Medical Biology Lecture Notes PDF

College of Medicine Lecturer: Zahraa Ch. Hameed Medical Biology/ Lecture: 4, 5 Cell Structures and Function (Cytoplasm and Organelles) Learning Objectives: - The students will learn...

College of Medicine Lecturer: Zahraa Ch. Hameed Medical Biology/ Lecture: 4, 5 Cell Structures and Function (Cytoplasm and Organelles) Learning Objectives: - The students will learn the composition of the cell. - Define the cell cytoplasm. - Explain the types of cell organelles. - Explain the Cytoskeleton of the cell. The cytoplasm The cytoplasm is a jelly-like substance of the cell. It consists of up to 90% water. In a eukaryotic cell, the cytoplasm is made up of the cytosol, the vesicles, the cytoskeleton, the inclusions, and the organelles except for the nucleus. The cytoplasm of a eukaryotic cell is that part of the cell between the cell membrane and the nuclear envelope. In fact, the cytoplasm and the nucleus make up the protoplasm. The main function:  To hold together the organelles which make up the cytoplasm.  It also nourishes the cell by supplying it with salts and sugars.  It provides a medium for metabolic reactions to occur. Cellular Organelles Mitochondrion The mitochondrion: mitochondrion, singular) is a double membrane- bound organelle found in all eukaryotic organisms. They are typically round to oval in shape and range in size from 0.5 -10μm. Mitochondria have been described as "the powerhouse of the cell" because they generate most of the cell's supply of adenosine triphosphate (ATP), used as a source of chemical energy. The number of mitochondria per cell varies widely; for example, in humans, erythrocytes (red blood cells) do not contain any mitochondria, whereas liver cells and muscle cells may contain hundreds or even thousands of Mitochondria. 1 College of Medicine Lecturer: Zahraa Ch. Hameed Medical Biology/ Lecture: 4, 5 Structure of Mitochondria  Outer membrane: is a relatively simple phospholipid bilayer, containing protein structures called porins which render is permeable.  Inner membrane: is more complex in structure than the outer membrane as it contains the complexes of the electron transport chain and the ATP synthase complex. The inner membrane has unfolding's called the cristae that increase the surface area for the complexes and proteins that aid in the production of ATP.  Inter-membrane space: It has an important role in the primary function of mitochondria, which is oxidative phosphorylation happen.  The matrix: is a complex mixture of enzymes that are important for the synthesis of ATP molecules. It also contains mitochondrial DNA and ribosome. Mitochondrial DNA Mitochondria are independent organelles they have their own DNA and ribosomes. They have circular DNA similar to bacteria and replicate by fission. Mutations in the mitochondrial DNA leads to a number of diseases such as: Progressive muscular disorders and some cases of Alzheimer’s disease. 2 College of Medicine Lecturer: Zahraa Ch. Hameed Medical Biology/ Lecture: 4, 5 Function of Mitochondria Structure of mitochondria Function Outer mitochondrial Transfer of nutrients to mitochondrion. membrane (e.g lipids) Inner membrane Stores membrane proteins that allow for energy production. Inter- membrane space Stores large proteins allowing for cellular respiration. Matrix Contains enzymes that allow for the production of ATP (energy). The endoplasmic reticulum The endoplasmic reticulum (ER) is a series of interconnected membranous sacs and tubules that collectively modifies proteins and synthesizes lipids. The double membranes of smooth and rough ER form sacs called cisternae. Protein molecules are synthesized and collected in the cisternal space (lumen). The RER: performs many essential cellular functions, including protein synthesis, calcium (Ca+) storage and release, and signaling to other organelles. The SER: is more tubular than. SER is devoted almost exclusively to the synthesis of lipids and in some cases to the metabolism of them and associated products. SER is also involved in the production of steroid hormones (testosterone); it also plays a large part in detoxifying compounds (such as drugs) in the liver. 3 College of Medicine Lecturer: Zahraa Ch. Hameed Medical Biology/ Lecture: 4, 5 Golgi body: The Golgi apparatus is named for Camillo Golgi, who discovered its presence in cells in 1898. It is a cellular organelle that packages and sorts proteins and other molecules before they are sent to their final destination. The Golgi apparatus consists of a stack of three to twenty slightly curved saccades (cisternae) whose appearance can be compared to a stack of pancakes. Function: 1. Golgi apparatus modifies proteins that it receives from the RER. 2. Transport lipids to vital parts of the cell and creates lysosomes. 3. Production of glucosaminoglycans which go on to form parts of connective tissues. 4. In all, the Golgi apparatus is involved in processing, packaging, and secretion. Ribosomes The ribosome is a complex molecular machine, found within all living cells, that serves as the site of biological protein synthesis (translation). Ribosomes link amino acids together in the order specified by messenger RNA (mRNA) molecules. Ribosomes consist of two major components: the small ribosomal subunit, which reads the mRNA, and the large subunit, which joins amino acids to form a polypeptide chain. Each subunit is composed of one or more ribosomal RNA (rRNA) molecules and a variety of ribosomal proteins. 