How cells communicate

How cells communicate

• Cells live in environments with numerous signals.
• Cels take up materials from the environments across plasma membranes.

Transport across membranes

• Two factors determine whether materials can cross the plasma membrane.
  • Size: Small molecules can cross the plasma membrane faster than larger molecules.
  • Attraction to water: hydrophobic molecules can easily cross the plasma membrane compared to hydrophilic molecules.

Crossing the border

• Molecules that cross the plasma membrane diffuse across the plasma membrane until they reach equilibrium.
• Diffusion can occur with or without involving transport proteins.
• Diffusion is divided into two categories:
  • Simple diffusion: diffusion across membranes without help from transport protein.
  • Facilitated diffusion occurs when transport proteins facilitate diffusion of molecules across membranes. This can further be divided into active or passive transport
• Channel proteins are built like straws of soda.
  • Channel proteins inserted in membranes form water-filled channels through the membranes. Channel proteins that remain all the time are called gated channels.
  • Carrier proteins: they transport certain molecules across the membranes.
  • Each carrier protein has a certain binding site specific for the molecule to be transported.

Going with the flow

• Osmosis is the process by which water molecules move from a highly concentrated region to a region of low concentration.
• Osmosis is a passive process that does not require energy input from the cell.
• Membrane relative concentration is divided into 3 terms:
  • Hypertonic solutions have a greater solute concentration.
  • Hypotonic solutions; have a lower solute concentration.
  • Isotonic solutions; have an equal solute concentration.

It is an uphill battle

• Cells use active transport to move molecules against their concentration gradient.
• Active transport needs energy input from the cell which is available in the form of adenosine triphosphate (ATP).
• Carrier proteins that perform active transport are called pumps.
• One common active transport protein is the sodium-potassium pump.

Cell-cell interactions

• Animal cells are connected and to the extracellular matrix around them.
• The cells are connected depending on their functions.
• The types of cellular connections include:
  • Tight junctions which bring cells tightly together. They provide strength and structure to the tissues. Proteins pass through both cell membranes making them appear like they are sown together. Tight junctions are important in mucous membranes and skin tissues.
  • Anchoring junctions hold cells tightly together but allow movement of materials through intracellular space.
  • There are three anchoring junction proteins:
    • Desmosomes
    • Hemidesmosomes
    • Adherens junctions.

Sending and receiving signals

• Signals can be produced in one cell and initiate a response in another cell.
• Signals that travel to target cells over long distances are called hormones.
• To respond to signals, cells receive signals through receptors.
• There are two types of receptors:
  • Cell surface receptors
  • Intracellular receptors

Signal transduction

• Signal transduction is the process by which ligands bind to receptors on the cell surface and cause desired changes within the cell.
• Signal transduction steps include the following:
  • The ligand recognizes and binds to its receptor.
  • The receptor changes shape and becomes ready to cause intracellular changes.
  • Signal amplification is activated by second messengers when the receptor induces intracellular changes.
  • The second messengers enable a cell response leading changes in cellular behavior.

 Enzyme-linked receptors

• These are receptors on the cell surface that have an intracellular domain linked to an enzyme to catalyze intracellular changes.
• The commonly studied enzyme-linked receptor is a tyrosine kinase.
• Signal transduction steps for tyrosine kinase include the following:
  • The ligand binds to the tyrosine receptor.
  • The receptor undergoes phosphorylation and activates the intracellular enzyme.
  • The receptor then activates the membrane-associated protein called Ras.
  • Ras begins the phosphorylation cascade.

G proteins

• This is a signaling pathway that involves the activation of G proteins that are associated with GTP.
• G protein signal transduction steps include the following:
  • The receptor binds the primary messenger.
  • The G protein on the receptor is activated.
  • A section of the G protein travels across the membrane and associates with an enzyme.
  • The enzyme stimulates the generation of second messenger molecules.

Deactivating the signal

• Several molecules downregulate signaling pathways;
  • Phosphatases are enzymes that cleave phosphates from molecules. This hence inactivates the molecules.
  • Activated Ras and G proteins. They convert GTP to GDP hence causing inactivation.
  • Short-lived second messengers. These messengers are degraded faster hence deactivating the signaling pathway.

