1. Phospholipids and the phospholipid bilayer

The key molecule in the membrane is a phospholipid. 

  • Like triglycerides (fats and oils), phospholipids are built around a molecule of glycerol, a 3-carbon alcohol, (shown in pink at number 2 in the diagram).
  • Attached to the glycerol on one side are two fatty acids (shown in yellow at number 3). The two fatty acids make up the molecule’s “tail.”
  • On the other side of the molecule (number 1 in the diagram) is a phosphate-bearing “head.”

 

simple phospholipid, no numbers
A phospholipid. The fatty acid tails are on the right, in orange. The phosphate-bearing head is on the left, in red

Typically, phospholipids are represented by diagrams like the one shown on the right. Whatever orientation you see this in, you should immediately be able to recognize it as a phospholipid.

In terms of how phospholipids interact with water, they have have a split personality.

  • The fatty acid tails are non-polar, making them hydrophobic (water fearing). In other words, they act like a fat or an oil, avoiding water.
  • The heads are polar. This makes them hydrophilic (water loving). In other words, the heads interact with water through forming hydrogen bonds with whatever water molecules they bump into.

As a result, phospholipids, when mixed with water, spontaneously organize themselves into a few orientations. The one you need to understand is called a bilayer. The bilayer is a membrane that defines a sphere (which you can think of as an empty cell). If we sliced open the sphere, here’s what we’d see:

06a_circular bilayer with water

How does this work? Let’s just look at a tiny slice of the sphere.

07_slice of membrane

  1. The hydrophilic heads of the phospholipids orient themselves so that the heads are in contact with water.
  2. The hydrophobic tails cluster together. This is the same clustering that happens in a droplet of oil mixed in water. All of the tails create a hydrophobic, water-free zone.

One of the key ideas of the cell theory is that all cells come from other cells. Today, new cells are formed as a parent cell divides into two smaller daughter cells. But for understanding the origin of life, it’s important to understand that if some non-living process can create phospholipids, then these phospholipids can spontaneously organize themselves into cell-like structures. Add some genetic material, and a system for taking in energy and for cell replication (the origin of both of which requires a lot of explanation), then you have a plausible scenario for the emergence of the first cells.

We’ll learn more about the origin of life later in this course, but, for now, use these flashcards to make sure that you understand the basic structural element of membranes: the phospholipid bilayer.

2. Flashcards: The Phospholipid Bilayer

[qdeck random = “true” style=”width: 528px; border: 2px solid black; ” qrecord_id=”sciencemusicvideosMeister1961-Phospholipid Bilayer Flashcards (M8)”]

[h] Flashcards: The phospholipid bilayer

[i] If you haven’t used a set of flashcards on sciencemusicvideos before, here’s what you need to know.

  • Click ‘Check Answer’ to see the answer to each card.
  • If you know it, click ‘Got it.”
  • If you don’t know it as well as you’d like, click ‘Need more practice,’ and that card will go to the bottom of the deck so you can practice it again.
  • ‘Shuffle’ lets you shuffle the deck.

[!] Card 4+++++++++++++++++++++++++++++++++++[/!]

[q]The molecules that make up that basic structure of the membrane are called __________.

[textentry]

[a]The molecules that make up that basic structure of the membrane are called phospholipids.

[!] Card 5+++++++++++++++++++++++++++++++++++[/!]

[q]The molecule shown below is a__________.[textentry]

[a]The molecule shown below is a phospholipid.

[!] Card 5+++++++++++++++++++++++++++++++++++[/!]

[q]The two fatty acid chains make up the phospholipid’s “_________”.[textentry]

[a]The two fatty acid chains make up the phospholipid’s “tail.”

[!] Card 6+++++++++++++++++++++++++++++++++++[/!]

[q]The phosphate containing part makes up the phospholipid’s “_________”.[textentry]

[a]The phosphate containing part makes up the phospholipid’s “head“.

[!] Card 7+++++++++++++++++++++++++++++++++++[/!]

[q]A more scientific way to say “water-fearing” is __________.

[textentry]

[a]A more scientific way to say “water-fearing” is hydrophobic.

