# Cell Membranes

Biology I · Cells and Tissues · https://tryals.app/learn/biology-i/cell-membranes

## A Mosaic That Flows

The **fluid mosaic model** describes the membrane as a two-dimensional fluid: a phospholipid bilayer in which proteins float, diffuse laterally, and are held in place only when the cell anchors them. Lipids exchange places with their neighbours millions of times a second; flipping between the two leaflets, by contrast, is rare and requires an enzyme, which is what keeps the two faces chemically different.

Membrane **fluidity** is tuned by three things:

| Factor | More fluid when |
|---|---|
| Temperature | Higher |
| Fatty-acid saturation | More unsaturated |
| Cholesterol | Buffers both ways |

**Unsaturated** fatty acids carry a double bond that puts a kink in the tail, so neighbouring lipids pack less tightly. This is why organisms living in the cold have more unsaturated membrane lipids, and why cooking oils from cold-adapted sources stay liquid in a refrigerator.

**Cholesterol** is the interesting case: it is a fluidity *buffer*, not a simple increaser or decreaser. At high temperature it restrains movement and reduces fluidity; at low temperature it wedges between tails and prevents the tight packing that would otherwise solidify the membrane.

Membrane **proteins** fall into two classes by how they sit. **Integral** proteins span the bilayer, held by a hydrophobic belt of amino acids; extracting one requires disrupting the membrane. **Peripheral** proteins sit on a surface, attached to lipid heads or to integral proteins, and can be removed by changing the salt concentration.

Their functions cover almost everything a membrane does: transport, enzymatic activity, signal reception, cell recognition, intercellular junction and anchorage to the cytoskeleton.

Membranes are **asymmetric**: the two leaflets differ in lipid composition, and the carbohydrates of the glycocalyx face outward without exception. That outward-facing sugar coat is what makes cell recognition possible, including the ABO blood groups.

> **Common pitfall:** treating cholesterol as simply "making membranes less fluid". It works in both directions depending on temperature, which is why it is described as a buffer. A membrane without it would be far more sensitive to temperature change than one with it.

## Practice questions

5 of this lesson's 11 practice questions, with answers. The full set is in the app.

### 1. A membrane phospholipid exchanges places with a neighbour about $10^7$ times per second, but flips between leaflets about once per hour. How many times faster is lateral exchange than flipping, given an hour is 3600 s? Give the answer as a power of ten.

**Answer:** 10.6 (within ±0.4)

**Why:** Lateral exchange happens $10^7 \times 3600 = 3.6 \times 10^{10}$ times an hour against a single flip, roughly $10^{10.6}$ times faster. Flipping is so rare because it drags a hydrophilic head through the hydrophobic core, and cells use dedicated flippase enzymes when they need it done.

Page: https://tryals.app/practice/biology-i/cell-membranes/a-membrane-phospholipid-exchanges-places-with-a-neighbour-about-10

### 2. Sort each protein by how it associates with the membrane.

**Answer:**

- Integral protein: Spans the entire bilayer, Held by a hydrophobic belt of amino acids, Requires detergent to extract
- Peripheral protein: Sits on one surface only, Can be removed by changing salt concentration

**Why:** Integral proteins penetrate the hydrophobic core and need detergent to extract; peripheral ones are held by weaker surface interactions and come away with a salt change. The extraction method is how the two are distinguished experimentally.

Page: https://tryals.app/practice/biology-i/cell-membranes/sort-each-protein-by-how-it-associates-with-the-membrane

### 3. A membrane at low temperature is stiffening toward a solid. Set the effect cholesterol has on its fluidity here, from -1 (reduces it further) to +1 (raises it back).

**Answer:** 1 (within ±0.25)

**Why:** In the cold cholesterol *raises* fluidity, giving $+1$, by wedging between tails and preventing tight packing. At high temperature it does the opposite, acting in both directions is precisely what makes it a buffer.

Page: https://tryals.app/practice/biology-i/cell-membranes/a-membrane-at-low-temperature-is-stiffening-toward-a-solid-set-the

### 4. Arrange these membrane components from smallest to largest.

**Answer:**

1. A single phospholipid molecule
2. A cholesterol-stabilised lipid raft
3. An integral membrane protein complex
4. A whole cell membrane

**Why:** The membrane is organised at several scales, from individual lipids through local rafts and protein complexes to the whole sheet. Rafts matter because they concentrate particular proteins into functional patches rather than letting them diffuse freely.

Page: https://tryals.app/practice/biology-i/cell-membranes/arrange-these-membrane-components-from-smallest-to-largest

### 5. Cell membranes maintain distinct inner and outer leaflets despite being fluids in which components diffuse rapidly. What accounts for this stable asymmetry?

A. Carbohydrate chains physically obstruct molecules from flipping leaflets
B. Hydrophobic interactions permanently anchor proteins to one leaflet side
C. Transverse movement of lipids across the bilayer is exceptionally rare
D. Cytoskeletal attachments strictly immobilise all inner-leaflet molecules

**Answer:** C. Transverse movement of lipids across the bilayer is exceptionally rare

**Why:** Lateral diffusion happens millions of times per second, but transverse flipping across the hydrophobic core requires enzymatic catalysis. Carbohydrates and the cytoskeleton localise specific components, but intrinsic bilayer dynamics preserve overall leaflet asymmetry.

Page: https://tryals.app/practice/biology-i/cell-membranes/cell-membranes-maintain-distinct-inner-and-outer-leaflets-despite