Cell Physiology PDF

Cell Physiology By: Assist. Prof. Dr. Salam Haji, Physiology, Ph.D Cell Physiology  Cells are the microscopic fundamental units of all living things. Animal cells, including human being, are eukaryotic cells that have...

Cell Physiology By: Assist. Prof. Dr. Salam Haji, Physiology, Ph.D Cell Physiology  Cells are the microscopic fundamental units of all living things. Animal cells, including human being, are eukaryotic cells that have membrane-bound organelles and true nucleus, which contains the genetic material and is enclosed by the nuclear envelope. They are multicellular organisms with numerous different types of cells that work together to maintain life. There are a number of specialized collections of cells that consist in organ systems serving for different functions. For example, a gastrointestinal system to digest and absorb food; a respiratory system to take up O2 and eliminate CO2, etc…  To understand the function of organs and other structures of the body, it is essential that we first understand the basic organization of the cell and the functions of its parts.  Structures of the Cell The cell contains various structural components known as “organelles” that are suspended within a gelatinous matrix, the cytoplasm, which is surrounded by cell membrane. The main organelles are as follows :  Cell membrane  Nucleus  Endoplasmic reticulum  Golgi apparatus  Lysosomes  Peroxisomes  Mitochondria  Microfilaments and microtubules. Cell membrane:- The boundary of the cell, sometimes called the plasma membrane, separates internal metabolic events from the external environment and controls the movement of materials into and out of the cell. This membrane is very selective about what it allows to pass through; this characteristic is referred to as "selective permeability." For example, it allows oxygen and nutrients to enter the cell while keeping toxins and waste products out. cell membrane The plasma membrane is a double phospholipid membrane, or a lipid bilayer, with the hydrophobic tails pointing toward the inside of the membrane and the hydrophilic heads forming the inner and outer surfaces of the membrane. The basic lipid bilayer (double layered of lipids) is composed of phospholipid molecules. One end of each phospholipid molecule (phosphate end ) is soluble in water; known as hydrophilic heads forming the inner and outer surfaces of the membrane. The other end (fatty acid end) is soluble only in fats, known as hydrophobic tails pointing toward the inside of the membrane. Fat-soluble substances, such as oxygen, carbon dioxide, and alcohol, can penetrate this portion of the membrane.  Fatty acids Hydrophilic heads hydrophobic tails Cell membrane Hydrophilic heads Functions of cell membrane:- 1) The maintenance of cell shape and structure. 2) A transport function (selective permeability) of ions and macromolecules, to maintain cytosolic ionic composition, osmotic pressure and pH (around 7.2– 7.4). 3) Intercellular communication, involving signal transduction, i.e. the detection of chemical signals (messengers) from other cells. 4) Intercellular adhesion by fusion of the membrane with other cell membranes via specialised junctions. 5) Directed cell movement Structures of cell membrane Cell membranes are composed primarily of lipids and proteins. Lipids are the major components of membranes. Carbohydrates comprise 5%–10% of cell membranes. They consist of glycolipids and glycoproteins and form the glycocalyx coat on the surface of the plasma membrane (carries surface receptors). According to the fluid mosaic model, cell membranes possess the form of a continuous fluid but stable lipid bilayer. The fluidity of the membrane is determined by the degree of unsaturation of the constituent fatty acids. Cytoplasm and Its Organelles The cytoplasm is a thick, clear, jelly-like substance present inside the cell membrane. It is filled with minute and large dispersed particles, and organelles. The liquid component of the cytoplasm surrounding the organelles and other insoluble cytoplasmic structures in an intact cell is called cytosol, where a wide variety of cell processes take place.  Subcellular organelles  Nucleus The nucleus contains most of the genes that control the cell. It is generally most obvious organelles in a eukaryotic cell, averaging about 5 micrometer in diameter. The nuclear envelope, which is a double membrane, encloses the nucleus, separating its content from cytoplasm.  Ribosomes Ribosomes are the protein synthesisers of the cell. Two types of ribosomes exist within the cell: free ribosome (located in the cytosol and bound ribosome (attached to endoplasmic reticulum).  