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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Overview: Life at the Edge
• The plasma membrane
– Is the boundary that separates the living cell from its nonliving
surroundings
• The plasma membrane exhibits selective permeability
– It allows some substances to cross it more easily than others
Figure 7.1
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Membrane Models: Scientific Inquiry
• Scientists studying the plasma membrane
– Reasoned that it must be a phospholipid
bilayer
Figure 7.2
Hydrophilic
head
Hydrophobic
tail
WATER
WATER
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• The Davson-Danielli sandwich model of
membrane structure
– Stated that the membrane was made up of a
phospholipid bilayer sandwiched between two
protein layers
– Was supported by electron microscope
pictures of membranes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Fluid Mosaic Model
• In 1972, Singer and Nicolson
– Proposed that membrane proteins are
dispersed and individually inserted into the
phospholipid bilayer
Figure 7.3
Phospholipid
bilayer
Hydrophobic region
of protein
Hydrophobic region of protein
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Freeze-fracture studies of the plasma
membrane
– Supported the fluid mosaic model of
membrane structure
Figure 7.4
A cell is frozen and fractured with a knife. The fracture plane often follows
the hydrophobic interior of a membrane, splitting the phospholipid bilayer
into two separated layers. The membrane proteins go wholly with one of
the layers.
Extracellular layer Cytoplasmic layer
APPLICATION A cell membrane can be split into its two layers, revealing the
ultrastructure of the membrane’s interior.
TECHNIQUE
Extracellular
layer
Proteins
Cytoplasmic
layer
Knife
Plasma
membrane
These SEMs show membrane proteins (the “bumps”) in the two layers,
demonstrating that proteins are embedded in the phospholipid bilayer.
RESULTS
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Fluidity of Membranes
• Phospholipids in the plasma membrane
– Can move within the bilayer
Figure 7.5 A
Lateral movement
(~107 times per second)
Flip-flop
(~ once per month)
(a) Movement of phospholipids
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Proteins in the plasma membrane
– Can drift within the bilayer
EXPERIMENT Researchers labeled the plasma mambrane proteins of a mouse
cell and a human cell with two different markers and fused the cells. Using a microscope,
they observed the markers on the hybrid cell.
Membrane proteins
Mouse cell
Human cell
Hybrid cell
Mixed
proteins
after
1 hour
RESULTS
CONCLUSION The mixing of the mouse and human membrane proteins
indicates that at least some membrane proteins move sideways within the plane
of the plasma membrane.
Figure 7.6
+
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• The type of hydrocarbon tails in phospholipids
– Affects the fluidity of the plasma membrane
Figure 7.5 B
Fluid Viscous
Unsaturated hydrocarbon
tails with kinks
Saturated hydro-
Carbon tails
(b) Membrane fluidity
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• The steroid cholesterol
– Has different effects on membrane fluidity at
different temperatures
Figure 7.5 (c) Cholesterol within the animal cell membrane
Cholesterol
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 7.7
Glycoprotein
Carbohydrate
Microfilaments
of cytoskeleton Cholesterol Peripheral
protein
Integral
protein
CYTOPLASMIC SIDE
OF MEMBRANE
EXTRACELLULAR
SIDE OF
MEMBRANE
Glycolipid
Membrane Proteins and Their Functions
• A membrane
– Is a collage of different proteins embedded in
the fluid matrix of the lipid bilayer
Fibers of
extracellular
matrix (ECM)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Integral proteins
– Penetrate the hydrophobic core of the lipid
bilayer
– Are often transmembrane proteins, completely
spanning the membrane
EXTRACELLULAR
SIDE
Figure 7.8
N-terminus
C-terminus
a Helix
CYTOPLASMIC
SIDE
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• An overview of six major functions of
membrane proteins
Figure 7.9
Transport. (left) A protein that spans the membrane
may provide a hydrophilic channel across the
membrane that is selective for a particular solute.
(right) Other transport proteins shuttle a substance
from one side to the other by changing shape. Some
of these proteins hydrolyze ATP as an energy ssource
to actively pump substances across the membrane.
Enzymatic activity. A protein built into the membrane
may be an enzyme with its active site exposed to
substances in the adjacent solution. In some cases,
several enzymes in a membrane are organized as
a team that carries out sequential steps of a
metabolic pathway.
Signal transduction. A membrane protein may have
a binding site with a specific shape that fits the shape
of a chemical messenger, such as a hormone. The
external messenger (signal) may cause a
conformational change in the protein (receptor) that
relays the message to the inside of the cell.
(a)
(b)
(c)
ATP
Enzymes
Signal
Receptor
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Cell-cell recognition. Some glyco-proteins serve as
identification tags that are specifically recognized
by other cells.
Intercellular joining. Membrane proteins of adjacent cells
may hook together in various kinds of junctions, such as
gap junctions or tight junctions (see Figure 6.31).
Attachment to the cytoskeleton and extracellular matrix
(ECM). Microfilaments or other elements of the
cytoskeleton may be bonded to membrane proteins,
a function that helps maintain cell shape and stabilizes
the location of certain membrane proteins. Proteins that
adhere to the ECM can coordinate extracellular and
intracellular changes (see Figure 6.29).
(d)
(e)
(f)
Glyco-
protein
Figure 7.9
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Synthesis and Sidedness of Membranes
• Membrane proteins and lipids
– Are synthesized in the ER and Golgi apparatus
ER
Figure 7.10
Transmembrane
glycoproteins
Secretory
protein
Glycolipid
Golgi
apparatus
Vesicle
Transmembrane
glycoprotein
Membrane glycolipid
Plasma membrane:
Cytoplasmic face
Extracellular face
Secreted
protein
4
1
2
3
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Concept 7.2: Membrane structure results in
selective permeability
• A cell must exchange materials with its
surroundings, a process controlled by the
plasma membrane

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membrane_structure.ppt

  • 1. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  • 2. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Overview: Life at the Edge • The plasma membrane – Is the boundary that separates the living cell from its nonliving surroundings • The plasma membrane exhibits selective permeability – It allows some substances to cross it more easily than others Figure 7.1
  • 3. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Membrane Models: Scientific Inquiry • Scientists studying the plasma membrane – Reasoned that it must be a phospholipid bilayer Figure 7.2 Hydrophilic head Hydrophobic tail WATER WATER
  • 4. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The Davson-Danielli sandwich model of membrane structure – Stated that the membrane was made up of a phospholipid bilayer sandwiched between two protein layers – Was supported by electron microscope pictures of membranes
  • 5. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Fluid Mosaic Model • In 1972, Singer and Nicolson – Proposed that membrane proteins are dispersed and individually inserted into the phospholipid bilayer Figure 7.3 Phospholipid bilayer Hydrophobic region of protein Hydrophobic region of protein
  • 6. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Freeze-fracture studies of the plasma membrane – Supported the fluid mosaic model of membrane structure Figure 7.4 A cell is frozen and fractured with a knife. The fracture plane often follows the hydrophobic interior of a membrane, splitting the phospholipid bilayer into two separated layers. The membrane proteins go wholly with one of the layers. Extracellular layer Cytoplasmic layer APPLICATION A cell membrane can be split into its two layers, revealing the ultrastructure of the membrane’s interior. TECHNIQUE Extracellular layer Proteins Cytoplasmic layer Knife Plasma membrane These SEMs show membrane proteins (the “bumps”) in the two layers, demonstrating that proteins are embedded in the phospholipid bilayer. RESULTS
  • 7. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Fluidity of Membranes • Phospholipids in the plasma membrane – Can move within the bilayer Figure 7.5 A Lateral movement (~107 times per second) Flip-flop (~ once per month) (a) Movement of phospholipids
  • 8. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Proteins in the plasma membrane – Can drift within the bilayer EXPERIMENT Researchers labeled the plasma mambrane proteins of a mouse cell and a human cell with two different markers and fused the cells. Using a microscope, they observed the markers on the hybrid cell. Membrane proteins Mouse cell Human cell Hybrid cell Mixed proteins after 1 hour RESULTS CONCLUSION The mixing of the mouse and human membrane proteins indicates that at least some membrane proteins move sideways within the plane of the plasma membrane. Figure 7.6 +
  • 9. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The type of hydrocarbon tails in phospholipids – Affects the fluidity of the plasma membrane Figure 7.5 B Fluid Viscous Unsaturated hydrocarbon tails with kinks Saturated hydro- Carbon tails (b) Membrane fluidity
  • 10. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The steroid cholesterol – Has different effects on membrane fluidity at different temperatures Figure 7.5 (c) Cholesterol within the animal cell membrane Cholesterol
  • 11. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 7.7 Glycoprotein Carbohydrate Microfilaments of cytoskeleton Cholesterol Peripheral protein Integral protein CYTOPLASMIC SIDE OF MEMBRANE EXTRACELLULAR SIDE OF MEMBRANE Glycolipid Membrane Proteins and Their Functions • A membrane – Is a collage of different proteins embedded in the fluid matrix of the lipid bilayer Fibers of extracellular matrix (ECM)
  • 12. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Integral proteins – Penetrate the hydrophobic core of the lipid bilayer – Are often transmembrane proteins, completely spanning the membrane EXTRACELLULAR SIDE Figure 7.8 N-terminus C-terminus a Helix CYTOPLASMIC SIDE
  • 13. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • An overview of six major functions of membrane proteins Figure 7.9 Transport. (left) A protein that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute. (right) Other transport proteins shuttle a substance from one side to the other by changing shape. Some of these proteins hydrolyze ATP as an energy ssource to actively pump substances across the membrane. Enzymatic activity. A protein built into the membrane may be an enzyme with its active site exposed to substances in the adjacent solution. In some cases, several enzymes in a membrane are organized as a team that carries out sequential steps of a metabolic pathway. Signal transduction. A membrane protein may have a binding site with a specific shape that fits the shape of a chemical messenger, such as a hormone. The external messenger (signal) may cause a conformational change in the protein (receptor) that relays the message to the inside of the cell. (a) (b) (c) ATP Enzymes Signal Receptor
  • 14. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cell-cell recognition. Some glyco-proteins serve as identification tags that are specifically recognized by other cells. Intercellular joining. Membrane proteins of adjacent cells may hook together in various kinds of junctions, such as gap junctions or tight junctions (see Figure 6.31). Attachment to the cytoskeleton and extracellular matrix (ECM). Microfilaments or other elements of the cytoskeleton may be bonded to membrane proteins, a function that helps maintain cell shape and stabilizes the location of certain membrane proteins. Proteins that adhere to the ECM can coordinate extracellular and intracellular changes (see Figure 6.29). (d) (e) (f) Glyco- protein Figure 7.9
  • 15. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Synthesis and Sidedness of Membranes • Membrane proteins and lipids – Are synthesized in the ER and Golgi apparatus ER Figure 7.10 Transmembrane glycoproteins Secretory protein Glycolipid Golgi apparatus Vesicle Transmembrane glycoprotein Membrane glycolipid Plasma membrane: Cytoplasmic face Extracellular face Secreted protein 4 1 2 3
  • 16. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Concept 7.2: Membrane structure results in selective permeability • A cell must exchange materials with its surroundings, a process controlled by the plasma membrane