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Lipids

Brother Gregory speaks to his class,

The subject for today is the structure and function of fatty acids and lipids. You must follow the lesson, answer the questions, then complete the required research investigations.

"Let us begin .......

"The average human adult is covered by a remarkable organ; the skin.  This highly specialized organ serves many functions that range from protection against attacks, a sensitive interface between the outside world and the world inside the body, temperature regulation, location of hair, storage of lipids and insulation.

"Furthest from the outermost layer of the skin is the hypodermis.  This is a layer of adipocyte cells and connective tissue that stores large amounts of lipids and thus carries out the two important functions of energy storage and thermal insulation.

"The adipose tissue is a loose collection of specialized cells (the adipocytes), which are embedded in a mesh of collagen fibers.

"There are two kinds of adipose tissue; white and brown.  The distribution of both these kinds is not uniform, but white adipose tissue is the most common. The main role of white adipose tissue is to collect, store and then release lipids as a source of energy for the cell and for the organism.

"Energy is stored in the covalent bonds holding atoms together in molecules. This energy, like the water behind a dam, is not obvious when the molecules are not reacting, but when they collide with one another, break apart and then reform into new, different molecules, this "chemical reaction" is always accompanied by a change in the amount of free energy.

"Most natural chemical reactions take place when molecules with low bond energies collide and crash into one another. Because of the low bond energy it does not take much effort to rip them apart. However, when they reform into newer molecules, they tend to form those kinds of molecules that are more stable. These stable molecules have a much larger bond energies holding them together.

"These differences in bond energies are due to the equal or unequal sharing of electrons in the covalent bonds holding the molecules together.

"Shared electrons in a covalent bond are pulled towards the positively charged protons at the centers of the two atoms. Electronegativity is a way of quantifying the amount of pull on the shared electrons in a covalent bond towards one atomic center, or the other.

"As a chemical reaction takes place, molecules in which the electrons are more or less equally shared by atoms are rearranged into new arrangements in which the electrons are not so equally shared by both sets of atoms.

"One example of this kind of reaction is when organic molecules react with molecular oxygen. At the beginning of the reaction electrons are equally shared by carbon and/or hydrogen atoms, but during the reaction these are rearranged to form molecules such as carbon dioxide and water, molecules in which the electrons are not equally shared.

As you proceed through this lesson, test yourself as you go by answering some of these "true/false" questions.

Hydrocarbons and Fatty Acids

"Hydrocarbons are compound molecules consisting entirely of the elements hydrogen and carbon.  Carbon atoms form linked chains to which hydrogen atoms are attached.

"Saturated hydrocarbon molecules (called alkanes) are molecules in which all the atoms in the structure are linked by single covalent bonds.

"Unsaturated hydrocarbon molecules (called, alkenes, alkynes and dienes) are molecules in which some of the carbon atoms can be linked by double or even triple covalent bonds.

"Hydrocarbon molecules are very rich in energy. They are high efficiency fuels that liberate a lot of useful and useable energy when burnt in oxygen, but unfortunately they are very hydrophobic and will not dissolve easily in water.

"Nature modifies pure hydrocarbons to make them more accessible to cells and organisms.

"A fatty acid is an organic acid (carboxylic acid -COOH), attached to a long aliphatic chain of either –CH2- units (saturated) or –CH=CH- units (unsaturated). Natural fatty acids, those found in organic tissues, usually have an even number of carbon atoms in the structure because of the way they are made or synthesized in the specialized adipose cells.

"Free fatty acids are those not joined with any other kind of molecules and are usually formed from the breakdown of a larger, more complex lipid molecule such as a di- or triglyceride.  This gives the free fatty acids and a molecule of glycerol.

"Fatty acids are made two carbon atoms at a time. Many of the metabolic processes taking place in cells, including the breakdown of carbohydrates for energy, result in the production of a two-carbon molecular fragment called an acetyl group (CH3-CO-). This is a very tiny molecular fragment that could easily get lost in the soup of similar tiny molecules which pack the cytoplasm of all metabolically active cells, so it is joined to a much larger molecule called CoEnzyme A (CoA).

"This hybrid molecule, acetyl-CoA, is a central player in the synthesis of all fatty acids.

"Fatty acids are made by repeatedly joining together the two-carbon fragments found in acetyl-CoA and then reducing the (-CO-) part of the molecule to (-CH2-). In this way, the hydrocarbon chain, which will become the hydrophobic, energy storing part of the fatty acid, grows two-carbons at a time as the cycle of joining reactions is repeated over and over again.

"Free fatty acids are either taken in from the digestive system directly, or are made in the liver. If the diet is rich in triglycerides (from a fatty meal), these are transported in the form of chylomicrons (from the gut) or lipoproteins (from the liver) to regions near the adipose tissue. An enzyme, lipoproteinlipase (LPL), is secreted by the adipocytes into the fluid surrounding each cell where it breaks down the triglycerides into free fatty acids and glycerol.

"The fatty acids are then transported across the adipocyte membrane into the cytoplasm of the cell. Here they are combined with a molecule of CoEnzyme A to form a thioester and then sequentially joined to a second and a third fatty acid to form mono-, di- and triglycerides. The final products are placed for longer-term storage into a large, central lipid vacuole.

Triglycerides and Phospholipids

"Lipids consisting of three molecules of fatty acids covalently bonded to one molecule of glycerol are termed triglycerides (or triacylglycerols), but are more commonly known as fats and oils.

"These molecules are the ultimate type of energy storage molecule because of their high calorific value and the fact that they can also thermally protect an animal's body from loss of heat in cold climates.

"However, they are found in both animals and plants and constitute an important part of the human diet. The division into fats and oils is arbitrary and depends on the physical state of these molecules at room temperature. Those molecules that are usually solid at room temperature (plant or animal) are termed fats, while those that are liquids at room temperature are termed oils. These latter are usually found in plants, but fish also store liquid forms of triglycerides (fish oils).

"Phospholipids are modified diglycerides and are usually found as an important component in cell membranes.

"In phospholipids a molecule of phosphoric acid (a phosphate group) replaces the third fatty acid molecule in a standard triglyceride molecule creating a diglyceride molecule that is no longer neutral (it carries a negative charge). The fatty acid parts of the molecule may be saturated or unsatruated.

"The phosphate group is then further modified by being joined to either a molecule of ethanolamine, choline or serine.

Required Readings
for the lesson
e-textbook

Table of Contents -|- Skin -|- Adipose

Electronegativity -|- Bond energy -|- hydrocarbons

Fatty Acids -|- fatty acid synthesis -|- Lipogenesis

Fats -|- Phospholipids

Science@a Distance
© 2005, Professor John Blamire