Photosynthesis: Light Reactions (For UG students)

 

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Nitrogen Metabolism

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Biological Nitrogen Fixation

 

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 Enzyme

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Metabolism 

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Metabolism is the sum of all chemical transformations taking place in a cell or organism.

It occurs through a series of enzyme-catalyzed reactions that constitute metabolic pathway.

In this multistep sequence, the product of one step serves as the substrate for the next step. These metabolic intermediates are called metabolites.

Multiple metabolic pathways remain interconnected to form a meaningful well-coordinated network that determines the overall functioning of the cell.

Complexity of metabolic pathways as depicted in KEGG: Kyoto Encyclopedia of Genes and Genomes

 

             

Catabolism

• The degradative phase of metabolism 
• Organic nutrient molecules (carbohydrates, fats, and proteins) are converted into smaller, simpler end products (such as lactic acid, CO2, NH3) 
• Release energy:
• A part is conserved in the form of ATP and reduced electron carriers (NADH, NADPH, and FADH2)
• The rest is lost as heat

Anabolism

• The biosynthetic phase of metabolism
• Small, simple precursors are built up into larger and more complex molecules (lipids, polysaccharides, proteins, and nucleic acids)
• Require input of energy (phosphoryl group transfer potential of ATP and reducing power of NADH, NADPH, and FADH2)

Some pathways can be either anabolic or catabolic, depending on the energy conditions in the cell. They are referred to as amphibolic pathways.

Example: Citric acid cycle

Metabolic pathways may linear, branched or cyclic.

Catabolic pathways are convergent and anabolic pathways divergent.

• Converging catabolism: convert several starting materials into a single product
• Diverging anabolism: yielding multiple useful end products from a single precursor
• Cyclic pathway: one starting component is regenerated in a series of reactions that converts another starting component into a product

Metabolic processes are regulated in three principal ways:

• by controlling the accessibility of substrates
• by controlling the amounts of enzymes
• by controlling their catalytic activities

Controlling the accessibility of substrates

• At low substrate concentration (near or below Km), rate of reaction depends on substrate concentration.
• Controlling the flux of substrates also regulates metabolism. 
• The transfer of substrates from one compartment of a cell to another (e.g., from the cytosol to mitochondria) can serve as a control point.
• Compartmentalization segregates opposed reactions. For example, fatty acid oxidation takes place in mitochondria, whereas fatty acid synthesis takes place in the cytosol. 

Controlling the amounts of enzymes

• The amount of a particular enzyme depends on:
• its rate of synthesis
• its rate of degradation
• The level of most enzymes is adjusted primarily by changing the rate of transcription of the genes encoding them.

Controlling their catalytic activities

The catalytic activity of enzymes is controlled by: 

• Reversible allosteric control: Key enzymes in many biosynthetic pathways is allosterically inhibited by a metabolic intermediate or coenzyme or the  ultimate product of the pathway. 
• Reversible covalent modification: Activation/deactivation of enzymes by phosphorylation/deposphorylation of particular residues.
• Hormones-mediated reversible modification of key enzymes: Hormones or growth factors coordinate metabolic relations between different tissues, often by regulating the reversible modification of key enzymes.

Classification of the Bryophytes (Goffinet, Buck and Shaw; 2008)

Mosses display a wide range of structural diversity from which relationships and lineages can be inferred. Historically, bryophyte systematics has focused on peristome teeth complexity and sex organ distribution to define taxonomic units.

Modern classifications reflect systematic concepts proposed by Fleischer (1920) and Brotherus (1924, 1925), wherein the peristomate mosses were classified as follows:

• Nematodontous
• Arthrodontous

Based on the position of perichaetia, arthrodontous mosses were sub-divided into:

• Acrocarpous 
• Pleurocarpous

Based on the architecture of the outer ring of peristome teeth, arthrodontous mosses were sub-divided into:

• Haplolepideous
• Diplolepideous

Sphagnum, Andreaea, Andreaeobryum, and Takakia represent additional groupings distinguished by the presence of a pseudopodium and the mode of sporangial dehiscence.

