are useful part of the centrosomes, they contribute in coordination of organizing the microtubules in the cytoplasm. They are other organelles of the cell that are not found here but these are main ones that everyone has to describe, and in further rea readings, you can read more about them.  Lectures of cell Biology ( master stage ) : First semester 2023 – 2024 Asst. Prof. Dr. Hayder M. Abbas Questions for evaluation 1. What is the difference between animal and plant cells? 2. Describe the process of protein synthesis by ribosomes 3. What is the difference between cell membrane and cell wall? 4. Centrioles facilitate cell division by fixing spindles fibers, and are not found in plant cell, explain how plant cells divide without centrioles. 5. Life is based on cells. What do you think on this statement? References 1. Bailey, R. (2015). What Is the Structure and Function of the Nucleus?. [online] 2. Baker, R. (2015). Eukaryotic Animal Cell Structure: A Visual Guide. [online] HubPages. Available at: http://hubpages.com/hub/What-Are-Cells-Made-Of [Accessed 14 Jan. 2015]. 3. Buzzle.com. (2015). [online] Available at: http://www.buzzle.com/images/diagrams/heart- wall.jpg [Accessed 27 Jan. 2015]. 4. Ispolatov, I., Ackermann, M. and Doebeli, M. (2011). Division of labour and the evolution of multicellularity. Proceedings of the Royal Society B: Biological Sciences, 279(1734), pp.1768- 1776.  Lectures of cell Biology ( master stage ) : First semester 2023 – 2024 Asst. Prof. Dr. Hayder M. Abbas 0. INTRODUCTION 0.1 Cell biology history The discovery of the microscope influenced the discovery of cells. The microscopist and physicist from England Robert Hook (1635-1702) took the first description of cells in 1665. His scientific experiment conducted by making thin slices of cork and matched the boxy partitions he totally observed to the cells in a monastery. Hook observed open empty spaces but he and other scientists made their suggestions by saying that these spaces can be used to transport fluid in a living plant. They did not confirm that it is a basic unit of living organisms that they were observing. Marcello Malpighi (1628–1694), and Hooke's colleague, Nehemiah Grew (1641–1712), continued making strong researches on plant cells, and put out the cellular structure in a plant body. Grew matched cellular empty spaces to the gas bubbles in rising bread and made his suggestions saying that they have the same process in their formation. Animals’ cells were discovered later because it was essential for thin sections to facilitate viewing under the microscope but were difficulty to prepare. Nowadays, scientists interested in biology were totally convinced that living things are made of fundamental units. It created curious to know what those units are. Microscope took its improvement to make their observation clearly and assisted to know more on cells and microscope chosen to be an important instrument to study life on the planet. The Dutch microscopist Antony van Leeuwenhoek (1632–1723) published his researches and observations in 1676 about single-cell organisms, or "little animalcules" the name given by him to these single celled animals. He has been respected first scientist observed red blood cells and even sperm cells in a microscope. Leeuwenhoek discovered and said many on his microorganisms, however, hundred years passed without guessing connection between cellular livings and cells that build plants and animals. Researches continued developing and reach in 1824 where Frenchman Henri Milne-Edwards put out his suggestions on animal tissues, according him animal tissues are structured like an array of globules (the basic structure of all animal tissues was an array of "globules). Henri Dutrochet (1776–1847) identified the relationship between plant and animal cells explicit, and mentioned his proposition saying that a cell was both just a structural and physiological unit, and clearly defined that everything comes from cells. Dutrochet in his proposal, he proposed that new cells come from old cells, and François Raspail (1794–1878) echoed this idea proposed by Dutrochet and said to be his contemporary, Raspail known as the first person who supported in mentioning one of the two main tenets of cell theory: Omnis cellula e cellula, which means "Every cell is derived from another cell." However, despite this ringing and famous phrase, the proposed mechanism on generation of cells has not been true. He contributed on chemical composition of cells and become the father and founder of cell biochemistry. In 1832 Barthelemy Dumortier (1797–1878) French scientist entered his description on described on binary fission in plants and was the idea to cell division in common sense. He took his careful  Lectures of cell Biology ( master stage ) : First semester 2023 – 2024 Asst. Prof. Dr. Hayder M. Abbas observation to the formation of a mid-line partition structure of both original and new cell, and Dumortier noted and took in considerations, it was as