Ch5 Cell Membranes 3f5w1r
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1
Chapter 5
Membrane Structure, Synthesis and Transport Key Concepts:
Membrane Structure
Fluidity of Membranes
Synthesis of Membrane Components
Membrane Transport
Transport Proteins
Exocytosis and Endocytosis 2
Membrane: The fluid mosaic model Characteristics of the membrane: Fluidity: membrane is fluid (why?) Selective permeability: membrane is selectively permeable (why?) Components of the membrane Membrane transport: ive transport: ive diffusion & Facilitated diffusion Active transport: Primary active transport & Secondary active transport Transport of larger molecules: Exocytosis & Endocytosis: Endocytosis: Receptor mediated endocytosis, Pinocytosis & Phagocytosis Function and types of transport proteins: Channels Transporters Types of transporters: Uniporter, Symporter, Antiporter Specific examples of transport: Sodium Potassium Pump
Membrane Structure
The framework of the membrane is the phospholipid bilayer
Phospholipids are amphipathic molecules
Hydrophobic (water-fearing) region faces in
Hydrophilic (water-loving) region faces out
Membranes also contain proteins and carbohydrates
The two leaflets (halves of bilayer) are asymmetrical, with different amounts of each component 4
Fluid-mosaic model
Membrane is considered a mosaic of lipid, protein, and carbohydrate molecules
Membrane resembles a fluid because lipids and proteins can move relative to each other within the membrane
5
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Extracellular environment Carbohydrate Glycolipid Integral membrane protein
Phospholipid bilayer
Glycoprotein
Extracellular leaflet
Cytosolic leaflet
Peripheral membrane proteins
Cytosol
Cholesterol (found only in animal cells)
HO
Polar
Nonpolar
Polar
6
Proteins bound to membranes Integral or intrinsic membrane proteins Transmembrane
Region(s) are physically embedded in the hydrophobic portion of the phospholipid bilayer
Lipid-anchored
proteins
proteins
An amino acid of the protein is covalently attached to a lipid
Peripheral or extrinsic membrane proteins Noncovalently
bound either to integral membrane proteins that project out from the membrane, or to polar head groups of phospholipids 7
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Extracellular environment Lipid
Transmembrane α helix Transmembrane protein
4 5
3 6
2 7 1
Lipidanchored protein
Peripheral membrane protein
Cytosol
8
Approximately 25% of All Genes Encode Transmembrane Proteins
Membranes are important medically as well as biologically
Computer programs can be used to predict the number of transmembrane proteins
Estimated percentage of membrane proteins is substantial: 20–30% of all genes may encode transmembrane proteins
This trend is found throughout all domains of life including archaea, bacteria, and eukaryotes
Function of many genes is unknown – study may provide better understanding and better treatments for disease
Transmission Electron Microscopy (TEM)
Biological sample is thin sectioned and stained with heavy-metal dyes
Dye binds tightly to the polar head groups of phospholipids, but not to the fatty acyl chains
This makes membranes resemble railroad tracks Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Membrane bilayer 11
Freeze Fracture Electron Microscopy (FFEM) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
A
specialized form of TEM used to analyze the interior of the phospholipid bilayer
Sample
is frozen in liquid nitrogen and fractured with a knife
Due
to the weakness of the central membrane, the leaflets separate into the P face (Protoplasmic face next to the cytosol) and the E face (Extracellular face)
Can
provide significant detail about membrane protein form
Direction of fracture
Transmembrane protein
P face exposed E face exposed
Cytosolic leaflet
P face
E face Extracellular leaflet E face
P face
© The McGraw-Hill Companies, Inc./Al Tesler, photographer
Fluidity of Membranes
Membranes are semifluid
Most lipids can rotate freely around their long axes and move laterally within the membrane leaflet
But ―flip-flop‖ of lipids from one leaflet to the opposite leaflet does not occur spontaneously
Flippase requires ATP to transport lipids between leaflets 13
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Flippase Lateral movement
Flip-flop
Rotational movement ATP (a) Spontaneous lipid movements
ADP +
Pi
(b) Lipid movement via flippase
14
Lipid rafts
Certain lipids associate strongly with each other to form lipid rafts
A group of lipids floats together as a unit within the larger sea of lipids in the membrane
Composition of lipid raft is different than rest of membrane High
concentration of cholesterol
Unique
set of membrane proteins 15
Factors affecting fluidity
Length of fatty acyl tails Shorter
acyl tails are less likely to interact, which makes the membrane more fluid
Presence of double bonds Double
bond creates a kink in the fatty acyl tail, making it more difficult for neighboring tails to interact and making the bilayer more fluid
Presence of cholesterol Cholesterol Effects
tends to stabilize membranes
vary depending on temperature
16
Experiments on lateral movement
Larry Frye and Michael Edidin experiment, 1970
Demonstrated the lateral movement of membrane proteins
Mouse and human cells were fused
Temperature treatment – 0°C or 37°C
Mouse membrane protein H-2 fluorescently labeled
Cells at 0°C – label stays on mouse side
Cells at 37°C – label moves over entire fused cell 17
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
1
Add agents that cause mouse cell and human cell to fuse. Mouse cell
Human cell H-2 mouse protein
2
Lower the temperature to 0°C and add a fluorescently labeled antibody that recognizes the mouse H-2 protein in the plasma membrane. Observe with a fluorescence microscope. H-2 protein is unable to move laterally and remains on one side of the fused cell.
