Domain Eukarya

Key Features of Eukaryotes

Eukaryotes are a highly diverse group of organisms, ranging from single-celled protists to complex multicellular organisms such as plants and animals. They are unified by several cellular characteristics that distinguish them from prokaryotes (Bacteria and Archaea).

• Nuclear envelope surrounds the genetic material, separating it from the cytoplasm.

• Cells are generally larger and contain more organelles than prokaryotic cells.

• Possess an extensive cytoskeleton for structural support and intracellular transport.

• Reproduce both sexually and asexually.

• Multicellularity evolved independently multiple times within Eukarya.

Protists

Definition and Diversity

"Protists" refers to all eukaryotes that are not land plants, fungi, or animals. This group is paraphyletic, meaning it includes a common ancestor but not all its descendants. Protists are extremely diverse and are primarily found in aquatic or moist environments.

Importance of Protists

• Medical Importance: Some protists cause human diseases (e.g., Giardia, amoebic dysentery, malaria).

• Ecological Importance: Photosynthetic protists are major primary producers in aquatic ecosystems, fixing atmospheric CO2 into sugars. Many are part of plankton and some produce toxins that can accumulate in food chains (e.g., domoic acid).

• Evolutionary Importance: Studying protists helps us understand the evolution and diversification of plants, fungi, and animals.

Slime Molds

Slime molds exemplify the complexity of protist life cycles, alternating between sexual (diploid) and asexual (haploid) reproduction.

Origin of Mitochondria and Chloroplasts

Endosymbiotic Hypothesis

The endosymbiotic hypothesis proposes that mitochondria and chloroplasts originated from free-living bacteria engulfed by ancestral eukaryotic cells. Over time, these bacteria became permanent organelles within the host cell.

• Mitochondria and chloroplasts are characteristic of eukaryotic cells.

• Peptidoglycan is found in bacterial cell walls, not in eukaryotic algae or plants, and helps distinguish these groups.

Green Algae and Land Plants

Ecological and Societal Importance of Plants

• Produce oxygen via photosynthesis.

• Form the base of terrestrial food webs.

• Provide food, materials (cotton, wood, paper), and pharmaceuticals.

• Store carbon and influence global carbon cycling.

• Affect soil stability, water movement, and ecosystem processes.

• Humans have altered plants through artificial selection and domestication.

The Transition from Water to Land

Evolutionary Challenges and Adaptations

Transitioning from aquatic to terrestrial environments required several key adaptations:

• Minimizing water loss: Adaptations to reduce water loss while allowing gas exchange.

• Structural support: Development of support structures to resist gravity (e.g., lignin and tracheids in vascular tissue).

• UV protection: Mechanisms to minimize damage from increased ultraviolet radiation.

• Reproduction without water: Adaptations to protect gametes and embryos from desiccation (e.g., amniotic egg in animals, protective structures in plants).

Key Plant Adaptations

• Lignin: A tough polymer providing rigidity and waterproofing vascular tissues.

• Tracheids: Long, tapered water-conducting cells in the xylem of vascular plants.

These adaptations allowed plants to grow vertically and compete for sunlight.

Where Did Land Plants Come From?

Shared Traits of Green Algae and Land Plants

• Chlorophyll a and chlorophyll b

• Carotenoids

• Similar thylakoids (internal membranes for photosynthesis)

• Similar cell walls, sperm, and peroxisomes

Thylakoids

Internal membranes in chloroplasts where the light reactions of photosynthesis occur, producing ATP and NADPH.

Peroxisomes

• Membrane-bound organelles containing enzymes for metabolism and detoxification.

• Break down harmful compounds and produce hydrogen peroxide.

• Participate in photorespiration and manage reactive oxygen species.

Major Groups in Plant Evolution

Alternation of Generations

Life Cycle Overview

All land plants alternate between two multicellular stages: the haploid gametophyte and the diploid sporophyte. This process is called alternation of generations.

Basic Cycle

1. The diploid sporophyte (2n) produces haploid spores (n) by meiosis.

2. Spores divide by mitosis to form multicellular haploid gametophytes (n).

3. Gametophytes produce haploid gametes (n) by mitosis.

4. Two gametes fuse during fertilization, forming a diploid zygote (2n).

5. The zygote divides by mitosis to become a multicellular diploid sporophyte (2n).

Spores vs. Gametes vs. Zygotes

Key distinction: Spores grow; gametes fuse.

Gametophyte vs. Sporophyte Dominance

• Mosses: Gametophyte dominant

• Ferns, Gymnosperms, Angiosperms: Sporophyte dominant

The dominant generation (most conspicuous and long-lived) varies among plant groups.

Main Ideas to Know

• Eukaryotes share features such as a nuclear envelope and complex cellular organization.

• Protists are diverse, paraphyletic, and important medically, ecologically, and evolutionarily.

• Land plants evolved from green algae and required adaptations for terrestrial life.

• Vascular tissue enabled larger plant bodies and improved transport.

• Dependence on external water for fertilization decreases across plant evolution.

• All land plants alternate between multicellular haploid gametophytes and diploid sporophytes.

• Sporophytes produce spores by meiosis; gametophytes produce gametes by mitosis.

• Mosses are gametophyte dominant; ferns, gymnosperms, and angiosperms are sporophyte dominant.