Structure of the membrane
A membrane is a surface structure that encloses the cell and cell organelles.
The membranes include the cell membrane, tonoplasts, nuclei membrane, mitochondrial membrane and chloroplast membrane.
The membranes have a common basic structure which regulates the movement of materials in and out of the cells.
The cell membrane is made up of a phospholipid layer sandwiched by two protein layer(it is a lipo-protein layer) the overall thickness of the cell membrane is about 7.5 nm thick.
The membrane is perforated by small pores that allow the passage of substances in and out of the cells.
Properties of the cell membrane
a) The cell membrane is semi permeable- The pores that occur on the cell membrane allows the passage of the small size molecules but does not allow the passage of the large sized molecules. Such a membrane is said to be selectively permeable or semi-permeable.
In particular, when a cell is surrounded by a dilute sugar solution, the small sized water molecules will enter the cell but the larger sugar molecules will not pass through the cell membrane.
In contrast, the cell wall is permeable as it allows both sugar and water molecules to pass through it; it has larger pores. This property of selectively permeability enables the cell membrane to select what enters and leaves the cell.
b) The cell membrane is sensitive to changes in temperature and pH- Cell membranes are made up of protein. Proteins are adversely affected by extreme changes in temperature and pH. Changes in temperature and pH will alter the structure of the cell membrane thereby hindering the normal functioning of the cell membrane. High temperature denatures (destroys) the proteins thereby impairing the functions of the cell membrane.
c) The cell membrane possesses electric charges- The cell membrane has both positive and negative charges. These charges affect the manner in which substances move in and out of the ells. The charges also enable the cell to detect changes in the environment.
Physiological Processes of the Cell membrane
In this section, we discuss the various physiological processes through which materials move in and out of the cells across the cell membrane.
Materials move in and out of the cells through three main physiological processes:
c) Active transport