The classification of the Bryophyta is undergoing constant revisions, particularly in the light of phylogenetic inferences. The classification proposed by Goffinet, Buck, and Shaw (2008) builds on those presented by Buck & Goffinet (2000) and Goffinet & Buck (2004). Their classification is outlined below:

BRYOPHYTA

        SUPERCLASS I

                CLASS TAKAKIOPSIDA: Leaves divided into terete filaments; capsules dehiscent by a single longitudinal spiral slit; stomata lacking. Example: Takakia

Takakia sp.

        SUPERCLASS II

                        CLASS SPHAGNOPSIDA: Branches usually in fascicles; leaves composed of a network of chlorophyllose and hyaline cells; setae lacking; capsules elevated on a pseudopodium; stomata lacking. Example: Sphagnum

Sphagnum

        SUPERCLASS III

                CLASS ANDREAEOPSIDA: Plants on acidic rocks, generally autoicous; cauline central strand absent; calyptrae small; capsules valvate, with four valves attached at apex; seta absent, pseudopodium present; stomata lacking. Example: Andreaea

Andreaea

                CLASS ANDREAEOBRYOPSIDA: Plants on calcareous rocks, dioicous; cauline central strand lacking; calyptrae large and covering whole capsule; capsules valvate, apex eroding and valves free when old; stomata lacking; seta present. Example: Andreaeobryum

Andreaeobryum

        SUPERCLASS V

         CLASS OEDIPODIOPSIDA: Leaves unicostate; calyptrae cucullate; capsule symmetric and erect, neck very long; stomata lacking; capsules gymnostomous. Example: Oedipodium

Oedipodium

                    CLASS POLYTRICHOPSIDA: Plants typically robust, dioicous; cauline central strand present; stems typically rhizomatous; costa broad, with adaxial chlorophyllose lamellae; peristome nematodontous, mostly of (16)32–64 teeth. Example: Polytrichum

Polytrichum

                    CLASS TETRAPHIDOPSIDA: Leaves unicostate; calyptrae small conic; capsule symmetric and erect, neck short; peristome nematodontous, of four erect teeth. Example: Tetraphis

Tetraphis

                       CLASS BRYOPSIDA: Plants small to robust; leaves costate or not, typically lacking lamellae; capsules operculate; peristome at least partially arthrodontous. Example: Bryum

Bryum

Features of the classification system:

• The rank of superclass is adopted to unite all arthrodontous mosses in one taxon (i.e. Superclass V).
• Although this system of classification aimed at accepting only monophyletic taxa, given the limited number of ranks available, paraphyletic taxa were incorporated (e.g. Bryanae). 
• Effort was made to resolve the relationships among genera of the Hypnanae.
• Due to the lack of sequence data and availability of information of many taxa at regional level only, the authors have been unable to expand their classification system to a global scale.                                        

Further reading: 

Buck WR, Shaw AJ, Goffinet B. Morphology, anatomy, and classification of the Bryophyta. In: Bryophyte Biology: Second Edition, ed. B. Goffinet & A. J. Shaw. Cambridge University Press; 2008. p. 55–138.

Economic Importance of Bryophyte

Medicinal Importance of Bryophytes

Bryophytes have been found to possess medicinal properties, making them a valuable resource for traditional and modern medicine.

Antibacterial and Antiviral Properties:

Many bryophyte species, such as Sphagnum, Marchantia, and Conocephalum, have been found to exhibit antibacterial and antiviral activities. These properties make them useful for treating bacterial infections.

Antifungal and Antiparasitic Properties:

Bryophytes, such as Polytrichum and Neckera, have been found to possess antifungal and antiparasitic properties. These properties make them useful for treating fungal infections and parasitic diseases.

Anti-Inflammatory and Antioxidant Properties:

Many bryophyte species, such as Marchantia and Conocephalum, have been found to exhibit anti-inflammatory and antioxidant activities and are used to treat inflammatory diseases and prevent oxidative stress.

Wound Healing and Surgical Dressings:

Sphagnum has been used for centuries as a wound dressing and surgical material due to its high absorptive power and antiseptic properties. Other bryophytes, such as Polytrichum, have also been found to promote wound healing.

