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Photosynthesis, Cell Communication, and the Chromosomal Basis of Inheritance: Core Concepts and Mechanisms

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Chapter 10: Photosynthesis

Chemical Formula and Chloroplast Structure

Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy, producing glucose and oxygen from carbon dioxide and water. This process occurs in the chloroplasts of plant cells.

  • Overall Reaction:

  • Chloroplast Structure:

    • Envelope: Double membrane (inner and outer)

    • Stroma: Thick fluid inside chloroplast; site of the Calvin cycle

    • Thylakoids: Flattened sacs within the stroma; contain chlorophyll

    • Grana: Stacks of thylakoids

    • Thylakoid Membrane: Contains photosystems I & II; site of light reactions

Pigments in Photosynthesis

  • Chlorophyll a: Primary pigment; directly involved in light reactions

  • Chlorophyll b: Accessory pigment; broadens light absorption

  • Carotenoids: Accessory pigments; reflect yellow/orange, provide photoprotection

  • Accessory pigments transfer energy to chlorophyll a

Stages of Photosynthesis

  1. Light Reactions (Light-dependent): Occur in thylakoid membrane; produce ATP and NADPH

  2. Calvin Cycle (Light-independent): Occurs in stroma; uses ATP and NADPH to synthesize sugars

Light Reactions: Key Details

  • Location: Thylakoid membrane

  • Key Events:

    • Water is split, providing electrons and protons; O2 is released

    • NADP+ is reduced to NADPH

    • ATP is produced via photophosphorylation

    • ATP and NADPH are sent to the Calvin cycle

Electron Flow Pathways

  • Cyclic Electron Flow:

    • Uses Photosystem I only

    • Produces ATP only (no NADPH or O2)

    • Electrons cycle back to cytochrome complex, building a proton gradient for ATP synthesis

    • Used when extra ATP is needed

  • Linear Electron Flow (Main Pathway):

    • Uses both Photosystem II (PS II) and Photosystem I (PS I)

    • Produces ATP, NADPH, and O2

    • Key Steps:

      1. Photon excites pigment; electron transferred to primary acceptor

      2. Water splits, providing electrons, protons, and O2

      3. Electrons move from PS II to PS I via electron transport chain

      4. Proton gradient forms, driving ATP synthesis

      5. PS I re-excites electrons, which reduce NADP+ to NADPH

Photosystems

  • Photosystem II (PS II): First in sequence; absorbs light at 680 nm; splits water and initiates electron flow

  • Photosystem I (PS I): Second; absorbs light at 700 nm; re-energizes electrons to produce NADPH

Calvin Cycle (Stroma)

The Calvin cycle uses ATP and NADPH to convert CO2 into sugars. It consists of three main steps:

  1. Carbon Fixation: CO2 attaches to RuBP (5C) via Rubisco, forming 3-PGA

  2. Reduction: 3-PGA is converted to G3P using ATP and NADPH; one G3P exits as sugar

  3. Regeneration: Remaining G3P regenerates RuBP (requires ATP)

  • Energy Requirement for 1 Glucose: 18 ATP and 12 NADPH

Alternative Carbon Fixation Mechanisms

Type

Mechanism

Adaptation

Example

C3

Normal Calvin cycle

Photorespiration (inefficient in hot/dry)

Rice

C4

CO2 fixed into 4C compound; spatial separation (mesophyll → bundle-sheath)

Reduces photorespiration; better in heat/drought

Corn, sugarcane

CAM

CO2 uptake at night (stored); Calvin cycle during day

Reduces water loss; temporal separation

Cactus, pineapple

  • Key Comparison: C3 = inefficient/photorespiration; C4 = spatial separation; CAM = temporal separation

Chapter 11: Cell Communication

Overview of Cell Communication

Cells communicate using chemical signals to coordinate activities. This process involves three main steps:

