1. Cell structure and Function
1.6 a , Chemical constituents of cell- Lipids
Molecules of the cell
Dr V Malathi and Mrs Sushumna Rao
Lipids
- These are next abundant macromolecules of the cell. They are composed of carbon, hydrogen and may also oxygen, nitrogen, sulfur and phosphorus.
- They are energy-storing molecules.
- They are structural components of membranes and hormones.
Fatty Acids
These are type of lipids that contain long chain hydrocarbon with carboxylic acid functional group. They are hydrophobic and nonpolar. They are of two types as
- Saturated Fatty acids: These are fatty acids with hydrocarbon chains that contain only single bonds. As they have the greatest number of hydrogen atoms possible they are therefore referred as ,“saturated” (with hydrogen). They have straight flexible back bone. Lipids containing saturated fatty acids are solids at room temperature.
- Unsaturated Fatty acids : These are fatty acids with hydrocarbon chains containing at least one double bond . They are called unsaturated because they have fewer hydrogen atoms. Unsaturated fatty acids because of their double bonds have bend in their carbon skeleton. Lipids containing unsaturated fatty acids are liquids.
Triacylglycerides
These are fattyacids chemically linked to a glycerol molecule.
These are called simple lipids as they are composed only of two compounds namely glycerol and fattyacids.
These are primarily present in the adipose tissue and sebum.
They are energy storing molecules and has more calorific value than carbohydrates and proteins.
The hydrophilic portion of the phospholipid is often referred to as a polar “head,” and the long hydrocarbon chains as nonpolar “tails.”
As these contain both molecule hydrophobic portion and a hydrophilic moiety, they are said to be amphipathic.
This amphipathic nature enables them to form unique functional structures in aqueous environments.
The amphipathic nature of phospholipids enables them to form uniquely functional structures in aqueous environments like
Miscelles : These are spherical assemblies containing a hydrophobic phospholipid tail in the interior and the polar head groups on the outer surface.
Lipid-bilayer sheets, or unit membranes: These are large, two-dimensional assemblies of phospholipids congregated tail to tail. The cell membranes of cells are made of lipid-bilayer sheets.
These are lipid spheres formed by lipid bilayer sheets.
“Miscelles” by Openstax is licensed under CC BY 4.0
Glycolipids
A polar head groupcan be more complex than a simple phosphate moiety. Glycolipids are examples in which carbohydrates are bonded to the lipids’ head groups.
Isoprenoids
The are branched lipids, also referred to as terpenoids,
These are formed by chemical modifications of the isoprene molecule.
These lipids play a wide variety of physiological roles in plants and animals.
For example, they are constituents of plant pigments like beta carotene, xanthophylls, Fragrances like menthol, camphor.
Long-chain isoprenoids are also found in hydrophobic oils and waxes.
Waxes are typically water resistant and hard at room temperature, but they soften when heated and liquefy if warmed adequately. The sebaceous glands of hair follicles in the human skin secrete sebum, which consists mainly of triacylglycerol, wax esters, and the hydrocarbon squalene.
“Isoprenoids” by Openstax is licensed under CC BY 4.0
Steroids, Sterol, Cholesterol
These are complex ring structure lipids.
They are found in cell membrane.
Some function as hormones example Oestrogen.
The most common types of steroids are , which are steroids containing an OH group. These are mainly hydrophobic molecules, but also have hydrophilic hydroxyl groups.
Cholesterol is the most common sterol found in animal tissues.
Its structure consists of four rings with a double bond in one of the rings, and a hydroxyl group at the sterol-defining position.
Cholesterol strengthens cell membrane.
Prokaryotes generally do not produce cholesterol, although bacteria produce similar compounds called , which are also multiringed structures that strengthen bacterial membranes.
Fungi and some protozoa produce a similar compound called ergosterol, which strengthens the cell membranes of these organisms.
Test your Understanding
Lipid spheres made of bilayer sheets
Branched lipids also known as terpenoids
Most common type of steroids
Compounds similar to cholesterol produced by bacteria