AP Bio Unit 4 - Cell Communication and Cell Cycle

Paracrine Signaling (eukaryotes)

A type of local signaling in which a cell secretes signaling molecules that travel only short distances to influence nearby target cells. The secreted molecules, called local regulators, diffuse through the extracellular fluid and bind to receptors on neighboring cells, triggering a response. A key example is the release of growth factors, which stimulate nearby cells to grow and divide. Paracrine signaling is distinct from synaptic signaling (which occurs across a narrow synapse between nerve cells) and endocrine signaling (in which hormones travel long distances through the circulatory system).

Synaptic Signaling (eukaryotes)

A specialized type of local signaling that occurs in the nervous system, in which an electrical signal moving along a nerve cell triggers the release of neurotransmitter molecules into the synapse — the narrow space between the nerve cell and its target cell (often another nerve cell or muscle cell). The neurotransmitters diffuse across the synapse and bind to receptors on the target cell, triggering a response. Many medications that treat depression, anxiety, and PTSD work by affecting this signaling process. Synaptic signaling differs from paracrine signaling in its high specificity — neurotransmitters are directed at a precise target cell across a very narrow gap rather than diffusing broadly to nearby cells.

Endocrine Signaling (eukaryotes)

A type of long-distance signaling in which specialized endocrine cells release hormones that travel via the circulatory system to reach target cells throughout the body. Unlike local regulators, hormones can travel great distances, but they only trigger a response in specific target cells that have the appropriate receptor proteins to recognize and bind them. Hormones vary widely in size and type — for example, the plant hormone ethylene is a tiny 6-atom hydrocarbon gas, while insulin is a large protein with thousands of atoms. Endocrine signaling is slower and more diffuse than synaptic signaling, but allows for coordinated responses across distant tissues and organs.

Hormones (eukaryotes)

Long-distance signaling molecules used by both animals and plants that are released by specialized cells and travel to distant target cells to trigger a response. In animals, hormones travel via the circulatory system, while in plants they may move through vascular vessels, diffuse through cells, or travel as a gas through the air. Despite traveling throughout the body and reaching most cells, hormones only trigger responses in specific target cells that have the appropriate receptor proteins to recognize them. Hormones vary widely in size and chemical nature — for example, ethylene (a plant hormone) is a tiny 6-atom hydrocarbon gas, while insulin (a mammalian hormone) is a large protein — reflecting the diversity of hormonal signaling across organisms.

Signal Reception

The first stage of cell signaling, in which a target cell detects a signaling molecule from outside the cell by means of a receptor protein. The signaling molecule, acting as a ligand, binds to a complementary site on the receptor — like a hand in a glove — causing the receptor to undergo a change in shape that activates it. Most receptors are plasma membrane proteins that bind water-soluble ligands too large to cross the membrane, while others are intracellular receptors that bind hydrophobic signaling molecules (such as steroid hormones) that can pass through the membrane. Reception ensures that only specific target cells respond to a given signal, even if that signal is present throughout the body.

Signal Transduction

The second stage of cell signaling, in which the signal detected by the receptor is converted into a form that can bring about a specific cellular response. This typically involves a signal transduction pathway — a sequence of molecular changes in a series of relay molecules (often proteins) that pass the signal from the receptor to the final response. The signal is commonly transmitted through protein shape changes, often brought about by phosphorylation (addition of phosphate groups). A key advantage of multi-step pathways is signal amplification — each molecule in the pathway can activate numerous molecules at the next step, resulting in a geometric increase in activated molecules by the end of the pathway, as well as providing more opportunities for coordination and control.

Cellular Response (Nuclear and Cytoplasmic Responses)

The third and final stage of cell signaling, in which the transduced signal triggers a specific activity within the cell. The response can occur in the nucleus — where the final activated molecule may act as a transcription factor, turning specific genes on or off to regulate protein synthesis — or in the cytoplasm, where it may affect activities such as opening or closing ion channels, changing the activity of metabolic enzymes, or rearranging the cytoskeleton. The cell-signaling process ensures that these responses occur in the right cells, at the right time, and in proper coordination with other cells of the organism.

Nuclear responses — where the final activated molecule acts as a transcription factor, turning specific genes on or off to produce new proteins. This is a slower but longer-lasting response since it involves gene expression and protein synthesis.

