Essential Cell Biology 6th. Edt: CH. 16 & 17 Essential concepts

How do cells in multicellular organisms communicate?

Through extracellular chemical signals; hormones travel in blood to distant cells, while most other signals act over short distances or through direct cell–cell contact.

What must an extracellular signal molecule do to influence a target cell?

It must interact with a receptor protein on or in the target cell that specifically recognizes it.

What do most cell-surface receptors do with extracellular signals?

They transduce (convert) the extracellular signal into intracellular signals, usually organized into signaling pathways.

What are the three main classes of cell-surface receptors?

• (1) Ion-channel-coupled receptors,

• (2) G-protein-coupled receptors (GPCRs),

• (3) enzyme-coupled receptors.

What do GPCRs and enzyme-coupled receptors do when activated?

They activate intracellular signaling pathways, which then activate effector proteins that alter cell behavior.

Why is turning off signaling pathways important?

Each activated component must be inactivated or removed for the pathway to be able to function again.

How do G proteins function as molecular switches?

They transmit signals onward briefly, then switch themselves off by hydrolyzing their bound GTP to GDP.

What can G proteins directly regulate or activate in the plasma membrane?

They can directly regulate ion channels, activate/inactivate adenylyl cyclase (affecting cyclic AMP levels), or activate phospholipase C (generating IP3 and diacylglycerol).

What does IP3 do, and how does Ca²⁺ act as a second messenger?

IP3 opens Ca²⁺ channels in the ER membrane, releasing Ca²⁺ into the cytosol; Ca²⁺ then acts as a second messenger by altering the activity of Ca²⁺-responsive proteins like calmodulin and CaM-kinases.

What activates PKA, and what activates PKC?

A rise in cyclic AMP activates PKA; Ca²⁺ and diacylglycerol together activate PKC.

How do PKA, PKC, and CaM-kinases alter protein activity?

They phosphorylate selected signaling and effector proteins on serines and threonines, altering their activity; different cell types are affected differently based on their protein composition.

What are receptor tyrosine kinases (RTKs) and what do they do?

RTKs are enzyme-coupled receptors that phosphorylate themselves and intracellular signaling proteins on tyrosines; the phosphotyrosines then serve as docking sites for intracellular signaling proteins.

How do most RTKs relay signals to the nucleus via Ras?

Most RTKs activate the monomeric GTPase Ras, which activates a three-protein MAP-kinase module that relays the signal from the plasma membrane to the nucleus.

How do Ras mutations contribute to cancer?

Ras mutations keep Ras and the Ras–MAP-kinase pathway constantly active, continuously stimulating cell proliferation; this is a common feature of many human cancers.

How do some RTKs promote cell growth and survival via PI 3-kinase?

They activate PI 3-kinase, which phosphorylates inositol phospholipids in the plasma membrane, creating docking sites that recruit signaling proteins like the protein kinase Akt.

How does the Notch receptor send signals directly to the nucleus?

When activated, part of the Notch receptor migrates from the plasma membrane directly to the nucleus, where it regulates transcription of specific genes.

How do steroid hormones and nitric oxide signal differently from most molecules?

They are small or hydrophobic enough to cross the plasma membrane and directly activate intracellular proteins, usually transcription regulators or enzymes.

How do plants use cell-surface receptors for growth and development?

They use enzyme-coupled cell-surface receptors similar to those in animals, which often act by relieving transcriptional repression of specific genes.

What happens to most animal cells in the absence of extracellular signals?

Most animal cells are programmed to kill themselves through apoptosis in the absence of extracellular survival signals.

What is the current limitation in understanding how cells integrate multiple signals?

It is still not understood how a cell integrates all of the many extracellular signals it receives simultaneously to generate an appropriate response.

What three components make up the eukaryotic cytoskeleton?

Intermediate filaments, microtubules, and actin filaments.

What are intermediate filaments, and what is their primary function?

Intermediate filaments are stable, ropelike polymers built from fibrous protein subunits; they give cells mechanical strength.

What role does the nuclear lamina play, and where else are intermediate filaments found?

The nuclear lamina supports and strengthens the nuclear envelope; other intermediate filaments are distributed throughout the cytoplasm.

How do intermediate filaments interact with actin filaments and microtubules?

They are cross-linked to actin filaments and microtubules by linker proteins, forming a filamentous network that absorbs and distributes mechanical forces.

What is the basic structure of a microtubule, and what is structural polarity?

Microtubules are stiff, hollow tubes formed by globular tubulin dimers; structural polarity means they have a slow-growing minus end and a fast-growing plus end.

Where do microtubules grow from, and what role do γ-tubulin complexes play?

Microtubules grow from organizing centers; γ-tubulin complexes within these centers serve as nucleation sites that cap the minus ends of microtubules.

What is dynamic instability in microtubules?

Dynamic instability is the behavior where microtubules rapidly alternate between phases of growth and shrinkage.

How does GTP hydrolysis promote microtubule disassembly?

GTP hydrolysis reduces the affinity of tubulin dimers for their neighbors, destabilizing the microtubule and promoting disassembly.

How do microtubule-binding proteins contribute to cell function?

They modulate microtubule dynamics and organization, enabling formation of the mitotic spindle during cell division and contributing to polarity in neurons and epithelial cells.

What are kinesins and dyneins, and how do they function?

Kinesins and dyneins are motor proteins that use ATP hydrolysis to move unidirectionally along microtubules, carrying organelles, vesicles, and other cargo to specific locations.

What is the structure of eukaryotic cilia and flagella, and what causes their beating?

Cilia and flagella contain a bundle of stable microtubules; their rhythmic beating is caused by bending of the microtubules driven by a dynein motor protein.

What is the basic structure of an actin filament?

Actin filaments are helical polymers of globular actin monomers; they are more flexible than microtubules and are generally found in bundles or networks.

What is the polarity of actin filaments, and what controls their assembly/disassembly?

Actin filaments have a fast-growing plus end and a slow-growing minus end; assembly and disassembly are controlled by ATP hydrolysis and various actin-binding proteins.

What determines the varied arrangements and functions of actin filaments?

The diversity of actin-binding proteins, which can control polymerization, cross-link filaments, attach them to membranes, or move filaments relative to each other.

What is the cell cortex, and what does it do?

The cell cortex is a concentrated network of actin filaments beneath the plasma membrane; it controls cell shape and surface movement, including crawling.

How do actin and microtubules cooperate to establish cell polarity?

Actin at the cell surface and microtubules in the interior cooperate to establish and maintain cell polarity, enabling directed migration, asymmetric divisions, and embryonic development.

What are myosins, and how do myosin I and myosin II differ in nonmuscle cells?

Myosins use ATP hydrolysis to move along actin filaments; myosin I carries organelles or vesicles along actin tracks, while myosin II causes adjacent actin filaments to slide past each other in contractile bundles.

How is skeletal muscle structured at the filament level?

Skeletal muscle cells contain repeating arrays of overlapping actin and myosin II filaments organized into highly ordered myofibrils.

What initiates muscle contraction, and how is the signal delivered to myofibrils?

A sudden rise in cytosolic Ca²⁺ initiates contraction; Ca²⁺ binds to Ca²⁺-binding proteins associated with actin filaments, which deliver the signal to the myofibrils.