25.2 Green Algae: Precursors of Land Plants
3 min read•june 14, 2024
Green algae and land plants share key traits that hint at their common ancestry. Chloroplasts, cellulose cell walls, and starch storage are hallmarks of these photosynthetic organisms. Their life cycles alternate between haploid and diploid stages, with flagellated sperm for aquatic reproduction.
, a group of algae, are the closest living relatives of land plants. They share features like oogamous reproduction, phragmoplasts, and . DNA analysis has confirmed this relationship, revealing the evolutionary journey from water to land and the adaptations that made it possible.
Green Algae and Land Plants
Traits of algae and land plants
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- Chloroplasts containing chlorophyll a and b pigments enable photosynthesis (green color)
- Originated from endosymbiotic event where ancestral eukaryotic cell engulfed cyanobacteria
- Cell walls made of cellulose provide structural support and protection
- Some land plants also incorporate for additional strength
- Starch stored in plastids serves as energy reserve polysaccharide (glucose polymer)
- life cycle with distinct haploid gametophyte and diploid sporophyte stages
- Gametophyte produces gametes (sperm and eggs) for sexual reproduction
- Sporophyte produces spores that develop into new gametophytes
- Flagellated sperm cells swim through water to reach eggs for fertilization (aquatic reproduction)
Charophytes as plant relatives
- Multicellular thalli with branching filaments resemble early land plant morphology
- Oogamous sexual reproduction involves large, non-motile eggs and smaller, flagellated sperm (similar to bryophytes)
- forms during cell division guides cell plate formation (also in land plants)
- Microtubule structure that separates daughter cells
- connections allow communication and transport between cells ()
- Contain chlorophyll a, b and carotenoid pigments in plastids (like land plants)
- Cell walls contain cellulose, pectins and hemicelluloses (similar composition to land plants)
- Share many genetic markers and have closely related gene sequences
- Phylogenetic analyses consistently place charophytes as sister group to land plants
DNA analysis in plant evolution
- Morphological traits can be misleading due to convergent evolution (similar adaptations in unrelated organisms)
- Comparing DNA sequences of nuclear, chloroplast and mitochondrial genomes provides evolutionary insights
- Phylogenetic trees constructed from molecular data reveal evolutionary relationships and distances between taxa
- Monophyletic groups consist of an ancestor and all its descendants (natural classification)
- DNA evidence resolves uncertainties in plant classification based on morphology alone
- Confirms charophytes as closest living relatives of land plants (consistent with other evidence)
- Molecular clock analyses estimate timing of divergence events and origins of key adaptations
- Transition to terrestrial habitats
- Development of vascular tissues, seeds, flowers, etc.
- Advances in sequencing technologies and bioinformatics continue to reshape our understanding of plant evolution and diversity
Adaptations for terrestrial life
- : Waxy layer on plant surfaces that reduces water loss
- : Pores in plant surfaces that regulate gas exchange and water loss
- Reproductive structures:
- : Female reproductive organs that produce and protect eggs
- : Male reproductive organs that produce sperm
- : Structures that produce and contain spores
- : General term for reproductive organs that produce gametes
Key Terms to Review (26)
Alternation of generations: Alternation of generations is a biological life cycle that occurs in plants and some algae, involving a switch between a haploid gametophyte stage and a diploid sporophyte stage. This cycle allows for genetic diversity and adaptation to different environments, with each generation playing distinct roles in reproduction and survival.
Antheridia: Antheridia are the male reproductive structures found in various plants, especially in non-flowering species. They are responsible for producing and releasing sperm cells for fertilization. Antheridia play a critical role in the reproduction of early plant life, connecting the evolution of green algae to the development of land plants and influencing the life cycles of bryophytes and seedless vascular plants.
Archegonia: Archegonia are the female reproductive structures found in certain plants, specifically non-flowering plants like bryophytes and seedless vascular plants. They play a crucial role in the reproduction process by producing and housing the egg, which is fertilized by sperm during sexual reproduction. Archegonia are typically flask-shaped and can be found on gametophytes, which are the dominant stage in the life cycles of these plants.
Chara: Chara is a genus of green algae that resembles higher plants and is often considered a close relative to land plants. It is characterized by its filamentous structure, complex life cycle, and the presence of specialized reproductive structures called gametangia. Chara plays a significant role in understanding the evolutionary transition from aquatic to terrestrial environments due to its morphological and biochemical similarities to land plants.
Charophytes: Charophytes are a group of green algae that are closely related to land plants and are considered their most direct ancestors. These freshwater organisms share several key traits with land plants, such as the structure of their chloroplasts, the formation of a phragmoplast during cell division, and the presence of sporopollenin in their reproductive structures. This connection highlights their significant role in the evolutionary transition from aquatic to terrestrial life.
Chlorophytes: Chlorophytes are a group of green algae belonging to the phylum Chlorophyta, which play a critical role as precursors to land plants. These organisms are primarily aquatic and exhibit a range of forms, from unicellular to multicellular, and they are known for their chlorophyll a and b pigments, which allow them to perform photosynthesis efficiently. Their similarities with land plants in cellular structure and reproductive strategies provide insights into the evolutionary transition from aquatic to terrestrial life.
Coleochaete scutata: Coleochaete scutata is a green alga belonging to the Coleochaetophyceae class, known for its significance as a model organism in understanding the evolutionary transition from aquatic to terrestrial life. This species exhibits characteristics similar to land plants, such as the presence of a multicellular structure and a unique method of reproduction, highlighting its role as a crucial link in the evolution of land plants.
