2.3 Plasma Membrane

The plasma membrane is a fluid mosaic—mainly a phospholipid bilayer with hydrophilic phosphate heads facing water and hydrophobic fatty-acid tails inside (EK 2.3.A.1, LO 2.3.B). Embedded proteins (integral/transmembrane and peripheral) and steroids like cholesterol make the membrane fluid and selectively permeable (EK 2.3.A.2, EK 2.3.B.1). What it actually does: - Barrier/regulator: keeps unwanted things out and controls what enters (selective permeability). - Transport: channels, carrier proteins and aquaporins move ions, polar molecules, and water (passive and active transport). - Communication: receptor proteins bind signals (hormones, ligands) to trigger pathways. - Recognition & adhesion: glycoproteins/glycolipids allow cell ID and tissue formation. - Structural: cholesterol modulates fluidity; cytoskeleton anchors membrane proteins. On the AP exam, you’ll be asked to describe these components and functions (LO 2.3.A, LO 2.3.B). For quick review, see the Unit 2 study guide (cell-size: https://library.fiveable.me/ap-biology/unit-2/cell-size/study-guide/3oB8hJyGwvYACz8XlUmG) and the Unit 2 overview (https://library.fiveable.me/ap-biology/unit-2). For practice problems, try (https://library.fiveable.me/practice/ap-biology).

Phospholipids are amphipathic—they have a polar, hydrophilic “head” (a phosphate group plus glycerol) and two nonpolar, hydrophobic fatty-acid “tails.” The phosphate head is charged or polar, so it forms favorable interactions (ion-dipole/hydrogen bonds) with water inside and outside the cell. The fatty-acid tails are nonpolar and avoid water, so they pack together away from the aqueous environments. That chemical difference drives phospholipids to arrange into a bilayer (EK 2.3.A.1): heads face the cytosol and extracellular fluid, tails face the membrane interior. This arrangement creates selective permeability (keeps polar/charged molecules out unless aided by proteins) and provides the fluid matrix in which integral proteins (with matching hydrophobic/hydrophilic regions) embed (EK 2.3.A.2, EK 2.3.B.1). You’ll see this concept on the exam in MC and FRQ items about membrane structure and transport (Unit 2). For a quick refresher, check the Topic 2 study guide (https://library.fiveable.me/ap-biology/unit-2/cell-size/study-guide/3oB8hJyGwvYACz8XlUmG) and more unit review (https://library.fiveable.me/ap-biology/unit-2); practice problems are at (https://library.fiveable.me/practice/ap-biology).

Think of the plasma membrane like a fluid, moving mosaic tile floor. The “tiles” are phospholipids arranged in a bilayer: hydrophilic (polar) heads face water inside and outside the cell, hydrophobic (nonpolar) tails tuck inward (EK 2.3.A.1). Embedded in that bilayer are proteins—integral/transmembrane proteins that span the membrane and peripheral proteins that sit on the surface—plus cholesterol, glycoproteins, and glycolipids (EK 2.3.B.1). Everything can drift laterally, so the membrane’s fluidity lets proteins (ion channels, carrier proteins, aquaporins) move to do transport and signaling; cholesterol modulates fluidity; glyco- groups help recognition. That dynamic mixture is the “fluid mosaic.” On the AP exam you should be able to describe these components, explain how hydrophobic/hydrophilic regions determine placement, and connect structure to selective permeability (LO 2.3.A, LO 2.3.B). For a quick review, see the Topic 2 study guide (https://library.fiveable.me/ap-biology/unit-2/cell-size/study-guide/3oB8hJyGwvYACz8XlUmG) and more unit resources (https://library.fiveable.me/ap-biology/unit-2) or practice problems (https://library.fiveable.me/practice/ap-biology).

