Photosynthesis, Endosymbiosis, and Land Plant Evolution: Key Concepts and Structures

Photosynthesis (overall equation)

6CO2 + 6H2O + sunlight → glucose + 6O2; converts light energy into chemical energy stored in sugar

Light Reactions

The part of photosynthesis where membranes harvest light energy to synthesize ATP and NADPH

Cyanobacteria

Ubiquitous photosynthetic prokaryotes (no nucleus, circular chromosome); first organisms to perform oxygenic photosynthesis

Oxygenic photosynthesis

The typical, still-prevalent form of photosynthesis that releases O2 as a byproduct; cyanobacteria were the first organisms to do this

Nitrogen fixation

Cyanobacterial process converting N2 gas into NH4, NO2−, and NO3− ("fertilizer"), needed to make amino acids and nucleic acids

Heterocysts

Specialized cyanobacterial cells where nitrogen fixation occurs

Great Oxidation Event

The period (~2.4 billion years ago) when free O2 began accumulating significantly in Earth's atmosphere, largely due to cyanobacterial photosynthesis

Endosymbiosis

The process by which one cell engulfs another, and the engulfed cell becomes a permanent, integrated organelle

Primary endosymbiosis

A eukaryotic host cell engulfed a cyanobacterium, which became the chloroplast; occurred once in the ancestor of red algae, green algae, and glaucophytes

Secondary endosymbiosis

A eukaryote engulfed another eukaryote that already had a primary chloroplast (from primary endosymbiosis); seen in groups like euglenoids, diatoms, cryptomonads, haptophytes

Tertiary endosymbiosis

Repeated/serial capture of chloroplasts from different secondarily-derived algae, as seen in some dinoflagellates

Nucleomorph

A remnant/vestigial nucleus of an engulfed algal cell, retained in some secondary endosymbionts (e.g., cryptomonads)

Peptidoglycan (in chloroplast evolution)

Cell wall material from the ancestral cyanobacterium; lost in most plant lineages but retained in glaucophytes, and recently also found in mosses

Plastid

General term for a chloroplast or related organelle derived from endosymbiosis

Plantae (phylogenetic definition)

The clade of eukaryotes whose chloroplasts came from primary endosymbiosis; includes glaucophytes, red algae, green algae, and land plants

Clade

An ancestor plus ALL of its descendants (a monophyletic group)

Viridiplantae

"Green plants"; the clade with chlorophyll a AND b and starch storage, includes Chlorophytes and Streptophytes

Chlorophytes

"Typical green algae" with chlorophyll a & b and starch storage; mostly aquatic (marine/freshwater); e.g., Ulva, Volvox

Streptophytes

Clade containing some green algae plus all land plants; synapomorphies include phragmoplasts, plasmodesmata, and (in many lineages) oogamy and apical growth

Chlorophyll a

Photosynthetic pigment found in cyanobacteria, red algae, and all green plants

Chlorophyll b

Additional photosynthetic pigment found only in green plants (Viridiplantae), not red algae or cyanobacteria

Starch

Main carbohydrate storage molecule in green plants, made of 2 main polysaccharide types

Floridean starch

Storage carbohydrate made by red algae; similar to starch but lacks one of the two main polysaccharides

Phycoerythrin

Red pigment in red algae that absorbs green and blue light

Coralline red algae

Red algae with calcium carbonate (CaCO3) in their cell walls; important reef-builders and among the oldest eukaryote fossils (~600 Ma)

Phycoplast

Structure in Chlorophytes where microtubules form parallel to the plane of cell division

Phragmoplast

Structure in most Streptophytes where microtubules/ER assemble perpendicular to the future cell plate, acting as a scaffold for outward cell growth

Plasmodesmata

Channels that penetrate cell walls, connecting the cytoplasm of adjacent plant cells

Parenchyma

The basic tissue type in streptophytes, made of cells connected by plasmodesmata

Isogamy

Sexual reproduction where the two gametes look identical (no size/motility distinction)

Oogamy

Sexual reproduction with differentiated gametes: a large, non-motile egg and small, motile, flagellated sperm

Apical growth

Growth concentrated at the tips (apex) of branches; may have evolved more than once or been lost multiple times in streptophyte algae

