Cell Physiology Structure and Function PDF

Cell Physiology Structure and Function Lectures in Physiology College of Dentistry/1st semester/ 2nd year Dr. Entissar Mansour  Lect.1 General Physiology The goal...

Cell Physiology Structure and Function Lectures in Physiology College of Dentistry/1st semester/ 2nd year Dr. Entissar Mansour  Lect.1 General Physiology The goal of physiology is to explain the physical and chemical factors that are responsible for the origin development and progression of life. Each life from the very simple virus up to the latest tree or the complicated human beingh as its own functional characteristics. In human physiology we attempt to explain the specific characteristics and mechanisms of the human body that make it living being. The basic living unit of the body is the cell. All cells use oxygen as one of the major substances from which energy is divided; the oxygen combines with carbohydrates, fat or protein to release the energy required for cell function. Cell Physiology The basic living unit of the body is the cell. 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. Cell membrane (also called plasma membrane) The cell membrane is a protective sheath that envelops the cell body. It separates the fluid outside the cell called extracellular fluid (ECF) and the fluid inside the cell called intracellular fluid(ICF).It is a semipermeable membrane and allows free exchange of certain substances between ECF and ICF. According to the fluid mosaic model, the membrane made up primarily of proteins, 55%; phospholipids, 25%; cholesterol, 13%; other lipids, 4%; and carbohydrates, 3%.The cell membrane consists of two layers of phospholipids with cholesterol in between the two layers. The presence of cholesterol decreases the fluidity of the membrane, thus making it more stable. The hydrophilic polar negatively charged (water-soluble) heads of the phospholipids are facing the outer and the inner surfaces of the membrane while the two non-polar fatty acid tails of the phospholipids which are hydrophobic (water-insoluble) are facing the core of the membrane. Therefore, lipid-soluble substances (such as O2, CO2, nitrogen, anesthetic gases, steroid hormones, alcohol) cross cell membranes easily because they can dissolve in the hydrophobic lipid bilayer. In contrast, water-soluble substances (such as Na+, Cl-, glucose, urea, H2O) cannot dissolve in the lipid of the membrane, but cross through water-filled channels, or pores, or may be transported by carriers. 2  Lect.1 FUNCTIONS OF CELL MEMBRANE 1.Protective function: Cell membrane protects the cytoplasm and the organelles present in the cytoplasm. 2.Selective permeability: Cell membrane acts as a semipermeable membrane which allows only some substances to pass through it and acts as a barrier for other substances. 3.Absorptive function: Nutrients are absorbed into the cell through the cell membrane. 4.Excretory function: Metabolites and other waste products from the cell are excreted out through the cell membrane. 5.Exchange of gases : Oxygen enters the cell from the blood and carbondioxide leaves the cell and enters the blood through the cell membrane. 6.Maintenance of shape and size of the cell:Cell membrane is responsible for the maintenance of shape and size of the cell The Cell Membrane Proteins The membrane proteins are globular masses floating in the lipid bilayer, most of which are glycoproteins. Two types of proteins occur: integral proteins that protrude all the way through the membrane and peripheral proteins that are attached only to one surface of the membrane and do not penetrate all the way through. The integral proteins act as: 1_Structural channels (or pores) through which water molecules and water-soluble substances, especially ions, can diffuse between the extracellular and intracellular fluids. 2_Carrier proteins for transporting substances that otherwise could not penetrate the lipid bilayer (bind with molecules or ions that are to be transported; conformational changes in the protein molecules then move the substances through the interstices of the protein to the other side of the membrane). 3_ Enzymes (catalyze chemical reactions). 4_Receptors for water-soluble chemicals, such as peptide hormones, that do not easily penetrate the cell membrane. Interaction of cell membrane receptors with specific ligands that bind to the receptor causes conformational changes in the receptor protein. This, in turn, enzymatically 3  Lect.1 activates the intracellular part of the protein or induces interactions between the receptor and proteins in the cytoplasm that act as second messengers, thereby relaying the signal from the extracellular part of the receptor to the interior of the cell). Peripheral protein molecules are often attached to the integral protein, function almost entirely as : 1_Enzymes 2_Controllers of transport of substances through the cell membrane “pores. Carbohydrate of the Cell Membrane Carbohydrate molecules form a thin loose covering over the entire surface of the cell membrane called glycocalyx. Some carbohydrate molecules are attached with proteins and form glycoproteins and some are attached with lipids and form glycolipids. Functions of glycocalyx 1.The carbohydrate molecules are negatively charged and do not permit the negatively charged substances to move in and out of the cell. 2.The glycocalyx from the neighboring cells helps in the tight fixation of cells with one another. 3.Some of the carbohydrate molecules form the receptors for some hormones. 4  Lect.1 CYTOPLASM The cytoplasm is the fluid present in side the cell. It contains a clear liquid portion called cytosol which contains various substances like proteins, carbohydrates, lipids and electrolytes. Apart from these substances, many organelles are also present in cytoplasm. ORGANELLES IN CYTOPLASM The main organelles are as follows : 1- Endoplasmic reticulum 2- Golgi apparatus 3-Lysosomes 4-Peroxisomes 5-Centrosome and Centrioles 6- Mitochondria 7- Ribosomes 8- Cytoskeleton 9- Nucleus 1. ENDOPLASMIC RETICULUM The endoplasmic reticulum is of two types namely, rough endoplasmic reticulum and smooth endoplasmic reticulum. Functions of rough endoplasmic reticulum It is concerned with the protein synthesis in the cell, especially those secreted from the cell leukocytes. It also plays an important role in degradation of worn out cytoplasmic organelles like mitochondria Functions of smooth endoplasmic reticulum 1- It is responsible for synthesis of cholesterol and steroid 2- It is concerned with various metabolic processes of the cell because of the presence of many enzymes on the outer surface. 