SONGLyrical DNA
Heads to the Water
A song about how a cell membrane is built around water. The lyrics move from watery cytosol and extracellular fluid to amphipathic phospholipids, polar phosphate heads facing out and nonpolar tails packed inside, then membrane proteins with hydrophilic and hydrophobic regions. Select any highlighted line to see the biology behind it.
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Interactive lyrics
Lines with a dotted underline and a + marker have an explanation. Select one and the science appears beside the lyrics. Everything else is ordinary lyric text.
Intro
Intro
Line it right.
Build
Drop
Hold the cell together!
Hydrophilic outside!
Hydrophobic within!
Tails facing in!
Heads out!
Tails in!
Let the phospholipid drop begin!
Verse 1
They face each other, hidden from the scene,
Packed together in the membrane interior clean.
Build
Drop
Hold the cell together!
Hydrophilic outside!
Hydrophobic within!
Tails facing in!
Verse 2
Touching fatty acids from shore to shore.
Matching the membrane from surface to center again.
Break
Face the water.
Face the tails.
Hydrophilic channel?
Hydrophobic surface?
Membrane matches.
Final Drop
Hold the cell together!
Every region fits.
Polar where it is watery,
Nonpolar in the lipids.
Heads out!
Tails in!
Hydrophilic, hydrophobic, lock it in!
Heads out!
Tails in!
Outro
Polar heads facing
Both watery sides.
Fatty tails hidden.
Proteins fit in.
Tails facing in.
Concepts covered
Water on both sides of the membraneA plasma membrane sits between two watery solutions: the cytosol inside the cell and the extracellular fluid outside it. Both are mostly water, and water is a polar molecule, so both faces of the membrane are in constant contact with polarity. That is the condition the whole structure is built around, and it is why the phospholipid parts and the protein regions that meet water are polar or charged.
Amphipathic phospholipidsAmphipathic means one molecule has two opposite personalities. A phospholipid has a polar, hydrophilic phosphate region that interacts well with water, and nonpolar, hydrophobic fatty acid regions that do not. In a membrane the phosphate heads are oriented toward the aqueous environments inside and outside the cell, while the fatty acid regions face each other in the interior, no matter which way a diagram happens to be drawn.
Phospholipid bilayer structureTwo sheets of phospholipids stack tail to tail. The hydrophilic phosphate heads of one sheet are oriented toward the fluid outside the cell and the heads of the other sheet toward the cytosol, so both membrane surfaces are polar. The nonpolar fatty acid regions of the two sheets point inward and face each other, which gathers all the hydrophobic material into the middle and keeps it away from water on either side.
Hydrophobic effect and membrane self-assemblyNobody positions phospholipids one at a time. Dropped into water they sort themselves, because any arrangement that leaves hydrocarbon among water molecules is unstable and gets squeezed out. What survives is the one arrangement that satisfies both ends at once, which is why a bilayer assembles on its own rather than needing to be built. This tendency is called the hydrophobic effect.
Membrane fluidityThis is the fluid half of the fluid mosaic model. Membrane molecules are not cemented in position. The components of a plasma membrane can move around the surface of the cell within the membrane, drifting sideways past their neighbors. A membrane therefore behaves more like a moving sheet of liquid than a solid wall, while still holding the heads out and tails in pattern.
Polar phosphate-containing headThe head of a phospholipid contains a phosphate region that is polar and carries charge. Water is polar too, because the electrons in its bonds are shared unevenly between oxygen and hydrogen, so water molecules and the phosphate head attract each other and can form hydrogen bonds. Those favorable interactions are what hydrophilic means, and they are why the phosphate regions always face the aqueous environment.
Two hydrophilic membrane surfacesA membrane meets water on both sides, so it needs a hydrophilic surface on both sides. The heads of one layer are oriented toward the cytosol and the heads of the other toward the fluid outside the cell. The two solutions are not identical, since a cell keeps concentrations of ions and molecules inside that differ from those outside, but both are aqueous, and the polar phosphate heads interact with either one.
Hydrophobic fatty acid tailsFatty acid tails are long hydrocarbon chains, which makes them nonpolar and hydrophobic. They cannot form the favorable interactions with water that the polar phosphate regions can, so rather than staying spread out in the surrounding solutions they turn inward, away from the water on both sides. Those same hydrocarbon tails are what prevent ions and large polar molecules from crossing the membrane freely.
Membrane protein orientationProteins embedded in a membrane are not dropped in at random angles. Each has a definite orientation, because different regions of the same protein are chemically different. An embedded protein can be hydrophilic, and the regions that are hydrophilic are built from amino acids with charged and polar side groups, so those regions end up exposed to the cytosol rather than buried among the fatty acids.
Hydrophilic membrane channelsHydrophilic regions of a membrane protein have two places to go: exposed to the cytosol, or tucked inside the interior of the protein itself. When charged and polar side groups line an internal opening, they create a water friendly passage through a membrane whose core would otherwise repel them. Hydrophilic substances such as large polar molecules and ions move across using these embedded channels and transport proteins.
Hydrophobic regions of membrane proteinsThe surface of a protein where it touches the middle of the membrane is made of amino acids with nonpolar side groups. That hydrophobic surface is the part that interacts with the fatty acids in the interior of the membrane, so the protein is stable sitting inside the bilayer rather than being pushed out into the water. Matching chemistry, nonpolar against nonpolar, is what holds an embedded protein in place.
Amphipathic membrane proteinsEmbedded proteins can be hydrophilic, hydrophobic, or both at once. A protein that is both has hydrophilic regions of charged and polar side groups facing the cytosol or lining an internal channel, and hydrophobic regions of nonpolar side groups pressed against the fatty acids in the membrane interior. Being both is what allows one protein to span a membrane and work in two very different chemical environments.
Selective cell boundaryPlasma membranes separate the internal environment of the cell from the external environment, which is what lets a cell hold conditions inside that differ from its surroundings. It is not a sealed skin, though. Selective permeability is the result of the membrane having a hydrophobic interior, and small nonpolar molecules, including nitrogen, oxygen, and carbon dioxide, freely pass across it. That selectivity is what allows concentration gradients of solutes to form.
Why phospholipids are the lipid that builds membranesLipids as a class are typically nonpolar, hydrophobic molecules, and different lipids do very different jobs. Fats provide energy storage and support cell function. Phospholipids are the lipids that group together to form the lipid bilayers found in plasma and cell membranes. So when a membrane gets built, one specific member of the lipid family is doing the building, not lipids in general.
How water itself gets acrossThe nonpolar hydrocarbon tails of the phospholipids prevent ions and polar molecules from moving across the membrane, and yet water still gets through. Small polar molecules that carry no charge, like water and ammonia, pass through the membrane in small amounts. When a cell needs to move large quantities of water at once, aquaporins do that job instead.
The three R group categoriesEvery amino acid carries a variable R group, and R groups sort into exactly three chemical categories: hydrophobic and nonpolar, hydrophilic and polar, or ionic. Those three are the entire menu this section is quizzing. The interactions of the R groups along one stretch of a chain determine the structure and function of that region of the protein.
The mosaic half of the fluid mosaic modelA plasma membrane is not phospholipids alone. It consists of a structural framework of phospholipid molecules embedded with proteins, steroids such as cholesterol in vertebrate animals, glycoproteins, and glycolipids. That mixture of different kinds of molecule set into a single sheet is the mosaic half of the fluid mosaic model. Cholesterol in particular provides structural stability to animal cell membranes.
