cell structure
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Cell Structure
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Cell Theory
• All organisms are composed of one or more cells.
• Cells are the smallest living units of all living organisms.
• Cells arise only by division of a previously existing cell.
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Cell Characteristics• Genetic material
– single circular molecule of DNA in prokaryotes
– double helix located in nucleus in eukaryotes – nuclear envelope (double membrane
• Cytoplasm fills cell interior – – sugars, amino acids, – proteins - organelles
• Plasma membrane encloses – the cell – phospholipid bilayer
Phospholipid
Membraneproteins
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Generalized Eukaryotic Cell
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Cell Size
• Most cells are relatively small because as size increases, volume increases much more rapidly.
– longer diffusion time
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Visualizing Cells
• Resolution - minimum distance two points can be apart and still be distinguished as two separate points
– Compound microscopes - magnify in stages using multiple lenses
– Transmission electron microscope - electrons transmitted through specimen
– Scanning electron microscope - electrons beamed onto surface of the specimen
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Visualizing Cells
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Prokaryotic Cells• Simplest organisms
– Cytoplasm is surrounded by plasma membrane and encased in a rigid cell wall composed of peptidoglycan.
no distinct interior compartments gram-positive – thick single layer wall that
retains a violet dye from Gram stain procedure gram-negative – multilayered wall does not
retain dye Susceptibility of bacteria to antibiotics
depends on cell wall structure.
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Prokaryotic Cells
• Some use flagellum for locomotion– threadlike structures protruding from cell
surfaceBacterial cell wall
Flagellin
Rotarymotor
Sheath
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Eukaryotic Cells
• Characterized by compartmentalization by an endomembrane system, and the presence of membrane-bound organelles.
– Central vacuole – plants, storage– Vesicles (smaller)– Chromosomes - DNA and protein– Cytoskeleton (internal protein scaffolding)– Cell walls – plants and fungi
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Nucleus• Repository for genetic material• Directs activities of the cell• Usually single, some cells several, RBC none
– Nucleolus - region of intensive ribosomal RNA synthesis
• Surface of nucleus bound by two phospholipid bilayer membranes
– nuclear membrane– Nuclear pores – protein gatekeepers
Usually proteins going in and RNA going out
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Nucleus
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Chromosomes
• DNA of eukaryotes is divided into linear chromosomes.
– exist as strands of chromatin, except during cell division
– associated with packaging histones, packaging proteins
nucleosomes
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Endomembrane System• Compartmentalizes cell, channeling passage
of molecules through cell’s interior.– Endoplasmic reticulum
Rough ER - studded with ribosomes Smooth ER - few ribosomes
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Endoplasmic reticulum
• Largest internal membrane• Composed of Lipid bilayer• Serves as system of channels from the
nucleus• Functions in storage and secretion• Rough ER is “rough” because of associated
ribosomes (sites of protein synthesis• Smooth ER - lack associated ribosomes –
contained embedded enzymes, catalyze synthesis of carbohydrate and lipid molecules
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Endomembrane System• Golgi apparatus
– collection of Golgi bodies collect, package, and distribute molecules
synthesized at one location in the cell and utilized at another location
Front - cis , Back – trans Cisternae – stacked membrane folds
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Vesiclebuddingfrom roughendoplasmicreticulum
Fusionof vesiclewith Golgiapparatus
Migratingtransportvesicle
Protein
Proteins
Transportvesicle
Golgiapparatus
Cisternae
Ribosome
trans face
cis face
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Endomembrane System• Vesicles
– Lysosomes - membrane-bound vesicles containing digestive enzymes – from Golgi
– Microbodies - enzyme-bearing, membrane-enclosed vesicles.
Peroxisomes - contain enzymes that catalyze the removal of electrons and associated hydrogen atoms
Peroxisome – named for hydrogen peroxide produced as a by-product
Enzyme breaks down to water and oxygen
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Cytoplasm
PhagocytosisFood
vesicleGolgi
apparatus
Lysosomes
Plasmamembrane
Digestion ofphagocytizedfood particles
or cells
Endoplasmicreticulum
Transportvesicle
Old or damagedorganelle
Breakdownof old
organelleExtracellularfluid
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Ribosomes
• Ribosomes are RNA-protein complexes composed of two subunits that join and attach to messenger RNA.
– site of protein synthesis– assembled in nucleoli
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Organelles With DNA
• Mitochondria – bounded by exterior and interior
membranes– interior partitioned by cristae
• Chloroplasts– have enclosed internal compartments of
stacked grana, containing thylakoids– found in photosynthetic organisms
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Mitochondria
A. "Powerhouse of the cell" - cellular metabolism
B. Structure- outer and inner membranes, cristae
C. Have their own DNA
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Chloroplasts
•Chloroplasts are larger and more complex than mitochondria
•Grana – closed compartments of stacked membranes
•Thylakoids – disc shaped structure – light capturing pigment
•Stroma – fluid matrix
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Endosymbiosis
• Endosymbiotic theory suggests engulfed prokaryotes provided hosts with advantages associated with specialized metabolic activities.
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Theory of Endosymbiosis
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Evidence for the endosymbiont theory is that mitochondria and chloroplasts:
- Are appropriate size to be descendants of eubacteria. - Have inner membranes similar to those on prokaryotic
plasma membranes. - Replicate by splitting, as in prokaryotes. - DNA is circular and different from the DNA of the cell's
nucleus. - Contain their own components for DNA transcription and
translation into proteins . - Have ribosomes similar to prokaryotic ribosomes. - Molecular systematics lend evidence to support this theory. - Many extant organisms are involved in endosymbiotic
relationships.
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Cytoskeleton
• Network of protein fibers supporting cell shape and anchoring organelles
– Actin filaments cell movement
– Microtubules Hollow tubes Facilitate cell movement Centrioles – barrel shaped organelles occur in pairs – help assemble animal cell’s microtubules
– Intermediate filaments Stable - don’t break down
Actin
Microtubules
Intermediate filaments
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Cytoskeleton
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Plant Cells
• Central vacuole – often found in the center of a plant, and
serves as a storage facility for water and other materials
• Cell wall– primary walls – laid down while cell is
growing– middle lamella – glues cells together– secondary walls – inside the primary cell
walls after growth
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Plant Cell
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Animal Cells
• Animal cells lack cell walls.– form extracellular matrix
provides support, strength, and resilience
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