Biology Lecture 3 & 4 Cell Structure and Function PDF

Cell Structure and Function Biology Lecture 3 Prof. Abu Salim Mustafa PhD, FRCPath (UK) Department of Microbiology Faculty of Medicine Kuwait University Tel. 24636505 E-mail: [email protected] 1 Objectives Introduction to the cell and the structure and function of a eukaryotic cell organelles...

Cell Structure and Function Biology Lecture 3 Prof. Abu Salim Mustafa PhD, FRCPath (UK) Department of Microbiology Faculty of Medicine Kuwait University Tel. 24636505 E-mail: [email protected] 1 Objectives Introduction to the cell and the structure and function of a eukaryotic cell organelles 1. Nucleus 2. Chromosomes 3. Ribosomes and protein synthesis • Reading: Lecture notes and Biology by Sylvia S. Mader 2 Introduction to cells • Cells are the basic units of life and all organisms are made up of cells. • All of the chemistry and biomolecules are necessary but insufficient on their own to support life. • It is only when these components are brought together and organized into a cell that life is possible. 3 Cell theory • All organisms are composed of cells. • Cells are the basic units of structure and function in organisms. • Cells come only from preexisting cells because cells are self-reproducing. Types of cells existing in nature • Prokaryotic cells (Prokaryote): (Gk. pro, “before”; karyon, “kernel, nucleus”) are cells that lack a membrane-bound nucleus and organelles; and include the cell type within the domains Bacteria and Archea. • Eukaryotic cells (Eukaryotes) (Gk. eu, “true”): have a membrane-bound nucleus and membranous organelles; found in organisms within the domain Eukarya. Origin of the Eukaryotic Cell (animal and plant cells) Endosymbiotic theory: mitochondria and chloroplasts were independent prokaryotes that took up residence in a eukaryotic cell. Endosymbiosis was the first step toward the origin of the eukaryotic cell during the evolutionary history of life. Structure of a eukaryotic animal Cell Mitochondrion Cell membrane Lysosome • An animal cell is a system of interconnected organelles that work together to metabolize, regulate, and conduct life processes. • For example, the nucleus is a compartment that houses the genetic material within eukaryotic chromosomes and contains hereditary information. • The nucleus communicates with ribosomes in the cytoplasm, and the organelles of the endomembrane system—notably the endoplasmic reticulum and the Golgi apparatus— Vesicle communicate with one another. Golgi apparatus Nucleus Cytoplasm Endoplasmic reticulum 8 Nucleus: an oval structure near the center of most cells • is essential to the life and function of a eukaryotic cell • contains the genetic information (DNA) that is passed on between each generation of cells Nucleus DNA and RNA synthesis occurs in the nucleus. Three types of RNA are synthesized in the nucleus: Ribosomal RNA (rRNA) Messenger RNA (mRNA) Transfer RNA (tRNA) 9 Nucleus: Structure 10 Nuclear envelope • A double membrane, encloses the nucleus, separating its contents from the cytoplasm. • Each membrane is a lipid bilayer with associated proteins. • To communicate with the cytoplasm, the nuclear envelope is perforated by pores. 11 Nuclear pore • The pores in the nuclear envelope regulate the entry and exit of certain large macromolecules and particles, i.e. • the passage of proteins from the cytoplasm into the nucleus and • the passage of ribosomal subunits and mRNA out of the nucleus into the cytoplasm. 12 Chromatin (Chroma = color, teino = stretch) • The nucleus contains thread like structures known as chromatin in a semifluid medium called nucleoplasm. • Chromatins are a network of strands and contain DNA and proteins. 13 Chromosomes • Just before a cell divides, the chromatin condenses and undergoes coiling into rod-like structures called chromosomes (chroma = color and soma = body). • Chromosomes carry the genetic information. • Genes, composed of DNA, are units of heredity located on the chromosomes. 14 Chromosomes • The cells of a eukaryotic species have a characteristic number of chromosomes. • A typical human cell, for example, has 46 chromosomes in its nucleus. • The exceptions are the sex cells (egg and sperm), which have only 23 chromosomes in humans. • All of the cells of an individual have the same number of chromosomes, except the sex cells. 