[!] Card 8+++++++++++++++++++++++++++++++++++[/!]

[q]A more scientific way to say “water-loving” is __________.

[textentry]

[a]A more scientific way to say “water-loving” is hydrophilic.

[!] Card 9+++++++++++++++++++++++++++++++++++[/!]

[q]Is the tail of a phospholipid hydrophobic or hydrophilic? [textentry]

[a]The tail of a phospholipid is hydrophobic.

[!] Card 10+++++++++++++++++++++++++++++++++++[/!]

[q]Is the head of a phospholipid hydrophobic or hydrophilic? [textentry]

[a]The head of a phospholipid is hydrophilic.

[!] Card 11+++++++++++++++++++++++++++++++++++[/!]

[q]Is the tail of a phospholipid polar or non-polar? [textentry]

[a]The tail of a phospholipid is non-polar.

[!] Card 12+++++++++++++++++++++++++++++++++++[/!]

[q]Is the head of a phospholipid polar or non-polar? [textentry]

[a]The head of a phospholipid is polar.

[!] Card 13+++++++++++++++++++++++++++++++++++[/!]

[q]The arrangement of phospholipids shown below is called a  phospholipid ________ [textentry]

[a]The arrangement of phospholipids shown below is called a  phospholipid bilayer.

[x]

If you want more practice, please press the restart button below. Otherwise, follow the links below.
[restart]
[/qdeck]

3. Quiz: Phospholipids

[qwiz random = “true” style = “width: 528px; border: 3px solid black; ” qrecord_id=”sciencemusicvideosMeister1961-Phospholipids Quiz (M8)”]

[h] Quiz: Phospholipids
[i] Here’s how the quiz works:

  • Each question is multiple choice, but the entire quiz is like a series of flashcards.
  • If you get the question right, it comes off the deck.
  • If you get the question wrong, it goes to the bottom of the deck, so you can try it again.

[!] Question 1++++++++++++++[/!]

[q] Which of the following molecules makes up the basic structure of the membrane?

[c] protein

[c] carbohydrate

[c] nucleic acid

[c*] phospholipid

[f] No. While proteins are incredibly important in allowing membranes to carry out their functions, they’re not the key structural component. Next time, try to identify the molecule that makes up the membrane’s lipid bilayer.

[f] No. Carbohydrates are important parts of membranes, playing a key role in displaying membrane markers, However, they’re not the key structural component. Next time, try to identify the molecule that makes up the membrane’s lipid bilayer.

[f] No. Nucleic acids are connected with heredity and cellular control, but they’re not a key part of cell membranes. Next time, try to identify the molecule that makes up the membrane’s lipid bilayer.

[f] Exactly. Phospholipids form the lipid bilayer, which is the key structural element of cell membranes.

[!] Question 2++++++++++++++[/!]

[q] The molecule shown below is a _____________?

[c]protein

[c*]phospholipid

[c]carbohydrate

[c]nucleic acid

[f] No. Proteins are polymers of amino acids. In the molecule above, you can see fatty acids on the right side, a glycerol in the center, and a phosphate bearing “head” on the left. Which molecule fits that description?

[f] Excellent. The molecule above is a phospholipid.

[f] No. Carbohydrates are polymers or monomers of monosaccharides. In the molecule above, you can see fatty acids on the right side, a glycerol in the center, and a phosphate bearing “head” on the left. Which molecule fits that description?

[f]No. Nucleic acids are polymers of nucleotides. In the molecule above, you can see fatty acids on the right side, a glycerol in the center, and a phosphate bearing “head” on the left. Which molecule fits that description?

[!] Question 3++++++++++++++[/!]

[q]In the diagram of a phospholipid shown below, which shaded area is covering the fatty acids?

[c]1

[c]2

[c*]3

[f]No. Number 1 is covering the ionically charged “head” of the phospholipid molecule. The fatty acids are the long hydrocarbon chains.

[f]No. Number 2 is covering the glycerol. The fatty acids are the long hydrocarbon chains.

[f]Yes. Number 3 is covering the fatty acids, identified by their long hydrocarbon chains.

[!] Question 4++++++++++++++[/!]

