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NOTES: CH 6 – A Tour of the Cell
Overview: The Importance of Cells
● All organisms are made of cells
● The cell is the simplest collection of matter that can live
● Cell structure is correlated to cellular function
● All cells are related by their descent from earlier cells
Measurements1 centimeter (cm) = 10–2 meter (m) = 0.4 inch1 millimeter (mm) = 10–3 m1 micrometer (µm) = 10–3 mm = 10–6 m1 nanometer (nm) = 10–3 µm = 10–9 m
10 m
1 mHuman height
Length of somenerve andmuscle cells
Chicken egg
0.1 m
1 cm
Frog egg1 mm
100 µm
Most plant andanimal cells
10 µmNucleus
1 µm
Most bacteria
Mitochondrion
Smallest bacteria
Viruses100 nm
10 nmRibosomes
Proteins
Lipids
1 nmSmall molecules
Atoms0.1 nmU
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Lig
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Ele
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6.1 – Biologists use microscopes and the tools of biochemistry to study cells
Brightfield (unstained specimen)
50 µmBrightfield (stained specimen)
Phase-contrast
MICROSCOPES
1) Light Microscope
2) Electron Microscope (1950’s)
● Transmission Electron Microscope
● Scanning Electron Microscope
Light Microscope
● works by passing visible light through a thin section of specimen and then through glass lenses
● resolving power = 0.2 µm (size of small bacteria)
● max. magnification about 1000x
Electron Microscope (1950’s)
● uses electron beams which
have shorter wavelengths
of light
● resolving power = 0.2 nm
(most cell structures)
● magnification up to
40,000x
Electron Microscope1) Transmission Electron Microscope
-electrons transmitted through specimen are focused and image is magnified using electromagnets
-used to study internal cell structure
2) Scanning Electron Microscope
-electron beam scans the surface of a spec.
-useful for studying the surface of specimen in 3-D.
Disadvantages to EM…
● can only view dead cells (elaborate preparation)
● very expensive!
zooxanthellae cells cultured from coral
Aiptasia pulchella in a Scanning Electron
Microscope
WE CAN ALSO STUDY CELLS BY... ● Cell Fractionation =
disrupting cells to separate out cell organelles
● Centrifugation = spinning mixtures of cells and their parts at very high speeds; separates the components
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 (& are in the domain Eukarya)
Types of CELLS:
ALL CELLS:
● have a cell membrane
● have cytoplasm / cytosol
● have ribosomes (make
proteins)
● can reproduce & contain
genetic material (DNA /
chromatin / chromosomes)
CELLS CAN BE CLASSIFIED AS:
1) PROKARYOTES
2) EUKARYOTES
1) PROKARYOTES 2) EUKARYOTES
• oldest cells (3.5 billion years)
• single celled
• lack nucleus & membrane-bound organelles
• genetic material in a single, circular molecule (PLASMID) in region called NUCLEOID
• small (1-2 µm)
• Domains Bacteria and Archaea
• “newer” cells (1.5 billion years)
• single or multicellular
• have a “true” nucleus & membrane-bound organelles
• genetic material organized into CHROMOSOMES in NUCLEUS
• larger (2-1000 µm)
• Domain Eukarya, includes Kingdoms Protista, Fungi, Plantae, Animalia
A typicalrod-shapedbacterium
A thin section through thebacterium Bacilluscoagulans (TEM)
0.5 µm
Pili
Nucleoid
Ribosomes
Plasmamembrane
Cell wall
Capsule
Flagella
Bacterialchromosome
● although eukaryotic cells are larger than prokaryotes, there is a limit on cell size due to the logistics of carrying out cellular metabolism
Total surface area(height x width xnumber of sides xnumber of boxes)
6
125 125
150 750
1
11
5
1.2 66
Total volume(height x width x lengthX number of boxes)
Surface-to-volumeratio(surface area volume)
Surface area increases whileTotal volume remains constant
● The plasma membrane is a selective barrier that allows sufficient passage of oxygen, nutrients, and waste to service the volume of the cell
● The general structure of
a biological membrane is
a double layer of
phospholipids
PLASMA MEMBRANE: the boundary of every cell
Hydrophilicregion
Hydrophobicregion
Carbohydrate side chain
Structure of the plasma membrane
Hydrophilicregion
Phospholipid Proteins
Outside of cell
Inside of cell 0.1 µm
TEM of a plasma membrane
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
Flagellum
Centrosome
CYTOSKELETON
Microfilaments
Intermediate filaments
Microtubules
Peroxisome
Microvilli
