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Botany MIDTERMS (by:thea) e. Depending upon the environment it gets suitably modified to perform special Morphology of Stems...

Botany MIDTERMS (by:thea) e. Depending upon the environment it gets suitably modified to perform special Morphology of Stems functions like f. storage of foods, means of propagation, etc. General Structure External Structure Stem - aerial part of the plant ➔ Plumule - develops to form the stem ➔ consists of axis and the leaves Characteristics: 1. It is an ascending axis of the plant and phototropic in nature 2. It consists of nodes, internodes and buds 3. It gives rise to branches, leaves and flowers 4. Stems may be aerial, sub-aerial and underground Depending upon the presence of mechanical tissues, the stems may be: ➔ Weak - stems are thin and long, they are unable to stand erect a. Creepers or Prostate stem: grow flat on the ground with or without roots Ex. grasses / gokharu b. Climbers: are too weak to stand alone, they climb on the support with the help of tendrils, hooks, prickles or roots Ex. Piper betel / Piper longum c. Twinners: coil the support and grow further, thin and wiry Ex. ipomoea ➔ Herbaceous & Woody - normal stems and may Node be soft or hard and woody (sunflower / - The point where leaves, branches, or buds sugarcane / mango) emerge. a. Produce leaves and exposes them properly to sunlight for carrying out Internode photosynthesis - The segment of stem between two nodes. b. Conducts water and minerals from roots to leaves and buds Bud - A dormant or developing shoot that can grow c. Foods produced by leaves are into leaves or flowers; includes apical and lateral transported to non green parts of the buds. plant ➔ Lateral (or Axillary) Bud: A bud d. Produce flowers and fruits for pollination located in the leaf axil, capable of and seal dispersal developing into a branch or flower Botany MIDTERMS (by:thea) ➔ Terminal (or Apical) Bud: The bud located at the tip of the stem responsible Internal Structure for growth in length and producing new leaves or flowers Petiole - The stalk that attaches a leaf to the stem, allowing flexibility and support for the leaf. Pedicel - The small stalk that supports a single flower or fruit, connecting it to the main stem or inflorescence. Axil - The angle formed between the upper side of a leaf (or branch) and the stem from which it arises - houses axillary buds The bud scales of a terminal bud leave tiny scars around The remainder of the meristematic tissue, called ground the twig when they fall off in the spring. meristem, produces two tissues composed of parenchyma cells Counting the number of groups of bud scale scars on a Pith twig can tell one how old the twig is. - The central region, often made of parenchyma involved in storage and sometimes in transport. These scars come from a leaf that has stipules at the Cortex base of the petiole. - The layer beneath the epidermis, composed of parenchyma cells for storage and support. ➔ Stipules - often serve protective functions for the Apical Meristem: This region at the tip of the stem is developing leaf or bud and can play a responsible for the increase in length and produces new role in photosynthesis cells. - In some plants, they fall off as the buds expand in the spring, leaving tiny stipule Primary Tissues: The apical meristem generates five scars main types of primary tissues: Deciduous trees and shrubs Epidermis: The outer protective layer. - plants that lose their leaves seasonally, typically Primary Xylem: Transports water and minerals. in the fall Primary Phloem: Transports sugars and - have dormant axillary buds with leaf scars left nutrients. below them after the leaves fall Cortex: Provides support and storage. Pith: Central tissue, often involved in storage. Botany MIDTERMS (by:thea) Leaf and Bud Primordia: As the stem grows, structures ○ Sieve Tube Members: Transport sugars called leaf primordia (early leaf forms) and bud primordia and nutrients. (embryonic buds) develop in the axils of leaves. These ○ Companion Cells: Assist in the function will mature into functional leaves and buds. of sieve tubes. ○ Other components that help in nutrient transport. Cork Cambium (Phellogen) 1. Origin: A second cambium that arises within the cortex or from the epidermis/phloem. 