apparatus and plasma membrane (Fig. 2.2). In addition to them plant cells have
chloroplasts. There are many chemical reactions catalyzed by enzymes in the cell.
Basic functions of biomembranes are compartmentation and selective permeation for
those chemical reactions. Water occupies 66% of body weight in a human, and
aqueous environment is required even for living organism on land. Biomembranes
also provide hydrophobic environment for various functions of biomembranes.
2.2 Thermodynamics of Lipid Assemblies
When lipid molecules are dispersed in water the lipid molecules spontaneously
assemble, which is a process of thermodynamics. For this process, chemical potential of one lipid molecule and that of a lipid assembly composed of n lipid molecules
are considered as follows. Chemical potential of single lipid molecule in water μ W is
represented as follows, where μ 0W , X W and f W are standard chemical potential, mole
fraction and activity coefficient of single lipid molecule in water, respectively.
μ W ¼ μ 0W RT ln Xw þ R T ln f W μ 0micelle, n
Average chemical potential of a molecule in a micelle as lipid assembly, η micelle, n
is represented as follows where μ 0micelle, n and X n are standard chemical potential and
mole fraction of a lipid molecule in water, respectively.
Inverted cone
Cone
Cylinder
Micelle
Hwxagonal II
Lipid bilayer
Fig. 2.1 Shape of lipid molecule and structure of lipid assembly
Geometrical shape of lipid molecule determines structure of lipid assembly
20
2 Cells and Biomembranes
chloroplasts. There are many chemical reactions catalyzed by enzymes in the cell.
Basic functions of biomembranes are compartmentation and selective permeation for
those chemical reactions. Water occupies 66% of body weight in a human, and
aqueous environment is required even for living organism on land. Biomembranes
also provide hydrophobic environment for various functions of biomembranes.
2.2 Thermodynamics of Lipid Assemblies
When lipid molecules are dispersed in water the lipid molecules spontaneously
assemble, which is a process of thermodynamics. For this process, chemical potential of one lipid molecule and that of a lipid assembly composed of n lipid molecules
are considered as follows. Chemical potential of single lipid molecule in water μ W is
represented as follows, where μ 0W , X W and f W are standard chemical potential, mole
fraction and activity coefficient of single lipid molecule in water, respectively.
μ W ¼ μ 0W RT ln Xw þ R T ln f W μ 0micelle, n
Average chemical potential of a molecule in a micelle as lipid assembly, η micelle, n
is represented as follows where μ 0micelle, n and X n are standard chemical potential and
mole fraction of a lipid molecule in water, respectively.
Inverted cone
Cone
Cylinder
Micelle
Hwxagonal II
Lipid bilayer
Fig. 2.1 Shape of lipid molecule and structure of lipid assembly
Geometrical shape of lipid molecule determines structure of lipid assembly
20
2 Cells and Biomembranes