Cell Communication in Biology

Cell Communication

• Cell communication is fundamental to all living things
• Cells must respond appropriately to their environment
• Cell signaling in unicellular organisms influences how a cell responds to its environment and signals between organisms.
• Yeast cells identify their mates through cell signaling.
• Mating factors are secreted by each cell type, binding to receptors on the other cell type.
• Binding leads to changes in the cells causing their fusion.
• The nucleus of the fused cell contains genes from both cells.
• Cell communication is crucial for multicellular organisms for coordinated function.
• Cells must interpret various signals to coordinate their behaviors.
• Cells need to communicate to proliferate, differentiate, migrate, and maintain a functional state (homeostasis).
• Most animal cells both send and receive signals.
• Signals from other cells or the environment can be stimulatory (turning on a gene or protein) or inhibitory (turning off a gene or protein).
• During animal development, cells exchange signals to determine their specialized roles and positions.
• Incorrect cellular information processing causes diseases such as cancer, autoimmunity, and diabetes
• Cells communicate via chemical messengers.
• Communication requires ligands (extracellular signaling molecules) produced and released by signaling cells.
• Receptors bind to signaling molecules on the surface of target cells causing a cellular response via signal transduction pathways.
• Signal transduction pathways are a series of steps converting a signal on a cell surface into a specific cellular response.
• External signal is received and converted to another form to elicit a response.

Methods used by Cells to Communicate

• Direct contact: Cell-cell or cell-matrix interaction using membrance bound receptors.
• Indirect, chemical signaling:
  • Autocrine: Signaling molecules act on the same cell that produces them. Examples include, response of the immune system to foreign antigens, and cancer cells.
  • Paracrine: Signaling molecules act on neighboring target cells (short distance). Examples include neurotransmitters at synapses and cytokines causing inflammatory responses to infections.
  • Endocrine: Signaling molecules (hormones) travel through the circulatory system to act on distant target cells (long distance). Examples include progesterone, testosterone, and thyroid hormones.

Types of Signaling

• Direct contact:
  • Animal and plant cells connect cytoplasm of adjacent cells through gap junctions or plasmodesmata.
• Cell-cell recognition: Cells make physical contact through signal molecules embedded in their plasma membranes.

Cell-cell recognition

• Animal cells have MHC proteins on their surface to distinguish "self" from "other".
• Antigen-presenting cells (APCs) present antigens to T cells.
• T cell activation and signaling to other immune cells for maturity is caused by foreign antigen recognition.
• B cells present antigen to TH cells.
• T cell receptor recognizes the antigen releasing lymphokines.
• The B cell matures into a plasma cell releasing antibodies into the blood plasma.

Three kinds of chemical signaling

• Autocrine Signaling: Cells respond to signaling molecules they produce themselves. Examples include immune response to foreign antigens and cancer cells.
• Paracrine Signaling: Signaling molecules act on neighboring cells. Examples include neurotransmitters at synapses and cytokines causing inflammatory responses.
• Endocrine Signaling: Signaling molecules (hormones) travel through the circulatory system to act on distant cells. Examples include progesterone, testosterone, and thyroid hormones. The pituitary gland produces FSH and LH hormones coordinating the maturation process of ovules and endometrium.

Signal Transduction Pathways

• A signal transduction pathway is a series of steps by which a signal on a cell’s surface is converted into a specific cellular response.
• Signal molecules bind to receptor proteins.
• The receptor protein activates an intracellular pathway, often involving a series of intracellular signaling proteins.
• These signaling proteins interact with target proteins, changing their activity and altering the cell's behavior.
• The signal molecules are amplified and distributed affecting several processes in parallel.

External Signals

• Extracellular signals alter the activity of various cellular proteins to change the cell behavior.
• A typical cell is exposed to hundreds of different signaling molecules.
• Each cell must respond selectively to different signals, according to its specialized function.
• A cell's response depends on whether it has a receptor for that signal.
• Limited range of signals can still influence cellular behavior.
• Cell responses to signaling can be fast or slow. Fast responses do not involve gene expression (changes in cell movement, secretion, or metabolism). Slow responses involve gene expression and new protein synthesis (cell growth and division).

Intracellular Receptors

• Intracellular receptors are cytoplasmic or nuclear proteins.
• Receptors are often in the cytoplasm until a ligand binds triggering their movement to the nucleus.
• Signal molecules are lipid-soluble molecules.
• Steroid receptors have three domains: hormone-binding, DNA-binding, and a domain interacting with coactivators.
• Steroid Receptor Superfamily are transcription factors. They function as either activators or repressors of transcription.
• nitric oxide (NO) is a hydrophobic signal and triggers smooth muscle relaxation by diffusing across the membrane and binding to an enzyme inside the target cell
• The mechanism involves acetylcholine release from nerve terminals. Acetylcholine, stimulates the release of NO which rapidly diffuses, activating guanylyl cyclase, resulting in cyclic GMP (cGMP) production. This cGMP triggers muscle relaxation.
• Hormones like cortisol, estradiol, testosterone, and thyroxine are hydrophobic signals that rely on intracellular receptors.
• Cortisol affects the cell by activating the receptor protein which moves into the nucleus and activates gene transcription.

Extracellular Signals

• Extracellular signals are generally hydrophilic molecules and large polypeptide hormones that can't pass through the phospholipid bilayer.
• They need cell-surface receptors.
• Some hydrophobic signals are small or hydrophobic molecules that can diffuse through the plasma membrane.
• They bind to intracellular receptors, which alter the activity of intracellular enzymes or proteins in the cytosol or nucleus.
• Most steroid receptors are transcription regulators, binding to promoters and turning on specific genes.

Explore the intricate processes of cell communication essential for all living organisms. This quiz covers how unicellular organisms signal to one another and how multicellular organisms coordinate activities through signaling. Understand the significance of cell signaling in growth, differentiation, and maintaining homeostasis.