ap bio unit 4

• 3 types of cell communication: direct contact, local signaling, and long-distance signaling
• direct contact: communication through cell junctions; signaling substances and other material dissolved (small ions and molecules) in the cytoplasm can freely pass between adjacent cells
• animal cells use gap junctions for direct contact
• plant cells use plasmodesmata (channels through cell walls) for direct contact (found in ALL plants)
• ex. of direct contact: immune cells; antigen presenting cells (APCs) communicate to T cells through direct contact
• 2 types of local signaling: paracrine and synaptic
• paracrine: signaling with nearby cells; secretory cells release local regulators (ex. growth factors) via exocytosis to an adjacent cell
• synaptic: specific kind of paracrine signaling involving nerve cells and neurotransmitters
• synaptic signaling: occurs in animal nervous systems; neurons secrete neurotransmitters and they diffuse across the synaptic cleft (space between nerve cell and target cell)
• long-distance signaling: animals and plants use hormones for this type of signaling

  • travel across membranes
• long-distance signaling in plants: release hormones that travel in the plant's vascular tissue (xylem and phloem) or through the air to reach target tissues
• long-distance signaling in animals: use endocrine signaling, specialized glands release hormones into the circulatory system where they reach target cells
• ex. of long-distance: insulin; this is released by the pancreas into the bloodstream where it circulates through the body and binds to target cells (ex. of endocrine signaling in animals)
• hormones are hydrophobic (so that they can directly pass through membrane)
• 3 stages of cell signaling: reception, transduction, and response
• reception (1st stage): the detection and receiving of a ligand by a receptor in the target cell (binding is highly specific)

  1. when ligand binds to receptor, the receptor activates via a conformational change to allow it to interact with other cellular molecules
  2. initiates transduction signal
  3. receptors can be in the plasma membrane or intracellular
• plasma membrane receptors: most common type of receptor involved in signal pathways

  • binds to ligands that are polar and large
  • ex. ligand-gated ion channels and G protein coupled receptors (GPCRs)
• intracellular receptors: found in cytoplasm or nucleus of target cell

  • binds to ligand that pass through membrane (hydrophobic molecules)
  • ex. steroid and thyroid hormones, gases like nitric oxide (hydrophobic)
• transduction (2nd stage): the conversion of an extracellular signal to an intracellular signal that will bring about a cell response

  • receptor protein changes and activates intracellular molecules (2nd messengers)
  • requires a signal transduction pathway
• signal transduction pathway: regulates protein activity through phosphorylation (protein kinase, relays signal inside cell) and dephosphorylation (enzyme phosphatase)

  • during transduction, signal is amplified
• second messengers: small, non-protein molecules and ions that help relay the message and amplify the response (cyclic AMP is common 2nd messenger)
• response (3rd stage): the final molecule in the signaling pathway converts the signal to a response that will alter a cell process

  • ex. protein that can alter membrane permeability, enzyme that will change a metabolic process, and protein that turns genes on or off
• ligand gated ion channels: located in the plasma membrane, important in nervous system

  • receptors that act as a "gate" for ions
  • when a ligand binds to a receptor, the "gate" opens or closed, allowing the diffusion of specific ions
  • initiates a series of events that lead to a cell response
• transcription and translation are involved in gene expression
• transcription (DNA to RNA) occurs in nucleus of cell and translation (RNA to protein) occurs in the cytoplasm and ER
• homeostasis: the state of relatively stable internal conditions; body maintains homeostasis through feedback loops
• positive feedback: a hormone acts to directly or indirectly cause increased secretion of the hormone
• examples of positive feedback:

  • child labor (oxytocin)
  • blood clotting (blood clumps to stop blood loss)
  • fruit ripening
• negative feedback: when a hormone acts to directly or indirectly inhibit further secretion of the hormone of interest
• examples of negative feedback:

  • sweat (body temp regulation)
  • blood sugar
  • breathing rate
• homeostatic inhibitors:

  • genetic disorders
  • drug or alcohol abuse
  • intolerable conditions (extreme heat or cold)
• DNA associates with and wraps around proteins known as histones to form nucleosomes
• strings of nucleosomes form chromatin
• chromatin is the non-condensed form, and after DNA replication, chromatin condenses to form a chromosome (densely packed)
• chromosomes are densely packed to allow for easier cell division
• sister chromatids: two identical chromosomes that are joined by a centromere
• centromere: the region on each sister chromatid where they are most closely attached
• when counting chromosomes, count the number of centromeres
• kinetochore: proteins attached to the centromere that link each sister chromatid to the mitotic spindle
• prokaryotes: singular, circular DNA