if he was going to provide clear understanding on the development of cells, “seems to us to provide a perfectly clear explanation of the origin and development of cells, which was still in obscurity explanations. His observation directed to rejection of the idea said that new cell comes from within old ones. Hugo von Mohl (1805–1872), is the one who discovered cell division despite Dumortier who preceded him. Von Mohl mentioned the word protoplasm as a material contained in the cell. Cell nucleus is also an important part of the cell and was discussed firstly by a Czech, Franz Bauer, in 1802 and was named in 1831 by Robert Brown (1773–1858) from Scotland, and also entered other parts of nucleus description. Schleiden and Schwann, took researches on cell theory and outlined their marks and contribution in 1838 and 1839. In 1838 Matthais Schleiden (1804–1881) clarified his proposition saying that each plant types or elements is made of cells. In 1839 a fellow German, Theodor Schwann (1810–1882), came up with propositions on animals’ structure. His proposition was that all structural elements in animals are cell set products, which means that, are made by cells. Contribution of Schwann seems as imitating what cell theory on plant has suggested. He declared that the laws governing cells were the same or identical in both animals and plants. The Czech Jan Purkyňe (1787–1869), or Purkinje, has also contributed on cell theory and was single cytologist in his day and known as one of the most important formulators of cell theory. He used Schwann theory to explain his contribution. His proposition was that animals were made of cells and cell products and this is applied to plants. Other scientists also contributed to cell theory but these are main ones. 0.2 Cell definition and overview The cell is the smallest basic unit of all living organisms. They independently do their activities, they replicate to and divide. They are also known to be building blocks of life. The science dealing with cell study is known as cell biology or cytology. A human being is known to have more than 10 trillion of cells mathematically it is 1013cells and seen by means a microscope, means that you cannot see them by a naked eye. All living organisms are composed of cells. Cells have various forms and shapes, utilities and visibility. Cells have abilities of metabolic process and this give them ability of living independently and play a huge role in living things. Scientists and various researchers strove to understand how cell itself plays interesting functions in all livings things and how its absence leads to inexistence of living organisms. We better know that there are animal cells and plant cells, and these cells has high percentage of similarities, however, some few differences has already mentioned within their structures. Cells are made of identical types of molecular building block and share some common characteristics. Even if cells have various common features, we take in consideration different and various cell types and this classification and categorization of cells is known as cellular diversity. This diversity of cells differs in kinds of organisms and within metazoan or multicellular livings themselves. Commonly known characteristics shared by cells are like using the same carbon in macromolecules which is the main component within cells. It includes carbohydrates, proteins,  Lectures of cell Biology ( master stage ) : First semester 2023 – 2024 Asst. Prof. Dr. Hayder M. Abbas lipids and two nucleic acids found within nucleus of living cells. All cells have DNA in which genetic characters are located and known as a genetic material of living organisms. All living organisms use their genetic material (DNA) to make proteins where it decodes before making proteins and these proteins help in energy of a cell helping also in metabolic activities of cells. Cells have ability to grow and divide despite some of cells found in multicellular organisms that lost their ability to divide and example can be given like on neurons that cannot divide. Both animal and human cells possess different parts or organelles, and each connects with cell components by means of intracellular membrane. DNA separates from cytoplasm by nuclear membrane shaping an important large organelle called nucleus. Organelle like mitochondria which is important organelles that play a huge role in cell activities like generating Adenosine triphosphate (ATP) which a useful component in providing essential energy that facilitate various biochemical reactions that lead to formation molecules from smaller units and an example that can be taken is formation of protein through amino acids. In multicellular organisms, cells are pointed to do and perform specific different functions. Those functions are like secretion and movements. Various molecules contribute in these functions talked above. Muscular cells of animals and humans themselves assured for synthesis of proteins that facilitate their