Plasma Membrane

The idea, then worked examples solved step by step, then practice questions whose numbers change every time you try them.

The idea

The plasma membrane: a boundary with a building plan

A cell is wrapped in a plasma membrane. It separates the inside of the cell from the outside, and it helps the cell keep its internal environment steady. This topic asks what the membrane is built from and why that arrangement works. The answer comes from one chemical fact: some parts of the membrane like water and some do not.

Phospholipids: heads that like water, tails that do not

A phospholipid has two regions. Its polar, hydrophilic head contains a phosphate group, and it is attracted to water. Its two tails are fatty acids, which are nonpolar and hydrophobic, and they stay away from water. A molecule with both kinds of region is called amphipathic, or amphiphilic.

In a watery setting, phospholipids arrange themselves so that each head faces water and the tails hide from it. The polar hydrophilic phosphate regions are oriented toward the aqueous environment outside the cell and the aqueous environment inside it, while the nonpolar hydrophobic fatty acid regions face each other in the interior of the membrane. The result is a lipid bilayer, a double layer of phospholipids. Think of drops of oil in water: oil gathers together and avoids the water, and the tails behave the same way.

Embedded proteins: some regions like water, some do not

Proteins are embedded in the bilayer. An embedded protein can be hydrophilic, with charged and polar side groups, or hydrophobic, with nonpolar side groups, or both. A protein that spans the membrane is usually both, and its regions line up with the surroundings.

The picture shows the rule. The hydrophobic regions of the protein make up the part of its surface that touches the fatty acid tails in the interior of the membrane. The hydrophilic regions of the protein are either inside the protein itself or exposed to the watery cytosol (the cytoplasm), or to the watery fluid outside the cell. A hydrophilic region buried among the fatty acid tails would be out of place, like a swimmer in a pool of oil.

The fluid mosaic model

The fluid mosaic model describes the plasma membrane as a structural framework of phospholipid molecules that is embedded with proteins, steroids (such as cholesterol in vertebrate animals), glycoproteins and glycolipids. A glycoprotein is a protein with carbohydrate chains attached, and a glycolipid is a lipid with carbohydrate chains attached; these carbohydrates are on the outer surface of the membrane. All of these components can move around the surface of the cell within the membrane. That movement is the reason for the word fluid, and the many different parts side by side are the reason for the word mosaic, like the tiles in a mosaic picture.

The picture shows four members of the mosaic. The carbohydrate chains of a glycoprotein stand out on the outer surface, a channel protein spans the membrane, cholesterol lies alongside the phospholipids, and a peripheral protein sits on the inner surface. The word passive printed beside the channel belongs to a later topic: it says that a substance moving through an open channel goes down its concentration gradient without the cell spending energy. Cholesterol also tends to buffer the effect of temperature on an animal cell membrane: it stops the membrane from becoming too rigid when it is cold and from becoming too runny when it is warm.

The membrane is also not stiff like a wall: it is more like a crowd of floating tiles, which is why a membrane can flow and reseal when it is pierced by a fine needle. Temperature and the shape of the fatty acid tails affect the fluidity as well. Tails with double bonds are kinked, so they pack less tightly than straight tails, and a membrane with more kinked tails stays fluid at lower temperatures.

Warning.

Three traps to leave behind. First, the tails do not face the water: the heads face the watery outside and inside, and the tails face each other in the middle. Second, the membrane is not a fixed layer with proteins stuck in place: its components can move around within the membrane. Third, cholesterol is not a phospholipid and it is not a protein: it is a steroid, with four fused carbon rings, that lies alongside the phospholipids.

Adapted from Biology by OpenStax (Rice University), licensed under CC BY 4.0. Access for free at https://openstax.org/details/books/biology

Worked examples

Each example is the kind of question the AP exam asks on this topic, worked step by step.

Example.

1. Which way do the parts face? Phospholipids are mixed into water and arrange themselves into a double layer. Describe which part of each phospholipid faces which environment, and explain why.

A phospholipid has a polar, hydrophilic head that contains a phosphate group, and two nonpolar, hydrophobic fatty acid tails.
Hydrophilic regions are attracted to water, so the heads are oriented toward the aqueous environment on the two outer faces of the double layer.
Hydrophobic regions stay away from water, so the tails of the two layers face each other in the interior.
The double layer that results is a lipid bilayer. Its interior is hydrophobic and its two surfaces are hydrophilic.

Result.

Heads face the watery outside and inside; the fatty acid tails face each other in the middle.

Example.

2. Placing a protein. A protein is found in a plasma membrane. Its middle section is made of amino acids with nonpolar side groups, and each of its two ends has amino acids with charged and polar side groups. How is the protein expected to sit in the membrane, and why?

Nonpolar side groups are hydrophobic. The hydrophobic regions of a membrane protein make up the part of its surface that interacts with the fatty acids in the interior of the membrane, so the nonpolar middle section is expected to sit among the fatty acid tails.
Charged and polar side groups are hydrophilic. Hydrophilic regions are exposed to the watery solution, so each end is expected to stick out of the bilayer, one into the cytosol and one into the fluid outside the cell.
The protein therefore spans the membrane, with each region in the environment that it suits.

Result.

The nonpolar middle lies among the tails; the charged and polar ends are exposed to the watery solutions on the two sides.

Example.

3. Reading the fluid mosaic. A diagram of a plasma membrane from an animal cell shows a bilayer with a protein spanning it, a protein attached to the inner surface, a steroid among the tails and carbohydrate chains on the outer surface attached to proteins. Name each part and say what the model says about how they behave.

The framework is the phospholipid bilayer. The steroid among the tails is cholesterol, which is found in vertebrate animals and tends to buffer the effect of temperature on the membrane.
The carbohydrate chains attached to proteins on the outer surface form glycoproteins. If they were attached to lipids instead, they would form glycolipids.
All of these components can move around the surface of the cell within the membrane. This movement is the fluid part of the name, and the mix of different components is the mosaic part.

Result.

Bilayer framework, cholesterol, glycoproteins and proteins, all able to move within the membrane.

Example.

4. Cold and fluidity. Cell P has membranes whose phospholipids have mostly straight, saturated tails. Cell Q has membranes in which many tails have double bonds. Both are cooled to the same low temperature. Which membrane is expected to stay more fluid, and why?

A double bond puts a kink in a fatty acid tail, so the tails of cell Q cannot pack closely together.
Straight tails, as in cell P, pack tightly when cooled, and the membrane becomes dense and rigid.
Kinked tails keep some space between the phospholipids, so the membrane of cell Q stays fluid at a lower temperature.

Result.

The membrane of cell Q, because its kinked tails pack less tightly.

Practice

Answer AP-style questions on plasma membrane. Each answer is checked, with the full solution, and every try brings new numbers. Practice needs a free account: it keeps your progress.

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