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Transcript
Cell Structure The Cell ESSENTIAL to the study of biology Simplest form of life Every organism’s basic unit of structure and function Named by Robert Hooke in 1665 after observing cork cells (cell walls) under microscope. Prokaryotic vs. Eukaryotic Cells Prokaryote “before” “nucleus”/ NO NUCLEUS/few organelles Bacteria DNA is concentrated in nucleoid (non membranebound) Eukaryote “true” “nucleus” / many membranous organelles Protists, plants, fungi, animals Nucleus with nuclear membrane holds DNA Why so small? Metabolism requires that cells stay small As a cell grows, its volume grows proportionately more than its surface area Cells need a high surface area to volume ratio to exchange materials with their environment through plasma membrane. Compartmental Organization of Cells Compartments (ORGANELLES) provide different local environments (pH, etc.) Incompatible but equally important processes can occur next to each other in different “rooms” Cytoskeleton & Related Organelles Cytoskeleton Maintains shape of cell Responsible for movement of cell and movement of organelles within cell Made of three types of protein fibers: Microtubules, microfilaments, & intermediate filaments Cytoskeleton & Related Organelles Microtubules Hollow tubes Made up of A- and Btubulin Responsible for: • Cell motility • cilia/flagella • Chromosome movements (mitosis) • centrioles • Movement of organelles • Maintenance of cell shape Cytoskeleton & Related Organelles Components of Cytoskeleton: Microtubules – 25 nm diameter Intermediate Filaments – 8 – 12 nm diameter Microfilaments – 7 nm diameter Cytoskeleton & Related Organelles Intermediate Filaments Made up of fibrous proteins Made up of keratin Responsible for: • Structural support • Maintenance of cell shape • Anchors nucleus & certain organelles Cytoskeleton & Related Organelles Microfilaments Made up of 2 intertwined strands of actin Responsible for: • • • • • Muscle contraction Cytoplasmic streaming Cell motility (pseudopodia) Cell division (cleavage furrow) Maintenance of/changes in cell shape Centrioles Only found in animal cells Visible only during cell division 9+0 arrangement of microtubules May give rise to cilia & flagella May be involved in formation of spindle fibers in animal cells, but not plants! Flagella and Cilia microtubule Structures for cell motility Flagella (long & few in #) Cilia (short & numerous) 9 + 2 internal structure Basal body has 9+0 structure like centrioles Figure 4.25 Page 73 dynein Cellular Organelles Nucleus: “control center” of the cell Surrounded by a nuclear envelope Contains DNA Nucleolus: site of ribosome synthesis Cellular Organelles Ribosomes Site of protein assembly Free and bound ribosomes • Free: float through cytoplasm (make proteins for use inside that cell) • Bound: attached to Rough ER (make proteins to be transported out of the cell) Cellular Organelles Endoplasmic Reticulum: Made up of membranous tubules and cisternae (sacs) Smooth ER: NO ribosomes attached • Synthesis and transport of lipids • Controls glucose glycogen conversion in liver & muscles • Detoxification of drugs and other poisons • Sarcoplasmic reticulum (muscle ER) stores calcium needed in muscle contraction. Rough ER: ribosomes attached • Synthesis & transport of proteins Endomembrane System Smooth and Rough ER Endomembrane System Golgi Apparatus: Products of the Endoplasmic Reticulum are modified and stored here Modifies & packages proteins Endomembrane System Lysosomes: Used by cells to digest macromolecules Sac of hydrolytic enzymes Apoptosis: • Programmed cell death Usually found only in animal cells Endomembrane System Vacuoles: Food vacuoles (storage) Contractile vacuoles (pump extra water out of cells in freshwater protists) Central vacuole (plant cells) • Stores organic compounds, inorganic ions (K+, Cl-), and water • Surrounded by tonoplast Endomembrane System Peroxisomes: Contain enzymes that transfer hydrogen from various substances to oxygen, producing H2O2 as a byproduct Various functions: • Break fatty acids down into smaller molecules for cellular respiration • Detoxify alcohol in liver Energy-related organelles Mitochondria Site of cellular respiration (Energy from the breakdown of organic molecules is used to phosphorylate ADP to produce ATP) “powerhouse of the cell” More metabolic activity = more mitochondria Energy-related organelles Mitochondrial Structure: Outer membrane Inner membrane: • Cristae = large surface area makes more efficient at producing energy Intermembrane space Mitochondrial matrix Energy-related organelles Chloroplasts: Found in plants and eukaryotic algae Site of photosynthesis Contain the green pigment chlorophyll Energy-related organelles Chloroplast Structure Thylakoids • Grana = stacks of thylakoids • (Light Dependent Phase) Stroma • Fluid outside the thylakoids • (Calvin Cycle) Cellular Organelles Cell Wall Found only in plant cells Protects the cell Maintains cell shape Prevents excessive uptake of water Holds plant up against gravity Primary Cell Wall-thin; cellulose Secondary Cell Wallthicker; found in woody plants Cellular Organelles Extracellular Matrix: Found in animal cells Made up of glycoproteins (collagen) & proteoglycans • Proteins + carbohydrates Provides support and anchorage for cells Differs from one cell type to another Intercellular Junctions Neighboring cells are connected to one another Plant