Bio Tool #6 · DE Bio 101 Unit 5 & Biology I Module 2 · The plasma membrane
Four Classes, One Membrane
You met the four macromolecule classes as four separate boxes. The plasma membrane is where all four are built into one working thing. Tap any part to see what it is and what it does — then watch the parts do their jobs.The four macromolecule classes converge in one structure. Read the plasma membrane as an assembly — phospholipids and cholesterol, the proteins that do most of the work, the carbohydrate glycocalyx, and the nucleic acids that author all of it — then work through transport, tonicity and junctions.
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The parts, by class
Tap any part of the membrane, or a name under it. Then tap a class chip to light up everything made of that class.Tap any component, or its name below. The class chips filter the mosaic by macromolecule class.
Tap a part of the membrane, or a name above, to see what it is and what it does.
Not here — but the author of everything here
DNAin the nucleusmRNAthe copyRibosomeson the rough ERVesiclefrom the GolgiPlasma membranethe destination
No nucleic acid sits in the membrane itself. But every protein here started as a gene. The instructions travel: DNA in the nucleus is copied into mRNA, which is carried to ribosomes on the rough ER; the new protein is threaded into the ER membrane as it's built; then a vesicle carries it to the surface, where it becomes part of the plasma membrane. The sugar name tags get added along the way, in the ER and Golgi.Nucleic acids are absent from the membrane but author it. A gene is transcribed to mRNA; ribosomes on the rough ER translate it and insert the new membrane protein into the ER membrane as it is made. Vesicles carry it through the Golgi — where the sugar chains are added, making glycoproteins — to the plasma membrane, where the vesicle fuses. The protein face that pointed into the ER lumen ends up facing outside the cell.
Getting across
Some things cross the membrane on their own, downhill, for free. Others have to be pushed uphill, and that costs energy. Pick a passenger and watch.Transport splits on two questions: which way relative to the gradient, and does it cost energy? Passive transport runs down the gradient for free; active transport runs against it and spends ATP. Pick a solute.
Downhill — passiveFrom the crowded side toward the emptier side. Free: no energy spent, whether or not a protein helps.Down the concentration gradient. No ATP, whether by simple diffusion or through a channel or carrier (facilitated diffusion).
Uphill — activeToward the side that's already crowded. Costs energy: the cell's energy molecule, ATP, paid to a pump.Against the concentration gradient. Costs ATP, spent by a pump (primary active transport).
Pick a passenger above. The badge answers the one question that matters: downhill for free, or uphill at a cost?
Water follows solute
Water moves toward the side with more dissolved stuff. Change what's outside the cell and watch the cell.Osmosis moves water toward higher solute concentration. Tonicity describes the outside solution relative to the cell.
Where cells meet
Cells also link to their neighbors — some joins seal, some fasten them together, and some open tunnels for direct signals.
Four kinds of junction
Tight junctions — seal neighboring cells so nothing leaks between them (e.g., lining the gut).
Desmosomes — rivets that fasten cells into a sheet that can take stress (e.g., skin, muscle).
Gap junctions — tunnels that let ions and small molecules pass straight from one cell to the next, for direct communication (e.g., heart muscle).
Plasmodesmata — the plant version: channels through the cell walls connecting plant cells.
Quick check
Five items. Pick an answer and the reasoning appears, right or wrong. Nothing is sent anywhere.