Treatment of Respiratory Diseases:

Marchantia, Neckera, and Polytrichum has been traditionally used to treat respiratory diseases, such as pulmonary tuberculosis and bronchitis. Conocephalum has also been found to exhibit anti-inflammatory and antioxidant activities that may help alleviate respiratory diseases.

Treatment of Liver and Kidney Diseases:

Polytrichum has been traditionally used to treat kidney and gall bladder stones. Other bryophytes, such as Marchantia, have also been found to exhibit hepatoprotective and nephroprotective activities.

Anticancer Properties:

Some bryophyte species, such as Conocephalum, have been found to exhibit anticancer properties, although more research is needed to confirm these findings.

Active component of medicinally important bryophytes:

Marchantia: marchantin A

Polytrichum: polytrichumol

Conocephalum: conocephalumol

Neckera: neckeral

Sphagnum: sphagnol

Importance of Bryophytes in Ornamental Purposes

Bryophytes are gaining popularity for their unique textures and colors, making them ideal for ornamental purposes. Bryophytes can be used in a range of ornamental applications, including:

• Terrariums and vivariums
• Floral arrangements and bouquets
• Ground cover and landscaping
• Indoor and outdoor containers

Key Benefits:

• Unique textures and colors
• Low maintenance
• Versatile
• Sustainable and eco-friendly
• Air purification

Examples of Ornamental Bryophytes

Sphagnum

Hypnum

Polytrichum

Rhytidiadelphus

Bryophytes in Peat Formation

Bryophytes, especially Sphagnum, play a crucial role in peat formation, which has significant economic and environmental importance.

Fuel Source: Peat is used as a fuel source, particularly in power plants and for domestic heating.

Horticulture: Peat is used as a soil amendment in horticulture, due to its high water-holding capacity and acidity.

Other Uses: Peat is also used in water filtration, as a component in potting mixes, and in the production of activated carbon.

Bryophytes as a Food Source

Bryophytes, including mosses, liverworts, and hornworts, are a vital food source for various animals and humans.

Animal feed:

Deers, reindeers, caribou, musk ox, arctic geese, lemmings, rodents, birds like such as grouse and ptarmigan and insects like springtails and beetles feed on mosses and liverworts, particularly during winter when other food sources are scarce.

Human Consumption:

• Traditional Medicine: In some cultures, bryophytes are used in traditional medicine to treat various ailments.
• Food Source: In some parts of the world, like Japan and Hawaii, mosses are considered a delicacy and are eaten raw or cooked.
• Supplements: Some bryophytes, like Sphagnum, are used as dietary supplements due to their high nutritional value.

Nutritional Value: Bryophytes are rich in fiber, protein and vitamins and minerals making them a valuable food source.

Bryophytes as Indicators

Bryophytes can serve as early warning systems for environmental pollution and degradation. Hence, they are used to monitor environmental health and track changes over time.

Types of Indicators:

• Air Quality Indicators
• Water Quality Indicators
• Soil Quality Indicators

How Bryophytes Indicate Environmental Health

• Heavy Metal Accumulation: Bryophytes can accumulate heavy metals, such as lead and mercury, in their tissues, indicating pollution levels.
• pH Tolerance: Bryophytes can tolerate a wide range of pH levels, making them useful indicators of soil and water acidity.
• Sensitivity to Pollutants: Bryophytes are sensitive to pollutants, such as sulfur dioxide and ozone, making them useful indicators of air pollution.

Examples of Indicator Bryophytes:

• Sphagnum: Indicates acidic and oxygen-poor conditions.
• Polytrichum: Indicates dry and nutrient-poor conditions.
• Marchantia: Indicates moist and nutrient-rich conditions

Bryophytes as Packing and Construction Materials

Bryophytes have been used for centuries as packing and construction materials due to their unique properties.

Packing Materials:

• Dried Mosses: Dried mosses, such as Sphagnum, have been used as packing materials for fragile items, like glassware and electronics.
• Insulation: Bryophytes have been used as insulation materials in buildings, reducing heat loss and energy consumption.
• Shipping Live Materials: Bryophytes have been used to keep live materials, like plants and animals, moist and secure during shipping.

