Unit 11: Cell Signaling and Communication
How a single molecule outside a cell turns into a coordinated, massively amplified response inside it.
9 topics · 30 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
Why Cells Need Receptors Instead of Direct Entry
- How it worksHow does a cell-surface receptor let a hydrophilic hormone trigger changes inside the cell without ever crossing the membrane?
- Common mistakeWhy isn't every signaling molecule required to enter a cell in order to have an effect on it?
- Going furtherWhat happens to a hormone's effect when its target cell has no matching receptor at all?
Your turn1 of 2
A hydrophilic hormone circulating in the blood cannot cross a target cell's lipid membrane on its own, yet it still changes what that cell is doing within seconds of arriving. A student assumes the hormone must somehow be sneaking across the membrane after all, since it clearly has an effect inside the cell. How does a cell-surface receptor let it work without ever crossing the membrane?G-Protein-Coupled Receptors and the On/Off Switch
- How it worksHow does a ligand binding a GPCR from outside the cell change its shape enough to activate a G protein inside?
- How it worksWalk me through a G protein swapping GDP for GTP and splitting apart once it's activated by a GPCR.
- Common mistakeWhy isn't a G protein itself the enzyme that carries out the cell's final response?
- Going furtherWhat happens to G-protein signaling once the activated G protein's own GTPase activity hydrolyzes its bound GTP?
Your turn1 of 2
A ligand binds a GPCR on the outside of a cell, and within moments a G protein on the inside of the membrane has been activated. A student assumes the ligand itself must somehow physically reach through the membrane to touch the G protein directly. How does binding on the outside actually activate a G protein on the inside?Second Messengers: cAMP and Amplification
- How it worksHow does activated adenylyl cyclase convert ATP into the second messenger cyclic AMP?
- How it worksWalk me through cyclic AMP activating protein kinase A to change the behavior of dozens of target proteins.
- Worked exampleFind how many cAMP molecules one active adenylyl cyclase could make in 10 seconds at a rate of 500 per second.
- Going furtherWhat happens to a cell's cAMP signal once the enzyme phosphodiesterase starts breaking it down?
Your turn1 of 2
Find how many cAMP molecules one active adenylyl cyclase could make in 32 seconds at a rate of 654 per second.Calcium as a Signaling Ion
- How it worksHow does opening a calcium channel let a steep gradient flood the cytoplasm with a signal in an instant?
- Going furtherWhat happens to a muscle cell's contraction machinery the moment calcium floods in from the sarcoplasmic reticulum?
- Common mistakeWhy isn't calcium's role in signaling about providing structural building material the way it does in bone?
Your turn1 of 2
A cell keeps its resting cytoplasmic calcium concentration about a thousand times lower than the calcium concentration outside or stored in internal compartments. A student assumes opening a calcium channel should therefore only produce a slow, gradual rise in cytoplasmic calcium, since ions typically diffuse gradually. Why does opening a calcium channel instead flood the cytoplasm almost instantly?Receptor Tyrosine Kinases and Dimerization
- How it worksHow does ligand-induced dimerization let two receptor tyrosine kinases phosphorylate each other?
- Common mistakeWhy isn't a single, unpaired receptor tyrosine kinase capable of activating its own signaling pathway?
- Going furtherWhat happens to a cell's growth signaling when a mutation locks a receptor tyrosine kinase permanently dimerized?
Your turn1 of 2
A growth factor binds two separate receptor tyrosine kinase molecules on a cell's surface, pulling them together into a pair. A student assumes this pairing must just be incidental crowding, with each receptor still working independently on its own. What does ligand-induced dimerization actually accomplish?The Ras/MAPK Kinase Cascade
- How it worksHow does an activated receptor tyrosine kinase switch on the small G protein Ras to start the cascade?
- How it worksWalk me through Ras activating a chain of three kinases that each phosphorylate and activate the next one.
- Common mistakeWhy isn't Ras itself the enzyme that ends up phosphorylating the pathway's final target proteins?
- Going furtherWhat happens to the MAPK cascade's output when one kinase in the middle of the chain gets permanently activated?
Your turn1 of 2
An activated receptor tyrosine kinase sits at the cell membrane, while the small G protein Ras is anchored nearby, also at the membrane. A student assumes the receptor must directly transform into Ras somehow to pass the signal along. How does the activated receptor actually switch Ras on?Signal Amplification: One Ligand into Thousands of Molecules
- How it worksHow does each enzyme activated in a signaling cascade go on to activate many copies of the next enzyme in line?
- Worked exampleFind the total molecules affected after three cascade steps if each activated enzyme activates 100 copies of the next one, starting from a single receptor.
- Ties togetherHow does a signaling cascade's step-by-step amplification use the same multiplying logic as an enzyme turning over thousands of substrate molecules per second?
Your turn1 of 2
Find the total molecules affected after 4 cascade steps if each activated enzyme activates 142 copies of the next one, starting from a single receptor.Turning a Signal Off: Phosphatases and Desensitization
- How it worksHow does a phosphatase removing a phosphate group reverse what a kinase just did to a target protein?
- Going furtherWhat happens to a receptor's sensitivity when it stays exposed to a hormone for hours instead of just minutes?
- Common mistakeWhy isn't a signal automatically switched off just because the hormone that started it is still bound?
Your turn1 of 2
A kinase adds a phosphate group to a target protein, switching on its activity, and later that same protein's activity switches back off. A student assumes the phosphate group must simply fall off on its own over time, with no dedicated enzyme needed to remove it. What actually reverses what the kinase did?How a Mutated Ras Protein Drives Cancer
- How it worksHow does a mutation that blocks Ras's own GTPase activity leave it locked in its active, GTP-bound state?
- Going furtherWhat happens to a cell's rate of division when its Ras protein is stuck signaling growth all the time?
- Ties togetherHow does a stuck-on Ras protein's runaway signaling connect to the same cell-cycle machinery a normal growth signal switches on?
Your turn1 of 2
A mutation in the Ras gene keeps a cell's growth signal permanently switched on, even with no growth factor present at all. A student assumes any mutation affecting Ras should simply make it produce a stronger version of its normal, temporary signal, not a permanent one. What does this particular cancer-driving mutation actually do?