Cell Signaling: Ligands, Receptors, and Pathways in Biology

What is a ligand?

A signal molecule that produces a response in a cell.

What are target cells?

Cells affected by a ligand.

What type of ligands do not bind to membrane proteins?

Nonpolar ligands.

Where are intracellular receptors located?

Inside a cell.

What is a signal transduction pathway?

A series of steps by which a signal from outside the cell is converted into a specific cellular response.

What are the three stages of cell signaling?

Reception, Transduction, and Response.

What happens when a ligand binds to a receptor?

It triggers the start of a signal transduction pathway.

What is the role of reversible binding in ligand-receptor interactions?

It allows cells to stop responding to a signal after the appropriate response has occurred.

What are G-Protein Linked Receptors?

Plasma membrane receptors that work with a mobile membrane protein called a G-protein.

What happens to G-proteins when GDP is replaced by GTP?

The G-protein becomes activated.

What is the function of ligand-gated ion channels?

They change shape and open when a ligand binds, allowing ions to diffuse in or out of the cell.

What initiates a signal transduction pathway?

The binding of a ligand to a receptor.

What is the significance of similar structures among G-proteins and G-protein linked receptors?

They suggest that these proteins evolved very early in life on Earth.

What is the role of kinases in signal transduction?

They are enzymes that add phosphate groups to proteins.

What is the role of phosphatases in signal transduction?

They are enzymes that remove phosphate groups from proteins.

What is an activated intracellular hormone receptor?

It can act as a transcription factor, turning on specific genes inside the cell.

What type of ligands can readily cross the membrane?

Small or hydrophobic ligands.

What is the effect of acetylcholine on skeletal muscle cells?

It binds to ligand-gated ion channels, allowing Na+ and Ca2+ ions to diffuse into the cell, causing muscle contraction.

Why do only some nearby cells respond to a ligand?

Only cells with receptors for the ligand will respond.

What is the importance of cell signaling in unicellular organisms?

It influences how the cell responds to its environment.

What is the importance of cell signaling in multicellular organisms?

It coordinates the activities within individual cells that support the function of the organism as a whole.

What type of receptors are found on the cell surface?

Membrane receptors.

What is the function of the ligand-binding domain of a receptor?

It recognizes a specific chemical messenger.

What is allosteric regulation?

It is when a protein's shape changes in one area as a result of a molecule binding elsewhere on the protein.

What are examples of hydrophobic messengers?

Steroid hormones such as testosterone and estrogen.

What is the likely identity of the small dots in the neuron membrane?

They represent Na+ ions.

What do multistep pathways amplify?

A signal, producing a large cellular response.

What are two advantages of multistep pathways?

More opportunities for coordination and regulation of cellular responses.

What is the result of one active protein kinase in a phosphorylation cascade?

100 final active proteins.

What are three characteristics of secondary messengers in cell signaling?

Small, non-protein, hydrophilic.

What happens when calcium ions are released into the cytosol?

They activate proteins.

What is the first step in the signal transduction pathway involving calcium ions?

Ligand binds to G-protein linked receptor.

What is a phosphorylation cascade?

A signaling pathway involving a series of protein kinases phosphorylating and activating other kinases.

What is the role of protein phosphatases?

They remove phosphates from proteins, turning off the signal transduction pathway.

What is the 'first messenger' in a signaling pathway?

The extracellular signal molecule (ligand) that binds to the receptor.

What is cyclic AMP (cAMP)?

One of the most widely used second messengers.

How does adenylyl cyclase function in signal transduction?

It converts ATP to cAMP in response to an extracellular signal.

What are two common responses initiated by activated proteins?

Muscle contraction and cell division.

How do liver and heart cells respond differently to epinephrine?

They have different signal transduction pathways despite having receptors for epinephrine.

What are three possible responses a cell could have to a ligand?

1) Opening of ion channels, 2) Changes in gene expression, 3) Activation or inhibition of enzymes.

What would happen if a cell was exposed to a toxin that inhibited adenylyl cyclase?

No cAMP would be produced, stopping the transduction pathway before a cellular response.

What is signal specificity?

The response of a cell to a signal depends on its specific collection of proteins.

How does epinephrine affect heart muscle cells?

It causes them to contract, leading to a faster heartbeat.

How does epinephrine affect liver cells?

It causes them to break down glycogen and release glucose into the blood.

What are the benefits of signal transduction pathways?

They allow cells to respond to signal molecules that cannot cross the plasma membrane.

What do signal amplifiers do in cellular responses?

They help amplify a signal for more efficient cellular responses and reduce energy waste in producing signal molecules.

What is the effect of correct signal transduction processes on an organism?

They are under strong selective pressure and increase an organism's fitness.

What is signal disruption caused by anthrax?

The poison produced by anthrax binds and inhibits adenylyl cyclase, blocking signal transduction.

What happens in Type 1 Diabetes regarding insulin?

The pancreas produces little or no insulin, preventing the activation of Glut-4 protein.

What is the issue in Type 2 Diabetes?

The signaling initiated by insulin receptors is defective and does not activate Glut-4.