Cytoplasmic responses — where the signal directly affects existing proteins in the cytoplasm without involving gene expression at all. For example:

Ligand

A molecule that specifically binds to another, often larger molecule, such as a receptor protein. In cell signaling, the ligand is typically the signaling molecule (such as a hormone or neurotransmitter) that binds to a complementary site on a receptor protein, triggering a shape change that activates the receptor and initiates a cellular response. Ligand binding is highly specific — each ligand is complementary in shape to a particular receptor site, like a hand in a glove — ensuring that only the correct receptor is activated by a given signaling molecule.

G Protein-Coupled Receptor (GPCR)

A cell-surface transmembrane receptor that works with the help of a G protein (a protein that binds GTP) to transmit signals from outside the cell to the inside. When a ligand binds to the extracellular side of the GPCR, the receptor changes shape and activates a G protein on the cytoplasmic side, which then diffuses along the membrane and binds to an enzyme, altering its activity and triggering the next step in the signaling pathway. GPCRs are the largest family of cell-surface receptors in mammals, with over 1,500 identified, and are involved in diverse functions including embryonic development, sight, smell, and taste. Many important signaling molecules use GPCRs, including epinephrine, other hormones, and neurotransmitters, and up to 60% of all medicines exert their effects by influencing G protein pathways.

G Protein

A protein that binds the energy-rich molecule GTP (similar to ATP) and acts as an intermediary in signal transduction between a GPCR and its target enzyme. When a ligand activates a GPCR, the receptor binds to and activates the G protein, which then diffuses along the membrane and binds to an enzyme, altering the enzyme's shape and activity to trigger the next step in the signaling pathway. The G protein's activity is self-limiting — it eventually hydrolyzes GTP, returning to its inactive form and ensuring the signal is only temporary. G proteins are remarkably similar in structure across many species, suggesting they evolved very early in the history of life.

Ligand-Gated Ion Channel

A type of membrane receptor protein that contains a region acting as a "gate" for ions, which opens or closes depending on whether a specific ligand is bound to the receptor. When a signaling molecule binds to the extracellular side of the receptor, the channel opens, allowing specific ions (such as Na⁺ or Ca²⁺) to rapidly diffuse through, directly changing the ion concentration inside the cell and affecting its activity. When the ligand dissociates, the channel closes and ion flow stops. Ligand-gated ion channels are especially important in the nervous system, where neurotransmitters bind to them at synapses, triggering electrical signals that propagate down the length of the receiving cell.

Intracellular Receptors

Receptor proteins located inside the cell, either in the cytoplasm or nucleus, that bind signaling molecules which are hydrophobic enough to pass directly through the plasma membrane. Examples include receptors for steroid hormones and thyroid hormones in animals, as well as nitric oxide (NO) in both animals and plants. Once a signaling molecule binds to its intracellular receptor, the activated hormone-receptor complex typically enters the nucleus and acts as a specific transcription factor, binding to specific genes and directly regulating their transcription into mRNA. Because the receptor itself carries out transduction, intracellular receptor pathways are simpler than those involving plasma membrane receptors.

Protein Kinase

An enzyme that transfers phosphate groups from ATP to a protein, a process called phosphorylation, which typically causes a shape change in the target protein that switches it from inactive to active. Protein kinases are central to signal transduction pathways — many relay molecules in these pathways are protein kinases that act on other protein kinases, creating a phosphorylation cascade that amplifies and transmits the signal. About 2% of human genes code for protein kinases, and a single cell may have hundreds of different kinds, each specific to a different protein. Abnormal protein kinase activity can cause uncontrolled cell division and contribute to the development of cancer.

Phosphorylation Cascade

A series of sequential protein phosphorylations in a signal transduction pathway, in which each protein kinase phosphorylates and activates the next protein kinase in the series, ultimately leading to a cellular response. Each step involves the transfer of a phosphate group from ATP, causing a shape change that activates the next molecule in the chain. A key advantage of this cascade is signal amplification — each activated kinase can phosphorylate many molecules at the next step, resulting in a geometric increase in activated molecules by the end of the pathway. The cascade is reversible — protein phosphatases can remove phosphate groups, returning proteins to their inactive form and turning off the pathway when the original signal is no longer present.

Protein Phosphatases

Enzymes that remove phosphate groups from proteins, a process called dephosphorylation, returning them to their inactive form. They serve as the "off switch" in signal transduction pathways, rapidly inactivating protein kinases when the original signal is no longer present. By dephosphorylating proteins, phosphatases also make protein kinases available for reuse, allowing the cell to respond again to future signals. Together with protein kinases, phosphatases form a molecular switch system — the activity of any regulated protein at a given moment depends on the balance between active kinases and active phosphatases in the cell.