Hydrophilic vs hydrophobic membrane proteins: Hydrophilic regions have charged or polar side chains and face aqueous environments (cytosol or extracellular space) or are buried in interior pockets of the protein. Hydrophobic regions have nonpolar side chains and sit in or interact with the phospholipid bilayer’s fatty-acid core. So membrane proteins often have both: transmembrane (integral) proteins have hydrophobic stretches that span the bilayer and hydrophilic loops that stick out to bind ligands, form channels, or interact with the cytoskeleton; peripheral proteins are mostly hydrophilic and attach to membrane surfaces via interactions with lipid heads or other proteins. These properties explain placement/function (transporters, receptors, ion channels vs surface enzymes) and are core to the fluid-mosaic/ selective-permeability ideas in EK 2.3.A.2 and EK 2.3.B.1 (CED Topic 2.3). For extra review of membrane concepts, check the Unit 2 study guide (https://library.fiveable.me/ap-biology/unit-2/cell-size/study-guide/3oB8hJyGwvYACz8XlUmG) and more practice problems (https://library.fiveable.me/practice/ap-biology).

Proteins fit into the phospholipid bilayer because their surfaces have regions that match the membrane’s chemistry (LO 2.3.A, 2.3.B). Integral (transmembrane) proteins have hydrophobic amino-acid stretches—often alpha helices—whose nonpolar side chains interact with the fatty-acid tails in the membrane interior. Their hydrophilic parts (charged or polar side chains) stick out into the aqueous cytosol or extracellular space as loops or domains. Peripheral proteins don’t cross the bilayer; they attach to membrane lipids or to the exposed surfaces of integral proteins. Because the membrane is a “fluid mosaic,” these components can move laterally, and cholesterol and cytoskeletal anchors affect that fluidity and positioning. Examples you should memorize: ion channels, carrier proteins, and aquaporins as transmembrane proteins; glycoproteins/glycolipids for cell ID (EK 2.3.A.1, EK 2.3.A.2, EK 2.3.B.1). For a quick topic review check the Unit 2 study guide (https://library.fiveable.me/ap-biology/unit-2/cell-size/study-guide/3oB8hJyGwvYACz8XlUmG) and more unit resources (https://library.fiveable.me/ap-biology/unit-2). Practice Qs: https://library.fiveable.me/practice/ap-biology.

Cholesterol’s role in the plasma membrane is to tune membrane fluidity and stability so the cell can maintain homeostasis. As a steroid interspersed among phospholipids (fluid mosaic model), cholesterol fits between fatty-acid tails and does two key things: at high temperatures it restrains phospholipid movement so the membrane isn’t too fluid; at low temperatures it prevents tight tail packing so the membrane doesn’t become too rigid. That buffering keeps selective permeability and membrane protein function consistent. Cholesterol also decreases permeability to small polar molecules and helps form lipid rafts—ordered microdomains that concentrate certain proteins for signaling and trafficking. These points map directly to EK 2.3.B.1 and LO 2.3.B in the CED. For a quick unit review check the Unit 2 overview (https://library.fiveable.me/ap-biology/unit-2) and practice membrane questions at (https://library.fiveable.me/practice/ap-biology). For a related study guide on cell size and membranes see (https://library.fiveable.me/ap-biology/unit-2/cell-size/study-guide/3oB8hJyGwvYACz8XlUmG).

Phospholipids are amphipathic: a polar, hydrophilic phosphate head and two nonpolar, hydrophobic fatty-acid tails. In water they spontaneously arrange so the heads face the aqueous environment (outside and inside the cell) and the tails face each other, forming a phospholipid bilayer. This orientation minimizes energetically unfavorable interactions between water and hydrophobic tails while maximizing favorable interactions between water and polar heads (EK 2.3.A.1). In small droplets or detergents they can form micelles, but in cell membranes the bilayer creates the core structural framework of the fluid mosaic—allowing embedded proteins, cholesterol, glycolipids, and glycoproteins to move laterally (EK 2.3.B.1). That bilayer is key for selective permeability and membrane fluidity; cholesterol modulates fluidity and proteins provide transport and signaling functions. For a compact review of Topic 2.3 and related practice, check the Unit 2 study guide (https://library.fiveable.me/ap-biology/unit-2/cell-size/study-guide/3oB8hJyGwvYACz8XlUmG) and the unit overview (https://library.fiveable.me/ap-biology/unit-2). For more practice problems, see (https://library.fiveable.me/practice/ap-biology).

