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Physiology of Cell Membranes
Cell membranes are phospholipid bilayers embedded with integral proteins. Phospholipids have a
polar head and a nonpolar tail. As a result of this composition, the phospholipid molecules naturally
form a bilayer with the heads facing the watery environment or interior of the cell and the nonpolar
tails facing each other in the middle of the bilayer. This arrangement makes the cell membrane
selectively permeable to different molecules based on their chemical composition. The integral
proteins embedded in the membrane are responsible for many membrane functions including:
transport, communication, enzyme reactions and tissue junction formation.
Transport through the cell membrane occurs in two basic ways. In passive transport, molecules
pass through the cell membrane along a concentration gradient from high concentrations to low
concentrations. Passive transport requires no energy and includes simple diffusion (movement of
solutes) and osmosis (movement of water). Examples of molecules that can diffuse include lipidsoluble molecules(ex. steroids) and respiratory gases(oxygen and carbon dioxide). Water can easily
move through the cell membrane because it is small. In active transport, a substance is transported
against its concentration gradient(from low to high concentrations). This requires energy in the form of
ATP. Examples of molecules that require an active transport mechanism include water-soluble
molecules like ions, many sugars and proteins.
The human cell is bounded by a cell, or plasma membrane. Inside the cell are membrane-bound
organelles and the gel-like cytosol(cytoplasm). Each organelle has a specific metabolic
function. Many of these organelles cooperate to produce a large variety of proteins which are used
during the normal course of cell functions.
Cell Lifespan and Division
The primary function of cells is to make proteins, particularly enzymes, which control the cell's
processes. About 90% of the cell's lifespan is involved in synthesizing proteins for growth, repair and
other cellular processes. This stage of the cell cycle is called interphase. As the cells die they must
divide to replace themselves. The division occurs in two stages:mitosis (duplication and separation
of the genetic material in the nucleus) and cytokinesis (division of the cytoplasm and the organelles.)
The mitotic stages are prophase, metaphase, anaphase, telophase. Mitotis and Cytokinesis
results in two genetically identical daughter cells. Use the image below as a guide to the stages of
cell division.