Second Messengers

Small, nonprotein, water-soluble molecules or ions that relay signals from the cell's plasma membrane receptors to the interior of the cell as part of a signal transduction pathway. The term "second messenger" distinguishes them from the "first messenger" — the extracellular signaling molecule that binds to the membrane receptor. Because they are small, second messengers can rapidly diffuse throughout the cell, quickly spreading the signal. The two most common second messengers are cyclic AMP (cAMP) and calcium ions (Ca²⁺). For example, when epinephrine binds to a GPCR, it triggers the production of cAMP, which can be boosted 20-fold within seconds, broadcasting the signal throughout the cytoplasm and activating protein kinase A.

3 Parts to the Signal Transduction Pathway

Reception: A signaling molecule (ligand) binds to a receptor protein, usually on the cell surface

Transduction: The signal is converted and relayed through a series of molecules inside the cell (a "cascade")

Response: The final output — could be gene expression, enzyme activation, cell movement, etc.

Quorum Sensing

A process by which bacteria detect their population density using secreted chemical signals called autoinducers. Once the signals reach a threshold concentration, bacteria collectively change gene expression to coordinate behaviors like biofilm formation or virulence.

Cell Division

The process by which a parent cell divides into two or more daughter cells. It is the basis of growth, reproduction, and tissue repair. In eukaryotes, this occurs through mitosis (producing genetically identical somatic cells) or meiosis (producing genetically diverse gametes).

G0 Phase

A resting state where cells exit the cell cycle and are not actively preparing to divide. Cells may enter G0 temporarily (e.g., waiting for a growth signal) or permanently (e.g., fully differentiated cells like neurons).

Cell Cycle

The ordered series of events a cell goes through to grow and divide. It consists of interphase (G1, S, and G2 phases — where the cell grows and replicates its DNA) followed by mitotic phase (M phase — where the cell divides).

Interphase

The longest stage of the cell cycle, during which the cell grows and prepares for division. It consists of three phases: G1 (cell growth), S (DNA replication), and G2 (further growth and division prep).

Mitosis

The process of nuclear division in eukaryotic cells that produces two genetically identical daughter cells. It consists of four stages: prophase, metaphase, anaphase, and telophase, followed by cytokinesis. Used for growth, repair, and asexual reproduction.

G1 Phase

The first gap phase of interphase where the cell grows, produces proteins, and carries out normal functions. It is also where the G1 checkpoint occurs, determining whether conditions are favorable for the cell to proceed with DNA replication.

S Phase

The "synthesis" phase of interphase where the cell replicates its entire DNA, resulting in duplicated chromosomes (each consisting of two identical sister chromatids). Centrosomes are also duplicated during this phase.

G2 Phase

The second gap phase of interphase where the cell continues to grow and produces proteins needed for division. The G2 checkpoint verifies that DNA replication was completed accurately before the cell enters mitosis.

Sister Chromatids

Two identical copies of a chromosome joined together at the centromere after DNA replication. They are separated during cell division to ensure each daughter cell receives a complete set of genetic information.

Centromere

The region of a chromosome where sister chromatids are held together and where the spindle fibers attach (via the kinetochore) during cell division to pull chromatids apart.

Cyclins

Regulatory proteins whose concentrations rise and fall throughout the cell cycle. They bind to and activate cyclin-dependent kinases (CDKs), which trigger progression through the different stages of the cell cycle.

Cyclin-Dependent Kinases (CDKs)

Enzymes that drive the cell cycle forward by phosphorylating target proteins. They are only active when bound to their corresponding cyclin, making their activity dependent on cyclin concentration.

Cell Cycle Checkpoints

Quality control points in the cell cycle where the cell verifies that conditions are met before proceeding. The three main checkpoints are the G1, G2, and spindle (M) checkpoints, which check for DNA damage, replication errors, and proper spindle attachment, respectively.

Apoptosis

Programmed cell death in which a cell deliberately destroys itself in a controlled way. It is essential for development, immune function, and eliminating damaged or potentially cancerous cells, preventing them from dividing uncontrollably.

Cancer

A disease caused by uncontrolled cell division resulting from mutations in genes that regulate the cell cycle, such as proto-oncogenes and tumor suppressor genes. Cancerous cells ignore checkpoint signals, divide indefinitely, and can invade other tissues (metastasis).

