Biology Study Notes

CO₂ levels historically high, now approx. $0.04\%$ of the atmosphere.
Plants utilize CO₂ as a primary reactant in photosynthesis to create organic molecules (sugars), using light energy and water.
This biochemical process converts light energy into chemical energy, primarily in the form of glucose, releasing oxygen as a byproduct.

Early eukaryotes, specifically ancient protists resembling cyanobacteria, adapted to use O₂ and began performing photosynthesis.
This led to a significant accumulation of oxygen in the atmosphere, an event known as the Great Oxygenation Event, which dramatically changed Earth's environment and favored aerobic life.

Continental movements (plate tectonics) significantly affect global climate patterns, ocean currents, and precipitation distribution.
The large landmasses create more extreme continental climates with hotter, drier conditions in continental interiors due to distance from moderating oceanic influences.
Mountain ranges formed by continental collision can also create rain shadows, further altering local climates.

The Permian extinction was the largest mass extinction event, leading to the loss of up to $90\%$ of all species.
Following this event, reptiles (including the ancestors of dinosaurs) became dominant due to their adaptations to drier conditions.
The note also mentions a later separate event: a meteor impact believed to have caused the Cretaceous-Paleogene (K-Pg) extinction, which led to widespread sediment upheaval, blocked sunlight, and intensified volcanic activity, resulting in the demise of non-avian dinosaurs and subsequent major diversification of mammals and flowering plants.

Non-vascular plants (like mosses and liverworts) remain non-dominant due to their reliance on moist environments for reproduction and lack of true roots, stems, and leaves.
Vascular plants emerged over $450$ million years ago, developing specialized tissues for transport and structural support, allowing them to colonize diverse terrestrial environments and grow taller.
Xylem: A vascular tissue that primarily transports water and dissolved minerals from the roots upwards to the rest of the plant.
Phloem: A vascular tissue responsible for transporting sugars (nutrients) produced during photosynthesis from leaves to other parts of the plant where they are needed for growth or storage.
Today, flowering plants (angiosperms) are dominant, largely due to their effective reproductive strategies and co-evolution with pollinators. Previous extinctions, like the K-Pg event, often cleared ecological niches, enabling such diversifications.

Gymnosperms literally mean "naked seeds" because their seeds are not enclosed within an ovary.
They were the first seed plants, evolving adaptations like pollen and seeds that allowed for reproduction in dry environments, freeing them from the need for water for fertilization.
Examples include conifers, cycads, and ginkgoes, which produce cones containing either male (pollen) or female (ovule) reproductive structures.

Plants and plant-related organisms like yeast (which is a fungus, but extensively used in genetic research due to conserved eukaryotic pathways) are useful models for testing cancer treatments due to their rapid reproduction rates and well-understood genetic systems.
Researchers can study cellular processes, gene regulation, and the effects of potential therapeutic compounds on shared fundamental biological mechanisms conserved across diverse eukaryotic life.

Plants are broadly classified into four major groups based on their evolutionary history, reproductive methods, and structural characteristics:
1. Bryophytes: Non-vascular plants (mosses, liverworts, hornworts)
2. Pteridophytes: Seedless vascular plants (ferns, horsetails)
3. Gymnosperms: Seed plants without flowers (conifers, cycads)
4. Angiosperms: Flowering plants (the most diverse group)

Meiosis is a type of cell division that reduces the number of chromosomes by half, producing haploid gametes (sperm and egg) or spores.
Plants exhibit an alternation of generations, cycling between a multicellular diploid sporophyte stage and a multicellular haploid gametophyte stage.
The sporophyte undergoes meiosis to produce haploid spores, which then develop into gametophytes. The gametophyte produces gametes by mitosis.

The moss life cycle is dominated by the gametophyte generation, which is the larger, longer-lived, and photosynthetic stage.
The haploid gametophyte produces male (antheridia) and female (archegonia) reproductive structures, which require water for sperm to swim to the egg.
Fertilization results in a diploid zygote, which develops into a sporophyte (spore-producing stage). The sporophyte is typically small, non-photosynthetic, and dependent on the gametophyte for nutrition.
The sporophyte undergoes meiosis to produce haploid spores, which are dispersed and germinate into new gametophytes.

Ferns are typically homosporous, meaning they produce only one type of spore, which develops into a bisexual gametophyte (prothallus).
The fern gametophyte (prothallus) is a small, heart-shaped, photosynthetic, and independent structure that contains both male (antheridia) and female (archegonia) reproductive structures.
Cross-fertilization is common, where sperm from one gametophyte fertilizes an egg from another.
The young sporophyte grows out of the gametophyte, eventually becoming the dominant, large, and independent fern plant we typically recognize.

The ovule is a critical structure containing the female gametophyte.
It consists of an outer protective integument, the nucellus (nutritive tissue), and the embryo sac (which contains the egg cell and polar nuclei).
Following fertilization, the ovule develops into a seed, which comprises:
- The embryo: The undeveloped plant within the seed, developed from the zygote.
- The endosperm: A nutritive tissue, formed from the fusion of a sperm with polar nuclei, providing food for the developing embryo.
- The seed coat: A protective layer derived from the integuments of the ovule.

Flowers are the reproductive organs of angiosperms, designed for efficient pollination.
Key components include:
- Stamen (male reproductive part): Composed of the anther (produces pollen) and the filament (supports the anther).
- Carpel (female reproductive part): Consists of the stigma (receives pollen), the style (connects stigma to ovary), and the ovary (contains ovules).
Other parts include sepals (outer protective structures) and petals (often brightly colored to attract pollinators).
Pollination is the transfer of pollen from the anther to the stigma, followed by fertilization where sperm from the pollen fuse with the egg and polar nuclei within the ovule.

Double fertilization is a unique process characteristic of angiosperms, involving two separate fusion events:
1. One sperm cell from the pollen tube fertilizes the egg cell, forming a diploid zygote ( $2n$ ) that will develop into the embryo.
2. The other sperm cell fuses with two polar nuclei within the embryo sac, forming a triploid ( $3n$ ) primary endosperm nucleus, which develops into the endosperm (the nutritive tissue for the embryo).
This process ensures that endosperm development only begins after successful fertilization of the egg cell, optimizing resource allocation.