Cell Physiology PDF

Cell Physiology Ass. Prof. Dr. Betilay Topkara Arslan, Ph.D School of Medicine Department of Physiology Learning Outcomes 1. Describe the structure and the functions of the cell membrane 2. List the functions of the membrane proteins 3. Name the cellular organelles and state...

Cell Physiology Ass. Prof. Dr. Betilay Topkara Arslan, Ph.D School of Medicine Department of Physiology Learning Outcomes 1. Describe the structure and the functions of the cell membrane 2. List the functions of the membrane proteins 3. Name the cellular organelles and state their functions in the cell 4. Describe the relationship between organelles of the cell 5. Describe the cellular cytoskeleton and their contribution to cell structure and function 6. Give examples of distinct types of cells with their characteristic structure and functions. 7. Describe the structure and function of the nucleus and explain the formation of ribosomes  Cells Cells are the structural and functional units of all living organisms. To build a human, about 100 trillion cells connect and interact, forming dynamic tissues, organs, and organ systems. Each diagram shows an image magnified by a factor of ten in an imaginary progression from a thumb, through skin cells, to a ribosome, to a cluster of atoms forming part of one of the many protein molecules in our body. Details of molecular structure, as shown in the last two panels, are beyond the power of the electron microscope Molecular Biology of the Cell. 4th edition. Major parts of the cell: The nucleus, Cytoplasm The nucleus is separated from the cytoplasm by a nuclear membrane. The cytoplasm is separated from the surrounding fluids by a cell membrane, also called the plasma membrane. The different substances that make up the cell are collectively called protoplasm. Protoplasm is composed mainly of five basic substances: water, electrolytes, Illustration of cell structures visible with a light microscope. proteins, lipids, carbohydrates. Water Most cells, are comprised mainly of water in a concentration of 70% to 85%. Many cellular chemicals are dissolved in the water. Ions Important ions in the cell: potassium, magnesium, phosphate, sulfate, bicarbonate, and smaller quantities of sodium, chloride, and calcium. The ions provide inorganic chemicals for cellular reactions and are necessary for the operation of some cellular control mechanisms. Proteins They constitute 10% to 20% of the cell mass. These proteins can be divided into two types, structural proteins and functional proteins Structural Proteins : Long filaments These filaments form microtubules; Which provide the cytoskeletons of cellular organelles such as cilia, nerve axons, the mitotic spindles of cells undergoing mitosis, Thin filamentous tubules that hold the parts of the cytoplasm and nucleoplasm together in their respective compartments. Fibrillar proteins are found outside the cell, especially in the collagen and elastin fibers of connective tissue, and elsewhere, such as in blood vessel walls, tendons, and ligaments. Functional Proteins : Composed of combinations of a few molecules in tubular-globular form They are mostly enzymes Mobile Adherent to membranous structures and they catalyze specific intracellular chemical reactions. Lipids: Soluble in fat solvents Important lipids are phospholipids and cholesterol (constitute only about 2% of the total cell mass). Phospholipids and cholesterol are mainly insoluble in water and therefore are used to form the cell membrane and intracellular membrane barriers that separate the different cell compartments. Some cells contain large quantities of triglycerides (neutral fats). In fat cells (adipocytes), triglycerides often account for as much as 95% of the cell mass. The fat stored in these cells represents the body’s main storehouse of energy-giving nutrients that can later be used to provide energy wherever it is needed in the body. Carbohydrates: Play a major role in cell nutrition They have structural functions 1% of their total mass (may increase to as much as 3% in muscle cells and, occasionally, to 6% in liver cells) In dissolved glucose form they are always present in the surrounding extracellular fluid. A small amount of carbohydrate is stored in cells as glycogen, which used rapidly to supply the cell’s energy needs Cell Structure  The cell contains highly organized physical structures called intracellular organelles, The physical nature of each organelle is as important as the cell’s chemical constituents for cell function Membranous Organelles: cell membrane, nuclear membrane, endoplasmic reticulum, mitochondria, lysosomes, Golgi apparatus  Cell Membrane (Plasma Membrane) The cell membrane envelops the cell and is a thin, pliable, elastic structure only 7.5 to 10 nanometers thick. It is composed almost entirely of proteins and lipids. 