[q]In the diagram of a phospholipid shown below, which shaded area is covering the molecule’s polar “head.”

[c*]1

[c]2

[c]3

[f]Yes. Number 1 is covering the ionically charged “head” of the phospholipid molecule. The charged molecular pieces in this region make it polar (as opposed to the rest of the molecule).

[f]No. Number 2 is covering the glycerol. The polar head can be identified by the ionic charges found in this region.

[f]No. Number 3 is covering the fatty acids. The polar head can be identified by the ionic charges found in this region.

[!] Question 5++++++++++++++[/!]

[q]In the diagram of a phospholipid shown below, which shaded area is covering the glycerol?

[c]1

[c*]2

[c]3

[f]No. Number 1 is covering the ionically charged “head” of the phospholipid molecule. The glycerol is a 3 carbon alcohol. Where do you see a 3 carbon unit?

[f]Yes. Number 2 is covering the glycerol.

[f]No. Number 3 is covering the fatty acids. The glycerol is a 3 carbon alcohol. Where do you see a 3 carbon unit?

[!] Question 6++++++++++++++[/!]

[q]In the diagram of a phospholipid shown below, which shaded area is covering a hydrophobic portion of the molecule?

[c]1

[c]2

[c*]3

[f]No. Number 1 is covering the ionically charged “head” of the phospholipid molecule. The charged ions in this part of the molecule make it polar. Because polar molecules interact with water (which is also polar), that also makes region 1 hydrophilic.

[f]No. Number 2 is covering the glycerol. This 3-carbon unit serves as the central framework that holds the rest of the molecule together. See if you can find a part of this molecule that is truly hydrophobic (water-fearing) because of its chemical structure.

[f]Yes. Number 3 is covering the fatty acids. The long hydrocarbon chains (chains of carbon atoms with hydrogens attached) make this region of the molecule non-polar. Because non-polar molecules don’t interact with water, that also makes this region hydrophobic.

[!] Question 7++++++++++++++[/!]

[q]In the diagram of a phospholipid shown below, which shaded area is covering a hydrophilic portion of the molecule?

[c*]1

[c]2

[c]3

[f]Yes. Number 1 is covering the ionically charged “head” of the phospholipid molecule. The charged ions in this part of the molecule make it polar. Because polar molecules interact with water (which is also polar), that also makes region 1 hydrophilic.

[f]No. Number 2 is covering the glycerol. This 3-carbon unit serves as the central framework that holds the rest of the molecule together. See if you can find a part of this molecule that is truly hydrophilic (water-loving) because of its chemical structure.

[f]No. Number 3 is covering the fatty acids. The long hydrocarbon chains (chains of carbon atoms with hydrogens attached) make this region of the molecule non-polar. Because non-polar molecules don’t interact with water, that also makes this region hydrophobic. See if you can find a part of this molecule that is truly hydrophilic (water-loving) because of its chemical structure.

[!] Question 8++++++++++++++[/!]

[q labels = “top”]Drag the terms below onto the appropriate places on the phospholipid molecule below.

 

[l]hydrophobic

[fx] No.  Please try again.

[f*] Great!

[l]hydrophilic

[fx] No.  Please try again.

[f*] Correct!

[!] Question 9++++++++++++++[/!]

[q labels = “top”]Drag the terms below onto the appropriate places on the phospholipid molecule below.

 

[l]non-polar

[fx] No.  Please try again.

[f*] Great!

[l]polar

[fx] No.  Please try again.

[f*] Correct!

[!] Question 10++++++++++++++[/!]

[q labels = “top”]Drag the terms below onto the appropriate places on the phospholipid molecule.

[l]hydrophobic

[fx] No, that’s not correct.  Please try again.

[f*] Great!

[l]hydrophilic

[fx] No, that’s not correct.  Please try again.

[f*] Correct!

[!] Question 11++++++++++++++[/!]

[q labels = “top”]Drag the terms below onto the appropriate places on the phospholipid molecule below.

 

[l]polar

[fx] No, that’s not correct.  Please try again.

[f*] Great!

[l]non-polar

[fx] No.  Please try again.

[f*] Correct!