Concept/Term	Description/Answer
Adaptor Proteins	Proteins that act as accessories to main proteins in a signal transduction pathway.
Amplification	The process of increasing voltage, power, or current, especially in electronics.
Autophosphorylation	The phosphorylation of a kinase protein catalyzed by its own enzymatic activity.
cAMP	Cyclic adenosine monophosphate, a molecule that plays a major role in controlling enzyme-catalyzed processes in living cells.
Calcium	An essential ion that serves as a second messenger in signal transduction pathways, regulates neurotransmitter release, muscle contraction, and fertilization.
Calmodulin	A protein that binds calcium and is involved in regulating various cellular activities.
Cell Surface Receptors	Integral membrane proteins that facilitate communication between the cell and the external environment.
Dephosphorylation	The process of removing phosphate groups from an organic compound, often resulting in deactivation.
Diacylglycerol (DAG)	A glyceride molecule consisting of two fatty acid chains bonded to a glycerol molecule, often involved in signal transduction pathways.
Direct Contact Signaling	A form of signaling that occurs over short distances through gap junctions, allowing for rapid communication between cells.
Effector	An organ or muscle that becomes active in response to nerve impulses.
Endocrine Signaling	Signaling in which endocrine cells release hormones that act on distant target cells, typically involving longer distances and slower responses compared to other forms of signaling.
Enzymatic Receptor	A transmembrane receptor that, upon binding of an extracellular ligand, triggers an enzymatic reaction within the cell.
Enzymes	Complex proteins that act as catalysts in specific biochemical reactions.
G protein-coupled receptors	Transmembrane receptors that sense external molecules and activate intracellular signaling pathways.
Gap Junctions	Specialized intercellular connections between animal cells, allowing direct communication and transfer of small molecules and ions.
Gated Ion Channels	Protein channels in the cell membrane that can open or close, altering the membrane potential by regulating the flow of specific ions.
Gene Regulators	Molecules or proteins involved in controlling gene expression, determining which genes are activated or repressed and influencing the timing, duration, and extent of gene expression.
Intracellular Receptors	Receptors located inside the cell, typically in the cytoplasm or nucleus, that can bind to signaling molecules and initiate cellular responses.
IP3	Inositol trisphosphate, a second messenger that binds to and opens ligand-gated calcium channels in the endoplasmic reticulum.
Kinase	An enzyme that adds a phosphate group to another molecule, typically activating or deactivating the target molecule.
Ligand	An ion or molecule that binds specifically to a receptor site on another molecule.
Local Signaling	Signaling mechanisms that occur over short distances, such as through the release of growth factors or synaptic signaling in the nervous system.
Mitogen-Activated Protein Kinase (MAPK)	Protein kinases that respond to extracellular signals and regulate various cellular activities.
Multi-step pathway	A series of sequential steps or reactions involved in signal transduction, often leading to complex cellular responses.
Nucleotide exchange	Proteins that promote the exchange of nucleoside diphosphates for nucleoside triphosphates bound to other proteins.
Paracrine Signaling	Signaling between nearby cells, where the signal-releasing cell affects neighboring cells.
Phosphatase	An enzyme that removes phosphate groups from molecules, often resulting in deactivation.
Phosphorylation	The addition of a phosphate group to a molecule, often activating or deactivating it.
PIP2	Phosphatidylinositol 4,5-bisphosphate, a minor phospholipid component of cell membranes involved in various signaling pathways.
Ras	A protein network in the reticular formation that functions in alertness or arousal.
Receptors	Organs or molecules that detect and respond to specific stimuli. In cell biology, receptors are often proteins that bind to signaling molecules, initiating a cellular response.
Receptor Tyrosine Kinase (RTK)	High-affinity cell surface receptors that respond to polypeptide growth factors, cytokines, and hormones by initiating intracellular signaling cascades.
Response Protein	A cellular stress response related to the endoplasmic reticulum, typically involving the unfolded protein response and chaperone-mediated protein folding.
Secondary Messengers	Small molecules or ions that relay signals from the cell surface receptors to target molecules within the cell, amplifying and diversifying the signal.
Signal Molecule	A chemical substance involved in transmitting information between cells.
Signal Transduction	The process by which a mechanical or chemical stimulus is converted into a specific cellular response.
Synaptic Signaling	Cell-cell communication that occurs across chemical synapses in the nervous system, involving the release and reception of neurotransmitters.
Target Proteins	Biomolecules within cells that are regulated and influenced by biologically active compounds.
Adenylyl cyclase	An enzyme that converts adenosine triphosphate (ATP) to cyclic adenosine monophosphate (cAMP) in response to a chemical signal.
Apoptosis	A programmed cell death process triggered by the activation of specific enzymes that break down cellular components.
Cyclic AMP	Cyclic adenosine monophosphate, a common intracellular signaling molecule and second messenger.
Diacylglycerol	A second messenger produced by the cleavage of certain phospholipids in the plasma membrane.
G protein-coupled receptor (GPCR)	A type of signal receptor protein located in the plasma membrane that, upon binding to a signaling molecule, activates a G protein and initiates intracellular signaling.
G protein	A guanosine triphosphate (GTP)-binding protein that relays signals from a plasma membrane receptor to other components of signal transduction pathways within the cell.
Hormone	A chemical signal produced by endocrine cells and transported via the bloodstream to target cells, regulating various physiological processes.
Inositol triphosphate (IP3)	A second messenger molecule that acts as an intermediate between nonsteroidal hormones and an increase in cytoplasmic calcium concentration.
Ligand	A molecule that specifically binds to a receptor and triggers a biological response.
Ligand-gated ion channel	A protein pore in the plasma membrane that opens or closes in response to the binding of a specific ligand, thereby controlling the flow of specific ions across the membrane.
Local regulator	A chemical messenger that influences nearby cells, typically acting over short distances.
Protein kinase	An enzyme that adds phosphate groups from adenosine triphosphate (ATP) to proteins, often regulating their activity.
Protein phosphatase	An enzyme that removes phosphate groups from proteins, often reversing the effects of protein kinases and deactivating the proteins.
Reception	The process of a target cell detecting a signal molecule, usually through the binding of the molecule to a receptor protein on the cell surface.
Receptor tyrosine kinase	A receptor protein located in the plasma membrane that, upon binding to a signal molecule, catalyzes the transfer of phosphate groups from ATP to tyrosine residues on the receptor, initiating intracellular signaling pathways.
Response	The cellular changes or behaviors resulting from signal transduction and activation of specific pathways.
Scaffolding protein	A large relay protein that facilitates and enhances the efficiency of signal transduction by providing a platform for the assembly of multiple signaling molecules and enzymes.
Second messenger	A small, nonprotein, water-soluble molecule or ion that relays signals from cell surface receptors to target molecules within the cell, amplifying and diversifying the signal.
Signal transduction pathway	A series of molecular events and reactions that transmit and amplify a signal from the extracellular environment to the interior of the cell, leading to a specific cellular response.
Transduction	The conversion of an extracellular signal into an intracellular signal or the transmission of signals between cells, often involving a cascade of biochemical reactions.
Abiotic	Referring to nonliving factors in an environment, such as temperature, light, soil composition, etc.
Abscisic acid (ABA)	A plant hormone that inhibits growth, induces seed dormancy, and promotes tolerance to drought and other environmental stresses.
Action potential	An electrical signal that travels along the membrane of excitable cells, such as neurons, initiating the propagation of nerve impulses.
Action spectrum	A graph depicting the relative effectiveness of different wavelengths of radiation in driving a specific biological process.
Auxin	A plant hormone that influences cell elongation, root formation, secondary growth, and fruit development, among other processes.
Avirulent	Describing a pathogen that is not capable of causing severe harm or death to the host organism.
Biotic	Pertaining to living factors in an environment, including organisms and their interactions.
Blue-light photoreceptor	A type of light receptor found in plants that detects blue light and triggers various responses, such as phototropism and inhibition of hypocotyl elongation.
Brassinosteroid	A class of plant steroid hormones that promote cell elongation, inhibit leaf abscission, and regulate xylem differentiation.
Circadian rhythm	A biological rhythm with a period of approximately 24 hours, persisting even in the absence of external cues.
Cytokinin	A plant hormone involved in cell division, delaying aging, regulating differentiation pathways, and controlling apical dominance.
Day-neutral plant	A plant species that is not sensitive to changes in day length or photoperiod and does not require specific day lengths to flower

