Transcript Cell Membrane Structure & Function

Cell Membrane
Structure and
Function
Cystic Fibrosis
•most common autosomal recessive
disease affecting Caucasian populations
– •incidence of 1 in 2000 births
•Clinical features
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•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
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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)
extracellular fluid
(watery environment)
phospholipid
hydrophilic
heads
hydrophobic
tails
bilayer
hydrophilic
heads
cytoplasm
(watery environment)
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)
recognition protein
receptor protein
transport protein
binding site
phospholipid
bilayer
carbohydrate
phospholipid
cholesterol
protein filaments
cytoplasm (inside)
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Types of Membrane
Proteins
1. Transport proteins
–
regulate the movement of water-soluble
molecules across the membrane
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Channel proteins
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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
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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
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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
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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.
drop of dye
pure water
2 Dye molecules
diffuse into the
water; water
molecules diffuse
into the dye.
3 Both dye molecules
and water molecules
are evenly dispersed.
(a) simple diffusion
(extracellular fluid)
(cytoplasm)
Transport across
membranes
Passive transport…(cont.)
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2. Osmosis
• a. Isotonic
• b. Hypertonic
• c. Hypotonic
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(a)
selectively
permeable
membrane
H2O
free water
molecule: can
fit through pore
sugar
bound water molecules
clustered around sugar:
cannot fit through pore
pore
(b)
selectively permeable
membrane
sugar molecule
water molecule
pure water
bag
bursts
10 micrometers
(a) isotonic solution
equal movement of water
into and out of cells
(b) hypertonic solution
net water movement
out of cells
(c) hypotonic solution
net water movement
into cells
Transport across
membranes
Passive transport…(cont.)
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3. Facilitated diffusion
(b) facilitated diffusion through a channel
ions
proteins forming
permanent
hydrophilic channel
channel
protein
(c) facilitated diffusion through a carrier
amino acids,
sugars,
small proteins
carrier
protein
Carrier protein
has binding
site for
molecule.
(extracellular fluid)
(cytoplasm)
Molecule
enters binding
site.
Carrier protein changes
Carrier protein
shape, transporting molecule resumes original
shape.
across membrane.
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
ATP ATP
recognition
binding Transport protein
site
site
binds ATP and
Ca2+
Ca2+.
(cytoplasm)
Transport protein
Transport protein uses
resumes original
energy from ATP to
change shape and move shape.
ion across membrane.
(a)
pinocytosis
(extracellular fluid)
1
2
3
vesicle containing
extracellular
fluid
(cytoplasm)
cell
(b)
phagocytosis
food particle
pseudopod
1
2
3
particle
enclosed in vesicle
secreted
material
(extracellular fluid)
plasma membrane
plasma membrane
2
3
1
vesicle
(cytoplasm)
0.2 micrometer
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