CHAPTER 17: Unit 2. Lipids Types

Lipids Types: Two major Categories: Fatty Acids and Steroids

Lipids are divided into two large categories:

a) Fatty Acids:

Fatty acids are carboxylic acids that have aliphatic chain (from 4 to 22 carbon atoms or more) which is not polar (considered as the tail of the molecule) and a polar end of carboxylic acid group (considered as the head of the molecule).

They are 3 types of fatty acids:

1. 1.  Saturated fatty acids:
      Are made of chain of aliphatic chain (from 4 to 22 carbon atoms or more). All bonds are single bonds in saturated fatty acids and the head of such fatty acid is polar (-COOH) and the tail of it is nonpolar (the rest of the molecule: the long chain). An example of saturated fatty acids is given below:
   2. Monounsaturated fatty acids:
      Are fatty acid with one double bond with polar head and nonpolar tail. An example of monounsaturated fatty acids is given below:
   3. Polyunsaturated fatty acids:
      Are fatty acid with two or more double bond with polar head and nonpolar tail. An example of polyunsaturated fatty acids is given below:

      The unsaturated fatty acids have cis and trans isomers because of the presence of the double bond(s). The trans isomer is represented by straight chain while the cis isomer is represented by kink or curved chain

      Reference: https://chemistry.stackexchange.com/questions/60735/what-makes-trans-fats-more- harmful-than-saturated-ones

      The cis isomers cannot stack themselves one molecule above the because of “kink” situation of the double bonds. The trans isomers, on the other hand can stack themselves on molecule above the another without any restriction. This yields to higher stability, In general trans unsaturated fatty acids have higher melting and boiling points compared with the cis isomers with comparable molecular weight.

      The video illustrates the different types and structures of the fatty acids:

      https://www.youtube.com/watch?v=O9lL2KStW9s

      Lipids based on Fatty Acids molecules are divided into 4 classes:

      1. Waxes
      2. Triacylglycerol
      3. Glycerophospholipids
      4. Sphingolipids 

      1.  Waxes are lipids made of one fatty acid connected to one long chain alcohol. Waxes are considered as esters. 

      

      Reference: http://chemistry.elmhurst.edu/vchembook/554wax.html 

      The video below illustrates the concept of waxes:  

      https://www.youtube.com/watch?v=Ilmh6QT5JTU 

      2. Triacylglycerol is made of one alcohol (glycerol) with three hydroxyl groups connected to three fatty acids. This reaction yields three esters (3 fatty acids + 1 glycerol). The three fatty acids can be monounsaturated, polyunsaturated or 
      Glycerol         Fatty acids with different saturations 

      Reference: https://chem.libretexts.org/Courses/University_of_Kentucky/UK%3A_CHE_103_-
      _Chemistry_for_Allied_Health_(Soult)/Chapters/Chapter_14%3A_Biological_Molecules/14.2
      %3A_Lipids_and_Triglycerides

      The video below illustrates the concept of Triacylglycerol:

      https://www.youtube.com/watch?v=OpyTJbzA7Fk 

      3. Glycerophospholipids are made of four components
      Glycerol, 2 fatty acids, phosphate and small amino group:

       

      The video below illustrates the concept of Glycerophospholipids:

      https://www.youtube.com/watch?v=asV-N1YeDFQ 

      The aggregation of phospholipids can aggregate or condense into two forms: a linear layers form and a micelle form: 
      Aggregate structures are called liposomes

      

      Reference: https://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/lipids.htm#fatacid 

      4. Sphingolipids are made sphingosine, a fatty acid, phosphate and a small amino group. Note that

      The video below illustrates the concept of Sphingolipids.

      https://www.youtube.com/watch?v=NwcxKTP1KXw 
      sphingolipids do not have a glycerol in their backbone

       Reference: https://chem.libretexts.org/Bookshelves/Biological_Chemistry/Supplemental_Module s_(Biological_Chemistry)/Lipids/Non-glyceride_Lipids/Sphingolipids
      The sphingosine structure is given below:

      

      Reference:  https://pubchem.ncbi.nlm.nih.gov/compound/Sphingosine
        

b) Steroids:

Steroids are the second category of lipids. Steroids are made of basic unit of four fused cyclic rings. Three of these rings are six – member rings and the fourth is five – member ring connected to one of these cyclic rings as shown in the figure below: The basic unit in steroids is also called steroids skeleton (nucleus).