3-It is concerned with the storage and metabolism of calcium 4-It is also concerned with catabolism& detoxification of toxic substances like some drugs and carcinogens (cancer producing substances) in liver. 5  Lect.1 2- Golgi Apparatus Functions of Golgi Apparatus 1-It is concerned with the processing and delivery of substances like proteins and lipids to different parts of the cell 2-It functions like a post office because,it packs the processed materials into the secretory granules, secretory vesicles, and lysosomes 3- It also functions like a shipping department of the cell because it sorts out and labels the materials for distribution to their proper destinations 3- Lysosomes Functions of Lysosomes 1-Digestion of unwanted substances With the help of hydrolytic enzymes like proteases, lipases, amylases and nucleases, lysosome digests and removes the unwanted substances 2-Removal of excess secretory products in the cells Lysosomes in the cells of the secretory glands play an important role in the removal of excess secretory products by degrading the secretory granules. 3-Secretory function–Secretory lysosomes Recently, lysosomes having secretory function called secretory lysosomes are found in some of the cells, particularly in the cells of immune system. The conventional lysosomes are modified into secretory lysosomes by combining with secretory granules 6  Lect.1 4- Peroxisomes Peroxisomes are otherwise called as microbodies.These are pinched off from endoplasmic reticulum. Peroxisomes contain some oxidative enzymes such as catalase, urate oxidase and D-amino acidoxidase. Functions of Peroxisomes 1.Degrade the toxic substances like hydrogen peroxide and other metabolic products by means of detoxification 2.Form the major site of oxygen utilization in the cells 3.Break down the excess fatty acids 4.Accelerate gluconeogenesis from fats 5.Degrade purine to uric acid 6.Participate in the formation of myelin and bile acids. 5- Centrosome and Centrioles The centrosome is situated near the center of the cell close to the nucleus. It consists of two cylindrical structures called centrioles which are responsible for the movement of chromosomes during cell division. 6- MITOCHONDRION The mitochondrion is a rod or oval shaped structure with a diameter of 0.5 to 1 μ. It is covered by a double layered membrane. Functions of Mitochondrion 1-The mitochondrion is called the‘power house of the cell’ because it produces the energy required for the cellular functions.The energy is produced by oxidation of the food substances like proteins, carbohydrates and lipids by the oxidative enzymes. During oxidation, water and carbon dioxide are produced with release of energy.The released energy is stored in mitochondria and used later for synthesis of ATP. 2- The components of respiratory chain in the mitochondrion are 7  Lect.1 responsible for the synthesis of ATP by utilizing the energy through oxidative phosphorylation. The ATP molecules defuse throughout the cell from mitochondrion. Whenever energy is needed for cellular activity, the ATP molecules are broken down 3- Apoptosis 7- RIBOSOMES The ribosomes are small granular structures with a diameter of 15 nm. The ribosomes are made up of proteins (35%) and RNA(65%).The RNA present in ribosomes is called ribosomal RNA (rRNA). Functions of Ribosomes Ribosomes are called protein factories because of their role in the synthesis of proteins. Messenger RNA (mRNA) passes the genetic code for protein synthesis from nucleus to the ribosomes. The ribosomes, in turn arrange the amino acids into small units of proteins. The ribosomes attached with endoplasmic reticulum are involved in the synthesis of proteins like the enzymatic proteins, hormonal proteins, lysosomal proteins and the proteins of the cell membrane.The free ribosomes are responsible for the synthesis of proteins in hemoglobin, peroxisome and mitochondria. 8- Cytoskeleton The cytoskeleton of the cell is acomplex network that gives shape, support and stability to the cell. It is also essential for the cellular movements and the response of the cell to external stimuli. The cytoskeleton consists of three major protein components viz. a. Microtubules b. Intermediate filaments c. Microfilaments A- Microtubules Functions of microtubules: 1. Determine the shape of the cell 2. Give structural strength to the cell 3. Responsible for the movements of centrioles and the complex cellular structures like cilia 4. Act like conveyer belts which allow the movement of granules, vesicles, protein molecules and some organelles like mitochondria to different parts of the cell 5. Form the spindle fibers which separate the chromosomes during mitosis 8  Lect.1 B-Intermediate Filaments The intermediate filaments form a network around the nucleus and extend to the periphery of the cell.These filaments are formed by fibrous proteins and help to maintain the shape of the cell. The adjacent cells are connected by intermediate filaments by desmosomes. C-Microfilaments Microfilaments are long and fine thread like structures which are made up of non tubular contractile proteins called actin and myosin. Actin is more abundant than myosin. Functions of microfilaments Microfilaments: 1.Give structural strength to the cell 2.Provide resistance to the cell against the pulling forces 3.Responsible for cellular movements like contraction, gliding &cytokines is (partition of cytoplasm during cell division). NUCLEUS Nucleus is present in those cells which divide and produce enzymes. The cells with nucleus are called eukaryotes and those without nucleus are known as Prokaryotes (e.g. red bloodcells). Functions of nucleus 1.Controls all the activities of the cell 2. Synthesizes RNA 3. Forms subunits of ribosomes 4. Sends genetic instruction to the cytoplasm for protein synthesis through mRNA 5.Controls the cell division through genes 6. Stores the hereditary information (in genes) and transforms this information from one generation of the species to the next. 9