biology-diffusion
Simple diffusion
The term simple diffusion refers to a process whereby a substance passes through a membrane without the aid of an intermediary such as a integral membrane protein.
The force that drives the substance from one side of the membrane to the other is the force of diffusion.
In order for substances to pass through a cell membrane by simple diffusion it must penetrate the hydrophobic core of the phospholipid bilayer.
The types of molecules that can do this are themselves substantially hydrophobic in nature such as carbon dioxide, oxygen or ethanol.
In the figure below the green triangle indicates a concentration gradient of carbon dioxide. The blue arrow indicates the direction of net flow of carbon dioxide. The carbon dioxide penetrates the phospholipid bilayer without the aid of an intermediary molecule. You should be aware that the relative sizes of the molecules in this figure are not correct. The carbon dioxide molecules are much smaller than the phospholipids.
Osmosis
Osmosis is the movement of water molecules from an area of high water concentration (weak/dilute solution) to an area of low water concentration (strong/concentrated solution) through a partially permeable membrane.
The Effects of Osmosis
1. If the water concentration of the cells cytoplasm is lower then that of the medium (i.e. the medium is a hypotonic solution) surrounding the cell then osmosis will result in the cell gaining water. The water molecules are free to pass across the cell membrane in both directions, but more water molecules will enter the cell than will diffuse out with the result that water enters the cell, which will then swell up and could possibly burst.
2. If the water concentration inside the cell is the same as that in the surrounding medium (i.e. the medium is a isotonic solution) there will exist a dynamic equilibrium between the number of molecules of water entering and leaving the cell and so the cell will retain its original size.
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The term simple diffusion refers to a process whereby a substance passes through a membrane without the aid of an intermediary such as a integral membrane protein.
The force that drives the substance from one side of the membrane to the other is the force of diffusion.
In order for substances to pass through a cell membrane by simple diffusion it must penetrate the hydrophobic core of the phospholipid bilayer.
The types of molecules that can do this are themselves substantially hydrophobic in nature such as carbon dioxide, oxygen or ethanol.
In the figure below the green triangle indicates a concentration gradient of carbon dioxide. The blue arrow indicates the direction of net flow of carbon dioxide. The carbon dioxide penetrates the phospholipid bilayer without the aid of an intermediary molecule. You should be aware that the relative sizes of the molecules in this figure are not correct. The carbon dioxide molecules are much smaller than the phospholipids.
Osmosis
Osmosis is the movement of water molecules from an area of high water concentration (weak/dilute solution) to an area of low water concentration (strong/concentrated solution) through a partially permeable membrane.
The Effects of Osmosis
1. If the water concentration of the cells cytoplasm is lower then that of the medium (i.e. the medium is a hypotonic solution) surrounding the cell then osmosis will result in the cell gaining water. The water molecules are free to pass across the cell membrane in both directions, but more water molecules will enter the cell than will diffuse out with the result that water enters the cell, which will then swell up and could possibly burst.
2. If the water concentration inside the cell is the same as that in the surrounding medium (i.e. the medium is a isotonic solution) there will exist a dynamic equilibrium between the number of molecules of water entering and leaving the cell and so the cell will retain its original size.
For