Noncommercial Joint-Stock Company “KAZAKH NATIONAL MEDICAL UNIVERSITY NAMED AFTER
S.D. ASFENDIYAROV "
a course of biophysics
Methodical instructions
Page 1 of 5
Theme-2: Biophysical methods for studying the structure and functions of biological
membranes on the model of an electric capacitor.
Purpose of the lesson:
To check and consolidate the knowledge about the main properties of biological membranes and
their functions. To master the methods of problems solving on this theme.
Teaching problems:
- physical and chemical peculiarities of the structure of membrane structures and mechanisms of
their functioning;
- solving of problems on the theme.
Main questions of the theme:
1.
2.
3.
4.
5.
6.
General concepts about biological membranes.
Modern concepts about the structure of membrane.
Model of Davson and Danielli, mosaic model, liquid-crystal model and etc.
Main functions of biological membranes.
Types and functions of membrane proteins.
Phasic transitions.
Cell membrane
p.1 Illustration of a Eukaryotic cell membrane
The cell membrane or plasma membrane is a biological membrane that separates the interior of
all cells from the outside environment.[1] The cell membrane is selectively permeable to ions and
organic molecules and controls the movement of substances in and out of cells.[2] It basically
protects the cell from outside forces. It consists of the lipid bilayer with embedded proteins. Cell
membranes are involved in a variety of cellular processes such as cell adhesion, ion
Noncommercial Joint-Stock Company “KAZAKH NATIONAL MEDICAL UNIVERSITY NAMED AFTER
S.D. ASFENDIYAROV "
a course of biophysics
Methodical instructions
Page 2 of 5
conductivity and cell signaling and serve as the attachment surface for several extracellular
structures, including the cell wall, glycocalyx, and intracellular cytoskeleton. Cell membranes can
be artificially reassembled.[3][4][5]
Function
The cell membrane or plasma membrane surrounds the cytoplasm of animal and plant cells,
physically separating the intracellular components from the extracellular environment.
Fungi, bacteria and plants also have the cell wall which provides a mechanical support for the cell
and precludes the passage of larger molecules. The cell membrane also plays a role in anchoring
the cytoskeleton to provide shape to the cell, and in attaching to the extracellular matrix and other
cells to help group cells together to form tissues.
The membrane is selectively permeable and able to regulate what enters and exits the cell, thus
facilitating the transport of materials needed for survival. The movement of substances across the
membrane can be either "passive", occurring without the input of cellular energy, or active,
requiring the cell to expend energy in transporting it. The membrane also maintains the cell
potential. The cell membrane thus works as a selective filter that allows only certain things to come
inside or go outside the cell. Cell employs a number of transport mechanisms that involve
biological membranes:
Fluid mosaic model
According to the fluid mosaic model of S.J. Singer and G.L. Nicolson (1972), which replaced the
earlier model of Davson and Danielli, biological membranes can be considered as a twodimensional liquid in which lipid and protein molecules diffuse more or less easily.[6] Although the
lipid bilayers that form the basis of the membranes do indeed form two-dimensional liquids by
themselves, the plasma membrane also contains a large quantity of proteins, which provide more
structure. Examples of such structures are protein-protein complexes, pickets and fences formed by
the actin-based cytoskeleton, and potentially lipid rafts.
Lipid bilayer
Diagram of the arrangement of amphipathic lipid molecules to form a lipid bilayer. The
yellow polar head groups separate the grey hydrophobic tails from the aqueous cytosolic and
extracellular environments.
Lipid bilayers form through the process of self-assembly. The cell membrane consists primarily of a
thin layer of amphipathic phospholipids which spontaneously arrange so that the hydrophobic "tail"
regions are isolated from the surrounding polar fluid, causing the more hydrophilic "head" regions
to associate with the intracellular (cytosolic) and extracellular faces of the resulting bilayer. This
forms a continuous, spherical lipid bilayer. Forces such as van der Waals, electrostatic, hydrogen
Noncommercial Joint-Stock Company “KAZAKH NATIONAL MEDICAL UNIVERSITY NAMED AFTER
S.D. ASFENDIYAROV "
a course of biophysics
Methodical instructions
Page 3 of 5
bonds, and noncovalent interactions, are all forces that contribute to the formation of the lipid
bilayer. Overall, hydrophobic interactions are the major driving force in the formation of lipid
bilayers.
Lipid bilayers are generally impermeable to ions and polar molecules. The arrangement of
hydrophilic heads and hydrophobic tails of the lipid bilayer prevent polar solutes (e.g. amino acids,
nucleic acids, carbohydrates, proteins, and ions) from diffusing across the membrane, but generally
allows for the passive diffusion of hydrophobic molecules. This affords the cell the ability to
control the movement of these substances via transmembrane protein complexes such as pores,
channels and gates.
Membranes serve diverse functions in eukaryotic and prokaryotic cells. One important role is to
regulate the movement of materials into and out of cells. The phospholipid bilayer structure (fluid
mosaic model) with specific membrane proteins accounts for the selective permeability of the
membrane and passive and active transport mechanisms. In addition, membranes in prokaryotes and
in the mitochondria and chloroplasts of eukaryotes facilitate the synthesis of ATP through
chemiosmosis.
Membrane lipids and proteins have great mobility, that is, they are able to diffuse due to thermal
motion. If the movement of their molecules occurs within the same membrane layer, then this
process is called lateral diffusion; if their molecules move from one layer to another, then the
process is called a “flip-flop” transition.