[!] Question 1++++++++++++++[/!]

[q] Which of the following best describes how phospholipids arrange themselves in the cell membrane?

[c*] The phospholipids form a bilayer where the hydrophilic heads face outward toward the watery exterior or the watery cytoplasm, and the hydrophobic tails cluster together in a water-free zone.

[c] The phospholipids form a bilayer where the hydrophobic heads form hydrogen bonds with one another on an inner layer, with the hydrophobic tails facing outward toward the cell’s watery exterior, or the watery cytoplasmic interior.

[c] Interlocking bonds form between the hydrophobic heads and the hydrophilic tails. This is repeated on the inside and outside of the membrane, enabling it to form a double layer.

[c] The phospholipids form a bilayer with the hydrophobic heads facing out and interacting with the watery cell exterior or the watery cytoplasm, while the hydrophilic tails form an inner layer.

[f] That’s exactly right. Just remember ‘heads out,’ ‘tails in,’ and you’ll understand the structure of the membrane’s phospholipid bilayer.

[f] That’s not right. The key thing to remember about the membrane’s structure is that it’s built around a phospholipid bilayer. Phospholipid molecules arrange themselves into a bilayer in which the phospholipids’ hydrophobic tails are facing inward, creating a water-free zone, and the hydrophilic heads are facing outwards.

[f] That’s not right. The key thing to remember about the membrane’s structure is that it’s built around a phospholipid bilayer. Phospholipid molecules arrange themselves into a bilayer in which the phospholipids’ hydrophobic tails are facing inward, creating a water-free zone, and the hydrophilic heads are facing outwards.

[f] No, but you’re close. As you seem to understand, the phospholipids do form a bilayer. But their arrangement is different from what you chose. In the bilayer, the phospholipids’ hydrophobic tails are facing inward, creating a water-free zone, and the hydrophilic heads are facing outwards.

[!] Card 4+++++++++++++++++++++++++++++++++++[/!]

[q]The molecules that make up that basic structure of the membrane are called __________.

[hangman]

[c]phospholipids

[f]Yes! The molecules that make up that basic structure of the membrane are called phospholipids.

[!] Card 5+++++++++++++++++++++++++++++++++++[/!]

[q]The molecule shown below is a__________.

[hangman]

[c]phospholipid

[f]Perfect! The molecule shown below is a phospholipid.

[!] Card 5+++++++++++++++++++++++++++++++++++[/!]

[q]The two fatty acid chains make up the phospholipid’s “_________”.

[hangman]

[c]tail

[f]Yes! The two fatty acid chains make up the phospholipid’s “tail.”

[!] Card 6+++++++++++++++++++++++++++++++++++[/!]

[q]The phosphate containing part makes up the phospholipid’s “_________”.

[hangman]

[c]head

[f]Perfect! The phosphate containing part makes up the phospholipid’s “head“.

[!] Card 7+++++++++++++++++++++++++++++++++++[/!]

[q]A more scientific way to say “water-fearing” is __________.

[hangman]

[c]hydrophobic

[f]Perfect! A more scientific way to say “water-fearing” is hydrophobic.

[!] Card 8+++++++++++++++++++++++++++++++++++[/!]

[q]A more scientific way to say “water-loving” is __________.

[hangman]

[c]hydrophilic

[f]Yes! A more scientific way to say “water-fearing” is hydrophilic.

[!] Card 9+++++++++++++++++++++++++++++++++++[/!]

[q]Is the tail of a phospholipid hydrophobic or hydrophilic?

[hangman]

[c]hydrophobic

[f]Correct! The tail of a phospholipid is hydrophobic.

 

 

[!] Card 13+++++++++++++++++++++++++++++++++++[/!]

[q]The arrangement of phospholipids shown below is called a  phospholipid ________

[hangman]

[c]bilayer

[f]Fantastic! The arrangement of phospholipids shown below is called a  phospholipid bilayer.

[x]

[restart]

[/qwiz]

Links

Depending on where you came from…

  1. Continue on to Cell Membrane Structure (the next tutorial in this series about cell membranes)
  2. Return to Lipids (in Biochemistry)
  3. Watch my Membranes!  music video