Fuse cells
Incubate cell at 37°C, then cool to 0°C and add a fluorescently labeled antibody that recognizes the mouse H-2 protein in the plasma membrane. Observe with a fluorescence microscope. Due to lateral movement at 37°C, the mouse H-2 protein is distributed throughout the fused cell surface.
Fluorescent dye H-2
Antibody
18
Not all integral membrane proteins can move
Depending on the cell type, 10–70% of membrane proteins may be restricted in their movement
Integral membrane proteins may be bound to components of the cytoskeleton, which restricts the proteins from moving laterally
Membrane proteins may be also attached to molecules that are outside the cell, such as the interconnected network of proteins that forms the extracellular matrix 19
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fiber in the extracellular matrix Extracellular matrix
Plasma membrane Linker protein Cytosol
Cytoskeletal filament 20
Glycosylation
Process of covalently attaching a carbohydrate to a protein or lipid Glycolipid – carbohydrate to lipid Glycoprotein – carbohydrate to protein
Can serve as recognition signals for other cellular proteins
Often play a role in cell surface recognition
Helps protect proteins from damage 21
Membrane Transport
The plasma membrane is selectively permeable
Allows the age of some ions and molecules but not others
This structure ensures that:
Essential molecules enter
Metabolic intermediates remain
Waste products exit
22
Ways to move across membranes ive transport Requires no input of energy – down or with gradient diffusion – Diffusion of a solute through a membrane without transport protein
ive
diffusion – Diffusion of a solute through a membrane with the aid of a transport protein
Facilitated
Active transport Requires energy – up or against gradient
23
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
ATP ADP + Pi
(a) Diffusion—ive transport
(b) Facilitated diffusion—ive transport
(c) Active transport
24
Phospholipid bilayer barrier
Barrier to hydrophilic molecules and ions due to hydrophobic interior
Rate of diffusion depends on chemistry of solute and its concentration
Example: Diethylurea diffuses 50 times faster through the bilayer than urea, due to nonpolar ethyl groups Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
O NH2
C Urea
O NH2
CH3
CH2
NH
C
NH
CH2
CH3
Diethylurea 25
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Artificial bilayer
Gases
High permeability
CO2 N2 O2
Very Ethanol small, uncharged molecules
Water Moderate permeability
H2O
Urea
H2NCONH2
Low permeability
Sugars Polar organic molecules Ions
Very low permeability
Charged polar molecules and macromolecules
Na+, K+, Mg2+, Ca2+, Cl– Amino acids ATP Proteins Polysaccharides Nucleic acids (DNA and RNA)
26
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Cells maintain gradients
Glucose
Living cells maintain a relatively constant internal environment different from their external environment
Transmembrane gradient
Concentration of a solute is higher on one side of a membrane than the other
Plasma membrane (a) Chemical gradient for glucose—a higher glucose concentration outside the cell
–
Ion electrochemical gradient
Both an electrical gradient and chemical gradient
+
+
+
– +
+
+
+
+
–
–
Cl– Na+ K+
– +
+
–
– –
+
+
+ – +
+
–
+
–
+
–
–
+ –
+
–
Plasma membrane
+
+ +
+ +
–
+
+
(b) Electrochemical gradient for Na+—more positive charges outside the cell and a higher Na+ concentration outside the cell
Tonicity
Isotonic Equal
water and solute concentrations on either side of the membrane
Hypertonic Solute
concentration is higher (and water concentration lower) on one side of the membrane
Hypotonic Solute
concentration is lower (and water concentration higher) on one side of the membrane 28
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
The solute concentration outside the cell is isotonic to the inside of the cell.
Solute
Cytosol (a) Outside isotonic
The solute concentration outside the cell is hypertonic to the inside of the cell.
(b) Outside hypertonic
The solute concentration outside the cell is hypotonic to the inside of the cell.
