membrane structure function - university of...
TRANSCRIPT
1
MEMBRANE STRUCTURE AND FUNCTION
�������������� �
• selective permeability • permits some substances to cross it
more easily than others
Figure 7.1
���� ���������•Scientists studying the plasma membrane
•Reasoned that it must be a phospholipid bilayer
Figure 7.2
HydrophilicheadHydrophobictail
WATER
WATER
2
����������������
• In 1972, Singer and Nicolson• Proposed that membrane proteins are
dispersed and individually inserted into the phospholipid bilayer
Figure 7.3
Phospholipidbilayer
Hydrophobic region of protein
Hydrophobic region of protein
����� �����������������������
• Freeze-fracture studies of the plasma membrane
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 bilayerinto 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
Extracellularlayer
Proteins
Cytoplasmiclayer
Knife
Plasmamembrane
These SEMs show membrane proteins (the “bumps”) in the two layers, demonstrating that proteins are embedded in the phospholipid bilayer.
RESULTS
�������������������� ��
• 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
3
� ����������������������������������������
• Affects the fluidity of the plasma membrane
Figure 7.5 B
Fluid Viscous
Unsaturated hydrocarbontails with kinks
Saturated hydro-Carbon tails
(b) Membrane fluidity
������ ��������• 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 cellHybrid cell
Mixedproteinsafter1 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
+
�����������
• Has different effects on membrane fluidity at different temperatures
Figure 7.5 (c) Cholesterol within the animal cell membrane
Cholesterol
Low Temperature- Interferes with packing of membrane lipids
High Temperature- Hinders transverse movement (fluidity)
4
Figure 7.7
Glycoprotein
Carbohydrate
Microfilamentsof cytoskeleton Cholesterol Peripheral
proteinIntegral
proteinCYTOPLASMIC SIDE
OF MEMBRANE
EXTRACELLULAR
SIDE OFMEMBRANE
Glycolipid
���� �������� �
Fibers of
extracellularmatrix (ECM)
Transmembrane Proteins
• Structure of transmembrane protein
Functions of Transmembrane Proteins
• Transport Enzymes
Example: Na+K+ Pump Example: Tyrosine Kinase Receptor
5
Functions of Transmembrane Proteins
• Signal Intercellular Transduction Joining
Example: Hormone Receptor Example: Tight Junction
Functions of Transmembrane Proteins
• Cell-Cell Cytoskeleton- ECM Recognition
Transmission of extracellularstimuli
Sidedness of Plasma Membranes
6
����������������������������� �
�������� • tendency for molecules of any
substance to spread out evenly into the available space
Figure 7.11 A
One solute
Molecules of dye Membrane (cross section)
Net diffusion Net diffusion Equilibrium
(a)
��������
• Substances diffuse down their concentration gradient
Figure 7.11 B
Two solutes
(b)
Net diffusion
Net diffusion
Net diffusion
Net diffusion Equilibrium
Equilibrium
7
�������� �� ��������
• Passive transport is diffusion across a membrane
• Osmosis is the passive transport of water
• Cell survival depends on balancing water uptake and loss
Osmosis• The movement of water from a region of high water
concentration to a region of lower water concentration through a semipermeable membrane
Higher Solute Concentration(Hypertonic)
Fewer H2O molecules
Lower SoluteConcentration(Hypotonic)
More H2O molecules
��������������������������������������������
• Hypotonic solution is the solution that loses the water
• Hypertonic solution is the solution that gains the water
• Isotonic solutions have the same water concentration
8
�����!��� ���� ������
Figure 7.13
Hypotonic solution Isotonic solution Hypertonic solution
Animal Cell
(a)
H2O H2O H2O H2O
Lysed Normal Shriveled
Plant Cell
(b)
H2OH2OH2OH2O
Turgid (normal) Flaccid Plasmolyzed
Filling vacuole 50 µm
(a) A contractile vacuole fills with fluid that enters froma system of canals radiating throughout the cytoplasm.
Contracting vacuole
(b) When full, the vacuole and canals contract, expellingfluid from the cell.
�����"������
� ���������
�������������������� �� ������������ �#���• Active transport is the pumping of
solutes against their gradients
9
��� ��������� �
• Provide corridors that allow a specific molecule or ion to cross the membrane
Figure 7.15
EXTRACELLULARFLUID
Channel proteinSolute
CYTOPLASM
A channel protein (purple) has a channel through which water molecules or a specific solute can pass.
(a)
�������������� �
• Undergo a subtle change in shape that translocates the solute-binding site across the membrane
Figure 7.15
Carrier proteinSolute
A carrier protein alternates between two conformations, moving asolute across the membrane as the shape of the protein changes.The protein can transport the solute in either direction, with the net movement being down the concentration gradient of the solute.
(b)
$� ���#�• Some ion pumps generate voltages
across membranes
10
����� �#���• Couples the transport of one solute to
another
%�&'(&���������#
��������� ������������ �#������#����
Figure 7.17
Passive Transport Active Transport
Diffusion Facilitated Diffusion
ATP
11
!��)���� �#���
• Bulk transport across the plasma membrane occurs by exocytosis and endocytosis
• Large proteins• Cross the membrane by different
mechanisms
!��)��� �#���
• Exocytosis• Transport vesicles migrate to the
plasma membrane, fuse with it, and release their contents
• In endocytosis• The cell takes in macromolecules by
forming new vesicles from the plasma membrane
EXTRACELLULARFLUID
PseudopodiumCYTOPLASM
“Food” or other particle
Foodvacuole
1 µm
Pseudopodiumof amoeba
Bacterium
Food vacuole
An amoeba engulfing a bacterium viaphagocytosis (TEM).
PINOCYTOSIS
Pinocytosis vesiclesforming (arrows) ina cell lining a smallblood vessel (TEM).
0.5 µm
Pinocytosis
Plasmamembrane
Vesicle
Phagocytosis
��������#������� ���������
Figure 7.20
PHAGOCYTOSIS
12
0.25 µm
RECEPTOR-MEDIATED ENDOCYTOSIS
Receptor
Ligand
Coat protein
Coatedpit
Coatedvesicle
A coated pitand a coatedvesicle formedduringreceptor-mediatedendocytosis(TEMs).
Plasmamembrane
Coatprotein
Receptor-mediated endocytosis
��������#������� ���������