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Unit 4
10–15% of examCells are always sending and receiving signals to coordinate what they do, whether it's a hormone traveling through your blood or bacteria sensing how many neighbors they have. In this unit you'll follow a signal from the receptor to the cell's response, see how feedback keeps your body in balance, and learn how the cell cycle copies and divides a cell under tight control. When that control breaks down, the result can be cancer.
Longer videos that cover the whole unit. Good for a first pass or a final review.
Cells communicate by touching each other directly or by releasing chemical signals. Local signals, such as neurotransmitters, act on nearby cells, while hormones such as insulin travel long distances through the body to reach their target cells.
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Signaling starts when a ligand (a signal molecule) binds a specific receptor, which may sit on the cell surface, like a G protein-coupled receptor, or inside the cell. The receptor changes shape and sets off a relay inside the cell, often a chain of proteins switched on by adding phosphate groups (a phosphorylation cascade) plus small relay molecules called second messengers, like cyclic AMP (cAMP). The relay amplifies the signal into a response such as gene expression, secretion or cell growth.
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A signaling pathway can switch genes on or off, change how a cell behaves, or even trigger programmed cell death (apoptosis). Because each step depends on the one before it, a mutation in the receptor or a chemical that blocks or activates any step can change the whole outcome.
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Negative feedback pushes a variable back toward its set point, like insulin and glucagon keeping your blood sugar steady. Positive feedback pushes a change even further from where it started, like contractions getting stronger during childbirth or ripening fruit giving off ethylene that speeds up more ripening.
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The cell cycle runs through interphase (G₁ growth, S phase when DNA is copied, and G₂ preparation), then mitosis and cytokinesis, and some cells leave the cycle to rest in a non-dividing G₀ stage. During mitosis (prophase, metaphase, anaphase, telophase), sister chromatids are pulled apart so each daughter cell gets an identical set of chromosomes. That's how your body grows and heals, and how some organisms reproduce asexually.
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Checkpoints act like inspections that must be passed before the cell moves on to the next phase, and proteins called cyclins team up with cyclin-dependent kinases (CDKs) to drive the cycle forward. When these controls are disrupted, cells may divide out of control and form cancer, or they may undergo apoptosis.
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