Plant Biology: Cells, Transport, and Growth

Domains of Life

• All living organisms are categorized into three primary groups: Bacteria, Archaea, and Eukarya.
• Classification is based on cellular structure, genetic makeup, and evolutionary relationships.

Bacteria

• These are prokaryotic cells.
• Bacteria are single-celled organisms.
• They lack membrane-bound organelles.
• Bacteria feature diverse metabolic pathways.

Archaea

• These are prokaryotic cells, similar to bacteria, but genetically distinct.
• Archaea are often found in extreme environments, and are thus extremophiles.
• They possess unique lipid membranes and cell wall composition.

Eukarya

• Eukarya are eukaryotic cells with a true nucleus.
• They have membrane-bound organelles.
• Eukarya includes single-celled (e.g., protists) and multicellular organisms (plants, animals, fungi).

Prokaryotes

• Prokaryotes are organisms without a nucleus.
• Both Bacteria and Archaea are included.
• They are generally smaller and simpler than eukaryotes.

Eukaryotes

• These are organisms with a defined nucleus.
• They feature a more complex cellular organization.
• Eukaryotes can be unicellular or multicellular, including plants, animals, and fungi.

Water and Sugar Transport

• Water and sugar transport in plants is essential for their growth and survival.
• It involves the movement of water through xylem and sugars through phloem.
• Transportation is driven by differences in water potential and active transport mechanisms.

Xylem

• Xylem transports water and minerals from roots to other parts of the plant.
• It is composed of tracheids and vessel elements.
• Xylem operates under negative pressure generated by transpiration.

Phloem

• Phloem transports sugars (mainly sucrose) and other organic compounds from sources (e.g., leaves) to sinks (e.g., roots, fruits).
• It contains sieve tubes and companion cells.
• Phloem utilizes the pressure flow hypothesis for loading and unloading sugars.

Water Potential

• Water potential represents the free energy of water and influences water movement.
• Total water potential equals pressure potential plus solute potential.
• Water moves from areas of higher water potential to lower water potential.

Sugar Transport

• Sugars are actively transported into phloem during loading.
• Phloem unloading occurs at sink tissues where sugars are needed.
• The same plant part can act as a source or sink depending on developmental stage and season.

Plant Growth

• Plant growth involves the processes by which plants increase in size and develop structures.
• This includes primary growth (lengthening) from apical meristems and secondary growth (thickening) from lateral meristems, along with the organization of plant tissues and organs.

Meristems

• Meristems are undifferentiated plant tissue.
• Cells divide indefinitely by mitosis.
• They give rise to new cells that differentiate into various tissues.

Apical Meristems

• Located at tips of roots and shoots.
• They are responsible for primary growth.

Lateral Meristems

• They are responsible for secondary growth, increasing diameter.

Primary Growth

• This involves lengthening of the plant.
• Results in primary tissues such as epidermis, ground tissue, and vascular tissue.
• It occurs at the tips of roots and stems.

Secondary Growth

• This involves thickening of the plant.
• Results in secondary tissues, primarily through the activity of the vascular cambium and cork cambium.
• Important for woody plants, leading to annual growth rings.

Plant Body Organization

• Composed of organized parts including: root system and shoot system.
• Root System: anchors the plant and absorbs water/nutrients.
• Shoot System: Includes stems, leaves, and reproductive structures like flowers.

Tissue Categories

• Dermal Tissue: protective outer layer.
• Ground Tissue: Involved in storage, photosynthesis, and support.
• Vascular Tissue: xylem (water transport) and phloem (sugar transport).

Developmental Aspects

• Continuous development without cell migration.
• Environmental factors significantly influence growth.
• Plants can regenerate parts, demonstrating plasticity.

Transport Mechanisms

• Water and nutrients are transported via xylem and phloem.
• Water movement is driven by transpiration and water potential gradients.
• Sugar transport occurs from sources (where produced) to sinks (where needed).

Plant Structures

• Plant structures encompass the various components and systems that make up plants, including their growth patterns, types of tissues, and reproductive strategies.
• Understanding these structures is essential for studying plant biology and evolution.

Gymnosperms

• These are seed plants with "naked seeds.”
• Ovules are exposed at pollination.
• Sporangia are located in cones.
• Xylem consists only of tracheids.
• They possess diverse leaf types.

Angiosperms

• These are seed plants with seeds enclosed in fruits.
• They feature the presence of flowers.
• They are the most diverse group of plants (e.g., grasses, trees).
• Evolutionary advantages include seed protection, food supply, dispersal, and dormancy.

Vascular Tissues

• Composed of xylem and phloem.
• Xylem transports water and contains tracheids and vessel elements.
• Phloem moves sugars from sources to sinks.
• Essential for support and nutrient transport.

Non-Vascular Plants

• Includes bryophytes (mosses, liverworts).
• The dominant gametophyte stage.
• Require water for fertilization.
• They lack true vascular tissue.

Plant Growth

• Primary growth increases length via apical meristems.
• Secondary growth increases diameter via lateral meristems.
• Meristems are undifferentiated tissues that allow continuous growth.

Tissue Types

• Dermal: protective outer layer (epidermis, cuticle).
• Ground: involved in storage, photosynthesis, and support (parenchyma, collenchyma, sclerenchyma).
• Vascular: conducting tissues (xylem and phloem).

Developmental Features

• Continuous development without cell migration.
• Environmental factors influence growth and regeneration.
• Programmed cell death plays a role in structure and function.

