Chapter 3

Fertilization

Timeline and Location:

• Secondary oocyte ovulated ~day 14 of menstrual cycle

• Fertilization occurs in the ampulla (widest part of fallopian tube)

• Oocyte viable for fertilization up to 24 hours after ovulation

Fertilization Process

1. Sperm binds to secondary oocyte

2. Releases acrosomal enzymes to penetrate:

   • Corona radiata (outer layer of follicular cells)

   • Zona pellucida (glycoprotein layer surrounding oocyte)

3. First sperm to contact oocyte membrane forms acrosomal apparatus (tube-like structure)

4. Sperm pronucleus enters oocyte after completion of meiosis II

Cortical Reaction:

• Release of calcium ions (Ca²⁺) after sperm penetration

• Two purposes:

  1. Depolarizes membrane → prevents polyspermy (fertilization by multiple sperm)

  2. Increases metabolic rate of new diploid zygote

• Depolarized, impenetrable membrane = fertilization membrane

Key Concept: The zygote is now diploid (2n), containing genetic material from both parents.

Twins

Dizygotic (Fraternal) Twins:

• Two different eggs fertilized by two different sperm

• Each zygote implants separately

• Each has own placenta, chorion, and amnion

• Genetically similar as any siblings

• Placentas may fuse if implantation sites are close

Monozygotic (Identical) Twins:

• Single zygote splits into two

• Genetically identical

• Incomplete division → conjoined twins

Classification by Shared Structures:

• Monochorionic/Monoamniotic: both Amnion and Chorion shared

• Monochorionic/Diamniotic: Amnion seperate and chorion shared

• Dichorionic/Diamniotic: Both Amnion and Chorion seperate

Key Concept: Earlier splitting = more separate structures and lower risk.

Cleavage

Definition: Rapid mitotic cell divisions of the zygote without overall growth.

Key Features:

• First cleavage marks transition from zygote to embryo (no longer unicellular)

• Total size remains unchanged initially

• Increases two important ratios:

  • Nuclear-to-cytoplasmic (N:C) ratio ↑

  • Surface area-to-volume ratio ↑ → better gas/nutrient exchange

Two Types of Cleavage:

Type	Characteristics
Indeterminate	Cells can develop into complete organisms; enables monozygotic twinning
Determinate	Cell fate is already determined; cells committed to specific differentiation

Mnemonic Aid: Indeterminate = Indecisive (cells haven't decided what to become yet)

Blastulation

Process:

• After several divisions, solid mass of cells = morula (looks like a mulberry)

• Morula undergoes blastulation → blastula (hollow ball of cells)

• Fluid-filled cavity = blastocoel

Mammalian Blastula (Blastocyst):

Structure	Location	Fate
Trophoblast	Surrounds blastocoel	Forms chorion and placenta
Inner cell mass	Protrudes into blastocoel	Forms the organism

Clinical Correlation - Ectopic Pregnancy:

• Blastula implants outside uterus

95% occur in fallopian tube

• Not viable; surgical emergency if tube ruptures

Implantation

Process:

• Blastula burrows into endometrium (uterine lining)

• Trophoblast cells form interface with maternal blood supply

Extraembryonic Membranes:

Structure	Function
Chorion	Forms placenta; outer membrane around amnion
Amnion	Thin, tough membrane filled with amniotic fluid (shock absorber)
Yolk sac	Supports embryo until placenta functional; site of early blood cell development
Allantois	Early fluid exchange; contributes to umbilical cord formation

Placental Development:

• Trophoblast → chorionic villi (finger-like projections into endometrium)

• Villi develop into placenta (site of maternal-fetal gas exchange)

Umbilical Cord:

• Formed from remnants of yolk sac and allantois

• Two arteries (carry deoxygenated blood/waste away)

• One vein (carries oxygenated blood/nutrients toward fetus)

• Encased in gelatinous substance (Wharton's jelly)

Clinical Correlation - Amniocentesis:

• Aspiration of amniotic fluid for fetal cell analysis

• Screens for chromosomal abnormalities

• Recommended for pregnant individuals >35 (higher nondisjunction risk)

Gastrulation

Definition: Generation of three distinct germ layers from the blastula.

Process (using sea urchin model):

1. Small invagination forms in blastula

2. Cells move toward invagination → blastocoel eliminated

3. Creates tube through embryo = archenteron (primitive gut)

4. Opening of archenteron = blastopore

Fate of Blastopore:

Group	Blastopore Becomes
Deuterostomes (humans)	Anus
Protostomes	Mouth

Mnemonic: Deuterostome starts with "deu" like "duo" (two) → develops anus ("number two")

Result: Three-layered structure = gastrula

Primary Germ Layers

Ectoderm (Outermost):

• Integument: epidermis, hair, nails

• Epithelia of nose, mouth, lower anal canal

• Lens of eye

• Nervous system (brain, spinal cord)

• Adrenal medulla (contains nervous tissue)

• Inner ear

Mnemonic: "Attracto"-derm — things that attract us to others (cosmetic features, "smarts")

Mesoderm (Middle):

• Musculoskeletal system

• Circulatory system

• Most of excretory system

• Gonads

• Muscular/connective tissue layers of digestive and respiratory systems

• Adrenal cortex

Mnemonic: "Means"-oderm — means of getting around (bones, muscle, circulation, gonads)

Endoderm (Innermost):

• Epithelial linings of digestive and respiratory tracts (including lungs)

• Pancreas

• Thyroid

• Bladder and distal urinary tracts

• Parts of liver

Mnemonic: Linings of "endernal" (internal) organs

Germ Layer Derivatives 

Germ Layer	Major Derivatives
Ectoderm	Epidermis, hair, nails, nervous system, adrenal medulla, lens of eye, inner ear
Mesoderm	Muscles, bones, circulatory system, kidneys, gonads, adrenal cortex
Endoderm	Lining of GI and respiratory tracts, lungs, liver, pancreas, thyroid, bladder

High-Yield: Adrenal Gland Dual Origin

• Adrenal cortex → Mesoderm

• Adrenal medulla → Ectoderm (contains nervous tissue)

Differentiation and Induction

Selective Transcription:

• All cells have same genes; only certain genes transcribed in each cell type

• Explains how cells with identical DNA become specialized

Induction:

• One group of cells influences fate of nearby cells

• Mediated by inducers (chemical substances)

• Inducers diffuse from organizing cells to responsive cells

• Ensures proximity of different cell types in organs

Neurulation

Definition: Development of the nervous system from ectoderm.

