Chapter 2: THE CELLULAR LEVEL OF ORGANIZATION

Chapter 2: THE CELLULAR LEVELOF ORGANIZATION#CellCellis the basic living structural and functional unit of the body.

Cytology - It is a branch of science concerned with a study of cells

Cell Theory explains about:All living organisms are composed of cell and cell products.Cell is the basic unit of structure & function of all living organisms.All cells come from the division of preexisting cell.An organism as a whole can be understood through the collective activities & interactions of its cells.

2 Major Classes of Cell:Prokaryotic Cell are the simplest and smallest cellsfirst to arise in biological evolution, some 3 billion years agogenerally solitary with the nuclear material unenclosed in a membrane

Eukaryotic CellMuch larger, more complex with a wider range of diversity and differentiationArose perhaps a billion years after the prokaryotesMulticellular and provided with nuclear membrane

ComparisonProkaryotesEukaryotesOrganismBacteriaProtist, Fungi, Plants, AnimalsOrganellesFew or NoneNucleus, Mitochondria, ER, etc.DNACircular in CytoplasmLinear/ Circular bounded by a membraneRNA and ProteinRNA and Protein synthesize in same compartmentRNA synthesize in nucleus, protein synthesized in cytoplasmCell DivisionBinary FissionMitosis/ meiosisPhysiologic Properties of the CellIrritability/ ExcitabilityConductivityContractilityAbsorption and secretionExcretionRespirationGrowth and ReproductionOrganization

PARTS OF A CELLCytoplasm , Nucleus, Cell MembraneThe plasma membrane/ cell membraneForms the cells flexible outer surface, separating the cells internal environment (inside the cell) from the external environment (outside the cell). It is a selective barrier that regulates the flow of materials into and out of a cell. This selectivity helps establish and maintain the appropriate environment for normal cellular activities. The plasma membrane also plays a key role in communication among cells and between cells and their external environment.The cytoplasmconsists of all the cellular contents between the plasma membrane and the nucleus. This compartment has two components: cytosol and organelles. Cytosol, the fluid portion of cytoplasm, contains water, dissolved solutes, and suspended particles. Surrounded by cytosol are several different types of organelles. Each type of organelle has a characteristic shape and specific functions. Examples include the cytoskeleton, ribosomes, endoplasmic reticulum, Golgi complex, lysosomes, peroxisomes, and mitochondria.The nucleusis a large organelle that houses most of a cells DNA. Within the nucleus, each chromosome, a single molecule of DNA associated with several proteins, contains thousands of hereditary units called genes that control most aspects of cellular structure and function.CytosolThe cytosol (intracellular fluid) is the fluid portion of the cytoplasm that surrounds organelles and constitutes about 55% of total cell volume. Although it varies in composition and consistency from one part of a cell to another, cytosol is 7590% water plus various dissolved and suspended components. Among these are different types of ions, glucose, amino acids, fatty acids, proteins, lipids, ATP, and waste products, some of which we have already discussed. Also present in some cells are various organic molecules that aggregate into masses for storage. These aggregations may appear and disappear at different times in the life of a cellThe CytoskeletonThe cytoskeleton is a network of protein filaments that extends throughout the cytosol. Three types of filamentous proteins contribute to the cytoskeletons structure, as well as the structure of other organelles. In the order of their increasing diameter, these structures are microfilaments, intermediate filaments, and microtubules.

MICROFILAMENTSMICROFILAMENTS These are the thinnest elements of the cytoskeleton. They are composed of the protein actin, and are most prevalent at the edge of a cell. Microfilaments have two general functions: They help generate movement and provide mechanical support. With respect to movement, microfilaments are involved in muscle contraction, cell division, and cell locomotion, such as occurs during the migration of embryonic cells during development, the invasion of tissues by white blood cells to fight infection, or the migration of skin cells during wound healing. Microfilaments provide much of the mechanical support that is responsible for the basic strength and shapes of cells. They anchor the cytoskeleton to integral proteins in the plasma membrane. Microfilaments also provide mechanical support for cell extensions called microvilli, non-motile, microscopic fingerlike projections of the plasma membrane. Within each microvillus is a core Of parallel microfilaments that supports it. Because they greatly increase the surface area of the cell, microvilli are abundant on cells involved in absorption, such as the epithelial cells that line the small intestine.

INTERMEDIATE FILAMENTSINTERMEDIATE FILAMENTS As their name suggests, these filaments are thicker than microfilaments but thinner than microtubules. Several different proteins can compose intermediate filaments, which are exceptionally strong. They are found in parts of cells subject to mechanical stress, help stabilize the position of organelles such as the nucleus, and help attach cells to one another

MICROTUBULESMICROTUBULES These are the largest of the cytoskeletal components and are long, unbranched hollow tubes composed mainly of the protein tubulin. The assembly of microtubules begins in an organelle called the centrosome (discussed shortly). The microtubules grow outward from the centrosome toward the periphery of the cell.