chapter 6: a tour of the cell
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Chapter 6: A Tour of the Cell. Technology to study cells. light microscopes – pass visible light through specimen and lenses. magnification – ratio of image size to actual size. resolution – clarity of image; minimum distance between two distinguishable points. - PowerPoint PPT PresentationTRANSCRIPT
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Chapter 6:A Tour of the Cell
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Technology to study cells
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light microscopes – pass visible light through specimen and lenses
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magnification – ratio of image size to actual size
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resolution – clarity of image; minimum distance between two
distinguishable points
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electron microscopes – focus beams of electrons through or onto specimen
– resolution 100x better than light microscopes
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scanning electron microscope (SEM) – studying external structures
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transmission electron microscope (TEM) – studying internal structures
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Cell Fractionation – take cells apart and isolate organelles
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Uses a centrifuge; spin test tubes very fast, separates cell components by size
and density
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Surface are to volume ratio:– limits cell size because as cells get bigger,
their volume increases faster than their surface area
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– surface area important for transport of substances through the membrane
microvilli in intestineincreases surface area for absorption
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All cells have:
• cytosol – semifluid substance containing organelles and dissolved nutrients
• plasma membrane – selective barrier
• chromosomes – packaged DNA
• ribosomes – make proteins
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Prokaryotic Cells
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Prokaryotic Cells
• smaller than eukaryotic cells• no membrane-bound organelles• no nucleus (nucleoid – region containing
prokaryotic DNA)• small ribosomes• circular DNA• plasmids
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Bacterial conjugation using pili
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Eukaryotic Cells
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nucleus – contains DNA
• nuclear envelope – double membrane that encloses nucleus
• nuclear pores – holes in the nuclear envelope. Allow passage of large molecules.
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• chromosomes – made of chromatin, a complex of proteins and DNA
• nucleolus – rRNA synthesized, ribosomes assembled
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ribosomes – synthesize proteins• made of ribosomal RNA
(rRNA) and protein• cells that synthesize many
proteins have many ribosomes
• either free-floating in cytosol (make proteins for cell’s use) or bound to rough ER (make proteins for secretion)
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endomembrane system• more than half the total membrane of the cell• consists of membranous tubules and sacs (cisternae)• lumen – interior cavity of cisternae
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smooth endoplasmic reticulum (ER)• no bound ribosomes• synthesizes lipids (phospholipids, oils,
steroids)• stores calcium ions, especially in muscles
(important to muscle contraction)
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Enzymes detoxify drugs and poisons, especially in the liver
• add hydroxyl groups to drugs; makes them more soluble
• drug tolerance due to proliferation of smooth ER in addicts; higher doses required to achieve the same effect
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rough endoplasmic reticulum (ER) • has bound ribosomes• continuous with nuclear envelope
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• helps in synthesis of secretory proteins (proteins made for secretion), especially glycoproteins – proteins that have carbohydrates on them
• adds carbohydrates to glycoproteins, sends them in transport vescicles (sacs of membrane) to Golgi
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• Also the membrane factory of cell; makes new membrane for itself that becomes vescicles; these eventually become part of cell membrane
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Golgi apparatus – products of ER modified, stored and then shipped
• flattened sacs (cisternae)
• cis face – receiving side
• trans face – shipping side
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• vescicles from ER fuse with cis face
and empy contents into lumen of
cisternae• products of ER modified in Golgi:
– modifies carbohydrates
– alters protein structure
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• Golgi makes some macromolecules• products transferred from one cisternae to
another, eventually arrive at trans face.• products sorted and “addressed” for where
they will go
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• vescicles bud off trans face and carry contents to cell membrane for export or to different parts of the cell
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Lysosomes – digest• membrane sac of hydrolytic enzymes• digests molecules and worn-out cell parts
(autophagy)
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phagocytosis – food particle engulfed by cell and contained
in vescicle
– vescicle merges with lysosome and
is digested
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Tay-Sachs disease – lysosomal disorder in humans, allows lipids to
accumulate in cells.