Process:

1. Notochord forms from mesoderm along long axis (primitive spine)

   • Remnants persist in intervertebral discs

2. Notochord induces overlying ectoderm to form:

   • Neural folds (elevations)

   • Neural groove (depression between folds)

3. Neural folds fuse → neural tube

4. Neural tube becomes central nervous system (brain and spinal cord)

Neural Crest Cells

• Located at tips of neural folds

• Migrate outward to form:

  • Peripheral nervous system (sensory ganglia, autonomic ganglia, Schwann cells)

  • Adrenal medulla (note: ectoderm origin like nervous system)

  • Calcitonin-producing thyroid cells

  • Melanocytes (skin pigment cells)

• Ectodermal cells then migrate over neural tube to cover nervous system

Clinical Correlations

Condition	Description	Outcome
Spina bifida	Neural tube fails to close; spinal cord exposed	Variable: asymptomatic to severe disability
Anencephaly	Brain fails to develop; skull remains open	Universally fatal

Prevention: Folate (folic acid) supplementation before and during early pregnancy (neurulation occurs often before pregnancy detected)

Problems in Early Development

Teratogens:

• Substances interfering with development

• Effects depend on:

  • Genetics of embryo

  • Route of exposure

  • Length of exposure

  • Rate of placental transmission

  • Exact identity of teratogen

Examples:

• Alcohol

• Prescription drugs

• Viruses, bacteria

• Environmental chemicals (polycyclic aromatic hydrocarbons)

Maternal Health Factors:

• Diabetes/hyperglycemia: Fetus may grow too large; hypoglycemic after birth (excess insulin production)

• Folic acid deficiency: Neural tube defects (spina bifida, anencephaly)

Key Concept: Outcomes are variable and somewhat unpredictable.

Mechanisms of Development

Cell Specialization

Three Stages:

Stage	Characteristics
Specification	Reversibly designated as specific cell type
Determination	Irreversibly committed to specific lineage
Differentiation	Cell changes structure, function, biochemistry to match cell type

Determination Mechanisms:

• Asymmetric distribution of mRNA/proteins during cleavage

• Exposure to morphogens (signaling molecules from nearby cells)

Differentiation:

• Cell now produces specialized products needed for function

• Changes in structure, biochemistry, and function

Stem Cells and Potency

Definition: Undifferentiated cells that can give rise to differentiated cells.

Potency Spectrum:

Potency	Capabilities	Examples
Totipotent	Any cell type (embryo + placenta)	Early embryonic cells (up to morula)
Pluripotent	Any cell type except placental	Inner cell mass cells
Multipotent	Multiple types within a group	Hematopoietic stem cells (blood cells)

Key Concept: As differentiation progresses, potency decreases: Totipotent → Pluripotent → Multipotent

Stem Cell Research Applications:

• Potential regeneration of: spinal cord (after injury), heart (after heart attack)

• Challenges:

  • Ethical concerns (embryonic stem cells)

  • Immunologic rejection

  • Difficulty controlling differentiation

  • Risk of cancer

Adult Stem Cell Advantages:

• Less ethical controversy

• Can use patient's own cells → reduced rejection risk

• Limited to multipotent (fewer cell types possible)

Cell-Cell Communication

Four Types of Signaling:

Type	Target	Mechanism
Autocrine	Same cell that secreted signal	Cell acts on itself
Paracrine	Local nearby cells	Diffusion in local area
Juxtacrine	Adjacent cell	Direct cell-to-cell contact
Endocrine	Distant tissues	Hormones via bloodstream

Key Terminology:

• Inducer: Cell or molecule that signals differentiation

• Responder: Cell receiving the signal

• Competent: Able to respond to the inducing signal

Inducers and Morphogens

Growth Factors:

• Peptides promoting differentiation and mitosis

• Function on specific cell types (determined by competence)

Example - Eye Development:

1. PAX6 expressed in head ectoderm

2. Optic vesicle (from brain) contacts overlying ectoderm

3. Induces formation of lens placode (future lens)

4. Lens placode reciprocally induces optic vesicle → optic cup (future retina)

5. Optic cup induces lens placode → cornea and lens

Key Concept: Reciprocal development — induction often bidirectional

Morphogen Gradients:

• Morphogens diffuse from source

• Concentration gradient created (high near source, low far away)

• Multiple morphogens secreted simultaneously

• Unique combinations of exposure → specific cell types

Common Morphogens:

• TGF-β (Transforming Growth Factor beta)

• Shh (Sonic hedgehog)

• EGF (Epidermal Growth Factor)

Cell Migration

Process:

• Cells disconnect from adjacent structures

• Migrate to correct location

Examples:

• Anterior pituitary: Originates from oral ectoderm → migrates to below hypothalamus

• Neural crest cells: Migrate extensively throughout body forming:

  • Sensory and autonomic ganglia

  • Adrenal medulla

  • Schwann cells

  • Melanocytes

  • Calcitonin-producing thyroid cells

Cell Death

Apoptosis (Programmed Cell Death):

Feature	Description
Process	Cell shrinks, forms membrane-bound apoptotic blebs
Fate	Blebs broken into apoptotic bodies; digested by other cells
Contents	Contained by membranes (no leakage)
Outcome	Materials recycled; no inflammation

Examples in Development:

• Removal of webbing between fingers and toes

• Sculpting of anatomical structures

Necrosis (Cell Death from Injury); Apoptosis vs Necrosis

Feature	Apoptosis	Necrosis
Cause	Programmed/signals	Injury
Contents	Contained in blebs	Leaked into environment
Effect	No inflammation	Tissue irritation, immune response
Organization	Orderly	Chaotic

Regeneration

Definition: Ability to regrow lost or damaged body parts.