What is juxtacrine signaling?

Signaling that requires direct contact between cells without the secretion of ligands.

Give an example of juxtacrine signaling.

Plant and animal cells use cell junctions (plasmodesmata and gap junctions) to connect cytoplasm directly.

What is autocrine signaling?

A signaling process where the chemical signal affects the same cells that release it.

Provide an example of autocrine signaling.

Cancer cells secrete growth factors that bind to their own membrane receptors, promoting faster division.

What is paracrine signaling?

A signaling process where chemical signals diffuse to and affect nearby cells.

What are morphogens?

Ligands produced in embryos that create concentration gradients for cell differentiation.

What type of signaling do neurons use to communicate?

Paracrine signaling, through neurotransmitters across the synapse.

What is quorum sensing in bacteria?

A process of cell-to-cell communication that allows bacteria to share information about cell density.

What initiates the transcription of genes in quorum sensing?

The binding of autoinducers to receptors on nearby bacteria.

What is the role of helper T cells in adaptive immunity?

They bind to antigens on antigen-presenting cells (APCs) to initiate an immune response.

What is the function of cytokines released by APCs?

They activate the bound helper T cell through paracrine signaling.

What is the result of activated helper T cells releasing their own cytokines?

They signal themselves to proliferate (multiply) through autocrine signaling.

What is endocrine signaling?

Long distance communication involving the secretion of hormones into the blood.

How do morphogens affect embryonic cells?

They create different responses based on the concentration of receptors activated.

What is the significance of the concentration of autoinducers in quorum sensing?

It determines when specific genes start being transcribed in bacteria.

What is the outcome of helper T cells activating other white blood cells?

They produce a targeted attack on the antigen.

What is the relationship between cell density and gene transcription in quorum sensing?

Gene transcription is dependent on cell density and the concentration of autoinducers.

What is the role of signaling molecules in adaptive immunity?

They help activate immune responses against specific antigens.

What happens when the density of bacteria cells is high enough in quorum sensing?

Specific genes start being transcribed simultaneously in all bacteria.

What is the primary function of neurotransmitters?

To travel across the synapse and bind to receptors on other neurons.

What type of signaling is involved when helper T cells bind to APCs?

Juxtacrine signaling.

What is the effect of morphogen concentration gradients on developing cells?

They provide spatial references for differentiation into various cell types.

What is long distance communication in cells?

Endocrine signaling involving secretion of hormones into the blood.

What do hormones refer to?

Chemical signals that travel in the blood.

What effect does insulin have on liver, adipose, and muscle cells?

Insulin causes those cells to transport glucose from the blood into their cells.

What type of molecules are most hormones?

Proteins, except for sex hormones like testosterone and estrogen, which are steroids.

What is the role of the endocrine system?

It is made up of glands that secrete hormones into the blood.

What is insulin and where is it secreted from?

Insulin is a hormone secreted by the pancreas.

What triggers the pancreas to secrete insulin?

High levels of blood glucose.

What do fat cells do with glucose?

They use it to synthesize fatty acids and glycerol to build triglycerides.

What do liver and muscle cells do with glucose?

They use it to synthesize glycogen, which stores glucose for later use.

What is human growth hormone (GH) and where is it produced?

GH is produced by the pituitary gland in the brain.

What role does GH play in the body?

It stimulates muscle and bone growth and signals fat cells to break down stored fat.

How does a thermostat act as a negative feedback mechanism?

It turns on the AC when the temperature goes above the setpoint and the heater when it goes below.

What happens when body temperature is too high?

Skin blood vessels dilate and sweat glands are activated.

What is homeostasis?

The ability of an organism to maintain a stable internal environment despite external changes.

What is dynamic homeostasis?

An interplay between outside factors that change the internal environment and internal control mechanisms.

What do negative feedback mechanisms do?

They maintain dynamic homeostasis by regulating physiological processes.

What is the response of the body to increased physical activity?

Increased respiration to remove CO2 from the blood.

What is the role of abscisic acid in plants during water scarcity?

It causes guard cells to shrink and close stomata, reducing transpiration.

What is the difference between negative and positive feedback mechanisms?

Negative feedback does the opposite of the stimulus; positive feedback amplifies the stimulus.

What happens during a blood vessel injury?

Cells release signals that attract platelets to help with blood clotting.

What is thermoregulation?

Maintaining a stable body temperature through negative feedback mechanisms.

What happens when body temperature is too low?

Skin blood vessels constrict and skeletal muscles shiver to produce heat.

What is transpiration in plants?

The movement of water from roots to leaves, leading to evaporation.

What do guard cells do?

They control the opening and closing of stomata on leaves.

What is positive feedback during childbirth?

The baby's pressure on the cervix causes the release of oxytocin, which stimulates contractions.

What is the role of ethylene in ripening fruit?

It accelerates the ripening of nearby unripened fruit.

What are the two main phases of the cell cycle?

Interphase and Mitotic Phase (M phase)

What occurs during interphase?

Cell growth and copying of chromosomes in preparation for cell division.