1 a Tour of the Cell
A Tour of the Cell
Chapter 6
• Objectives • Distinguish between prokaryotic and eukaryotic cells. • Distinguish between free and bound ribosomes in terms of location and function. • List the components of the endomembrane system, and describe the structure and functions of each component. • Briefly describe the roles of the mitochondrion, chloroplast and peroxisome. • Describe the functions of the cytoskeleton and compare the structure and functions of microtubules, microfilaments, and intermediate filaments.
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• Explain how the ultrastructure of cilia and flagella relates to their functions. • Describe the basic structure of a plant cell wall. • Describe the structure and function of the extracellular matrix in animal cells. • Describe four different intercellular junctions
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1 Introduction
• Cell is smallest functional unit of life – cell theory: • all organisms composed of cells • all existing cells arise from pre-existing cells • Common characteristics of all cells: – cell contents surrounded by plasma membrane – cytoplasm consists of semifluid matrix – organelles are embedded in cytoplasm – contain genes in the form of DNA
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Fundamental Features of Life
• Features of cells are characteristic of life – cells are highly structured – cell structure and function are related – cell membranes separate the cell from the external environment – each cell has DNA as the genetic material – each cell carries out metabolism
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Cell Organization
• Two types of cells-prokaryotic and eukaryotic • Prokaryotic: – small – lacks nucleus • the DNA is in the nucleoid region – cytoplasm surrounded by plasma membrane and outer cell wall – flagella and pili may be present
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2 Cell Organization
• Eukaryotic – Larger than prokaryotic cells – complex internal structure with membranous and non-membranous organelles • membranous: nucleus, endoplasmic reticulum, Golgi apparatus,mitochondria, lysosomes and peroxisomes • non-membranous: ribosomes, microtubules, centrioles, flagella and cytoskeleton
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3 • The plasma membrane functions as a selective barrier – allows sufficient passage of nutrients and waste
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A Panoramic View of the Eukaryotic Cell • Eukaryotic cells have extensive and elaborately arranged internal membranes, which form organelles • Plant and animal cells have most of the same organelles – animal cells surrounded by plasma membrane only
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4 – plant cells surrounded by plasma membrane and rigid cell wall; also have central vacuole and chloroplasts
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5 Eukaryotic Cell Organization
• Eukaryotic organelles form four functional groups: – manufacture – breakdown – energy processing – support, movement and communication
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• Manufacturing – nucleus – ribosomes – endoplasmic reticulum: rough and smooth – Golgi apparatus
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The Nucleus: Genetic Library of the Cell • Nucleus is cell’s genetic control center – surrounded by double membrane • contains pores to allow passage of material between nucleus and cytoplasm – DNA normally present as strands of chromatin – During cell division, chromatin coils up to form chromosomes – Nucleolus-organelle within nucleus-responsible for ribosome synthesis
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6 Ribosomes: Protein Factories in the Cell • Ribosomes synthesize proteins – two populations of ribosomes within cells • free ribosomes – suspended in cytosol • bound ribosomes – attached to the outside of endoplasmic reticulum or nuclear membrane
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7 The Endoplasmic Reticulum: Biosynthetic Factory • Endomembrane system – smooth endoplasmic reticulum synthesizes lipids, metabolizes carbohydrates, detoxifies drugs and poisons • stores Ca 2+ in muscle cells – rough endoplasmic reticulum synthesizes proteins on attached ribosomes which are distributed by transport vesicles • known as secretory proteins – many modified in rough ER by addition of carbohydrates • also synthesize membrane proteins and phospholipids
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The Golgi Apparatus: Shipping and Receiving Center • Golgi apparatus finishes, sorts and ships cell products transported in vesicles from ER – consists of flattened membranous sacs called cisternae • Functions of the Golgi apparatus include – modification of the products of the rough ER – manufacture of certain macromolecules
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8 • Breakdown – lysosomes – vacuoles
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Lysosomes: Digestive Compartments • Lysosomes contain enzymes to digest food and wastes – defective lysosomes cause fatal diseases • Lysosomes carry out intracellular digestion by – phagocytosis – autophagy
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9 Vacuoles: Diverse Maintenance Compartments • Vacuoles function in general cell maintenance – a plant or fungal cell may have one or several vacuoles – food vacuoles are formed by phagocytosis – contractile vacuoles pump excess water out of protist • Central vacuoles are found in plant cells – hold reserves of important organic compounds and water
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10 The Endomembrane System: A Review • The organelles involved in synthesis of macromolecules form the endomembrane system – primarily responsible for protein synthesis • lipids synthesized to form transport vesicles which fuse with other membrane-bound organelles
