Notes
How cells communicate
- Direct contact: gap junctions (animals), plasmodesmata (plants), and cell-surface molecules touching.
- Short distance: local regulators, like neurotransmitters at a synapse.
- Long distance: hormones travel through the blood to target cells.
Signal transduction
1. Reception: a ligand (signal molecule) binds a receptor. Nonpolar ligands like steroids can cross the membrane and bind receptors inside the cell. 2. Transduction: the signal is relayed and often amplified through a cascade, such as G protein-coupled receptors activating second messengers (cAMP) or protein kinases phosphorylating other proteins. 3. Response: the cell changes, for example turning on genes, opening channels, or activating enzymes.
- A mutation in a receptor or relay protein can block a response or turn it on permanently (a common cause of cancer).
Feedback
- Negative feedback reverses a change to keep conditions stable (body temperature, blood glucose with insulin and glucagon).
- Positive feedback amplifies a change (contractions during childbirth, fruit ripening).
The cell cycle
Interphase: $\mathrm{G_1}$ (growth), S (DNA replication), $\mathrm{G_2}$ (preparation). Then mitosis (prophase, metaphase, anaphase, telophase) and cytokinesis. Cells can also exit to $\mathrm{G_0}$, a non-dividing state.
- Mitosis produces two genetically identical diploid cells for growth, repair and asexual reproduction.
- Checkpoints (at $\mathrm{G_1}$, $\mathrm{G_2}$ and the metaphase spindle checkpoint) stop the cycle if DNA is damaged or chromosomes aren't attached.
- Cyclins and cyclin-dependent kinases (CDKs) control progress through the cycle.
- Cancer comes from loss of cell-cycle control, often mutations in genes like $p53$.
Topics in this unit
Each topic has its own short notes and practice.
Practice
Lipid-soluble signals diffuse through the bilayer.
One activated kinase can phosphorylate many targets.
The response counteracts the original change.
S stands for synthesis of DNA.
Spindle fibers pull sister chromatids to opposite poles in anaphase.
The pathway is stuck "on," as in many cancers.
The $\mathrm{G_1}$ checkpoint checks for DNA damage before replication.
Mitosis produces genetically identical cells with the same chromosome number.
Free response
Write your answer first, then open the scoring guide.
Free-response question
Epinephrine binds a G protein-coupled receptor on liver cells, which leads to breakdown of glycogen into glucose.
(a) Describe the role of the G protein in this pathway.
Show the answer and scoring guide
When epinephrine binds, the receptor changes shape and activates the G protein (it binds GTP). The active G protein activates adenylyl cyclase, which makes cAMP.
Check off the points you earned:
(b) Explain how one epinephrine molecule can cause the release of thousands of glucose molecules.
Show the answer and scoring guide
Each step of the cascade activates many molecules of the next step (cAMP activates kinases that phosphorylate many enzymes), so the signal is amplified.
Check off the points you earned:
(c) Predict the effect of a drug that blocks the receptor's ligand-binding site.
Show the answer and scoring guide
Epinephrine couldn't bind, so the cascade wouldn't start and glycogen wouldn't be broken down in response to the signal.
Check off the points you earned: