Human Physiology I · Cellular Physiology
Cell Membranes and Membrane Proteins
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The Plasma membrane The cell's outer boundary Full entry → is a fluid mosaic of Amphipathic phospholipids Molecules with hydrophilic head + hydrophobic tails Full entry → arranged in a bilayer, with Cholesterol Steroid lipid in the membrane Full entry → modulating fluidity and a variety of proteins embedded in or attached to it. It is selectively permeable: small, lipid-soluble, nonpolar molecules cross easily, while ions and large polar molecules require Channels Watery pores for ions/water Full entry →, Carriers Proteins that bind and shuttle solutes Full entry →, or Pumps Energy-using transporters Full entry →. Membrane proteins act as channels, carriers, pumps, Receptors Proteins that bind signals Full entry →, Enzymes Proteins that catalyze reactions Full entry →, and Cell-adhesion molecules Proteins that stick cells together Full entry →, and carry Glycoproteins Proteins with attached sugars Full entry → and Glycolipids Lipids with attached sugars Full entry → that form the Glycocalyx Carbohydrate coat on the cell surface Full entry →. A molecule's lipid solubility, expressed as its partition coefficient, predicts how readily it diffuses across the lipid core.
Why this matters
Many drugs are designed around lipid solubility and the partition coefficient. Lipid-soluble drugs cross membranes (and the blood-brain barrier) readily, whereas charged drugs often cannot—so a drug's chemistry dictates its distribution. Membrane receptors are also the targets of a large fraction of pharmaceuticals: the same receptor proteins that normally bind hormones and neurotransmitters can bind drug molecules. These principles explain why a drug reaches its target; prescribing decisions, dosing, and monitoring remain the responsibility of trained clinicians.
The college version
1. The Phospholipid Bilayer and the Fluid Mosaic Model
Phospholipids are amphipathic: they have a hydrophilic (water-loving) head and two hydrophobic (water-fearing) fatty-acid tails. In water they self-assemble into a phospholipid bilayer, tails facing inward, heads facing the aqueous environments. The fluid mosaic model describes this as a two-dimensional fluid in which membrane proteins float and move laterally—a mosaic of proteins within a fluid lipid sea. Cholesterol, wedged among the tails, acts as a fluidity buffer: it reduces fluidity at high temperature and prevents over-stiffening at low temperature.
2. Membrane Proteins: Integral versus Peripheral
Integral membrane proteins are embedded within the bilayer, often spanning it (transmembrane proteins); removing them requires disrupting the membrane. Peripheral membrane proteins sit on the membrane surface, attached to integral proteins or lipid heads, and are removed more easily. Functionally, proteins include channels (watery pores for ions), carriers (bind and shuttle solutes), pumps (use energy to move solutes uphill), receptors (bind signals and trigger responses), enzymes (catalyze reactions at the membrane), and cell-adhesion molecules (anchor cells to each other or to the matrix).
3. The Glycocalyx and Selective Permeability
Glycoproteins (proteins with carbohydrate) and glycolipids (lipids with carbohydrate) extend their sugars outward to form the glycocalyx—a carbohydrate coat involved in cell recognition, protection, and adhesion. Together with the bilayer, this produces selective permeability: the membrane lets some substances through and blocks others. Lipid solubility is the key predictor of passive crossing; the partition coefficient (a substance's solubility in oil relative to water) quantifies this—higher values mean easier membrane crossing. Membrane-protein specificity means each channel or carrier is selective for particular solutes, so different cell types import and export different substances.
How it works
- Amphipathic phospholipids self-assemble into a bilayer.
- Cholesterol inserts among tails, tuning fluidity.
- Integral and peripheral proteins populate the membrane.
- Carbohydrates on proteins and lipids form the glycocalyx.
- Lipid-soluble solutes pass directly; polar/charged solutes use proteins.
- Specificity of channels, carriers, and pumps shapes each cell's transport profile.
Common confusions
| Do not confuse | With | Difference |
|---|---|---|
| Integral protein | Peripheral protein | Integral is embedded; peripheral sits on the surface |
| Channel | Carrier | A channel is an open pore; a carrier binds and changes shape |
| Carrier | Pump | A carrier may be passive; a pump always uses energy |
| Glycoprotein | Glycolipid | Sugar attached to protein vs. to lipid |
| Lipid solubility | Membrane-protein specificity | Lipid solubility is a property of the solute; specificity is a property of the protein |
Memory aids
"Channel, Carrier, Pump, Receptor, Enzyme, Adhesion = CCPREA" — the six functional classes of membrane proteins. For crossing rules: "Lipid-Loving Leaves Liberally" — lipid-soluble solutes cross the bilayer freely.
