Unit 4: Cell Communication Flashcards
ligand
A molecule (e.g., hormone, neurotransmitter) that binds to a receptor to initiate a response. Signaling molecules could be proteins, small peptides, amino acids, nucleotides, steroids, fatty acid derivatives, nitric oxide, carbon monoxide. The receptor binding with the ligand often causes a conformational change in the receptor, activating downstream pathways.
Receptor
A protein on the cell membrane or within the cell that detects and binds the ligand
Signaling cells
Cells that produce ligands
Target Cells
Cells that receive the signal
Cytoplasmic receptor
Inside the cell, small or nonpolar molecules (e.g., steroids)., Ligand-receptor complex directly influences DNA transcription.
Membrane bound receptor
Embedded in the plasma membrane, Large or polar molecules (e.g., peptides), binding triggers intracellular signaling cascades
Plasmodesmata
Plants construct channels between cells called plasmodesmata that allow ligands to move directly from one cell to another throughout the plant structure. This is juxtacrine signaling.
Autocrine
A cell signals to itself, releasing signals that bind to its own receptors.
Juxtacrine
Requires direct contact between cells. Signal molecules are membrane-bound.
Paracrine
Signals act on nearby cells.
Endocrine
Signals travel through the bloodstream to distant target cells.
Quorum Sensing
A process where bacteria communicate using chemical signals to regulate gene expression based on population density. Paracrine, as the signaling molecules act on nearby bacterial cells. Bacteria release small chemical signals into the environment. As the population of bacteria grows, the concentration of signal molecules increases. Once the concentration of autoinducers reaches a threshold, bacteria detect them via receptors and alter their gene expression collectively. This allows bacteria to coordinate group behaviors like bioluminescence
Step 1: Reception
- Ligand binds to the receptor
- Most receptor proteins are in the cell membrane but some are inside the cell
- Hydrophilic (polar) ligands bind to plasma membrane receptors
- Small or hydrophobic (nonpolar) ligands can pass through the membrane and attach to intracellular receptors (ex. steroid hormones like testosterone)
G-Protein Coupled Receptors
G proteins bind the energy-rich GTP (very similar to ATP- source of energy). GPCR systems are extremely widespread and diverse in their functions. Signaling molecule binds to the receptor. G protein changes conformation an GDP is physically replaced by GTP. G protein diffuses laterally to activate an enzyme. Transduction starts when the enzyme is activated until GTP is hydrolyzed and G protein detaches
Ion Channel Receptors
An ion channel receptor acts as a gate that opens and closes when the receptor changes shape, When a signal molecule binds as a ligand to the receptor, the gate allows specific ions, such as Na+ or Ca2+, through a channel in the receptor. Ex: acetylcholine receptor, a Na+ channel in skeletal muscle cells → muscle contraction
Protein Kinase Receptor
Membrane receptors that transfer phosphate groups from ATP to another protein. Can trigger multiple signal transduction pathways at once. Ex: insulin, cell cycle
Step 2: Transduction
- A series of chemical reactions that link ligand binding to the target cell response. It relays signals.
- The goal is to ensure that the target cells have the appropriate response to a ligand binding
- Transduction can have multiple pathways
- Can amplify a signal (make it larger) by activating multiple copies of the next component in the pathway
- Provide more opportunities for coordination and regulation
- At each step in a pathway, the signal is transduced into a different form, commonly a conformational change in a protein
Secondary Messengers
Secondary messengers are small molecules/ions that relay and amplify signals received by receptors to proteins.
Calcium
Calcium is a common secondary messenger. Ca2+ can function as a second messenger because its concentration in the cytosol is normally much lower than the concentration outside the cell; a small change in the number of calcium ions represents a relatively large percentage change in calcium concentration
Protein kinase
Enzymes that use ATP to add phosphate groups (phosphorylate) intracellular proteins. This leads to structural changes that activate or inactive a protein.
Phosphorylation Cascade
In this process, a series of protein kinases add a phosphate to the next one in line, activating it. This leads to amplification of the signal. Phosphatase enzymes then remove the phosphates
Step 3: Response
There can be many possible responses to a cell signal:
- Activate or inactivate intracellular proteins
- Trigger multiple receptors and different responses
- Example of responses: turn transcription on/off or regulate activity of proteins in cytoplasm
- What can happen if the protein synthesis is altered? Cell behavior is altered - protein production can be altered
what can stop the response from happening?
- no more ligands
- ligand stops binding
- reception happens but something goes wrong in transduction (like an interfering molecule) and response doesn’t happen
How are steroid and protein hormones different?
steroid hormones are hydrophobic while protein hormones are hydrophilic
