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Cell Structure Chapter 4 Cell Theory • Cells were discovered in 1665 by Robert Hooke. • Early studies of cells were conducted by • Mathias Schleiden (1838) - plant cells •Theodor Schwann (1839) - animal cells • Schleiden and Schwann proposed the Cell Theory. 2 Cell Theory • Cell Theory 1. All organisms are composed of cells. 2. Cells are the smallest living things. 3. Cells arise only from pre-existing cells. • All cells today represent a continuous line of descent from the first living cells. 3 Cell Theory • Cell size is limited. • As cell size increases, it takes longer for material to diffuse from the cell membrane to the interior of the cell. • Surface area-to-volume ratio: as a cell increases in size, the volume increases 10x faster than the surface area 4 Cell Theory 5 Cell Theory • Microscopes are required to visualize cells. • Light microscopes can resolve structures that are 200nm apart. • Electron microscopes can resolve structures that are 0.2nm apart. 6 Fig. 4.2 Cell Theory • All cells have certain structures in common: 1. Genetic Material – in a nucleoid or nucleus 2. Cytoplasm – a semifluid matrix 3. Plasma Membrane – a phospholipid bilayer 8 Phospholipids • Chapter 3: Phospholipids are Amphiphilic molecules Polar Head Group Nonpolar Hydrocarbon Tail 101 Page 63 Two Cell Types 1. Prokaryotic Cells 2. Eukaryotic Cells • Recall Three Domains (Ch. 1) • Defined by cell type 1.Eukarya • • • • Plantae Fungi Animlia Protista 2.Bacteria 3.Archaea Eukaryotic Prokaryotic 11 11 1. Prokaryotic Cells Prokaryotic Cells • Origin: ‘pro’-before; ‘karyote’ - nut • Lack a membrane-bound nucleus. - genetic material is present in the nucleoid • Two types of prokaryotes: 1. Archaea 2. Bacteria 12 1. Prokaryotic Cells • Prokaryotic Cell Characteristics: • Simplest organisms - simple internal organization • Very small (1 to 10 microns across) • Genetic material in the nucleoid • No membrane-bound organelles • Capsules • Cytoplasm 13 1. Prokaryotic Cells • Prokaryotic and Eukaryotic Characteristics: • DNA, RNA • Ribosomes • Plasma membrane • Cell walls (bacteria, archaea) • Flagella • Pilli 14 1. Prokaryotic Cell Structure 15 Prokaryotic Cell Structure • Prokaryotic cell walls – Surround and protect cell and maintain cell shape – Composed of polysaccharides (sugar coated) • Bacterial cell walls composed of peptidoglycan • Archaean cell walls lack peptidoglycan. 16 Recall Ch. 3 Polysaccharides b. Function 1. Structural Molecules Cellulose - plant cell walls Chitin – Fungi cell walls Peptidoglycan - Bacterial cell walls 22 Prokaryotic Cell Structure Bacterial cell walls composed of peptidoglycan • Two Types of Bacterial Cell Walls 1. Gram Positive 2. Gram Negative • Gram Positive/Gram Negative type is determined by cell cell wall structure and the Gram Stain Reaction • Gram Positive Bacteria Stain Purple • Gram Negative Bacteria Stain Pink 18 Gram + vs. Gram • Gram + Bacteria stain Purple • Gram – Bacteria stain Pink Courtesy: Dr. O’Steen Prokaryotic Cell Structure Flagella (singular, flagellum) • Whip-like proteins attached to cell wall used for locomotion • Present in some prokaryotic cells - one to several flagella on a single cell • Rotary motion of flagellum propels the cell through fluid environment • Flagella powered by protein motors - uses energy of a proton gradient 20 Flagella Structure 21 2. Eukaryotic Cells • Eukaryotic Cells Origin: ‘eu’ - true, good; ‘karyote’ - nut • Possess a membrane-bound nucleus. - genetic material is highly organized within double-layer nuclear envelope - DNA never leaves the nuclear envelope • Types of eukaryotes divided into 4 kingdoms: 1. Plantae 2. Fungi 3. Animalia 4. Protista 22 2. Eukaryotic Cells • Eukaryotic Cell Characteristics: • More complex organisms • highly organized structure (compartmentalization) known as endomembrane system • Typically larger than prokaryote (10-100 microns) • Genetic material in the membrane-bound nucleus • Many membrane-bound organelles • Cytoplasm • Cytoskeleton 23 2. Eukaryotic Cells • Eukaryotic and Prokaryotic Characteristics: • DNA, RNA • Ribosomes • Plasma