Source: Campbell Biology, Chapter 7 (Sections 7.1–7.2)
Tags: membrane structure, fluid mosaic model, phospholipid bilayer, membrane fluidity, cholesterol, integral proteins, peripheral proteins, selective permeability, transport proteins, channel proteins, carrier proteins, amphipathic
Difficulty: Introductory to Intermediate | Prerequisites: Basic chemistry (polarity, bonds), Chapter 5 (macromolecules, especially lipids and proteins), Chapter 6 (cell organelles, especially ER and Golgi apparatus).
Every cell is wrapped in a plasma membrane that controls what enters and exits. This chapter explains what the membrane is made of, why it behaves like a fluid, and how its structure gives rise to selective permeability. You need a working understanding of polar vs. nonpolar molecules, lipid structure, and the basic functions of proteins. If any of that is fuzzy, revisit Chapters 5 and 6 before going further.
Cell membranes are built from a phospholipid bilayer studded with proteins and some carbohydrates. The whole assembly is fluid, with components drifting laterally, and its composition determines what can cross. Small nonpolar molecules slip through the lipid layer easily; ions and polar molecules need help from specific transport proteins.
Amphipathic
Having both a hydrophilic (water-loving) region and a hydrophobic (water-fearing) region within the same molecule. Think of it as a molecule with a split personality: one end mixes happily with water, the other avoids it.
Fluid mosaic model
The accepted model of membrane structure: a mosaic of proteins floating in (or attached to) a fluid bilayer of phospholipids. In simple terms, picture a moving crowd of different molecules drifting in a sea of lipids.
Phospholipid bilayer
Two back-to-back layers of phospholipids forming the core of the membrane. The hydrophobic tails face inward (away from water) and the hydrophilic heads face outward (toward water on both sides).
Integral protein
A membrane protein that penetrates into or spans the hydrophobic interior of the lipid bilayer. Most are transmembrane proteins that pass all the way through. Think of them as embedded anchors in the membrane.
Transmembrane protein
An integral protein that spans the entire membrane, with portions exposed on both the cytoplasmic and extracellular sides. Its hydrophobic middle region (typically an alpha helix of 20 to 30 nonpolar amino acids) sits within the bilayer.
Peripheral protein
A protein loosely bound to the surface of the membrane, usually attached to exposed parts of integral proteins rather than embedded in the lipid bilayer itself.
Cholesterol
A steroid molecule wedged between phospholipids in animal cell membranes that acts as a fluidity buffer, restraining movement at high temperatures and preventing tight packing at low temperatures.
Glycoprotein
A protein with short, branched carbohydrate chains covalently bonded to it, found on the extracellular surface of the membrane. In simple terms, a protein wearing a sugar tag that other cells can recognise.
Glycolipid
A lipid with a covalently bonded carbohydrate chain, found on the extracellular face of the membrane. Functions in cell-cell recognition alongside glycoproteins.
Selective permeability
The property of a membrane that allows some substances to cross more easily than others. The membrane is not simply open or closed; it is selective, based on size, polarity, and available transport proteins.
Transport protein
A transmembrane protein that helps specific ions or polar molecules cross the membrane. These come in two main forms: channel proteins and carrier proteins.
Channel protein
A transport protein with a hydrophilic tunnel through which specific molecules or ions pass. Think of it as a selective doorway. Aquaporins (water channels) are a classic example.
Carrier protein
A transport protein that changes shape to shuttle a substance across the membrane. Unlike a channel, it grips the passenger and physically moves it through.
Aquaporin
A channel protein specifically dedicated to the rapid transport of water molecules across the membrane.
Supramolecular structure
A structure made of many molecules organised into a higher level of complexity, with emergent properties that the individual molecules do not have on their own. The membrane is a prime example.
The plasma membrane is built from three classes of molecule: phospholipids (the most abundant), proteins, and carbohydrates. Phospholipids and most membrane proteins are amphipathic, meaning each has both hydrophobic and hydrophilic regions. This dual nature is what makes the bilayer possible: hydrophobic tails face inward, hydrophilic heads face the aqueous environment on both sides.
The fluid mosaic model describes the membrane as a mosaic of diverse proteins drifting in a fluid phospholipid bilayer. Proteins are not randomly scattered. They cluster in long-lasting, specialised patches that carry out shared functions.
How the membrane stays fluid
Phospholipids are held together by hydrophobic interactions, which are weaker than covalent bonds.
Most lipids and some proteins shift laterally (side to side) within their layer.
