Cell Membrane Function The purpose of cells membranes is to?
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Transcript of Cell Membrane Function The purpose of cells membranes is to?
Cell Membrane Function
• The purpose of cells membranes is to?
Large and small substances move across cell membranes in fundamentally different ways.
• Small molecules-
A. Passive transport (simple and facilitated diffusion)
B. Active transport
• Large molecules(endo and exocytosis)
A membrane’s molecular organization results in selective permeability
• Membrane permeability is influenced by size, chemical composition and charge/polarity of the molecule trying to cross the membrane
a. Membranes are more permeable to small molecules than larger ones
b. Membranes are more permeable to hydrophobic molecules
c. Membranes are most permeable to uncharged/nonpolar molecules
Simple Diffusion
• Defined-the spontaneous net movement of a substance from an area of its higher concentration to an area of its lower concentration until an equilibrium is achieved
• Diffusion occurs because of the second law of thermodynamics
LE 7-11a
Molecules of dye Membrane (cross section)
WATER
Net diffusion Net diffusion Equilibrium
Diffusion of one solute
LE 7-11b
Net diffusion Net diffusion Equilibrium
Diffusion of two solutes
Net diffusion Net diffusion Equilibrium
Osmosis
• Osmosis is a special case of diffusion
• It involves the diffusion of water across a differentially permeable membrane
• Cell and tissues can gain or lose water by osmosis depending on the type of environment they exist in
Effect of solute on cell solutions
• The solute concentration of the environment determines whether a cell gains or loses water
• The addition of solute lowers the concentration of water (makes it less than 100%)
• Three terms describe the tendency of one solution to gain or lose water to another solution
a. Hypertonic (salty) solutions tend to gain water from hypotonic solutions (less salty)
b. Isotonic solutions gain and lose water to one another at the same rate.
LE 7-12Lowerconcentrationof solute (sugar)
Higherconcentrationof sugar
Same concentrationof sugar
Selectivelypermeable mem-brane: sugar mole-cules cannot passthrough pores, butwater molecules can
H2O
Osmosis
LE 7-UN140
“Cell”
0.03 M sucrose
0.02 M glucose
0.01 M sucrose
0.01 M glucose
0.01 M fructose
Environment
Cell survival depends on balancing water uptake and loss
• Plant and animal responses to being placed in
• A. hypertonic solution
• B. hypotonic solutions
• C. Isotonic solutions
LE 7-13
Animalcell
Lysed
H2O H2O H2O
Normal
Hypotonic solution Isotonic solution Hypertonic solution
H2O
Shriveled
H2OH2OH2OH2OPlantcell
Turgid (normal) Flaccid Plasmolyzed
LE 7-14
Filling vacuole50 µm
50 µmContracting vacuole
Traffic across membranes
• A membrane’s molecular organization results in selective permeability
• Passive transport is diffusion across a membrane• Osmosis is the passive transport of water• Cell survival depends on balancing water uptake and loss• The solute concentration of the environment determines
whether a cell gains or loses water• Specific proteins facilitate the passive transport of selected
solutes (facilitated diffusion)• Active transport is the pumping of solutes against their gradients• Some ion pumps generate voltage across membranes• In cotransport, a membrane protein couples the transport of one
solute to another• Exocytosis and endocytosis transport large molecules
How do small molecules move across cell membranes?
• Passive Transport is diffusion across a membrane
A. Simple diffusion-membrane is permeable; highlow concentration; no energy required
B. Facilitated diffusion-diffusion-membrane is impermeable (carrier molecule required) highlow concentration; no energy required
LE 7-17a
Diffusion Facilitated diffusion
Passive transport
Facilitated Diffusion
• Specific proteins facilitate the passive transport of selected solutes (facilitated diffusion)
• Hydrophilic channels
• Rotating carriers (conformational changes)
LE 7-15a
EXTRACELLULARFLUID
Channel protein Solute
CYTOPLASM
LE 7-15b
Carrier protein Solute
Active Transport
• Active transport is the pumping of solutes against their gradients
A. Membrane is impermeable (carrier required); movement from low concentration to high concentration; energy required
B. Sodium/potassium pump (neurons)
C. Plants often actively transport nutrients from soil into the root cell (advantage of doing this?)
LE 7-17b
ATP
Active transport
Solution A (.2M glucose) is separated from solution B (.4 M glucose) by a membrane which is impermeable to glucose .
Which solution is hypertonic?
1 2 3 4
21%
9%12%
58%1. A
2. B
3. Both A and B
4. Neither A nor B
Solution A (.2M glucose) is separated from solution B (.4 M glucose) by a membrane which is impermeable to glucose .
Which solution will have a net gain of water?
1 2 3 4
47%
11%
3%
39%
1. A
2. B
3. Both A and B
4. Neither A nor B
In the Na+/K+ pump, the ATPase enzyme is activated by
1 2 3 4 5
16%
6%
29%29%
19%
1. Release of K+
2. Binding of K+
3. Binding of Na+
4. Release of Na+
5. phosphorylation
Figure 8.15 The sodium-potassium pump: a specific case of active transport
LE 7-16
Cytoplasmic Na+ bonds tothe sodium-potassium pump
CYTOPLASMNa+
[Na+] low[K+] high
Na+
Na+
EXTRACELLULARFLUID
[Na+] high[K+] low
Na+
Na+
Na+
ATP
ADP
P
Na+ binding stimulatesphosphorylation by ATP.
