Cell Physiology Lec (1) PDF

Cell physiology Lec (1) Human Physiology By Dr. Wassan.M. Husain  Introduction Human physiology is the study of human organs and of the cells that make them up. An understanding of human physiology is helpful in a variety of fields, such as me...

Cell physiology Lec (1) Human Physiology By Dr. Wassan.M. Husain  Introduction Human physiology is the study of human organs and of the cells that make them up. An understanding of human physiology is helpful in a variety of fields, such as medicine, fitness, and biology  What is physiology? Physiology tests how organs and systems within the body work, how they communicate, and how they combine their efforts to make conditions favorable for survival. The study of physiology is, in the other word, the study of life. It asks questions about the internal workings of organisms and how they interact with the world around them. The major systems covered in the study of human physiology are as follows: Circulatory system, Digestive/excretory system, Endocrine system, Musculoskeletal system, Nervous system, Renal/urinary system, Reproductive system and Respiratory system  Cell physiology: The cell is the basic living unit of the body. Each organ is an aggregate of many different cells held together by intercellular supporting structures. Each type of cell is specially adapted to perform one or a few particular functions. Organization of the Cell A typical cell, as seen by the light microscope it is consist of two major parts are the nucleus and the cytoplasm. The nucleus is separated from the cytoplasm by a nuclear membrane, and the cytoplasm is separated from the surrounding fluids by plasma membrane. The different substances that make up the cell are collectively called protoplasm. Protoplasm is composed mainly of five basic substances: water, electrolytes, proteins, lipids, and carbohydrates.  Physical Structure of the Cell The cell is not only a bag of fluid, enzymes, and chemicals; it also contains highly organized physical structures, called intracellular organelles. The physical nature of each organelle is an important as the cell’s chemical constituents for cell function.  Cell Membrane The cell membrane (also called the plasma membrane), which envelops the cell, is a thin, elastic structure only 7.5 to 10 nanometers thick. It is composed almost entirely of proteins and lipids Chemical composition of Cell Membrane Is composed of lipid & protein, which are mixed together and give shape of mosaic model. The ratio of lipid to protein is variable depending of the type and function of cell. Ex: nerve cell has high lipid contains were as membrane of RBC are high protein. Lipid contain: cholesterol & phospholipid. The function of cholesterol is to give fluidity to cell membrane and prevent cell down. The major lipids are phospholipids such as phosphatidylcholine, phosphatidylserine, and phosphatidylethanolamine. Phospholipid has duple layer and two reactions. (polar) Hydrophilic reaction of the head of the phospholipid (is relatively soluble in water). ( nonpolar) Hydrophobic reaction of the tail of the phospholipid.(relatively insoluble). The possession of both hydrophilic and hydrophobic properties makes the lipid an amphipathic molecule. The head because they are hydrophilic there are directed to the location were water is present (extra cellular fluid) and the tails because they are hydrophobic there are directed to the interior (intra cellular). The membrane proteins are classified into two categories: Integral proteins or intrinsic membrane proteins Peripheral proteins or extrinsic membrane proteins. They exist as separate globular units and many pass through or are embedded in one leaflet of the membrane (eg, integral proteins), whereas peripheral proteins are associated with the inside or outside of the membrane. The amount of protein varies significantly with the function of the cell but makes up on average 50% of the mass of the membrane; that is, there is about one protein molecule per 50 of the much smaller phospholipid molecules. The membrane occasionally perforated to form membrane pore; the diameter is between 7-10 A˚. These pores used to transport materials across membrane. Membrane proteins have several functions to cell and there are many types: 1. Structural proteins or cell adhesion molecules (CAMs) that anchor cells to their neighbors or to basal laminas 2. Carrier proteins: transporting substances down electrochemical gradients by facilitated diffusion. 3. Pump proteins: actively transporting ions across the membrane 4. Ions channel proteins: which, when activated, permit the passage of ions into or out of the cell 5. Enzyme proteins: catalyzing reactions at the surfaces of the membrane 6. Receptor proteins that bind ligands or messenger molecules, initiating physiological changes inside the cell. 7. Glycoproteins: such as RBC antigens.  Transport across the Cell Membrane One of the great wonders of the cell membrane is its ability to regulate the concentration of substances inside the cell. These substances include ions such as Ca++, Na+, K+, and Cl–; nutrients including sugars, fatty acids, and amino acids; and waste products, particularly carbon dioxide (CO2), which must leave the cell.  