contraction, but in non contractile cells these proteins are not synthesized. An example here is skin cells. Cells are different biochemically in multicellular livings, and also notification of shape difference is important as cells have different forms. Our look can be addressed to red blood cells that are small and disc in shape while neuron or nerve cells are long in shape, and all these forms and shapes are known as cell morphology. In organisms with many cells ( multicellular) cells tend to be classified in different groups or tissues basing on their responsibilities and functions. You better know that cells and tissues organize to make organs and organs to organ systems that participate in performing different functions. Example is digestive and cardiovascular system. Living cells always work their activity; they always need energy and this allows them to make nutrients that will continue to facilitate cell activities in synthesis of new molecules. Remember that they make molecules and transport them in different parts of the cell and all need energy with this they also expulse waste. if the process is done in appropriate manner, cells get growing and enter division. Cell activities can allow a cell to take new shapes in terms of responding to environment and also in interacting with other cells in the process called cellular communication or cell signaling. This big process needs essentials movement of molecules to maintain and organize and coordinate in cell 1. CELL DIVERSITY AND CLASSIFICATION 1.1 Cell Diversity Cells are found in different organisms, and each organisms has its special cells depending on its specie. However, cells are very diverse in size, shape and their internal structure and this applied to cells found in the same organisms. This diversity of cells is influenced by their roles and function within organism’s body.  Lectures of cell Biology ( master stage ) : First semester 2023 – 2024 Asst. sst. Prof. Dr. Hayder M. Abbas 1.1.2 Cell Shape Cells have different shapes due to appropriate function. Comparison can be found below where you find cells with long extensions like nerve cells that facilitate in sending and receiving impulses. You can find other cells which are flat or platlike, most of these cells are body cells and their function is protecting and covering body surface. Thus cells develop in size according recommended function within the body of a living. Cells have different shapes. Nerve cell cellss have long extensions. Skin cells have a shape which is flat and platelike. Egg cells have shape which is like sphere, and some bacteria are rod in shape. Some plant cells are rectangular. Figure 1: Shape of cells 1.1.3 Cell Size In all livings Cells aree different both in shape and size. Some cell can be seen without using magnification instruments as they enough to be seen in size. One example that we say is a neuron cell of giraffe which is 2 meters in length. 1.2 Different types of animal cells There are number of different kinds of animal cells and like skin, muscle, and blood. 1.2.1 Skin cells The skin cells of animals are categorized in two and those are keratinocytes and melanocytes – and the suffix ‘cyte’ means a cell. Keratinocytes have a big num number ber in all skin cells and have rate of 90% of all skin cells and is responsible in production of a protein known as ‘keratin’. Keratin is responsible in making effective layers of the skin in term of body protection. It can also participate in hair and nails formation.  Lectures of cell Biology ( master stage ) : First semester 2023 – 2024 Asst. sst. Prof. Dr. Hayder M. Abbas Figure 2: Skin cells Another important skin cell is Melanocyte which is responsible in melanin production and melanin is responsible in skin color determination. Melanocytes are located under keratinocytes in the part of lower layer of skin cells and after producing melanin, melanin gets transported up to the surface layers of the cells. The number of melanpocytes in the skin, determine how darker you are. Darker skinned means that you have thousands of melanocytes. 1.2.2 Muscle cells Myocytes, tes, muscle fibers or muscle cells are long tubular cells and have the responsibility of facilitating movement of an organism. Muscle cells are like cardiac muscles, smooth muscle cells, and skeletal muscle cells. In these cells, skeletal muscle cells are known to be the most common type of muscle cells with responsibilities of facilitating movements that are conscious in the body. Coming to cardiac muscle cells, they manage movement of contractions of the heart, and lastly, smooth muscle cells assist in ma managing naging subconscious movements of tissues including uterus, stomach, and the blood vessels 1.2.3 Blood cells Figure 3: Blood cells In the blood we find types of cells are classified in two categories: those are white bl blood cells and red blood cells. The estimations show that red blood cells occupy 99.9% of all blood cell found in the blood. Red blood cells help in facilitating distribution of oxygen in all parts of the body. It is also known that red blood cells have no nucleus and this make them different with other animal cells. White blood cells are known to be immunity of livings. They can kill invaders of our body and others that are harmful to the body.  