Steroids Skeleton (nucleus)

Reference:https://chem.libretexts.org/Courses/University_of_South_Carolina_-_Upstate/USC_Upstate%3A_CHEM_U109_-_Chemistry_of_Living_Things_(Mueller)/13%3A_Lipids/13.4%3A_Steroids

An example of such steroids (lipids that are not based on fatty acids) is Cholesterol;

Cholesterol

Reference:https://chem.libretexts.org/Courses/University_of_South_Carolina_- _Upstate/USC_Upstate%3A_CHEM_U109_- _Chemistry_of_Living_Things_(Mueller)/13%3A_Lipids/13.4%3A_Steroids

a) Cholesterol :

Cholesterol is manufactured naturally in the human body. It is very component of cellular membranes (including myelin and brain and nerve tissues). Cholesterol is manufactured in the liver and it is also found in the bile salts and in the human skin which can be converted into D vitamin when the human skin is exposed directly to the sun.

Cholesterols in the body can be divided into classes:

1. Low Density Lipoprotein (LDL): Also called bad cholesterol: Higher concentration of LDL leads to cholesterol deposits in the arteries which in turn leads to its blockage and therefore a stroke might develop.
2. High Density Lipoprotein (HDL): Also called good cholesterol: It carries out cholesterol from the body to the liver where the liver can work on this cholesterol and remove it from the body.

The amount of cholesterol in human is less than 200 mg/dL. Above 200 mg/dL is considered as a health risk situation.

Cholesterol plays a major role in our body. The video below explains this importance:

b) Lipoprotein:

Lipoprotein has the general structure: Low Density Lipoprotein (LDL) structure is shown below.

Lipoprotein has the general structure: High Density Lipoprotein (HDL) structure is shown below.

Reference: http://mesacharter.blogspot.com/2015/

The videos below illustrate the LDL and HDL functions:

https://www.youtube.com/watch?v=BBdMpu3VOCM

https://www.youtube.com/watch?v=q0YiPqmsXRg

Lipoproteins are called “Transporting Lipids”. As mentioned above higher concentrations of such Low density lipoprotein (LDL) can lead to the obstruction of the arteries and clocking and eventually strokes. The higher density lipoproteins help in emulsifying and dissolving the cholesterol and help to transport it.Lipoproteins help also to move lipids through the blood stream to the liver and other storing tissues where they can be used as energy.The figure below shows LDL and HDL in the artery:

Reference: https://pbscholesterol101.weebly.com/structure-and-functional-differences-between-hdl-and-ldl.html

The structures of several of LDL’s are given below:

Reference: https://www.semanticscholar.org/paper/Low-Density-Lipoprotein-(LDL)-Antioxidant-from-Osorio-Londo%C3%B1o/493536ddd16b5bfd919f005e5bfc79c670bc6075/figure/0

c) Bile salts:

Bile salts have a steroid skeleton or nucleus and they have five- or eight-carbon side-chain terminating in a carboxylic fatty acid, and several hydroxyl groups, the number and orientation these hydroxyl group will yield to different types of Bile Salts.

Bile salts have a steroid skeleton or nucleus and they have five- or eight-carbon side-chain terminating in a carboxylic fatty acid, and several hydroxyl groups, the number and orientation these hydroxyl group will yield to different types of Bile Salts.

Reference: https://www.caymanchem.com/product/16942/glycochenodeoxycholic-acid-(sodium-salt)

Bile salts act as a detergent to dissolve the insoluble cholesterol. This process is called cholesterol absorption by bile salts. The bile salts therefore help in the digestion of fats and oils by emulsify them into micelles which will be easily carried out.Some bile salt are acting as hormones steroidsThe video below explains the synthesis and the function of Bile salts in our body:https://www.youtube.com/watch?v=wAa6a1AKdog

d) Steroid Hormones:

Steroid Hormones are produced in the adrenal gland. The liver is producing these types of hormones from fat, carbohydrates and proteins.

Steroid hormones have several functions such as controlling the inflammations, the metabolism, the immune system, water balance as well as sexual characteristics and fighting sickness.