Na+
Na+
Na+
K+
Phosphorylation causesthe protein to change itsconformation, expelling Na+
to the outside.
P
Extracellular K+ bindsto the protein, triggeringrelease of the phosphategroup.
PP
Loss of the phosphaterestores the protein’soriginal conformation.
K+ is released and Na+
sites are receptive again;the cycle repeats.
K+
K+
K+
K+
K+
LE 7-18
H+
ATP
CYTOPLASM
EXTRACELLULARFLUID
Proton pump
H+
H+
H+
H+
H+
+
+
+
+
+
–
–
–
–
–
Co-transport
• In co-transport, a membrane protein couples the transport of one solute to another
• In plants, transport of sucrose into cells is coupled to the active transport of H+ ions out of the cell
LE 7-19
H+
ATP
Proton pump
Sucrose-H+
cotransporter
Diffusionof H+
Sucrose
H+
H+
H+
H+
H+
H+
+
+
+
+
+
+
–
–
–
–
–
–
Movement of large molecules/cells into and out of cells
• Exocytosis and endocytosis transport large molecules into and out of cells
• Exocytosis-out
• Endocytosis-in
a. Pinocytosis
b. Phagosytosis
c. Receptor-mediated endocytosis
LE 7-20b
Plasmamembrane Pinocytosis
vesicles forming(arrows) in a celllining a smallblood vessel(TEM).
0.5 µm
Vesicle
PINOCYTOSIS
LE 7-20a
CYTOPLASM
Pseudopodium
“Food” orother particle
EXTRACELLULARFLUID
Bacterium
Food vacuole
An amoeba engulfing a bacterium viaphagocytosis (TEM)
Pseudopodiumof amoeba
1 µm
Food vacuole
PHAGOCYTOSIS
LE 7-20c
Receptor
RECEPTOR-MEDIATED ENDOCYTOSIS
Ligand
Coatedpit
Coatedvesicle
Coat protein
Coat protein
Plasmamembrane
0.25 µm
A coated pitand a coatedvesicle formedduringreceptor-mediatedendocytosis(TEMs).
Familial Hypercholesterolemia
• Symptoms/consequences
• Causes
• Genetics
Membrane Structure and Function
• Membrane structure• Membrane models have evolved to fit
new data (science as a process)• A membrane is a fluid mosaic of lipids,
proteins and carbohydrates• There is a lot of experimental evidence
that favors the fluid mosaic model of membrane structure.
History of Membrane Models
• Overton (1875) –Membranes contain lipids (like dissolve like)
• Langmuir(1917)-Membranes have amphipathic lipids (phospholipids)
• Gorter and Grendel(1925)-Phospholipid bilayer
• Davson and Danielli (1935)-Phospholipids and proteins (sandwich)
Figure 8.1 Artificial membranes (cross sections)
LE 7-2
Hydrophilichead
Hydrophobictail
WATER
WATER
Figure 8.2 Two generations of membrane models
History of Membrane Models (continued)
• Robertson (1950)-Electron micrographs showing “trilaminate” structure
• Problems with current models
• Singer and Nicholson (1975)-Fluid mosaic model
Figure 8.19 The three types of endocytosis in animal cells
Fluid Mosaic Model
• Consistent with all observations of membrane properties to date
Figure 7-01
LE 7-5
Lateral movement(~107 times per second)
Flip-flop(~ once per month)
Viscous
Movement of phospholipids
Fluid
Unsaturated hydrocarbontails with kinks
Membrane fluidity
Saturated hydro-carbon tails
Cholesterol
Cholesterol within the animal cell membrane
In sucrose co-transport in plants, the active transport of sucrose into plant cells is couple to
1 2 3 4
25% 25%25%25%1. Facilitated diffusion
2. ATP hydrolysis
3. A proton pump
4. 1 and 3
This model of membrane structure consisted of a phospholipid bilayer sandwiched between 2 layers of
protein:
1 2 3 4 5
20% 20% 20%20%20%1. Gorter and Grendle
2. Davson and Danielli
3. Singer and Nicholson
4. Overton
5. Robertson
An increased synthesis of phospholipids containing unsaturated fatty acids may be an adaptation by plants to:
1 2 3 4 5
20% 20% 20%20%20%1. Predators
2. Decreasing sunlight
3. Hypertonic environments
4. Cooling temperatures
5. Warming temperatures
LE 7-4
Knife
Cytoplasmic layerExtracellular layer
Cytoplasmic layer
Plasmamembrane
Extracellular layer
Proteins
LE 7-6
Membrane proteins
Mixedproteinsafter1 hourHybrid cell
Human cell
Mouse cell
Figure 8.9 Some functions of membrane proteins
LE 7-8
EXTRACELLULARSIDEN-terminus
C-terminusCYTOPLASMICSIDE
Helix