Chapter 8—Membrane Structure & Function · Carbs—mmm…Delicious! I. Membrane Structure...
Transcript of Chapter 8—Membrane Structure & Function · Carbs—mmm…Delicious! I. Membrane Structure...
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Chapter 8—Membrane
Structure & Function
Proteins, Lipids, & a Splash of Carbs—mmm…Delicious!
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I. Membrane Structure
Phospholipids are—
amphipathic—containing
both a hydrophobic and a hydrophilic region
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History of the Plasma Membrane
1935—1970 1972
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Freeze-Fracture
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Membranes are Fluid
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Evidence for the Drifting of
Membrane Proteins
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Membranes are Mosaics
Each type of membrane has a unique collection of proteins & carbs
Membrane carbohydrates allow for cell to cell recognition (ex. glycolipids/glycoproteins)
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2 Types of Membrane Proteins
• Integral Proteins—– Penetrate the hydrophobic core of the lipid
bilayer
• Peripheral Proteins—– Not embedded in lipid bilayer
– Loosely bound to the surface of the membrane
– Held in place by cytoskeleton or ECM
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Transmembrane Protein
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Sidedness of the Plasma
Membrane
Membranes have distinct inside
and outside faces
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Functions of Membrane Proteins
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II. Traffic across Membranes
• Membranes are selectively permeable on the basis of:– Type of substance
– Amount of substance
– Rate of movement
• Lipid Bilayer—– Permeable to:
• hydrophobic molecules (hydrocarbons, CO2, O2)
– Not Permeable to:• Hydrophilic molecules (polar, ions, H2O, sugars)
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How do hydrophilic, polar
substances get into cells?
• Transport Proteins—
– Span the membrane
– Allow a certain substance to cross the
membrane (very selective)
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Passive Transport
Diffusion is a spontaneous process (no NRG input
required) that occurs due to thermal motion (heat)
Any substance will diffuse down its own concentration gradient (this increases entropy)
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Osmosis—passive transport of H2O
across a membrane
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Water always moves: Hypotonic → Hypertonic
High water potential → Low water potential
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Water Balance of Living Cells
Aquaporins—water channel proteins that cause osmosis
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Osmoregulation—control of water
balance
• Contractile
vacuole
in Paramecium
(lives in hypotonic
pondwater)
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Facilitated Diffusion—passive transport
using proteins
How is a transport protein similar to an enzyme?
--specific for one type of molecule (“substrate”)
--can be saturated (transporting at maximal rate)
--can be inhibited by an “imposter”
Difference? – T.P.s cause physical transport (not chemical reactions)
Channel Gated
Channel
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Active Transport—requires energy and
proteins
Sodium-
Potassium
Pump
(Animal Cells)
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Let’s Review…
(against the
concentration gradient)
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Electrochemical Gradients
• The combination of forces acting on an ion:– Chemical force—the ion’s concentration gradient– Electrical force—the effect of the membrane potential
on the ion’s movement• Membrane potential = voltage (separation of charges) across
a membrane – This is electrical potential energy
– -50 → -200 millivolts
(Generally the inside of a cell is negative compared to outside)So, anions tend to move….and cations tend to move…
Thus, ions diffuse down their electrochemical gradients…
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Electrogenic Pump—protein that stores energy by
generating voltage (charge separation) across a
membrane
Example: Proton Pump found in plants, bacteria, and fungi
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Active Transport—requires energy and
proteins
Electrogenicpumps store energy that can be used for cellular work
Example:
Na+/K+ pump stores negative
charge on the inside of cell
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Cotransport
Coupling the “downhill”diffusion of one
substance to the “uphill”
transport of another against its
concentration gradient
Example:
Sucrose/H+
cotransporter in plants
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But what about the ‘big’ stuff?
• Exocytosis—
– Secretion of
macromolecules by fusion of vesicles with
the plasma membrane
• Endocytosis—
– Uptake of macromolecules by
formation of vesicles from the plasma
membrane
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3 Types of Endocytosis
Phagocytosis—
”cellular eating”
Non-specific
Pinocytosis—
”cellular drinking”
Non-specific
Receptor-mediated endocytosis—
Very specific
Ligand binds to receptor