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Chapter 7 notes
Membrane Structure and Function
Concept 7.1
Most abundant lipids in membranes are phospholipids.- phospholipids are amphipathic (head is hydrophilic, tail is hydrophobic )
Phospholipids and proteins are arranged in the “fluid mosaic model”: membrane is fluid w/ proteins embedded in or attached to the bilayer
Concept 7.1
Hydrophilichead
WATER
Hydrophobictail
WATER
Concept 7.1
Phospholipids and proteins are arranged in the “fluid mosaic model”: membrane is fluid w/ proteins embedded in or attached to the bilayer- disproved the Davson-Danielli “sandwich” model
Concept 7.1
Phospholipidbilayer
Hydrophobic regionsof protein
Hydrophilicregions of protein
Concept 7.1
The membrane is fluid-membranes are not static sheets of molecules locked in place-the membrane is held together primarily by hydrophobic interactions
Concept 7.1
(a) Movement of phospholipids
Lateral movement(107 times per second)
Flip-flop( once per month)
Concept 7.1
(b) Membrane fluidity
Fluid
Unsaturated hydrocarbontails with kinks
Viscous
Saturated hydro-carbon tails
Concept 7.1
Membranes are mosaics of structure and function- proteins are embedded in the fluid matrix; the lipid bilayer is the main fabric of the membrane, but proteins determine its specific fcn.
Concept 7.1Fibers ofextracellularmatrix (ECM)
Glyco-protein
Microfilamentsof cytoskeleton
Cholesterol
Peripheralproteins
Integralprotein
CYTOPLASMIC SIDEOF MEMBRANE
GlycolipidEXTRACELLULARSIDE OFMEMBRANE
Carbohydrate
Concept 7.1
Two major types of membrane proteins:- Integral proteins: penetrate the hydrophobic core of the bilayer; many are transmembrane proteins
- Peripheral proteins: appendages loosely bound to the surface of the membrane
Concept 7.1
N-terminus
C-terminus
HelixCYTOPLASMICSIDE
EXTRACELLULARSIDE
Concept 7.1
Membrane carbohydrates are important for cell-cell recognition- cell-cell recognition is the ability of a cell to distinguish one type of neighboring cell from another- membrane carbohydrates are usually oligosaccharides (can vary greatly)
Concept 7.1
(a) Transport (b) Enzymatic activity (c) Signal transduction
ATP
Enzymes
Signal transduction
Signaling molecule
Receptor
Concept 7.1
(d) Cell-cell recognition
Glyco-protein
(e) Intercellular joining (f) Attachment to the cytoskeleton and extracellular matrix (ECM)
Concept 7.2
Hydrophobic molecules can cross the bilayer with ease. However, ions and polar molecules cannot pass through because they are hydrophilic.- proteins play keys roles in regulating transportation.
Concept 7.2
Transport proteins: allow hydrophilic molecules to enter and exit the cell.
The selective permeability of a membrane depends on the specific transport proteins built into the membrane.
Concept 7.3
Passive transport involves diffusion across a membrane.- Diffusion: the tendency for molecules of any substance to spread out into available space- any substance will move down a [gradient]. [high] [low]
Concept 7.3
Molecules of dye Membrane (cross section)
WATER
Net diffusion Net diffusion
(a) Diffusion of one solute
Equilibrium
Concept 7.3
(b) Diffusion of two solutes
Net diffusion
Net diffusion
Net diffusion
Net diffusion
Equilibrium
Equilibrium
Concept 7.3
Passive transport: diffusion of a substance across a biological membrane. (no energy is used)
Osmosis is the passive transport of water- sln. w/ a higher [solute] = hypertonic- sln. w/ a lower [solute] = hypotonic- slns. w/ equal [solute] = isotonic
Concept 7.3
Concept 7.3
Organisms without cell walls that live in hypertonic or hypotonic environments must have adaptations for osmoregulation, the control of water balance
Concept 7.3
Organisms with cell walls- turgid (very firm) when placed in a hypotonic sln.- flacid (limp) if the sln. is isotonic- plasmolysis (shriveled) occurs when put in a hypertonic sln.
Concept 7.3Hypotonic 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
Concept 7.3
Facilitated diffusion: passive transport of molecules through transport proteins- each protein is specific for the solute it transports
Concept 7.3EXTRACELLULAR FLUID
Channel protein
(a) A channel protein
Solute CYTOPLASM
Solute Carrier protein
(b) A carrier protein
Concept 7.4
Active transport: movement of molecules across a membrane against the gradient (uses ATP)- sodium-potassium pump: movement of 3 Na+ for every 2 K+ ions
Concept 7.4
2
EXTRACELLULAR
FLUID [Na+] high [K+] low
[Na+] low [K+] high
Na+ Na+
Na+
Na+ Na+
Na+
CYTOPLASM ATP
ADP P
Na+ Na+
Na+
P 3
K+
K+ 6
K+
K+
5 4
K+
K+
P P
1
Concept 7.4
Some ion pumps generate voltage across membranes- membrane potential: the voltage across a membrane- electrogenic pump: a transport protein that generates voltage across a membrane (ex. Sodium-potassium pump)
Concept 7.4
The main electrogenic pump for plants and fungi is a proton pump which transports H+ ion out of the cell.
Concept 7.4
EXTRACELLULARFLUID
H+
H+
H+
H+
Proton pump
+
+
+
H+
H+
+
+
H+
–
–
–
–
ATP
CYTOPLASM
–
Concept 7.4
In cotransport, a ATP powered pump can drive the transport of other solutes.1) active transport of a substance against a gradient2) cotransport through a protein w/ 2nd substance
Concept 7.4
Proton pump
–
–
–
–
–
–
+
+
+
+
+
+
ATP
H+
H+
H+
H+
H+
H+
H+
H+
Diffusionof H+Sucrose-H+
cotransporter
Sucrose
Sucrose
Concept 7.5
Exocytosis: the secretion of macromolecules by the fusion of vesicles with the plasma membrane
Endocytosis: the cell takes in macromolecules by forming new vessicles- 3 types: phagocytosis, pinocytosis, and receptor-mediated endocytosis
PHAGOCYTOSIS
CYTOPLASM EXTRACELLULARFLUID
Pseudopodium
“Food” orother particle
Foodvacuole Food vacuole
Bacterium
An amoeba engulfing a bacteriumvia phagocytosis (TEM)
Pseudopodiumof amoeba
1 µm
Concept 7.5