chapter 7 membrane structure and function. overview: life at the edge the plasma membrane is the...
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Chapter 7Chapter 7
Membrane Structure and Function
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Overview: Life at the Edge
• The plasma membrane is the boundary that separates the living cell from its surroundings
• The plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Concept 7.1: Cellular membranes are fluid mosaics of lipids and proteins
• Phospholipids are the most abundant lipid in the plasma membrane
• Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regions
• The fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it
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Membrane Models: Scientific Inquiry
• Membranes have been chemically analyzed and found to be made of proteins and lipids
• Scientists studying the plasma membrane reasoned that it must be a phospholipid bilayer
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Fig. 7-2
Hydrophilichead
WATER
Hydrophobictail
WATER
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• In 1935, Hugh Davson and James Danielli proposed a sandwich model in which the phospholipid bilayer lies between two layers of globular proteins
• Later studies found problems with this model, particularly the placement of membrane proteins, which have hydrophilic and hydrophobic regions
• In 1972, J. Singer and G. Nicolson proposed that the membrane is a mosaic of proteins dispersed within the bilayer, with only the hydrophilic regions exposed to water
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Fig. 7-3
Phospholipidbilayer
Hydrophobic regionsof protein
Hydrophilicregions of protein
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• Freeze-fracture studies of the plasma membrane supported the fluid mosaic model
• Freeze-fracture is a specialized preparation technique that splits a membrane along the middle of the phospholipid bilayer
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Fig. 7-4
TECHNIQUE
Extracellularlayer
KnifeProteins Inside of extracellular layer
RESULTS
Inside of cytoplasmic layer
Cytoplasmic layerPlasma membrane
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The Fluidity of Membranes
• Phospholipids in the plasma membrane can move within the bilayer
• Most of the lipids, and some proteins, drift laterally
• Rarely does a molecule flip-flop transversely across the membrane
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Fig. 7-5
Lateral movement(~107 times per second)
Flip-flop(~ once per month)
(a) Movement of phospholipids
(b) Membrane fluidity
Fluid Viscous
Unsaturated hydrocarbontails with kinks
Saturated hydro-carbon tails
(c) Cholesterol within the animal cell membrane
Cholesterol
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Fig. 7-5a
(a) Movement of phospholipids
Lateral movement(107 times per second)
Flip-flop( once per month)
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Fig. 7-6
RESULTS
Membrane proteins
Mouse cellHuman cell
Hybrid cell
Mixed proteinsafter 1 hour
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• As temperatures cool, membranes switch from a fluid state to a solid state
• The temperature at which a membrane solidifies depends on the types of lipids
• Membranes rich in unsaturated fatty acids are more fluid that those rich in saturated fatty acids
• Membranes must be fluid to work properly; they are usually about as fluid as salad oil
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Fig. 7-5b
(b) Membrane fluidity
Fluid
Unsaturated hydrocarbontails with kinks
Viscous
Saturated hydro-carbon tails
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• The steroid cholesterol has different effects on membrane fluidity at different temperatures
• At warm temperatures (such as 37°C), cholesterol restrains movement of phospholipids
• At cool temperatures, it maintains fluidity by preventing tight packing
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Fig. 7-5c
Cholesterol
(c) Cholesterol within the animal cell membrane
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Membrane Proteins and Their Functions
• A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer
• Proteins determine most of the membrane’s specific functions
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Fig. 7-7
Fibers ofextracellularmatrix (ECM)
Glyco-protein
Microfilamentsof cytoskeleton
Cholesterol
Peripheralproteins
Integralprotein
CYTOPLASMIC SIDEOF MEMBRANE
GlycolipidEXTRACELLULARSIDE OFMEMBRANE
Carbohydrate
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• Peripheral proteins are bound to the surface of the membrane
• Integral proteins penetrate the hydrophobic core
• Integral proteins that span the membrane are called transmembrane proteins
• The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids, often coiled into alpha helices
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Fig. 7-8
N-terminus
C-terminus
HelixCYTOPLASMICSIDE
EXTRACELLULARSIDE
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• Six major functions of membrane proteins:
– Transport
– Enzymatic activity
– Signal transduction
– Cell-cell recognition
– Intercellular joining
– Attachment to the cytoskeleton and extracellular matrix (ECM)
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Fig. 7-9
(a) Transport
ATP
(b) Enzymatic activity
Enzymes
(c) Signal transduction
Signal transduction
Signaling molecule
Receptor
(d) Cell-cell recognition
Glyco-protein
(e) Intercellular joining (f) Attachment to the cytoskeleton and extracellular matrix (ECM)
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The Role of Membrane Carbohydrates in Cell-Cell Recognition