Type	Description	Example Species
Complete regeneration	Lost tissue replaced with identical tissue	Salamanders, newts
Incomplete regeneration	New tissue not identical in structure/function	Humans (typically)

Human Regenerative Capacity by Organ:

Organ	Capacity
Liver	High (can regenerate up to 50% loss; living donor transplants possible)
Kidney	Moderate (can repair nephron tubules; easily overwhelmed)
Heart	Little to none (scarring after injury like heart attack)

Mechanism in Highly Regenerative Species:

• Retain extensive clusters of stem cells throughout body

• Stem cells migrate to injury site for regrowth

Senescence and Aging

Cellular Senescence:

• Cells fail to divide after ~50 divisions (in vitro)

• Caused by telomere shortening

• Telomeres: Ends of chromosomes

  • High guanine-cytosine content

  • "Knot off" chromosome ends

  • Protect against DNA unraveling and information loss

  • Difficult to replicate → shorten with each division

Telomerase:

• Enzyme that synthesizes telomere ends

• Reverse transcriptase (makes DNA from RNA template)

• Expressed in:

  • Germ cells

  • Fetal cells

  • Cancer cells (allows indefinite division)

• Not expressed in most somatic cells → senescence

Organismal Senescence:

• Changes in body's ability to respond to environment

• Accumulation of chemical/environmental damage over time

Fetal Circulation

Placental Function

Key Principle: Maternal and fetal blood do not mix (different blood types possible)

Exchange Mechanisms:

• Diffusion (preferred method):

  • Water

  • Glucose

  • Amino acids

  • Inorganic salts

• Requires concentration gradient

  • Higher O₂ in maternal blood than fetal blood

Fetal Hemoglobin (HbF):

• Higher oxygen affinity than adult hemoglobin (HbA)

• Enhances oxygen transfer from mother to fetus

• Helps retain oxygen in fetal circulation

Additional Placental Functions:

1. Waste removal: CO₂ and waste move from fetus to mother

2. Immune protection: Maternal antibodies cross placenta (passive immunity)

3. Endocrine organ: Produces:

   • Progesterone

   • Estrogen

   • hCG (human chorionic gonadotropin) — maintains pregnancy

Clinical Correlation - TORCHES Infections

Pathogens that can cross placental barrier:

• TOxoplasma gondii

• Rubella

• Cytomegalovirus

• HErpes/HIV

• Syphilis

Umbilical Vessels

Critical Concept - Proper Definitions:

Vessel	Direction	Oxygenation	Carries
Umbilical arteries (2)	Fetus → Placenta	Deoxygenated	Carries waste products
Umbilical vein (1)	Placenta → Fetus	Oxygenated	Carries nutrients, oxygen

Key Insight:

• Umbilical arteries = exception to "arteries carry oxygenated blood" rule

• Umbilical vein = exception to "veins carry deoxygenated blood" rule

• Oxygenation occurs at placenta, NOT fetal lungs

Three Fetal Shunts

Three Fetal Shunts

Rationale: Fetal lungs and liver are nonfunctional; must bypass these organs

Shunt	Location	Function	Blood Flow
Foramen ovale	Between right and left atria	Bypasses lungs	Right atrium → Left atrium (skip right ventricle)
Ductus arteriosus	Between pulmonary artery and aorta	Bypasses lungs	Pulmonary artery → Aorta
Ductus venosus	Connecting umbilical vein to IVC	Bypasses liver	Umbilical vein → Inferior vena cava

Pressure Dynamics:

• Fetus: Right heart pressure > Left heart pressure (drives blood through shunts)

• After birth: Pressure reverses → shunts close

• First breath → lungs expand → pulmonary resistance drops

Fate of Shunts After Birth: Fetal structure vs. Adult Remnant

Fetal Structure	Adult Remnant
Foramen ovale	Fossa ovalis
Ductus arteriosus	Ligamentum arteriosum
Ductus venosus	Ligamentum venosum

Gestation and Birth

Overview

• Human gestation: ~280 days (40 weeks)

• Divided into three trimesters

• General rule: Larger animals = longer gestation, fewer offspring

First Trimester (Weeks 1-13)

Week-by-Week Development:

Time	Developmental Milestones
Week 3	Heart begins beating (~day 22)
Weeks 3-4	Eyes, gonads, limbs, liver begin forming
Week 5	Embryo ~10 mm
Week 6	Embryo ~15 mm
Week 7	Cartilaginous skeleton begins ossifying into bone
Week 8	Most organs formed; brain fairly developed; embryo → fetus
End of Month 3	Fetus ~9 cm

Key Events:

• Organogenesis: Major organs develop

• Most susceptible period for teratogenic effects

Second Trimester (Weeks 14-26)

Key Developments:

• Tremendous growth (most significant size increase)

• Fetus begins moving in amniotic fluid

• Face takes on human appearance

• Toes and fingers elongate

By End of Month 6: Fetus measures 30-36 cm

Third Trimester (Weeks 27-40)

Key Developments:

• Continued rapid growth (months 7-8)

• Extensive brain development

• Antibody transfer:

  • Highly selective active transport from mother

  • Begins earlier but highest in 9th month

  • Provides passive immunity for newborn

• Growth rate slows; fetus becomes less active (less room)

Premature Birth:

• Survival possible as early as 24 weeks

• Severe complications common (respiratory, GI, nervous systems incomplete)

Birth (Parturition)

Hormonal Control:

• Prostaglandins: Coordinate uterine smooth muscle contractions

• Oxytocin: Peptide hormone promoting contractions (positive feedback loop)

Three Stages of Labor:

Stage	Events	Description
1	Cervical thinning and dilation; "water breaking"	Amniotic sac ruptures; cervix effaces and dilates
2	Birth of fetus	Strong uterine contractions push fetus through birth canal
3	Expulsion of placenta and umbilical cord	"Afterbirth" — membranes and placenta delivered

Key Terminology:

• Parturition: Process of giving birth

• Afterbirth: Placenta and fetal membranes expelled after delivery

• Positive feedback: Oxytocin → contractions → more oxytocin release

Summary of Key Developmental Stages

Stage	Key Features
Zygote	Diploid, unicellular
Morula	Solid ball of cells
Blastula	Hollow ball with blastocoel; trophoblast + inner cell mass
Gastrula	Three germ layers; archenteron and blastopore
Neurula	Neural tube formation; notochord present

Fertilization

• Location: Ampulla of fallopian tube

• Process: Sperm penetrates corona radiata and zona pellucida using acrosomal enzymes

• Acrosomal apparatus: Tube-like structure formed when sperm contacts oocyte membrane; injects pronucleus

• Cortical reaction: Calcium ion release causes:

  • Membrane depolarization (prevents polyspermy)

  • Increased metabolic rate of diploid zygote

• Fertilization membrane: The now depolarized, impenetrable membrane

Twins

Type	Mechanism	Genetic Similarity	Structures
Dizygotic (Fraternal)	2 eggs + 2 sperm	Like any siblings	Each has own placenta, chorion, amnion
Monozygotic (Identical)	1 zygote splits	Genetically identical	Varies by timing of split

