Photosynthesis, Mitosis, Meiosis, and Mendelian Genetics: Core Concepts and Mechanisms

Photosynthesis

Overview of Photosynthesis

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 is essential for life on Earth as it provides the organic molecules and oxygen required by most organisms.

• Overall Equation: The general equation for photosynthesis is: $6CO_2 + 12H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6O_2 + 6H_2O$

• Endergonic Reaction: Photosynthesis requires an input of energy (sunlight), making it an endergonic process. The products have more potential energy than the reactants.

Redox Reactions: In photosynthesis, carbon dioxide is reduced to glucose, and water is oxidized to oxygen.

Light Reactions

The light reactions occur in the thylakoid membranes of chloroplasts and convert solar energy into chemical energy in the form of ATP and NADPH. Water is split, releasing oxygen as a byproduct.

• Photosystems: Light energy is absorbed by chlorophyll in photosystem II (PSII) and photosystem I (PSI).

• Electron Transport Chain: Excited electrons move through the electron transport chain, generating a proton gradient used to produce ATP (photophosphorylation).

• NADPH Formation: Electrons are ultimately transferred to NADP+, forming NADPH.

The Calvin Cycle

The Calvin cycle, also known as the light-independent reactions, takes place in the stroma of the chloroplast. It uses ATP and NADPH from the light reactions to fix carbon dioxide and synthesize glucose.

• Phases: The Calvin cycle consists of three main phases: carbon fixation, reduction, and regeneration of RuBP.

• Key Enzyme: Rubisco catalyzes the fixation of CO2 to ribulose bisphosphate (RuBP).

• Output: For every three CO2 molecules fixed, one molecule of glyceraldehyde-3-phosphate (G3P) is produced, which can be used to form glucose and other organic compounds.

Photorespiration and Adaptations

Photorespiration is a process where rubisco adds O2 instead of CO2 to RuBP, leading to a decrease in photosynthetic efficiency. Plants have evolved mechanisms to minimize photorespiration:

• C4 Plants: These plants spatially separate carbon fixation and the Calvin cycle. CO2 is initially fixed into a four-carbon compound in mesophyll cells, then transported to bundle-sheath cells where the Calvin cycle occurs.

• CAM Plants: These plants temporally separate carbon fixation and the Calvin cycle. CO2 is fixed at night into organic acids and released during the day for use in the Calvin cycle.

Mitosis

Overview and Function

Mitosis is a type of cell division that results in two genetically identical daughter cells. It is essential for growth, tissue repair, and asexual reproduction in multicellular organisms.

• Key Functions: Growth, wound repair, and asexual reproduction.

• Somatic Cells: Mitosis produces somatic (body) cells that carry out various functions in the organism.

The Cell Cycle

The cell cycle consists of interphase (G1, S, G2) and the mitotic (M) phase, which includes mitosis and cytokinesis.

• Interphase: The cell grows (G1), replicates its DNA (S), and prepares for division (G2).

• M Phase: Mitosis divides the nucleus, and cytokinesis divides the cytoplasm.

Phases of Mitosis

Mitosis is divided into several phases: prophase, metaphase, anaphase, and telophase, followed by cytokinesis.

• Prophase: Chromosomes condense, spindle fibers form.

• Metaphase: Chromosomes align at the metaphase plate.

• Anaphase: Sister chromatids separate and move to opposite poles.

• Telophase: Nuclear envelopes reform around the chromosomes.

• Cytokinesis: Division of the cytoplasm, differing in plants (cell plate formation) and animals (cleavage furrow).

Meiosis

Overview and Function

Meiosis is a specialized form of cell division that reduces the chromosome number by half, producing four genetically unique gametes. It is essential for sexual reproduction and genetic diversity.

• Two Divisions: Meiosis I (homologous chromosomes separate) and Meiosis II (sister chromatids separate).

• Genetic Variation: Crossing over and independent assortment increase genetic diversity.

Key Events in Meiosis

• Prophase I: Homologous chromosomes pair and exchange segments (crossing over).

• Metaphase I: Homologous pairs align independently (independent assortment).

• Anaphase I: Homologous chromosomes separate (law of segregation).

• Meiosis II: Sister chromatids separate, similar to mitosis.

Comparison of Mitosis and Meiosis

Feature	Mitosis	Meiosis
Number of cell divisions	One	Two
Number of daughter cells	Two	Four
Genetic composition	Identical to parent	Genetically unique
Role	Growth, repair, asexual reproduction	Gamete production, genetic diversity

Mendelian Genetics

Basic Principles

Gregor Mendel's experiments with pea plants established the fundamental laws of inheritance, including the law of segregation and the law of independent assortment.

• Law of Segregation: Each individual has two alleles for each gene, which segregate during gamete formation so that each gamete carries only one allele.

• Law of Independent Assortment: Alleles of different genes assort independently during gamete formation.

Extensions to Mendelian Genetics

• Incomplete Dominance: Heterozygotes have an intermediate phenotype.

• Codominance: Both alleles are fully expressed in heterozygotes (e.g., ABO blood groups).

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

• Epistasis: One gene affects the expression of another gene.

• Polygenic Inheritance: Multiple genes contribute to a single trait, resulting in continuous variation (e.g., skin color, height).