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At the end of this section, you should be able to:
(c) Describe the structures and properties of the following biomolecules and explain how these are related to their roles in living organisms: phospholipid
🕐Estimated time for this section: 15 minutes
Phospholipids
Phospholipids
Recall from the previous section about triglycerides, which consists of 3 fatty acid chains joined to 1 glycerol.
A phospholipid has a similar structure as triglycerides, with the exception of 1 fatty acid chain between substituted with a phosphate group. So it has 2 fatty acid chains and 1 phosphate group joined to a glycerol.
💡What is a Phosphate?
💡What is a Phosphate?
A phosphate is an anion (negatively charged) with the chemical formula: PO43−
It is an essential mineral for life and can be obtained through the food we eat.
Some phosphate rich food are food rich in protein, like meat, fish, and nuts.
Phosphate can also be derived from phosphoric acid, H3PO4, through the dissociation of 3 hydrogen ions. (see image on the right)
Recall from functions of organelles, where do you think are phospholipids synthesised in the cell?
Smooth endoplasmic reticulum (SER)! Recall that the SER is the site of lipids synthesis.
💡Formation of phospholipids
💡Formation of phospholipids
Similar to triglycerides, to form a phospholipids, 1 glycerol would need to undergo condensation reaction with 2 fatty acids and 1 phosphoric acid.
Watch the video from 0:00 - 2:43 to see how the condensation reaction occurs between glycerol and phosphoric acid to form a phospholipid. (note that the video shows 2 fatty acids already joined to the glycerol)
Note that the name of the bond formed between phosphate and glycerol should be phosphoester bond (not phosphate ester bond)
In the formation of one phospholipid from 1 glycerol, 2 fatty acids and 1 phosphoric acid, how many water molecules would be released?
There will be 3 condensation reactions, 2 for the fatty acids and 1 for the phosphoric acid. 1 water molecule is released per condensation reaction. Thus there would be a total of 3 water molecules released from the formation of 1 phospholipid.
💡Properties of Phospholipids
💡Properties of Phospholipids
Phospholipids are commonly shown as a circle with 2 lines.
The sphere/circle represents the phosphate 'head' (along with the glycerol backbone), while the lines represent the hydrocarbon 'tails'.
Recall from properties of lipids,
fatty acids are non-polar and hydrophobic
phosphate is a charged ion, and thus, is hydrophilic
since phospholipids have both, hydrophobic and hydrophilic region, it is considered to be amphipathic.
Watch the following video again to see how phospholipids behave in an aqueous environment.
Keep in mind that:
Hydrophilic head 'loves water' and would want to be in contact with water.
Hydrophobic tails 'hates water' and want to avoid water, and prefer to be in contact with other hydrophobic tails.
How do you think phospholipids would behave in oil?
The structures would be inverted!
💡Functions of phospholipids
💡Functions of phospholipids
Watch the video from 7:35 - 13:03 to gain an understanding about the function of phospholipids.
The main function of phospholipids lies in the formation of the phospholipid bilayer, which is the basic structure of all membranes found in eukaryotic cells.
The amphipathic property of the phospholipid allow for the assembly of the bilayer with a hydrophobic core made up of the hydrophobic fatty acid tails. This allows the membrane to be semipermeable and control the movement of substances across the membrane.
More details about the function of the membrane will be explored in the chapter of Membrane structure and Cellular Transport.
Phospholipids synthesised can be further modified before being transported to the cell surface membrane, recall from functions of organelles, which organelle does modification of phospholipids occur and subsequent transport to the cell surface membrane?
Phospholipids synthesised in the SER are transported to the Golgi apparatus to undergo modification, before they are packed into (membrane renewal) vesicles, which are transported to the cell surface membrane, where the newly synthesised and modified phospholipids will be incorporated to the cell surface membrane.
🖊️Check Your Understanding
🖊️Check Your Understanding
1) Illustrate the structural differences between triglyceride and phospholipids.
2) Describe the difference in properties between triglycerides and phospholipids.
3) Attempt qns 11-14 in this SLS lesson.
2) Triglcyeride is hydrophobic but phospholipid is ampthipathic.
Triglyceride is hydrophobic because it is largely made up of hydrophobic hydrocarbon tail.
Phospholipid is amphipathic as it is composed of hydrophilic phosphate head and hydrophobic hydrocarbon tail.
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