2. Function: Produces cork cells that are impregnated with suberin, making them waterproof and protective. 3. Cork Tissue: Forms the outer bark, reducing water loss and protecting against injury. 4. Gas Exchange: While cork tissue cuts off supplies to the epidermis, lenticels develop for gas exchange, allowing oxygen and carbon dioxide movement. Steles Cambium: A thin layer of meristematic tissue between 1. Definition: The central cylinder of primary xylem and phloem, responsible for secondary growth xylem, primary phloem, and pith (if present) in (increase in thickness) younger stems and roots. 2. Types: ○ Protostele: A solid core of vascular tissue; common in primitive plants. ○ Siphonostele: Tubular structure with pith; typical in ferns. ○ Eustele: Discrete vascular bundles found in most flowering plants and conifers. Herbaceous Dicotyledonous Stems Secondary Xylem: - characterized by their primarily primary structure, distinct vascular arrangement, and the The cells produced toward the center of the potential for secondary growth through cambium stem differentiate into: development, enabling efficient transport and ○ Tracheids: Water-conducting cells. support ○ Vessel Elements: Specialized cells that also transport water. ○ Fibers: Provide structural support. ○ Other components involved in the overall function of the xylem. Secondary Phloem: The cells produced toward the outside of the stem develop into: Botany MIDTERMS (by:thea) General Characteristics Primary vs. Secondary Growth 1. Annuals: Herbaceous dicots often die after one 1. Initial Development: Young stems of growing season, with green, nonwoody stems. herbaceous and woody dicots show similar 2. Primary Tissues: Most of the tissues are arrangements of primary tissues. primary, though some may develop secondary 2. Cambium Development: As vascular cambium tissues through cambium activity. and cork cambium develop in woody plants, notable differences arise, particularly in the Vascular Bundles formation of secondary xylem (wood). 1. Arrangement: Vascular bundles are discrete Wood Formation and arranged in a cylinder, separating the cortex from the pith. 1. Seasonal Growth: 2. Continuous Rings: In some plants, like ○ In temperate climates, growth occurs foxgloves, xylem and phloem form continuous mainly in spring and summer. rings instead of separate bundles. ○ Spring Wood: Composed of large vessel elements produced early in the Cambium Activity growing season. ○ Summer Wood: Characterized by 1. Procambium: Initially produces primary xylem smaller vessel elements and a higher and phloem. proportion of tracheids. 2. Vascular Cambium: Later develops between 2. Annual Rings: The alternating layers of spring primary tissues, adding secondary xylem and and summer wood form annual rings, which phloem. indicate the age of the tree and provide insights 3. Variability: The cambium can either form a into past environmental conditions (e.g., wider narrow ring that connects bundles or be rings during years of abundant rainfall). confined to individual bundles, allowing for flexibility in growth. Environmental Indicators Woody Dicotyledonous Stems The width of annual rings can reflect climatic - characterized by their thick, rigid structure and conditions and events (e.g., droughts, pest the presence of secondary growth damage, fires). Increment borers can be used to determine a tree's age without cutting it down, allowing for conservation of the tree. Vascular Rays 1. Structure: Radiating lines from the center of the trunk made of parenchyma cells, aiding lateral transport of nutrients and water. 2. Function: Facilitate lateral conduction and food storage, with distinct xylem and phloem rays. Botany MIDTERMS (by:thea) Although heartwood contributes strength, it does not affect the tree's overall function. Trees can survive and thrive even if heartwood rots away, demonstrating their resilience. For example, large holes can be cut into trees like coastal redwoods without causing harm. Sapwood and Heartwood: ➔ Sapwood: Acts as the functional part of the xylem for water transport, forming at the same rate as heartwood develops. Its width varies by species. ➔ Heartwood: The darker, inner wood that no longer conducts water, providing strength but not functionality. Softwood vs. Hardwood: ➔ Softwood: Found in pines and other conifers, primarily