Construction Materials:

• Chinking Material: Bryophytes, like Sphagnum, have been used as chinking materials to seal gaps between logs in log cabins.
• Roofing Material: Bryophytes have been used as roofing materials, providing insulation and waterproofing.
• Wall Construction: Bryophytes have been used as a component in wall construction, providing insulation and structural support.
• Fire prevention: Nordic people used the aquatic moss Fontinalis antipyretica between the chimney and walls to prevent fires.

Benefits:

• Sustainable
• Eco-Friendly
• Insulation Properties
• Fire Resistance

Examples: Sphagnum, Polytrichum, Hypnum

Bryophyte as Absorbent Material

• A layer of Sphagnum is used in hiking boots for cushioning the foot and absorbing moisture and odor. Dry Sphagnum is used in diapers and in cradles to keep babies clean and warm.
• It is also used to make beddings, mattresses, cushions, and pillows by stuffing mosses into coarse linen sacks.

Research and Education

• Bryophytes are used as model organisms in scientific research, particularly in the fields of ecology, evolution, and developmental biology.
• They are also used in educational settings to teach students about plant biology, ecology, and conservation.

Role of Bryophytes in Ecology and Plant Succession

The ability of bryophytes to stabilize soil, to trap and hold moisture, to exchange cations, and to tolerate desiccation together with their high totipotency makes them a very important contributor in plant succession and ecology.

Succession is an orderly sequence of changes in an open habitat in which organisms interact with the environment for development of a stable and climax community.

Plant succession taking place on a particular habitat is called a sere, its various intermediate stages are called seral stages, and the communities representing the stages are called seral communities.

The importance of bryophytes in ecology and plant succession is outlined below: 

As aids in soil conservation: 

Mosses prevent sheet erosion of soil. They grow densely forming a mat or carpet-like structure. It bears the impact of the falling raindrops, holds much of the rainwater, and thus the amount of run-off water is considerably reduced. The intertwined moss stems and the underground rhizoids firmly binds the soil particles together to a considerable depth of around 6 to 8 inches. This minimizes soil erosion even on steep hill sides.

Formation of soil and vegetation cover: 

In a xeric succession which begins on a bare rock surface the bryophytes, mosses in particular, form an important seral community. The bryophytes and the lichens in the primary succession led to soil formation through accumulation of organic matters and prepare rocky out crops for secondary succession of herbs, shrubs, and trees. The pioneer community is the crustose lichen and followed by the first seral community, the folios lichen. Acid secreted by lichens attack the rock and provide bits of soil. Additional soil particles maybe formed by weathering or be blown in from somewhere else. Thus, the accumulation of soil particles, organic matter, and humus results in the development of a fine soil layer over the underlying rock. This favors the entrance and colonization of mosses, thus beginning the moss stage.

Funaria hygrometrica is the first to colonize the high pH and high potash substratum. It is followed by Polytricum juniperinum, P. piliferum, Ceratodon sp., Tortula sp., and Grimmia sp. These mosses have an added advantage due to presence of minute green leaf-like structures with accelerated photosynthesis and presence of slender multicellular branched rhizoids that compete with folios lichens for absorption of water and nutrients. The increased rate of photosynthesis and absorption produces organic matter at higher pace thus accelerating the soil formation process.

Bog succession: 

Mosses play an important role in bog succession from open water to climax forest. Moses, like Sphagnum, grow over the water surface with their intertwined stems forming a thick mat together with the dead and partiality decayed gametophytic tissue. This provides a substratum for the germination of seeds or propagules of many floating and rooted hydrophytes. In course of time the partially decomposed Sphagnum and the hydrophytes form a dense surface covering over the water forming the so-called quacking bogs. Later bogs are converted into swamps capable of supporting relatively large trees and eventually such swamps are replaced by forest growth. The 'feather mosses' like Hylocomium splendens and Thuidium delicatulum, serve as seed beds for native forest trees and wildflowers.

Rock builders: 

Certain mosses growing in association with other aquatic plants contain large amount of calcium carbonate. The plants bring about decomposition of bicarbonate ions by abstracting free carbon dioxide. The insoluble calcium carbonate precipitates and hardens, forming calcareous rock like deposits. This deposition grows and extends over areas of several 100 square feet. This travertine rock deposits are extensively used as building stone. 