ENDOPLASMIC RETICULUM (ER
Rough ER Smooth ER
MitochondrionLysosome
Golgi apparatus
Ribosomes:
Plasma membrane
Nuclear envelope
NUCLEUS
In animal cells but not plant cells: LysosomesCentriolesFlagella (in some plant sperm)
Nucleolus
Chromatin
Roughendoplasmicreticulum
In plant cells but not animal cells: ChloroplastsCentral vacuole and tonoplastCell wallPlasmodesmata
Smoothendoplasmicreticulum
Ribosomes(small brown dots)
Central vacuole
Microfilaments
IntermediatefilamentsMicrotubules
CYTOSKELETON
Chloroplast
Plasmodesmata
Wall of adjacent cell
Cell wall
Nuclearenvelope
Nucleolus
Chromatin
NUCLEUS
Centrosome
Golgiapparatus
Mitochondrion
Peroxisome
Plasmamembrane
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
● houses the information / instructions for cell functioning and maintenance …the “control center” of the cell
● averages 5 µm in diameter
Close-up of nuclearenvelope
Nucleus
Nucleolus
Chromatin
Nuclear envelope:Inner membraneOuter membrane
Nuclear pore
Porecomplex
Ribosome
Pore complexes (TEM) Nuclear lamina (TEM)
1 µm
Rough ER
Nucleus
1 µm
0.25 µm
Surface of nuclear envelope
NUCLEAR ENVELOPE ● double membrane which encloses
the nucleus
-each of the 2 membranes is a phospholipid bilayer w/specific proteins
-is perforated by pores which regulate molecular traffic into and out of the nucleus
-RNA and proteins enter or leave the nucleus through these pores
-breaks down prior to cell division
CHROMATIN ● fibrous, threadlike
complex of DNA and histone proteins which make up chromosomes in eukaryotic cells
CHROMOSOMES
● compacted, coiled up chromatin;
● visible under microscope;
● form just prior to cell division;
● human cells have 46 chromosomes (23 pairs)
NUCLEOLUS ● dense, spherical region in the nucleus
-visible in a nondividing cell-may be 2 or more per cell -packages ribosomal subunits from:1) rRNA: transcribed in nucleolus2) RNA produced elsewhere in nucleus-ribosomal subunits pass through nuclear pores to the cytoplasm where assembly into ribosomes is completed
RIBOSOMES
● cytoplasmic organelle; site of protein synthesis
-made of RNA and protein
-made in the nucleolus
-cells with high rates of protein synthesis have large numbers of nucleoli & ribosomes (e.g. human liver cells have millions)
● Ribosomes carry out protein synthesis in two locations:
-in the cytosol (free ribosomes)
-attached to the outside of the endoplasmic reticulum (ER) or the nuclear envelope (bound ribosomes)
RIBOSOMES
Ribosomes
0.5 µm
ER Cytosol
Endoplasmicreticulum (ER)
Free ribosomes
Bound ribosomes
Largesubunit
Smallsubunit
Diagram ofa ribosome
TEM showing ERand ribosomes
Cell Organelles (continued)
The Endomembrane System
**all structures are essentially compartments, closed off by their membranes from the cytoplasm
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 vesicles
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
ENDOPLASMIC RETICULUM (ER): ● extensive network of tubules and sacs
● used for transport and/or modification of proteins;
● can be ROUGH (ribosomes) or SMOOTH (no ribosomes)
Rough ER: ● manufactures secretory
proteins and membranes ;● proteins made here may
be modified (i.e. folded into theirtertiary structure)
● usually closer in to nucleusthan smooth ER
Smooth ER:
● synthesizes lipids, phospholipids, steroids
● participates in carbohydrate metabolism
● detoxifies drugs and poisons
● stores calcium ions (for muscle
contraction)
Ribosomes
Smooth ER
Rough ER
ER lumen
Cisternae
Transport vesicle
Smooth ER Rough ER
Transitional ER
200 nm
Nuclearenvelope
● 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
The Golgi Apparatus: Shipping and Receiving Center
GOLGI APPARATUS:
● cis face (forming face; faces the rough ER) receives products by accepting transport vesicles from the rough ER
● trans face (maturing face; faces the
cell membrane) pinches off vesicles
from the Golgi and transports
molecules to other sites
trans face(“shipping” side ofGolgi apparatus) TEM of Golgi apparatus
0.1 µm
Golgi apparatus
cis face(“receiving” side ofGolgi apparatus)
Vesicles coalesce toform new cis Golgi cisternae Vesicles also
transport certainproteins back to ER
Vesicles movefrom ER to Golgi
Vesicles transport specificproteins backward to newerGolgi cisternae
Cisternalmaturation:Golgi cisternaemove in a cis-to-transdirection
Vesicles form andleave Golgi, carryingspecific proteins toother locations or tothe plasma mem-brane for secretion