The frequency of molecule jumps due to lateral diffusion is   2 3
D
,
A
Where: D is the coefficient of lateral diffusion; A is the area occupied by one molecule on the
surface of the membrane. The sedentary life of a molecule in one position is inversely proportional
A
to the hopping frequency:   1 /  
2 3D
In this case, the mean quadratic displacement of molecules during time t is: S  2 Dt
Phospholipids forming lipid vesicles
Lipid vesicles or liposomes are circular pockets that are enclosed by a lipid bilayer. These structures
are used in laboratories to study the effects of chemicals in cells by delivering these chemicals
directly to the cell, as well as getting more insight into cell membrane permeability. Lipid vesicles
and liposomes are formed by first suspending a lipid in an aqueous solution then agitating the
mixture through sonication, resulting in a vesicle. By measuring the rate of efflux from that of the
inside of the vesicle to the ambient solution, allows researcher to better understand membrane
permeability. Vesicles can be formed with molecules and ions inside the vesicle by forming the
vesicle with the desired molecule or ion present in the solution. Proteins can also be embedded into
the membrane through solubilizing the desired proteins in the presence of detergents and attaching
them to the phospholipids in which the liposome is formed. These provide researchers with a tool to
examine various membrane protein functions.
Proteins
Noncommercial Joint-Stock Company “KAZAKH NATIONAL MEDICAL UNIVERSITY NAMED AFTER
S.D. ASFENDIYAROV "
a course of biophysics
Methodical instructions
Page 4 of 5
The cell membrane has large content of proteins, typically around 50% of membrane
volume[9] These proteins are important for cell because they are responsible for various biological
activities. Approximately a third of the genes in yeast code specifically for them, and this number is
even higher in multicellular organisms.[8]
The cell membrane, being exposed to the outside environment, is an important site of cell-cell
communication. As such, a large variety of protein receptors and identification proteins, such as
antigens, are present on the surface of the membrane. Functions of membrane proteins can also
include cell-cell contact, surface recognition, cytoskeleton contact, signaling, enzymatic activity, or
transporting substances across the membrane.
Most membrane proteins must be inserted in some way into the membrane. For this to occur, an Nterminus "signal sequence" of amino acids directs proteins to the endoplasmic reticulum, which
inserts the proteins into a lipid bilayer. Once inserted, the proteins are then transported to their final
destination in vesicles, where the vesicle fuses with the target membrane.
Permeability
The permeability of a membrane is the rate of passive diffusion of molecules through the
membrane. These molecules are known as permeant molecules. Permeability depends mainly on
the electric charge and polarity of the molecule and to a lesser extent the molar mass of the
molecule. Due to the cell membrane's hydrophobic nature, small electrically neutral molecules pass
through the membrane easier than charged, large ones. The inability of charged molecules to pass
through the cell membrane results in pH partition of substances throughout the fluid
compartments of the body.
The biological membranes on the model of an electric capacitor.
The membrane in its structure resembles a flat capacitor, the plates of which are formed by surface
proteins, and the role of the dielectric is played by the lipid bilayer. Using the formula of a flat
capacitor, we can estimate the dielectric constant from the hydrophobic and hydrophilic regions of
the membranes, knowing the limits of the change in the thickness of the membrane.
С
 0 S
d
Where: ε- is the dielectric constant, ε0- is the electric constant, s- is the area, d- is the distance
between the plates.
The physical properties of the membranes
•
The density of the lipid bilayer is 800 kg / m3, which is lower than that of water.
•
Dimensions. By electron microscopy data, membrane thickness (L) ranging from 4 to 13 nm,
and various cell membranes characterized by different thickness.
•
Viscosity. The lipid layer of the membrane has a viscosity η = 30-100 mPa*s (corresponding
to the viscosity of vegetable oil).
The membranes have a high electrical resistivity (about 107 Ohm * m) and a high specific capacitance
(approximately 0.5 * 10-2 F / m2). The dielectric constant of membrane lipids is 2.
Noncommercial Joint-Stock Company “KAZAKH NATIONAL MEDICAL UNIVERSITY NAMED AFTER
S.D. ASFENDIYAROV "
a course of biophysics
Methodical instructions
Page 5 of 5
Membranes contain a large number of different proteins. Their number is so great that the surface
tension of the membrane is closer to the surface tension at the protein-water interface (   10 4 N / m )
than lipid-water (   10 2 N / m ). The concentration of membrane proteins depends on the type of cell.
Problems:
1. Calculate the time  of sedentary life and the frequency of hoppings  from one membrane layer
to another one of the lipids of sarcoplasmic reticulum membrane if the coefficient of lateral
diffusion is D=45 µm2/sec and the area of one phospholipid molecule is А=1,9 nm2 .
2. Calculate the root mean square displacement of protein molecules in 2 sec if the coefficient of
lateral diffusion for them is approximately 10-12 mm2/sec.
3. How will the electrical (specific) capacity of membrane change at its transition from liquidcrystal state to the gel if it is known that in liquid-crystal state the thickness of hydrophobic layer is
3,9 nm and in the state of gel it is 4,7 nm. Permittivity of lipids  2.
1.
2.
3.
4.
5.
References:
Suzanne A.K. Introduction to physics in modern medicine. USA: Taylor@Francis Group,2009.
422 pages.
An Introduction by Roland Glaser. Biophysics. Second edition. Springer.2012
Patrick F.Dillon. Biophysics. Cambridge University Press. 2012
Daniel Goldfarb. Biophysics DeMystified. 2011 by the McGraw-Hill Company. USA
Philip Nelson. Biological Physics. 2004.
Additional:
1. www. Sciencedirect.com.
2. www.thecochranelibrary.com.
3. www.springerlink.com.
4. www.webofknowledge.com.
5. http://dlib.eastview.com.