The basics
What it covers: How cells communicate (cell signaling and signal transduction), how they maintain stability (feedback), and how they divide (the cell cycle and its regulation).
Exam weight: 10–15% of the AP Biology exam.
The big question: How do cells receive information from their environment, respond appropriately, and decide whether to divide?
Big Ideas covered: Energetics (BI 2), Information Storage & Transmission (BI 3), Systems Interactions (BI 4).
Key topics at a glance
Four Types of Signaling
Direct contact (gap junctions, plasmodesmata), paracrine (local diffusion), synaptic (neuron → target), endocrine (hormones through bloodstream).
Three Stages of Signal Transduction
Reception — ligand binds receptor. Transduction — signal relayed and amplified through cascades. Response — cell changes (gene expression, enzyme activation, etc.).
Receptor Types
GPCRs (G protein-coupled), RTKs (receptor tyrosine kinases), ligand-gated ion channels, and intracellular receptors (for hydrophobic ligands like steroids).
Second Messengers & Cascades
Small molecules (cAMP, Ca²⁺) relay and amplify the signal inside the cell. Phosphorylation cascades magnify the response — one ligand can trigger millions of response molecules.
Negative Feedback
The product of a process inhibits the process. Maintains homeostasis at a set point. Examples: insulin/glucose, body temperature, blood pH.
Positive Feedback
The product amplifies the process. Pushes toward completion or a peak. Examples: labor contractions, blood clotting, action potentials.
Cell Cycle Phases
Interphase (G₁ → S → G₂; growth and DNA replication) and the M phase (mitosis + cytokinesis). Most of the cell's life is in interphase.
Mitosis: PMAT
Prophase (chromosomes condense), metaphase (align at equator), anaphase (chromatids separate), telophase (new nuclei form). Then cytokinesis splits the cell.
Cell Cycle Checkpoints
G₁ (size, nutrients, DNA damage), G₂ (DNA replication complete), M (chromosomes attached). Cyclins + CDKs drive progression; p53 halts on damage.
Cancer = Failed Regulation
Mutations in proto-oncogenes (gain of function) or tumor suppressors like p53 (loss of function) let damaged cells divide. Apoptosis normally cleans up dangerous cells.
The key terms you must know
- Ligand / Receptor — signaling molecule and the protein it binds. The lock-and-key of cell communication.
- Signal transduction — converting an extracellular signal into an intracellular response through a cascade.
- Second messenger — small molecule (cAMP, Ca²⁺) that relays and amplifies the signal inside the cell.
- Phosphorylation cascade — chain of kinases each activating the next; the most common way signals are relayed and amplified.
- Negative feedback — product inhibits process; maintains homeostasis.
- Positive feedback — product amplifies process; drives system to a peak or completion.
- Interphase (G₁, S, G₂) — the cell grows, replicates DNA, prepares to divide. Takes up most of the cell cycle.
- Mitosis (PMAT) — prophase, metaphase, anaphase, telophase — division of the nucleus into two identical sets.
- Sister chromatids — identical DNA copies joined at the centromere, separated during anaphase.
- Cell cycle checkpoints — G₁, G₂, and M points where the cell verifies conditions before proceeding.
- Cyclin / CDK — regulatory proteins that drive the cell into the next phase when paired up.
- p53 — tumor suppressor; stops the cell cycle on DNA damage or triggers apoptosis. Mutated in over half of cancers.
- Apoptosis — programmed cell death; orderly self-destruction of damaged or unneeded cells.
Key themes to remember
- Reception → Transduction → Response is universal. Almost every signaling question follows these three stages — name them when you can.
- Signal amplification is exponential. One ligand can produce millions of response molecules because each cascade step activates many of the next.
- Negative feedback maintains; positive feedback escalates. If a system stays near a set point, it's negative feedback. If it accelerates to a peak, it's positive.
- The cell cycle is tightly regulated for a reason. Uncontrolled cell division is cancer. Checkpoints exist to prevent that.
- External signals control internal decisions. Growth factors (signals from outside) often determine whether a cell enters the cell cycle in the first place.
- Structure–function still applies. Receptors are specific to their ligands because of shape. Cyclins activate CDKs because of how they bind.
Common exam traps
- Steroid hormones bind INSIDE the cell. They're hydrophobic, so they cross the plasma membrane and find intracellular receptors. Don't confuse with hydrophilic hormones (which use membrane receptors).
- Positive feedback doesn't mean "good." It means "amplifying." Labor and blood clotting both use positive feedback — they're not "good" or "bad," they just escalate.
- Signal transduction needs SIGNAL AMPLIFICATION, not many ligands. Even ONE ligand can trigger a huge response because each step magnifies the signal.
- Mitosis ≠ cell cycle. Mitosis is just one phase (M). The cell cycle includes G₁, S, G₂, and M.
- "DNA replication" happens in S phase, not mitosis. By the time mitosis starts, the DNA is already doubled. Mitosis just separates it.
- Cancer mutations come in two flavors. Proto-oncogenes mutate to become OVERACTIVE (gain of function). Tumor suppressors mutate to become INACTIVE (loss of function). Both lead to uncontrolled division.
- p53 doesn't fix DNA itself. It HALTS the cell so other proteins can repair the DNA — or triggers apoptosis if damage is too severe.
- Apoptosis is NOT necrosis. Apoptosis is orderly, intentional, and doesn't cause inflammation. Necrosis is messy, accidental cell death from injury.