Peatland ecology:

Bryophytes play an important role information of peatland. The factors for the development of peat as follows:

Nutrient absorption capacity: Nitrates and phosphate are the limiting factors in peat formation. Bryophytes can absorb nitrates and phosphate very easily then other vascular plants. 

Water holding capacity: Bryophytes can absorb water and hold it for longer period of time. Some morphological structures help them in this purpose. This enables continuous photosynthetic activity even in the dry season. 

Decomposition: In peat and, growth rate of peat moses is greater than their decomposition rate. This happens only if there is oligotrophication.

Acidification: Some peat moses like Sphagnum secret uronic acids. Protons are liberated from the uronic acid resulting in acidification of the environment.

Water Retention and Purification:

Bryophytes have the ability to absorb and retain large amounts of water, helping to regulate water cycles. They also aid in purifying water by filtering out impurities and excess nutrients.

Unifying Features of Archegoniates

Archegoniates are a group of non-vascular and vascular plants that reproduce via archegonia, which are specialized female reproductive structures that produce and house egg cells. This group includes:

• Mosses (Bryophyta)
• Liverworts (Marchantiophyta)
• Ferns (Polypodiophyta)
• Most gymnosperms (e.g., conifers, cycads)

Archegoniates are considered among the most ancient plant groups, with fossil records dating back to the Devonian period (around 416 million years ago).

Structure of the Archegonium:

• It is a flask-shaped structure.
• It consists of a neck and a swollen base called a venter.
• The venter contains the egg.
• The neck protrudes above the surface of the gametophyte.
• It is composed of four-tier of cells that form an opening called the neck canal.
• The neck canal cells form a passage for the entry of sperms.
• The neck canal cells on maturity, sometimes lyse to provide an easy entry for the sperm.

Structure of archegonium

Archegoniates, a diverse group of plants, share several key characteristics that highlight their evolutionary adaptations and biological significance.

Ancestral Origin and Shared Features

• Ancestral Origin: Likely originated from a monophyletic group of ancient aquatic green algae.
• Reproductive Structures: Possess distinct sexual organs – female archegonium and male antheridium.
• Photosynthetic Pigments: Contain chloroplasts with chlorophyll a, chlorophyll b, and carotene.
• Life Cycle: Exhibit a multicellular gametophytic and sporophytic generations with a heteromorphic alternation of generations.
• Embryo Protection: Provide protection to their embryos.

Adaptations for Terrestrial Life

• Motile Gametes: Male gametes are flagellated and motile in bryophytes and pteridophytes. Female gametes (eggs) are non-motile.
• Dependence on Water for Fertilization: Bryophytes and pteridophytes rely on fluid water for fertilization, while gymnosperms utilize a pollen tube.
Soil Exploration, Anchorage and Nutrient Uptake: Differentiated rhizoids and roots provided strong anchorage and efficient absorption of nutrients and water, supporting growth and development on land.
• Evolution of vasculature: Development of efficient vascular systems for transport of water and nutrients.
Land Adaptations: Developed mechanisms to regulate internal water and nutrient balance, reducing dependence on external environmental conditions.
Specialized Spore Dispersal: Developed diverse spore dispersal mechanisms, enhancing genetic variation and facilitating colonization of new terrestrial habitats.
Desiccation-Resistant Spores: Spores became increasingly resistant to water loss, particularly in seed plants, ensuring survival in dry environments.

Structural and Functional Innovations

• Photosynthetic Efficiency: Increased green surface areas enhanced chlorophyll availability and increased photosynthetic efficiency.
• Transpiration Mechanism: Developed transpiration to regulate internal temperatures.
• Vascular System: An efficient vascular system evolved to transport water and nutrients throughout the plant body.
• Waxy Cuticle and Stomata: Minimized water loss and regulated gaseous exchange.
• Structural Support: Differentiated tissues with thickened cell walls supported an erect growth habit.
• Spore Dispersal: Evolved efficient spore dispersal mechanisms to spread and colonize diverse habitats.

Origin of Alternation of Generations

In a life cycle, when the two generations: sexual (haploid) and asexual (diploid) generations alternate with each other, by the production of gametes and spores respectively, it is called alternation of generations. However, the origin of alternation of generations is unclear.

