The Cell Membrane

It has been known for many years that cells and their nuclei are surrounded by membranes. With the use of the electron microscope, it was found that many organelles also had a membrane structure. Organelles allow for specialisation within a cell and greater efficiency. The structure of the cell surface membrane is the same as the membranes around organelles. Chemical analysis of cell membranes showed that they contained two types of molecules:

• Phospholipids: with water-hating (hydrophobic) tails and water loving (hydrophilic) heads.
• Proteins: also with some sections of the molecules that are hydrophilic and hydrophobic. These help in transportation across the membrane, provide structural support and form enzymes.

Before continuing with our study guide, have a look at two AS/A level sites on membrane structure. Membrane 1 - and Membrane 2

Movement across membranes can be by:

Active transport requires energy to carry substances from low to high concentrations.
Osmosis is the movement of water from an area of high water potential to an area of lower water potential through a partially permeable membrane. It is a passive process. Pure water has a water potential of 0 kPa. The addition of solutes to water gives it a more negative value.
Exocytosis is the transport of substances out of the cells, and endocytosis (e.g.Phagocytosis and pinocytosis) is the transport of substances into the cell.

Articles on the cell membrane -