• Cells recognize each other by binding to surface molecules, often carbohydrates, on the plasma membrane
• Membrane carbohydrates may be covalently bonded to lipids (forming glycolipids) or more commonly to proteins (forming glycoproteins)
• Carbohydrates on the external side of the plasma membrane vary among species, individuals, and even cell types in an individual
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Synthesis and Sidedness of Membranes
• Membranes have distinct inside and outside faces
• The asymmetrical distribution of proteins, lipids, and associated carbohydrates in the plasma membrane is determined when the membrane is built by the ER and Golgi apparatus
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Fig. 7-10
ER1
Transmembraneglycoproteins
Secretoryprotein
Glycolipid
2Golgiapparatus
Vesicle
3
4
Secretedprotein
Transmembraneglycoprotein
Plasma membrane:
Cytoplasmic face
Extracellular face
Membrane glycolipid
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Selectively permeable- allows certain substances to pass through
By 2 ways: active or passive transport
Passive- downhill
Active- uphill (needs energy)
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Passive: downhill reaction• Simple diffusion• Osmosis• Facilitated diffusion• Filtration
Active: uphill reaction, needs ATP• Exocytosis• Endocytosis
- Pinocytosis- Phagocytosis
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No Barrier:
• Substances “spread out”
• High concentration to low concentration
e.g.: Red dye placed in glass of water
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• Substances diffuse
• High concentration to low concentration
• Pores in membrane must be large
• “Down the concentration gradient”
• Dynamic equilibrium, equal rates in both directions
Biological membrane:
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Carrier proteins:
• Bind specific molecule & change shape
• Pass molecule through middle of protein
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Osmosis- diffusion of a water through a semi-permeable membrane
• Moves down concentration gradient
e.g., Two sugar solutions of different concentrations separated by porous membrane which lets water through but not sugar
What will happen?
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• More concentrated to less concentrated
• Until concentration same on both sides: isotonic
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Concentration of solute less: solution is hypotonic.
Concentration of solute greater: solution is hypertonic.
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Animal cells No cell walls
Isotonic environment: Influx of water equals the efflux of water No change in cell shape
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Hypotonic solution: Water enters cell Bursts, or lyses
Hypertonic solution: Water leaves cell Shriveled, or crenate
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Glomerular filtration
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Passive transport & facilitated diffusion do NOT require
ATP
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DOES require the input of
ATP
Transport proteins AGAINST concentration
gradient
outside cell
inside cell
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ATP ATP ADP + P ADP + Pii + Energy + Energy
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mucus
http://www.1lecture.com/Physiology/Endocytosis%20and%20Exocytosis/index.html
Goblet cell
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Nerve Axon at Rest
Na+/K+ Pumphttp://www.1lecture.com/Biochemistry/cotransport/
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• Ion channels - Voltage gated- Chemically gated- Mechanically gated
• Porins - Larger - Less specific
• Aquaporins - water
Channel Proteins
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Channel Proteins:Ion Channels
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Channel Proteins:Porins
• MAC• Barrel shaped protein
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Channel Proteins:Aquaporins
H2O
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Membrane Permeability
Cell membrane:
selectively permeable
4 factors that determine permeability
1. lipid solubility
2. molecular size
3. polarity
4.charge
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Lipid solubilityMost important factor
Hydrophobic molecules
•Passively diffuse
•Hydrocarbons, carbon dioxide, & oxygen
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Molecular Size and Polarity
Larger molecules, less permeable Lower kinetic energy Small pore sizes in the membrane
Polar molecules hydrophilic, less permeable Very small, polar uncharged (water) molecules can diffuse
-+
Molecular Size
Polarity
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ChargeCharged molecules hydrophilic, less permeable Surrounded by coat of water (hydration shell), increases the size
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You should now be able to:
1. Define the following terms: amphipathic molecules, aquaporins, diffusion
2. Explain how membrane fluidity is influenced by temperature and membrane composition
3. Distinguish between the following pairs or sets of terms: peripheral and integral membrane proteins; channel and carrier proteins; osmosis, facilitated diffusion, and active transport; hypertonic, hypotonic, and isotonic solutions
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4. Explain how transport proteins facilitate diffusion
5. Explain how an electrogenic pump creates voltage across a membrane, and name two electrogenic pumps
6. Explain how large molecules are transported across a cell membrane
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings