Overview of Cell Structure
There are hundreds of different kinds of cells in an animal body. But all the cells share a common cell structure such as cytoplasm, cell membrane, and nucleus.
The outermost layer of the cell is the cell membrane / plasma membrane. This cell structure is an ultra-thin layer. It is a partially permeable membrane which controls the movement of substances in and out of the cell. This movement of substances is a selective process as it only allows for certain molecules to be transported.
The plasma membrane is useful for the cell for the following reasons:
- Separates cellular contents from external environment
- Excretes waste products
- Gains nutrients
- Generates essential ionic gradients
- Maintains a proper pH and ionic concentration in cell
- Secretes useful substances
- Has receptors for recognizing chemicals like neurotransmitters and hormones
The plasma membrane is basically a phospholipid bilayer. Its unique orientation allows it to have polar and nonpolar properties. This is due to the structure of phospholipids, which contain polar heads that are hydrophilic (water-loving) and nonpolar fatty acids chains (tails) that are hydrophobic (water-hating). The hydrophilic heads face outwards into the environment and hydrophobic tails face towards each other.
Plasma Membrane Structure
Carbohydrate chains are bound to phospholipids or protein molecules like antennae. Carbohydrates are known as ‘glycolipids’ when they attached to phospholipid molecules and ‘glycoproteins’ when they attached to proteins. Glycoproteins and proteins protrude the membrane.
There are cholesterol molecules embedded in the membrane. Some proteins are embedded in it forming channels to pass substances like ions and certain polar molecules across either direction of it while some proteins penetrate only a part of the cell membrane.
Some protein molecules float on the fluid phospholipid bilayer and some proteins are embedded giving a mosaic appearance. Therefore, this structure of plasma membrane is known as ‘fluid mosaic model’. The more cholesterol molecules there are in the plasma membrane, the more fluid it becomes.
Cytoplasm is the living material of the cell. The texture is an intermediate between solid and liquid, therefore it’s known to be a jelly-like material. It hosts complex structures and these structures are known to be different kinds of organelles. These organelles are not visible through a light microscope. A higher magnification is needed to examine those structures and they are visible through an electron microscope.
The nucleus is the largest organelle in the cell and it is enclosed by an envelope of two membranes. It is the controlling unit of the cell. There are pores in the membrane known as nuclear membranes. There is a nucleolus and almost all the living cells contain a nucleus. A membrane-bound nucleus is seen in the eukaryotic cell.