Concept 5: Cell Signaling

5.1 The Plasma Membrane and Cell Signaling

• In multicellular organisms, cell-to-cell communication is crucial for coordinating activities.

• Plasma membranes facilitate communication, which is also vital for many unicellular organisms.

5.2 Local and Long-Distance Signaling

5.2.1 Local Signaling

• Eukaryotic cells communicate via direct contact; animal cells use gap junctions and plant cells use plasmodesmata.

• Local regulators like growth factors stimulate nearby cells through paracrine signaling.

5.2.2 Synaptic Signaling

• A specialized form of local signaling in the nervous system.

• An electrical signal triggers the release of neurotransmitters, which diffuse across synapses to target cells.

5.2.3 Endocrine Signaling

• Long-distance signaling uses hormones that travel through the circulatory system.

• Hormones can vary widely in size and shape, allowing diverse responses among target cells.

5.3 The Three Stages of Cell Signaling

• Reception: Binding of a signaling molecule (ligand) to a receptor protein.

• Transduction: Conversion of the signal into a cellular response through transduction pathways.

• Response: The final cellular activity in response to the signal.

5.4 Reception of Signaling Molecules

5.4.1 Receptors in the Plasma Membrane

• Water-soluble signaling molecules typically bind to membrane receptors:

  • G protein-coupled receptors (GPCRs): Activate G proteins, which bind to GTP and trigger signal transduction pathways.

  • Ligand-gated ion channels: Change shape to allow ion flow upon ligand binding, crucial in the nervous system.

5.4.2 Intracellular Receptors

• Found in the cytoplasm or nucleus; hydrophobic messengers like steroid hormones can cross membranes to bind and activate these receptors.

5.5 Signal Transduction Pathways

• Signal transduction involves multiple steps to amplify the signal.

• Activated receptors can relay the signal through a cascade, often through protein phosphorylation.

5.6 Small Molecules as Second Messengers

• After the first messenger (signal molecule) binds, second messengers (e.g., cAMP, calcium ions) diffuse within cells.

• cAMP activates protein kinase A, triggering various cellular responses.

5.7 Cellular Responses

• Responses can involve the regulation of gene expression or modifications of cellular activities (e.g., enzyme activation).

Concept 9: The Cell Cycle

9.1 Key Roles of Cell Division

• Distinguishes living organisms from nonliving matter; essential for growth, repair, and reproduction.

9.2 Cell Division Mechanisms

• Most cell division results in genetically identical daughter cells.

• Eukaryotic chromosomes consist of chromatin, packed into distinct structures.

9.3 Phases of the Cell Cycle

• Mitotic (M) phase: Mitosis and cytokinesis.

• Interphase: Divided into G1, S, and G2 phases. DNA synthesis occurs only during S phase.

9.4 Mitosis Stages

• Mitosis is further divided into five phases: Prophase, Prometaphase, Metaphase, Anaphase, and Telophase.

9.5 The Mitotic Spindle

• A microtubule structure controlling chromosome movement during mitosis.

9.6 Cytokinesis

• Animal cells undergo cleavage to form a cleavage furrow, while plant cells form a cell plate.

9.7 Cellular Regulation of the Cell Cycle

• The cell cycle is controlled by a molecular system with internal checkpoints (G1, G2, M).

  • External signals can promote division or inhibit it (density-dependent inhibition).

9.8 Cancer Cells and Cell Cycle Control

• Cancer cells ignore typical cell cycle regulations; they can proliferate indefinitely, forming tumors.

• Treatment options include radiation, chemotherapy, and personalized medical treatments based on tumor DNA sequencing.