Glycoproteins are membrane proteins with short carbohydrate chains attached; glycolipids are phospholipids with carbohydrate chains. Both stick out from the extracellular surface and are part of the glycocalyx. In the fluid-mosaic plasma membrane (EK 2.3.B.1 / LO 2.3.B), they help with cell recognition/communication (receptor binding, immune ID), cell–cell adhesion, and forming surface markers (like blood type). Their carbohydrate portions are hydrophilic and face the aqueous outside, contributing to membrane asymmetry and specific interactions while proteins’ hydrophobic regions still span the bilayer (EK 2.3.A.2). On the AP exam, know their roles in signaling, selective permeability, and recognition. For a quick unit review see the Topic 2 study guide (https://library.fiveable.me/ap-biology/unit-2/cell-size/study-guide/3oB8hJyGwvYACz8XlUmG) and try practice problems (https://library.fiveable.me/practice/ap-biology).

Think of the membrane like a 2D liquid made of phospholipids with proteins floating in it—that’s the fluid mosaic model (LO 2.3.B). Phospholipids have hydrophilic heads facing water and hydrophobic tails stuck together; the tails interact by weak hydrophobic forces, so lipids and many proteins can move laterally (fluid) but don’t fall apart (structure) because those hydrophobic interactions hold the bilayer together (EK 2.3.A.1). Cholesterol fits between tails in animal membranes and buffers fluidity (keeps it from being too fluid at high temp or too rigid at low temp). Some proteins are anchored to the cytoskeleton or extracellular matrix, and lipid rafts concentrate certain lipids/proteins—both add local structure and function (EK 2.3.A.2). For AP exam points, be ready to name these components and explain how they let the membrane be fluid yet selectively permeable (see the Topic 2.3 CED goals). For extra practice, check the Unit 2 study guide (https://library.fiveable.me/ap-biology/unit-2/cell-size/study-guide/3oB8hJyGwvYACz8XlUmG) and more problems (https://library.fiveable.me/practice/ap-biology).

Because phospholipids are amphipathic—they have a polar, hydrophilic phosphate head and nonpolar, hydrophobic fatty acid tails—the heads want to face the watery environments (extracellular fluid and cytosol) while the tails avoid water. So the tails pack together in the membrane interior, held by hydrophobic interactions; this arrangement minimizes unfavorable contact between water and nonpolar tails and is therefore energetically favorable (CED EK 2.3.A.1). That inward-facing, nonpolar core also creates a selective barrier to polar or charged molecules, while membrane proteins with hydrophobic regions fit into that interior (CED EK 2.3.A.2), which is a key part of the fluid mosaic model (LO 2.3.B). For a quick review of Topic 2.3 and related practice, see the Unit 2 study guide (https://library.fiveable.me/ap-biology/unit-2/cell-size/study-guide/3oB8hJyGwvYACz8XlUmG) and more Unit 2 resources (https://library.fiveable.me/ap-biology/unit-2).

The plasma membrane controls what goes in and out by being selectively permeable—a fluid mosaic of a phospholipid bilayer plus proteins, cholesterol, and carbs (LO 2.3.A & LO 2.3.B). Phospholipids form a hydrophobic interior that blocks most polar or charged molecules, while hydrophilic heads face the water inside and outside (EK 2.3.A.1). Embedded transmembrane proteins provide specific routes: ion channels and aquaporins let ions or water pass by facilitated diffusion; carrier proteins change shape to move polar molecules; and pumps (like Na+/K+ ATPase) use ATP for active transport. Cholesterol modulates membrane fluidity, affecting permeability. Glycoproteins/glycolipids help cell recognition and signaling. Peripheral proteins and cytoskeletal anchors maintain membrane shape and localize proteins. For AP exam: be ready to describe each component’s role (phospholipids, integral/peripheral proteins, cholesterol, glycoproteins) and link structure to selective permeability. Need more practice? Check the Topic 2 study guide (https://library.fiveable.me/ap-biology/unit-2/cell-size/study-guide/3oB8hJyGwvYACz8XlUmG) and Unit 2 resources (https://library.fiveable.me/ap-biology/unit-2) or try practice questions (https://library.fiveable.me/practice/ap-biology).

