Communication

-2 Methods—

Cell-to-cell contact

• Plasmodesmata— channels that connect the cytoplasm of plant cells through their cell walls (used in immune system)

• Gap Junction— channels that connect the cytoplasm of animal cells

• temporary contact is used in the immune system —> immune cells activate when a cell has an antigen that binds to the receptor

Chemicals

• Ligand— chemical messenger, can be polar (water-soluble) or nonpolar (lipid-soluble)

• target cells have the receptors for ligands

• e.g. neurotransmitters cross synapses between neurons, hormones are used over long distances through the bloodstream

-Signal Transduction Pathway

1. Signal Reception

   • ligand binds to receptor, triggers shape change

2. Signal Transduction

   • steps that convert the signal into a form that can create a response

   • usually activating proteins with phosphate

   • can amplify signal

3. Response

   • any cell activity (e.g. gene activation, apoptosis, cell growth)

• G protein-coupled receptor (GPCR)— a ligand binds to receptor, activated receptor binds + activates G protein, G protein carries GTP to an enzyme, enzyme is activated, triggers next step (all temporary)

• Ligand-gated ion channel— ligand binds to channel to open it and allow ions through (temporary)

• Kinase— protein that activates other proteins by adding a phosphate to them (can cause phosphorylation cascades)

• Phosphatase— protein that deactivates other proteins by dephosphorylating them

• Second messenger— small, non-protein molecules or ions that help carry signal through cytoplasm (e.g. cAMP)

• signaling pathways mostly either regulate protein synthesis by activating/deactivating genes in the nucleus or regulate protein activity of proteins functioning outside the nucleus (e.g. opening/closing ion channels)

-Bacterial Communication

• Quorum Sensing— regulates gene expression in response to bacteria population density

• bacteria produce autoinducers that are released into the environment

• certain levels of autoinducers alter gene expression

• e.g. bioluminescence

Cell Division

• somatic cell = body cell, diploid

• divide when the body grows or heals

• Regulation

  • Maturation Promotion Factor— example of signal telling cell when to divide

    • all cells have cyclin-dependent kinase, concentration doesn’t fluctuate, but they’re usually inactive

    • cyclin activates CDK, cyclin concentrations vary

    • cyclin + CDK = MPF —> high MPF causes cell division

    • cyclin is degraded to stop division

  • Checkpoints

    • if there are issues, cell undergoes apoptosis (controlled cell death)

  • Cancer— uncontrolled cell division

    • Tumor-suppressor gene— codes for protein that stops cell division when there is major DNA damage

    • Proto-oncogene— codes for protein that stimulates cell division

      • should NOT always be turned on (becomes an oncogene)