cells: Plasmodesmata: • Channels in the cell wall through which strands of cytoplasm pass through and connect the living contents of adjacent cells Intercellular Junctions (Animal Cells) Tight junctionsmembrane proteins interlock Desmosomes, (anchoring junction)intermediate filaments “sew” membranes together Gap junctionschannels align allowing materials to flow between cells Intercellular Junctions Tight junctions: Membranes of neighboring cells are fused Form a continuous “belt” around a cell Example: junction between epidermis of the skin Intercellular Junctions Desmosomes Anchoring junctions Act as rivets Muscle cells are held together by desmosomes. What happens when a muscle is torn? Intercellular Junctions Gap junctions Communicating junctions Cytoplasmic channels between adjacent cells Salts, sugars, AAs, etc. can pass through Membrane Structure & Function Functions of the Cell Membrane Isolates the cytoplasm from the external environment Regulates the flow of materials into and out of the cell Communicates with other cells The Fluid Mosaic Model Currently accepted model of the cell membrane Proposed by Singer and Nicolson in 1972 Phospholipid bilayer Hydrophilic “head” – exposed to the outside Hydrophobic “tail” – hides inside Membrane proteins are randomly dispersed in phospholipid bilayer Fluid Mosaic Model Fluidity of the Membrane The lipids and proteins can drift throughout the membrane Membrane is NOT stiff/rigid Cholesterol makes the membrane stronger by limiting the movement of phospholipids Membrane as a Mosaic Lipid bilayer has membrane proteins embedded in it Integral proteins Go through the membrane (both sides) Peripheral proteins attached to the surface of the membrane Selective Permeability of the Cell Membrane The cell membrane can “choose” what enters and exits a cell “Gatekeeper of the Cell” Passive Transport Def’n: Diffusion of a substance that does NOT require the input of energy by the cell 3 types of passive transport: Diffusion Osmosis Facilitated diffusion Diffusion Movement of molecules from high to low concentration until equilibrium is reached. Passive Transport= no energy required What substances may diffuse across membrane? Nonpolar (non-charged) molecules; small polar molecules Diffusion Each substance diffuses down its OWN concentration gradient and is unaffected by concentration gradients of other substances Diffusion Does all movement stop once equilibrium is reached?? NO!! Equal rates in all directions Osmosis Def’n: The passive transport of water across a selectively permeable membrane Hyper-, hypo-, iso- tonic • RELATIVE TERMS!! • Always referring to solute concentration Water moves from areas of lower concentration of solutes (hypotonic) to areas of higher solute concentration (hypertonic) Osmosis in Plant and Animal Cells Animal Cells: Cell crenate/shrinks Cytolysis • Occurs when a cell is in a hypotonic solution • Water goes from solution into cell Plant Cells: Turgid- vacuole swells Flaccid-vacuole shrinks (plasmolysis) Facilitated Diffusion Def’n: The diffusion of large molecules across the cell membrane using transport proteins Glucose; ions Does NOT require an input of energy Solute is still moving down its concentration gradient Facilitated Diffusion Transport proteins are specific for their solutes Transport proteins can become saturated Some are gated channels: Chemical or electrical stimulus causes them to open Example Which direction will sucrose move? Which direction will glucose move? Which direction will fructose move? Pop Quiz – Passive Transport 1. 2. 3. 4. What is passive transport across the cell membrane? Describe 3 types of passive transport discussed yesterday. If a blood cell is placed in a hypotonic solution what would be the result? What type of solution is preferred in plant cells. Active Transport Def’n: The pumping of solutes against their gradients Requires an input of energy by the cell Used so cells can “stockpile” extra supplies (storage of any substance in a very high concentration; i.e. Iodine stored in thyroid; glycogen stored in liver Sodium/Potassium Pump Electrogenic Pumps Voltage across membranes = stored energy that can be used for cellular work Sodium-Potassium Pump: 3 Na+ OUT of the cell for every 2 K+ pumped in Net transfer of one positive charge from cytoplasm to extracellular fluid Very important for transferring signals between nerve cells Sodium-Potassium Pump Cotransport Substance that has been pumped across a membrane can do work as it “leaks” back by diffusion Another substance “hitches a ride” Endocytosis & Exocytosis Def’n: The movement of large molecules (polysaccharides, proteins, etc.) across the membrane using vesicles Endocytosis = cell takes in macromolecules Exocytosis = cell secretes macromolecules Endocytosis Cell takes in macromolecules by forming vesicles made from the plasma membrane Phagocytosis = “cell eating” Large molecules Pinocytosis = “cell drinking” Small molecules & liquids Receptor-mediated endocytosis = seeks out specific molecules Exocytosis The cell secretes macromolecules by the fusion of vesicles with the plasma membrane Used to release hormones, chemical signals, etc. Pop Quiz 2 1. 2. 3. 4. What is Active Transport across the cell membrane? What does it mean when molecules are moving up the concentration gradient? Describe (in detail) how the sodium potassium pump works in nerve cells. What is the difference between phagocytosis and pinocytosis?