15 A karyotype is an individual's complete set of chromosomes 16 Nucleolus: a dark-staining spherical body inside the nucleus. • In the nucleolus • a special type of RNA called ribosomal RNA (rRNA) is synthesized from instructions in the DNA. • proteins imported from the cytoplasm are assembled with rRNA into large and small ribosomal subunits. • The subunits of ribosomes exit the nucleus through the nuclear pores to the cytoplasm • where a large and a small subunit can assemble into a ribosome. 17 Ribosomes: Protein Factories in the Cell Ribosomes are small particles in the cytoplasm where protein synthesis occurs. They are made in the nucleolus and function as the site of protein synthesis in the cytoplasm. Consist of rRNA and protein molecules, which make up two subunits: large and small subunits. Each subunit has its own mix of proteins and rRNA. 18 Ribosomes: Free and Bound • Free ribosomes are suspended in the cytoplasm. • Bound ribosomes are attached to the endoplasmic reticulum. • Both types of ribosomes participate in proteins synthesis. • Cells that have high rates of protein synthesis have large number of ribosomes. 19 Ribosomes: Free and Bound • Free ribosomes make proteins that function within the cytoplasm; e.g. enzymes that catalyze the first steps of sugar breakdown. • Bound ribosomes make proteins that are exported out of the cell or become part of membranes and lysosomes. 20 Protein synthesis; role of nucleus and ribosomes The nucleus directs protein synthesis by synthesizing messenger RNA (mRNA) from DNA. The mRNA is then transported to the cytoplasm via the nuclear pores. mRNA contains a coded message from DNA indicating the correct sequence of amino acids in a protein. 21 Protein synthesis; role of nucleus and ribosomes In the cytoplasm, ribosomes translate the mRNA′s genetic message into the primary structure of a specific polypeptide. 22 22 Thank you Thank you 23 Biology Lecture 4 Cell Structure and function Prof. Abu Salim Mustafa PhD, FRCPath (UK) Department of Microbiology Faculty of Medicine Kuwait University Tel. 24636505 E-mail: [email protected] 24 Objectives To explain the structure and function of the organelles of 1. The Endomembrane System Endoplasmic reticulum Golgi apparatus Vesicles Lysosomes 2. Microbodies and vacuoles Peroxisomes Vacuoles • Reading: Lecture notes and Biology by Sylvia S. Mader 25 The Endomembrane System • Consists of the nuclear envelope, the membranes of the endoplasmic reticulum, the Golgi apparatus, and several types of vesicles. • This system compartmentalizes the cell so that particular enzymatic reactions are restricted to specific regions and overall cell efficiency is increased. • The vesicles transport molecules from one part of the system to another. 26 Endoplasmic Reticulum (ER): Biosynthetic Factory • An extensive membranous network in eukaryotic cells • It is physically continuous with the nuclear envelope • The membrane of the ER encloses a continuous compartment called the ER lumen (or cisternal space). Endoplasmic reticulum cisternal space 27 Endoplasmic reticulum • There are two distinct, but connected, regions of ER that differ in structure and function: • Rough ER has ribosomes on the outer surface of the membrane and appears rough through the electron microscope. • Smooth ER is continuous with rough ER and its outer surface lacks ribosomes. 28 Functions of Rough ER • Many types of specialized cells secrete proteins produced by ribosomes attached to rough ER, e.g. • islet cells in the pancreas secrete the protein insulin, a hormone, into the bloodstream. • Most secretory proteins are glycoproteins. • The carbohydrate is attached to the protein in the lumen of ER. 29 Functions of Smooth ER • The smooth ER of various cell types functions in: • synthesis of lipids (including triglycerides, phospholipids, and steroids) • metabolism of carbohydrates • detoxification of drugs and poisons • storage of calcium ions • The functions of smooth ER are dependent on the particular cell, e.g. • in the testes, it produces the steroid known as testosterone • in the liver, it detoxifies drugs 30 Functions of ER • ER is a membrane factory for the cell. • The ER membrane expands and is transported in the form of transport vesicles. 31 The Golgi Apparatus Receiving and Shipping Center A center of manufacturing, warehousing, sorting, and shipping. Here, products of the ER are received, modified, stored and then sent to other destinations. 32 The Golgi Apparatus • Consists of stacks of slightly curved, flattened saccules, or cisternae, which are not physically connected. • The inner face (cis) is directed toward the ER, and the outer (trans) face is toward the plasma membrane. • The cis and trans face act as the receiving and shipping departments of the Golgi apparatus. • Transport vesicles move material from the ER (both rough and smooth ER) to the Golgi apparatus. • Vesicles are frequently seen at the edges of the saccules. 