Cell Communication
Cell Signaling
Direct Contact: Cells are touching other cells
1. Animal Cells – gap junctions
2. Plant Cells - plasmodesmata
3. Cell-Cell Recognition – immune response
Local Regulators: a substance that influences cells in the vicinity
1. Paracrine Signaling – secretes molecules into extracellular fluid next to nearby target cell
2. Synaptic Signaling – secretes neurotransmitters between neurons
Long Distance Signaling: (also called endocrine signaling) hormones
are released into vessels for long distance transport to a target cell
Signal Transduction Pathway
Reception: The target cells detection of a signal (ligand) which binds to a surface protein
Transduction: The conversion of the signal to a form that can cause a cellular response
Response: The triggering of a specific cellular response
Types of Receptors
1. G-Protein Linked Receptors: a GTP binding protein with a secondary structure of 7 alpha-helices
that acts as an on/off switch
Ex. Epinephrine between neurons
2. Enzymatic Receptors: signal molecules combine to the
receptors to form a dimer. Phosphates are added to the
receptors. These in turn activate relay proteins which in
turn activate signal transductions pathways
Ex. Growth hormones on cells
3. Gated Ion Channels: protein pores that are opened or closed in response to chemical signals that
allow or block specific ions
Ex. Calcium intake in neurons which cause the neurotransmitter vesicles to fuse with the
synaptic membrane
4. Intracellular Responses: these involve proteins that are dissolved in the
cytoplasm of the cell. The signal molecules must be able to dissolve
through the plasma membrane
Ex. Steroid hormones
Transduction: Relay signals from receptors to cellular responses. This is usually done by protein kinases (an
enzyme that transfers phosphate groups from ATP to a protein) and includes the use of secondary
messengers.
Cellular Responses
1. Signal Amplification: increases the amount of response by a signal
Ex. Glycogen breakdown by epinephrine
2. Specificity: different cells will act differently in response to the same signal molecule
Ex. Epinephrine affects liver cells by causing glycogen breakdown but cause heart cells to contract