Reproductive Structures

• Gametes: male and female reproductive cells.
• Spores: haploid cells for reproduction.
• Pollen: male gametophyte for fertilization.
• Seeds: embryo encased in protective layers.

Plant Defenses

• Plant defenses are mechanisms that protect plants from herbivores, pathogens, and competitors.
• These defenses can be static or inducible, involving physical barriers, chemical toxins, and systemic responses to damage or infection.

Static Defense

• Physical Defense: Dermal tissue acts as a barrier.
• Chemical Defense: production of toxins and defensins deters herbivores.

Inducible Defense

• Energy Efficiency: Lower energy demand but involves a time lag.

Wound Response

• Systemin is produced after wounding.
• It travels through phloem to trigger jasmonic acid production.
• Jasmonic acid activates defense gene transcription, leading to proteinase inhibitor production.

Pathogen Response

• Gene-for-Gene Hypothesis: plants recognize specific pathogen genes (Avr) with corresponding resistance genes (R).
• Systemic Acquired Resistance (SAR): provides temporary whole-plant resistance to diseases.
• Hypersensitive Response: involves sacrificing cells around the infection site to prevent spread and signal other parts of the plant for defense activation.

Tropisms and Hormones

• Tropism: directional growth in response to stimuli (e.g., light, gravity).
• Hormonal Regulation: various hormones like auxin, cytokinins, and ethylene guide growth and responses to environmental changes.

Plant Hormones

• Plant hormones are chemical signals that regulate growth and development in plants.
• They are produced in various locations within the plant and can act locally or be transported to other areas, influencing processes such as tropisms, cell division, and responses to environmental stimuli.

Auxin

• It promotes stem elongation and leaf formation.
• Auxin is involved in cell expansion and differentiation of vascular tissue.
• It regulates gravitropism and phototropism.

Cytokinins

• Stimulate cell division and differentiation.
• Found in actively dividing tissues and interact with auxin for balanced growth.

Ethylene

• Regulates fruit ripening and leaf abscission (falling).
• Plays a role in senescence (aging) of plant tissues.

Abscisic Acid

• Acts as a growth suppressant.
• Induces bud and seed dormancy.
• Regulates stomatal closure during water stress.

Gibberellin

• Promotes stem elongation and seed germination.
• Influences flowering and fruit development.

Photoperiodism

• This is the response to changes in day length affecting flowering and growth cycles.
• Involves phytochrome signaling related to light exposure.

Tropisms

• Directional growth responses to environmental stimuli (e.g., light, gravity).
• Auxin distribution plays a key role in these movements.

Thigmomorphogenesis and Thigmotropism

• Growth response to touch and contact.
• Reversible responses influenced by turgor pressure.

Plant Tissues

• Plant tissues are organized groups of cells that perform specific functions essential for plant growth and survival.
• They are categorized into three main types: dermal, ground, and vascular tissues, each serving distinct roles in the plant's structure and function.

Dermal Tissue

• Function: Protection of plant surfaces.
• Components: epidermis (outer layer), cuticle (waxy covering), specialized cells (guard cells, trichomes, root hairs).

Ground Tissue

• Function: Storage, photosynthesis, and support.
• Types: Parenchyma, Collenchyma, Sclerenchyma.
• Parenchyma: large vacuoles, involved in storage and photosynthesis.
• Collenchyma: flexible support with unevenly thickened walls.
• Sclerenchyma: rigid support with thick secondary walls.

Vascular Tissue

• Function: Transport of water, nutrients, and sugars.
• Components: xylem and phloem.
• Xylem: transports water and minerals and consists of tracheids and vessel elements.
• Phloem: moves sugars from sources to sinks and includes sieve tubes and companion cells.

Cell Types

• Meristematic Cells: undifferentiated cells capable of division and differentiation.
• Parenchyma Cells: involved in storage and metabolic functions.
• Collenchyma Cells: provide flexible support.
• Sclerenchyma Cells: offer rigid structural support.

Plant Adaptations

• Plant adaptations are evolutionary changes that enable plants to survive and thrive in terrestrial environments.
• These adaptations address challenges such as desiccation, nutrient transport, and structural support while facilitating reproduction and growth.

Environmental Challenges

• Need for hydration and moisture retention.
• Requirement for efficient transport systems (water and nutrients).
• Support against gravity and environmental stressors.
• Desiccation prevention through structures like cuticles and stomata.

Sporophyte vs Gametophyte

• Sporophyte Generation: dominant in vascular plants; diploid phase producing spores.
• Gametophyte Generation: dominant in non-vascular plants (bryophytes); haploid phase producing gametes.

Evolutionary Adaptations

• Development of vascular tissues (xylem and phloem) for efficient water and nutrient transport.
• Formation of seeds in seed plants providing protection and nourishment to embryos.
• Evolution of flowers and fruits in angiosperms enhancing reproductive success and dispersal.

Plant Groups

• Bryophytes: non-vascular, gametophyte dominant, require water for reproduction.
• Tracheophytes: vascular plants with sporophyte dominance.
• Lycophytes: seedless, vascular tissue present.
• Pterophytes: ferns and relatives, also seedless, require water for fertilization.
• Seed Plants: include gymnosperms and angiosperms, characterized by seeds and advanced reproductive structures.

Gymnosperms

• Feature "naked seeds" with exposed ovules and sporangia in cones.

Angiosperms

• Feature "seeds in a vessel,” presence of flowers and fruits and are the most diverse group.

Explore fundamental concepts in plant biology. Topics include cell characteristics, water and sugar transport mechanisms, and the roles of meristems and vascular tissues. Understand primary differences between cell types and the evolutionary significance of plant structures.