1.6 a , Chemical constituents of cell- Lipids

Dr V Malathi; Mrs Sushumna Rao

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.

A diagram showing a triglyceride is made of a glycerol and three fatty acids. Glycerol is a 3 carbon chain with an OH on each carbon. The H on each OH is highlighted. Fatty acids are long carbon chains with a C that has an OH and a double bonded O at the end. The OH of this C is highlighted. Three fatty acids are shown. Each fatty acid binds to one of the O’s from the OH groups on each Carbon on glycerol. The result is a triglyceride (or neutral fat) and 3 water molecules.

“Triglyceride” by Openstax is licensed under CC BY 4.0

Phospholipids

These are complex lipids.

These are composed of glycerol esterified with two fatty acids

The third binding position of glycerol is attached with a modified phosphate group.

A drawing of a phospholipid as a large circle with 2 rectangles projecting from the bottom. The circle is labeled hydrophilic head and contains glycerol (which contains 3 carbons). Attached ot one of these carbons is a phosphate (which is a phosphorus attached to 4 oxygen atoms). The rectangles at the bottom are both long carbon chains labeled as hydrophobic tails. One of the chains is a straight zig-zag line and is labeled saturated fatty acid. The other has a double bond that creates a bend in the line; this is labeled unsaturated fatty acid.

“Phospholipids” by Openstax is licensed under CC BY 4.0

Unlike other lipids Phospholipids are hydrophilic as they have a negatively charged phosphate group which can attract water.

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.

A lipid bilayer sheet is when there are 2 rows of phospholipids across each other forming a flat surface. The polar heads of all phospholipids are towards the outside of the sheet, and the nonpolar tails are towards the inside. This lipid-bilyaer can also form a sphere. The lipid-bilayer forms the surface of the sphere; the polar heads are on the outside of the sphere and lining the inside space of the sphere. Lipids can also form a single-layer sphere where the outside of the sphere is the polar heads and the nonpolar tails fill the center of the sphere.

“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.

Alpha-pinene is a carbon ring with added carbon projections. Camphor is a carbon ring with added carbon projections and a double bonded oxygen on one carbon. Isophrene is a 4 carbon chain with another carbon attached to carbon 2. Limonene is a carbon ring with a carbon attached to on one end and another carbon attached to the other end; this carbon has 2 carbons attached to it. Menthol i s a carbon ring with a carbon attached to on one end and another carbon attached to the other end; this carbon has 2 carbons attached to it. One more carbon corner has an OH group. Beta-carotene is two carbon rings attached by a long carbon chain.

“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.