  1. Reception: Ligand binds to receptor, causing a shape change

  2. Transduction: Signal relayed through intracellular proteins (relay molecules) in a cascade

  3. Response: Cellular effect (e.g., gene expression, enzyme activation)

Key Terms

  • Ligand: Signaling molecule that binds specifically to a receptor

  • Relay Molecules: Intracellular proteins transmitting the signal

Quorum Sensing

  • Bacterial communication via chemical signals

  • Coordinates group behaviors (e.g., biofilm formation, toxin release)

  • Potential target for new antibiotics

Types of Cell Signaling

  • Local Signaling: Between nearby cells

  • Distant Signaling: Signals (hormones) travel via bloodstream (endocrine system)

Local Signaling Types

  • Direct: Through cell junctions or membrane-bound ligands

  • Paracrine: Molecules diffuse short distances (e.g., growth factors)

  • Synaptic: Neurons release neurotransmitters across synapse

Distant Signaling

  • Endocrine: Hormones released into bloodstream; only target cells with correct receptor respond

Types of Receptors

  • Surface Membrane Receptors:

    • G-protein Coupled Receptors (GPCR): 7-pass proteins; bind water-soluble ligands

    • Receptor Tyrosine Kinases (RTKs): Dimerize and autophosphorylate; activate multiple pathways; linked to cancer if defective

    • Gated Ion Channels: Open/close in response to ligand; allow ion flow (Na+, K+, Ca2+)

  • Intracellular Receptors: In cytoplasm/nucleus; bind hydrophobic ligands (e.g., steroid hormones); can act as transcription factors

Signal Transduction: Key Players

  • Protein Kinases: Add phosphate groups (phosphorylation)

  • Protein Phosphatases: Remove phosphate groups (dephosphorylation)

  • Second Messengers: Small, non-protein, water-soluble molecules (e.g., cAMP, Ca2+) that amplify signals

Switching Signaling On/Off

  • Phosphorylation: Activates proteins

  • Dephosphorylation: Inactivates proteins

Second Messengers

  • cAMP: Produced from ATP by adenylyl cyclase; activates protein kinase A (PKA)

  • Cholera: Toxin locks G protein active; continuous cAMP production; causes severe dehydration

  • Ca2+ Ions: Released from internal stores (via IP3); common second messenger

Aspects of Cellular Response

  • Amplification: Small signal yields large response

  • Specificity: Different cells respond differently to same signal

  • Efficiency: Scaffolding proteins organize pathway

  • Termination: Signal stops when ligand is removed

Cellular Responses

  • Can occur in cytoplasm or nucleus

  • Includes gene expression, enzyme activation, ion channel regulation

Apoptosis (Programmed Cell Death)

  • Definition: Controlled cell self-destruction via signaling pathways

  • Process: Activates caspases; cell components packaged and removed

  • Benefits: Development, removal of damaged/infected cells, tissue health

  • Diseases: Too much apoptosis (Parkinson's, Alzheimer's); too little (cancer)

Chapter 12: Mitosis

Purpose of Mitosis

  • Unicellular Organisms: Reproduction

  • Multicellular Organisms: Growth, development, tissue repair, replacement of cells

Key Definitions

  • Genome: Complete DNA set in a cell

  • Somatic Cells: Body cells; diploid

  • Gametes: Reproductive cells; haploid

  • Sister Chromatids: Identical chromosome copies post-replication

  • Cohesins: Proteins holding sister chromatids together

  • Centromere: Region where chromatids attach

  • Centrosome: Organelle organizing microtubules

  • Mitotic Spindle: Microtubule structure moving chromosomes

  • Kinetochore: Protein complex on centromere for spindle attachment

  • Pac-Man Mechanism: Microtubules shorten at kinetochore, pulling chromosomes

  • Benign Tumor: Non-invasive abnormal cell mass

  • Malignant Tumor: Cancerous, invasive cell mass

  • Metastasis: Spread of cancer cells to new locations

Phases of the Cell Cycle

  1. Interphase (≈90%): G1 (growth), S (DNA replication), G2 (preparation)

  2. M Phase: Mitosis (nuclear division) and cytokinesis (cytoplasm division)

Mitosis Phases

  • Prophase: Chromosomes condense, centrosomes move, spindle forms

  • Prometaphase: Nuclear envelope breaks down, spindle attaches to kinetochores

  • Metaphase: Chromosomes align at metaphase plate

  • Anaphase: Cohesins cleaved, sister chromatids separate, move to poles

  • Telophase: Nuclei form, chromosomes decondense, spindle disassembles

  • Cytokinesis: Animal cells: cleavage furrow; Plant cells: cell plate forms

Cell Cycle Control System

  • Regulated by checkpoints and signals

  • Checkpoints:

