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Membrane Structure & Function (Learning Objectives)
• Review the basic function and biochemical composition of the plasma membrane. p p
• Learn the fluid state of membranes and the movement of its lipids and proteins.
• Explain the importance of cholesterol in maintaining fluidity of animal cells membranes.
• Review the mosaics of membrane structures that dictate its function.
• Summarize the functions of membrane proteins.
• Compare and contrast movement of small and large molecules across the plasma membrane.
• Explain the basis of selective membrane permeability and the distinguishing features of substances that can move across freely and those that require protein transporters.
• Compare and contrast passive transport (diffusion and facilitated diffusion) with active transport.
• Explain the driving force behind diffusion and osmosis. Anticipate the movement of water into & out of the cells as a function of the concentration of water soluble substances.
E l i th l f t i t t i i ifi l l th b• Explain the role of protein transporters in moving specific molecules across the membrane down their concentration gradient and uphill against that gradient.
• Describe the role of ATP and phosphorylation in active transport.
• Compare and contrast exocytosis with endocytosis and the three types of endocytosis• Compare and contrast exocytosis with endocytosis and the three types of endocytosis.
MEMBRANE STRUCTURE AND FUNCTION
Membranes divide the cell into compartments where different chemical cellular activities canwhere different chemical cellular activities can take place.
The plasma membrane is the outer boundary ofThe plasma membrane is the outer boundary of the cell and is selectively permeable
It t l th fl f b t i t t fIt controls the flow of substances into or out of the cell
Outside of cell
Cytoplasm
M 200
,000
Figure 5.10
TEM
Membranes are made of
h h li id bilCH2
CH
CH3
CHCH3N
+Hydrophilic head
phospholipids bilayerOne hydrophilic head
CH2CH3
OO O–P
OCH2CHCH2
Phosphategroup
Two hydrophobic tailsCH2
CH2
CH
CH2
CH2
CH
22
C O C OO O
CH2
CH2
CH2
CH2
CHCH2
CH2
CH2
CH2
CH2 SymbolCH2
CH2
CH2
CH2
CH2
CH2
CHCH
CH2CH
CH2
Symbol
CH2
CH2
CH2
CHCH2
CH2CH2CH2CH2CH2
Figure 5.11A
CH2
CH3
CH3
Hydrophobic tails
– The heads of the phospholipid bilayer face outward– The heads of the phospholipid bilayer, face outward and the tails face inward
WaterHydrophilicheads
Hydrophobictails
W
Figure 5.11B
Water
In addition to phospholipids membranesIn addition to phospholipids, membranes contain proteins that determine the function of the membraneof the membrane
Hydrophilic regionof proteinof protein
Phospholipidbilayerbilayer
Hydrophobic region of protein
Cholesterol is a steroid lipid with a carbon skeleton consisting of four fused carbon rings present in plasma membranes of ganimal cells
• Cholesterol is wedged between phospholipid molecules in the plasma membrane of animalsmolecules in the plasma membrane of animals cells.
• At warm temperatures, it restrains the movementAt warm temperatures, it restrains the movement of phospholipids and reduces fluidity.
• At cool temperatures, it maintains fluidity by p , y ypreventing tight packing.
Ch l t lCholesterolCholesterol within the animal cell membrane
A fluid mosaic of phospholipids and proteins
Membrane fluidity affects activity of membrane boundMembrane fluidity affects activity of membrane‐bound enzymes. http://www.insidecancer.org/
Fibers of the extracellular matrix
Carbohydrate(of glycoprotein)
Glycoprotein
Plasmamembrane
Glycolipid
Phospholipid
Figure 5.12Microfilamentsof cytoskeleton Cholesterol
Proteins
Cytoplasm
Functions of plasma membrane proteinsFunctions of plasma membrane proteins
SignalEnzymes
Signal
R tReceptorATP
Transport Enzymatic activity Signal transduction
Enzymes catalyze chemical reactionsEnzymes catalyze chemical reactions
Figure 5.13A
Receptors receive chemical messages from other cells
Messenger molecule
RReceptor
Activatedl l
Figure 5.13B
molecule
Transporters move substances across the bmembrane
ATP
Figure 5.13C
ATP
Traffic Across Membranes
A. Ions and monomers (small molecules)Move physically through the membrane
B Macromolecules and large particlesB. Macromolecules and large particlesmove across inside vacuoles & vesicles
Traffic of ions and monomers
Selective permeability depends on interaction of that molecule with the hydrophobic core and presence of specific proteins
• non‐polar molecules can pass:
• polar and ionic molecules and inorganic ions are assisted by membrane proteins
Traffic of ions and monomers
Selective permeability depends on interaction of that molecule with the hydrophobic core and presence of specific proteins
• non‐polar molecules can pass:
hydrocarbons, CO2, and O2
• polar and ionic molecules and inorganic ions are assisted by membrane proteins– nutrients (monomers of sugars and amino acids) and metabolic
waste products
N + K+ C 2+ d Cl– Na+, K+, Ca2+, and Cl‐
Fig 8 16 Both diffusion and facilitated diffusion are forms of passive transport of moleculesFig. 8.16 Both diffusion and facilitated diffusion are forms of passive transport of molecules down their concentration gradient, while active transport requires an investment of energy to move molecules against their concentration gradient.
