Chapter 6 A Tour of the Cell

Overview: The Importance of Cells All organisms are made of cells Cell structure is correlated to cellular function All cells are related by their descent from earlier cells There is an opinion that cell is the simplest formation that can live (but others say it’s virus)

Concept 6.2: Eukaryotic cells have internal membranes that compartmentalize their functions The basic structural and functional unit of every organism is one of two types of cells: prokaryotic or eukaryotic Only organisms of the domains Bacteria and Archaea consist of prokaryotic cells Protists, fungi, animals, and plants all consist of eukaryotic cells

Comparing Prokaryotic and Eukaryotic Cells Basic features of all cells: Plasma membrane Cytosol Chromosomes (carry genes) Ribosomes (make proteins)

Prokaryotic cells have no nucleus In a prokaryotic cell, DNA is in an unbound region called the nucleoid Prokaryotic cells lack membrane-bound organelles

Eukaryotic cells have DNA in a nucleus that is bounded by a membranous nuclear envelope Eukaryotic cells have membrane-bound organelles Eukaryotic cells are generally much larger than prokaryotic cells

The plasma membrane is a selective barrier that allows sufficient passage of oxygen, nutrients, and waste to service the volume of the cell

A Panoramic View of the Eukaryotic Cell A eukaryotic cell has internal membranes that partition the cell into organelles Plant and animal cells have most of the same organelles

ENDOPLASMIC RETICULUM (ER LE 6-9a ENDOPLASMIC RETICULUM (ER Nuclear envelope Flagellum Rough ER Smooth ER Nucleolus NUCLEUS Chromatin Centrosome Plasma membrane CYTOSKELETON Microfilaments Intermediate filaments Microtubules Ribosomes: Microvilli Golgi apparatus Peroxisome Mitochondrion Lysosome In animal cells but not plant cells: Lysosomes Centrioles Flagella (in some plant sperm)

LE 6-9b Nuclear envelope Rough endoplasmic NUCLEUS reticulum Nucleolus Chromatin Smooth endoplasmic reticulum Centrosome Ribosomes (small brown dots) Central vacuole Golgi apparatus Microfilaments Intermediate filaments CYTOSKELETON Microtubules Mitochondrion Peroxisome Plasma membrane Chloroplast Cell wall Plasmodesmata Wall of adjacent cell In plant cells but not animal cells: Chloroplasts Central vacuole and tonoplast Cell wall Plasmodesmata

The nucleus contains most of the DNA in a eukaryotic cell Concept 6.3: The eukaryotic cell’s genetic instructions are housed in the nucleus and carried out by the ribosomes The nucleus contains most of the DNA in a eukaryotic cell Ribosomes use the information from the DNA to make proteins

The Nucleus: Genetic Library of the Cell The nucleus contains most of the cell’s genes and is usually the most conspicuous organelle The nuclear envelope encloses the nucleus, separating it from the cytoplasm

Ribosomes: Protein Factories in the Cell Ribosomes are particles made of ribosomal RNA and protein Ribosomes carry out protein synthesis in two locations: In the cytosol (free ribosomes) On the outside of the endoplasmic reticulum (ER) or the nuclear envelope (bound ribosomes)

http://genome.imim.es/courses/Madrid04/exercises/ensembl/images/ribosome.jpg

Ribosomes ER Cytosol Endoplasmic reticulum (ER) Free ribosomes Bound ribosomes Large subunit 0.5 µm Small subunit TEM showing ER and ribosomes Diagram of a ribosome

Components of the endomembrane system: Concept 6.4: The endomembrane system regulates protein traffic and performs metabolic functions in the cell Components of the endomembrane system: Nuclear envelope Endoplasmic reticulum Golgi apparatus Lysosomes Vacuoles Plasma membrane These components are either continuous or connected via transfer by vesicles

LE 6-12 Smooth ER Nuclear envelope Rough ER ER lumen Cisternae Ribosomes Transitional ER Transport vesicle 200 nm Smooth ER Rough ER

The Endoplasmic Reticulum: Biosynthetic Factory The endoplasmic reticulum (ER) accounts for more than half of the total membrane in many eukaryotic cells The ER membrane is continuous with the nuclear envelope There are two distinct regions of ER: Smooth ER, which lacks ribosomes Rough ER, with ribosomes studding its surface

Functions of Smooth ER The smooth ER Synthesizes lipids Metabolizes carbohydrates Stores calcium Detoxifies poison

Functions of Rough ER The rough ER Has bound ribosomes Produces proteins and membranes, which are distributed by transport vesicles Is a membrane factory for the cell

http://sun.menloschool.org/~cweaver/cells/c/rough_er/picture.jpg

The Golgi Apparatus: Shipping and Receiving Center The Golgi apparatus consists of flattened membranous sacs called cisternae Functions of the Golgi apparatus: Modifies products of the ER Manufactures certain macromolecules Sorts and packages materials into transport vesicles

http://micro.magnet.fsu.edu/cells/golgi/images/golgifigure1.jpg

Lysosomes: Digestive Compartments A lysosome is a membranous sac of hydrolytic enzymes Lysosomal enzymes can hydrolyze proteins, fats, polysaccharides, and nucleic acids Lysosomes also use enzymes to recycle organelles and macromolecules, a process called autophagy

Vacuoles: Diverse Maintenance Compartments Vesicles and vacuoles (larger versions of vacuoles) are membrane-bound sacs with varied functions A plant cell or fungal cell may have one or several vacuoles

