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Chapter 6: A Tour of the Cell
Overview: The Importance of Cells
• The cell theory
o The __________________________________ is the basic unit of living things
o All organisms are made of cells
o All cells come from pre-existing cells
Cell structure is correlated to cellular function
• To study cells, biologists use __________________________________ and the tools of __________________________________
• Though usually too small to be seen by the unaided eye, cells can be complex
• Scientists use microscopes to visualize cells too small to see with the naked eye
• __________________________________ microscope: visible light passes through a specimen and then through glass
lenses, which magnify the image
o The minimum resolution of an LM is about 200 nanometers (nm), the size of a small bacterium
• Two basic types of electron microscopes (EMs) are used to study subcellular structures
o __________________________________ electron microscopes (SEMs): focus a beam of electrons onto the surface
of a specimen, providing images that look 3D
o __________________________________ electron microscopes (TEMs): focus a beam of electrons through a
specimen
 TEMs are used mainly to study the internal ultrastructure of cells
Isolating Organelles by Cell Fractionation
• Cell fractionation takes cells apart and separates the major organelles from one another
• Ultracentrifuges fractionate cells into their component parts
• Cell fractionation enables scientists to determine the functions of organelles
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
• Only organisms of the domains Bacteria and Archaea consist of prokaryotic cells
Eukaryotic
• Protists, fungi, animals, and plants all consist of eukaryotic cells
Comparing Prokaryotic and Eukaryotic Cells
• Basic features of all cells:
o Plasma membrane
o Semifluid substance called the __________________________________
o __________________________________ (carry genes)
o __________________________________ (make proteins)
• Prokaryotic cells:
o Have no __________________________________
o DNA is in an unbound region called the __________________________________
o Lack membrane-bound organelles
• Eukaryotic cells:
o Have DNA in a nucleus that is bounded by a membranous __________________________________
o Contain membrane-bound organelles
• Eukaryotic cells are generally much larger than prokaryotic cells
• The logistics of carrying out cellular metabolism sets limits on the size of cells
A Panoramic View of the Eukaryotic Cell
• Plant and animal cells have most of the same organelles
The Nucleus: Genetic Library of the Cell
• The nuclear envelope encloses the nucleus, separating it from the cytoplasm
o The nuclear membrane is a double layered membrane perforated by __________________________________
o Pores are stabilized by encompassing __________________________________
• The nucleus contains most of the cell’s genes in either a chromatin or chromosome format
o __________________________________: expanded, can still be transcribed
o __________________________________: condensed, utilized for replication
• The nucleus contains the __________________________________
• The nucleolus assembles __________________________________
Ribosomes: Protein Factories in the Cell
• Ribosomes are assembled in the nucleolus
o They are molecules composed of ribosomal RNA and protein
o They are composed of one __________________________________ subunit and one
__________________________________ subunit
• Ribosomes carry out protein synthesis in two locations:
o __________________________________ ribosomes: in the cytosol
o __________________________________ ribosomes: on the outside of the endoplasmic reticulum (ER) or the
nuclear envelope
The endomembrane system regulates protein traffic and performs metabolic functions in the cell
• Components of the endomembrane system:
1. Nuclear envelope
2. Endoplasmic reticulum
3. Golgi apparatus
4. Lysosomes
5. Vacuoles
6. Plasma membrane
• These components are either continuous or connected via transfer by __________________________________
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
o The ER membrane is convoluted, thee folds are known as __________________________________
o Cisternae are hollow, the hollow space is called the __________________________________
• There are two distinct regions of ER:
o __________________________________ ER: lacks ribosomes
o __________________________________ ER: studded with ribosomes
Functions of Smooth ER
• Smooth ER
o Synthesizes __________________________________ for membranes
o Metabolizes __________________________________
o Stores __________________________________
o __________________________________ poison
Functions of Rough ER
• Rough ER
o Produces __________________________________ and membranes
o Assembles __________________________________
The Golgi Apparatus: Shipping and Receiving Center
• The Golgi apparatus consists of flattened membranous sacs called cisternae
o The side which receives materials is the __________________________________ face
o The side which transports materials is the __________________________________ face
• Functions of the Golgi apparatus:
1. Modifies products of the ER
2. Manufactures certain macromolecules
3. Sorts and packages materials into transport __________________________________
Lysosomes: Digestive Compartments
• A lysosome is a membranous sac of __________________________________
• Lysosomal enzymes can hydrolyze:
o Proteins
o Fats
o Polysaccharides
o Nucleic acids
• Lysosomes also use enzymes to recycle organelles and macromolecules, a process called
__________________________________
Phagocytosis and Autophagy
• Phagocytosis
1. Cell encompasses usable material via __________________________________, forming a vesicle
2. Vesicle fuses with a lysosome, forming a __________________________________
3. Enzymes digest material and either dispose or recycle it
• Autophagy
1. Damaged organelle is enclosed in a membrane, forming a vesicle
2. Vacuole fuses with a lysosome, forming a phagolysosome
3. Enzymes digest material and either dispose or recycle it
Vacuoles: Diverse Maintenance Compartments
• Vesicles are membrane-bound sacs with varied functions
• A plant cell or fungal cell may have one or several vacuoles
o Food vacuoles: formed by __________________________________
o Contractile vacuoles: found in many freshwater protists, pump excess water out of cells
o Central vacuoles: found in many plant cells, hold organic compounds and __________________________________
The Endomembrane System: A Review
• The endomembrane system is a complex and dynamic player in the cell’s compartmental organization