Oncogene

A mutated or overexpressed gene that promotes excessive cell division. Oncogenes are derived from normal proto-oncogenes that have been altered by mutation, essentially acting as a stuck "on" switch for cell growth.

Tumor Suppressor Gene

A gene that normally slows or stops cell division and promotes apoptosis. When mutated and inactivated, these genes lose their braking function, allowing cells to divide uncontrollably. A classic example is p53.

Stages of Mitosis

Prophase — Chromatin condenses into visible chromosomes, the spindle forms, and the nuclear envelope breaks down.

Metaphase — Chromosomes align along the metaphase plate (cell's midline), attached to spindle fibers at their kinetochores.

Anaphase — Sister chromatids are pulled apart to opposite poles of the cell by shortening spindle fibers.

Telophase — Nuclear envelopes reform around each set of chromosomes, and chromosomes begin to decondense. Typically followed by cytokinesis.

Chromatin

The complex of DNA and histone proteins that makes up chromosomes. It exists in a loosely coiled form during interphase to allow gene expression, and condenses tightly during cell division.

Spindle Fibers

Protein structures made of microtubules that form during cell division. They attach to chromosomes at the kinetochore and are responsible for moving and separating chromosomes to opposite poles of the cell.

Kinetochore

A protein complex that assembles on the centromere of each chromosome, serving as the attachment site for spindle fibers during cell division to facilitate chromosome movement.

Metaphase Plate

The imaginary plane at the center of the cell where chromosomes align during metaphase, ensuring each daughter cell receives an equal set of chromosomes when they are pulled apart.

Cytokinesis

The division of the cytoplasm following mitosis, producing two separate daughter cells. In animal cells, this occurs via a cleavage furrow; in plant cells, a cell plate forms down the middle. Not part of mitosis but still housed as part of the mitotic phase (M-phase)

M Phase

The mitotic phase of the cell cycle where the cell undergoes mitosis (nuclear division) followed by cytokinesis (cytoplasmic division), producing two genetically identical daughter cells.

Cleavage Furrow

A pinching inward of the cell membrane in animal cells during cytokinesis, formed by a ring of actin filaments that contracts until the cell is split into two daughter cells.

Cell Plate

A structure that forms down the middle of dividing plant cells during cytokinesis, built from vesicles produced by the Golgi apparatus. It eventually develops into a new cell wall separating the two daughter cells.

Taxis

A directed movement of an organism toward or away from a stimulus, where the direction of movement depends on the direction of the stimulus — the organism actively moves closer (positive) or further away (negative) from the source.

Common Examples:

Type	Stimulus	Example
Phototaxis	Light	Moths flying toward light
Chemotaxis	Chemicals	Bacteria moving toward glucose
Thermotaxis	Temperature	Organisms moving toward warmth
Geotaxis	Gravity	Roots growing downward

Catalytic (Enzyme-Linked) Receptor

A transmembrane receptor that acts as both a receptor and an enzyme — when a signaling molecule binds to its extracellular side, it directly activates enzymatic activity on its intracellular side, triggering a cellular response without the need for a separate G-protein. G proteins have to go through a middleman (G protein + secondary messengers), but catalytic/enzyme receptors do the job themselves without needed a middleman.

Negative Feedback

A regulatory mechanism where the output of a system inhibits or reduces the original stimulus, bringing the system back toward a set point and maintaining homeostasis.

Classic Examples:

Example	Stimulus	Response	Feedback
Thermoregulation	Body temp too high	Sweating	Cooling brings temp back down
Blood glucose	High glucose	Insulin released	Glucose drops back to normal
Thyroid hormone	Low thyroid hormone	TSH released	Hormone levels rise → TSH stops

Positive Feedback

A regulatory mechanism where the output of a system amplifies or enhances the original stimulus, driving the process further in the same direction until a completion point is reached — the opposite of negative feedback.

Example	Stimulus	Response	Feedback
Childbirth	Baby pushes on cervix	Oxytocin released	More contractions → more pressure → more oxytocin
Blood clotting	Vessel damage	Platelets aggregate	More platelets recruit even more platelets
Fruit ripening	Ethylene gas released	Fruit ripens	Ripening releases more ethylene → ripens surrounding fruit
Action potential	Na⁺ enters cell	Membrane depolarizes	More Na⁺ channels open → more depolarization