Some workers opine that the alternation of the sporophytic and gametophytic generations came into effect with the successful establishment of land habit and when transformation of the ancestral algae to the ancestral at archegoniates occurred. The other opinion states that it was already a well-established phenomenon in the algae, before the transformation of the archegoniates occurred.

There are two opposing theories for this phenomenon, both of which are supported by the data in the fossil records. These two theories are the antithetic theory and the homologous theory. In both the cases, the gametophytic phase is considered to have evolved earlier. So, the mode of origin of the sporophyte is the main issue of consideration.

The late-nineteenth/early-twentieth century debated over homologous versus antithetic alternation of generations. Supporters of both theories, at first, used Coleochaete as a model for the origin of land-plant life cycles. The early debate focused on the morphological interpretation of the sporophyte and on whether vascular cryptogams had bryophyte-like ancestors. The terms of the debate shifted after the discovery that the alternation of morphological generations was accompanied by an alternation of chromosome number (Haig, 2008).

Supporters of homologous alternation now promoted a model in which land plants had been derived from an algal ancestor with an isomorphic alternation of haploid and diploid generations whereas supporters of antithetic alternation favored a model in which land plants were derived from a haploid algal ancestor with zygotic meiosis. Modern evidence that embryophytes are derived from charophycean green algae is more compatible with an updated version of the antithetic theory (Haig, 2008).

Antithetic theory:

This theory is also known as interpolation or intercalation theory. 

Proposer: Celakovsky, 1874 

Supporters: Bower, Chamberlain and others 

Theory: This theory assumes that the gametophyte, i.e. the sexual generation is the original or historically prior generation while the sporophyte or spore producing generation is an entirely new phase, derived from the progressive elaboration of the zygote of some algal ancestors and has been intercalated or interpolated into the life cycle in course of evolution, between the successive events of fertilization (syngamy) and meiosis (reduction division) and is thus different in structure from the gametophyte from its very inception.

Chamberlain (1935) states, 'The alternation of generation viewed as an alternation of n and 2n phases of the life history strictly antithetic.'

Evidence and proposals: 

• The sporophyte of the land plants originated from the green algae which already had alternation of generations and were of diplohaplontic type.
• Delay in the zygotic meiosis could have late to the development of multicellular sporophyte (Bower, 1908).
• The sporophyte of the land plants evolved as a new component of the life cycle and from algae which lacked alternation of generations.

Arguments: 

• The free-living sporophyte becoming parasitic on the gametophyte is confusing.

Intercalation of prolonged diploid phase led to the evolution of the sporophyte (Antithetic theory)

Homologous theory:

This theory is also known as modification or transformation theory.

Proposer: Pringsheim (1878)

Supporters: Scott, Goebel, Zimmermann, Fritsch, Stebbing, Events, Church and others

Theory: This theory holds that the sporophytic and gametophytic generations are fundamentally similar in nature and the sporophyte is a direct modification of the gametophyte and is not a new structural type. Thus, this theory supports the homologous alternation of generation which are phylogenetically similar but differs in the presence or absence of the sex organs. 

The advocate of this theory pointed to the fact that in algae the gametophytic plant reproduces by both the methods of reproduction and bears the asexual spores as well as sexual gametes. In course of evolution, these two functions became separated in two distinct individuals producing spores and gametes and were designated as the sporophyte and gametophyte respectively.

Evidences and proposals: 

• Presence of isomorphic alternation of generations between the spore producing and gamete producing individuals in algae. Example: Ulva
• Initially, both the sporophyte and the gametophyte where independent and free living. Gradually the sporophyte became parasitic on the gametophyte and got reduced.
• The phenomenon of apogamy and apospory also supports this theory.

Arguments: 

• Simultaneous and successful migration of both the gametophytes and sporophyte from water to land is impossible. 
• The two generations in algae and the land plants are analogous but not homologous.

