Download Lecture 5 The Cell membrane and Membrane Proteins The cell

10/21/10
The cell
Lecture 5
The Cell membrane
and
Membrane Proteins
Ameoba-­‐ single celled organism A single human cell The Cell is the simplest collec4on of ma9er that can live Cells 4ssue organ Cellular membranes
•  A cell is the simplest
collection of matter that
can live
Phospho-lipids
•  Plasma membrane
separates the living cell
from its non-living
environment
•  Cell membranes are
fluid mosaics of lipids
and proteins
•  Exhibit “selective
permeability” – allowing
some substances to
cross more easily than
others
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Plasma membrane is made up of a phospholipid bilayer Model of the cell membrane
Cell membrane
Membrane proteins
• 
• 
• 
• 
Phospholipids- fabric of the cell membrane
Proteins –determine membranes specific functions
Different types of cells-different types of proteins
Different organelles within a cell-different proteins
• Lipids are always moving sideways
• Proteins drift slowly
• Cholesterol act as a temperature buffer keeping the
membrane fluid at moderate temperatures
• Membranes must be fluid to work
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Membrane proteins
2 types of membrane protein
1.  Integral proteins
penetrate the
hydrophobic core, many
are “transmembrane”
Transmembrane protein
eg Bacteriorhodopsin
2.  Peripheral proteins are
not embedded in the
membrane, they are
often bound to exposed
integral proteins or
loosely to the surface of
the membrane
Membrane proteins
Membrane protein func4ons Hydrophilic channel or Carrier protein Tight junc4ons formed between cells Team of enzymes Glycoprotein-­‐ ID tag Stablises loca4on of certain proteins 3
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Transport across the membrane Transport across a membrane
•  Regulation of transport across the cell
membrane-essential its to existence
Eg. Muscle cells Sugars, amino acids, oxygen, ions in out Carbon dioxide, ions, metabolic waste •  Cell membrane is selectively permeable
Selective Permeabilty
•  Membrane proteins – key role in regulating
transport
Diffusion-passive transport
•  Diffusion- tendency for molecules of any substance to
spread evenly into available space
•  Any substance will diffuse down its concentration
gradient- spontaneous process – no energy required
•  Small hydrophobic molecules will diffuse across the
membrane
Eg. Oxygen crosses into cells performing cellular respira4on 4
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Selective permeability
•  Charged and polar molecules can cross the
membrane by passing through transport proteins
•  Transport proteins span the membrane
•  Transport proteins are very specific
•  2 types- channel and carrier
Channel protein eg. aquaporin Carrier protein eg glucose Facilitated Diffusion-Passive Transport
channel transporter eg. aquaporin •  Diffusion of hydrophilic
solutes across the
membrane must be
facilitated by transport
proteins
Ion channel eg. Sodium chanel •  Transport proteins are
very specific
•  Channel proteins provide
hydrophilic corridors
•  Ion channels
•  Gated ion channelsstimulus regulated
Facilitated Diffusion-Passive
Transport
•  Carrier proteinschange shape in
order to translocate
substances across
Carrier protein eg. Glucose transporter the membrane
•  Changes in shape –
triggered by the
binding and release of
the transported
molecule
Ligand gated ion channel eg. neurotransmi9ers Diffusion-passive transport
•  Uniport-movement of one
solute from one side to the
other
•  Co-transport
–  Symport-two solutes in
the same direction
–  Antiport-two solutes in
opposite directions
•  Solutes will diffuse down
their concentration gradient
Facilitated Diffusion is s4ll passive because the solute moves down the concentra4on gradient 5
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Active transport-uses energy
•  Movement of substances against their concentration
gradient across the membrane
•  From a low concentration to a high concentration
Active transport
•  ATP supplies the energy for active transport
•  ATP can transfer its terminal phosphate group
directly to the transport protein-transfer of energy
•  Eg of a carrier protein that requires ATP to transport
is the sodium-potassium pump
•  Requires work- cell uses energy- active
•  All active transporters are carrier proteins
•  Active transport enables a cell to maintain higher internal
concentrations of molecule compared to the external
environment
•  Eg. Low [Na+] and high [K+] compared with [extracellular]
Sodium-potassium pump
Membrane potential
•  is the difference in voltage (or electrical potential
difference) between the interior and exterior of a cell
•  Resting membrane potential -inside of the cell has a
negative voltage (millivolts; mV) with respect to the outside
of the cell (0 mV).
Sodium-­‐potassium pump = electrogenic pump = Generates a voltage across the membrane -­‐More nega4vely charged inside a cell than outside -­‐creates an “electrochemical” gradient 6
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Membrane potential
•  Voltage across the membrane =
“membrane potential”
-storage of electrical potential
energy
•  This potential energy can be
tapped into by the cell to carry
out work
•  eg.:
the diffusion of H+ ions back
down their electrochemical
gradient can be coupled to the
active transport of Sucrose
against its concentration
gradient
In Summary...Transport across the membrane Membrane potential
•  Upon stimulation of the cell, this
negative voltage inside the cell
(negative membrane potential)
may become
–  positive-depolarisation
–  More negative-hyperpolarisation
•  Important in
–  Neurons-Generation of a
membrane potential will induce
electric current (action potential) to
flow rapidly to other points in the
membrane
–  Mitochondria-the polarisation state
determines life or death.
Depolarisation =activation of cell
death
Summary
1.  Proteins - What are they? And why are they
important?
2.  The building blocks (aa’s) and how they are
connected
3.  The hierarchy of protein structure
4.  Three-dimensional protein structure
5.  Proteins in disease
6.  Enzymes as an example of complex proteins
7.  Cell membranes and the role of proteins
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Biology
Campbell and Reece
•  Chapters 5, 7 and 8
http://www.tcd.ie/Biology_Teaching_Centre/
BY1010/modules/BY1011
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