the cell membrane ap chapter 7. overview: life at the edge the plasma membrane is the boundary that...
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The Cell Membrane
• AP Chapter 7
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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
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Composition of the cell membrane: lipids (phospholipids and cholesterol) and proteins
• Phospholipids are the most abundant lipid in the plasma membrane
- they are amphipathic molecules, contain 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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Development of the Fluid Mosaic Model• In 1935, Hugh Davson and James Danielli proposed a
sandwich model in which the phospholipid bilayer lies between two layers of globular proteins
• 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
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Sandwich model of the cell membrane
What is wrong with this model?
Proteins are mostly hydrophobic.All phospholipids are not alike.
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Fluid Mosaic Model
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Fig. 7-2
Hydrophilichead
WATER
Hydrophobictail
WATER
This slide shows how the hydrophobic andhydrophilic regions are set up.
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Fig. 7-3
Phospholipidbilayer
Hydrophobic regionsof protein
Hydrophilicregions of protein
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Double phospholipid layer
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Different types of phospholipids
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Different types of phospholipids making up the membrane
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Fig. 7-4
TECHNIQUE
Extracellularlayer
KnifeProteins Inside of extracellular layer
RESULTS
Inside of cytoplasmic layer
Cytoplasmic layerPlasma membrane
Freeze-fracture studies of the plasma membrane supported the fluid mosaic model
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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 (very rarely transverse flip-flops)
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
fluidity of membrane
fluidity of cell 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
Biology Animations
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Fig. 7-6
RESULTS
Membrane proteins
Mouse cellHuman cell
Hybrid cell
Mixed proteinsafter 1 hour
This experiment shows how the proteins can move about the membrane.
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The fluidity of the membrane depends on temperature and types of lipids making up the membrane.
• Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids
• Membranes must be fluid to work properly; they are usually about as fluid
as salad oil.
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Fig. 7-5b
(b) Membrane fluidity
Fluid
Unsaturated hydrocarbontails with kinks
Viscous
Saturated hydro-carbon tails
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• Would you expect an amoeba that lives in a pond in a cold northern climate to have a higher or lower percentage of saturated fatty acids in its membranes during the summer as compared to the winter?
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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
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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, 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
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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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Intercellular joiningE-selectin is a transmembrane protein expressed by endothelial cells
that binds to an oligosaccharide expressed on the surface of leukocytes
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Construct a cell membraneTry this at home!
constructing a cell membrane
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The Role of Membrane Carbohydrates in Cell-Cell Recognition
• Cells recognize each other by binding to surface molecules, usually carbohydrates
• 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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Membranes are bifacial• Carbohydrates (making glycoproteins and
glycolipids) on outer surface• Peripheral proteins generally on cytoplasmic
surface• Proteins have a distinct orientation, ie…
receptor proteins oriented at surface, enzyme proteins oriented toward cytoplasm
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Synthesis and Sidedness of Membranes
• Membranes have distinct inside and outside faces – determined when the membrane is built by the ER and Golgi.
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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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Can you guess where you would find this cell?
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This cell?
Notice how thin-walledthey are.
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Need a hint?
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These cells?