Cell Communication and Signal Transduction

Cells use direct contact or chemical signaling to communicate. Direct contact involves physical touch between cells, like immune cells or plant cells connected by plasmodesmata. Chemical signaling uses messenger molecules released from one cell to signal another.

Local regulators target cells in close proximity to the signal-emitting cell. Examples include neurotransmitters, which signal between neurons, and morphogens, which guide embryonic development.

Long-distance signaling involves signals that travel from one cell type to another, often across long distances. Hormones, such as insulin or estrogen, are examples of long-distance messengers.

Signal transduction pathways are chains of molecules that relay a signal from the outside of a cell to the inside, triggering specific cellular responses.

A ligand is a signaling molecule that binds to a specific receptor protein on the target cell. Binding triggers a change in the receptor's shape, initiating signal transduction.

Second messengers are molecules that relay and amplify the intracellular signal within the cell, often triggered by a receptor protein activation. Examples include cyclic AMP (cAMP).

Signal transduction pathways can be influenced by the environment, leading to different responses depending on the context. For example, microbes use quorum sensing to coordinate behaviors based on population density.

Signal transduction pathways can trigger a variety of cellular responses, including changes in gene expression, cell growth, secretion of molecules, and even cell death.

A signaling pathway's response can be altered by changes in signal transduction. This may involve mutations in receptor proteins or signaling pathway components, impacting downstream components and signal transduction. Additionally, external chemicals can activate or inhibit the pathway by interfering with its components.

Feedback mechanisms are biological processes that regulate internal environments (homeostasis) and responses to internal and external changes. Negative feedback maintains homeostasis by counteracting changes, bringing the system back to its set point.

Positive feedback mechanisms amplify responses and processes in organisms. Instead of returning to a set point, they further activate the stimulus, leading to a larger response and system change.

The cell cycle is a highly regulated process of growth and reproduction in eukaryotes. It consists of interphase (G1, S, G2), mitosis, and cytokinesis. Cells can enter a non-dividing state (G0) and re-enter the cycle if needed.

Mitosis is a process of cell division that ensures a complete genome is transferred from a parent cell to two identical daughter cells. This plays a crucial role in growth, tissue repair, and asexual reproduction.

Mitosis occurs in a sequence of steps: prophase, metaphase, anaphase, and telophase. This ensures that each daughter cell receives a full set of chromosomes.

Checkpoints are internal controls that regulate progression through the cell cycle. They ensure that the process is properly regulated and that errors are minimized. Interactions between cyclins and cyclin-dependent kinases (CDKs) control the cell cycle.

Disruptions to the cell cycle can lead to serious issues like cancer or programmed cell death (apoptosis). This can occur due to errors in regulation, leading to abnormal or uncontrolled cell growth.