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• Energy Processing – mitochondria – chloroplasts – peroxisomes
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11 The Evolutionary Origins of Mitochondria and Chloroplasts • Mitochondria and chloroplasts have similarities with bacteria – Enveloped by a double membrane – Contain free ribosomes and circular DNA molecules – Grow and reproduce somewhat independently in cells
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• The Endosymbiont theory – An early ancestor of eukaryotic cells engulfed a nonphotosynthetic prokaryotic cell, which formed an endosymbiont relationship with its host – The host cell and endosymbiont merged into a single organism, a eukaryotic cell with a mitochondrion – At least one of these cells may have taken up a photosynthetic prokaryote, becoming the ancestor of cells that contain chloroplasts 35
12 Mitochondria: Chemical Energy Conversion • Mitochondria: – found in all eukaryotic cells, except anaerobic protozoans – surrounded by double membrane • a smooth outer membrane • an inner membrane folded into cristae – site of cellular respiration
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Chloroplasts: Capture of Light Energy • Chloroplasts are specialized members of a family of closely related plant organelles called plastids – contain chlorophyll – found in plants and algae – site of photosynthesis • convert solar energy to chemical energy
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13 • Surrounded by double membrane • Separate inner membrane system divided into two parts – thylakoids, membranous sacs – stroma, the internal fluid
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Peroxisomes: Oxidation
• Peroxisomes are involved in the breakdown of fatty acids and amino acids – Detoxify blood toxins in liver and kidney • Examples – alcohol, peroxide
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14 • Support, Movement, and Communication – cytoskeleton – centrioles and centrosomes – cilia and flagella – intracellular connections
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Roles of the Cytoskeleton: Support, Motility, and Regulation • Internal cell skeleton-cytoskeleton – composed of 3D meshwork of proteins extending through the cytoplasm – gives mechanical support to the cell
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15 Components of the Cytoskeleton
• There are three components of the cytoskeleton – rods of globular proteins-microfilaments – ropelike strands of fibrous proteins-intermediate filaments – hollow tubes of globular proteins-microtubules
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16 Microtubules
• Microtubules provide anchors for organelles, act as conveyor belts and form cilia and flagella
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Centrosomes and Centrioles
• The centrosome is considered to be a “microtubule-organizing center” – In animals contains a pair of centrioles
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17 Cilia and Flagella
• Cilia and flagella – function to move whole cell
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18 • Cilia and flagella share a common ultrastructure – structure consists of 9 microtubule doublets arranged around central pair (9+2)
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• Movement of cilia and flagella occurs when arms consisting of the protein dynein move the microtubule doublets past each other
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19 Microfilaments (Actin Filaments)
• Microfilaments are built from molecules of the protein actin – microfilaments cause contraction of muscle cells – they also function in amoeboid movement, cytoplasmic streaming and support for cellular projections
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20 Intermediate Filaments
• Intermediate filaments – support cell shape – fix organelles in place
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Eukaryotic Cell Surface Organization • Extracellular components and connections between cells help coordinate cellular activities • Cell surfaces protect, support and join cells
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Cell Walls of Plants
• In plants, neighboring cells joined to form interconnected and coordinated group – cell walls are multi-layered • composed of mixtures of polysaccharides and proteins
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21 The Extracellular Matrix (ECM) of Animal Cells • In multi-cellular animals cells are often surrounded by sticky mixture of polysaccharides and proteins-no support • Functions of the ECM include – support – adhesion – movement – regulation
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22 Intercellular Junctions
• Several type of junctions between cells – In plants, plasmodesmata are channels through cell walls connecting cytoplasm of adjacent cells
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• In animal cells, there are three types of cellular connections – tight junctions-leak proof – anchoring junctions-join cells and allow passage of materials in spaces between cells or anchor to extracellular matrix – communicating junctions-provide channels between cells
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23 The Cell: A Living Unit Greater Than the Sum of Its Parts • Cells rely on the integration of structures and organelles in order to function – For example, a macrophage’s ability to destroy bacteria involves the whole cell, coordinating components such as the cytoskeleton, lysosomes, and plasma membrane
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