🐇Honors Biology Unit 14 – Plant Structure, Function & Reproduction

Plant Cell Structure

• Plant cells have a cell wall composed of cellulose that provides structural support and protection
• The cell membrane is a semi-permeable barrier that controls the movement of substances in and out of the cell
• Chloroplasts are organelles that contain chlorophyll and are the site of photosynthesis
  • Chloroplasts have a double membrane and internal thylakoid membranes where light-dependent reactions occur
  • The stroma is the fluid-filled space surrounding the thylakoids where the light-independent reactions take place
• Vacuoles are large, membrane-bound organelles that store water, nutrients, and waste products
  • The central vacuole can occupy up to 90% of the cell volume in mature plant cells
• The nucleus contains the cell's genetic material (DNA) and directs cellular activities
• Mitochondria are the powerhouses of the cell, generating ATP through cellular respiration
• The endoplasmic reticulum (ER) is a network of membranes involved in protein and lipid synthesis and transport
• Ribosomes are the sites of protein synthesis and can be found free in the cytoplasm or attached to the ER

Plant Tissues and Organs

• Meristematic tissue consists of actively dividing cells found in root and shoot tips, responsible for plant growth
  • Apical meristems are located at the tips of roots and shoots and contribute to primary growth (lengthening)
  • Lateral meristems (cambium) are found in stems and roots, responsible for secondary growth (thickening)
• Dermal tissue is the outer protective layer of the plant, including the epidermis and periderm
  • The epidermis is a single layer of cells that covers the plant's surface and may have specialized structures like stomata and trichomes
  • The periderm replaces the epidermis in older stems and roots, providing protection and support
• Ground tissue fills the space between dermal and vascular tissues, including parenchyma, collenchyma, and sclerenchyma
  • Parenchyma cells are the most common, involved in photosynthesis, storage, and secretion
  • Collenchyma cells have unevenly thickened walls and provide structural support in growing parts of the plant
  • Sclerenchyma cells have thick, lignified walls and provide mechanical support in mature parts of the plant
• Vascular tissue is responsible for the transport of water, nutrients, and sugars throughout the plant
  • Xylem tissue conducts water and dissolved minerals from the roots to the leaves and provides structural support
  • Phloem tissue transports sugars and other organic compounds from the leaves to other parts of the plant

Roots and Nutrient Absorption

• Roots anchor the plant in the soil and absorb water and nutrients
• The root system consists of a taproot (in dicots) or fibrous roots (in monocots)
  • Taproots are a single, thick primary root with smaller lateral roots branching off (carrots)
  • Fibrous roots are a network of thin, branching roots of similar size (grasses)
• The root cap protects the delicate apical meristem as the root grows through the soil
• Root hairs are extensions of epidermal cells that increase the surface area for water and nutrient absorption
• The endodermis is a layer of cells with Casparian strips that regulate the flow of water and solutes into the vascular cylinder
• Mycorrhizae are symbiotic associations between fungi and plant roots that enhance nutrient absorption
  • Ectomycorrhizae form a network around the root, while endomycorrhizae penetrate the root cells
• Nitrogen-fixing bacteria (Rhizobia) form symbiotic relationships with legumes, converting atmospheric nitrogen into a usable form for plants

Stems and Transport Systems

• Stems provide support, elevate leaves for light capture, and transport water, nutrients, and sugars between roots and leaves
• The stem contains vascular bundles arranged in a ring (in dicots) or scattered throughout the ground tissue (in monocots)
  • Each vascular bundle consists of xylem (toward the center) and phloem (toward the outside)
• The xylem is composed of tracheids and vessel elements that conduct water and provide structural support
  • Tracheids are elongated cells with tapered ends and pits for water movement
  • Vessel elements are shorter, wider cells that stack end-to-end to form continuous tubes
• The phloem is composed of sieve tube elements and companion cells that transport sugars and other organic compounds
  • Sieve tube elements are living cells with perforated end walls (sieve plates) for the movement of sugars
  • Companion cells are closely associated with sieve tube elements and provide metabolic support
• The vascular cambium is a lateral meristem that produces secondary xylem (wood) and secondary phloem (bark)
• Lenticels are pores in the stem's periderm that allow gas exchange between the stem and the atmosphere
• Transpiration is the loss of water vapor from the plant through stomata, creating a pull that drives water transport through the xylem