Lectures of cell Biology ( master stage ) : First semester 2023 – 2024 Asst. Prof. Dr. Hayder M. Abbas 1.2.4 Nerve cells Nerve cells are also known as neurons, they are known as basic and main cells in the nervous system. Only human brain stores 100 billion nerve cells. They carry impulses of animal cells and responsible in delivering and receiving signals by means of their dendrites and axons. 1.2.5 Fat cells Fat cells, also called adipocytes or lipocytes, and are responsible in storing fats and lipids which will facilitate energy store in animal’s body. Fat cells are categorized in white fat cells and brown fat cells. The different is made from their ways of storing lipids. White fat cells store one large lipid drop while brown fat cells store smaller and multiple droplets of lipids spreading in the whole body of the cell. Questions for evaluation 0. What do you understand by the term cell diversity? 1. Explain the difference between keratinocytes and melanocytes. 2. Explain the effects of low melanin production on human skin color 3. What are main shapes of cells 4. Fat cells are categorized in two classes. What are they? Mention their differences 5. Describe blood cells and give their proper functions. What makes difference on red blood cells to other types of cells of humans? References 1. Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P. Molecular Biology of the Cell (4th ed.). Garland. ISBN 0-8153-3218-1. 2. Campbell Biology—Concepts and Connections. Pearson Education. 2009. 3. Cooper GM. The cell: a molecular approach (2nd ed.). Washington, D.C: ASM Press. ISBN 0- 87893-102-3. 4. Dennis, Michael Aaron (1989). "Graphic Understanding: Instruments and interpretation in Robert Hooke's Micrographia". Science in Context 3 (2): 309–364. 5. Jim B, Cooper M, Hunter M, Jardine L, (2003). London's Leonardo: The Life and Work of Robert Hooke. Oxford University Press. ISBN 0-19-852579-6. CYTO-SKELETON AND CELL MOTILITY The cytoskeleton is a network of fibers that forms the "infrastructure" of eukaryotic cells, prokaryotic cells, and archaeans. Cytoskeleton was previously thought to be a feature only of eukaryotic cells, but homologues to all the major proteins of the eukaryotic cytoskeleton have recently been found in prokaryotes. Although the evolutionary relationships are so distant that they are not obvious from protein sequence comparisons alone, the similarity of their three- dimensional structures and similar functions in maintaining cell shape and polarity provides strong evidence that the eukaryotic and prokaryotic cytoskeletons are truly homologous unlike some structural differences in bacteria. In eukaryotic cells, these fibers consist of a complex mesh of protein filaments and motor proteins that aid in cell movement and stabilize the cell. Cell motility is the extra cellular (cell itself) and intracellular movements of the cell which include moving along surfaces, through a tissue and the movement of inner cell components. Typical examples of cellular movement may include extracellular (cell movement) such as; movement of cells from one point to another inthe embryo during embryonic development, movement of cells in to wound during wound healing, contraction of muscle cells, separation of cells during cell division (formation daughter cells) and intracellular movements ( cell components) such as membrane-bound vesicles in to the cell during cell eating( phagocytosis or endocytosis) and chromosomal movement during cell division ( mitosis). The cytoskeleton is responsible for cell shape, motility (movement) of the cell as a whole, and motility of organelles within a cell. There are three types of filaments in the cytoplasm of most eukaryotic cells (vertebrate cells): microfilaments, microtubules, and intermediate filaments. All of these filament systems share a critical feature: They are composed of proteins that have the unique property of being able to self-assemble into a filamentous network. Imagine a pile of bricks that could assemble by themselves into a wall; the proteins that make up the fibers of the cytoskeleton are able to do just this. The proteins that make each of the three different filament systems assemble into only the structure characteristic of that filament. Unlike the human skeleton, the cytoskeleton is extremely dynamic, meaning the filament systems are able to lengthen or shorten very rapidly. This dynamic nature of the cytoskeleton is necessary for cells to be able to change shape, complete cell division, or migrates, and representsone of the cytoskeleton's most important features. Each of the self-assembling proteins has a characteristic concentration, called the "critical concentration," below which the monomer state is favored and above which the polymer state is favored. Increasingly, the subunit concentration favors filament building, and decreasing it favors filament deconstruction. This property allows the cell to rapidly control cytoskeleton structure. 