63
Cell Membrane Structure and
Cell Membrane Structure and
Function
Cystic Fibrosis
•most common autosomal recessive disease affecting Caucasian populations – •incidence of 1 in 2000 births
•Clinical features – •pulmonary obstruction and infection – •recurrent and persistent infections – •exocrine pancreatic dysfunction – •infertility, especially in males
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Diagnosis
• failure to thrive -child fails to gain weight despite a good appetite
• sweat test -chloride concentration greater than 60 meq/liter
• life expectancy now often >30 years
• this was the disease most people thought would be a good candidate for gene therapy
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Metabolic Basis of Disease• defect in a single gene on chromosome 7 that
encodes a cAMP-regulated chloride channel
• called Cystic Fibrosis Transmembrane Conductance Regulator (CFTR)
• resides on the apical surface of epithelial cells lining the airways, intestines, vas deferens, sweat ducts, and pancreatic ducts
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Genetics and Epidemiology• autosomal recessive
• 4-5% of Americans have at least one CF allele
• >900 different mutations (.F508, ~70% of CF alleles)
• ~50% of the patients are homozygous for the .F508 mutation
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Consequences of Disease
• Loss of CFTR function
– fluid secretion is insufficient
– protein portions of the secretions become viscous
– results in obstruction in the ducts and eventual organ dysfunction
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Lung Disease• accounts for much of the morbidity and nearly all the
mortality from the disease
• bacterial infections become soon after birth, and remain difficult to stop
• Pseudomonas aeruginosais the main agent in patients 18 or older
• bacterial resistance to antibiotics results in mucoid coating of lungs, and subsequent decline in lung
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Cystic Fibrosis
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Membrane Functions
• Isolate the cell’s contents from the external environment
• Regulate traffic in and out of the cell
• Communicate with other cells
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II. Plasma membrane structure and functions
The phospholipid bilayer and isolation
1. Impermeable to water-soluble and polar molecules, ions
2. Permeable to small and nonpolar molecules
3. Lipids oriented with polar heads facing out
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tails(hydrophobic)
head(hydrophilic)
hydrophobictails
hydrophilicheads
hydrophilicheads
extracellular fluid(watery environment)
cytoplasm(watery environment)
phospholipid
bilayer
Membrane Structure and Function
• Membranes are “fluid mosaics” with proteins embedded in or attached to the membrane
• Proteins can move within the fluid lipid bilayer
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extracellular fluid (outside)
cytoplasm (inside)protein filaments
recognition protein receptor protein transport protein
binding sitephospholipid
bilayer
phospholipid cholesterolcarbohydrate
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Types of Membrane Proteins
1. Transport proteins
– regulate the movement of water-soluble molecules across the membrane
• Channel proteins
• Carrier proteins
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Types of Membrane Proteins
2. Receptor Proteins
• trigger cellular response when specific molecules bind to them
Nervous system
Endocrine system
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Types of Membrane Proteins
3. Recognition proteins
• act as ID tags and cell surface attachment sites
– the immune system
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• Key Classes of Membrane Proteins •• A.Transport channels-Intergral
– 1. Membrane selects what substances will enter – 2. Take up molecules present in high concentration– 3 Part of protein extends through bilayer – 4. May be non polar helix beta-pleated sheets of non polar
amino acids – 5. Non polar portion held within interior of bilayer – 6. Polar ends protrude from both sides of membrane
• B. Enzymes – 1. Chemical reactions carried out on interior surface of membrane
• 2. Enzymes attached directly to membrane-carry out actions within the cell
• 3. Cell surface receptors-glycoproteins, glycolipids, oligosaccharides
– a. Membranes sensitive to chemical messages – b. Receptor proteins on surface act as antennae-
interpresurroundings– c. Receptors for extra cellular signals are also
single-pass anchors – d. Portion of receptor that sticks outward binds
with molecules – e. Binding induces changes in part of protein on
the inside
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• C. Glycoproteins-MH-ID tags -recognition
– a.self recognition-cells to other cells – b. Creates proteins/carb chains shaped for specific person
– c. self recognition– 2. Glycolipid-
» a. Tissue recognition» b. Lipid/carb chain» c. Specific for each tissue» d. Blood types
• D. Cell adhesion proteins – a. Cells use certain proteins to glue
themselves to one another – b. Some are detachable, others are permanent
• 4. Attachments to the cytoplasm-peripheral – a. Surface proteins may interact with other
cells – b. Often linked to cytoskeleton by proteins
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The Extracellular Matrix
• Structure: – Glycoproteins– Collagen– Glycolipids
• Functions:– Support, adhesion, movement– In Embryonic development - guides
movement of cells
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Intracellular Junctions
Tight junctions– Epithelial cells (surface): Seal cells together– Prevent molecules from ‘leaking’ through between
cells --> Keep integral proteins in place
• Adherins junctions:– Transmembrane proteins that span 2 cells– Hold cardiac cells together – May be responsible for contact inhibition
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More junctions• Gap Junctions: permit flow of substances b/w cells
flow of ions – allow all cells of heart to contract @ same time– allow muscles in uterus to contract simultaneously during
childbirth
• Desmosomes: patches that hold cells together– Breakdown of desmosomes may contribute to metastasis of
cancer
• Plasmodesmata:– Allow for continuous stream of cytoplasm b/w plant cells
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Transport across membranes
• Passive transport is a function of molecular size, lipid solubility, and size of the concentration gradient
1. Simple diffusion
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1 A drop of dye is placed in water.
2 Dye molecules diffuse into the water; water molecules diffuse into the dye.
drop of dye
pure water
3 Both dye molecules and water molecules are evenly dispersed.
(a) simple diffusion
(extracellular fluid)
(cytoplasm)
Transport across membranes
Passive transport…(cont.)
– 2. Osmosis• a. Isotonic • b. Hypertonic • c. Hypotonic
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free water molecule: can fit through pore
bound water moleculesclustered around sugar:cannot fit through pore
pore
sugar
H2O
bagbursts
selectively permeable membrane
water molecule
pure water
sugar molecule
(a)
selectively permeable membrane
(b)
(a) isotonic solution (b) hypertonic solution (c) hypotonic solution10 micrometers
equal movement of waterinto and out of cells
net water movement out of cells
net water movement into cells
Transport across membranes
Passive transport…(cont.)
– 3. Facilitated diffusion
ions
(b) facilitated diffusion through a channel
channelprotein
proteins formingpermanenthydrophilic channel
(extracellular fluid)
(cytoplasm)Carrier protein has binding site for molecule.
amino acids,sugars,
small proteins
(c) facilitated diffusion through a carrier
carrier protein
Molecule enters binding site.
Carrier protein changesshape, transporting moleculeacross membrane.
Carrier protein resumes original shape.
Transport across membranes
Energy-requiring transport
1. Active transport • Ion gradients and energy
production
2. Endocytosis
3. Exocytosis
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(extracellular fluid)
Transport protein binds ATP and Ca2+.
Transport protein uses energy from ATP to change shape and moveion across membrane.
Transport protein resumes original shape.
(cytoplasm)
transport protein
ATP binding
site
recognitionsite
ATP
Ca2+
(extracellular fluid)
(cytoplasm)
food particle
particle enclosed in vesicle
phagocytosis
vesicle containing extracellular fluid
cell
pseudopod
pinocytosis
(a)
(b)
133
2
1 32
(extracellular fluid)
(cytoplasm)
vesicle
plasma membrane
1
32
plasma membranesecretedmaterial
0.2 micrometer
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