    • G1: Size, nutrients, DNA damage

    • G2: DNA replication completion

    • M: Chromosome attachment to spindle

  • Internal Regulators: Cyclins (regulate timing), CDKs (activated by cyclins), MPF (cyclin-CDK complex)

  • External Regulators: Growth factors, density-dependent inhibition, anchorage dependence

Cancer and Loss of Control

  • Cancer cells ignore checkpoints, divide uncontrollably, form tumors

  • May produce own growth signals, ignore external signals, or have defective control systems

Chapter 13: Meiosis

Overview and Key Definitions

Meiosis is a specialized cell division in germ cells that halves the chromosome number, producing haploid gametes. Fertilization restores diploidy, maintaining chromosome number across generations.

  • Fertilization: Fusion of gametes to form diploid zygote

  • Haploid (n): One chromosome set

  • Diploid (2n): Two chromosome sets

  • Autosomes: Non-sex chromosomes

  • Sex Chromosomes: X and Y; determine sex

  • Homologous Chromosomes: Maternal and paternal chromosomes with same genes

  • Sister Chromatids: Identical copies post-replication

  • Synapsis: Pairing of homologs in prophase I

  • Crossing Over: Exchange of DNA between non-sister chromatids

  • Chiasmata: X-shaped crossing over points

  • Recombinant Chromosomes: Chromosomes with mixed parental DNA

  • Gene: DNA segment controlling trait

  • Allele: Gene variant

  • Locus: Gene location on chromosome

  • Karyotype: Chromosome display for abnormalities

Structure of Meiosis

  • Two divisions: Meiosis I (reductional), Meiosis II (equational)

  • One DNA replication (before Meiosis I)

  • Result: 1 diploid cell → 4 haploid, genetically unique cells

Meiosis I

  • Prophase I: Homologs pair (synapsis), crossing over occurs

  • Metaphase I: Homolog pairs align; independent assortment

  • Anaphase I: Homologs separate; sister chromatids stay together

  • Telophase I + Cytokinesis: Two haploid cells (chromosomes still duplicated)

Meiosis II

  • Similar to mitosis

  • Sister chromatids separate, producing four haploid cells

Sources of Genetic Variation

  1. Crossing over (prophase I)

  2. Independent assortment (metaphase I)

  3. Random fertilization

  4. Mutations

Meiosis vs. Mitosis Comparison

Feature

Mitosis

Meiosis

Divisions

1

2

Daughter Cells

2

4

Ploidy

Diploid

Haploid

Genetic Identity

Identical

Different

Types of Reproduction

  • Asexual: No gametes; clones; e.g., hydra

  • Sexual: Meiosis + fertilization; genetic variation

Sexual Life Cycles

  • Animals: Diploid dominant; only gametes are haploid

  • Fungi/Protists: Haploid dominant; only zygote is diploid

  • Plants: Alternation of generations; both haploid and diploid multicellular stages

Chapter 14: Mendelian Genetics

Mendel's Principles and Key Terms

  • Gene: Unit of heredity

  • Allele: Gene variant

  • Locus: Gene position

  • Genotype: Genetic makeup (e.g., AA, Aa, aa)

  • Phenotype: Observable trait

  • Homozygous: Same alleles

  • Heterozygous: Different alleles

  • Carrier: Heterozygous for recessive trait

  • Dominant/Recessive: Dominant expressed if present; recessive only if no dominant allele

  • Hybridization: Crossing organisms

  • Punnett Square: Tool for predicting offspring genotypes/phenotypes

Mendel's Laws

  • Law of Segregation: Two alleles separate during gamete formation

  • Law of Independent Assortment: Genes for different traits assort independently (if not linked)

