In multicellular organisms, cells send and receive chemical messages constantly to coordinate the actions of distant organs, tissues, and cells. The ability to send messages quickly and efficiently enables cells to coordinate and fine-tune their functions.
9.1: Signaling Molecules and Cellular Receptors
Chemical signals are released by signaling cells in the form of small, usually volatile or soluble molecules called ligands. A ligand is a molecule that binds another specific molecule, in some cases, delivering a signal in the process. Ligands can thus be thought of as signaling molecules. Ligands interact with proteins in target cells, which are cells that are affected by chemical signals; these proteins are also called receptors.
What property prevents the ligands of cell-surface receptors from entering the cell?
- The molecules bind to the extracellular domain.
- The molecules are hydrophilic and cannot penetrate the hydrophobic interior of the plasma membrane.
- The molecules are attached to transport proteins that deliver them through the bloodstream to target cells.
- The ligands are able to penetrate the membrane and directly influence gene expression upon receptor binding.
The secretion of hormones by the pituitary gland is an example of _______________.
- autocrine signaling
- paracrine signaling
- endocrine signaling
- direct signaling across gap junctions
Why are ion channels necessary to transport ions into or out of a cell?
- Ions are too large to diffuse through the membrane.
- Ions are charged particles and cannot diffuse through the hydrophobic interior of the membrane.
- Ions do not need ion channels to move through the membrane.
- Ions bind to carrier proteins in the bloodstream, which must be removed before transport into the cell.
Endocrine signals are transmitted more slowly than paracrine signals because ___________.
- the ligands are transported through the bloodstream and travel greater distances
- the target and signaling cells are close together
- the ligands are degraded rapidly
- the ligands don't bind to carrier proteins during transport
What is the difference between intracellular signaling and intercellular signaling?
Intracellular signaling occurs within a cell, and intercellular signaling occurs between cells.
How are the effects of paracrine signaling limited to an area near the signaling cells?
The secreted ligands are quickly removed by degradation or reabsorption into the cell so that they cannot travel far.
What are the differences between internal receptors and cell-surface receptors?
Internal receptors are located inside the cell, and their ligands enter the cell to bind the receptor. The complex formed by the internal receptor and the ligand then enters the nucleus and directly affects protein production by binding to the chromosomal DNA and initiating the making of mRNA that codes for proteins. Cell-surface receptors, however, are embedded in the plasma membrane, and their ligands do not enter the cell. Binding of the ligand to the cell-surface receptor initiates a cell signaling cascade and does not directly influence the making of proteins; however, it may involve the activation of intracellular proteins.
Cells grown in the laboratory are mixed with a dye molecule that is unable to pass through the plasma membrane. If a ligand is added to the cells, observations show that the dye enters the cells. What type of receptor did the ligand bind to on the cell surface?
An ion channel receptor opened up a pore in the membrane, which allowed the ionic dye to move into the cell.
9.2: Propagation of the Signal
Once a ligand binds to a receptor, the signal is transmitted through the membrane and into the cytoplasm. Continuation of a signal in this manner is called signal transduction. Signal transduction only occurs with cell-surface receptors because internal receptors are able to interact directly with DNA in the nucleus to initiate protein synthesis. When a ligand binds to its receptor, conformational changes occur that affect the receptor’s intracellular domain.
Where do DAG and IP3 originate?
- They are formed by phosphorylation of cAMP.
- They are ligands expressed by signaling cells.
- They are hormones that diffuse through the plasma membrane to stimulate protein production.
- They are the cleavage products of the inositol phospholipid, PIP2.
What property enables the residues of the amino acids serine, threonine, and tyrosine to be phosphorylated?
- They are polar.
- They are non-polar.
- They contain a hydroxyl group.
- They occur more frequently in the amino acid sequence of signaling proteins.
The same second messengers are used in many different cells, but the response to second messengers is different in each cell. How is this possible?
Different cells produce different proteins, including cell-surface receptors and signaling pathway components. Therefore, they respond to different ligands, and the second messengers activate different pathways. Signal integration can also change the end result of signaling.
What would happen if the intracellular domain of a cell-surface receptor was switched with the domain from another receptor?
The binding of the ligand to the extracellular domain would activate the pathway normally activated by the receptor donating the intracellular domain.
9.3: Response to the Signal
Inside the cell, ligands bind to their internal receptors, allowing them to directly affect the cell’s DNA and protein-producing machinery. Using signal transduction pathways, receptors in the plasma membrane produce a variety of effects on the cell. The results of signaling pathways are extremely varied and depend on the type of cell involved as well as the external and internal conditions. A small sampling of responses is described below.
What is the function of a phosphatase?
- A phosphatase removes phosphorylated amino acids from proteins.
- A phosphatase removes the phosphate group from phosphorylated amino acid residues in a protein.
- A phosphatase phosphorylates serine, threonine, and tyrosine residues.
- A phosphatase degrades second messengers in the cell.
How does NF-κB induce gene expression?
- A small, hydrophobic ligand binds to NF-κB, activating it.
- Phosphorylation of the inhibitor Iκ-B dissociates the complex between it and NF-κB, and allows NF-κB to enter the nucleus and stimulate transcription.
- NF-κB is phosphorylated and is then free to enter the nucleus and bind DNA.
- NF-κB is a kinase that phosphorylates a transcription factor that binds DNA and promotes protein production.
Apoptosis can occur in a cell when the cell is ________________.
- no longer needed
- infected by a virus
- all of the above
What is the effect of an inhibitor binding an enzyme?
- The enzyme is degraded.
- The enzyme is activated.
- The enzyme is inactivated.
- The complex is transported out of the cell.
What is a possible result of a mutation in a kinase that controls a pathway that stimulates cell growth?
If a kinase is mutated so that it is always activated, it will continuously signal through the pathway and lead to uncontrolled growth and possibly cancer. If a kinase is mutated so that it cannot function, the cell will not respond to ligand binding.
How does the extracellular matrix control the growth of cells?
Receptors on the cell surface must be in contact with the extracellular matrix in order to receive positive signals that allow the cell to live. If the receptors are not activated by binding, the cell will undergo apoptosis. This ensures that cells are in the correct place in the body and helps to prevent invasive cell growth as occurs in metastasis in cancer.
9.4: Signaling in Single-Celled Organisms
Within-cell signaling allows bacteria to respond to environmental cues, such as nutrient levels, some single-celled organisms also release molecules to signal to each other.
Which type of molecule acts as a signaling molecule in yeasts?
- mating factor
- second messenger
Quorum sensing is triggered to begin when ___________.
- treatment with antibiotics occurs
- bacteria release growth hormones
- bacterial protein expression is switched on
- a sufficient number of bacteria are present
What characteristics make yeasts a good model for learning about signaling in humans?
Yeasts are eukaryotes and have many of the same systems that humans do; however, they are single-celled, so they are easy to grow, grow rapidly, have a short generation time, and are much simpler than humans.
Why is signaling in multicellular organisms more complicated than signaling in single-celled organisms?
Multicellular organisms must coordinate many different events in different cell types that may be very distant from each other. Single-celled organisms are only concerned with their immediate environment and the presence of other cells in the area.