Monochorionic/Monoamniotic: Share both chorion and amnion (highest risk)
Monochorionic/Diamniotic: Share chorion, separate amnions
Dichorionic/Diamniotic: Separate chorions and amnions (lowest risk, earliest split)

Cleavage

Definition: Rapid mitotic divisions without overall growth

Key Features:

• First cleavage: zygote → embryo (no longer unicellular)

• Increases N:C ratio and surface area-to-volume ratio

• Indeterminate cleavage: Cells retain ability to form complete organism (enables monozygotic twins)

• Determinate cleavage: Cell fate already committed to specific lineage

Blastulation

Morula → Blastula (Blastocyst)

Stage	Description
Morula	Solid mass of cells
Blastula/Blastocyst	Hollow ball with fluid-filled blastocoel

Two Cell Populations in Blastocyst:

• Trophoblast: Surrounds blastocoel → forms chorion and placenta

• Inner cell mass: Protrudes into blastocoel → forms the organism

Ectopic Pregnancy: Blastula implants outside uterus (>95% in fallopian tube); surgical emergency

Implantation and Extraembryonic Membranes

Structure	Function
Chorion	Forms placenta; outer protective membrane
Amnion	Produces amniotic fluid (shock absorber)
Yolk sac	Supports embryo before placenta; early blood cell development
Allantois	Early fluid exchange; contributes to umbilical cord

Placental Development:

• Trophoblast forms chorionic villi penetrating endometrium

• Villi create maternal-fetal interface (blood does NOT mix)

• Umbilical cord: Two arteries (deoxygenated away), one vein (oxygenated toward fetus)

Amniocentesis: Aspiration of amniotic fluid to test fetal cells for chromosomal abnormalities

Gastrulation

Process:

• Invagination of blastula → archenteron (primitive gut) forms

• Blastopore = opening of archenteron

• Blastocoel eliminated as three layers establish

Blastopore Fate:

• Deuterostomes (humans): Blastopore → anus

• Protostomes: Blastopore → mouth

Result: Three primary germ layers established in gastrula

Neurulation

Sequence:

1. Notochord (mesoderm) forms along long axis

2. Notochord induces overlying ectoderm → neural folds and neural groove

3. Neural folds fuse → neural tube (becomes CNS)

4. Neural crest cells at fold tips migrate → PNS and other structures

Neural Crest Derivatives:

• Sensory and autonomic ganglia

• Adrenal medulla

• Schwann cells

• Melanocytes

• Calcitonin-producing thyroid cells

Clinical:

• Spina bifida: Incomplete neural tube closure (variable severity)

• Anencephaly: Brain fails to develop (fatal)

• Prevention: Folate/folic acid supplementation

Problems in Early Development

Teratogens: Substances interfering with development

• Examples: Alcohol, prescription drugs, viruses, bacteria, environmental chemicals

• Effects vary by: genetics, route, length of exposure, placental transmission

Maternal Conditions:

• Diabetes/hyperglycemia: Macrosomia (large fetus), neonatal hypoglycemia

• Folic acid deficiency: Neural tube defects

Cell Specialization

Stage	Definition
Specification	Reversible designation as specific cell type
Determination	Irreversible commitment to specific lineage
Differentiation	Cell changes structure/function/biochemistry via selective transcription

Determination Mechanisms:

• Asymmetric distribution of mRNA/proteins during cleavage

• Morphogens (signaling molecules) from nearby cells

Competency: Ability of cell to respond to morphogen/inducer

Stem Cell Potency

Type	Differentiation Capacity
Totipotent	All cell types including placental structures
Pluripotent	All three germ layers and derivatives (not placenta)
Multipotent	Specific subset of cell types

Potency decreases as differentiation progresses: Totipotent → Pluripotent → Multipotent

Cell–Cell Communication

Signal Type	Target	Mechanism
Autocrine	Same cell	Cell acts on itself
Paracrine	Local cells	Diffusion in nearby area
Juxtacrine	Adjacent cell	Direct contact stimulation
Endocrine	Distant tissues	Hormones via bloodstream

Induction Terminology:

• Inducer: Cell/molecule that releases signals

• Responder: Cell receiving signals (must be competent)

• Reciprocal induction: Both tissues induce further differentiation in each other

• Growth factors: Peptides promoting differentiation and mitosis

Signaling via Gradients: Morphogen concentration determines cell fate (high near source, low far away)

Cell Migration, Death, and Regeneration

Cell Migration: Cells disconnect and travel to correct location (e.g., neural crest, anterior pituitary)

Apoptosis vs. Necrosis:

Feature	Apoptosis	Necrosis
Trigger	Programmed/signals	Injury
Process	Formation of apoptotic blebs → digested by other cells	Cell rupture
Contents	Contained in membranes	Leaked into environment
Inflammation	None	Present
Function	Sculpting structures (e.g., removing finger webbing)	Pathological

Regeneration:

Organ	Regenerative Capacity
Liver	High (can regenerate 50% loss)
Kidney	Moderate (nephron repair; easily overwhelmed)
Heart	Low (scarring after injury)

Senescence and Aging

Cellular Senescence:

• Cells stop dividing after ~50 divisions

• Caused by telomere shortening (telomeres protect chromosome ends)

• Telomerase: Reverse transcriptase that extends telomeres

  • Expressed in germ cells, fetal cells, cancer cells

  • Allows indefinite division

Organismal Senescence: Accumulation of molecular/cellular changes and environmental damage over time

Fetal Circulation

Placental Function

Key Principle: Maternal and fetal blood DO NOT mix

Exchange:

• Oxygen, CO₂: Passive diffusion (concentration gradients)

• HbF has higher O₂ affinity than adult HbA → enhances O₂ transfer and retention

• Nutrients (glucose, amino acids) and waste exchanged

• Maternal antibodies transferred → passive immunity

Endocrine Functions: Placenta secretes estrogen, progesterone, hCG

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Umbilical Vessels

Vessel	Direction	Oxygenation
Umbilical arteries (2)	Fetus → Placenta	Deoxygenated
Umbilical vein (1)	Placenta → Fetus	Oxygenated

Critical: Gas exchange occurs at placenta, NOT fetal lungs

Three Fetal Shunts

Shunt	Connection	Bypasses
Foramen ovale	Right atrium → Left atrium	Lungs
Ductus arteriosus	Pulmonary artery → Aorta	Lungs
Ductus venosus	Umbilical vein → IVC	Liver

