AP® Biology review sheet from Aim for Five (aimforfive.com/bio/units/2/2-3)
Unit 2 · Topic 2.3
2.3 Plasma Membrane
The plasma membrane is a phospholipid bilayer with proteins and other molecules embedded in it. Its two-sided structure, with water-loving heads outside and water-fearing tails inside, sets the boundary of the cell, and its proteins carry out the membrane's jobs. The fluid mosaic model describes how all these parts can drift around within the layer.
Key terms
- phospholipid bilayer
- hydrophilic
- hydrophobic
- fluid mosaic model
- membrane protein
- glycoprotein
The phospholipid bilayer
A phospholipid has a hydrophilic (water-loving) head containing a phosphate group and two hydrophobic (water-fearing) fatty acid tails (1.5). A molecule with both a hydrophilic part and a hydrophobic part is called amphipathic.
Both the inside of the cell (the cytosol) and the outside are watery. So phospholipids line up in two layers: the polar heads face the water on each side, and the nonpolar tails face each other in the middle, away from water. This bilayer forms on its own because it's the most stable arrangement in water.
Membrane proteins
Proteins embedded in the membrane do most of its work. Some span the whole membrane (integral or transmembrane proteins), and others sit on one surface (peripheral proteins).
A protein's position fits its R groups (1.7). The parts of a transmembrane protein that touch the fatty acid tails have mostly nonpolar, hydrophobic R groups. The parts that stick out into the cytosol or the outside fluid, and any water-filled channel through the middle of the protein, have polar and charged, hydrophilic R groups.
- Transport: channels and carriers let specific ions and polar molecules cross (2.6, 2.8).
- Receiving signals: receptors bind signal molecules and pass the message inside (4.2).
- Enzymes: some membrane proteins catalyze reactions right at the membrane.
- Cell recognition and attachment: proteins help cells identify each other and stick to neighboring cells or to fibers outside the cell.
Other parts of the mosaic
Cholesterol, a steroid (1.5), is wedged between phospholipids in the membranes of vertebrate animals. It helps keep the membrane stable: at warm temperatures it restrains phospholipid movement, and at cool temperatures it keeps the tails from packing too tightly. That keeps the membrane's fluidity in a working range.
Glycoproteins (proteins with short sugar chains attached) and glycolipids (lipids with sugar chains attached) have their sugars facing the outside of the cell. They work like name tags that let cells recognize each other. Your immune system uses them to tell your own cells from foreign ones, and your ABO blood type is set by sugar chains like these on your red blood cells.
The fluid mosaic model
The fluid mosaic model is the accepted description of membrane structure. 'Mosaic' because the membrane is a patchwork of many different molecules: phospholipids, proteins, cholesterol, glycoproteins and glycolipids. 'Fluid' because these molecules aren't locked in place. Phospholipids and many proteins drift sideways within their layer, so the membrane behaves more like a thin film of oil than a solid wall.
Fluidity matters. Membrane proteins need to move and change shape to work, and a membrane that is too stiff or too runny doesn't function well. Unsaturated fatty acid tails (kinked) make a membrane more fluid; saturated tails (straight) make it less fluid. Temperature matters too: warmer membranes are more fluid.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Testing the 'fluid' part of the model
In a classic type of experiment, a mouse cell and a human cell are fused into one hybrid cell. The mouse membrane proteins are tagged with a green fluorescent label and the human proteins with a red label. Right after fusion, green is on one half of the hybrid cell and red is on the other. Predict what researchers see after 40 minutes at 37 °C, and what they'd see if the experiment were run in the cold.
Show the solutionHide the solution
- Step 1: Identify the model being tested: the fluid mosaic model says membrane proteins can drift sideways within the bilayer.
- Step 2: Predict at 37 °C: proteins from both cells should drift and mix, so the red and green labels end up spread around the whole cell.
- Step 3: Recall the effect of temperature: cold makes the lipid tails pack more tightly and slows molecular motion, so the membrane is less fluid.
- Step 4: Predict in the cold: the proteins would mix much more slowly, so the two colors would stay mostly separate for longer.
Answer: At 37 °C the colors intermix over the whole membrane, showing that proteins move within a fluid bilayer. In the cold, mixing is much slower because the membrane is less fluid.
- Example 2
Which way does it face? (classic trap)
A diagram shows a membrane protein with a region of mostly charged amino acids buried in the middle of the bilayer and a region of nonpolar amino acids sticking out into the cytoplasm. Is this diagram realistic?
Show the solutionHide the solution
- Step 1: Recall the environment: the middle of the bilayer is hydrophobic fatty acid tails; the cytoplasm is watery.
- Step 2: Charged R groups are hydrophilic, so they interact with water, not with hydrophobic tails.
- Step 3: Nonpolar R groups are hydrophobic, so they would be buried in the bilayer or the protein's core, not exposed to the cytoplasm.
- Step 4: The diagram has them backward.
Answer: No. The nonpolar region should sit in the hydrophobic interior of the bilayer, and the charged region should face the watery cytoplasm or outside fluid.
Common mistakes
- Drawing the bilayer with tails facing the water. Heads face the water on both sides; tails face each other in the middle.
- Thinking the membrane is rigid or that its parts stay in fixed positions. The 'fluid' in fluid mosaic means the parts drift sideways.
- Putting the sugar chains of glycoproteins on the inside of the cell. They face outward, where they act in cell recognition.
- Saying cholesterol only makes membranes stiffer. It steadies fluidity in both directions, depending on temperature.
On the exam
- Expect diagram questions that ask you to label hydrophilic and hydrophobic regions, or to explain why a protein sits where it does.
- When explaining a membrane component's role, connect structure to function: amphipathic phospholipids form the barrier, proteins do the transporting and signaling, and carbohydrate chains on the outside do recognition.
Connected topics
Videos
Check yourself
4 questions on 2.3 Plasma Membrane. Pick an answer to see if you got it, and why.
A researcher labels the carbohydrate chains of glycoproteins on the plasma membrane of a living animal cell. Where would most of the label be found, and what is the most likely function of these chains?
Two cells, one with membrane proteins tagged with a green marker and one with proteins tagged with a red marker, are fused into a single cell. After 40 minutes at 37 °C, the green and red markers are evenly mixed over the whole membrane. Which feature of the fluid mosaic model does this result support?
| Temperature (°C) | Fused cells with fully mixed markers after 40 min (%) |
|---|---|
| 37 | 90 |
| 25 | 45 |
| 15 | 10 |
| 4 | 0 |
Experimental data: a mouse cell with green-tagged membrane proteins was fused with a human cell with red-tagged membrane proteins. Fused cells were kept at different temperatures, and 200 cells per temperature were checked after 40 minutes.
Which of the following best explains the trend in the data?
The experiment at 15 °C is repeated using cells whose membrane phospholipids contain a much higher proportion of unsaturated fatty acids. Which result is most likely?
0 of 4 answered