Saturday, December 7, 2013

Structure of Cells

Despite their differences, however, all cells share certain organizational and functional features. Every cellhas a plasma membrane. A plasma membrane is selectively permeable, allows only certain materials tocross. All cell membranes, including the plasma membrane, consist mainly of lipids. The plasmamembrane encloses a fluid or jellylike mixture of water, sugars, ions, and proteins called cytoplasm. Some or all of a cell’s metabolism occurs in the cytoplasm, and the cell’s internal components are suspended in it. All cells start out life with DNA, although a few types of cells lose it as they mature.

Cell type
Biologists have categorized cells into two general types: eukaryotic and prokaryotic cells.
The cells of plants, animals, fungi, protozoa, and algae are eukaryotic, and are placed in a category called Eucarya . All eukaryotic cells have their genetic material surrounded by a nuclear membrane forming the cellular nucleus. They also have a large number and variety of complex organelles, each specialized in the metabolic function it performs. In general, they are large in comparison to Prokaryotic cells. These cell types do not have a nuclear membrane; therefore they lack a cellular nucleus. In addition, they display unique chemical and metabolic characteristics but do not have the variety and number of organelles seen in eukaryotes.

Lipid Bilayer
Lipids—mainly phospholipids—make up the bulk of a cell membrane. A phospholipid consists of a phosphate containing head and two fatty acid tails. The polar head is hydrophilic, which means that it interacts with water molecules. The nonpolar tails are hydrophobic, so they do not interact with water molecules, but they do interact with the tails of other phospholipids. Lipid bilayers are the basic structural and functional framework of all cell membranes, gives membrane it's fluidity.



Fluid mosaic

Other molecules, including steroids and proteins, are embedded in or associated with the lipid
bilayer of every cell membrane. Most of these molecules move around the membrane more or less freely. A cell membrane behaves like a two dimensional liquid of mixed composition, so we
describe it as a fluid mosaic. The “mosaic” part of the name comes from a cell membrane’s mixed composition of lipids and proteins. The fluidity occurs because the phospholipids in a cell membrane are not bonded to one another. They stay organized as a bilayer as a result of collective hydrophobic and hydrophilic attractions.