(c) Outside hypotonic
29
Osmosis
Water diffuses through a membrane from an area with more water to an area with less water
If the solutes cannot move, water movement can make the cell shrink or swell as water leaves or enters the cell
Osmotic pressure – the tendency for water to move into any cell 30
2% sucrose solution
1 liter of distilled water
1 liter of 10% sucrose solution
1 liter of 2% sucrose solution
aka: crenate
Hypertonic Conditions
Isotonic Conditions
Outside the cell
Inside the cell
Isotonic The solution and cell are isotonic
Hypertonic
The solution is hypertonic to the cell
Hypotonic
The solution is hypotonic to the cell
Osmosis in animal cells
aka crenation
Osmosis in plant cells
aka: plasmolysis
Which solution is hypertonic to the other?
the cell contents
the environment
Transport proteins
Transport proteins enable biological membranes to be selectively permeable (will allow diffusion or not)
2 classes
Channels (porins)
Transporters
Channel Proteins
Form an open ageway, normally polar inside.
i.e. Aquaporins
Osmosis in animal cells Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Animal cells must maintain a balance between extracellular and intracellular solute concentrations to maintain their size and shape Crenation – shrinkage of a cell in a hypertonic solution Osmotic Lysis – swelling and bursting of a cell in a hypotonic solution
Cells are initially in an isotonic solution. Red blood cell Cells maintain normal shape. Place in hypotonic solution.
Place in hypertonic solution.
H2O H2O
Cells undergo shrinkage (crenation) because water exits the cell.
Cells swell and may undergo osmotic lysis because water is taken into the cell.
(a) Osmosis in animal cells
39
Osmosis in plant cells Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
A cell wall prevents major changes in cell size Turgor pressure – pushes plasma membrane against cell wall
Cell is initially in an isotonic solution. Vacuole
Plant cell
Cells maintain normal shape. Place in hypertonic solution.
Place in hypotonic solution.
H2O
H2O
Maintains shape and size
Plasmolysis – plants wilting because water leaves plant cells
Volume inside the plasma membrane shrinks, and the membrane pulls away from the cell wall (plasmolysis) due to the exit of water.
A small amount of water may enter the cell, but the cell wall prevents major expansion.
(b) Osmosis in plant cells
40
Osmosis in freshwater protists
Freshwater protists like Paramecium have to survive in a strongly hypotonic environment To prevent osmotic lysis, contractile vacuoles take up water and discharge it outside the cell Using vacuoles to remove excess water maintains a constant cell volume
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Filled contractile vacuole 60 μm
Vacuole after expelling water 60 μm (all): © Carolina Biological Supply/Visuals Unlimited
41
Agre Discovered That Osmosis Occurs More Quickly in Cells with Transport Proteins That Allow the Facilitated Diffusion of Water
Water can ively diffuse across plasma membranes, but some cell types allow water to move across the membrane much faster than predicted
Peter Agre and colleagues first identified a protein that was abundant in red blood cells, bladder, and kidney cells
Channel-forming Integral Membrane Protein, 28kDa (CHIP28)
Unlike controls, frog oocytes that expressed CHIP28 swelled up and lysed when put in a hypotonic medium
CHIP28 was renamed Aquaporin, since it forms a channel that allows water to through the membrane
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Experimental level
1
Add an enzyme (RNA polymerase) and nucleotides to a test tube that contains many copies of the CHIP28 gene. This results in the synthesis of many copies of CHIP28 mRNA.
Conceptual level CHIP28 mRNA
RNA polymerase
Enzymes and nucleotides
CHIP28 DNA
2
Inject the CHIP28 mRNA into frog eggs (oocytes). Wait several hours to allow time for the mRNA to be translated into CHIP28 protein at the ER membrane and then moved via vesicles to the plasma membrane.
CHIP28 protein
Frog oocyte Nucleus
3
4
CHIP28 protein is inserted into the plasma membrane.
CHIP28 mRNA
Ribosome
Cytosol
Place oocytes into a hypotonic medium and observe under a light microscope. As a control, also place oocytes that have not been injected with CHIP28 mRNA into a hypotonic medium and observe by microscopy .
Control
THE DATA
Oocyte rupturing
Oocyte
3–5 minutes CHIP28 protein
Control
CHIP28
Control
CHIP28
Courtesy Dr. Peter Agre. From GM Preston, TP Carroll, WP Guggino, P Agre (1992), “Appearance of water channels in Xenopus oocytes expressing red cell CHIP28 protein,” Science, 256(5055):385–7
Transport Proteins
Transport proteins are transmembrane proteins that provide a ageway for the movement of ions and hydrophilic molecules across membranes
Two classes based on type of movement Channels Transporters
44
Channels Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Form an open ageway for the direct diffusion of ions or molecules across the membrane
Most are gated
example: Aquaporins
When a channel is open, a solute directly diffuses through the channel to reach the other side of the membrane. Gate opened Gate closed
45
Transporters Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Also known as carriers
Conformational change Hydrophilic pocket
Conformational change transports solute across membrane Principal pathway for uptake of organic molecules, such as sugars, amino acids, and nucleotides
Solute
For transport to occur, a solute binds in a hydrophilic pocket exposed on one side of the membrane. The transporter then undergoes a conformational change that switches the exposure of the pocket to the other side of the membrane, where the solute is then released.