Movement across membranes
I Passive transportI. Passive transportFrom an area of high concentration to one with lower concentration down awith lower concentration, down a concentration gradient, no ATP required
II. Active transportFrom an area of low concentration to oneFrom an area of low concentration to one with higher concentration, requires ATP
I. Passive transport is diffusion across a membrane‐ without work by the cellwithout work by the cell‐ Spreading from areas of high concentration to areas of low concentrationareas of low concentration
EquilibriumMembraneMolecules of dye EquilibriumMembraneMolecules of dye
EquilibriumFigure 5.14A
Figure 5.14B
I. Passive TransportI. Passive Transport
1. Simple diffusion (gases & hydrocarbons)
2. Osmosis diffusion of solvent (H2O)2. Osmosis diffusion of solvent (H2O)
3. Facilitated diffusion (via protein transporters)
Solvation of ionic compounds in waterSolvation of ionic compounds in water
http://programs.northlandcollege.edu/biology/Biology1111/animations/dissolve htmlBiology1111/animations/dissolve.html
Osmosis
http://physioweb.med.uvm.edu/bodyfluids/osmp //p y / y /osis.htm
Osmosis is the diffusion of water across a membraneWater travels from a solution of lower soluteWater travels from a solution of lower solute concentration to one of higher solute concentration
L Hi hEqual
Lowerconcentration
of solute
Higherconcentration
of solute
concentrationof solute
H2OSolute molecule
Selectively permeabley pmembrane
Watermolecule
Solute molecule with
Figure 5.16
cluster of water molecules
Net flow of water
Water balance between cells and their surroundings is crucial to organisms
Osmosis causes cells to shrink in hypertonic solutionsswell in hypotonic solutionsswell in hypotonic solutionsnot change in isotonic solutions
In isotonic solutions
Animal cells are normal, but plant cells are limp
Isotonic solution Hypotonic solution Hypertonic solution
H2O H2OH2O H2O
Isotonic solution Hypotonic solution Hypertonic solution
l(1) Normal (2) Lysed (3) Shriveled
Animalcell
H2O H2O H2O H2OPlasma
membrane
Plantcell
(4) Flaccid (5) Turgid(6) Shriveled
(plasmol ed)
Figure 5.17
( ) ( ) g(plasmolyzed)
Transport proteins may facilitate diffusion across b b idi h lmembranes by providing passages or channels
Solutemoleculemolecule
Transport
Figure 5.15
Transportprotein
II Active transport uses cellular energyII. Active transport uses cellular energy – Transport proteins can move solutes against a concentration gradientconcentration gradient
– ATP provides a phosphate group to change the shape of the transporter protein
Transportprotein
the transporter protein
PP PProtein
changes shapePhosphatedetaches
ATPADPSolute
Solute binding1 Phosphorylation2 Transport3 Protein reversion4Figure 5.18
B. Transport of large macromolecules across l b (E t i d E d t i )plasma membrane (Exocytosis and Endocytosis)
• Uptake of macromolecules is known asUptake of macromolecules is known as endocytosis
• Secretion or excretion of macromolecules is known as exocytosis e.g. insulin (protein hormone) by pancreatic cells.
Endocytosis and exocytosishttp://highered mcgrawhttp://highered.mcgraw-
hill.com/sites/0072437316/student_view0/chapter6/animations.html#r6/animations.html#
In exocytosis a vesicle fuses with the membraneIn exocytosis, a vesicle fuses with the membrane and expels its contents
Fluid outside cell
ProteinVesicle
CytoplasmFigure 5.19A
In endocytosis a membrane vesicle folds inwardIn endocytosis, a membrane vesicle folds inward enclosing material from the outside
Vesicle forming
Figure 5.19B
Endocytosis can occur in three waysEndocytosis can occur in three ways
• Phagocytosis
• Pinocytosis• Pinocytosis
• Receptor‐mediated endocytosis
Plasma membrane
Pseudopodium of amoebaFood being ingested Material bound to receptor proteins
PIT
00
,500
Phagocytosis Pinocytosis Receptor‐mediated endocytosis
CytoplasmTEM 54,00
TEM 96,
LM 230
Figure 5.19C
Faulty membranes can overload the blood with ycholesterol– Harmful levels of cholesterol can accumulate in theHarmful levels of cholesterol can accumulate in the blood if membranes lack cholesterol receptors
Ph h li id t lLDL particle Phospholipid outer layer
Vesicle
Cholesterol
Protein
ReceptorPlasma
b CytoplasmReceptorprotein
membrane
Figure 5.20