Mitochondria: Chemical Energy Conversion
• Mitochondria are in nearly all eukaryotic cells
o They are the site of cellular __________________________________
• They have a double membrane structure, with a convoluted inner membrane
o The folds are known as __________________________________
 Cristae increase the __________________________________, thus increasing the amount of area that
cellular respiration can occur
 The inner membrane creates two compartments: intermembrane space and mitochondrial matrix
Chloroplasts: Capture of Light Energy
• Chloroplasts are found in leaves and other green organs of plants and in algae
• Chloroplast structure includes:
o __________________________________: membranous sacs
o Thylakoids contain the green pigment __________________________________
 Thylakoids are stacked into __________________________________
 Stacking increase the __________________________________, thus increasing the amount of area that
photosynthesis can occur
• __________________________________: internal fluid
Peroxisomes: Oxidation
• Peroxisomes are specialized metabolic compartments bounded by a single membrane
• Peroxisomes produce __________________________________ and convert it to __________________________________
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
o It organizes the cell’s structures and activities, anchoring many organelles
• It is composed of three types of molecular structures:
1. __________________________________: thickest
2. Microfilaments: also called actin filaments, thinnest
3. Intermediate filaments: diameters in a middle range
Roles of the Cytoskeleton: Support, Motility, and Regulation
• The cytoskeleton:
o Aids in cell shape maintenance
o Interacts with motor proteins to produce motility
 Inside the cell, vesicles can travel along “monorails” provided by the cytoskeleton
Centrosomes and Centrioles
• Microtubules grow out from a __________________________________ near the nucleus
• The centrosome is a “microtubule-organizing center”
o In animal cells, the centrosome has a pair of centrioles, each with nine triplets of microtubules arranged
in a ring
 Microtubules control the beating of cilia and flagella, locomotor appendages of some cells
 Cilia and flagella differ in their beating patterns
• Cilia and flagella share a common ultrastructure:
o __________________________________: microtubules sheathed by a plasma membrane
o Basal body: anchors the cilium or flagellum
o Motor proteins: called __________________________________, drives the bending movements of a cilium or
flagellum
• How dynein “walking” moves flagella and cilia:
o Dynein arms alternately grab, move, and release the outer microtubules
o Protein cross-links limit sliding
o Forces exerted by dynein arms cause doublets to curve, __________________________________ the cilium or
flagellum
Microfilaments (Actin Filaments)
• Microfilaments are solid rods about 7 nm in diameter, built as a twisted double chain of actin subunits
o Bear tension, resisting pulling forces within the cell
o Form a 3D network just inside the plasma membrane to help support the cell’s shape
• Example:
o Bundles of microfilaments make up the core of microvilli of intestinal cells
• Microfilaments that function in cellular motility contain the protein myosin in addition to actin
o In muscle cells, thousands of actin filaments are arranged parallel to one another
o Thicker filaments composed of myosin interdigitate with the thinner actin fibers
• Amoeboid movement: localized contraction brought about by actin and myosin
o __________________________________ (cellular extensions) extend and contract through the reversible
assembly and contraction of actin subunits into microfilaments
• Cytoplasmic streaming: circular flow of cytoplasm within cells
o Speeds distribution of materials within the cell
o In plant cells, actin-myosin interactions and sol-gel transformations drive cytoplasmic streaming
Intermediate Filaments
• Intermediate filaments range in diameter from 8–12 nanometers
o Support cell shape and fix organelles in place
o More permanent cytoskeleton fixtures than the other two classes
Plasma Membrane
• Plasma membrane: a selective barrier that allows sufficient passage of oxygen, nutrients,
and waste to service the volume of the cell
o The general structure of a biological membrane is a double layer of __________________________________
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:
1. __________________________________ of plants
2. __________________________________ (ECM) of animal cells
3. Intercellular junctions
Cell Walls of Plants
• The cell wall is an extracellular structure that distinguishes plant cells from animal cells
• Functions include:
1. Protects the plant cell
2. Maintains its shape
3. Prevents excessive uptake of __________________________________
• Plant cell walls are made of __________________________________ fibers embedded in other polysaccharides and
protein
• Plant cell walls may have multiple layers:
o Primary cell wall: relatively thin and flexible
o Middle __________________________________: thin layer between primary walls of adjacent cells
o Secondary cell wall (in some cells): between the plasma membrane and the primary cell wall
• __________________________________ are channels between adjacent plant cells
The Extracellular Matrix (ECM) of Animal Cells
• Animal cells lack cell walls but are covered by an elaborate extracellular matrix (ECM)
• Functions of the ECM include:
1. Support
2. Adhesion
3. Movement
4. Regulation
The ECM is made up of glycoproteins and other macromolecules
• The ECM is composed of:
o __________________________________: provides tensile strength
o __________________________________: used to bind to integral proteins
o __________________________________: recognized in cell to cell communication
Intercellular Junctions
• Neighboring cells in tissues, organs, or organ systems often adhere, interact, and communicate through direct
physical contact
• Intercellular junctions facilitate this contact
Plants: Plasmodesmata
• Plasmodesmata are channels that perforate plant cell walls
• Through plasmodesmata, water and small solutes (and sometimes proteins and RNA) can pass from cell to cell
Animals: Tight Junctions, Desmosomes, and Gap Junctions
• __________________________________ junctions: membranes of neighboring cells are pressed together, preventing
leakage of extracellular fluid
• __________________________________: (anchoring junctions) fasten cells together into strong sheets
• __________________________________ junctions: (communicating junctions) provide cytoplasmic channels between
adjacent cells