Exam 3 Study Guide: Photosynthesis, Cell Division, and Genetics

Photosynthesis

Sum Equation for Photosynthesis

Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy. The overall equation summarizes the transformation of carbon dioxide and water into glucose and oxygen.

• Sum Equation: $6\text{CO}_2 + 6\text{H}_2\text{O} + \text{light energy} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$

• Importance: Provides energy and organic molecules for most life forms.

Redox Reactions in Photosynthesis

Redox reactions involve the transfer of electrons. In photosynthesis, water is oxidized and carbon dioxide is reduced.

• Oxidation: Loss of electrons (water loses electrons).

• Reduction: Gain of electrons (carbon dioxide gains electrons).

• Importance: Drives the formation of glucose and oxygen.

Light and Pigments

Light is absorbed by pigments, which initiate photosynthesis. Different wavelengths are absorbed to varying degrees.

• Most Important Colors: Red and blue wavelengths are most effective for photosynthesis.

• Pigments: Molecules like chlorophyll absorb light energy.

• Chlorophyll: Main pigment; absorbs red and blue light, reflects green.

Light-Dependent and Light-Independent Reactions

Photosynthesis consists of two main stages: light-dependent reactions and the Calvin Cycle (light-independent reactions).

• Light-Dependent Reactions: Occur in thylakoid membranes; produce ATP, NADPH, and O2.

• Calvin Cycle: Occurs in stroma; uses ATP and NADPH to fix CO2 into glucose.

• Relationship: Products of light-dependent reactions fuel the Calvin Cycle.

Electron Transport Chain in Light-Dependent Reactions

The electron transport chain transfers electrons, generating ATP and NADPH.

• ATP Production: Via chemiosmosis and ATP synthase.

• NADPH Production: Electrons reduce NADP+ to NADPH.

• Oxygen: Produced as a byproduct from splitting water.

Rubisco and Photorespiration

Rubisco is the enzyme that catalyzes the first step of the Calvin Cycle. It can also cause photorespiration, which reduces efficiency.

• Rubisco: Fixes CO2 but can bind O2, leading to photorespiration.

• Photorespiration: Consumes O2 and releases CO2, wasting energy.

• CAM and C4 Photosynthesis: Adaptations that minimize photorespiration by spatial or temporal separation of carbon fixation.

Cell Division

Mitosis

Mitosis is the process by which a cell divides to produce two genetically identical daughter cells.

• Replicated Chromosome Structure: Each chromosome consists of two sister chromatids joined at the centromere.

• Phases: Prophase, Metaphase, Anaphase, Telophase.

• Cytokinesis: Division of cytoplasm; differs in plants (cell plate formation) and animals (cleavage furrow).

• Cell Cycle Regulation: Checkpoints (G1, G2, M) ensure proper division.

• Mitosis and Cancer: Uncontrolled mitosis leads to tumor formation.

Binary Fission

Binary fission is a simple cell division process in prokaryotes (bacteria and archaea).

• Process: DNA is replicated, cell splits into two identical cells.

• Organisms: Occurs in prokaryotes.

Meiosis

Meiosis produces four genetically unique daughter cells, essential for sexual reproduction.

• Homologous Pair: Two chromosomes, one from each parent, with the same genes.

• Allele: Different versions of a gene.

• Phases: Meiosis I (separates homologs), Meiosis II (separates sister chromatids).

• Genetic Reshuffling: Crossing over and independent assortment.

• Errors: Nondisjunction leads to abnormal chromosome numbers (e.g., Down syndrome).

Genetics

Key Genetics Terms

Understanding genetics requires familiarity with foundational terminology.

• Gene: Unit of heredity; sequence of DNA encoding a trait.

• Genome: Entire set of genetic material.

• Heredity: Transmission of traits from parents to offspring.

• Character: Observable feature (e.g., flower color).

• Trait: Variant of a character (e.g., purple or white flowers).

• True-breeding: Organisms that produce offspring identical to themselves.

• Hybrid: Offspring of two different true-breeding parents.

• Heterozygous: Two different alleles for a gene.

• Homozygous: Two identical alleles for a gene.

• Recessive/Dominant: Dominant allele masks recessive allele in phenotype.

• P, F1, F2 Generations: Parental, first filial, and second filial generations.

• Phenotype: Observable traits.

• Genotype: Genetic makeup.

Gregor Mendel and His Experiments

Gregor Mendel is the father of modern genetics. He used pea plants to study inheritance.

• Method: Crossed true-breeding plants and analyzed offspring.

• Significance: Established basic laws of inheritance.

Mendel’s Laws of Inheritance

Mendel described three fundamental laws governing inheritance.

• Law of Segregation: Each individual has two alleles for each gene, which segregate during gamete formation.

• Law of Independent Assortment: Genes for different traits assort independently during gamete formation.

• Law of Dominance: One allele may mask the expression of another.

Monohybrid vs. Dihybrid Crosses

Monohybrid crosses involve one trait; dihybrid crosses involve two traits.

• Monohybrid: 1 gene, 2 alleles.

• Dihybrid: 2 genes, 4 alleles.

• Example: Monohybrid: flower color; Dihybrid: flower color and seed shape.

Test Crosses

Test crosses determine the genotype of an individual with a dominant phenotype.

• Method: Cross with homozygous recessive individual.

• Outcome: Offspring ratios reveal genotype.

Multiple Alleles

Some genes have more than two alleles, increasing genetic diversity.

• Example: ABO blood group system.

Sex-Linked Traits

Traits determined by genes on sex chromosomes, often X-linked.

• Example: Color blindness, hemophilia.

• Inheritance: Males more frequently affected by X-linked recessive traits.

Exceptions to Mendel’s Laws

Some inheritance patterns deviate from Mendel’s laws.

• Genetic Linkage: Genes close together on a chromosome are inherited together.

• Incomplete Dominance: Heterozygote shows intermediate phenotype.

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

Punnett Squares

Punnett squares are diagrams used to predict genetic crosses.

• Setup: Place parental alleles on axes, fill in offspring genotypes.

• Application: Used for monohybrid, dihybrid, and test crosses.

Blood Types and Rh Incompatibility

Blood types are determined by multiple alleles and codominance. Rh incompatibility can cause issues in pregnancy.

• ABO System: A, B, AB, O types.

• Rh Factor: Positive or negative; incompatibility can lead to hemolytic disease of the newborn.

Lethal Alleles

Lethal alleles cause death when present in certain genotypes.

• Example: Homozygous dominant or recessive may be lethal.

Chromosomal Theory of Inheritance

This theory links genes to chromosomes, explaining inheritance patterns.

• Key Point: Genes are located on chromosomes, which segregate during meiosis.

Chromosomal Structural Rearrangements

Structural changes in chromosomes can affect gene expression and lead to disorders.

• Translocations: Segment of one chromosome moves to another.

• Inversions: Segment of chromosome is reversed.

• Effects: Can disrupt gene function, cause genetic diseases.

Term	Definition	Example
Gene	Unit of heredity	Flower color gene
Allele	Variant of a gene	Purple or white allele
Phenotype	Observable trait	Purple flowers
Genotype	Genetic makeup	PP, Pp, pp
Homozygous	Two identical alleles	PP or pp
Heterozygous	Two different alleles	Pp

Additional info: Academic context and definitions were expanded for clarity and completeness. Table was inferred from key terms listed in the guide.