chapter 8—membrane structure & function · carbs—mmm…delicious! i. membrane structure...
TRANSCRIPT
Chapter 8—Membrane
Structure & Function
Proteins, Lipids, & a Splash of Carbs—mmm…Delicious!
I. Membrane Structure
Phospholipids are—
amphipathic—containing
both a hydrophobic and a hydrophilic region
History of the Plasma Membrane
1935—1970 1972
Freeze-Fracture
Membranes are Fluid
Evidence for the Drifting of
Membrane Proteins
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)
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
Transmembrane Protein
Sidedness of the Plasma
Membrane
Membranes have distinct inside
and outside faces
Functions of Membrane Proteins
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)
How do hydrophilic, polar
substances get into cells?
• Transport Proteins—
– Span the membrane
– Allow a certain substance to cross the
membrane (very selective)
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)
Osmosis—passive transport of H2O
across a membrane
Water always moves: Hypotonic → Hypertonic
High water potential → Low water potential
Water Balance of Living Cells
Aquaporins—water channel proteins that cause osmosis
Osmoregulation—control of water
balance
• Contractile
vacuole
in Paramecium
(lives in hypotonic
pondwater)
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
Active Transport—requires energy and
proteins
Sodium-
Potassium
Pump
(Animal Cells)
Let’s Review…
(against the
concentration gradient)
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…
Electrogenic Pump—protein that stores energy by
generating voltage (charge separation) across a
membrane
Example: Proton Pump found in plants, bacteria, and fungi
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
Cotransport
Coupling the “downhill”diffusion of one
substance to the “uphill”
transport of another against its
concentration gradient
Example:
Sucrose/H+
cotransporter in plants
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
3 Types of Endocytosis
Phagocytosis—
”cellular eating”
Non-specific
Pinocytosis—
”cellular drinking”
Non-specific
Receptor-mediated endocytosis—
Very specific
Ligand binds to receptor