Lipids

Lipids

Lipids are the one class of large biological molecules that does not include true polymers.

Hydrophobic

The unifying feature of lipids is that they mix poorly, if at all, with water.

Water-insoluble molecules

Lipids are water-insoluble, primarily nonpolar biological molecules composed mostly of hydrocarbons.

Neutral lipids

Neutral lipids are stored and used as an energy source.

Phospholipids

Phospholipids form cell membranes.

Steroids

Steroids serve as hormones that regulate cellular activities.

Energy-storage molecules

Neutral lipids, found in cells as energy-storage molecules, have no charged groups (nonpolar).

Types of neutral lipids

There are three types of neutral lipids: oils, fats, and waxes.

Oils

Oils are liquid at biological temperatures.

Fats

Fats are semisolid.

Waxes

Waxes are water insoluble and solids.

Glycerol

Glycerol is a three-carbon alcohol with a hydroxyl group attached to each carbon.

Fatty acid

A fatty acid consists of a carboxyl group attached to a long carbon skeleton.

Triacylglycerol

In a fat, three fatty acids are joined to glycerol by an ester linkage, creating a triacylglycerol, or triglyceride.

Saturated fatty acid

A fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated or unsaturated.

Unsaturated fatty acid

Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, from 4 to 28.

Classes of esters

Exist as three main classes of esters: triglycerides, phospholipids, and cholesteryl esters.

Triglycerides

Triglycerides form by dehydration synthesis between three-carbon glycerol (an alcohol) and three fatty acid side chains.

Ester linkage

A covalent bond - ester linkage forms between the -COOH group of the fatty acid and the -OH group of the glycerol.

Nonpolar triglyceride

The polar groups of glycerol are eliminated, forming a nonpolar triglyceride.

Saturated fats

Fats made from saturated fatty acids are called saturated fats and are solid at room temperature.

Unsaturated Fats

Fats made from unsaturated fatty acids that are liquid at room temperature.

Monounsaturated Fatty Acid

A fatty acid with one double bond.

Polyunsaturated Fatty Acid

A fatty acid with more than one double bond.

Hydrogenation

The process of converting unsaturated fats to saturated fats by adding hydrogen.

Trans Fats

Unsaturated fats with trans double bonds that may contribute more than saturated fats to cardiovascular disease.

Thermoregulation

The process by which a layer of fatty tissue just under the skin acts as insulation in mammals and birds.

Plant Fats

Fats that are usually unsaturated.

Animal Fats

Fats that are typically saturated.

Kink in Hydrocarbon Chain

A bend created by double bonds in unsaturated fatty acids.

Fatty Acid Chain Length

Most common fatty acids have chains of 14 to 22 carbons.

Healthier Fats

Unsaturated fats are considered healthier than saturated fats in the human diet.

Hydrogenated Vegetable Oils

Plant oils converted commercially to saturated fats through the process of hydrogenation.

Atherosclerosis

A condition that may be contributed to by a diet rich in saturated fats through plaque deposits.

Insulation in Animals

A function of triglycerides that helps maintain body temperature.

Waterproofing Bird Feathers

A function of triglycerides that helps make bird feathers waterproof.

Oiliness of Fatty Acids

As chain length increases, fatty acids become less water-soluble and oilier.

Double Bonds in Fatty Acids

Nearly every double bond in naturally occurring fatty acids creates a kink in the hydrocarbon chain.

Saturated and Unsaturated Fats

Saturated fats are found in solid animal fats such as butter.

Insulation

A layer of fatty tissue just under the skin acts as insulation in mammals and birds (e.g., penguins).

Waterproofing

Triglycerides help make bird feathers waterproof.

Structure of Waxes

Waxes consist of a long-chain fatty acid linked through an ester oxygen to a long-chain alcohol (e.g., cetyl alcohol).

Protective Coatings

Waxy coatings help animals keep skin, hair, or feathers protected, lubricated, and pliable.

Plant Waxes

Plants secrete waxes that form a protective exterior layer, which reduces water loss and resists infective agents.