Steroid Hormones are fat – soluble and they pass through the cell membrane and hence can cause changes inside the targeted cell. The cell membrane of a cell is made of phospholipids bilayer (fat insoluble) and hence preventing other molecule from diffusing into the targeted cell. Inside the cell, the steroid Hormones can bind to the specific receptors found in the cytoplasm of the targeted cell. The receptors bound to steroid hormones can move into the nucleus of the targeted cell and bind itself with new specific receptor in the chromatin. This new receptor bound with receptor – steroid hormone will form new steroid hormone –receptor called mRNA [message Ribonucleic Acid]. This process is called transcription

Steroid Hormones

Reference: https://www.britannica.com/science/steroid

The video below illustrates the steroids hormones:https://www.youtube.com/watch?v=J2u9Td8_hB8

e) Corticosteroids: Adrenal Corticosteroids

Corticosteroids are chemicals produced in the adrenal glands which is located above the kidney. The corticosteroids increase the blood glucose concentrations and facilitate the production of glycogen in the liver. They help the kidney balancing the electrolytes concentrations and water amount.

An example of such corticosteroids is cortisol which helps under stress to increase the blood glucose and hence regulate the concentrations of fats, carbohydrates and protein metabolisms.

Synthesized corticosteroids are used to reduce inflammation and fighting asthma and arthritis.

Side effects of the usage of such synthesized corticosteroids are:

thin skin, susceptibility to bruising, high or increased blood pressure, susceptibility to infections, build-up of fat around the face, chest and abdomen, thinning of the limbs, osteoporosis (thinning of the bones) leading to bone fractures, particularly in the spine, fluid retention and diabetes.

Reference: https://www.betterhealth.vic.gov.au/health/conditionsandtreatments/Hormones-cortisol-and-corticosteroids

Some synthesized Corticosteroids are given below:

Reference: https://www.ncbi.nlm.nih.gov/books/NBK279156/

The videos below illustrate the function of the adrenal glands.https://www.youtube.com/watch?v=4WJj8IE83oM

https://www.youtube.com/watch?v=prQlI38jcOI

e) Cell Membranes 

Cell Membranes is membrane which is semi preamble and it supports the cell and its integrity and shape.  It allows selectively ions and organic molecules to go through in and out of the cell. The very important function of the cell membrane is to protect the cell.

The lipid bilayer in cell membrane structure is shown below:

Typical Cell Membrane Composition

Reference:https://www.assignmentpoint.com/science/biology/function-and-structure-of-cell-membrane.html

Lipid Bilayer in Cell membranes

This bilayer is a thin polar membrane made of two layers of lipid molecules. This membrane is a flat sheet and it helps the cell to protect itself. The carbohydrates in the lipid bilayer can communicate with steroid hormones and neurotransmitters.

Reference:https://chem.libretexts.org/Bookshelves/Biological_Chemistry/Supplemental_Modules_(Biological_Chemistry)/Lipids/Applications_of_Lipids/Lipid_Bilayer_Membranes

The video below illustrates the function of the lipid bilayer.https://www.youtube.com/watch?v=9ILSsW3tXr8

They are 2 transportation mechanisms of molecules and ions going in and out of the cell membranes:Passive Transport Mechanism: It occurs when ions and molecules are transported from area with higher concentrations to area with lower concentrations.There are two types of Passive Transport Mechanism:a.)Diffusion:Small molecules such as O2 and CO2 H2O etc. move via diffusion in and out cell from the area with higher concentrations (outside the cell) to the area with lower concentrations (inside the cell).b.)Facilitated Diffusion:Certain and specific molecules can use the channels present in the protein that extended from the outer of the bilayer to the other side of the bilayer. Molecules and ions are transported through the Facilitated Diffusion are Cl–, HCO3–, and glucose. Such ions have higher concentrations outside of cell compared with lower concentrations inside the cell.Active Transport Mechanism:K+, Na+, Ca2+ can be transported from the area of lower concentrations to the area of higher of concentrations against their concentration’s gradient. Such ions have higher concentrations inside the cell than outside of the cell. They can move into cell (higher concentrations) from outside the cell (lower concentrations) by effect of nerve pulses conduction.The videos below illustrate these types of transport within the bilayer cell membranehttps://www.youtube.com/watch?v=UgN76naeA1Q

https://www.youtube.com/watch?v=Ptmlvtei8hw

https://www.youtube.com/watch?v=J5pWH1r3pgU

Active transport needs ATP (adenosine triphosphate) as source of energy to move ions and molecules around.Reference: https://socratic.org/questions/how-is-facilitated-diffusion-different-from-active-transport