Mechanism: Higher right heart pressure in fetus drives blood through shunts; reverses after birth

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Gestation and Birth

Trimesters

Trimester	Key Events
First	Organogenesis: Heart beats (~day 22), eyes/gonads/limbs/liver form, brain develops; cartilage → bone (week 7); embryo → fetus (week 8)
Second	Tremendous growth; movement begins; face becomes human; digits elongate
Third	Rapid growth and brain development; antibody transfer (highest in 9th month); growth slows

---

Birth (Parturition)

Hormonal Control: Prostaglandins and oxytocin coordinate uterine contractions

Three Stages:

1. Cervical thinning/dilation: Amniotic sac ruptures ("water breaking")

2. Birth of fetus: Strong uterine contractions

3. Afterbirth: Expulsion of placenta and umbilical cord

Resonance

Definition and Representation

• Resonance structures: Two or more Lewis structures with same atom arrangement but different electron placement

• Represented with double-headed arrow (↔) between structures

• Actual structure = resonance hybrid (composite of all resonance forms)

• Example: SO₂ has three resonance structures; actual S–O bonds are identical and equivalent

Stability Rules for Resonance Contributors

Major contributor = most stable resonance structure; contributes most to hybrid character

Assessing Stability Using Formal Charge

Rule	Explanation
Small/no formal charges preferred	Structure with minimal formal charges is more stable
Minimize charge separation	Less separation between opposite charges = more stable
Negative charge on electronegative atoms	Negative formal charge on more electronegative atom (e.g., O) is more stable than on less electronegative atom (e.g., C)

Key Concept: Resonance delocalizes electrons and charges over π systems, increasing overall stability.

Exceptions to the Octet Rule

Three Categories of Exceptions

Exception Type	Elements Affected	Explanation
Incomplete Octet	H, He, Li, Be, B	Stable with fewer than 8 valence electrons
Expanded Octet	Period 3 and beyond	Can accommodate >8 electrons using d-orbitals; can form >4 bonds
Odd-Electron Species	Various	Cannot have 8 electrons on every atom (free radicals)

MCAT Tip: Don't automatically reject structures where central atom has >4 bonds—testmakers may test ability to recognize expanded octets.

Example: SO₄²⁻ — sulfur has 12 valence electrons, allowing formal charges of zero on three of five atoms. Has at least six resonance forms.

VSEPR Theory (Valence Shell Electron Pair Repulsion)

Purpose

Predicts three-dimensional molecular geometry of covalently bonded molecules based on Lewis structures

Core Principle

Electron pairs (bonding and nonbonding) in valence shell repel each other and arrange as far apart as possible to minimize repulsion.

Steps for Predicting Geometry

1. Draw Lewis dot structure

2. Count total bonding + nonbonding electron pairs around central atom

3. Arrange pairs to maximize separation

Electronic Geometries (All Electron Pairs)

Regions of Electron Density	Electronic Geometry	Ideal Bond Angles	Example
2	Linear	180°	BeCl₂
3	Trigonal planar	120°	BH₃
4	Tetrahedral	109.5°	CH₄
5	Trigonal bipyramidal	90°, 120°, 180°	PCl₅
6	Octahedral	90°, 180°	SF₆

Electronic vs. Molecular Geometry

Critical Distinction:

Geometry Type	What It Describes
Electronic geometry	Spatial arrangement of ALL electron pairs (bonding + lone pairs)
Molecular geometry	Spatial arrangement of ONLY bonding pairs (atoms)

Coordination number: Number of atoms bonded to central atom (determines molecular geometry)

Classic MCAT Comparison:

Molecule	Electronic Geometry	Molecular Geometry
CH₄	Tetrahedral	Tetrahedral
NH₃	Tetrahedral	Trigonal pyramidal
H₂O	Tetrahedral	Bent/angular

Why the Differences?

• All three have 4 electron pairs (tetrahedral electronic geometry)

• CH₄: 4 bonding, 0 lone pairs → tetrahedral molecular

• NH₃: 3 bonding, 1 lone pair → trigonal pyramidal molecular

• H₂O: 2 bonding, 2 lone pairs → bent/angular molecular

Effect of Lone Pairs on Bond Angles

Key Principle: Lone pairs exert MORE repulsion than bonding pairs because they reside closer to the nucleus.

Molecule	Lone Pairs	Actual Bond Angle
CH₄	0	109.5°
NH₃	1	~107°
H₂O	2	~104.5°

Pattern: More lone pairs → greater repulsion → smaller bond angles

Polarity of Molecules

Bond Polarity

ΔEN	Bond Type
< 0.5	Nonpolar (or very slightly polar)
0.5 – 1.7	Polar covalent
> 1.7	Ionic

Polar bonds: Unequal sharing; more electronegative atom = partial negative (δ⁻), less electronegative atom = partial positive (δ⁺)

Molecular Polarity

Critical Rule: Bond polarity ≠ molecular polarity

Bond Type	Molecular Polarity Possibilities
All nonpolar bonds	Always NONPOLAR
Polar bonds present	Can be POLAR or NONPOLAR

Determining Molecular Polarity:

• Consider molecular geometry

• Vector sum of bond dipole moments:

  • Cancel out (sum = 0) → Nonpolar molecule

  • Do not cancel (sum ≠ 0) → Polar molecule

Examples:

Molecule	Bonds	Geometry	Bond Dipoles	Molecular Polarity
CCl₄	4 polar C–Cl	Tetrahedral	Cancel (symmetrical)	Nonpolar
H₂O	2 polar O–H	Bent/angular	Do not cancel	Polar
CO₂	2 polar C=O	Linear	Cancel (180° apart)	Nonpolar

MCAT Warning: Spot a polar bond? The molecule MIGHT be polar. See only nonpolar bonds? The molecule MUST be nonpolar.