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Tay-Sachs
• Lipids accumulate in nervous tissue• Degeneration of mental and physical abilities• Seizures, paralysis• Death before age 4
Cherry-red spot on retina identifies Tay-Sachs
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Vacuoles
• membrane-bound sacs
• central vacuole – in plants, storage for nutrients and wastes, water
– membrane: tonoplast
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• food vacuoles – formed by phagocytosis
• contractile vacuoles – in protists, pump excess water out of cell
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Mitochondria – make cell energy • change molecular energy to cellular energy;
cell respiration
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• double membrane• outer membrane is
smooth• Inner membrane has
folds called cristae • intermembrane
space – between outer and inner membrane
• mitochondrial matrix – lumen within the inner membrane
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Chloroplasts – make carbohydrates
• a plastid (other plastids are amyloplasts (store starch in plants) and chromoplasts (contain pigments that color fruit and flowers)
• contain pigment chlorophyll
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• double membrane – outer membrane
smooth – inner membrane is
stacks of sacs called thylakoids• a stack of thylakoids is
a granum• fluid between granum
and outer membrame is stroma
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Peroxisomes
• sac containing enzymes that transfer hydrogen to oxygen, producing H2O2
• digestion of fats, detoxification of alcohol• not part of endomembrane system
(lysosomes are)
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Cytoskeleton
• support, maintain cell shape
• cell motility (movement): both movement of whole cell and parts of cell within.
• motor proteins – help cytoskeleton accomplish movement
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Microtubules • hollow tubes of 13 columns of tubulin dimers• 25 nm• -tubulin and -tubulin• cell shape (reists compression), cilia and
flagella, move chromosomes during cell division, organelle movement
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Microfilaments (actin filaments)
• 2 intertwined strands of actin• 7 nm • cell shape (resist force), muscle contraction,
cytoplasmic streaming, pseudopodia in amoeboid movement
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Intermediate filaments• thick cables of
fibrous protein• 8-12 nm • fibrous Keratin
protein• cell shape
(resist force), anchorage of nucleus and organelles
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Centosomes and centrioles• centrosome – region
near nucleus where microtubules grow out from
• centrioles – in animals, 9 sets of triplet microtubules that help organize mitotic spindle during cell division
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Cillia and flagella• 9 + 2 arrangement of microtubules• dynein arms are motor proteins
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flagella – a tail-like structure for cellular locomotion or moving liquid
past cell– made of microtubules
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cilia – a hair-like structure, for locomotion or moving liquid past cell
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• Dynein arms bend cilia and flagella• Dynein “walking”: arms of one microtubule
grip adjacent doublet, push it up, release, then repeat
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basal body – where cilium or flagellum is anchored to cell
• – 9 sets of triplet microtubules (9 x 3)• – a basal body of a sperm flagellum enters egg
and becomes a centriole
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Extracellular components of plants
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cell wall
• made of cellulose microfibrils and proteins. Protects, maintains shape, prevents too much water
• also in prokaryotes, fungi and some protists• primary cell wall – young cell wall. Thin and
flexible• secondary cell well – in woody plants. Grown
between membrane and primary wall.
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• middle lamella – between primary cell walls of adjacent cells. Rich in pectins (sticky polysaccharides). Glues cells together.
• plasmodesmata
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Animal Extracellular Matrix (ECM)
• Mostly glycoproteins; mainly collagen fibers
• Collagen embedded in a network of proteoglycans
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fibronectin – another glycoprotein in the ECM that binds integrins on cell
membrane
integrins – proteins that span the cell membrane and transmit info on changes
outside the cell to the cytoplasm
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• Changes in ECM my trigger changes in cells. • Integrins help relay signals to and from cells• Play role in coordinating behavior of all cells in
a tissue.
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Intercellular junctions
• Plasmodesmata (plants) – channels made by perforation in cell walls. Cytosol, water and nutrients passes through them, linking cells
• tight junctions (animals) – membranes of cells tightly pressed together, bound by proteins. Prevents leakage.
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tight juction
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• desmosomes (animals) – fasten cells together into a strong sheet. Like rivets.
• gap junctions (animals) – channels between cells through which flow ions, sugars, other molecules. Useful in cell communication.