AP Biology Unit 4 Study Guide

Cell communication

The process by which cells send, receive, and respond to signals

Direct cell-to-cell communication

Cell signaling through physical contact, such as plasmodesmata or gap junctions

Short-distance signaling

Communication using local regulators like paracrine signals or growth factors

Long-distance signaling

Communication through hormones traveling via the circulatory system

Ligand

A signaling molecule that binds to a receptor

Receptor

A protein that binds a specific ligand and initiates signaling

Reception

The binding of a ligand to its receptor

Signal transduction

A series of steps that convert a signal into a cellular response

Response

The specific action taken by the cell as a result of signaling

Second messenger

Small molecules that relay and amplify a signal inside the cell

Phosphorylation cascade

A series of protein kinases activating each other by phosphorylation

Steroid hormone receptor location

Inside the cell because steroid hormones are hydrophobic

Protein hormone receptor location

On the cell membrane because protein hormones are hydrophilic

Signal amplification

The increase in signal strength through cascades and second messengers

Environmental signal

A signal triggered by external conditions such as light or chemicals

Gene expression response

A cellular response that changes transcription or translation

Cytoplasmic response

A response involving enzyme activity or ion channel opening

Signal pathway disruption

A change in signaling due to altered ligand or receptor structure

Negative feedback

A mechanism that returns a system to homeostasis

Negative feedback example

Regulation of body temperature or blood glucose levels

Positive feedback

A mechanism that amplifies a response

Positive feedback example

Blood clotting or skin repair after injury

Cell cycle

The ordered sequence of events leading to cell division

Interphase

The phase where the cell grows and replicates DNA

G1 phase

Cell growth and preparation for DNA replication

S phase

DNA replication occurs

G2 phase

Final preparation for mitosis

Mitosis

Nuclear division producing two identical nuclei

Cytokinesis

Division of the cytoplasm into two daughter cells

Purpose of mitosis

Growth, repair, and asexual reproduction

Cytokinesis in animal cells

Occurs via a cleavage furrow

Cytokinesis in plant cells

Occurs through formation of a cell plate

G0 phase

A resting phase where cells exit the cell cycle

Cells that enter G0

Neurons and muscle cells

Cell cycle checkpoint

A control point that ensures proper conditions before progression

G1 checkpoint

Checks for cell size, nutrients, and DNA integrity

G2 checkpoint

Ensures DNA replication is complete

M checkpoint

Ensures spindle fibers attach correctly to chromosomes

Cancer and the cell cycle

Loss of checkpoint control leading to uncontrolled division

Apoptosis

Programmed cell death

Purpose of apoptosis

Prevents damaged or oversized cells from harming the organism

B cells

Produce antibodies in adaptive immunity

T cells

Recognize antigen fragments and coordinate immune response

Primary immune response

The initial response to an antigen

Secondary immune response

Faster and stronger response due to memory cells

Endocrine signaling

Hormones released into the bloodstream to reach distant cells

Example of endocrine signaling

Insulin regulating blood glucose

Insulin’s role in signaling

Acts as a ligand

Tetrodotoxin effect

Blocks sodium channels, preventing action potentials

Ion channel–linked receptor

A receptor that opens or closes an ion channel when a ligand binds

G protein–coupled receptor (GPCR)

A receptor that activates a G protein, which triggers a signaling pathway

Intracellular receptor

A receptor located in the cytoplasm or nucleus that binds small hydrophobic ligands

Example of ion channel receptor

Neurotransmitters opening Na⁺ channels in neurons

Example of GPCR

Epinephrine activating cAMP signaling

Example of intracellular receptor

Steroid hormones regulating gene transcription

cAMP

A second messenger that activates protein kinases

Calcium ions (Ca²⁺)

A second messenger involved in muscle contraction and secretion

IP₃

A second messenger that releases Ca²⁺ from the ER

Why second messengers are effective

They amplify signals and allow rapid cellular responses

Signal amplification example

One ligand activates many kinases through a cascade

Effect of blocking a receptor

Ligand cannot bind and no cellular response occurs

Effect of mutated receptor binding site

Ligand binding is reduced or eliminated

Effect of nonfunctional second messenger pathway

The signal cannot reach the target response

Effect of inability to degrade ligand

The signaling pathway remains active too long

Effect of inability to degrade receptor

The cell becomes overly sensitive to signaling

Role of protein kinases

They phosphorylate proteins to activate or deactivate them

Phosphorylation

The addition of a phosphate group to a protein

Dephosphorylation

The removal of a phosphate group to stop signaling

Role of phosphatases

They turn off signaling pathways

Specificity in cell signaling

Only cells with the correct receptor respond to a ligand

Immune cell signaling

B and T cells communicate using antigen recognition and signaling molecules

Antigen specificity

Each immune cell receptor binds one specific antigen

Memory cells

Long-lived immune cells that produce faster secondary responses

Reason secondary immune response is faster

Memory cells already recognize the antigen

Apoptosis in development

Removes unnecessary cells during embryonic development

Failure of apoptosis

Can lead to cancer or developmental abnormalities

Difference between apoptosis and necrosis

Apoptosis is controlled; necrosis is uncontrolled cell death

Cancer cell cycle disruption

Loss of checkpoint control and apoptosis

Effect of loss of G1 checkpoint

Damaged DNA is replicated

Effect of loss of M checkpoint

Chromosomes may not separate correctly

Cell fusion experiment result

G1 cell enters mitosis due to cytoplasmic signals

Role of cyclins

Regulate progression through the cell cycle

Cyclin-dependent kinases (CDKs)

Enzymes activated by cyclins to drive the cell cycle

High cyclin concentration

Triggers progression to next cell cycle phase

Low cyclin concentration

Prevents cell cycle progression

Environmental signal example

Light triggering photoreceptor signaling

Hormone signal example

Insulin binding to liver cell receptors

Signal transduction pathway summary

Ligand → receptor → transduction → response