46
Transporter types
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
A single solute moves in one direction.
Uniporter
Single molecule or ion
Symporter
or cotransporter
Two or more ions or molecules transported in same direction
(a) Uniporter
Two solutes move in the same direction.
(b) Symporter
Two solutes move in opposite directions.
Antiporter
Two or more ions or molecules transported in opposite directions
(c) Antiporter
47
Question A cell is placed in an hypertonic solution. Which way will the water move? a.
Into the cell
b.
Out of the cell
c.
No net movement
Question Gated channels which open when a chemical binds to it is – a.
Ligand gated channel
b.
Leakage channel
c.
Mechanically gated channel
d.
Voltage gated channel
e.
All can open in response to chemical binding
Question What type of transport protein can move 2 or more different molecules in opposite directions? a.
Uniporter
b.
Antiporter
c.
Symporter
d.
Multiporter
e.
Diporter
Active transport
Movement of a solute across a membrane against its gradient from a region of low concentration to higher concentration
Energetically unfavorable and requires the input of energy
Primary active transport uses a pump Directly
uses energy to transport solute
Secondary active transport uses a different gradient Uses
a pre-existing gradient to drive transport
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Extracellular environment
A pump actively exports H+ against a gradient.
ATP
A H+/sucrose symporter uses the H+ gradient to transport sucrose against a concentration gradient into the cell.
ADP + Pi Sucrose Cytosol
(a) Primary active transport
H+
(b) Secondary active transport
52
ATP-driven ion pumps generate ion electrochemical gradients Na+/K+-ATPase
Actively transports Na+ and K+ against their gradients using the energy from ATP hydrolysis 3 Na+ are exported for every 2 K+ imported into cell
Antiporter – ions move in opposite directions
Electrogenic pump – exports one net positive (+) charge
53
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Nerve cell
Extracellular environment
1 3 Na+ bind from cytosol. ATP is hydrolyzed. ADP is released and phosphate (P) is covalently attached to the pump, switching it to the E2 conformation.
3
3 Na+ are released outside of the cell.
3
2 K+ bind from outside of the cell.
E2
Na+/K+-ATPase
High [Na+] Low [K+]
ADP + Pi
2
4 Phosphate (Pi) is released, and the pump switches to the E1 conformation. 2 K+ are released into cytosol. The process 2 K+ repeats. E1
3 Na+
Na+ Extracellular environment
E2
E1
Pi
ATP
2 K+
2 K+ Low [Na+] High [K+] (a) Active transport by the Na+ / K+-ATPase
Cytosol
Cytosol ATP 3
ADP
P
Na+
(b) Mechanism of pumping
54
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55
Exocytosis and Endocytosis Used to transport large molecules such as proteins and polysaccharides
Exocytosis Material
inside the cell packaged into vesicles and excreted into the extracellular medium
Endocytosis Plasma
membrane invaginates (folds inward) to form a vesicle that brings substances into the cell Three types of endocytosis:
Receptor-mediated endocytosis Pinocytosis Phagocytosis 56
Exocytosis Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Golgi apparatus
Cargo Vesicle
Cytosol
1 A vesicle loaded with cargo is formed as a protein coat wraps around it.
Protein
2
coat The vesicle is released from the Golgi, carrying cargo molecules.
Extracellular environment
3 The protein coat is shed. 4 The vesicle fuses with the plasma membrane and releases the cargo to the outside.
Plasma membrane
57
Receptor-mediated endocytosis Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Cargo Invagination Coat protein
Cytosol
5
Cargo is released into the cytosol.
Receptor
Lysosome Extracellular environment
3 The protein coat is shed.
1
Cargo binds to receptor and receptors aggregate. The receptors cause coat proteins to bind to the surrounding membrane. The plasma membrane invaginates as coat proteins cause a vesicle to form.
4
The vesicle fuses with an internal organelle such as a lysosome.