Conclusion

Molecular phylogenetic data unambiguously relate charophycean green algae to the ancestry of monophyletic embryophytes and identify bryophytes as early divergent land plants. Comparison of reproduction in charophyceans and bryophytes suggests that the following stages occurred during evolutionary origin of embryophytic alternation of generations: 

• origin of oogamy
• retention of eggs and zygotes on the parental thallus
• origin of matrotrophy (regulated transfer of nutritional and morphogenetic solutes from parental cells to the next generation)
• origin of a multicellular sporophyte generation
• origin of non-fagellate, walled spores

The charophycean green algae, the closest extant relatives of embryophytes, possess a multicellular haploid body but lack a multicellular diploid body. Therefore, the sporophyte has been interpolated into a basically haploid life cycle. One could interpret this conclusion as a vindication of the antithetic theory championed by Bower (1908) and as a rejection of the homologous theory. However, debate continues about whether the sporophyte originated from a dispersed zygote or from a zygote that was retained on a maternal gametophyte (Graham and Wilcox, 2000; Haig, 2008).

Further reading:

Graham LK, Wilcox LW. The origin of alternation of generations in land plants: a focus on matrotrophy and hexose transport. Philos Trans R Soc Lond B Biol Sci. 2000 Jun 29;355(1398):757-66; discussion 766-7. doi: 10.1098/rstb.2000.0614. PMID: 10905608; PMCID: PMC1692790.

Haig, D. Homologous Versus Antithetic Alternation of Generations and the Origin of Sporophytes. Bot. Rev 74, 395–418 (2008). https://doi.org/10.1007/s12229-008-9012-x

Vashista B.R., Sinha A.K., Kumar A -Botany for degree students – Bryophyta.

Parihar N. -.Bryophytes: An Introduction to Embryophyta. Vol I

Alternation of Generations in Bryophyte

 Alternation of generations is a common component of all land plants, both vascular and non-vascular. The life cycle of bryophytes consists of two distinct phases, one of which is the green thalloid or leafy plant body, and the other is the sporogonium. In a life cycle, when the two generations, sexual (haploid) and asexual (diploid) generations, alternate with each other by the production of gametes and spores respectively, it is called alternation of generation.

In bryophytes, the gametophytic or gamete-producing generation is distinct in its morphology and is haploid having a single set of chromosomes. It is green, thalloid (liverworts) or leafy (mosses) and bears sex organs, i.e. antheridium (male) and archegonium (female). They produce male and female gametes i.e. antherozoids and eggs respectively, that unite sexually to form diploid zygote. 

The zygote is the first cell of the sporophytic generation. The zygote then divides to form an embryo which further differentiates to form the sporogonium. The sporophyte or spore-producing generation is morphologically different from the gametophyte and is diploid in nature. It is usually differentiated into foot, seta, and capsule. The diploid spore mother cells produced inside the capsule undergoes meiosis to form haploid spores which germinate to produce haploid gametophyte. 

In this way the life cycle is completed with an alternation of generations. This type of alternation of generations are termed as diplohaplontic life cycles. 

In bryophytes, the alternating individuals in the life cycle differs morphologically as well as physiologically. The gametophyte is independent while the sporophyte is attached and generally dependent fully or partially on the gametophyte. This kind of alternation of generation is known as heterologous or heteromorphic.

Life cycle of bryophyte showing distinct alternation of generations

Amphibious Nature of Bryophytes: Adaptations to Land Habit

Plants grow in two well-defined habitats: water (aquatic) and land (terrestrial.). Between these two habitats is a transitional zone, represented by the swamps and the areas where land and water meet. This zone is called the amphibious zone. Inhabiting this zone are the moses, liverworts, and hornworts which constitute a group of non-vascular land plants called the bryophytes.

Evidence supports the view that these early land plants descended from the algal ancestors which were green. 

This movement of plants from water to land required them to make several adaptations to survive this new and strange environment. These adaptive features are as follows:

Development of organs for anchorage and absorption of water: With the change of habitat the bryophytes unlike the algae are not bathed in water. So, absorption of water and nutrients through body surface is not possible anymore. This led to the development of rhizoides which help in absorption as well as anchorage of the plant to the substratum. Two types of rhizoids are found: smooth and pegged. In addition to this, scales are developed in bryophytes with increases absorptive surface. For example, simple rhizoids are present in anthocerotopsida. In Riccia, both simple as well as pegged rhizoids are present. Septate rhizoids, which increases absorptive function, are present in mosses.