Monohybrid and Dihybrid Crosses

  • Monohybrid Cross: One trait; F2 genotype ratio 1:2:1, phenotype ratio 3:1

  • Dihybrid Cross: Two traits; F2 phenotype ratio 9:3:3:1 (if independent assortment)

Probability Rules

  • Multiplication Rule: Probability of two independent events = product of probabilities

  • Addition Rule: Probability of either event = sum of probabilities

Extensions to Mendelian Genetics

  • Incomplete Dominance: Heterozygote is intermediate (e.g., pink flowers)

  • Codominance: Both alleles expressed (e.g., AB blood type)

  • Multiple Alleles: More than two alleles in population (e.g., ABO blood group)

  • Polygenic Inheritance: Multiple genes affect one trait (e.g., height)

  • Pleiotropy: One gene affects multiple traits (e.g., sickle-cell disease)

  • Epistasis: One gene masks/modifies another (e.g., coat color in mice)

  • Environmental Impact: Phenotype influenced by environment (e.g., hydrangea color)

Examples of Genetic Conditions

  • Recessive: Cystic fibrosis, sickle-cell disease, albinism

  • Dominant: Huntington's disease, achondroplasia, polydactyly

  • Multifactorial: Heart disease, cancer, mental illness

Genetic Testing

  • Fetal Testing: Amniocentesis, chorionic villus sampling, ultrasound

  • Newborn Testing: Phenylketonuria (PKU)

Chapter 15: Chromosomal Basis of Inheritance

Genes and Chromosomes

Genes are located on chromosomes, and their behavior during meiosis explains inheritance patterns, including linkage, sex-linked traits, and chromosomal disorders.

  • Linked Genes: On same chromosome; inherited together

  • Sex-linked Genes: On sex chromosomes (usually X)

  • Recombinants: Offspring with new trait combinations

  • Genetic Map: Diagram of gene locations

  • Map Unit: 1% recombination frequency

  • Nondisjunction: Failure of chromosomes to separate in meiosis

  • Aneuploidy: Abnormal chromosome number

  • Deletion/Duplication/Inversion/Translocation: Structural chromosome changes

Sex-linked Inheritance

  • Males express X-linked recessive traits more often (only one X)

  • Females need two recessive alleles to show trait

Chromosomal Systems of Sex Determination

System

Female

Male

Example

X-Y

XX

XY

Humans

X-0

XX

X

Grasshoppers

Z-W

ZW

ZZ

Birds

Haplo-diploid

Diploid

Haploid

Bees

Human X-linked Recessive Disorders

  • Color blindness

  • Hemophilia

  • Duchenne muscular dystrophy

  • Males affected more; females usually carriers

X-inactivation and Barr Body

  • In females, one X chromosome is randomly inactivated (Barr body)

  • Prevents double gene expression; females are genetic mosaics

Genomic Imprinting

  • Gene expression depends on parent of origin

  • Caused by DNA methylation (gene silencing)

Genetic Linkage and Mapping

  • Genes close together: low recombination, inherited together

  • 1 map unit = 1% recombination

Cytoplasmic (Mitochondrial) Inheritance

  • Genes in mitochondria/chloroplasts; inherited maternally

  • Not Mendelian inheritance

  • Example: mitochondrial diseases

Chromosomal Disorders: Aneuploidy

  • Down Syndrome (Trisomy 21): Extra chromosome 21

  • Klinefelter Syndrome (XXY): Male with extra X

  • Turner Syndrome (XO): Female missing one X

  • Trisomy X (XXX): Female with extra X

Chromosomal Disorders: Structural Changes

  • Deletion: Missing segment (e.g., Cri du chat)

  • Duplication: Repeated segment

  • Inversion: Segment reversed

  • Translocation: Segment moved to another chromosome (e.g., CML)

Key Concepts

  • Genes are on chromosomes; their position affects inheritance

  • Errors in chromosome number or structure cause genetic disorders

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