Concept/Term	Description/Answer
Gene-for-gene recognition	A widespread form of plant disease resistance involving recognition of pathogen-derived molecules by the protein products of specific plant disease resistance genes
Gibberellin	Any of a class of related plant hormones that stimulate growth in the stem and leaves, trigger the germination of seeds and breaking of bud dormancy, and stimulate fruit development
Gravitropism	A response of a plant or animal to gravity
Heat-shock protein	A protein that helps protect other proteins during heat stress; found in plants, animals, and microorganisms
Hypersensitive response	A plant’s localized defense response to a pathogen, involving the death of cells around the site of infection
Long-day plant	A plant that flowers (usually in late spring or early summer) only when the light period is longer than a critical length
Photomorphogenesis	Effects of light on plant morphology
Photoperiodism	A physiological response to photoperiod, the relative lengths of night and day; example is flowering
Phototropism	Growth of a plant shoot toward or away from light
Salicylic acid	A signaling molecule in plants that may be partially responsible for activating systemic acquiring resistance to pathogens
Senescence	The growth phase in a plant or plant part (as a leaf) from full maturity to death
Short-day plant	A plant that flowers (usually in late summer, fall, or winter) only when the light period is shorter than a critical length
Statolith	In plants, a specialized plastid that contains dense starch grains and may play a role in detecting gravity
Strigolactones	A class of plant hormone that inhibits shoot branching, triggers the germination of parasitic plant seeds, and stimulates the association of plant roots with mycorrhizal fungi
Systemic acquired resistance	A defensive response in infected plants that helps protect healthy tissue from pathogenic invasion
Thigmorphogenesis	A response in plants to chronic mechanical stimulation, resulting from increased ethylene production; example is thickening stems in response to strong winds
Thigmotropism	A directional growth of plant in response to touch
Triple response	A plant growth maneuver in response to mechanical stress, involving slowing of stem elongation, thickening of the stem, and a curvature that causes the stem to start growing horizontally
Tropism	A growth response that results in the curvature of whole plant organs toward or away from stimuli due to differential rates of cell elongation
Vernalization	The use of cold treatment to induce a plant to flower
Virulent	Describing a pathogen against which an organism has little specific defense
Adenophypophysis	Anterior pituitary
Adrenal gland	One of two endocrine glands located adjacent to the kidneys in mammals; endocrine cells in the outer portion (cortex) respond to ACTH by secreting steroid hormones that help maintain homeostasis during long-term stress; neurosecretory cells in the central portion (medulla) secrete epinephrine and norepinephrine in response to nervous inputs triggered by short-term stress
Adrenocorticotrophic hormone	A trophic hormone produced and secreted by the anterior pituitary that stimulates the production and secretion of steroid hormones by the adrenal cortex