33 The Golgi Apparatus • Protein-filled vesicles bud from rough ER • Lipid-filled vesicles bud from the smooth ER • A vesicle that buds from the ER can add its membrane and the contents of its lumen to the cis face by fusing with a Golgi membrane. • The trans face gives rise to vesicles, which pinch off and travel to other sites. 34 The Golgi Apparatus • Products of the ER are usually modified during their transit from the cis region to the trans region of the Golgi. • Proteins and phospholipids of membranes may be altered. • For example, various Golgi enzymes modify the carbohydrates first attached to the protein in rough ER. 35 The Golgi Apparatus • The Golgi removes some sugar monomers and substitutes others, producing a large variety of carbohydrates. • In addition to finishing work, The Golgi apparatus manufactures certain macromolecules by itself, e.g. • polysaccharides secreted by cells are Golgi products. 36 The Golgi Apparatus and Vesicles • The Golgi apparatus sorts and packages proteins and lipids in membrane-bounded structures known as vesicles that depart from the outer face. • Transport (secretory) vesicles proceed to plasma membrane where they become part of the membrane as they discharge their contents during secretion. Lysosome 37 Lysosomes: Digestive Compartments • Lysosomes are membranous sacs of hydrolytic enzymes that a cell uses to digest all types of macromolecules. • Hydrolytic enzymes and lysosomal membranes are made by ER and then transferred to the Golgi apparatus for further processing. • They arise by budding from the trans face of the Golgi apparatus. • They have a very low pH and contain powerful hydrolytic enzymes to digest macromolecules. • Lysosomes carry out intracellular digestion in a variety of circumstances. 38 Lysosomes: Functions • Amoebas and many other protists eat by engulfing smaller organisms or other food particles by phagocytosis. • The food vacuole formed in this way then fuses with a lysosome, whose enzymes digest the food. • Digestion products, including simple sugars, amino acids, and other monomers, pass into the cytosol and become nutrients for the cell. 39 Lysosomes: Functions Some human cells also carry out phagocytosis. Among them are macrophages, a type of white blood cell that helps defend the body by engulfing and destroying bacteria and other invaders. Lysosomes also use their hydrolytic enzymes to recycle the cell′s own organic material, a process called autophagy. 40 40 Autophagy • A damaged organelle or small amount of cytoplasm becomes surrounded by a membrane, and a lysosome fuses with this vesicle. • The lysosomal enzymes digest the enclosed material, and the organic monomers are returned to the cytosol for reuse. • With the help of lysosomes, the cell continually renews itself. A human liver cell, for example, recycles half of its macromolecules each week. 41 Microbodies and Vacuoles • Eukaryotic cells contain a variety of membrane-bound vesicles, called microbodies, that contain specialized enzymes to perform specific metabolic functions. • One example is the peroxisome. • In addition, cells may contain large storage areas called vacuoles. 42 Peroxisomes • Membrane-bound vesicles contain enzymes that are used to break down fatty acids. • The enzymes in peroxisomes are synthesized by free ribosomes and transported into a peroxisome from the cytoplasm. • As the enzymes within the peroxisome oxidize fatty acids, they produce hydrogen peroxide (H2O2), a toxic molecule. • Peroxisomes contain an enzyme called catalase that immediately breaks down H2O2 to water and oxygen. In germinating plant seeds, they oxidize fatty acids into molecules that can be converted to sugars needed by the growing plant. In leaves, peroxisomes can carry out a reaction that is opposite to photosynthesis— the reaction uses up oxygen and releases carbon dioxide. 43 Vacuoles • Membranous sacs usually store substances. • Fat cells (adipocytes) in animals contain a very large, lipid-filled vacuole that takes up nearly two-thirds of the volume of the cell. • Plant vacuoles contain water, sugars, salts, and water-soluble pigments and toxic molecules. • The pigments are responsible for many of the red, blue, or purple colors of flowers and some leaves. • The toxic substances help protect a land plant from herbivorous animals. 44 The End Thank you 45