Membrane fluidity depends a lot on temperature: as temperature rises, phospholipid fatty acids move more (higher kinetic energy) and the membrane becomes more fluid; as temperature drops, movement slows and the bilayer can stiffen or even enter a gel-like state (less fluid, decreased permeability). Two key modifiers: fatty-acid composition and cholesterol. More unsaturated tails (double bonds/kinks) increase fluidity at a given temp; more saturated tails (no kinks) pack tightly and decrease fluidity. Cholesterol acts as a buffer: at high temps it stabilizes the membrane and reduces excess fluidity; at low temps it prevents tight packing of phospholipids, so it increases fluidity. These ideas are part of the fluid mosaic model (EK 2.3.B.1) and show up on the AP exam in MC and FR questions about membrane structure, permeability, and adaptation. Review Topic 2.3 and related practice sets on Fiveable (unit review: https://library.fiveable.me/ap-biology/unit-2; topic study guide: https://library.fiveable.me/ap-biology/unit-2/cell-size/study-guide/3oB8hJyGwvYACz8XlUmG; practice: https://library.fiveable.me/practice/ap-biology).

We call it a "mosaic" because the membrane isn’t just one uniform substance—it’s a patchwork of different molecules (integral and peripheral proteins, cholesterol, glycoproteins, glycolipids) embedded in a phospholipid bilayer. Those pieces form a “mosaic” of different shapes and functions across the surface. Coupled with "fluid" (phospholipids and many proteins can move laterally), the fluid mosaic model describes a flexible bilayer with a mobile, varied set of components (EK 2.3.B.1; EK 2.3.A.2). That variability is why membranes can have ion channels, receptors, transporters, and lipid rafts in different spots—each patch does different jobs to maintain the cell’s internal environment (selective permeability, signaling, anchoring). For AP prep, connect this to LO 2.3.B and practice explaining how specific components (e.g., transmembrane proteins, cholesterol) contribute to membrane function. For a quick review, check the Unit 2 study guide (https://library.fiveable.me/ap-biology/unit-2/cell-size/study-guide/3oB8hJyGwvYACz8XlUmG) and more practice problems (https://library.fiveable.me/practice/ap-biology).

Integral (embedded) and peripheral membrane proteins differ in how they associate with the phospholipid bilayer and what they do. Integral proteins penetrate the hydrophobic core—many are transmembrane proteins with hydrophobic amino acids facing the fatty-acid tails and hydrophilic regions exposed to the aqueous sides. They form channels, carriers, pumps, receptors, and aquaporins that directly control selective permeability and transport (EK 2.3.A.2; fluid mosaic model EK 2.3.B.1). Peripheral proteins do not enter the bilayer; they bind loosely to membrane surfaces or to integral proteins via polar interactions. They often function in signaling, cytoskeleton anchoring, and enzyme activity at the membrane. Experimentally, integrals require detergents to remove, while peripherals can be released with high salt or pH changes. For AP review, connect this to LO 2.3.A and the fluid mosaic model—see the Unit 2 study guide (https://library.fiveable.me/ap-biology/unit-2) and Topic 2.3 resources on Fiveable for practice (https://library.fiveable.me/practice/ap-biology).

If the plasma membrane is damaged, the cell quickly loses its ability to maintain a controlled internal environment. Because the membrane’s phospholipid bilayer and embedded proteins (EK 2.3.A.1–A.2, EK 2.3.B.1) provide selective permeability, a breach lets ions and water flow in or out uncontrollably. Consequences include loss of electrochemical gradients (so ATP production and nerve/transport functions fail), osmotic swelling or lysis if water rushes in, leakage of cytosolic contents, and failure of membrane proteins (receptors, channels, carriers, aquaporins) needed for signaling and transport. Severe or irreparable damage usually leads to cell death (necrosis or triggers for apoptosis). Cells with smaller, localized damage may repair membranes using vesicle fusion or membrane-protein rearrangement, but large disruptions are fatal. This ties directly to AP LO 2.3.A/B (fluid mosaic, selective permeability) and is a common concept on the exam—practice applying it in membrane-permeability questions (see the Topic 2.3 study guide and related Unit 2 review: https://library.fiveable.me/ap-biology/unit-2/cell-size/study-guide/3oB8hJyGwvYACz8XlUmG and https://library.fiveable.me/ap-biology/unit-2). For extra practice, try problems at https://library.fiveable.me/practice/ap-biology.