ap bio unit 4

• ligands- molecules that bind specifically to other molecules (such as receptors)

  • original intercellular (between cells) signal is converted to an intracellular (within cells) signal
• 4 types of signaling found in multicellular organisms:

  1. paracrine signaling
  2. synaptic signaling
  3. autocrine signaling
  4. direct contact
  5. endocrine signaling
• paracrine signaling- cells communicate over short distances with neighbor cells

  • important during development
• synaptic signaling- type of paracrine signaling in which NERVE cells transmit signals

  • ligands are neurotransmitters

  
• autocrine signaling- cell signals to itself

  • ligand binds to receptors on its own surface
• endocrine signaling- over long distances through bloodstream

  • the circulatory system is used as a distribution network
  • signals are known as hormones
  • each endocrine gland released one ore more types of hormones
  • ex. pituitary gland released growth hormone

  
• direct contact signaling- gap junctions in animals and plasmodesmata in plants

  • these two types of water-filled channels allow small signaling molecules called intracellular mediators to diffuse between the two cells

  
• which type of signaling is displayed when immune cells use cell-surface markers to recognize the body's own cells and cells infected by pathogens?

  direct contact signaling
• quorum sensing in bacteria- cell signaling in prokaryotes

  • when bacteria monitor the density of the population (number of other bacteria in an area) based on chemical signals
  • when the signaling reaches a threshold level, all the bacteria in the population will change gene expression at the same time
• autoinducers- signaling molecules continually secreted by bacteria to announce their presence to their neighbors
• cell-cell junction in plant cells:

  • plasmodesmata- holes are punched in the cell walls of plants to allow direct cytoplasmic exchange between two cells

  
• cell-cell junction in animal cells:

  1. gap junctions- channels between neighboring cells allows transport of ions, H2O, etc.

  • gap junctions are important in cardiac muscle contraction

  1. tight junctions- watertight seal between two adjacent animal cells

  • purpose of tight junctions is to keep liquid from escaping between cells

  1. desmosomes

  
• types of ligands and receptors:

  • intracellular receptors- receptor proteins INSIDE of cell (ligands are usually small and hydrophobic)
  • cell-surface receptors- bind to ligands on OUTSIDE surface of cells
• ligand-gated ion channels: open when a ligand binds to the channel

  • when a ligand binds to the extracellular region of the channel, the protein's structure changes in such a way that ions of a particular type can pass through
  • neurons have ligand-gated channels that are bound by neurotransmitters

  
• G protein-coupled receptors (GPCRs):

  • when G protein is attached to GTP, it is active
  • when G protein is attached to GDP, it is inactive
• enzyme-linked receptors: cell-surface receptors with intracellular domains that are associated with an enzyme

  • ex. receptor tyrosine kinases (RTKs)

  
• steroid hormones are ligands that CAN PASS through membrane and bind to intracellular receptors in the nucleus or cytoplasm
• peptide (protein) ligands are polar and CANNOT cross the membrane

  • they bind to extracellular domains of cell-surface receptors
• signal transduction pathway:

  1. ligand binds to receptor
  2. 2nd messengers relay the signal inside the cell
  3. response

  
• phosphorylation: alters protein activity with addition of a phosphate group

  • kinases= ON (adds a phosphate to protein)
  • phosphatases = OFF (removes a phosphate from protein)
• 2nd messengers: small, non-protein molecules that pass along a signal initiated by the binding of a ligand

  • ex. calcium ions and cyclic AMP (cAMP)
• cellular response examples:

  • gene expression through growth factor signaling
  • cellular metabolism
  • apoptosis
• positive feedback loops: amplify the starting signal

  • ex. childbirth- hormone oxytocin amplifies contractions
  • fruit ripening
• negative feedback loops: returns to the set point

  • how homeostasis is maintained
  • ex. body temperature, blood pressure regulation
• interphase: cell grows and makes a copy of its DNA

  • consists of G1, S, and G2 phase

  S phase- DNA duplication, centrosome duplication which helps separate DNA during anaphase
• prophase:

  • chromosomes start to condense and become visible
  • mitotic spindle begins to form
  • nucleolus disappears

  
• kinetochore microtubules- microtubules that bind a chromosome

  centromeres- region of DNA where sister chromatids are most tightly connected

  
• metaphase-

  • all chromosomes line up at the metaphase plate
  • kinetochores are attached to microtubules from opposite spindle poles
  • spindle checkpoint that helps ensure that the sister chromatids will split evenly between the two daughter cells
• anaphase-

  • sister chromatids separate, pulled toward opposite ends of the cell
  • microtubules make cell longer
• telophase-

  • mitotic spindle is broken down
  • two new nuclei form
  • chromosomes decondense (stringy form)
• cytokinesis-

  • in animal cells, contractile ring contracts inward and cleavage furrow indentation is produced as a result
  • in plant cells, they divide in two by building a cell plate down the middle
• G1 checkpoint- G1/S transition

  • checks for cell size, nutrients, growth factors, and DNA damage

  G2 checkpoint- G2/M transition

  • checks for DNA damage and PROPER DNA REPLICATION

  spindle checkpoint- metaphase/anaphase transition

  • checks for chromosome attachment to spindle at metaphase plate
• G0 phase- non-dividing stage and resting stage for the cell (cell enters this phase after g1 if it doesn’t pass the checkpoint)
• cell cycle regulator: cyclin-dependent kinases (CDKs)

  • CDKs must be phosphorylated in order to be active
  • cyclin must bind

  
• metastasis- cancer cells gain the ability to migrate to other parts of the body

  benign tumor- mass of cells that divide too much and do not have the potential to invade other tissues

  malignant tumor- group of cels that divide excessively and can invade other tissues
• tumor suppressors- genes that normally block cell cycle progression
• prometaphase:

  • chromosomes become even more compact
  • nuclear envelope breaks down
  • mitotic spindle grows more

  
• juxtacrine = direct contact
• direct contact signaling: helper T cell binds to antigen presenting cell
• paracrine signaling: growth factor