Some lipids flip-flop across the bilayer from one leaflet to the other, though this is less common.
Phospholipids switch positions rapidly; proteins, being much larger, move more slowly.
Some proteins move in a directed way along cytoskeletal fibres. Others are immobile, anchored to the cytoskeleton or the extracellular matrix.
Temperature and lipid composition
As temperature drops, phospholipids pack more tightly and the membrane can solidify (like bacon grease cooling). The type of lipid matters:
Unsaturated hydrocarbon tails have kinks that prevent tight packing, keeping the membrane fluid at colder temperatures.
Saturated tails pack closely, increasing viscosity.
Cholesterol as a fluidity buffer
Cholesterol sits wedged between phospholipids in animal cell plasma membranes. Its effect depends on temperature:
At high temperature (e.g. 37 degrees C, body temperature): cholesterol restrains phospholipid movement, making the membrane less fluid.
At low temperature: cholesterol hinders tight packing of phospholipids, lowering the solidification point.
Net effect: cholesterol resists changes in membrane fluidity caused by temperature swings. It is a buffer.
Membrane fluidity affects both permeability and the ability of membrane proteins to move to where they are needed.
Evolutionary adaptations in lipid composition
Organisms have evolved membrane compositions suited to their environment. Fish living in extreme cold, for example, have membranes rich in unsaturated hydrocarbon tails, which keeps the membrane fluid. Some organisms can actively adjust their membrane lipid composition in response to changing temperature.
The membrane is a collage of different proteins, usually clustered in groups within the fluid lipid matrix. There are two major populations:
Integral proteins
Penetrate into the hydrophobic interior of the bilayer.
Most are transmembrane proteins, spanning the entire membrane.
Some extend only partway into the hydrophobic core.
Their hydrophobic regions consist of one or more stretches of nonpolar amino acids (typically 20 to 30 residues), coiled into alpha helices.
Hydrophilic regions are exposed to the aqueous solutions on either side. Some integral proteins have hydrophilic channels that allow polar substances through.
Peripheral proteins
Not embedded in the bilayer.
Loosely bound to the membrane surface, often to the exposed parts of integral proteins.
Anchoring and attachment
On the cytoplasmic side, some proteins are held in place by attachment to the cytoskeleton.
On the extracellular side, some proteins attach to fibres in the extracellular matrix via integrins.
Medical relevance: HIV and membrane proteins
The protein CD4 on the surface of immune cells helps HIV infect those cells. Research into people who are exposed to HIV but show no symptoms revealed that resistant individuals carry an unusual form of the gene coding for CCR5, an immune cell-surface protein. HIV must bind to both CD4 and CCR5 to enter and infect a cell. This is a clear example of how membrane protein identity determines cellular vulnerability.
Cell-cell recognition and carbohydrates
Cells recognise other cells by binding to molecules on the extracellular surface of the plasma membrane. These molecules usually contain carbohydrates. Membrane carbohydrates are typically short chains (up to about 15 monosaccharides) and branched.
Carbohydrates bonded to lipids form glycolipids.
Carbohydrates bonded to proteins (the majority) form glycoproteins.
Synthesis and sidedness
Membranes have distinct inside and outside faces. The two leaflets can differ in lipid composition, and each protein has a fixed directional orientation. This asymmetry is established during membrane synthesis in the ER and Golgi apparatus. When vesicles from the Golgi fuse with the plasma membrane, the inside face of the vesicle becomes continuous with the cytoplasmic face, and the outside face becomes the extracellular face. Carbohydrates of glycoproteins and glycolipids end up on the outer surface.
Selective permeability (Section 7.2)
The membrane is a supramolecular structure with emergent properties: it does things that its individual molecules cannot do alone.
Nonpolar molecules (hydrocarbons, CO2, O2) dissolve in the lipid bilayer and cross easily, without help from proteins.
The hydrophobic interior blocks ions and most polar molecules.
Polar molecules pass more slowly; ions cannot cross the bilayer on their own.
Transport proteins
Ions and polar molecules cross the membrane through transport proteins:
Channel proteins provide a hydrophilic tunnel. Example: aquaporins for water transport.
Carrier proteins grip a solute and change shape to shuttle it across. Example: the glucose transporter.
Transport proteins are specific: each one helps only certain substances cross. The selective permeability of a membrane therefore depends on two things: the barrier properties of the lipid bilayer, and the particular transport proteins built into it.