2 The vesicle is released in the cell.
58
NEXT QUIZ I
Chapter 4: Cell Membrane Structure and Function in Audesirk
http://wps.prenhall.com/esm_audesirk_bloe_7/17/4453/1140182.cw/index.html Media Activities 4.1 Membrane Structure and Transport Pre-quiz Activity Post-quiz 4.2 Osmosis Pre-quiz Activity Post-quiz
II Membranes and Transport in Hippocampus http://www.hippocampus.org/HippoCampus/Biology;jsessionid=A1F9977D639B10E4E8DA26617C1E0CB0 Cell Membranes: Overview
Membrane Structure Transport Mechanisms Membrane Proteins Cell Membranes: Summary
See separate PowerPoint slides for all figures and tables pre-inserted into PowerPoint without notes and animations.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
1
Chapter 5
Membrane Structure, Synthesis and Transport Key Concepts:
Membrane Structure
Fluidity of Membranes
Synthesis of Membrane Components
Membrane Transport
Transport Proteins
Exocytosis and Endocytosis 2
Membrane: The fluid mosaic model Characteristics of the membrane: Fluidity: membrane is fluid (why?) Selective permeability: membrane is selectively permeable (why?) Components of the membrane Membrane transport: ive transport: ive diffusion & Facilitated diffusion Active transport: Primary active transport & Secondary active transport Transport of larger molecules: Exocytosis & Endocytosis: Endocytosis: Receptor mediated endocytosis, Pinocytosis & Phagocytosis Function and types of transport proteins: Channels Transporters Types of transporters: Uniporter, Symporter, Antiporter Specific examples of transport: Sodium Potassium Pump
Membrane Structure
The framework of the membrane is the phospholipid bilayer
Phospholipids are amphipathic molecules
Hydrophobic (water-fearing) region faces in
Hydrophilic (water-loving) region faces out
Membranes also contain proteins and carbohydrates
The two leaflets (halves of bilayer) are asymmetrical, with different amounts of each component 4
Fluid-mosaic model
Membrane is considered a mosaic of lipid, protein, and carbohydrate molecules
Membrane resembles a fluid because lipids and proteins can move relative to each other within the membrane
5
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Extracellular environment Carbohydrate Glycolipid Integral membrane protein
Phospholipid bilayer
Glycoprotein
Extracellular leaflet
Cytosolic leaflet
Peripheral membrane proteins
Cytosol
Cholesterol (found only in animal cells)
HO
Polar
Nonpolar
Polar
6
Proteins bound to membranes Integral or intrinsic membrane proteins Transmembrane
Region(s) are physically embedded in the hydrophobic portion of the phospholipid bilayer
Lipid-anchored
proteins
proteins
An amino acid of the protein is covalently attached to a lipid
Peripheral or extrinsic membrane proteins Noncovalently
bound either to integral membrane proteins that project out from the membrane, or to polar head groups of phospholipids 7
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Extracellular environment Lipid
Transmembrane α helix Transmembrane protein
4 5
3 6
2 7 1
Lipidanchored protein
Peripheral membrane protein
Cytosol
8
Approximately 25% of All Genes Encode Transmembrane Proteins
Membranes are important medically as well as biologically
Computer programs can be used to predict the number of transmembrane proteins
Estimated percentage of membrane proteins is substantial: 20–30% of all genes may encode transmembrane proteins
This trend is found throughout all domains of life including archaea, bacteria, and eukaryotes
Function of many genes is unknown – study may provide better understanding and better treatments for disease
Transmission Electron Microscopy (TEM)
Biological sample is thin sectioned and stained with heavy-metal dyes
Dye binds tightly to the polar head groups of phospholipids, but not to the fatty acyl chains
This makes membranes resemble railroad tracks Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Membrane bilayer 11
Freeze Fracture Electron Microscopy (FFEM) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
A
specialized form of TEM used to analyze the interior of the phospholipid bilayer
Sample
is frozen in liquid nitrogen and fractured with a knife
Due
to the weakness of the central membrane, the leaflets separate into the P face (Protoplasmic face next to the cytosol) and the E face (Extracellular face)
Can
provide significant detail about membrane protein form
Direction of fracture
Transmembrane protein
P face exposed E face exposed
Cytosolic leaflet
P face
E face Extracellular leaflet E face
P face
© The McGraw-Hill Companies, Inc./Al Tesler, photographer
Fluidity of Membranes
Membranes are semifluid
Most lipids can rotate freely around their long axes and move laterally within the membrane leaflet
But ―flip-flop‖ of lipids from one leaflet to the opposite leaflet does not occur spontaneously
Flippase requires ATP to transport lipids between leaflets 13
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Flippase Lateral movement
Flip-flop
Rotational movement ATP (a) Spontaneous lipid movements
ADP +
Pi
(b) Lipid movement via flippase
14
Lipid rafts
Certain lipids associate strongly with each other to form lipid rafts
A group of lipids floats together as a unit within the larger sea of lipids in the membrane
Composition of lipid raft is different than rest of membrane High
concentration of cholesterol
Unique
set of membrane proteins 15
Factors affecting fluidity
Length of fatty acyl tails Shorter
acyl tails are less likely to interact, which makes the membrane more fluid
Presence of double bonds Double
bond creates a kink in the fatty acyl tail, making it more difficult for neighboring tails to interact and making the bilayer more fluid
Presence of cholesterol Cholesterol Effects
tends to stabilize membranes
vary depending on temperature
16
Experiments on lateral movement
Larry Frye and Michael Edidin experiment, 1970
Demonstrated the lateral movement of membrane proteins
Mouse and human cells were fused
Temperature treatment – 0°C or 37°C
Mouse membrane protein H-2 fluorescently labeled
Cells at 0°C – label stays on mouse side
Cells at 37°C – label moves over entire fused cell 17
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
1
Add agents that cause mouse cell and human cell to fuse. Mouse cell
Human cell H-2 mouse protein
2
Lower the temperature to 0°C and add a fluorescently labeled antibody that recognizes the mouse H-2 protein in the plasma membrane. Observe with a fluorescence microscope. H-2 protein is unable to move laterally and remains on one side of the fused cell.
