Select the six statements that accurately describe signal transduction pathways. A ligand, such as a hormone, binds to a specific cell surface receptor on a target cell. A receptor may pass on a signal by interacting with another protein or by acting as an enzyme. Phosphatases remove phosphoryl groups from polypeptides, regulating a cell's response. A receptor changes conformation upon binding, transmitting a signal across the cell membrane. Signal transduction cascades directly transmit a single stimulus to a single target. Signal transduction cascades, often involving protein kinases, amplify a signal intracellularly. A ligand phosphorylates protein residues, ending the signaling cascade inside the cell. A second messenger may carry a signal from the cell membrane to an organelle. A second messenger carries a signal from a tissue or organ to a target cell.

A ligand, such as a hormone, binds to a specific cell surface receptor on a target cell.

A receptor may pass on a signal by interacting with another protein or by acting as an enzyme.

Phosphatases remove phosphoryl groups from polypeptides, regulating a cell's response.

A receptor changes conformation upon binding, transmitting a signal across the cell membrane.

Signal transduction cascades, often involving protein kinases, amplify a signal intracellularly.

A second messenger may carry a signal from the cell membrane to an organelle.

Explanation:

All the above mentioned statements are correct. Although based on the type of receptor and ligand, cell signalling may differ a bit from cell to cell yet some basic changes that occur within the cell are same as mentioned above.

A receptor may be present intracellulary i.e. in the cytosol or nucleus, such receptors are known as intracellular receptors. For example: steroid, thyroxine hormone receptors etc. The ligands of intracellular receptors are hydrophobic so they can cross cell membrane directly.

Some receptors on the other hand are present on cell membrane and therefore they are known as cell surface receptors or extracellular receptors. For example: G protein coupled receptors (GPCR), receptor tyrosine kinase (RTK) etc. Some examples of their ligands are: neurotransmitters like GABA, acetylcholine etc, others are hormones like insulin, glucagon etc.

There are 3 major events of cell signalling which are mentioned as under: (1) reception;

(2) transduction; and

(3) response.  

Upon binding of ligand to it's respective receptor, a series of molecular interaction starts within the cell which leads to changes within a cell based on the type of hormone ultimately producing response as per the hormone. During such interaction, various proteins/enzymes may be involved which interact mainly through phosphorylation mechanism. Some secondary messengers like IP3, DAG, cAMP, cGMP, Ca² ⁺ relay signals received by these receptors so as to target molecules in the cytosol and/or nucleus which ultimately leads to generation of response.

Also once a response is generated, in order to terminate cell signaling, phosphatases remove phosphoryl groups from the protein molecules/enzymes. That is how cell signalling is regulated.

Example of cell signalling: Insulin signaling which involves RTK receptors.

step 1 ammonium can be absorbed by plants, but some is converted into nitrates which are better for plants to absorb. this is called nitrification. 2. step 2 the deer eats the plants and uses the nitrogen-containing proteins for cell growth. 3. step 3 bacteria in the soil conduct nitrogen fixation to convert nitrogen gas from the atmosphere into ammonium. 4. step 4 some of the nitrates are absorbed back into plants, but denitrification breaks down the nitrates into nitrogen gas released back into the atmosphere. 5. step 5 nitrogen is found in the urine, which gets broken down into ammonium through the process of ammonification. 6. step 6 plants absorb the ammonium and begin assimilation, the process to add the nitrogen to protein.