Rhizoids of Riccia

Protection against desiccation: A major problem due to change of habitat from water to land is desiccation. So, to protect it from desiccation, thick, compact, multicellular epidermis is developed in the bryophytes. In some liverworts the free surface of the epidermal cells is covered with waxy substances like cuticle which reduces water loss.

Gaseous exchange: In the terrestrial habitat the plants have to take carbon dioxide and oxygen from the atmospheric air. So numerous minute pores called airpores are developed on the upper surface of the thallus. These airpores facilitate gaseous exchange between atmospheric air and the interior of the thallus. In addition to this, air canals (vertical slits between pseudo-mesophylls) also help in gaseous exchange. For example, airpores are present in Marchantia. In Riccia, air chambers are present.

T.S. of Marchantia thallus showing air pores, air chambers and rhizoids

Protection of reproductive structures from drying and mechanical injury: As terrestrial habitat is more prone to mechanical injury and drying up the reproductive structures, i.e. sex organs in bryophytes become multicellular and jacketed. This gives additional protection to the eggs and the sperms.

The fertilized egg is retained within the archegonium where it obtains food and water and remains protected from drying as it develops into an embryo.  For example, in Marchantia, the antheridium is protected by antheridial wall while the archegonium is protected by archegonial jacket. The zygote is retained within the venter.

Marchantia thallus bearing reproductive structures

Development of vascular bundle-like structures in higher bryophytes: Primitive vascular system in the form of a conductive strand is present due to land adaptation. Bryophytes have been group into three main groups on the basis of water absorption:

Ectohydric: They can absorb water or solutes by body surface only. They do not possess well developed conducting strands. Example: leafy liverworts

Endohydric: They absorb water by rhizoids which are connected to well-developed conducting strands. Example: some moses and thalloid liverworts

Mixohydric: They absorb water and solutes both by body surface as well as rhizoids. Example: Funaria

Necessity of water to complete life cycle: Bryophytes cannot carry on their reproductive activities without sufficient moisture. Without water sex organs cannot mature and do not dehisce. Water is essential for transport of sperm to archegonium. This explains why the bryophytes usually inhabit moist shaded situations. 

Since the bryophytes usually grow in amphibious situations and cannot complete their life cycle without presence of external water they are called amphibians of the plant world.

Salient Features of Bryophytes

1. Presence of a distinct and well-defined heteromorphic alternation of generation in the life history that is gametophyte and sporophyte generations which follow one another in regular succession. Gametophyte is independent while sporophyte is dependent on gametophyte.
2. In their vegetative structure they have become completely adapted to land habit. However, they still require water for fertilization and hints to complete their life cycle. They are called the amphibians of plant Kingdom.
3. The plant body lacks true root, stem, and leaf. In lower bryophytes the plant body is thalloid containing rhizoids (Example: Riccia). In higher bryophytes, however, the gametophytic plant body is differentiated into roots like (rhizoids), stem like (caulid), and leaf like (phyllid) structures (Example: moss).
4. The bryophytes (like thallophytes) lack vascular bundles (which is characteristic feature of higher plants). However, vascular bundle-like structures in form of a strength maybe present in higher bryophytes like Pogonatum.
5. Sexual reproduction is highly oogamous. The sex organs are jacketed and multicellular (in contrast to algae where sex organs are unicellular and non-jacketed).
6. Female sex organ in form of archegonium appears for the first time in the bryophytes.
7. Sperms are biflagellate and both the flagella are whiplash type.
8. The fertilized egg is retained within the venter. It neither becomes independent of the parent gametophyte, nor passes into resting period (unlike algae).
9. The zygote divides to form embryo. The first division of the zygote is transverse, and the apex of the embryo develops from the outer cell. Such an embryogeny is called exoscopic (Embryo is absent in thallophyta).
10. The venter enlarges with the developing embryo into a protective multicellular structure called calyptra.
11. The embryo develops into sporophyte. It is differentiated into foot, seta, and capsule. In some species Seta is absent (Example: Corsinia).
12. The sporogonium is concerned with the production of wind-disseminated, non-motile, cutinized spores which belong to the category of gonospores or meiospores. This meiospores are of one kind, i.e. they are homospores.