Hydrophilic headThe head: choline, a phosphate group and glycerol. It is polar, so it is drawn to water.
Hydrophobic tailsThe two fatty acid tails. They are nonpolar, so they keep away from water.
Inside the molecule: 6 6
CholineThe small group at the top of the most common phospholipid, phosphatidylcholine. Other phospholipids carry a different group here, such as ethanolamine or serine.
Phosphate groupThe charged group joined to the glycerol. It gives a phospholipid its name.
GlycerolThe three-carbon backbone that joins the head to the two tails.
Saturated fatty acidA straight tail: its carbons are joined by single bonds only.
Unsaturated fatty acidA tail with at least one double bond. That puts a kink in it.
Double bondA double bond between two carbons, which bends the unsaturated tail. The kinks stop the tails packing tightly, so the membrane stays more fluid.
In a membrane: 2 2
Phospholipid bilayerTwo layers of phospholipids back to back, as in a cell membrane. Heads out to the water, tails inward.
WaterThe watery fluid on both sides of a membrane. It is inside and outside the cell. The heads face it.
How this list was compiled: Read 2026-09-27. (1) Wikipedia, Phospholipid, https://en.wikipedia.org/wiki/Phospholipid : a hydrophilic head containing a phosphate group and two hydrophobic tails from fatty acids, linked by an alcohol residue, usually glycerol; the phosphate group modified with choline, ethanolamine or serine. (2) Wikipedia, Lipid bilayer, https://en.wikipedia.org/wiki/Lipid_bilayer : two layers with the polar heads facing the water and the tails inside; an unsaturated double bond can put a kink in a tail, disrupting the packing and making the bilayer more fluid; a bilayer made of phosphatidylcholine. (3) OpenStax Biology 2e, 3.3 Lipids and 5.1 Components and Structure (used to check, nothing copied): the same parts, saturated tails straight and unsaturated tails kinked. Licence: own work
The drawing: Drawn for this site by script, 2026-09-27. Own work, nothing traced. Licence: own work
How it works
A phospholipid has two parts: a hydrophilic head and two hydrophobic tails. Start with those two, then the pieces of the molecule, then how many of them line up in a membrane.
Read the full guide
Head and tails
From the top: choline, the phosphate group, then glycerol, which the two fatty acid tails hang from. The head is polar and drawn to water; the tails are nonpolar and keep away from it.
In a membrane
Water sits on both sides of a cell membrane, so the phospholipids line up in two layers: heads out to the water, tails hidden in the middle.
Hydro means water, philic loving and phobic fearing: the head loves water, the tails fear it.
Questions and answers
What are the two parts of a phospholipid?
A hydrophilic head, which is drawn to water, and two hydrophobic fatty acid tails, which keep away from it. The head holds the phosphate group, and glycerol joins it to the tails.
Which part of a phospholipid is hydrophobic?
The two fatty acid tails. They are nonpolar, so in a membrane they point inward, away from the water, while the polar heads face out.
Why is one tail bent?
It is unsaturated: a double bond between two of its carbons puts a kink in it. Kinked tails cannot pack tightly, which keeps the membrane more fluid.