There are four major groups of organic compounds in cells

An organic compound is one that contains both carbon and hydrogen. Most carbon compounds in cells do contain hydrogen (an exception would be CO₂)

(1) Nucleic Acids

These are DNA and RNA, the genetic components of the cell. They are polymers (strings of single units called monomers) of nucleotides. We'll look at these in detail in lab 7 but will not be testing for them here.

(2) Carbohydrates

Carbohydrates are sugars or polymers of sugars. Another name for them is saccharides. Monosaccharides are just a single sugar like glucose or fructose. Disaccharides have two sugars linked together such as sucrose (table sugar with a glucose and a fructose), lactose (milk sugar with a glucose and galactose) and maltose (two glucose molecules linked together). Most of these simple sugars will react with Benedict's reagent that we'll be using today. Polysaccharides have many sugar monomers linked together. These three below are all made from chains of glucose (and do not react with Benedict's):

• Starch - a way that plants store energy. Lots of starch in potatoes. Starch gives a positive reaction with Iodine. Here's the chemical structure of starch.
• Glycogen - Similar to starch but found in animals. Our livers (and muscle cells) store glucose in this form for ready access to energy.
• Cellulose - this polymer of glucose uses a different linkage to connect glucoses that makes it's polymers much stiffer. We do not have the enzymes that can break this kind of linkage and so we can't digest cellulose. It is a major component of wood.

You might notice that mono- and disaccharides end in -"ose". You might also notice that Benedict's reagent and iodine are specific for certain subgroups of carbohydrates, not all "carbs". "Carbs" is not a correct answer when specifying what these reagents react with.

(3) Lipids

Lipids are a diverse group characterized largely by being hydrophobic (water hating).Three major categories of lipids are:

• Triglycerides - This is what we usually call "fat" (image) in animals or "oil" in plants and are the most efficient way to store chemical energy in a cell. They have a 3-carbon glycerol with each carbon linked to a fatty acid (fatty acids are usually 14-16 carbons long). This is one of the items that you want to be low when you have blood work done at the doctor's office. Animal fat tends to be solid at room temperature because the fatty acids are saturated and so pack tightly together. Mono and poly-unsaturated fats are found in plants. Unsaturated refers to double bonds (sharing of two pairs of electrons rather than one) between carbon atoms in the fatty acids and it puts a kink in the fatty acids so they don't pack so tightly. These kinks cause them to be liquid at room temperature.
• Phospholipids - if you replace one of the fatty acids of the fat above with a highly charged phosphate group (and usually something else), you get a molecule that likes water at the phosphate end (hydrophilic) but that still doesn't for most of the rest of the molecule (hydrophobic). These molecules spontaneously form lipid bilayers - membranes in other words. They are major components of all the membranes found in a cell.
• Sterols - these are somewhat different from other lipids, being hydrophobic molecules formed from four carbon rings. Cholesterol is an important component of animal cell membranes (and another item you don't want a high value of in your blood tests). A variety of hormones (testosterone and estrogen) are in this group.

Our test today is the famous "scientific brown paper" test. Aqueous (water) solutions evaporate where as oily solutions leave a translucent greasy spot on the paper.

(4) Proteins

Proteins are the little machines that could. Proteins can be enzymes (biological catalysts that speed up chemical reactions), structural (like the components that give cells their shape or keratin in hair or collagen in our tendons and ligaments), communicative - like some hormones (growth factors, insulin) and cell receptors that recognize those hormones (as well as molecules that transport materials in and out of cells). Just about anything that needs to be done is done by proteins. Their monomers are amino acids. We have about 20 amino acids we use to make our proteins. When two amino acids are connected to each other, a peptide bond if formed. We call polymers of amino acids polypeptides (just another word for protein).

• Enzymes
• Membrane Receptors and transporters
• Some hormones
• Cytoskeletal elements
• Lots of other stuff (like antibodies)

We will test for proteins by testing for the the peptide bond using Biuret Solution. This test would not be positive for amino acids since it tests for the bonds between them in a protein.

Chemistry

The monomers in all four of these groups are combined to make larger molecules in a reaction called condensation. This is where two molecules join to make a larger molecule and a small molecule. The small molecule in this case is water and so it's also called a "dehydration" reaction. The reverse of this reaction, where you break these molecules into smaller pieces combines water back into them and is called "hydrolysis". If you see "hydrolysed protein" on the ingredient list of food or cosmetics, you'll know what it means.

You don't need to know the details of the chemical reactions in these tests but you do need to be able to recognize the positive and negative results so know the table below.

From PSTCC's Official Practical Review

Know the basic premise of the 4 tests performed for organic compounds, and be able to identify a positive reaction.
Organic Compound	TEST	POSITIVE REACTION
Carbohydrates  simple-glucose	Benedict’s (must be heated)	Yellow/orange/brick red
Carbohydrates  complex-starch	Iodine	Blue/Black
Lipids	Paper Spot	Non-drying, translucent spot
Proteins	Biuret	violet

• Be able to give examples and characteristics of carbohydrates, lipids and proteins.
• Be able to explain steps that you would take to determine the types of organic compounds found in an unknown sample.
• What are the monomers for proteins?
• What are the monomers for carbohydrates?
• What are "polar" versus "nonpolar" compounds?
• How are monomers or subunits of organic compounds linked together to create polymers?