Leaves and Photosynthesis

• Leaves are the primary site of photosynthesis, where light energy is converted into chemical energy (sugars)
• The leaf blade (lamina) is the flat, expanded portion of the leaf specialized for light capture
  • The upper and lower epidermis protect the leaf and regulate gas exchange through stomata
  • Mesophyll tissue is composed of palisade and spongy layers, containing chloroplasts for photosynthesis
• The leaf veins are vascular bundles that transport water, nutrients, and sugars to and from the leaf
• Stomata are pores in the leaf epidermis that allow for gas exchange (CO2 in, O2 and water vapor out)
  • Guard cells surrounding the stomata regulate their opening and closing based on environmental factors (light, humidity, CO2 concentration)
• The light-dependent reactions of photosynthesis occur in the thylakoid membranes of chloroplasts
  • Light energy is captured by chlorophyll and other pigments, exciting electrons that power the synthesis of ATP and NADPH
• The light-independent reactions (Calvin cycle) occur in the stroma of chloroplasts
  • CO2 is fixed and reduced to form sugars using the energy-rich molecules ATP and NADPH produced in the light-dependent reactions
• C3, C4, and CAM photosynthesis are different adaptations for carbon fixation in various environments
  • C3 photosynthesis is the most common pathway, where CO2 is directly fixed by the enzyme RuBisCO (rice, wheat, soybeans)
  • C4 photosynthesis is an adaptation for hot, dry climates, using a two-step process to concentrate CO2 (corn, sugarcane)
  • CAM photosynthesis is an adaptation for arid environments, where stomata open at night to minimize water loss (cacti, pineapples)

Flowers and Reproduction

• Flowers are the reproductive structures of angiosperms (flowering plants)
• The main parts of a flower are the sepals, petals, stamens (male), and carpels (female)
  • Sepals are the outermost parts that protect the flower bud
  • Petals are often brightly colored to attract pollinators
  • Stamens consist of a filament and an anther, which produces pollen grains containing male gametes
  • Carpels consist of a stigma, style, and ovary, which contains ovules (female gametes)
• Flowers can be complete (having all four parts) or incomplete (missing one or more parts)
• Flowers can be perfect (having both male and female parts) or imperfect (having only male or female parts)
• Pollination is the transfer of pollen from the anther to the stigma, which can occur by wind, water, or animals (insects, birds, bats)
  • Self-pollination occurs when pollen is transferred within the same flower or between flowers on the same plant
  • Cross-pollination occurs when pollen is transferred between flowers on different plants, promoting genetic diversity
• Double fertilization is unique to angiosperms, where one sperm cell fuses with the egg to form a zygote, and another fuses with two polar nuclei to form the endosperm
  • The zygote develops into the embryo, while the endosperm provides nutrition for the developing seed

Seed Formation and Dispersal

• After fertilization, the ovule develops into a seed, and the ovary develops into a fruit
• A seed consists of an embryo, endosperm (in monocots) or cotyledons (in dicots), and a seed coat
  • The embryo is the young plant that will develop into a new individual
  • The endosperm or cotyledons provide nutrition for the developing embryo
  • The seed coat is a protective layer derived from the integuments of the ovule
• Seed dormancy is a state of reduced metabolic activity that prevents germination until favorable conditions are met
  • Dormancy can be broken by environmental cues such as light, temperature, moisture, or chemical signals
• Seed dispersal is the movement of seeds away from the parent plant, which can occur by various means
  • Wind dispersal: seeds are lightweight or have structures that allow them to be carried by the wind (dandelions, maples)
  • Water dispersal: seeds are buoyant or have structures that allow them to float on water (coconuts, mangroves)
  • Animal dispersal: seeds are eaten and passed through the digestive system, or they stick to animal fur or feathers (berries, burs)
  • Explosive dispersal: fruits or seeds are forcibly ejected from the plant (touch-me-nots, witch hazel)
• Seed banks are natural reservoirs of dormant seeds in the soil that can germinate when conditions are favorable
  • Seed banks help maintain genetic diversity and allow plant populations to recover after disturbances (fires, droughts)

Plant Growth and Development

• Plant growth is the irreversible increase in size and volume, resulting from cell division and cell expansion
• Primary growth occurs at the apical meristems and results in the lengthening of roots and shoots
  • The primary root and shoot apical meristems give rise to the primary plant body
• Secondary growth occurs at the lateral meristems (cambium) and results in the thickening of stems and roots
  • The vascular cambium produces secondary xylem (wood) and secondary phloem (bark)
  • The cork cambium produces the periderm, which replaces the epidermis in older stems and roots
• Plant hormones (phytohormones) are chemical messengers that regulate growth, development, and responses to environmental stimuli
  • Auxins promote cell elongation, apical dominance, and root formation (IAA)
  • Cytokinins promote cell division, delay senescence, and interact with auxins to control shoot and root growth
  • Gibberellins stimulate stem elongation, seed germination, and fruit development (GA)
  • Abscisic acid (ABA) regulates seed dormancy, stomatal closure, and stress responses (drought, cold)
  • Ethylene is a gaseous hormone that promotes fruit ripening, leaf abscission, and senescence
• Photoperiodism is the response of plants to the relative lengths of day and night, which influences flowering and other developmental processes
  • Short-day plants flower when the night is longer than a critical length (poinsettias, chrysanthemums)
  • Long-day plants flower when the night is shorter than a critical length (spinach, radishes)
  • Day-neutral plants flower regardless of the night length (tomatoes, dandelions)
• Tropisms are growth responses to directional stimuli, such as light (phototropism) and gravity (gravitropism)
  • Positive phototropism is the growth of shoots toward light, while negative phototropism is the growth of roots away from light
  • Positive gravitropism is the growth of roots downward, while negative gravitropism is the growth of shoots upward