Concept/Term	Description/Answer
Autocrine Signaling	Cell responds to its own signaling molecule
Paracrine Signaling	Signals act locally between cells that are close together
Endocrine Signaling	Signals from distant cells that affect other body regions
Direct Signaling	Signaling between neighboring cells through gap junctions or plasmodesmata
Internal Receptors	Found in the cytoplasm of target cells, respond to hydrophobic ligands
Cell-Surface Receptors	Integral proteins that bind to external signaling molecules and perform signal transduction
Enzyme-Linked Receptors	Cell-surface receptors associated with an enzyme
Ion Channel-Linked Receptors	Cell-surface receptors that open channels for specific ions
G-Protein-Linked Receptors	Cell-surface receptors that activate associated G-proteins
Ligands	Chemical signals produced by signaling cells that travel to target cells
Small Hydrophobic Ligands	Lipid-soluble ligands that can directly diffuse through the plasma membrane
Water-Soluble Ligands	Polar ligands that bind to cell-surface receptors
Signaling Pathways	Chains of events triggered by ligand-receptor binding
Phosphorylation	Addition of a phosphate group to a molecule
Second Messengers	Small molecules that propagate a signal after ligand-receptor binding
Gene Expression	Regulation of RNA transcription or protein translation
Increase in Cellular Metabolism	Activation of β-adrenergic receptors leads to an increase in cyclic AMP inside the cell
Cell Growth	Stimulation of cell division by growth factors binding to cell-surface receptors
Cell Death	Mechanism of programmed cell death, or apoptosis, initiated by damaged or potentially dangerous cells
Termination of Signaling Pathways	Degradation or removal of ligands to stop signal transmission
Steroid Hormones	Lipids with a hydrocarbon skeleton and functional groups, such as estradiol and testosterone
Cyclic AMP	Second messenger synthesized from ATP that activates cAMP-dependent kinase
Epidermal Growth Factor Receptors	Example of receptor tyrosine kinases involved in signaling