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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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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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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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• 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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– plant cells surrounded by plasma membrane and rigid cell wall; also have central vacuole and chloroplasts

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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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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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The Endoplasmic Reticulum: Biosynthetic Factory

• Endomembrane system– smooth endoplasmic reticulum synthesizes lipids,

metabolizes carbohydrates, detoxifies drugs and poisons

• stores Ca2+ 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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• 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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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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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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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

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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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• 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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• 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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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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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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Cilia and Flagella

• Cilia and flagella– function to move whole cell

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• 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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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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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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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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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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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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