Photosynthesis

Overview of Photosynthesis

Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy, primarily in the form of carbohydrates. In eukaryotes, this process occurs in the chloroplasts, which contain specialized membranes and pigments to capture light energy.

• Chloroplast Structure: Chloroplasts have three membranes, creating three distinct spaces. Most photosynthetic reactions occur in the thylakoid membrane (light-capturing reactions) and the stroma (space outside of the thylakoid) (Calvin cycle).

• Photosynthetic Pigments: Located in the thylakoid membrane, these molecules absorb specific wavelengths of light. The color of a pigment is due to the wavelengths it does not absorb.

• Major Pigment Classes:

  • Chlorophylls: Absorb purple/blue and red light; appear green.

  • Carotenoids: Absorb blue/green light; appear yellow, orange, or red. They also act as antioxidants, neutralizing free radicals formed by high-energy light.

Stages of Photosynthesis

• Light-Capturing Reactions: Occur in the thylakoid membrane. Convert light energy into chemical energy stored in ATP and NADPH.

• Calvin Cycle: Occurs in the stroma. Uses ATP and NADPH to fix carbon dioxide into carbohydrates.

• Interdependence: Both stages are linked by ATP/ADP and NADPH/NADP+ cycling. The Calvin cycle cannot proceed for long in the dark because it depends on the products of the light reactions.

Mechanism of Light Absorption and Energy Transfer

• When a pigment absorbs a photon, its electrons become excited. This energy can be transferred to adjacent pigment molecules via resonance energy transfer, but the electron itself is not transferred until it reaches the reaction center of a photosystem.

• At the reaction center, the excited electron is transferred to an electron carrier, converting light energy into chemical energy.

Photosystems and Electron Transport

• Photosystem II (PSII): Uses an electron transport chain to pump protons across the thylakoid membrane, creating an electrochemical gradient used by ATP synthase to produce ATP (photophosphorylation).

• Electrons lost from PSII are replaced by splitting water, releasing oxygen as a byproduct.

• Electrons from PSII are passed to Photosystem I (PSI), which further excites them with light energy and transfers them to NADP+ to form NADPH.

The Calvin Cycle

• Uses ATP and NADPH to convert CO2 into the 3-carbon sugar G3P (glyceraldehyde-3-phosphate).

• G3P can be used to synthesize sucrose (when photosynthesis is slow) or starch (when rapid).

• The first step is catalyzed by rubisco, which fixes CO2 to a 5-carbon sugar (RuBP), forming a 6-carbon intermediate that splits into two 3-carbon molecules.

• Most G3P is recycled to regenerate RuBP; some exits the cycle to form other organic molecules.

Photorespiration and Adaptations

• Photorespiration: Occurs when rubisco fixes O2 instead of CO2, wasting energy and releasing CO2. More common when leaf pores are closed to prevent water loss.

• C4 Plants: (e.g., corn, sugar cane) Spatially separate initial CO2 fixation (in mesophyll cells) from the Calvin cycle (in bundle sheath cells).

• CAM Plants: (e.g., cacti) Temporally separate CO2 fixation (at night) from the Calvin cycle (during the day).

• C3 Plants: Most plants; do not have these adaptations.

DNA Technology

Organization of Genetic Material

• Genome: All genetic material in a cell; in eukaryotes, divided among multiple chromosomes.

• Chromosome: A continuous double-stranded DNA molecule with associated proteins; contains many genes at specific loci.

• Gene: A segment of DNA coding for a protein or functional RNA.

• Allele: Different versions of a gene, leading to variation in traits.

• Humans have 23 pairs of chromosomes (22 autosomes, 1 pair of sex chromosomes).

Basic Tools of DNA Technology

• Restriction Enzymes: Cut DNA at specific sequences; some create "sticky ends" for easier recombination.

• PCR (Polymerase Chain Reaction): Amplifies specific DNA sequences using cycles of heating and cooling, primers, and DNA polymerase.

• Transgenic Organisms: Organisms with recombinant DNA from different sources. Used in agriculture, medicine (e.g., insulin production), and research.

• Plasmids: Small circular DNA molecules in bacteria; used as vectors to introduce new genes.

CRISPR/Cas9 Gene Editing

• Based on a bacterial defense system against viruses.

• Uses RNA sequences to guide the Cas9 enzyme to a specific DNA sequence, where it makes a cut.

• If no homologous DNA is provided, repair often leads to insertions/deletions (gene knockout). If homologous DNA is provided, new genes can be inserted.