Cisternae
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
Phagocytosis: lysosome digesting food
1 µm
Plasmamembrane
Food vacuole
Lysosome
Nucleus
Digestiveenzymes
Digestion
Lysosome
Lysosome containsactive hydrolyticenzymes
Food vacuolefuses withlysosome
Hydrolyticenzymes digestfood particles
Autophagy: lysosome breaking down damaged organelle
1 µm
Vesicle containingdamaged mitochondrion
Mitochondrionfragment
Lysosome containingtwo damaged organelles
Digestion
Lysosome
Lysosome fuses withvesicle containingdamaged organelle
Peroxisomefragment
Hydrolytic enzymesdigest organellecomponents
LYSOSOMES ● probably pinch off from the trans face of Golgi;
● are responsible for intracellular digestion;
● recycle the cell’s own organic material;
● destroy cells
Other Membrane-Bound Organelles:1) Vacuoles
2) Peroxisomes
VESICLES / VACUOLES: ● membrane-enclosed sac
used for storage and/or transport animal cells, vacuoles are small and look like vesicles
**Vacuoles in plants have special characteristics:-plant cells have a LARGE central vacuole that stores water and water-soluble organic compounds and inorganic ions (K+ and Cl-);
PLANT VACUOLES…
● contain soluble pigments in some cells (red and blue
pigments in flowers);
● play a role in plant growth by absorbing water and
elongating the cell;
PLANT VACUOLES…
● help protect from predators by storing waste
products that may also be poisonous compounds
● Are surrounded by a membrane called the
TONOPLAST
● some fresh-water protists
have a contractile vacuole
that pumps excess water
from the cell
5 µm
Central vacuole
Cytosol
Tonoplast
Central vacuole
Nucleus
Cell wall
Chloroplast
The Endomembrane System: A Review
● The endomembrane system is a complex and dynamic player in the cell’s compartmental organization
Nuclear envelope
Nucleus
Rough ER
Smooth ER
Nuclear envelope
Nucleus
Rough ER
Smooth ER
Transport vesicle
cis Golgi
trans Golgi
Nuclear envelope
Nucleus
Rough ER
Smooth ER
Transport vesicle
cis Golgi
trans Golgi
Plasma membrane
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
MITOCHONDRIA:
● sites of cellular respiration
● found in nearly all eukaryotic cells
● the # in cells varies and is related to the
cell’s metabolic activity
● inner membrane is convoluted
and contains proteins/enzymes
involved in cellular respiration
● inner membranes many
infoldings are called CRISTAE;
they increase the surface area
for cellular respiration
reactions to occur
● region within inner membrane is
the MITOCHONDRIAL MATRIX
Mitochondria in a human liver cell
Mitochondrion
Intermembrane space
Outer membrane
Inner membrane
Cristae
Matrix
100 nmMitochondrialDNA
Freeribosomes in themitochondrialmatrix
CHLOROPLASTS: (“the organelles that feed the world”)
● contain chlorophyll;● site of photosynthesis
(convert light energy into chemical energy;
● found in eukaryotic algae, leaves and other green plant organs;
● can change shape, move and divide
Chloroplasts: Capture of Light Energy
● Chloroplast structure includes:
-Thylakoids, membranous sacs
-Stroma, the internal fluid
Chloroplast
ChloroplastDNA
RibosomesStroma
Inner and outermembranes
Granum
Thylakoid1 µm
PEROXISOMES:
● contain special enzymes for specific metabolic pathways
● found in nearly all eukaryotic cells
● contain peroxide-producing enzymes
that transfer hydrogen ions to oxygen
producing hydrogen peroxide
● contain catalase enzyme which
converts / detoxifies hydrogen peroxide
to water
Chloroplast
Peroxisome
Mitochondrion
1 µm
6.6 - The cytoskeleton is a network of fibers that organizes structures and activities in the cell
● anchors and/or provides “tracks” for many organelles
● It is composed of three types of molecular structures:– Microtubules– Microfilaments– Intermediate filaments
Microtubule
Microfilaments0.25 µm
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
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 (movement)
● inside the cell, vesicles can travel along “monorails” provided by the cytoskeleton
● recent evidence suggests that the cytoskeleton may help regulate biochemical activities
Vesicle
Receptor formotor protein
Microtubuleof cytoskeleton
Motor protein(ATP powered)
ATP
0.25 µmMicrotubule Vesicles
Microtubules: ● straight, hollow rods made of protein called
TUBULIN;
● can serve as “tracks” to
guide organelle movement;