Endoplasmic Reticulum The endoplasmic reticulum (ER) consists ribosomes of network of membranous tubules and sacs that extend away from the nuclear Rough ER Smooth envelope. ER  There are two types of ER in structure and function: Structures of endoplasmic reticulum 1. Smooth ER The smooth ER does not contain ribosomes and functions in diverse metabolic process, including in lipid biosynthesis, carbohydrate metabolism, and detoxification of drugs and other poisons. 2. Rough ER: has ribosomes and is the site of protein synthesis  Golgi Apparatus The golgi apparatus is a membrane-bound organelle found in most cells. It is responsible for packaging proteins into vesicles prior to secretion and therefore plays a key role in the Diagram shows Golgi apparatus and synthesis of protein contained vesicles secretory pathway. The Golgi apparatus function involves the distribution of newly synthesised proteins and lipids from the endoplasmic reticulum to the plasma membrane, and different intracellular or extracellular targets by exocytosis.  Mitochondria Mitochondria are the organelles known as “power house of the cell” that involved in the generation of ATP to run the cellular activities. Mitochondria are the sites of cellular respiration, the elaborate catabolic process that generates ATP by extracting energy from sugars, fats, and other fuels with help of the oxygen. Mitochondria have an inner and outer membrane, with an inter-membrane space between them. The outer membrane contains proteins known as porins, which allow movement of ions into and out of the mitochondrion. The space within the inner membrane of the mitochondrion is known as the matrix, which contains the enzymes of the Krebs (TCA) and fatty acid cycles, alongside DNA, RNA, ribosomes and calcium granules. The inner membrane contains a variety of enzymes. It contains ATP synthase which generates ATP in the matrix, and transport proteins that regulate the movement of metabolites into and out of the matrix. The inner membrane is arranged into cristae in Structure of a mitochondrion order to increase the surface area available for energy production via oxidative phosphorylation.  Lysosomes Lysosomes are spherical, membrane-bound organelles that are generated by the golgi apparatus. Lysosomes contain hydrolytic enzymes (the lysosomal acid hydrolases), which require an acidic pH (around 4.8–5.0) to function as part of the recycling system of the cell. (comprising the intracellular digestive system). Their functions include: o Autophagy: the destruction of unwanted subcellular organelles. o Autolysis: the digestion of cells after death. o The release of enzymes by exocytosis in the extracellular environment to digest external material. o Intracellular digestion of proteins, carbohydrates, lipids  Peroxisome Peroxisomes are round or oval membrane-bound organelles, which contain high concentrations of oxidative enzymes, such as catalase, D-amino oxidase and urate oxidase. Peroxisomes are abundant in the liver and kidneys. Their functions include: The principal function of peroxisomes is to house many metabolic pathways that are involved in various aspects of lipid metabolism, including: Enzymes involved in the degradative oxidation (Beta oxidation of fatty acids). Formation of bile acid and cholesterol detoxification of reactive oxygen species, such as hydrogen peroxide, preventing tissue damage  Cytoskeleton: is a network of long fibers that make up the cell’s structural framework. The cytoskeleton is a complex, active network of interlinking protein filaments present in the cytoplasm of all cells. It extends from the cell nucleus to the cell membrane and composed of three main components, including: microfilaments, intermediate filaments and microtubules.  Microfilaments are polymers of the protein known as actin. Microtubules are composed of tubulin. Intermediate filaments are composed of various proteins, depending on the type of cell in which they are found. Function: making it strong 1) Maintains cell shape and strengthening it. 2) Anchors organelles and enzymes to specific regions of the cell. 3) Intracellular movements, including organelle, protein and vesicle transport within the cells. 4) Directional cell motility. 5) participating in cell division  Cilia and Flagella Structure They are Bundles of microtubules with plasma membrane. Some cells are propelled at velocities approaching 1 mm/second by the beating of cilia and flagella, flexible membrane extensions of the cell. Typically, cells possess one or two long flagella, whereas ciliated cells have many short cilia. For example, the mammalian spermatozoon has a single flagellum. In mammals, many epithelial cells are ciliated in order to sweep materials across the tissue surface. For instance, huge numbers of cilia cover the surfaces of mammalian respiratory passages to expel particulate material that collects in the mucus secretions of these tissues. Cytoplasm and Its Organelles Figure: shows a typical cell, the internal organelles in the cytoplasm and in the nucleus.