Common Phospholipid Structure

The most common phospholipid has a glycerol backbone linked to two fatty acid chains and a polar phosphate group, which is linked to another polar group.

Polarity of Phospholipids

The end of the molecule containing the fatty acids is nonpolar and hydrophobic, while the end with the phosphate group is polar and hydrophilic.

Phospholipid Arrangement in Water

In watery environments, only the polar ends of phospholipid molecules are exposed to water.

Phospholipid Bilayer

The phospholipid bilayer is a film of phospholipids two molecules thick and is the structural basis of membranes.

Orientation in Bilayer

In a bilayer, the polar groups face the surrounding water molecules at the surfaces, while the hydrocarbon chains form a nonpolar, hydrophobic region in the interior.

Amphipathic Molecules

Substances that contain both hydrophilic and hydrophobic regions are amphipathic.

Spontaneous Bilayer Formation

Phospholipids are amphipathic and spontaneously form bilayers.

Biological Importance of Amphipathic Nature

The amphipathic nature of many lipids is by far their most important biological feature.

Micelles

If mixed with water, amphipathic molecules spontaneously assume structures called micelles.

Lipid Bilayer Formation

In lipid bilayers, the hydrophilic heads are oriented outward while the hydrophobic tails are oriented toward each other inside the barrier formed by the hydrophilic heads.

Diagram of Lipid Micelles

The diagrams illustrate the orientation of hydrophilic heads and hydrophobic tails in lipid micelles.

Hydrophobic Tails

The hydrophobic tails interact with one another inside the spherical micelle.

Hydrophilic Heads

The hydrophilic heads interact with water outside the micelle.

Lipid Bilayers

In water, phospholipids form a two layered plane, with their hydrophobic tails oriented inward and their hydrophilic heads oriented outward.

Top Layer of Lipid Bilayer

The top layer has hydrophilic heads on top and hydrophobic tails directed down.

Bottom Layer of Lipid Bilayer

The bottom layer has hydrophobic tails on top, continuing end to end with the top layer's tails, and hydrophilic heads directed down.

Selective Permeability

Selective permeability means that some substances cross a membrane more easily than other substances do.

Factors Affecting Diffusion

The polarity, size, and charge of solutes affect their rate of diffusion across a membrane.

Membrane Permeability

This difference in membrane permeability is a critical issue because controlling what passes between the exterior and interior environments is a key characteristic of cells.

Bond Saturation

Bond saturation and hydrocarbon chain length affect membrane permeability.

Unsaturated Hydrocarbon Tails

Unsaturated hydrocarbon tails tend to reduce van der Waals interactions holding the hydrophobic tails together, weakening the barrier to solutes.

Saturated Hydrocarbon Tails

As the length of the saturated hydrocarbon tails increases, the forces that hold them together also increases, making the membrane denser and less permeable.

Permeability of Lipid Bilayers

Lipid bilayers are more permeable when they contain many short, kinked, unsaturated hydrocarbon tails.

Sterols

Sterols, the most common steroids, have a single polar -OH group linked to one end of the ring framework and a complex, nonpolar hydrocarbon chain at the other end.

Cholesterol

Cholesterol is an important component of animal cell membranes.

Phytosterols

Similar sterols (phytosterols) occur in plant cell membranes.

Steroid Hormones

Steroid hormones control development, behavior, and many internal biochemical processes.

Estradiol

Estradiol (Estrogen) is a female sex hormone that has an —OH in the position where testosterone (androgen) has an =O.

Testosterone

Testosterone (androgen) is a male sex hormone that has a methyl group (—CH3) that is absent from estradiol.

Chlorophylls

Chlorophylls are pigments that absorb light and help convert it to chemical energy in plants.

Carotenoids

Carotenoids are pigments that absorb light and help convert it to chemical energy in plants.

Glycolipids

Lipid groups combine with carbohydrates to form glycolipids.

Lipoproteins

Lipid groups combine with proteins to form lipoproteins, which have important structural and functional roles in cell membranes.