cell membrane

The Cell Membrane and Interactions with the Environment
Cells interact with their environment in a number of ways. Each cell needs to
obtain oxygen and other nutrients (carbohydrates, amino acids, lipid molecules,
minerals, etc.) from the environment, maintain water balance with its
surroundings, and remove waste materials from the cell. The boundary between
any cell and its environment (through which substances must pass) is the plasma
membrane, composed of phospholipid and protein molecules.
Plasma Membrane
The plasma membrane has a number of functions for a cell.
• Serves as the boundary between the cytoplasm of the cell and the external
environment, and selectively isolates the cell from the external environment.
• Maintains the cell's environment by regulating materials that enter or leave the
cell. (Anything that enters or leaves the cell must pass through the
membrane). We often say that a membrane is selectively or differentially
permeable for this reason.
• Provides mechanisms for cell-to-cell communication.
• Genetically unique cell recognition markers embedded in the plasma membrane
provide mechanisms for a cell to recognize itself and other cells of its
particular individual organism versus non-self (foreign materials). This is
important to the immune system and defense of the organism.
Although the plasma membrane forms the boundary of the cell, and surrounds the
cell, many internal structures of most cells also have their own membrane
boundaries. Much of what we say about membrane structure and function at this
time applies to all membranes.
The Fluid Mosaic Membrane Structure
The typical membrane structure consists of a phospholipid bilayer with a
number of proteins scattered throughout, along with some carbohydrates
(glycoproteins), glycolipids and sterols, similar to the way in which one does a
mosaic tile, hence the name.
Membrane Structure and Function - 2
Phospholipid Bilayer
A phospholipid has both polar and non-polar regions. The fatty acid "tails" of the
two phospholipid layers are oriented towards each other so that the hydrophilic
"heads", which contain the phosphate portion, face out to the environment as well
as into the cytoplasm of the cell's interior, where they form hydrogen bonds with
surrounding water molecules. Because the individual phospholipid molecules are not
bonded to each other, a membrane is flexible (or “fluid”), something which is
pretty important to its functions.
The fluidity of a membrane is crucial to its function. In caribou, circulation is
reduced in the lower legs to prevent excess heat loss during cold winters. The
membranes of the lower legs have more unsaturated fatty acids than those of the
upper legs to retain more fluidity in reduced temperatures. Brain cell membranes
in ground squirrels become more solid during hibernation. Phospholipids containing
more polyunsaturated fatty acids are more fluid than those with fewer
polyunsaturated fatty acids. Cholesterol in membranes reduces fluidity as well.
There are times when membranes need more or less fluidity, and molecular
composition provides for sure membrane flexibility.
Phospholipid Movement Unsaturated/Saturated With Cholesterol
Many materials that enter or leave the cell are water-soluble; the fatty acid layers
serve as a barrier to their free entry. Proteins in the membrane are required to
move these substances through the membrane. Lipids generally pass through the
membrane more easily.