• Applications include gene therapy, gene drives (e.g., making mosquitoes unable to carry malaria), and research.

• Gene therapy often uses viral vectors to deliver healthy alleles; CRISPR is being developed for more precise therapies.

Mitosis and Cancer

Cell Division in Eukaryotes

• Mitosis: Produces two genetically identical daughter cells for growth, repair, and asexual reproduction.

• Meiosis: Produces gametes with half the chromosome number for sexual reproduction.

• Cells are diploid (2n) if they have pairs of chromosomes, haploid (n) if only one of each.

• Most of the cell cycle is spent in interphase (growth and DNA replication).

Phases of Mitosis

1. Prophase: Chromosomes condense; spindle apparatus forms.

2. Prometaphase: Nuclear envelope dissolves; spindle fibers attach to kinetochores.

3. Metaphase: Chromosomes align at the cell's equator.

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

5. Telophase: Nuclear envelopes reform; chromosomes decondense.

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

• Binary Fission: Bacterial cell division; simpler due to circular chromosomes and lack of nucleus.

Cell Cycle Regulation and Cancer

• Cell division is tightly regulated by checkpoints (e.g., DNA damage, chromosome alignment).

• MPF (Maturation Promoting Factor): A cyclin-Cdk complex that triggers mitosis.

• Cancer: Results from uncontrolled cell division due to mutations in specific genes:

  • Proto-oncogenes: Normally promote cell division; mutated forms (oncogenes) are overactive.

  • Tumor Suppressor Genes: Inhibit cell division; loss of function leads to unchecked growth (e.g., p53).

  • Mutator Genes: Involved in DNA repair; mutations increase mutation rates.

• Cancer can metastasize (spread) and induce angiogenesis (blood vessel growth).

• Immunotherapy is a modern treatment that stimulates the immune system to target cancer cells.

Meiosis and Chromosomal Inheritance

Chromosome Number Variations

• Polyploidy: More than two sets of chromosomes (e.g., 4n, 8n).

• Aneuploidy: Abnormal number of chromosomes (e.g., trisomy = 2n+1, monosomy = 2n-1).

Phases of Meiosis

• Meiosis I: Homologous chromosomes separate; cells become haploid.

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

Genetic Diversity Mechanisms

• Independent Assortment: Random distribution of maternal and paternal chromosomes during meiosis I.

• Random Fertilization: Any sperm can fertilize any egg, increasing combinations.

• Crossing Over: Exchange of genetic material between homologous chromosomes at chiasmata during prophase I.

Consequences of Chromosome Abnormalities

• Most aneuploidies result in embryonic death; some (e.g., trisomy 21) can survive but with health issues.

• Nondisjunction: Failure of chromosomes to separate properly during meiosis, leading to aneuploidy.

• Mosaicism: Nondisjunction during mitosis in the embryo leads to some cells with abnormal chromosome numbers.

• Sex chromosome aneuploidies are generally less severe than autosomal ones.

• X Inactivation: In individuals with more than one X chromosome, all but one X is inactivated in each cell, leading to a mosaic pattern.

• Other species have different sex determination systems (e.g., ZW in birds, environmental in reptiles).

• Unequal Crossing Over: Can result in gene duplications or deletions.

Genetics Introduction

Basic Genetic Concepts

• Alleles: Different versions of a gene; each individual inherits two alleles per gene (one from each parent).

• Homozygous: Two identical alleles for a gene.

• Heterozygous: Two different alleles for a gene.

• Genotype: The genetic makeup (allele combination) of an individual.

• Phenotype: The observable traits resulting from the genotype.

• Dominant and Recessive Alleles: Dominant alleles mask the effect of recessive alleles in heterozygotes.

Mendelian Genetics

• Monohybrid Crosses: Crosses examining inheritance of a single trait; led to discovery of dominance and segregation.

• Chromosomal Theory of Inheritance: Genes are located on chromosomes; inheritance patterns are explained by chromosome behavior during meiosis.

• Punnett Squares: Tools to predict offspring genotypes and phenotypes based on parental genotypes.

• Principle of Segregation: Each gamete receives only one allele of each gene due to separation of homologous chromosomes during meiosis.

• Testcross: Crossing an individual of unknown genotype with a homozygous recessive to determine the unknown genotype.

Skills for Exam

• Interpret figures related to photosynthesis and predict outcomes of experimental changes (e.g., enzyme inhibition).

• Construct and interpret Punnett squares for single-gene, complete dominance scenarios.