Atomic and Molecular Orbitals

Quantum Number Review

Quantum Number	Symbol	Description
Principal	n	Energy level (shell)
Azimuthal	l	Subshell shape (s, p, d, f)

Orbital Shapes

Subshell	l value	Number of Orbitals	Shape
s	0	1	Spherical
p	1	3 (px, py, pz)	Dumbbell/barbell along x, y, z axes
d	2	5	Complex (do not memorize for MCAT)
f	3	7	Complex (do not memorize for MCAT)

Molecular Orbitals

Formation: Atomic orbitals combine → molecular orbitals (wave functions combine)

Sign Combination	Result
Same signs	Bonding orbital
Different signs	Antibonding orbital

Sigma (σ) and Pi (π) Bonds

Bond Type	Overlap Pattern	Electron Density	Rotation
Sigma (σ)	Head-to-head	Single linear accumulation between nuclei	Free rotation allowed
Pi (π)	Parallel electron clouds	Two parallel regions of electron density	No free rotation

Key Concept:

• Single bond = 1 σ bond

• Double bond = 1 σ + 1 π bond

• Triple bond = 1 σ + 2 π bonds

Ionic Bonds

Formation: Transfer of electron(s) from low IE element to high EA element

• Occurs between elements with ΔEN > 1.7 (typically metal + nonmetal)

• Cation: Positive ion (electron donor)

• Anion: Negative ion (electron acceptor)

Properties:

Property	Description
Structure	Crystalline lattices (organized arrays of ions)
In Water	Dissociate in water and polar solvents
Melting Point	High (strong electrostatic attractions)
Conductivity	Conduct electricity when dissolved or molten (ions are mobile)

Key Concept: Ionic bonds are the strongest type of chemical bond

Mnemonic: Ionic compounds "ION-ize" (dissociate) in water

Covalent Bonds

Formation

• Sharing of electrons between atoms

• Elements have similar electronegativities

• Can be nonpolar or polar

Bond Order

Bond Type	Bond Order	Strength	Energy	Length
Single	1	Weakest	Lowest	Longest
Double	2	↑	↑	↓
Triple	3	Strongest	Highest	Shortest

Key Relationship: As bond order increases:

• Bond strength increases

• Bond energy increases

• Bond length decreases

Bond Polarity:

• Nonpolar: ΔEN < 0.5 (equal or near-equal sharing)

• Polar: ΔEN = 0.5 – 1.7 (unequal sharing; δ⁺ and δ⁻)

• Ionic: ΔEN > 1.7 (electron transfer)

Polar Bond Characteristics:

• More electronegative atom: partial negative (δ−)

• Less electronegative atom: partial positive (δ+)

• Creates a dipole moment

Coordinate Covalent Bonds

Definition: One atom contributes both bonding electrons; the other contributes none

Key Context: Lewis acid–base chemistry

• Lewis acid: Electron pair acceptor

• Lewis base: Electron pair donor

Example: Formation of ammonium ion (NH₄⁺)

• NH₃ (has lone pair) + H⁺ (has no electrons) → NH₄⁺

• Nitrogen provides both electrons for the new N–H bond

Lewis Structures

Lewis Dot Symbols: Chemical representation of an atom's valence electrons

Steps for Drawing Lewis Structures:

1. Count total valence electrons

2. Draw skeletal structure (least electronegative atom in center)

3. Connect atoms with single bonds

4. Add remaining electrons as lone pairs (complete octets)

5. Form multiple bonds if needed to complete octets

Three Types of Electrons in Lewis Structures:

• Valence electrons: Total electrons available

• Bonding electrons: Electrons in bonds (each bond = 2 electrons)

• Nonbonding electrons: Lone pairs

Formal Charge

Formula:

FC = Valence electrons − Dots − Lines

Where:

• Valence electrons = number in neutral atom

• Dots = nonbonding electrons (lone pairs)

• Lines = number of bonds (each bond = 1 in this formula)

Interpretation:

• FC = 0: Atom has "normal" electron count (most stable)

• FC = +: Atom has fewer electrons than neutral (electron-deficient)

• FC = −: Atom has more electrons than neutral (electron-rich)

Key Principle: Best Lewis structure minimizes formal charges (sum of formal charges = overall charge)

Resonance Structures

Definition: Multiple possible electron configurations for a molecule with a π (pi) system

Key Features:

• Represent all possible configurations (stable and unstable)

• The true structure is a hybrid of all resonance forms

• Delocalization of electrons → increased stability

Example: Ozone (O₃)

• Double bond can be on either side

• True structure: both bonds are "1.5" bonds (partial double bond character)

Resonance Stabilization: Molecules with resonance are more stable than any single resonance form would suggest

VSEPR Theory

Full Name: Valence Shell Electron Pair Repulsion

Core Principle: Electrons (bonding and nonbonding) arrange to be as far apart as possible in 3D space

Key Rule: Nonbonding electrons exert more repulsion than bonding electrons

• Lone pairs are held closer to the nucleus

• Therefore: LP-LP repulsion > LP-BP repulsion > BP-BP repulsion

Electronic vs. Molecular Geometry:

Term	Definition
Electronic Geometry	Position of ALL electrons (bonding + nonbonding)
Molecular Geometry	Position of only BONDING pairs of electrons

Example: Water (H₂O)

• Electronic geometry: Tetrahedral (4 electron groups: 2 bonds + 2 lone pairs)

• Molecular geometry: Bent (only consider the 2 bonds)

Molecular Polarity

Determined by:

1. Polarity of individual bonds (dipole moments)

2. Overall molecular geometry (sum of dipole moments)

Rules:

• All polar molecules contain polar bonds

• Nonpolar molecules may contain:

  • Nonpolar bonds (ΔEN < 0.5)

  • Polar bonds with dipole moments that cancel (symmetrical arrangement)

Examples:

• CO₂: Polar bonds (C=O) but linear geometry → dipoles cancel → nonpolar molecule

• H₂O: Polar bonds (O–H) + bent geometry → dipoles don't cancel → polar molecule

Dipole Moment Formula:

p = qd

Where:

• p = dipole moment

• q = magnitude of charge

• d = displacement (distance) between charges

Sigma (σ) and Pi (π) Bonds

Bond Type	Overlap Pattern	Characteristics
Sigma (σ)	Head-to-head overlap	First bond formed; stronger; allows rotation
Pi (π)	Parallel electron cloud overlap	Second/third bond in multiple bonds; weaker; restricts rotation

Bond Composition:

• Single bond: 1 σ bond

• Double bond: 1 σ bond + 1 π bond

• Triple bond: 1 σ bond + 2 π bonds

Key Insight: Pi bonds create restricted rotation (important for cis/trans isomerism)

Intermolecular Forces

Strength Comparison

From Weakest to Strongest:

1. London dispersion forces (weakest)

2. Dipole–dipole interactions

3. Hydrogen bonds (strongest IMF)

4. Covalent bonds (stronger than all IMFs)

5. Ionic bonds (strongest overall)

Key Concept: Intermolecular forces << Covalent bonds < Ionic bonds

London Dispersion Forces

Characteristics:

• Weakest intermolecular force

• Present in ALL atoms and molecules

• Result from temporary/instantaneous dipoles

Factors Affecting Strength:

• Size of atom/structure: Larger atoms/molecules → stronger London forces

• Surface area: Greater surface contact → stronger London forces

Significance: Only intermolecular force in nonpolar molecules (e.g., noble gases, hydrocarbons)

Dipole–Dipole Interactions

Characteristics:

• Occur between oppositely charged ends of polar molecules

• Stronger than London dispersion forces

• Evident in solid and liquid phases

• Negligible in gas phase (molecules too far apart)

Orientation: Positive end (δ+) of one molecule aligns with negative end (δ−) of another

Hydrogen Bonds

Definition: Specialized subset of dipole–dipole interactions

Requirements:

• Hydrogen must be bonded to one of three electronegative atoms:

  • F (Fluorine)

  • O (Oxygen)

  • N (Nitrogen)

Mnemonic: FON — Hydrogen bonds with its "FON" (phone)

Characteristics:

• Strongest type of intermolecular force

• Can be intermolecular (between molecules) or intramolecular (within a molecule)

• Critical for:

  • Water's unique properties (high boiling point, surface tension)

  • DNA base pairing

  • Protein secondary/tertiary structure

Structural Proteins

General Characteristics

• Highly repetitive secondary structure

• Supersecondary structure (motif): Repetitive organization of secondary structural elements

• Often have a fibrous nature due to this regularity

Collagen

Property	Description
Structure	Trihelical fiber (three left-handed helices woven into a right-handed helix)
Location	Extracellular matrix of connective tissue
Function	Provides strength and flexibility
Distribution	Throughout body; major component of bone

Key Amino Acid: Glycine is essential for proper collagen folding (small size allows tight helix packing)

Clinical Correlation – Osteogenesis Imperfecta (Brittle Bone Disease):

• Caused by replacement of glycine with other amino acids

• Leads to improper collagen folding and cell death

• Results in bone fragility

Elastin

Property	Description
Location	Extracellular matrix of connective tissue
Function	Stretch and recoil like a spring; restores original tissue shape
Key Feature	Elasticity—allows tissues to return to original form after stretching

Examples of Elastin-Rich Tissues: Lungs, blood vessels, skin, ligaments

Keratins

Property	Description
Type	Intermediate filament proteins
Location	Epithelial cells
Function	Mechanical integrity of cells; regulatory functions
Primary Locations	Hair and nails

Key Concept: Keratins provide structural support and are the main protein in hair and nails

Actin

Property	Description
Type	Microfilament protein
Abundance	Most abundant protein in eukaryotic cells
Location	Microfilaments and thin filaments of myofibrils
Key Feature	Polarity: Positive and negative sides
Function of Polarity	Allows motor proteins (myosin) to travel unidirectionally like a one-way street

MCAT Relevance: Actin–myosin interaction is critical for muscle contraction, cytokinesis, and cell motility

Tubulin

Property	Description
Type	Microtubule protein
Functions	• Structural support • Chromosome separation (mitosis/meiosis) • Intracellular transport
Polarity	Negative end: near nucleus Positive end: periphery of cell

Motor Proteins Associated with Tubulin:

• Kinesin: Moves toward positive end

• Dynein: Moves toward negative end

Motor Proteins

General Characteristics

• Display enzymatic activity as ATPases

• ATP hydrolysis powers conformational changes for motor function

• Have transient interactions with actin or microtubules

Myosin

Property	Description
Interacts with	Actin
Structure	Single head and neck per subunit
Primary Role	Thick filament in myofibril (muscle contraction)
Additional Role	Cellular transport
Mechanism	Movement at the neck produces the power stroke of sarcomere contraction

Kinesin and Dynein

Feature	Kinesin	Dynein
Associated with	Microtubules	Microtubules
Structure	Two heads (at least one attached at all times)	Two heads (at least one attached at all times)
Direction	Toward positive end (+)	Toward negative end (−)
Key Function	Aligning chromosomes (metaphase); depolymerizing microtubules (anaphase)	Sliding movement of cilia and flagella

Neuron Example – Classic MCAT Application:

Direction	Motor Protein	Cargo
Anterograde transport(soma → synaptic terminal)	Kinesin (toward + end)	Vesicles with neurotransmitter
Retrograde transport(synaptic terminal → soma)	Dynein (toward − end)	Waste vesicles, recycled neurotransmitter

Key Concept: Both kinesin and dynein are involved in vesicle transport but move in opposite directions along microtubules

Mnemonic: Kinesin "kicks" vesicles outward (toward + end); Dynein "drags" them back inward (toward − end)

Binding Proteins

General Functions

• Transport or sequester molecules by binding to them

• Each has an affinity curve for its target molecule

Types of Binding Proteins

Type	Function	Affinity Pattern
Transport proteins	Bind/unbind to maintain steady-state concentrations	Varying affinity depending on environmental conditions
Sequestration proteins	Keep target bound at nearly 100%	High affinity across large concentration range

Examples of Binding Proteins:

• Hemoglobin: Oxygen transport (has oxyhemoglobin dissociation curve)

• Calcium-binding proteins: Regulate calcium signaling

• DNA-binding proteins: Often transcription factors

Cell Adhesion Molecules (CAMs)

General Characteristics

• Proteins on cell surfaces

• Aid in binding cells to extracellular matrix or other cells

• All are integral membrane proteins

• Three major families: cadherins, integrins, selectins

Cadherins

Property	Description
Type	Glycoproteins
Adhesion Dependence	Calcium-dependent
Function	Hold similar cell types together
Specificity	Type-specific (E-cadherin in epithelial cells, N-cadherin in nerve cells)

Key Concept: Cadherins mediate homotypic cell–cell adhesion (same cell types binding together)

Integrins

Property	Description
Structure	Two membrane-spanning chains: α (alpha) and β (beta)
Function	Bind to and communicate with extracellular matrix
Additional Role	Cellular signaling (promote cell division, apoptosis, etc.)

