AP Biology Unit 4 – Cell Communication and Cell Cycle: Ultimate Study Notes

Cell Communication

Purpose of Cell Communication

Why cells communicate

  • coordinate activities

  • respond to environmental changes

  • regulate growth and development

  • maintain homeostasis

Important concept

  • Cells can only respond to signals if they have the correct receptor.

AP Exam Trick

Different cells can respond differently to the same signal because they contain different proteins and receptors.

Types of Cell Signaling

Autocrine Signaling

  • The cell releases a signal and responds to its own signal.

  • Example:

    • Immune cells releasing cytokines.

Mnemonic: Auto = self

Paracrine Signaling

  • Signals affect nearby cells.

Characteristics

  • short distance signaling

  • local effect

  • signal molecules quickly degraded

Examples

  • growth factors

  • neurotransmitters

Mnemonic: Para = parallel / nearby

Endocrine Signaling

  • Signals travel long distances through the bloodstream.

  • Signaling molecules: hormones

Examples

  • insulin

  • adrenaline

  • thyroid hormone

Characteristics

  • slower response

  • affects many cells throughout the body

Mnemonic: Endocrine = entire body

AP Exam Tip: Endocrine signals are usually long lasting but slower.

Ligands and Receptors

Ligands

  • A ligand is a molecule that binds to a receptor.

  • Ligand binding causes a conformational change in the receptor.

  • Examples:

    • hormones

    • neurotransmitters

    • growth factors

Mnemonic: Ligand = key

Receptors

  • Proteins that receive signaling molecules.

  • Function: When a ligand binds, the receptor changes shape and activates a signaling pathway

Two main receptor types

  • membrane receptors

  • intracellular receptors

Mnemonic: Receptor = lock

Stages of Cell Signaling

Reception

  • Reception occurs when a ligand binds to a receptor protein.

  • Reception is highly specific.

  • If a signal cannot cross the membrane, it must bind to a membrane receptor.

What happens

  • receptor changes shape

  • signaling pathway begins

Transduction

  • The signal is converted and relayed through the cell.

Key processes

  • phosphorylation cascades

  • second messenger systems

Purpose

  • Signal amplification.

  • One signal can activate thousands of molecules.

Response

  • The cell carries out a specific action.

Possible responses

  • gene expression changes

  • enzyme activation

  • secretion of molecules

  • cell division

Membrane Receptors

G Protein-Coupled Receptors (GPCR)

  • The most common type of receptor in eukaryotic cells.

Mechanism

  1. Ligand binds receptor

  2. Receptor activates G protein

  3. G protein activates enzyme

  4. Second messenger produced

Example second messenger: cAMP

Mnemonic: “LGGE” Ligand → G protein → enzyme → effect

AP Exam Tip: GPCR pathways often involve signal amplification.

Receptor Tyrosine Kinases (RTK)

  • Receptors that activate protein kinases.

Mechanism

  1. Ligand binds receptor

  2. Two receptors join together (dimerization)

  3. Phosphate groups added

  4. Signaling proteins activated

Importance

  • cell growth

  • cell division

AP Exam Trick: Mutations in RTKs are commonly linked to cancer.

Ligand-Gated Ion Channels

  • Receptors that open ion channels when ligands bind.

Mechanism

  1. Ligand binds receptor

  2. Channel opens

  3. Ions flow across membrane

Examples: sodium channels in neurons

Result: Rapid signal transmission.

Intracellular Receptors

Characteristics

  • Location: Found inside the cell.

  • Signal molecules: Signals must be able to cross the membrane.

  • Examples:

    • steroid hormones

    • thyroid hormones

Mechanism

  • Step 1: Hormone enters cell.

  • Step 2: Hormone binds receptor.

  • Step 3: Hormone-receptor complex enters nucleus.

  • Step 4: Gene transcription changes.

AP Exam Tip: Intracellular signaling often directly affects gene expression.

Signal Transduction Pathways

Phosphorylation Cascades

  • A series of proteins activated by phosphate addition.

  • Enzymes involved: Kinases

  • Kinases add phosphate groups from ATP.

  • Opposite enzyme

    • Phosphatases remove phosphate groups.

Second Messengers

  • Small molecules that relay signals inside the cell.

  • Examples:

    • cAMP

    • Ca²⁺ ions

    • IP₃

Function

  • amplify signal

  • activate proteins

Cellular Responses

Types of Responses

Gene expression

Genes turned on or off.

Enzyme activation

Metabolic pathways activated.