● involved in separation of chromosomes in cell
division; make up CENTRIOLES;
Cilia and Flagella● Microtubules control the beating of cilia and
flagella, locomotor appendages of some cells
● Cilia and flagella differ in their beating patterns
FLAGELLA and CILIA:
● FLAGELLA: longer than cilia; usually found singly or in pairs; used to propel a cell
● CILIA: shorter than flagella; usually present in great numbers; wavelike motion used to sweep extracellular material over/away from cell
5 µm
Direction of swimming
Motion of flagella
15 µm
Direction of organism’s movement
Motion of cilia
Direction ofactive stroke
Direction ofrecovery stroke
● Cilia and flagella share a common ultrastructure:
-A core of microtubules sheathed by the plasma membrane
-A basal body that anchors the cilium or flagellum
-A motor protein called DYNEIN, which drives the bending movements of a cilium or flagellum
0.5 µm
Microtubules
PlasmamembraneBasal body
Plasmamembrane
0.1 µm
Cross section of basal body
Triplet
Outer microtubuledoublet
0.1 µm
Dynein arms
Centralmicrotubule
Cross-linkingproteins insideouter doublets
Radialspoke
Dynein “walking”
Microtubuledoublets ATP
Dynein arm
Wavelike motion
Cross-linkingproteins insideouter doublets
ATP
Anchoragein cell
Effect of cross-linking proteins
Microfilaments:● can exist as single filaments or in bundles;
● formed from the protein ACTIN;
● help the cell (or parts of the cell) to contract;
● they stabilize cell shape;
● Involved in “pinching” contractions during cell division;
● Involved in forming “pseudopodia” that enable some cells to move.
YellowYellow: nucleus: nucleus
GreenGreen: microfilaments throughout : microfilaments throughout cytoplasmcytoplasm
Cortex (outer cytoplasm):gel with actin network
Amoeboid movement
Inner cytoplasm: solwith actin subunits
Extendingpseudopodium
Muscle cell
Actin filament
Myosin filamentMyosin arm
Myosin motors in muscle cell contraction
Nonmovingcytoplasm (gel)
Cytoplasmic streaming in plant cells
Chloroplast
Streamingcytoplasm(sol)
Cell wall
Parallel actinfilaments
Vacuole
Intermediate Filaments
● Intermediate filaments range in diameter from 8–12 nanometers, larger than microfilaments but smaller than microtubules
● They support cell shape and fix organelles in place
● Intermediate filaments are more permanent cytoskeleton fixtures than the other two classes
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
Extracellular Structures:
● CELL WALL:
-semirigid structure
outside of cell membrane
of PLANT CELLS;
-consists of CELLULOSE
fibers + complex
polysaccharides
& proteins
-provides support, limits cell’s volume, and protects against fungi and/or microorganism infection.
Cell Walls of Plants
● Plant cell walls may have multiple layers:– Primary cell wall: relatively thin and flexible– Middle lamella: thin layer between primary walls
of adjacent cells– Secondary cell wall (in some cells): added
between the plasma membrane and the primary cell wall
● Plasmodesmata are channels between adjacent plant cells
Centralvacuole of cell
PlasmamembraneSecondarycell wall
Primarycell wall
Middlelamella
1 µm
Centralvacuole of cell
Central vacuoleCytosol
Plasma membrane
Plant cell walls
Plasmodesmata
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:– Support– Adhesion– Movement– Regulation
Extracellular Structures:● EXTRACELLULAR MATRIX:
– fibrous proteins such as COLLAGEN and glycoproteins are secreted by and surround cells;
– it holds cells together in tissues;– helps filter materials passing between different
tissues;– orients cell movement during
development;– involved in cell-cell signalling.
EXTRACELLULAR FLUID ProteoglycancomplexCollagen
fiber
Fibronectin
Integrin Micro-filaments
CYTOPLASM
Plasmamembrane
Polysaccharidemolecule
Carbo-hydrates
Coreprotein
Proteoglycanmolecule
Proteoglycancomplex
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
Interiorof cell
Interiorof cell
0.5 µm Plasmodesmata Plasma membranes
Cell walls
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
Tight junctions preventfluid from moving across a layer of cells
Tight junction
0.5 µm
1 µm
0.1 µm
Gap junction
Extracellularmatrix
Spacebetweencells
Plasma membranesof adjacent cells
Intermediatefilaments
Tight junction
Desmosome
Gapjunctions
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
5 µ
m
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