Quick review
Topic Recap
The plasma membrane is a selectively permeable phospholipid bilayer containing cholesterol and a mosaic of proteins, as described by the fluid mosaic model. Integral proteins (channels, carriers, pumps, receptors, enzymes, adhesion molecules) and peripheral proteins perform transport, signaling, and attachment, while surface glycoproteins and glycolipids form the glycocalyx. Selective permeability arises from the oily core, which admits lipid-soluble solutes (measured by the partition coefficient) but forces polar and charged solutes through specific proteins. This sets up the transport topics that follow.
Knowledge Check
- Why do phospholipids form a bilayer in water?
- What role does cholesterol play in membrane fluidity?
- Name three functional classes of membrane proteins.
- Which property best predicts whether a solute crosses the membrane without a protein?
- What is the glycocalyx made of?
Answers and Rationales
- Because they are amphipathic—hydrophilic heads face water while hydrophobic tails hide in the interior, producing a stable two-layered sheet.
- It buffers fluidity, reducing it at high temperature and preventing stiffening at low temperature.
- Channels, carriers, and pumps (also receptors, enzymes, cell-adhesion molecules).
- Lipid solubility (quantified by the partition coefficient): small, nonpolar, lipid-soluble solutes diffuse through the lipid core.
- Carbohydrates attached to glycoproteins and glycolipids on the outer surface, forming a recognition and protection coat.
Eli explains
The same idea, in plain words
Explain it like I’m 10
Picture the membrane as a soap-bubble wall made of two layers of tiny "tadpoles." Each tadpole has a water-loving head and a water-fearing tail, so the tails point inward and the heads face the water inside and outside the cell. Floating in this oily, flexible sheet are protein "doors," "mail slots," "pumps," and "antennae" that let specific things in, out, or send signals. Because the middle of the wall is oily, only oily or tiny substances slip through on their own; everything else needs a door. Where this stops being exact: the real membrane is not a static wall of doors—it flows, and its proteins drift, cluster, and get recycled constantly.
Simple Example
Oxygen crosses a cell membrane by itself because it is small and dissolves in the oily interior. A sodium ion, which is charged and water-loving, cannot cross that oily center and must use a channel protein instead.
Worked example
- A solute approaches the membrane; its fate depends on size, charge, and lipid solubility.
- Small, nonpolar solutes (O₂, CO₂, steroid hormones) dissolve into the lipid core and diffuse straight through.
- Polar or charged solutes cannot cross the hydrophobic core and are blocked.
- Such solutes cross only via channels (passive, specific pores) or carriers/pumps (which bind and transport them).
- Why it matters: Because crossing requires either lipid solubility or a specific protein, the membrane—not just concentration differences—controls what enters and leaves a cell, setting the stage for passive and active transport.
Key takeaways
- High yield: Phospholipids are amphipathic and self-assemble into a bilayer.
- High yield: The fluid mosaic model = mobile proteins in a fluid lipid bilayer.
- High yield: Cholesterol is a fluidity buffer (lowers at high temp, raises at low temp).
- High yield: Integral proteins span or embed; peripheral proteins sit on the surface.
- High yield: Lipid-soluble, nonpolar solutes cross unaided; ions and polar solutes need proteins.
- High yield: The partition coefficient measures lipid solubility and predicts permeability.
- Glycoproteins + glycolipids form the glycocalyx for recognition.
Study tools & related lessonsYou’ll learn to · Key vocabulary · Related
You’ll learn to
- Describe the structure of the plasma membrane using the fluid mosaic model.
- Explain how phospholipid and cholesterol composition controls membrane fluidity and selective permeability.
- Distinguish integral from peripheral membrane proteins and classify membrane proteins by function.
- Relate lipid solubility and the partition coefficient to how easily a substance crosses the membrane.
Key vocabulary
- Plasma membrane
- The cell's outer boundary
- Phospholipid bilayer
- Two-layered sheet of phospholipids
- Fluid mosaic model
- Proteins floating in a fluid lipid sea
- Amphipathic phospholipids
- Molecules with hydrophilic head + hydrophobic tails
- Cholesterol
- Steroid lipid in the membrane
- Membrane fluidity
- Ease of movement within the bilayer
- Integral membrane proteins
- Proteins embedded in the bilayer
- Peripheral membrane proteins
- Proteins on the membrane surface
- Channels
- Watery pores for ions/water
- Carriers
- Proteins that bind and shuttle solutes
- Pumps
- Energy-using transporters
- Receptors
- Proteins that bind signals
- Enzymes
- Proteins that catalyze reactions
- Cell-adhesion molecules
- Proteins that stick cells together
- Glycoproteins
- Proteins with attached sugars
- Glycolipids
- Lipids with attached sugars
- Glycocalyx
- Carbohydrate coat on the cell surface
- Selective permeability
- Allowing some solutes, not others
- Lipid solubility
- Ability to dissolve in lipid
- Partition coefficient
- Oil-to-water solubility ratio
- Membrane-protein specificity
- Each transporter is solute-selective
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