membrane • Cytoplasm • Cell walls (plantae, fungi, protista, not present in animal cells) • Flagella 24 Eukaryotic Cells 25 Eukaryotic Cells 26 Eukaryotic Cells Nucleus • Largest most definitive organelle in the cytoplasm • Surrounded by a nuclear envelope composed of 2 phospholipid bilayers • Stores the genetic material of the cell as long separate chains of DNA known as chromosomes • Cell DNA is organized with proteins to form chromatin 27 Eukaryotic Cells Nucleus • Cell DNA is organized with proteins to form chromatin - Chromosomes are tightly packed (condensed) with proteins inside the nucleus into nucleosomes - DNA is wound around histone proteins to resembles beads on a string 28 Fig. 4.9 Eukaryotic Cells Nucleolus (plural, nucleoli) • Dark staining zone within the nucleus • Composed of RNA • Synthesis of ribosomal RNA (rRNA) occurs here - rRNA is involved in the translation of DNA into protein 30 Eukaryotic Cells Nuclear Envelope • Composed of an inner and outer phospholipid bilayer - the outer layer is continuous with the membrane of the endoplasmic reticulum - an organelle for protein synthesis • Nuclear pores provide passage for proteins and rRNA into and out of the nucleus - DNA never leaves the nucleus 31 Eukaryotic Cells 32 Eukaryotic Cells Ribosomes • Present in prokaryotic and eukaryotic cells • Composed of ribosomal RNA and proteins • Found in the cytoplasm and attached to internal membranes of the endoplasmic reticulum • Important protein function in protein synthesis in the cell 33 Eukaryotic Cells Ribosomes • Composed of 2 subunits of ribosomal RNA (rRNA) and protein • The two subunits associate to form complete Ribosomes • Other types of RNA assist with protein synthesis: - mRNA - tRNA 34 Fig. 4.10 Endomembrane System Endomembrane system • A series of membranes throughout the eukaryotic cytoplasm • Divides cell into compartments where different cellular functions occur: 1. Endoplasmic Reticulum 2. Golgi Apparatus 3. Lysosomes 36 Endomembrane System 1. Endoplasmic Reticulum (ER) • Membranes that create a network of channels throughout the cytoplasm • Membrane continuous with the outer membrane of the nuclear envelope • Location for protein synthesis • Two Sections of the ER 1. Rough Endoplasmic Reticulum 2. Smooth Endoplasmic Reticulum 37 Endomembrane System 1. Rough Endoplasmic Reticulum (RER) • • • System of cytoplasmic membranes that create a network of channels throughout the cytoplasm Ribosomes are attached to the outside of the RER membrane giving it a rough appearance under the microscope Synthesis of proteins to be secreted out of the cell, or packaged and sent to lysosomes or plasma membrane - Proteins are synthesized into the RER channels (cisternal space) 38 Endomembrane System 2. Smooth Endoplasmic Reticulum (SER) • Relatively few associated ribosomes • Functions: - Synthesis of membrane lipids - Calcium storage - Detoxification of foreign substances 39 Endomembrane System 40 Endomembrane System Golgi Apparatus • Flattened stacks of interconnected membranes folds (cisternae) known individually as Golgi bodies • Located peripheral to the nucleus and ER • Function in packaging and distribution of materials to different parts of the cell • Synthesis of cell wall components 41 Fig. 4.12 • Cis face of Golgi faces and receives products from the ER • Trans face of Golgi releases secretory vesicles 43 Endomembrane System Lysosomes • Membrane bound vesicles containing digestive enzymes to break down macromolecules • Packaged and secreted by Golgi apparatus • Function to destroy cells or foreign matter that the cell has engulfed by phagocytosis 44 45 Endomembrane System Microbodies • Membrane bound vesicles that do not originate from the endomembrane system • Contain enzymes: - Glyoxysomes in plants contain enzymes for converting fats to carbohydrates - Peroxisomes contain oxidative enzymes such as H2O2 and catalase 46 Fig. 4.15 Endomembrane System Vacuoles • ‘Blank spaces’ • Membrane-bound structures with a variety of functions depending on the cell type • There are different types of vacuoles: - Central Vacuole in plant cells - Contractile Vacuole of some