The membrane’s lipid bilayer structure provides the first level of control. The phospholipids are tightly packed together, and the membrane has a hydrophobic interior. This structure causes the membrane to be selectively permeable. A membrane that has selective permeability allows only substances meeting certain criteria to pass through it unaided. In the case of the cell membrane, only relatively small, nonpolar materials can move through the lipid bilayer (remember, the lipid tails of the membrane are nonpolar). Some examples of these are other lipids, oxygen and carbon dioxide gases, and alcohol. However, water-soluble materials—like glucose, amino acids, and electrolytes—need some assistance to cross the membrane because they are repelled by the hydrophobic tails of the phospholipid bilayer. All substances that move through the membrane do so by one of two general methods, which are categorized based on whether or not energy is required. Passive transport is the movement of substances across the membrane without the expenditure of cellular energy. In contrast, active transport is the movement of substances across the membrane using energy from adenosine triphosphate (ATP).  Transport of materials across plasma membrane There are many methods for transport: 1. Diffusion. Passive transport 2. Osmosis. 3. Active transport. 4. Transport of very large molecules.  Diffusion. Passive transport: Movement of molecules across cell membrane from the region of higher concentration to areas of lower concentration. (Downhill movement). It doesn’t need energy of transport. This type of transport includes: a). simple diffusion: movement of molecules through the pore according to concentration gradient. The size of these molecule should be less than the size of pore, ex: Na, K, H2O, Ca. They all have less than 7 A˚. b). Facilitated Diffusion (carrier _ mediated transport): this is transport of material have size large than the size of pore. This is done by using carrier protein on the cell membrane, there are specific carriers for specific material. That transport also “downhill movement”.  Factors that influence passive transport: 1. Solubility: fat soluble materials are transport passively thought cell membrane because cell membrane contains a great amount of fat. 2. Size of molecules: must be smaller than (7_10 A˚) they can movement faster. 3. Charge of molecules: negative charge is faster than positive charge because the lining of the pore is positive charge. 4. Concentration gradient. 5. Surface area of diffusion.  Osmosis Transport of solvent through a semipermeable membrane from region of low solute concentration to higher one. The term tonicity is used to describe the osmolality of a solution relative to plasma. Solutions that have the same osmolality as plasma are said to be isotonic; those with greater osmolality are hypertonic; and those with lesser osmolality are hypotonic. In isotonic solution the concentration of water molecules is the same outside and inside the cells, and the cells maintain their normal shape and function. Osmosis occurs when there is an imbalance of solutes outside of a cell versus inside the cell. In hypertonic solution , Cells will shrivel as water leaves the cell via osmosis. In hypotonic solution , Cells will take on too much water and swell, with the risk of eventually bursting. Filtration Another mechanism besides diffusion to passively transport materials between compartments is filtration. Unlike diffusion of a substance from where it is more concentrated to less concentrated, filtration uses a hydrostatic pressure gradient that pushes the fluid—and the solutes within it—from a higher pressure area to a lower pressure area. Filtration is an extremely important process in the body. For example, the circulatory system uses filtration to move plasma and substances across the endothelial lining of capillaries and into surrounding tissues, supplying cells with the nutrients. Filtration pressure in the kidneys provides the mechanism to remove wastes from the bloodstream.  Active transport: Movement of molecules across cell membrane from the region lower concentration to areas of higher concentration (upper hill movement). This type is need expenditure energy, characterized of this transport is similar to the carrier transport, ex: sodium-potassium pump, which is also called Na+/K+ ATPase. Na+/K+ ATPase, transports sodium out of a cell while moving potassium into the cell. The Na+/K+ pump is an important ion pump found in the membranes of many types of cells. These pumps are particularly abundant in nerve cells, which are constantly pumping out sodium ions and pulling in potassium ions to maintain an electrical gradient across their cell membranes. Na+/K+ pump moves three Na+ ions out of the cell and two K+ ions into the cell for each ATP molecule that is used  Transport of very large molecules  More than 7-10 A˚ and not have carriers. Very large particles enter the cell by a specialized function of the cell membrane called endocytosis. Endocytosis (bringing “into the cell”) is the process of a cell ingesting material by enveloping it in a portion of its cell membrane, and then pinching off that portion of membrane Once pinched off, the portion of membrane and its contents becomes an independent, intracellular vesicle. A vesicle is a membranous sac—a spherical and hollow organelle bounded by a lipid bilayer membrane. Endocytosis often brings materials into the cell that must to be broken down or digested.  