cell membrane and function chapter 7 big idea #2: biological systems use energy to grow, reproduce,...
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Cell Membrane and Function
Chapter 7• Big Idea #2: Biological systems use energy to
grow, reproduce, and maintain dynamic homeostasis.
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Essential Knowledge
• 2B1:Cell membranes are selectively permeable due to their structure
• 2B2: Growth and dynamic homeostasis are maintained by the constant movement of
molecules across membranes
• 2B3: Eukaryotic cells maintain internal membranes that partition the cell into
specialized regions
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• Separates internal environment of cell from external environment.
• Is selectively permeable: some substances can cross more easily than others (small,
nonpolar, not charged particles)
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Semi-Permeability
• Is due to structure of membrane• Contains phosholipids, embedded proteins,
cholesterol, glycoproteins, and glycolipids
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Fig. 7-2
Hydrophilichead
WATER
Hydrophobictail
WATER
Made of a double phospholipid layer that has amphipathic molecules
(hydrophobic and hydrophilic areas)
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Fig. 7-3
Phospholipidbilayer
Hydrophobic regionsof protein
Hydrophilicregions of protein
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• Can be hydrophilic w/ charged and polar side groups.
• Can be hydrophobic with nonpolar side groups
Embedded Proteins
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Fig. 7-8
N-terminus
C-terminus
HelixCYTOPLASMICSIDE
EXTRACELLULARSIDE
Hydrophobic regions of an integral protein: made of 1 or more stretches of nonpolar amino acids, often
coiled into alpha helices
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Cholesterol• Steroid found in b/t
phospholipids.• Affects membrane
fluidity• High temps Less
fluid membrane.• Low temps Hinders
solidification of membrane by disrupting phospholipid packing
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Fig. 7-5a
(a) Movement of phospholipids
Lateral movement(107 times per second)
Flip-flop( once per month)
Membrane is fluid Moves
When Cold, Moves Less!!
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Cell to Cell Recognition
• Cells recognize each other by binding to surface molecules, often carbs, on membrane
• Membrane carbs may be covalently bonded to lipids (forming glycolipids) or to proteins
(forming glycoproteins)
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Permeability of the Lipid Bilayer
• Pass: Small, uncharged molecules and small nonpolar molecules (N2)
• Large hydrophilic, polar molecules (sugars) can cross w/ help of embedded channel or
transport proteins
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Aquaporins
• Channel protein that allows water to move across
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• Binds to molecules and changes shape to shuttle them across membrane…
extremely specific
Transport Proteins
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Cell Walls
• Provides a structural boundary and an internal permeability boundary.
Cell Wall
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• Bacteria Cell Wall: Made of peptidoglycan
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• Plant Cell Wall: Made of Cellulose
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• Fungi Cell Wall: Made of Chitin
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• Movement of molecules in and out of cell w/out
using energy.
• AKA Diffusion
Passive Transport
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• High to low concentrations!
• Often used for import of resources/ export of wastes
Concentration Gradient
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Osmosis• Passive transport process: movement
of water across cell membrane.• Again, it follows the magic rule!
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Lowerconcentrationof solute (sugar)
Fig. 7-12
H2O
Higher concentrationof sugar
Selectivelypermeablemembrane
Same concentrationof sugar
Osmosis
Osmosis Video Clip!
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Tonicity• Ability of a solution to cause a cell to gain or
lose water osmoregulation
• Isotonic solution: Solute concentration is the same as that inside the cell; no net water movement across the plasma membrane
• Hypertonic solution: Solute concentration is greater out of cell; cell loses water
• Hypotonic solution: Solute concentration is high inside cell; cell gains water
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Fig. 7-13
Hypotonic solution
(a) Animal cell
(b) Plant cell
H2O
Lysed
H2O
Turgid (normal)
H2O
H2O
H2O
H2O
Normal
Isotonic solution
Flaccid
H2O
H2O
Shriveled
Plasmolyzed
Hypertonic solution
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Facilitated Diffusion• A channel/carrier
protein carries molecules across (area of high to low conc.)
• Often charged and polar molecules cross here
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Fig. 7-15
EXTRACELLULAR FLUID
Channel protein
(a) A channel protein
Solute CYTOPLASM
Solute Carrier protein
(b) A carrier protein
Facilit
ate
d D
iffu
sio
n
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Active Transport• Energy-
requiring: materials move across cell membrane from low to high conc.
• Uses ATP• Done by
embedded proteins
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• Allows cells to maintain concentration gradients that differ from their surrounding
Ex: Sodium-Potassium Pump
http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_pump_works.html
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Fig. 7-17Passive transport
Diffusion Facilitated diffusion
Active transport
ATP
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Exocytosis• Internal vesicles fuse w/ plasma mebrane to
secrete large macromolecules out of cell.• Requires ATP!
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• Cell takes in macromolecules by forming new vesicles derived from plasma
membrane.
Endocytosis
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Eukaryotes Compartmentalize…
• Minimizes competing interactions and increases surface area.
• Keep reactions and enzymes localized• Bacteria and Archaea (ancient) can’t do this