protists - Storage Vacuoles 48 Fig. 4.16 Mitochondria Mitochondria • ‘Power house of the cell” • Present in all types of eukaryotic cells • Contain oxidative metabolism enzymes for the chemical reactions of cellular respiration - the transferring of energy within macromolecules to ATP 50 Mitochondria Structure • Surrounded by 2 membranes: 1. Smooth outer membrane 2. Folded inner membrane with layers (cristae) intermembrane space is located between the two membranes - matrix within the inner membrane - • Similar in structure and function to chloroplasts of photosynthesis • Contain their own DNA (Mitochondrial DNA) - Produce some essential proteins - Mitochondria undergo their own division51 Mitochondria 52 Chloroplasts • Organelles present in cells of plants and some other eukaryotes (Kingdom Protista) • Contain chlorophyll for photosynthesis • Surrounded by 2 membranes - Inner membrane forms sacs known as thylakoids - Stacks of thylakoids are known as Grana • Mitochondria, chloroplasts and similar DNA containing organelles known collectively as plastids 53 Chloroplasts 54 Mitochondria & Chloroplasts Endosymbiosis • Proposal that eukaryotic organelles evolved through a symbiotic relationship • One prokaryotic cell engulfed a second prokaryotic cell and a symbiotic relationship developed • Mitochondria and chloroplasts are thought to have evolved this way 55 Mitochondria & Chloroplasts • Evidence supports this endosymbiosis theory: • Mitochondria and chloroplasts: – – – – have 2 membranes possess DNA and ribosomes are about the size of a prokaryotic cell divide by process of simple fission similar to bacteria 56 Mitochondria & Chloroplasts 57 Cytoskeleton Cytoskeleton • Network of protein fibers found in all eukaryotic cells • Supports the shape of the cell • Keeps organelles in fixed locations • Helps move materials within the cell 58 Cytoskeleton • Three Fiber Types of Cytoskeleton: 1. Actin Filaments – responsible for cellular contractions, crawling, “pinching” - Composed of actin protein subunits 2. Microtubules – provide organization to the cell and move materials within the cell - Composed of tubulin protein subunits 3. Intermediate Filaments – provide structural stability - Composed of vimentin protein subunits 59 Cytoskeleton 60 Fig. 4.20a Cytoskeleton Centrosomes • Organelles that organize microtubule function within the cell - Cell division • Composed of two perpendicular centrioles - bundles of microtubules organized in 9 triplets 62 Fig. 4.21 Cell Movement • Cell movement takes different forms: • Crawling is accomplished via actin filaments and the protein myosin • Molecular motor proteins such as kinesin and dyein use ATP energy to move organelles in the cytoplasm along microtubule tracks • Flagella undulate to move a cell • Cilia can be arranged in rows on the surface of a eukaryotic cell to propel a cell forward64 Fig. 4.22 Cell Movement • The cilia and flagella of eukaryotic cells have a similar structure: • 9-2 structure: 9 pairs of microtubules surrounded by a 2 central microtubules • dynein protein motors slide the microtubules across each other causing them to undulate • Flagella attached to cell body at basal body • Cilia are usually more numerous than flagella on a cell 66 Cell Movement 67 Fig. 4.24b Fig. 4.24a Extracellular Structures • Extracellular structures: • Cell Walls: • plants • fungi • some protists • Extracellular Matrix • surrounding animal cells 70 Extracellular Structures Cell Walls • Present surrounding the cells of plants, fungi, and some protists • Composed of carbohydrates -Type of carbohydrate present in the cell wall vary depending on the cell type: - plant and protist cell walls composed of cellulose - fungal cell walls composed of chitin 71 Fig. 4.25 Extracellular Structures Extracellular matrix (ECM) • Surrounds animal cells - animal cells lack cell walls • Composed of glycoproteins and fibrous proteins such as collagen and elastin - provide a protective layer over the cell • Connected to plasma membrane via fibronectins • Connected to the cytoplasm via integrin 73 proteins present in the plasma membrane Extracellular Structures 74 75 76