Food vacuoles are formed by phagocytosis Central vacuoles, found in many mature plant cells, hold organic compounds and water

LE 6-15 Central vacuole Cytosol Tonoplast Nucleus Central vacuole Cell wall Chloroplast 5 µm

Endomembrane system Nucleus Rough ER Smooth ER Nuclear envelope

LE 6-16-2 Nucleus Rough ER Smooth ER Nuclear envelope cis Golgi Transport vesicle trans Golgi

LE 6-16-3 Nucleus Rough ER Smooth ER Nuclear envelope cis Golgi Transport vesicle Plasma membrane trans Golgi

Concept 6.5: Mitochondria and chloroplasts change energy from one form to another Mitochondria are the sites of cellular respiration Chloroplasts, found only in plants and algae, are the sites of photosynthesis Mitochondria and chloroplasts are not part of the endomembrane system Peroxisomes are oxidative organelles

Mitochondrion Intermembrane space Outer membrane Free ribosomes in the LE 6-17 Mitochondrion Intermembrane space Outer membrane Free ribosomes in the mitochondrial matrix Inner membrane Cristae Matrix Mitochondrial DNA 100 nm

Mitochondria: Chemical Energy Conversion Mitochondria are in nearly all eukaryotic cells They have a smooth outer membrane and an inner membrane folded into cristae The inner membrane creates two compartments: intermembrane space and mitochondrial matrix Some metabolic steps of cellular respiration are catalyzed in the mitochondrial matrix Cristae present a large surface area for enzymes that synthesize ATP

Chloroplasts: Capture of Light Energy The chloroplast is a member of a family of organelles called plastids Chloroplasts contain the green pigment chlorophyll, as well as enzymes and other molecules that function in photosynthesis Chloroplast structure includes: Thylakoids, membranous sacs Stroma, the internal fluid

LE 6-18 Chloroplast Ribosomes Stroma Chloroplast DNA Inner and outer membranes Granum 1 µm Thylakoid

Concept 6.6: The cytoskeleton is a network of fibers that organizes structures and activities in the cell The cytoskeleton is a network of fibers extending throughout the cytoplasm It organizes the cell’s structures and activities, anchoring many organelles It is composed of three types of molecular structures: Microtubules Microfilaments Intermediate filaments

Roles of the Cytoskeleton: Support, Motility, and Regulation The cytoskeleton helps to support the cell and maintain its shape It interacts with motor proteins to produce motility Recent evidence suggests that the cytoskeleton may help regulate biochemical activities

Components of the Cytoskeleton Microtubules are the thickest of the three components of the cytoskeleton Microfilaments, also called actin filaments, are the thinnest components Intermediate filaments are fibers with diameters in a middle range

Microtubules

Cilia and Flagella Microtubules control the beating of cilia and flagella, locomotor appendages of some cells

Vesicle ATP Receptor for motor protein Motor protein Microtubule (ATP powered) Microtubule of cytoskeleton

Microfilaments

Intermediate filaments

Microfilaments (Actin Filaments) Microfilaments are solid rods about 7 nm in diameter, built as a twisted double chain of actin subunits The structural role of microfilaments is to bear tension, resisting pulling forces within the cell They form a 3D network just inside the plasma membrane to help support the cell’s shape

Microfilaments that function in cellular motility contain the protein myosin in addition to actin In muscle cells, thousands of actin filaments are arranged parallel to one another Thicker filaments composed of myosin interdigitate with the thinner actin fibers

Muscle cell Actin filament Myosin filament Myosin arm LE 6-27a Muscle cell Actin filament Myosin filament Myosin arm Myosin motors in muscle cell contraction

These extracellular structures include: Cell walls of plants Concept 6.7: Extracellular components and connections between cells help coordinate cellular activities Most cells synthesize and secrete materials that are external to the plasma membrane These extracellular structures include: Cell walls of plants The extracellular matrix (ECM) of animal cells Intercellular junctions

The Extracellular Matrix (ECM) of Animal Cells Animal cells are covered by an elaborate extracellular matrix (ECM) The ECM is made up of glycoproteins and other macromolecules Functions of the ECM: Support Adhesion Movement Regulation

Proteoglycan complex EXTRACELLULAR FLUID Collagen fiber Fibronectin LE 6-29a Proteoglycan complex EXTRACELLULAR FLUID Collagen fiber Fibronectin Plasma membrane CYTOPLASM Integrin Micro- filaments

Collagens are, in most animals, the most abundant glycoproteins in the ECM. Collagen is the most abundant protein in the human body, making up about 25% of the total protein content. Collagens are present in the ECM as fibrillar proteins and give structural support to resident cells.

Fibronectins are proteins that connect cells with collagen fibers in the ECM, allowing cells to move through the ECM. Fibronectins bind collagen and cell surface integrins, causing a reorganization of the cell's cytoskeleton and facilitating cell movement.

Intercellular Junctions Neighboring cells in tissues, organs, or organ systems often adhere, interact, and communicate through direct physical contact Intercellular junctions facilitate this contact

Animals: Tight Junctions, Desmosomes, and Gap Junctions At tight junctions, membranes of neighboring cells are pressed together, preventing leakage of extracellular fluid Desmosomes (anchoring junctions) fasten cells together into strong sheets Gap junctions (communicating junctions) provide cytoplasmic channels between adjacent cells