Antiretroviral drugs for HIV target the interaction between the virus and membrane proteins CD4 and CCR5, exploiting the specificity of membrane protein binding. Understanding membrane fluidity also matters in medicine: general anaesthetics are thought to work partly by dissolving in the lipid bilayer and disrupting normal membrane protein function. In food science, the solidification behaviour of lipid membranes is the same principle behind why butter (saturated fats) is solid at room temperature while olive oil (unsaturated fats) remains liquid.
Students often think the membrane is static or rigid. It is fluid: phospholipids and many proteins are in constant lateral motion.
Students sometimes confuse integral and peripheral proteins by assuming "integral" means "important." Integral refers to being embedded in the bilayer, not to functional importance. Peripheral proteins are equally important functionally.
Cholesterol is often thought to always increase fluidity. It does not. At body temperature it actually reduces fluidity. Its role is as a buffer: it moderates fluidity in both directions.
Students sometimes assume that all molecules cross the membrane by the same mechanism. In fact, nonpolar molecules cross via the lipid bilayer directly, while ions and polar molecules require transport proteins. Each substance follows its own route.
⚠️ Be able to explain the fluid mosaic model in your own words and identify all three classes of membrane molecule (lipids, proteins, carbohydrates).
⚠️ Know the dual role of cholesterol at high vs. low temperatures. This is a classic exam question.
⚠️ Distinguish clearly between integral (transmembrane) and peripheral proteins, including where each sits and how each is attached.
⚠️ Understand why nonpolar molecules cross freely but ions and polar molecules need transport proteins. Be ready to give an example of each type of transport protein (channel vs. carrier).
⚠️ The concept of selective permeability often appears as a short-answer or essay prompt. Know that it depends on both the lipid bilayer and the specific transport proteins present.
True or False: The phospholipid bilayer is held together primarily by covalent bonds. (False, hydrophobic interactions.)
Fill in the blank: A molecule with both hydrophilic and hydrophobic regions is called ______. (Amphipathic.)
True or False: Cholesterol always increases membrane fluidity. (False, it acts as a buffer: reduces fluidity at high temp, prevents solidification at low temp.)
Fill in the blank: Proteins that are loosely attached to the membrane surface are called ______ proteins. (Peripheral.)
True or False: Oxygen (O2) needs a transport protein to cross the plasma membrane. (False, it is nonpolar and crosses the lipid bilayer directly.)
Q: Describe the fluid mosaic model. What are the key components and why is the membrane considered "fluid"?
A: The fluid mosaic model describes the membrane as a diverse mosaic of proteins embedded in or attached to a fluid bilayer of phospholipids, with carbohydrates on the outer surface. It is considered fluid because the phospholipids and many proteins move laterally within the plane of the membrane, held together by hydrophobic interactions rather than rigid covalent bonds.
Q: How does cholesterol affect membrane fluidity at body temperature (37 degrees C) compared to low temperature?
A: At body temperature, cholesterol restrains phospholipid movement, reducing fluidity. At low temperature, cholesterol hinders tight packing of phospholipids, preventing the membrane from solidifying. Cholesterol therefore acts as a fluidity buffer.
Q: Explain why a fish living in Arctic waters would have a different membrane lipid composition from one living in tropical waters.
A: Arctic fish need their membranes to remain fluid at very low temperatures. They achieve this by having a higher proportion of unsaturated hydrocarbon tails in their phospholipids. The kinks in unsaturated tails prevent tight packing, keeping the membrane fluid even in extreme cold.
Q: What is the difference between a channel protein and a carrier protein? Give one example of each.
A: A channel protein provides a hydrophilic tunnel through which specific molecules or ions pass (e.g. aquaporins for water). A carrier protein binds its passenger and changes shape to shuttle it across the membrane (e.g. the glucose transporter). Both are transport proteins, but they move substances by different mechanisms.
Q: Why can O2 and CO2 cross the membrane without transport proteins, while glucose cannot?
A: O2 and CO2 are small, nonpolar molecules that dissolve easily in the hydrophobic interior of the lipid bilayer. Glucose is a polar molecule that cannot pass through the hydrophobic core without the help of a specific carrier protein.
This material connects directly to Chapter 5 (macromolecules): the properties of lipids and proteins you learned there explain why the membrane assembles the way it does. It also connects to Chapter 6 (cell structure), because the ER and Golgi apparatus are where membranes are built and modified. Understanding membrane structure here is essential preparation for sections 7.3 to 7.5 (transport mechanisms), for Chapter 11 (cell signalling, which depends on membrane receptors), and for Chapter 9 (cellular respiration, which relies on membrane-embedded electron transport chains).
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