Clinical Examples:

• αIIbβ3 integrin: Allows platelets to bind fibrinogen → platelet activation → clot stabilization

• Other integrins: White blood cell migration, epithelial stabilization on basement membrane

Selectins

Property	Description
Binding Target	Carbohydrate molecules on other cell surfaces
Bond Strength	Weakest of the CAMs discussed
Location	White blood cells and endothelial cells lining blood vessels
Function	Host defense: inflammation and white blood cell migration

Key Concept: Selectins bind carbohydrates (not proteins); these are the weakest CAM interactions

CAM Family Comparison

CAM Type	Binding Target	Dependence	Bond Strength	Primary Function
Cadherins	Binding Target: Other cadherins (homotypic)	Dependence: Calcium	Bond Strength: Strong	Function: Hold similar cells together
Integrins	Binding Target: Extracellular matrix	Dependence: α/β chains	Bond Strength: Strong	Function: Cell–matrix adhesion and signaling
Selectins	Binding Target: Carbohydrates	—	Bond Strength: Weakest	Function: White blood cell migration, inflammation

Clinical Application:

• Cancer metastasis associated with unique CAM expression patterns

• Medications targeting CAMs: prevent metastasis, stop clotting in heart attacks

Immunoglobulins (Antibodies)

General Characteristics

• Proteins produced by B-cells

• Function: Neutralize targets and recruit other immune cells

• Y-shaped proteins

Structure

Component	Description
Heavy chains	Two identical chains
Light chains	Two identical chains
Connections	Disulfide linkages and noncovalent interactions
Antigen-binding region	Tips of the "Y"; specific polypeptide sequences
Constant region	Remainder of antibody; recruits other immune cells (e.g., macrophages)

Antibody Specificity

• Each antibody binds one, and only one, specific antigenic sequence

• This specificity is determined by the variable region at the tips of the Y

Three Outcomes of Antibody–Antigen Binding

Outcome	Description	Mechanism
Neutralization	Pathogen/toxin unable to exert effects on body	Antibody blocks active site or binding region
Opsonization	Pathogen marked for destruction	Constant region recruits other immune cells (macrophages)
Agglutination	Antigen–antibody complexes clump into large insoluble proteins	Cross-linking of multiple antigens by antibodies → phagocytosis

Key Mnemonic: NOA — Neutralize, Opsonize, Agglutinate

Ion Channels

General Characteristics

• Proteins creating specific pathways for charged molecules

• All permit facilitated diffusion (passive transport down concentration gradient)

• Used for molecules impermeable to membrane (large, polar, or charged)

• Three main types: ungated, voltage-gated, ligand-gated

Ungated Channels

Property	Description
Gating	No gates; unregulated
Example	All cells have ungated potassium channels
Consequence	Net efflux of K⁺ through channels unless K⁺ is at equilibrium

Voltage-Gated Channels

Property	Description
Regulation	Membrane potential change near the channel
Mechanism	Depolarization → protein conformation change → channel opens
Example	Voltage-gated sodium channels in neurons

Sodium Channel Kinetics:

1. Closed at resting potential

2. Depolarization → rapid opening

3. Voltage increase → rapid closing (inactivation)

Sinoatrial (SA) Node – Pacemaker Current:

• Voltage-gated nonspecific sodium–potassium channels

• As voltage drops → channels open → cell brought back to threshold → fires action potential

• This creates the rhythmic electrical activity of the heart

Ligand-Gated Channels

Property	Description
Regulation	Binding of a specific substance (ligand) causes opening or closing
Example	Neurotransmitters at postsynaptic membrane

GABA Example:

• GABA (γ-aminobutyric acid) = inhibitory neurotransmitter

• Binds to chloride channel → opens it

• Cl⁻ influx → hyperpolarization → decreased neuronal excitability

Transport Kinetics

Key Concept: Km and Vmax parameters apply to transporters (same as enzymes)

• Km: Solute concentration at which transporter functions at half of maximum activity

• Can be derived from Michaelis–Menten and Lineweaver–Burk equations

Facilitated Diffusion Characteristics:

• Passive transport (no ATP required)

• Down concentration gradient

• Through transmembrane protein pore

• Saturable (limited number of transporters)

Enzyme-Linked Receptors

General Characteristics

• Membrane receptors with catalytic activity

• Activated by ligand binding

Three Primary Protein Domains

Domain	Function
Membrane-spanning domain	Anchors receptor in cell membrane
Ligand-binding domain	Stimulated by appropriate ligand
Catalytic domain	Activated by conformational change; often initiates second messenger cascade

Receptor Tyrosine Kinases (RTKs)

Property	Description
Structure	Monomer that dimerizes upon ligand binding
Active Form	Dimer
Activity	Phosphorylates cellular enzymes, including itself (autophosphorylation)

Other Classes of Enzyme-Linked Receptors:

• Serine/threonine-specific protein kinases

• Receptor tyrosine phosphatases

G Protein-Coupled Receptors (GPCRs)

General Characteristics

• Large family of integral membrane proteins

• Characterized by seven membrane-spanning α-helices

• Differ in specificity of extracellular ligand-binding area

G Proteins

Property	Description
Type	Heterotrimeric G protein (three subunits: α, β, γ)
Name Origin	Linked to guanine nucleotides (GDP and GTP)
Function	Link receptor to effector in cell

Three Main Types of G Proteins

G Protein	Action	Effect
Gₛ	Stimulates adenylate cyclase	Increases cAMP levels
Gᵢ	Inhibits adenylate cyclase	Decreases cAMP levels
Gq	Activates phospholipase C	PIP2 → DAG + IP3; IP3 opens Ca²⁺ channels in ER → increases calcium

Mnemonic:

• Gₛ stimulates (s for stimulate)

• Gᵢ inhibits (i for inhibit)

• "Mind your p's and q's" → Gq activates phospholipase C

GPCR Activation Cycle

Step	Event
Inactive State	α subunit binds GDP; complexed with β and γ subunits
Step 1	Ligand binds GPCR → receptor engages G protein
Step 2	GDP replaced with GTP on α subunit; α dissociates from βγ
Step 3	Activated α subunit (with GTP) alters adenylate cyclase activity
	— αₛ subunit: activates adenylate cyclase
	— αᵢ subunit: inhibits adenylate cyclase
Step 4	GTP dephosphorylated to GDP on α subunit
Step 5	α subunit rebinds βγ subunits → G protein returns to inactive state

Key Concept: The GTP-bound α subunit is the active form; GTP hydrolysis to GDP turns it off