55% proteins, 25% phospholipids, 13% cholesterol, 4% other lipids, 3% carbohydrates. Guyton and Hall Textbook of Medical Physiology,14th ed. Cell Membrane Functions Physical Isolation A physical barrier between ECF and ICF. Regulation of exchange with the environment. Controls the entry of ions and nutrients. Communication between the Contains proteins to respond to the cell and its environment. changes in the external environment. Structural support. Cytoskeleton Cytoplasm and Its Organelles  The cytoplasm is filled with dispersed particles and organelles. Cytosol : Jelly-like fluid contains mainly dissolved proteins, electrolytes, and glucose. Dispersed in the cytoplasm are: neutral fat globules glycogen granules ribosomes secretory vesicles five important organelles the endoplasmic reticulum, the Golgi apparatus mitochondria lysosomes peroxisomes  The Endoplasmic Reticulum The endoplasmic reticulum (ER) is a network of interconnected membrane tubes (cisternae) that are a continuation of the outer nuclear membrane. Helps process molecules made by the cell and transports them to their specific destinations inside or outside the cell https://www.amoebasisters.com/gifs.html Rough endoplasmic reticulum has a granular appearance due to rows of ribosomes dotting its cytoplasmic surface. The rough ER is the main site of protein synthesis Smooth endoplasmic reticulum lacks ribosomes and appears as smooth membrane tubes. The smooth ER synthesizes lipids and, in some cells, concentrates and stores calcium ions.  Golgi Apparatus The Golgi Apparatus functions in association with the ER -has membranes -is prominent in secretory cells substances entrapped in the ER vesicles are transported from the ER to the Golgi apparatus https://www.amoebasisters.com/gifs.html The substances are then processed in the Golgi apparatus to form lysosomes, secretory vesicles, other cytoplasmic components Golgi participates in protein modification and packages these proteins into the vesicles. Lysosomes Lysosomes act as the digestive system of the cell. They use powerful enzymes (hydrolase) to break down: bacteria Damaged cell structures, Food particles that have been ingested by the cell https://www.amoebasisters.com/gifs.html  Hydrolytic enzymes break down organic compounds by combining hydrogen from a water molecule with one part of the compound and combining the hydroxyl portion of the water molecule with the other part of the compound For example, protein is hydrolyzed to form amino acids glycogen is hydrolyzed to form glucose, lipids are hydrolyzed to form fatty acids and glycerol Peroxisomes Their main function appears to be to degrade long-chain fatty acids and potentially toxic foreign molecules. Peroxisomes get their name from the fact that the reactions that take place inside them generate hydrogen peroxide (H2O2), a toxic molecule. The peroxisomes rapidly convert this peroxide to oxygen and water using the enzyme catalase. Peroxisomal disorders disrupt the normal processing of lipids and can severely disrupt neural function by altering the structure of nerve cell membranes. Secretory Vesicles One of the important functions of many cells is secretion of special chemical substances Secretory substances are formed by the ER–Golgi apparatus system are then released from the Golgi into the cytoplasm in the form of storage vesicles called secretory vesicles or secretory granules. Mitochondria They are called the “powerhouses” of the cell, ATP The basic structure of the mitochondrion is composed of two lipid bilayer–protein membranes: an outer membrane an inner membrane Many infoldings of the inner membrane form shelves or https://www.amoebasisters.com/gifs.html tubules called cristae oxidative enzymes are attached to cristae Mitochondria are self-replicative: Mitochondria has its own DNA (circular) Replicates independently of the nucleus through the process of binary fission If the cell needs more energy, it will replicate its mitochondria Mitochondria Functions: ATP Synthesis: It is the power house of the cell. It brings oxidation of food. Kreb's cycle reactions, electron transport system enzymes are located in mitochondria. By the oxidation of food, energy is liberated in the form of ATP (Oxidative phosphorilation takes place) Cell signaling Heat production: A fraction of the energy released by the oxidation of respiratory substrates is used to generate ATP, while a substantial proportion is released as heat. Controls cell cycle and cell growth Apoptosis (programmed cell death or cell suicide)  Cytoskeleton The cell cytoskeleton is a network of fibrillar proteins organized into filaments or tubules. These originate as precursor proteins synthesized by ribosomes in the cytoplasm. The