Phagocytosis (“cell eating”) is the endocytosis of large particles. Many immune cells engage in phagocytosis of invading pathogens, such as invading bacterial cells, phagocytize them, and digest them. Pinocytosis (“cell drinking”) brings fluid containing dissolved substances into a cell through membrane vesicles. Phagocytosis and pinocytosis take in large portions of extracellular material, and they are typically not highly selective in the substances they bring in Receptor-mediated endocytosis is endocytosis by a portion of the cell membrane that contains many receptors that are specific for a certain substance. Once the surface receptors have bound sufficient amounts of the specific substance (the receptor’s ligand), the cell will endocytose the part of the cell membrane containing the receptor-ligand complexes. Iron, a required component of hemoglobin, is endocytosed by red blood cells in this way. Iron is bound to a protein called transferrin in the blood. Specific transferrin receptors on red blood cell surfaces bind the iron-transferrin molecules, and the cell endocytoses the receptor-ligand complexes. Exocytosis Taking “out of the cell”, is the process of a cell exporting material using vesicular transport. Many cells manufacture substances that must be secreted, like a factory manufacturing a product for export. These substances are typically packaged into membrane-bound vesicles within the cell. When the vesicle membrane fuses with the cell membrane, the vesicle releases it contents into the interstitial fluid. The vesicle membrane then becomes part of the cell membrane. Cells of the stomach and pancreas produce and secrete digestive enzymes through exocytosis  Organelles 1- Mitochondria: has the ability to form the energy-rich compound ATP by oxidative phosphorylation. Mitochondria perform other functions, including a role in the regulation of apoptosis (programmed cell death), but oxidative phosphorylation is the most crucial. 2- Lysosomes The interior of lysosomes, is more acidic than the rest of the cytoplasm, and external material such as endocytosed bacteria, as well as worn-out cell components, are digested in them. The interior is kept acidic by the action of a proton pump, or H+ ATPase. This integral membrane protein uses the energy of ATP to move protons from the cytosol up their electrochemical gradient and keep the lysosome relatively acidic, near pH 5.0. Lysosomes can contain over 40 types of hydrolytic enzymes 3- Peroxisomes: Peroxisomes are similar physically to lysosomes, but they are different in two important ways. First, they are believed to be formed by self-replication (or perhaps by budding off from the smooth endoplasmic reticulum) rather than from the Golgi apparatus. Second, they contain oxidases rather than hydrolases. Several of the oxidases are capable of combining oxygen with hydrogen ions derived from different intracellular chemicals to form hydrogen peroxide (H2O2). Hydrogen peroxide is a highly oxidizing substance and is used in association with catalase, another oxidase enzyme presents in large quantities in peroxisomes, to oxidize many substances that might otherwise be poisonous to the cell. For instance, about half the alcohol a person drinks is detoxified by the peroxisomes of the liver cells in this manner. 4- Cytoskeleton: All cells have a cytoskeleton, a system of fibers that not only maintains the structure of the cell but also permits it to change shape and move. The cytoskeleton is made up primarily of microtubules, intermediate filaments, and microfilaments. Along with proteins that anchor them and tie them together. In addition, proteins and organelles move along microtubules and microfilaments from one part of the cell to another, propelled by molecular motors.  Cell adhesion molecules (CAMs) Cells are attached to the basal lamina and to each other by CAMs that are prominent parts of the intercellular connections. The unique structural and signaling functions of these adhesion proteins have been found to be important in embryonic development and formation of the nervous system and other tissues, in holding tissues together in adults, in inflammation and wound healing, and in the metastasis of tumors. Many CAMs pass through the cell membrane and are anchored to the cytoskeleton inside the cell. CAMs can be divided into four broad families: 1- integrins, heterodimers that bind to various receptors 2- adhesion molecules of the IgG superfamily of immunoglobulins. 3- cadherins, Ca2+-dependent molecules that mediate cell-to-cell adhesion 4- selectins, which have lectin-like domains that bind carbohydrates.  Intercellular connections Intercellular junctions that form between the cells in tissues can be broadly split into two groups: 1- junctions that fasten the cells to one another and to surrounding tissues, 2- junctions that permit transfer of ions and other molecules from one cell to another. Tight junctions: the types of junctions that tie cells together and endow tissues with strength and stability. Gap junction forms a cytoplasmic “tunnel” for diffusion of small molecules between two neighboring cells.  NUCLEUS & RELATED STRUCTURES The nucleus is made up in large part of the chromosomes, the structures in the nucleus that carry a complete blueprint for all the heritable species and individual characteristics of the animal. Except in germ cells, the chromosomes occur in pairs, one originally from each parent. Each chromosome is made up of a giant molecule of DNA. The DNA strand is about 2 m long, but it can fit in the nucleus because at intervals it is wrapped around a core of histone proteins to form a nucleosome. 1- Endoplasmic Reticulum (E R) a) Rough Endoplasmic Reticulum (RER): Involve with protein synthesis. So that found in large number in protein secreting cells (endocrine gland that secrete protein hormone). b) Smooth Endoplasmic Reticulum (SRE): Involve in steroidogenesis and detoxification, found in (endocrine gland that secrete steroid hormone). c) Sarcoplasmic Reticulum (SR): control the contraction and relaxation of muscle. 2- Ribosome: bound with RER to synthesis protein. 3- Golgi apparatus: glycosylation of protein.