Fuse cells
Incubate cell at 37°C, then cool to 0°C and add a fluorescently labeled antibody that recognizes the mouse H-2 protein in the plasma membrane. Observe with a fluorescence microscope. Due to lateral movement at 37°C, the mouse H-2 protein is distributed throughout the fused cell surface.
Fluorescent dye H-2
Antibody
18
Not all integral membrane proteins can move
Depending on the cell type, 10–70% of membrane proteins may be restricted in their movement
Integral membrane proteins may be bound to components of the cytoskeleton, which restricts the proteins from moving laterally
Membrane proteins may be also attached to molecules that are outside the cell, such as the interconnected network of proteins that forms the extracellular matrix 19
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fiber in the extracellular matrix Extracellular matrix
Plasma membrane Linker protein Cytosol
Cytoskeletal filament 20
Glycosylation
Process of covalently attaching a carbohydrate to a protein or lipid Glycolipid – carbohydrate to lipid Glycoprotein – carbohydrate to protein
Can serve as recognition signals for other cellular proteins
Often play a role in cell surface recognition
Helps protect proteins from damage 21
Membrane Transport
The plasma membrane is selectively permeable
Allows the age of some ions and molecules but not others
This structure ensures that:
Essential molecules enter
Metabolic intermediates remain
Waste products exit
22
Ways to move across membranes ive transport Requires no input of energy – down or with gradient diffusion – Diffusion of a solute through a membrane without transport protein
ive
diffusion – Diffusion of a solute through a membrane with the aid of a transport protein
Facilitated
Active transport Requires energy – up or against gradient
23
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
ATP ADP + Pi
(a) Diffusion—ive transport
(b) Facilitated diffusion—ive transport
(c) Active transport
24
Phospholipid bilayer barrier
Barrier to hydrophilic molecules and ions due to hydrophobic interior
Rate of diffusion depends on chemistry of solute and its concentration
Example: Diethylurea diffuses 50 times faster through the bilayer than urea, due to nonpolar ethyl groups Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
O NH2
C Urea
O NH2
CH3
CH2
NH
C
NH
CH2
CH3
Diethylurea 25
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Artificial bilayer
Gases
High permeability
CO2 N2 O2
Very Ethanol small, uncharged molecules
Water Moderate permeability
H2O
Urea
H2NCONH2
Low permeability
Sugars Polar organic molecules Ions
Very low permeability
Charged polar molecules and macromolecules
Na+, K+, Mg2+, Ca2+, Cl– Amino acids ATP Proteins Polysaccharides Nucleic acids (DNA and RNA)
26
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Cells maintain gradients
Glucose
Living cells maintain a relatively constant internal environment different from their external environment
Transmembrane gradient
Concentration of a solute is higher on one side of a membrane than the other
Plasma membrane (a) Chemical gradient for glucose—a higher glucose concentration outside the cell
–
Ion electrochemical gradient
Both an electrical gradient and chemical gradient
+
+
+
– +
+
+
+
+
–
–
Cl– Na+ K+
– +
+
–
– –
+
+
+ – +
+
–
+
–
+
–
–
+ –
+
–
Plasma membrane
+
+ +
+ +
–
+
+
(b) Electrochemical gradient for Na+—more positive charges outside the cell and a higher Na+ concentration outside the cell
Tonicity
Isotonic Equal
water and solute concentrations on either side of the membrane
Hypertonic Solute
concentration is higher (and water concentration lower) on one side of the membrane
Hypotonic Solute
concentration is lower (and water concentration higher) on one side of the membrane 28
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
The solute concentration outside the cell is isotonic to the inside of the cell.
Solute
Cytosol (a) Outside isotonic
The solute concentration outside the cell is hypertonic to the inside of the cell.
(b) Outside hypertonic
The solute concentration outside the cell is hypotonic to the inside of the cell.