precursor molecules then polymerize to form filaments. The cell cytoskeleton is composed of protein fibers called microfilaments, intermediate filaments, and microtubules. Large numbers of actin microfilaments frequently occur in the outer zone of the cytoplasm, called the ectoplasm, to form an elastic support for the cell membrane. Also, in muscle cells, actin, and myosin filaments are organized into a special contractile machine that is the basis for muscle contraction. Muscle, actin and myosin https://medicine.nus.edu.sg/phys/lab/TsaiSY_Lab/LSM4232-L1.html  Intermediate filaments are generally strong ropelike filaments that often work together with microtubules, providing strength and support for the fragile tubulin structures. Their functions are mainly mechanical, and they are less dynamic than actin microfilaments or microtubules. Koutakis et al. (2015). 10.1369/0022155415569348. Specific intermediate filaments found in various cells include desmin filaments in muscle cells, neurofilaments in neurons, and keratins in epithelial cells. Tjensvoll et al (2021). 268. 10.1007/s00415-020-10290-y. Microtubules: A special type of stiff filament composed of polymerized tubulin molecules is used in all cells to construct strong tubular structures, the microtubules. A major function of microtubules is to act as a cytoskeleton, providing rigid physical structures for certain parts of cells. The cell cytoskeleton not only determines cell shape but also participates in cell division, allows cells to move, and provides a tracklike system that directs the movement of organelles in the cells. Microtubules serve as the conveyor belts for the intracellular transport of vesicles, granules, and organelles such as mitochondria The Nucleus The nucleus is the control center of the cell sends messages to the cell to grow and mature, to replicate, or to die The nucleus contains large quantities of DNA, which comprise the genes The genes determine the characteristics of the cell's proteins, including the structural proteins, the intracellular enzymes that control cytoplasmic and nuclear activities The genes also control the reproduction of the cell  The genes also control and promote cell reproduction. The genes first reproduce to create two identical sets of genes; then the cell splits by a special process called mitosis to form two daughter cells, each of which receives one of the two sets of DNA genes. During mitosis, the chromatin material organizes in the form of highly structured chromosomes, which can then be easily identified using the light microscope The light microscopic appearance of the interphase nucleus (during the period between mitoses), revealing darkly staining chromatin material throughout the nucleoplasm  The nuclear membrane, also called the nuclear envelope, separates the nucleus from the cytoplasm This structure is composed of two bilayer membranes; the outer membrane is continuous with the ER, and the space between the two nuclear membranes is also continuous with the space inside the ER Both layers of the membrane are penetrated by several thousand nuclear pores  Nucleoli The nuclei in most cells contain one or more structures called nucleoli do not have a surrounding membrane The nucleoli contain large amounts of RNA and proteins of the type found in ribosomes A nucleolus becomes enlarged when the cell is actively synthesizing proteins Ribosomal RNA is stored in the nucleolus and transported through the nuclear membrane pores to the cytoplasm where it is used to produce mature ribosomes, which play an important role in the formation of proteins Ribosomes Ribosomes are the protein factories of a cell. On ribosomes, protein molecules are synthesized from amino acids, using genetic information carried by RNA messenger molecules from DNA in the nucleus. Ribosomes attached to the cytosolic surface of organelles are called fixed ribosomes. Those suspended free in the cytosol are free ribosomes.  Functional Properties of Cell Endocytosis—Ingestion by the Cell Substances can pass through the cell membrane in 3 different ways: By diffusion through membrane pores or membrane matrix With an active transport system consisting of enzyme systems and special carrier proteins, Large particles enter the cell by a specialized function of the cell membrane called endocytosis With endocytosis, which envelops the extracellular fluid with the cell by a specialized function of the cell membrane called endocytosis  Endocytosis Pinocytosis ingestion of minute particles that form vesicles of extracellular fluid and particulate constituents inside the cell cytoplasm. Phagocytosis the ingestion of large particles, such as bacteria, whole cells, or portions of degenerating tissue  Pinocytosis Pinocytosis occurs continually in the cell membranes