Cuticle: A cuticle is a protective, non-cellular layer found on the outer surface of both plants and invertebrates. In plants, it minimizes water loss; in arthropods, it forms part of the exoskeleton providing structural support and protection.
Cuticle: The cuticle is a protective, waxy layer that covers the surface of leaves and stems in many plants, helping to reduce water loss and protect against environmental stress. This adaptation is crucial for survival in terrestrial environments, as it aids in preventing desiccation and provides a barrier against pathogens.
Embryo retention: Embryo retention refers to the process where fertilized eggs or embryos are kept within the parent organism for a period of time instead of being immediately released into the environment. This adaptation provides a protective environment for developing embryos, allowing them to mature in a safe space, which is a significant evolutionary step seen in the transition from aquatic to terrestrial life.
Flagella: Flagella are long, whip-like structures that protrude from the surface of certain cells, primarily used for movement. These appendages are essential for many organisms, providing them with the ability to swim through liquids, and they play a crucial role in the biology of various protists and algae.
Gametangia: Gametangia are specialized organs or cells in which gametes are formed in plants, algae, and fungi. They play a crucial role in the sexual reproduction of seedless plants.
Gametangia: Gametangia are specialized reproductive structures found in certain organisms that produce gametes, which are the sex cells involved in sexual reproduction. These structures can be found in fungi and green algae, highlighting their crucial role in the life cycles of these organisms. In fungi, gametangia can vary in form and function, while in green algae, they serve as precursors to the more complex reproductive systems seen in land plants.
Lignin: Lignin is a complex organic polymer found in the cell walls of many plants, providing rigidity and structural support. It plays a crucial role in the evolution of land plants by enabling them to grow taller and thrive in diverse environments, as it contributes to the strength of vascular tissues and protects against pathogens and decay.
Oogamy: Oogamy is a form of sexual reproduction characterized by the production of two distinct types of gametes: a large, non-motile egg and a smaller, motile sperm. This reproductive strategy is significant as it facilitates the fusion of different gamete sizes, promoting genetic diversity and adaptability in certain organisms, particularly among some fungi and green algae.
Phragmoplast: The phragmoplast is a plant cell structure that forms during cytokinesis, the final stage of cell division, specifically in the formation of the cell plate. This structure consists of microtubules and is crucial for guiding the assembly of new cell walls between dividing daughter cells. In green algae, which are precursors to land plants, the presence of the phragmoplast highlights the evolutionary link between these groups and reflects their shared mechanisms of growth and division.
Plasmodesmata: Plasmodesmata are microscopic channels that traverse the cell walls of plant cells, facilitating communication and transport between them. These channels allow the movement of substances such as nutrients, signaling molecules, and genetic material.
Plasmodesmata: Plasmodesmata are microscopic channels that traverse the cell walls of plant cells, allowing for communication and transport of materials between adjacent cells. These structures play a crucial role in maintaining cellular activities by enabling the exchange of ions, nutrients, and signaling molecules, thereby facilitating the coordination of physiological processes across plant tissues.
Pyrenoid: A pyrenoid is a specialized structure found within the chloroplasts of certain algae, including green algae, that plays a critical role in the storage and metabolism of starch. This organelle is involved in photosynthesis and is crucial for carbon fixation, allowing algae to efficiently convert sunlight into energy. The presence of pyrenoids in green algae highlights their evolutionary significance as precursors to land plants, showcasing adaptations for life in varied environments.
Rhizoids: Rhizoids are root-like structures found in non-vascular plants that anchor the plant to the substrate and help in the absorption of water and nutrients. Unlike true roots, rhizoids do not have vascular tissues and primarily serve as stabilizers for plants like mosses and liverworts. They play a crucial role in the early adaptations of plants moving from aquatic to terrestrial environments.
Spirogyra: Spirogyra is a genus of green algae characterized by its filamentous structure and spiral arrangement of chloroplasts within its cells. This unique arrangement allows for efficient photosynthesis, making Spirogyra an important contributor to aquatic ecosystems and a key player in the evolutionary transition from algae to land plants.
Sporangia: Sporangia are specialized structures that produce and contain spores, which are crucial for the reproduction and dispersal of various plant and fungal species. These structures can be found in different life cycles, playing a vital role in the formation of spores that facilitate both sexual and asexual reproduction, depending on the organism's needs.
Stomata: Stomata are small openings found on the surfaces of leaves and stems that allow for gas exchange between the plant and its environment. They play a crucial role in regulating photosynthesis, respiration, and transpiration, influencing how plants interact with their surroundings and manage water loss.
Symplastic pathway: The symplastic pathway refers to the route taken by water and solutes through the cytoplasm of plant cells, interconnected via plasmodesmata. This pathway allows for the efficient movement of materials within plant tissues, crucial for maintaining cellular function and supporting physiological processes. In the context of green algae and the transition to land plants, the symplastic pathway illustrates how early plants adapted to nutrient uptake and water transport in a terrestrial environment.
Thallus: A thallus is a simple, undifferentiated body structure found in some plants, particularly algae, that does not have true stems, leaves, or roots. This structure enables organisms like green algae to adapt and thrive in aquatic environments, showcasing how early plant forms may have evolved before more complex structures developed in land plants.
Zygospores: Zygospores are a type of thick-walled, resting spore produced by certain fungi, particularly within the Zygomycota phylum, during sexual reproduction. These structures are crucial for survival during unfavorable conditions and contribute to the genetic diversity of the fungal population through the fusion of specialized hyphae from different mating types.