Chara (stonewort)

A streptophyte green alga historically considered sister to land plants due to shared traits like apical growth; life cycle is primarily haploid

Zygnematophyceae

Group of "other green algae" (e.g., desmids, Spirogyra) now considered by some recent phylogenies to be the closest algal relatives of land plants

Embryophytes

Another name for land plants within the streptophyte clade

Three Domains of Life

Bacteria, Archaea, and Eukarya — the three major divisions of life based on cell structure

Asgard archaea

A group of archaea (e.g., Lokiarchaeota) now understood to be the closest prokaryotic relatives of eukaryotes

Cuticle

A waxy coating (lipid/hydrocarbon polymer) on the epidermis of leaves and shoots that slows water loss; made exclusively by epidermal cells

Sporopollenin

An extremely chemically stable biopolymer forming the tough outer (exine) wall of spores and pollen, protecting against drying out and decay

Stomata

Closable openings (most lineages) that regulate gas exchange, balancing CO2 uptake against water loss through transpiration

Accessory pigments

Pigments like chlorophyll b and carotenoids that absorb light at wavelengths chlorophyll a doesn't, boosting photosynthetic efficiency

Flavonoids

Pigments that act as a natural sunscreen, absorbing UV radiation to protect the plant

Mycorrhizae

A mutually beneficial association between plant roots and fungi that promotes nutrient uptake from soil

Ectomycorrhizal (EM) fungi

Fungi that wrap around the outside of roots, increasing surface area for water and mineral absorption

Vesicular-arbuscular (VA) mycorrhizal fungi

Fungi that grow into root cells and exchange water and nutrients directly, forming structures called arbuscules

Gametangia

Organs that enclose gametes and prevent them from drying out (singular: gametangium)

Antheridium

The gametangium that produces motile sperm

Archegonium

The gametangium that produces the egg cell and houses the embryo after fertilization

Embryo (land plants)

The young, multicellular diploid sporophyte, protected inside the archegonium; defines the land plant clade "Embryophyta"

Alternation of generations

A life cycle with both a multicellular haploid stage (gametophyte) and a multicellular diploid stage (sporophyte); a synapomorphy for land plants

Gametophyte

The multicellular haploid (n) stage of the life cycle; produces gametes by mitosis inside gametangia

Sporophyte

The multicellular diploid (2n) stage of the life cycle; develops from the embryo and produces spores by meiosis in sporangia

Sporangium

A spore-producing structure on the sporophyte, where meiosis occurs to generate haploid spores (plural: sporangia)

Haplodiplontic life cycle

A life cycle with both multicellular haploid and multicellular diploid stages, characteristic of land plants (contrast: diplontic in humans, haplontic in algae like Chara)

Embryophyta

The scientific name for the land plant clade, named for the shared trait of protecting an embryo

Apical growth

Growth occurring at the tips of roots/shoots, allowing elongation and branching; found in land plants and some streptophyte algae

Clade

A group consisting of a common ancestor and all of its descendants (monophyletic group)

Bryophytes

Nonvascular land plants (e.g., mosses) with conspicuous, visible haploid and diploid life stages

Zygote

The diploid (2n) cell formed when a haploid sperm fertilizes a haploid egg

Isogamy

Fusion of gametes that are similar in size and form (seen in some Zygnematophyceae/"other green algae")

Oogamy

Fusion of gametes that differ in size/form -- a large, immobile egg fertilized by a small, motile sperm

Bryophytes

Non-vascular land plants; includes three lineages: hornworts, liverworts, and mosses

Alternation of generations

Life cycle with both multicellular haploid (gametophyte) and diploid (sporophyte) stages; a synapomorphy for the land plant clade

Gametophyte

The haploid (n) multicellular stage of the life cycle; produces gametes by mitosis; the dominant, free-living generation in bryophytes

Sporophyte

The diploid (2n) multicellular stage of the life cycle; produces spores by meiosis; in bryophytes it remains attached to and dependent on the gametophyte

Synapomorphy

A shared derived trait used to define a clade (e.g., alternation of generations for land plants)