is especially rapid in some cells (in macrophages) the pinocytotic vesicles are so small only 100 to 200 nm in diameter most of them can be seen only with an electron microscope Pinocytosis is the only way that very large macromolecules such as protein molecules enter the cell In receptor-mediated endocytosis, Mechanism of pinocytosis. molecules bind to receptors The receptors are concentrated in small pits on the outer surface of the cell membrane, called coated pits  On the inside of the cell membrane beneath these pits contains contractile filaments fibrillar protein called clathrin as well as other proteins, including contractile filaments of actin and myosin Once the protein molecules have bound with the receptors → the entire pit invaginates inward → fibrillar proteins surrounding the invaginating pit cause its borders to close Immediately after, it breaks off the cell surface and forms the pinocytic vesicle The energy required is provided from ATP This process also requires the presence of Ca+2 since it interacts with contractile microfilaments under the membrane in the formation of pinocytosis vesicles This provides the necessary force for the vesicle to move away from the membrane  Phagocytosis -Phagocytosis occurs in much the same way as pinocytosis Phagocytosis is a mechanism involving large particles Only certain cells have the capability of phagocytosis tissue macrophages some white blood cells Phagocytosis occurs in the following steps 1. Cell membrane receptors bind to the particle 2. The edges of the membrane around the attachment point protrude forward and swallows the object 3. Contractile fibrils surround the phagocytic vesicle → contract and push the vesicle inward 4. The phagocytosis vesicle leaves the membrane like a pinocytosis vesicle into the cell Lysosomes Digest Pinocytotic And Phagocytic Foreign Substances Inside The Cell Digest pinocytotic and phagocytic substances inside the cell After a pinocytotic/phagocytic vesicle appears inside a cell, lysosomes attach to the vesicle and empty their acid hydrolases to the inside of the vesicle Thus, a digestive vesicle is formed in which proteins, carbohydrates, lipids, and other substances are hydrolyzed. Digestive product amino acid, glucose, phosphate, etc. molecules diffuse into the cytoplasm The sediment remaining in the digestive vesicle, called the residual body, consists of indigestible substances Residual products are excreted by exocytosis Often this develops in the opposite direction from the cell membrane to endocytosis. Lysosomes and Regression of Tissues Tissues of the body often regress to a smaller size The uterus after pregnancy, mammary glands after lactation, non-functioning muscles gradually shrink. Lysosomes remove cells damaged by heat, cold, trauma and chemicals Damage to the cell also induces lysosomes to rupture The released hydrolases immediately begin to digest the surrounding organic substances Cell repair is provided if the damage is mild, but if the damage is severe, the entire cell is digested, a process called autolysis In this way, the cell is completely removed, a new cell is formed by mitotic reproduction of an adjacent cell to take the place of the old one The lysosomes also contain bactericidal agents that can kill phagocytized bacteria before they cause cellular damage. These agents include the following: lysozyme, which dissolves the bacterial cell wall; lysoferrin, which binds iron and other substances before they can promote bacterial growth; acid at a pH of about 5.0, which activates the hydrolases and inactivates bacterial metabolic systems. Autophagy and Recycling of Cell Organelles Autophagy “to eat oneself”. Autophagy is a housekeeping process whereby obsolete organelles and large protein aggregates are degraded and recycled. Worn-out organelles are transferred to lysosomes by double-membrane structures called autophagosomes. Once inside the lysosomes, the organelles are digested, and the nutrients are reused by the cell. Autophagy contributes to the routine turnover of cytoplasmic components; it is a key mechanism for tissue development, cell survival when nutrients are scarce, and maintenance of homeostasis. In liver cells, for example, the average mitochondrion normally has a life span of only about 10 days before it is destroyed Textbooks Hall, J. E. (2021). Guyton and hall textbook of medical physiology (14th ed.). W B Saunders.. Boron, W.F. and Boupaep, E.L. (2016) Medical Physiology. 3rd Edition, Elsevier Publisher, Philadelphia. Barrett K.E., & Barman S.M., & Brooks H.L., & Yuan J.J.(Eds.), (2019). Ganong's Review of Medical Physiology, 26e. McGraw Hill. Widmaier, Eric P. (2019). Vander, Sherman, & Luciano's human physiology : the mechanisms of body function. Boston :McGraw-Hill Higher Education.