Download biological membranes and membrane transport

BIOLOGICAL MEMBRANES
Associate Professor Dr. Wipa Suginta
School of Biochemistry, Institute of Science
References
1. Lehninger Principles of Biochemistry by David L. Nelson, Michael M. Cox.
2. Biochemistry by Jeremy M. Berg, John L. Tymoczko, Lubert Stryer.
3. Biochemistry by Donald Voet, Judith G. Voet.
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Overview of cellular membranes
Red-blood-cell plasma
membranes
Courtesy of Dr. Vincent Marches
The boundaries of cells are formed by plasma
membranes
Plasma membranes prevent molecules inside the
cell from leaking out and unwanted molecules from
diffusing in.
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Membrane permeability
Lipid bilayers are highly impermeable to ions and most polar molecules.
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Isolation of cellular membranes by
‘differential centrifugation’
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Isolation of cellular membranes by
‘density gradient centrifugation’
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Lipid vesicles can be formed from phospholipids
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Liposome for drug delivery
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Common features of biological membranes
1. Membranes are sheet-like structure. The
thickness is between 60 – 100 Å (6-10 nm).
2. Membrane lipid bilayers are barriers to the
polar molecules.
3. Membranes provide selective permeability
around the cell.
4. Specific proteins mediates distinct
functions, such as pumps, channels,
receptors, energy transducers, and enzymes.
5. Membranes are non-covalent assemblies.
6. Membranes are asymetric and fluidic.
7. Most cell membranes are electrically
polarized. For example, the intracellular
face has negative potential of ~60 mV.
8. Membrane potentials play a key role in
transport, energy conversion, and
excitability.
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Membrane transporters
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Energy transducer on the outer membrane of
mitochondria
ATP synthase (EC 3.6.3.14) is an important enzyme
that provides energy for the cell to use through the
synthesis of adenosine triphosphate (ATP). ATP is
the most commonly used "energy currency" of cells
from most organisms.
ATP synthase consists of 2 regions
• The FO portion is within the membrane.
• The F1 portion of the ATP synthase is above
the membrane, inside the matrix of the
mitochondria.
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Photosynthesis on the thylakoid membranes of plant cells
Photosynthesis is a process used by plants and other autotrophic organisms to convert light energy, normally
from the sun, into chemical energy that can be used to fuel the organisms' activities. Carbohydrates, such as
sugars, are synthesized from carbon dioxide during the process.
In plants, light-dependent reactions occur in the thylakoid membranes of the chloroplasts and use light energy
to synthesize ATP and NADPH.
In the light-independent (or "dark") reactions, the enzyme RuBisCO captures CO2 from the atmosphere and in a
process that requires the newly formed NADPH, called the Calvin-Benson Cycle, releases three-carbon sugars,
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which are later combined to form sucrose and
starch.
Membrane receptors
The insulin receptor is embedded in the cell membranes of muscle, fat cells and certain types of other cells. Its
function is to facilitate their uptake of glucose from the blood stream through special glucose transport proteins
that are normally present inside the cell in an inactive form. When the insulin molecule binds to the alpha
subunits of the receptor, it triggers a chain reaction within the cytosol that activates GLUT4 and causes it to be
translocated and inserted into the cell membrane.
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Chemical composition of cellular membranes
Three common types of membrane lipids
• Phospholipids
• Glycolipids
• Cholesterols
Membrane consists mainly of lipids and
proteins (mass ratio ranging from 1:4 to
4:1).
Membranes also contain carbohydrates
that are linked to lipids or proteins and
cholesterols.
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Asymmetry of carbohydrates in plasma membranes
Carbohydrates – Total asymmetry
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Asymmetry of lipids in plasma membranes
Lipids – partial asymmetry
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Different membranes have different lipid compositions
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Phospholipids are major membrane lipids
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Effect of temperature on membrane fluidity
Types of phospholipids that affect
membrane fluidity
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Bleaching experiments to demonstrate membrane
movement
Fluorescence Loss in Photobleaching (FLIP)
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Immunological experiments that demonstrate lateral movement of
lipids in membranes
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Movement of phospholipids in a bilayer
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Cholesterol in plasma membranes
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Asymmetry of proteins in plasma membranes
Proteins – Total asymmetry
Association of proteins with the cell membranes
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Plasma membrane proteins have a variety of functions
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Membrane proteins are covalently attached to lipids by
various acyl groups
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Transport of proteins to endoplasmic reticulum (ER)
Proteins incorporated into the plasma membrane, enzymes in lysosomes and proteins secreted to
the outside of the cell are synthesized in ribosomes attached to the ER membrane.
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See movie: http://csls-text.c.u-tokyo.ac.jp/flash/0544_1.html
Transport vesicles transport the membrane components and
secretory proteins.
The destination of a transport vesicle is determined by the type of SNARE protein. When the transport vesicle is
fused with the plasma membrane, proteins on the membrane stay on the cell surface, while those inside the
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transport vesicle are released to the outside
of the cell.
Topology prediction of integral membrane proteins
Hydropathy plot of glycophorin
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Hydropathy plot of bacteriorhodopsin
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Channel proteins can be formed from beta-strands
Structure of bacterial porin
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Visualization of membrane proteins by atomic force
)
microscopy (AFM
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Solubilization of membrane proteins by detergents
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Properties of detergents
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A common experimental system for studying the functions of transport proteins is liposomes
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containing a purified transport protein.
Liposome swelling assay is the technique to study
permeation of molecules across membranes
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