The cytoplasm in all the cells (both plant and animal) is surrounded by a living membrane called the plasma-lemma or plasma membrane. This membrane is vital to the cell as it controls the exit and entry of all substances to and from the cell.
The plasma membrane, as demonstrated by Mc Alear and Edwards (1958) in fungi, invigilates and maintains connection with the nucleus.
It is also believed that many of the cell and their bounding membrane are same as the plasma membrane these organelles are nucleus, mitochondria, chloroplasts etc.
The presence of the cell membrane is proved by the following evidences.
1. On injury to a cell, when protoplasm flows out, it remains as a discreet unit.
2. Observations under electron microscope have revealed the presence of a boundary membrane over the surface of cytoplasm.
1. Size of the membrane:
It is about 75A thick and is composed of three concentric layers. The middle layer is about 35A in thickness, while the outer layers on either side or 20A thick.
2. Chemical composition:
Membranes from different sources show variations in chemical composition. Essentially however all membranes are lipoproteinanceous. The components are held together in a thin sheet by non covalent bonds.
The ratio of protein to lipids varies depending on the type (Golgi membrane, plasma membrane etc) of membrane, type of organism (eukaryote, prokaryote, etc.) and the type of cell (epidermis, phoelm etc). In addition to these two, certain membranes also have carbohydrates.
Membranes have three different categories of proteins. These are – structural proteins, functional proteins (enzymes) and carrier proteins. Structural proteins maintain the structural frame work of the membranes. The average molecular weight of structural proteins found in membranes in 3×104.
Enzymes catalyzing various physiological activities are found in the membrane of mitochondria, endoplasmic reticulum etc. Carrier proteins are known to form a complex with the substances and help them to pass through the membrane.
The protein composition of Diagrammatic representation of cell membrane different membranes vary (as having two unit membranes has been pointed out already)
For instance plasma membranes have 50% while mitochondrial membranes have 75% proteins.
The proteins present in plasma membrane are categorized into two viz. integral proteins and peripheral proteins.
While integral proteins form the back bone of the membrane, peripheral proteins have a weak association maintained by weak electrostatic interaction.
The integral proteins are hydrophobic in nature as they have to penetrate the hydrophobic lipids to form the basic structure of the membrane. These proteins do not easily disassociate from the membrane and it requires harsh chemical procedures to accomplish it.
The integral proteins have a hydrophilic portion also which protrudes out of lhe lipoid bilayer. Peripheral proteins are associated either with the hydrophilic head of lipids or hydrophilic part of the integral proteins.
The Lipids of membranes are of several types, but all of them share a common character in being amphipatic i.e. having both hydrophilic and hydrophobic portions within a molecule.
There are three categories of membrane lipids-they are phosopholipids, glycolipids and sterols. While most membrane lipids are phospholipids, cholesterol does not have phosphates.
Phospholipids are of two categories viz. phosphoglycerides and sphingophospholipids. Phosphoglycerides have a glycerol back bone, while sphingophospholpids have the back bone derived from sphingosine an aminoalchobol having a long hydrocarbon chain.
Glycolipids contain one or many monosaccharide units attached to the lipids based on sphingenine or ceramide. There are three kinds of glycolipids viz., Cerebrosides, sulphatides and gangliosides.
Cerebrosides contain a single monosaccharide unit (galactose or glucose). Sulphatides are sulphuric acid esters of cerebrosides. Gangliosides are glycosphingolipids having sialic acid.
Sterols are steroid alcohols and are of three types – cholesterols (present in animal tissues), phytosterols (present in plants) and ergosterols (present in eukaryotic microorganisms).
Among the glycerophospholipids lecithin and cephalin are most abundant in higher plants and animals.