(c) Outside hypotonic
29
Osmosis
Water diffuses through a membrane from an area with more water to an area with less water
If the solutes cannot move, water movement can make the cell shrink or swell as water leaves or enters the cell
Osmotic pressure – the tendency for water to move into any cell 30
2% sucrose solution
1 liter of distilled water
1 liter of 10% sucrose solution
1 liter of 2% sucrose solution
aka: crenate
Hypertonic Conditions
Isotonic Conditions
Outside the cell
Inside the cell
Isotonic The solution and cell are isotonic
Hypertonic
The solution is hypertonic to the cell
Hypotonic
The solution is hypotonic to the cell
Osmosis in animal cells
aka crenation
Osmosis in plant cells
aka: plasmolysis
Which solution is hypertonic to the other?
the cell contents
the environment
Transport proteins
Transport proteins enable biological membranes to be selectively permeable (will allow diffusion or not)
2 classes
Channels (porins)
Transporters
Channel Proteins
Form an open ageway, normally polar inside.
i.e. Aquaporins
Osmosis in animal cells Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Animal cells must maintain a balance between extracellular and intracellular solute concentrations to maintain their size and shape Crenation – shrinkage of a cell in a hypertonic solution Osmotic Lysis – swelling and bursting of a cell in a hypotonic solution
Cells are initially in an isotonic solution. Red blood cell Cells maintain normal shape. Place in hypotonic solution.
Place in hypertonic solution.
H2O H2O
Cells undergo shrinkage (crenation) because water exits the cell.
Cells swell and may undergo osmotic lysis because water is taken into the cell.
(a) Osmosis in animal cells
39
Osmosis in plant cells Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
A cell wall prevents major changes in cell size Turgor pressure – pushes plasma membrane against cell wall
Cell is initially in an isotonic solution. Vacuole
Plant cell
Cells maintain normal shape. Place in hypertonic solution.
Place in hypotonic solution.
H2O
H2O
Maintains shape and size
Plasmolysis – plants wilting because water leaves plant cells
Volume inside the plasma membrane shrinks, and the membrane pulls away from the cell wall (plasmolysis) due to the exit of water.
A small amount of water may enter the cell, but the cell wall prevents major expansion.
(b) Osmosis in plant cells
40
Osmosis in freshwater protists
Freshwater protists like Paramecium have to survive in a strongly hypotonic environment To prevent osmotic lysis, contractile vacuoles take up water and discharge it outside the cell Using vacuoles to remove excess water maintains a constant cell volume
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Filled contractile vacuole 60 μm
Vacuole after expelling water 60 μm (all): © Carolina Biological Supply/Visuals Unlimited
41
Agre Discovered That Osmosis Occurs More Quickly in Cells with Transport Proteins That Allow the Facilitated Diffusion of Water
Water can ively diffuse across plasma membranes, but some cell types allow water to move across the membrane much faster than predicted
Peter Agre and colleagues first identified a protein that was abundant in red blood cells, bladder, and kidney cells
Channel-forming Integral Membrane Protein, 28kDa (CHIP28)
Unlike controls, frog oocytes that expressed CHIP28 swelled up and lysed when put in a hypotonic medium
CHIP28 was renamed Aquaporin, since it forms a channel that allows water to through the membrane
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Experimental level
1
Add an enzyme (RNA polymerase) and nucleotides to a test tube that contains many copies of the CHIP28 gene. This results in the synthesis of many copies of CHIP28 mRNA.
Conceptual level CHIP28 mRNA
RNA polymerase
Enzymes and nucleotides
CHIP28 DNA
2
Inject the CHIP28 mRNA into frog eggs (oocytes). Wait several hours to allow time for the mRNA to be translated into CHIP28 protein at the ER membrane and then moved via vesicles to the plasma membrane.
CHIP28 protein
Frog oocyte Nucleus
3
4
CHIP28 protein is inserted into the plasma membrane.
CHIP28 mRNA
Ribosome
Cytosol
Place oocytes into a hypotonic medium and observe under a light microscope. As a control, also place oocytes that have not been injected with CHIP28 mRNA into a hypotonic medium and observe by microscopy .
Control
THE DATA
Oocyte rupturing
Oocyte
3–5 minutes CHIP28 protein
Control
CHIP28
Control
CHIP28
Courtesy Dr. Peter Agre. From GM Preston, TP Carroll, WP Guggino, P Agre (1992), “Appearance of water channels in Xenopus oocytes expressing red cell CHIP28 protein,” Science, 256(5055):385–7
Transport Proteins
Transport proteins are transmembrane proteins that provide a ageway for the movement of ions and hydrophilic molecules across membranes
Two classes based on type of movement Channels Transporters
44
Channels Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Form an open ageway for the direct diffusion of ions or molecules across the membrane
Most are gated
example: Aquaporins
When a channel is open, a solute directly diffuses through the channel to reach the other side of the membrane. Gate opened Gate closed
45
Transporters Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Also known as carriers
Conformational change Hydrophilic pocket
Conformational change transports solute across membrane Principal pathway for uptake of organic molecules, such as sugars, amino acids, and nucleotides
Solute
For transport to occur, a solute binds in a hydrophilic pocket exposed on one side of the membrane. The transporter then undergoes a conformational change that switches the exposure of the pocket to the other side of the membrane, where the solute is then released.
