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 35

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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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