AP Biology · Unit 2 · Topic 2.3
Phospholipids arrange themselves without being told. Drop them in water and watch which way round they settle, then place proteins in the sheet.
Learning objectives 2.3.A Describe the roles of each of the components of the cell membrane in maintaining the internal environment of the cell. 2.3.B Describe the fluid mosaic model of cell membranes.
Narrated walkthrough of the “Place the proteins” tab. Captions are on by default.
The band across the middle is a membrane forming between two watery compartments. Drag each phospholipid into place. A phospholipid has a hydrophilic phosphate head, drawn as a circle, and two hydrophobic fatty acid tails. Water is above and below; the interior is not watery at all. New phospholipids arrive pointing either way: if one is the wrong way up, double-click or double-tap it, or press Turn it over.
Nothing directs this. A phospholipid has a region that water attracts and a region that water repels, and a bilayer is the only arrangement in water that satisfies both at once. Put one in upside down and the tails are forced into water they cannot mix with, which is why the tool marks it wrong.
The CED lists five kinds of component. Phospholipids are the sheet; the other four are embedded in it or attached to it.
Press Start the drift above and watch. The phospholipids keep their orientation but slide past each other in the plane of the membrane. That is the "fluid" half. The "mosaic" half is everything embedded in the sheet — proteins, glycoproteins, glycolipids and steroids, scattered rather than arranged in a pattern.
The bilayer is already built. Each protein below has a nonpolar band shown in the tail color and polar ends shown in the head color. Drag it up and down until every region is in an environment it can tolerate. A peripheral protein is hydrophilic all over, so it cannot enter the interior at all.
Answer in your own words before opening the answers.
1. A phospholipid has both a hydrophilic and a hydrophobic region, so in water this is the only arrangement that puts each region in an environment it tolerates.
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A cell wall is not a membrane and does not replace one. It lies outside the plasma membrane, gives the cell a structural boundary, and acts as a permeability barrier to some substances. Every wall here is made of a complex carbohydrate.