46
Transporter types
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
A single solute moves in one direction.
Uniporter
Single molecule or ion
Symporter
or cotransporter
Two or more ions or molecules transported in same direction
(a) Uniporter
Two solutes move in the same direction.
(b) Symporter
Two solutes move in opposite directions.
Antiporter
Two or more ions or molecules transported in opposite directions
(c) Antiporter
47
Question A cell is placed in an hypertonic solution. Which way will the water move? a.
Into the cell
b.
Out of the cell
c.
No net movement
Question Gated channels which open when a chemical binds to it is – a.
Ligand gated channel
b.
Leakage channel
c.
Mechanically gated channel
d.
Voltage gated channel
e.
All can open in response to chemical binding
Question What type of transport protein can move 2 or more different molecules in opposite directions? a.
Uniporter
b.
Antiporter
c.
Symporter
d.
Multiporter
e.
Diporter
Active transport
Movement of a solute across a membrane against its gradient from a region of low concentration to higher concentration
Energetically unfavorable and requires the input of energy
Primary active transport uses a pump Directly
uses energy to transport solute
Secondary active transport uses a different gradient Uses
a pre-existing gradient to drive transport
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Extracellular environment
A pump actively exports H+ against a gradient.
ATP
A H+/sucrose symporter uses the H+ gradient to transport sucrose against a concentration gradient into the cell.
ADP + Pi Sucrose Cytosol
(a) Primary active transport
H+
(b) Secondary active transport
52
ATP-driven ion pumps generate ion electrochemical gradients Na+/K+-ATPase
Actively transports Na+ and K+ against their gradients using the energy from ATP hydrolysis 3 Na+ are exported for every 2 K+ imported into cell
Antiporter – ions move in opposite directions
Electrogenic pump – exports one net positive (+) charge
53
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Nerve cell
Extracellular environment
1 3 Na+ bind from cytosol. ATP is hydrolyzed. ADP is released and phosphate (P) is covalently attached to the pump, switching it to the E2 conformation.
3
3 Na+ are released outside of the cell.
3
2 K+ bind from outside of the cell.
E2
Na+/K+-ATPase
High [Na+] Low [K+]
ADP + Pi
2
4 Phosphate (Pi) is released, and the pump switches to the E1 conformation. 2 K+ are released into cytosol. The process 2 K+ repeats. E1
3 Na+
Na+ Extracellular environment
E2
E1
Pi
ATP
2 K+
2 K+ Low [Na+] High [K+] (a) Active transport by the Na+ / K+-ATPase
Cytosol
Cytosol ATP 3
ADP
P
Na+
(b) Mechanism of pumping
54
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55
Exocytosis and Endocytosis Used to transport large molecules such as proteins and polysaccharides
Exocytosis Material
inside the cell packaged into vesicles and excreted into the extracellular medium
Endocytosis Plasma
membrane invaginates (folds inward) to form a vesicle that brings substances into the cell Three types of endocytosis:
Receptor-mediated endocytosis Pinocytosis Phagocytosis 56
Exocytosis Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Golgi apparatus
Cargo Vesicle
Cytosol
1 A vesicle loaded with cargo is formed as a protein coat wraps around it.
Protein
2
coat The vesicle is released from the Golgi, carrying cargo molecules.
Extracellular environment
3 The protein coat is shed. 4 The vesicle fuses with the plasma membrane and releases the cargo to the outside.
Plasma membrane
57
Receptor-mediated endocytosis Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Cargo Invagination Coat protein
Cytosol
5
Cargo is released into the cytosol.
Receptor
Lysosome Extracellular environment
3 The protein coat is shed.
1
Cargo binds to receptor and receptors aggregate. The receptors cause coat proteins to bind to the surrounding membrane. The plasma membrane invaginates as coat proteins cause a vesicle to form.
4
The vesicle fuses with an internal organelle such as a lysosome.
2 The vesicle is released in the cell.
58
NEXT QUIZ I
Chapter 4: Cell Membrane Structure and Function in Audesirk
http://wps.prenhall.com/esm_audesirk_bloe_7/17/4453/1140182.cw/index.html Media Activities 4.1 Membrane Structure and Transport Pre-quiz Activity Post-quiz 4.2 Osmosis Pre-quiz Activity Post-quiz
II Membranes and Transport in Hippocampus http://www.hippocampus.org/HippoCampus/Biology;jsessionid=A1F9977D639B10E4E8DA26617C1E0CB0 Cell Membranes: Overview
Membrane Structure Transport Mechanisms Membrane Proteins Cell Membranes: Summary
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