Cell division

Growth signals trigger mitosis.

Secretion

Cells release molecules.

Apoptosis

Programmed cell death.

  • Purpose

    • remove damaged cells

    • shape developing tissues

    • maintain tissue balance

    • Example

      • Removal of webbing between fingers during development.

AP Exam Tip: Apoptosis is controlled and beneficial, unlike accidental cell death.

Mnemonic: A = arranged death

Cell Cycle

Overview

Definition: The cell cycle is the process by which cells grow and divide.

Two main stages:

  • Interphase

  • Mitotic phase

Mnemonic: I Make Cells (Interphase → Mitosis → Cytokinesis)

Interphase

G1 Phase

Function: Cell growth.

Activities:

  • organelles produced

  • proteins synthesized

AP Exam Tip: Cells may leave the cycle into G0 phase.

S Phase

Function: DNA replication occurs.

Result: Each chromosome becomes two sister chromatids.

G2 Phase

Function: Preparation for mitosis.

Activities:

  • microtubules produced

  • DNA checked for errors

Mnemonic: Grow → Synthesize → Get ready

Mitosis

Overview

Purpose: Division of the nucleus.

Result:

  • Two genetically identical daughter cells.

Stages:

  • Prophase

  • Prometaphase

  • Metaphase

  • Anaphase

  • Telophase

Mnemonic: PMAT

Prophase

Events:

  • chromosomes condense

  • spindle fibers form

  • centrosomes move apart

Prometaphase

Events:

  • nuclear envelope breaks down

  • spindle fibers attach to kinetochores

Key structure:

  • Kinetochore = attachment site on chromosome.

Metaphase

Events:

  • Chromosomes align at the metaphase plate.

Important checkpoint:

  • Spindle checkpoint ensures correct attachment.

Mnemonic: M = middle

Anaphase

Events:

  • Sister chromatids separate.

  • Pulled toward opposite poles.

Mechanism:

  • Microtubules shorten.

Mnemonic: A = apart

Telophase

Events:

  • nuclei reform

  • chromosomes decondense

  • spindle disassembles

Mnemonic: Two nuclei form

Cytokinesis

Animal Cells

Process:

  • Cleavage furrow forms.

  • Membrane pinches inward.

Plant Cells

Process:

  • Cell plate forms.

  • New cell wall develops.

AP Exam Tip: Plant cells cannot pinch because of the rigid cell wall.

Cell Cycle Regulation

Cyclins

Definition: Proteins that regulate timing of the cell cycle.

Function: Cyclins activate CDKs.

Cyclin-Dependent Kinases (CDKs)

Definition: Enzymes that trigger cell cycle progression.

Activation: Cyclin binds CDK.

Mnemonic: Cyclin = timer, CDK = engine

Cell Cycle Checkpoints

G1 Checkpoint

Checks:

  • cell size

  • nutrients

  • DNA damage

G2 Checkpoint

Checks:

  • DNA replication completed correctly.

M Checkpoint

Checks:

  • Chromosomes properly attached to spindle.

AP Exam Tip: If damage is detected → cell cycle stops.

Cancer

Definition: Cancer is uncontrolled cell division.

Proto-Oncogenes

Definition: Genes that normally promote cell growth.

Mutation effect

Proto-oncogene → oncogene.

Result: excessive cell division.

Mnemonic: Proto promotes growth

Tumor Suppressor Genes

Definition: Genes that slow or stop the cell cycle.

Example:

  • p53 gene.

Mutation effect

  • Loss of regulation.

Mnemonic: Suppressor = stop

Density-Dependent Inhibition

Definition: Normal cells stop dividing when crowded.

Cancer cells ignore this signal.

Anchorage Dependence

Definition: Normal cells must attach to a surface to divide.

Cancer cells can divide without attachment.

AP Exam Tip: Often tested in cell culture experiments.

Big Picture Connections

Signal pathways and cell division

Signals can activate pathways that trigger cell division.

Example: Growth factor → receptor → kinase cascade → cell cycle activation.

Mutation effects

Mutations in signaling proteins or cell cycle regulators can cause:

• uncontrolled growth
• cancer

Quick Unit 4 Memory Tricks

Signal Pathway

RTR: Receive → Transmit → Respond

Mitosis

PMAT: Prophase → Metaphase → Anaphase → Telophase

Interphase

GSG: Grow → Synthesize → Get ready

Cyclin/CDK

Timer + Engine: Cyclin tells when, CDK makes it happen.