Antheridium

Structure on the gametophyte that produces sperm

Archegonium

Structure on the gametophyte that produces the egg and is where fertilization and early sporophyte development occur

Sporangium

Structure on the sporophyte where meiosis occurs and spores are produced

Zygote

The diploid cell formed when gametes fuse during fertilization; develops into the sporophyte

Spores

Haploid cells produced by meiosis in the sporangium; germinate to form the gametophyte

Osmosis (in bryophytes)

The undirected way bryophytes move water around their body, since they lack vascular tissue

Diffusion (in bryophytes)

How minerals are distributed throughout the small bodies of bryophytes, in the absence of a vascular system

Desiccation tolerance

Bryophyte strategy of surviving water loss (can lose >95% of cellular water) rather than avoiding it, then rapidly reviving upon rehydration

LEA proteins

"Late Embryogenesis Abundant" proteins produced during drying that stabilize cellular components and prevent damage to membranes and DNA

Monophyletic bryophytes

The hypothesis, increasingly supported by transcriptome-scale studies (2019, 2022), that hornworts, liverworts, and mosses together form a single clade

Paraphyletic bryophytes

The older view (e.g., Wickett et al. 2014, based on nuclear genes) that bryophytes are not a single clade but a grade of separately branching lineages

Setaphyte hypothesis

The hypothesis that mosses and liverworts together form a natural group, supported by similarities in sperm morphology (Renzaglia et al. 2018)

Hornworts

Bryophyte lineage (~100 species) with horn-shaped sporophytes, one chloroplast per cell, a basal sporophyte meristem, permanently open stomata, and cyanobacterial symbionts

Hornwort sporophyte meristem

Basal region of the hornwort sporophyte capable of indefinite cell division/growth

Cyanobacteria symbiosis (hornworts)

Nitrogen-fixing cyanobacteria that live in internal cavities of hornworts in a symbiotic relationship

Mosses

Bryophyte lineage (~15,000 species) with erect leafy gametophytes, hydroids, an unbranched sporophyte with a single capsule, and peristome teeth

Hydroids

Water-conducting cells in mosses that die and leave behind a channel through which water can move

Peristome teeth

Hygroscopic (humidity-responsive) tooth-like structures at the mouth of the moss capsule that aid gradual spore dispersal

Calyptra

Protective cap covering the developing sporangium (capsule) of a moss sporophyte

Seta

The stalk portion of a bryophyte sporophyte connecting the capsule to the gametophyte

Sphagnum

Genus of peat moss; creates and maintains peatlands, is a major global carbon sink, and has antimicrobial/preservative properties

Peatland

Acidic, cold, anaerobic wetland ecosystem maintained by Sphagnum that resists decay and stores an estimated 400 gigatons of carbon

Liverworts

Bryophyte lineage (~9,000 species) with flat leaf-like gametophytes, apical growth, no internal water conduction, no true stomata, and elaters

Gemmae

Asexual propagules produced by liverworts in splash cups for asexual reproduction

Elaters

Hygroscopic structures in liverwort sporangia that flex in response to humidity to assist spore dispersal

Hygroscopic

Describes a structure that changes shape in response to humidity (e.g., peristome teeth, elaters)

Flagellated sperm

Swimming, flagella-bearing sperm produced by bryophytes; requires water for fertilization to occur

Tracheophytes

Vascular plants; the clade of land plants with a sporophyte-dominant life cycle, tracheids, branching independent sporophytes, and true roots

Sporophyte-dominant life cycle

Life cycle in which the diploid (2n) sporophyte is the large, long-lived, independent phase — defines vascular plants (opposite of bryophytes, which are gametophyte-dominant)

Tracheid

Long, tapered, lignified xylem cell that is dead at maturity; transports water and minerals and provides structural support; a synapomorphy of tracheophytes

Phloem

Vascular tissue made of living cells that transports photosynthetic products (sugars)

Microphyll

Small leaf with a single, unbranched midvein; found in lycophytes, arranged spirally on the stem

Megaphyll

Leaf with a branched vein system; found in euphyllophytes; evolved from overtopped, flattened, and webbed branch systems

Sporangium

Structure/organ in which spores are produced by meiosis