4 College of Medicine Lecturer: Zahraa Ch. Hameed Medical Biology/ Lecture: 4, 5 Vacuole Vacuole is a membrane-bound organelle. The main function of vacuoles is:  Remove and store waste produced during autophagy (when part of the cell is broken down due to age or damage).  Store water and Store nutrients such as lipids, proteins, and carbohydrates  Remove and store harmful foreign products. Lysosome Lysosomes are membranous sacs filled with enzymes produced by the Golgi apparatus. Lysosomes digest many complex molecules such as carbohydrates, lipids, proteins, and nucleic acids, within the cell then recycles for other uses. Lysosomes are found in all cells of the body but are particularly numerous in white blood cells that engulf disease-causing microbes. In a process called auto digestion, parts of a cell may be broken down by the lysosomes. Some human diseases are caused by the lack of a particular lysosome enzyme. Tay-Sachs disease, occurs when an undigested substance collects in nerve cells, leading to developmental problems and death in early childhood. 5 College of Medicine Lecturer: Zahraa Ch. Hameed Medical Biology/ Lecture: 4, 5 Peroxisome Peroxisomes are small, membrane-enclosed cellular organelles containing oxidative enzymes that are involved in a variety of metabolic reactions, including several aspects of energy metabolism. Peroxisome protect the cell from cytotoxic product They are most abundantly found in detoxifying organs such as the liver and kidney cells. Cytoskeleton The cytoskeleton is a network of fibers throughout the cell's cytoplasm. The structure, function and dynamic behavior of the cytoskeleton can be very different, depending on the organism and cell type. Even within one cell the cytoskeleton can change through association with other proteins. It is typically divided into three categories based on the diameter and composition of the filaments: 1- Microfilaments (Actin filaments). 2- Intermediate filaments(Keratin filaments) 3- Microtubules. 6 College of Medicine Lecturer: Zahraa Ch. Hameed Medical Biology/ Lecture: 4, 5 Microfilaments: Microfilaments are solid rods made of a protein known as actin, long, extremely thin fibers that usually occur in bundles or other groupings. Actin filaments are involved in movement. Microvilli, which project from certain cells and can shorten and extend, contain actin filaments. Intermediate Filaments (Keratin filaments): Intermediate filaments, as their name implies, are intermediate in size between microtubules and actin filaments. Their structures and functions differ according to the type of cell. Microtubules: Microtubules are hollow cylindrical tubes, composed of polymers of a protein called tubulin. In nine triplet sets (star-shaped), they form the centriole, and in nine doublets oriented about two additional microtubules (wheel-shaped) they form cilia and flagella. The latter formation is commonly referred to as “9+2” arrangement, where in each doublet is connected to another by protein dynein. Microtubules help maintain the shape of the cell and act as tracks along which organelles move. During cell division, microtubules form spindle fibers, which assist in the movement of chromosomes. The centriole All cells capable of division contain a special organelle called a centriole. 7 College of Medicine Lecturer: Zahraa Ch. Hameed Medical Biology/ Lecture: 4, 5 The centriole is a cylindrical tube-like structure that is composed of 9 microtubules arranged in a very particular pattern. Two centrioles arranged perpendicular to each other are referred to as a centrosome. The centrosome plays a very important role in cell division. The centrioles are responsible for organizing the microtubules that position the chromosomes in the correct location during cell division. Cilia & Flagella Eukaryotic cilia and flagella are hair‐like cellular appendages composed of specialized microtubules and covered by a specialized extension of the cellular membrane. In eukaryotic cells, cilia and flagella contain the motor protein (dynein) and (microtubules), the core of each of the structure is termed the (axoneme) and contains two central microtubules that are surrounded by an outer ring of nine doublet microtubules. Dynein molecules are located around the axoneme. A plasma membrane surrounds the entire axoneme complex, which is attached to the cell at a structure termed the basal body. 8 College of Medicine Lecturer: Zahraa Ch. Hameed Medical Biology/ Lecture: 4, 5 Table: The differences between cilia and flagella : Cilia Flagella The number of cilia is The number of flagella is comparatively more (typically comparatively less (usually ranges ranges in the thousands) from 1 to 8) Cilia is usually shorter in length Flagella is comparatively longer in length Found in Eukaryotic cells Found in prokaryotic cells and eukaryotic cells Cilia are of two types: Non-motile Flagella are of three types: cilia and Motile cilia Bacterial flagella, Archaeal flagella and Eukaryotic flagella References: Madder, S., S., and Windelelspecht, M., Human biology, 15th ed. Mc Graw – hill, USA, 2018. Sylvia S. Mader, Biology, 6th ed. Mc Graw-Hill Education, USA,1999. 9