1.1.Microfilaments The microfilament (actin) system is a network of filaments 6 nanometers (nm) in diameter that are important for anchoring plasma membrane proteins, for producing cell movement, and for cell division. The base filament is composed of a protein called actin that is 42 kilodaltons (kd)in weight. Actin is also the protein that forms the thin filaments found in muscle. When purified actin is incubated in a test tube, 6 nm filamentous structures are formed. These threads consist ofsideby-side actin monomers that twist around each other in a helix. Inside cells, actin exists in two states, the monomeric protein, called G-actin (for globular actin) and the 6 nm filament, called Factin (for filamentous actin). The factor that determines the relative proportions of F- actin and Gactin is the concentration of actin protein. Each microfilament has a fast-growing, or "plus," end, and a slow-growing, or "minus," end. In most cells the plus ends of the filaments are oriented toward the edge of the cell. In this way rapid polymerization of actin monomers onto theplus ends of microfilaments can produce protrusions on the cell surface called pseudopods. These extensions are critical for the ability of cells to migrate in a directional fashion. Microfilaments exist in their highest concentration in association with the cell periphery, where they are believed to play an important role in anchoring membrane proteins. Microfilaments can also be organized into bundles, called stress fibers, which serve as contractile elements, somewhat like little muscles, within cells. These structures are important for maintaining connections between the cell and the surface on which it grows. In addition, these structures maybe important for producing contractility to generate directional force during cell motility. A third microfilamentbased structure, the contractile ring, is critical for the separation of a cell into its two progeny during cytokinesis. In most cells the concentration of actin exceeds the critical concentration for microfilament assembly, yet the actin is not entirely assembled into filaments. This occurs because cells make a variety of "actin-associated" or "actin-binding" proteins. One example of an actin binding protein is the G-actin-binding protein profilin. When bound to profilin, actin monomers cannot assemble into filaments. Binding of actin by profilin can effectively reduce the concentration of free actin monomer to below the critical concentration. The actin-binding activity of profilin is regulated in cells. Certain stimuli will cause profilin molecules to release their bound actin monomers, effectively increasing the concentration of actin and thereby stimulating actin assembly. Thus cells can control the relative proportions of G-actin and F-actin. In general, the functions of actin-associated proteins are to modify the properties of the microfilament network in cells. Some filament-associated proteins, for example the protein tropomyosin, bind along the length of the filament to stiffen it. There are also proteins such as villin or filamin that bind microfilaments together side by side to produce bundles of actin filaments. Other actin-binding proteins cross-link actin filaments to form meshlike structuressuch as those found in association with the cell membrane. Cells can also control the length of filaments through the action of proteins that can cut filaments to produce two shorter filaments. To keep the filaments a certain length, cells produce "capping" proteins that bind to the ends and prevent the addition of new actin subunits. By modulating the state of the microfilament networkthe cell can control the physical properties of the cytoplasm such as rigidity and viscosity. One ofthe most interesting types of actin-associated proteins is a family of enzymes, called myosins, which have the ability to convert chemical energy into movement. The characteristic property of these socalled myosin molecular motors is their ability to bind actin in an adenosine triphosphate– sensitive fashion and to produce movement of actin filaments. Over fifteen different types of myosin motors have been identified. Some of them, such as those involved in cytokinesis and cell motility, are two headed, meaning they have two actin- binding motordomains, while others have only one head. Some of these myosins are involved in the movement of membrane-bound vesicles along actin tracks. The best characterized of these molecular motors, myosin II, slides actin filaments past each other either to power contraction of the contractile ring or to produce cell migration. A different version of this myosin motor forms the thick filaments that are responsible for the contraction of muscle.