AP Biology Unit 4 Cheatsheet: Cell Communication and Cell Cycle

Direct Contact

• Cell-to-cell interaction (immune cells, APCs, T-cells)
• Requires physical proximity

Short Distance (Local Regulators)

• Neurotransmitters, morphogens, quorum sensing
• Target nearby cells only
• Diffusion-based

Long Distance (Hormones)

• Insulin, growth hormone, thyroid hormones, sex steroids
• Travel through bloodstream or other transport systems
• Reach distant target cells

Negative Feedback: Homeostasis

• Reduces initial stimulus; returns system to set point
• Counteracts perturbation
• Blood sugar: High blood glucose → insulin → cellular glucose uptake ↑ → normal; Low blood glucose → glucagon → glucose production/release ↑ → normal
• Operates at molecular, cellular, and organismal levels

Positive Feedback: Amplification

• Amplifies initial response; moves variable away from set point
• Stimulus intensifies → additional response → system change
• Lactation: Suckling → oxytocin → milk ejection (let-down) → more suckling (prolactin, not oxytocin, drives milk production)
• Childbirth: Uterine stretch → oxytocin → stronger contractions → more stretch
• Fruit ripening: Ethylene → enzyme changes → more ripening → more ethylene