composed of tracheids with no fibers. ➔ Hardwood: Found in most dicots, containing both tracheids and fibers, generally denser. Resin Canals: ➔ Scattered throughout the xylem and other tissues, these canals secrete resin, which is aromatic, antiseptic, and deters pests. They can also form in response to injury. Bark Structure: ➔ Bark: Comprises all tissues outside the vascular cambium, including inner bark (primary and secondary phloem) and outer bark (cork tissue). ➔ Older phloem layers become crushed and functionless, while the cork cambium produces new cork to replace sloughed-off tissues. Tyloses: As trees age, parenchyma cells can expand through pits in vessels and tracheids, forming Phloem Function: protrusions that fill these cavities. This process prevents further water conduction. ➔ The younger phloem transports sugars and nutrients from leaves to other parts of the plant. Heartwood vs. Sapwood: Historically, Native Americans used strips of young phloem for food. Heartwood: The darker, inner region that accumulates resins, gums, and pigments, Laticifers: providing structural support but no longer conducting water. ➔ Specialized latex-secreting cells found in various Sapwood: The lighter, outer layer near the plants. They form networks that can secrete cambium that remains functional and actively latex, which may help in wound closure and has transports water and nutrients. commercial value (e.g., rubber, chicle). Botany MIDTERMS (by:thea) ➔ Latex: a thick fluid that is white, yellow, orange, Phloem Composition: or red in color and consists of gums, proteins, sugars, oils, salts, alkaloidal drugs, enzymes, The phloem is composed entirely of sieve tubes and other substances. and companion cells, surrounded by a sheath of sclerenchyma cells that provide additional Monocotyledonous Stems support. - characterized by distinct structural features that differentiate them from dicots Parenchyma Tissue: The parenchyma between vascular bundles does not differentiate into distinct cortex and pith as in dicots but serves similar functions. Support Structures: A band of sclerenchyma cells beneath the epidermis and thicker-walled parenchyma cells contribute to the stem's ability to withstand mechanical stresses from weather and plant weight. Intercalary Meristem: Found at the base of each internode in grasses like wheat and rice, this meristem contributes to rapid stem elongation without increasing stem girth, leading to a columnar growth form. Palm Trees: Unlike many monocots, palms grow larger through the continued division and enlargement of parenchyma cells without developing a true cambium. Vascular Bundle Arrangement: Secondary Meristem: Each vascular bundle has xylem positioned Some monocots (like certain houseplants) towards the center and phloem closer to the develop a secondary meristem that produces surface. This arrangement helps with efficient parenchyma cells outward and vascular bundles transport of water and nutrients. inward, differing from the cambium in dicots. Xylem Structure: Commercial Fibers: In corn, the xylem typically contains two large Monocots provide important cordage fibers (e.g., vessels with smaller ones interspersed. The sisal) that are harvested differently from dicot first-formed xylem cells may stretch and fibers. The preparation involves scraping entire collapse, creating irregular air spaces. vascular bundles rather than retting. Botany MIDTERMS (by:thea) Stem Modification I. Underground modifications - serve various functions in plants, particularly for storage and vegetative reproduction ➔ Rhizome: - A horizontal, thick underground stem with nodes, internodes, and scale leaves. It has buds in the axils of scale ➔ Corm: leaves, allowing for new plant growth - Stout, vertical stems that bear buds in (e.g., ginger, turmeric). the axils of scaly leaves, producing new plants. They also have adventitious roots at the base (e.g., saffron, dioscorea). ➔ Tuber: - Swollen underground stems characterized by "eyes," which are II. Sub-aerial modifications of stems vegetative buds that can develop into - assist in vegetative reproduction new plants. Common examples include potatoes and aconite. ➔ Runner: - Horizontal stems that creep along the ground and root at their nodes, allowing new plants to develop (e.g., strawberry). ➔ Stolon: - Lateral branches from the base of the stem that grow horizontally, featuring nodes and internodes. They can develop into new plants (e.g., jasmine). ➔ Offsets: - Short, thick horizontal branches originating from leaf axils, characterized by rosette leaves and a cluster of roots, ➔ Bulb: promoting new growth (e.g., aloe). - Composed of fleshy overlapping scales ➔ Sucker: that store food. Bulbs develop new - Lateral branches from underground plants in spring from stored nutrients, stems that grow upward and can with adventitious roots at the base (e.g., garlic, onion). develop into new plants (e.g., banana, chrysanthemum). Botany MIDTERMS (by:thea) III. Aerial modification of stems - adaptations that allow plants to thrive above Uses of Stem ground ➔ Phylloclades: 1. Food and Spices: Underground stems like - These are flattened stems that perform potatoes, garlic, ginger, and onions are widely photosynthesis, taking on a leaf-like used in culinary applications. function. Common in xerophytes (desert plants), they may have small leaves or 2. Fodder: Stems of crops like jowar and rice are spines (e.g., Opuntia, Euphorbia). A important as animal feed. special form, called a cladode, has just one internode (e.g., asparagus). 3. Industrial Fibers: Stems of jute, hemp, and flax ➔ Thorns and Prickles: are harvested for their fibers, used in textiles - Thorns are hard, pointed structures and other products. derived from stem tissue, providing protection (e.g., lemon, duranta). 4. Sweeteners and Rubber: Sugarcane stems are - Prickles are superficial outgrowths from a primary source of sucrose, while latex from the outer stem tissue, sharp and often Hevea brasiliensis provides rubber. curved, found scattered on plants (e.g., roses, smilax). 5. Medicinal Woods: Wood from certain plants, ➔ Stem Tendrils: like sandalwood and guaiacum, is utilized for - These are modified stems that help medicinal purposes. support climbing plants. They can develop from terminal or axillary buds 6. Gum Production: Injuring stems can yield gums (e.g., grapevine (Vitis) and (e.g., gum acacia, gum tragacanth) for various passionflower (Passiflora)). industrial applications. ➔ Bulbils: - These are modifications of floral buds that enable vegetative propagation. They can develop into new plants (e.g., Dioscorea, Agave). Botany MIDTERMS (by:thea) Morphology of Leafs Internal Structure Leaves - flat, thin green, appendages to the stem, containing supporting and conducting strands in their structure. - develop in such a way that older leaves are placed at the base while the younger ones at the apex. Stomata - essential structures in the leaves of most plants, primarily located on the lower epidermis. ➔ Location and Variation: Most plants have stomata on the lower epidermis, which typically has a thinner cuticle than the upper epidermis. Some plants, like alfalfa and corn, have stomata on both surfaces, while others, like water lilies, may have them exclusively on the upper surface. Aquatic plants often lack stomata altogether on submerged leaves. Three main regions: 1. Epidermis: This is the outer layer of the leaf, ➔ Abundance: typically a single layer of cells that protects the Stomata are incredibly numerous, with internal structures. The epidermal cells usually counts ranging from about 1,000 to lack chloroplasts, as their main function is over 1.2 million per square protective. centimeter. For instance, an average 2. Mesophyll: This is the middle layer where most sunflower leaf can have around 2 million photosynthesis occurs, containing chloroplasts. stomata. 3. Veins/Vascular Bundles: These transport water, nutrients, and sugars throughout the leaf ➔ Structure: and connect to the plant’s broader vascular Each stoma is flanked by two guard system. cells, which are typically smaller than surrounding epidermal cells. These The epidermis has a waxy cuticle that helps prevent guard cells are unique because they water loss and protects against environmental damage. contain chloroplasts, allowing them to engage in photosynthesis. Under stress from pollution or pests, leaves can produce additional waxes for extra protection. ➔ Functions: Gas Exchange: Stomata regulate the Glands on the epidermis may secrete sticky exchange of gases (like oxygen and substances that can help deter herbivores or protect carbon dioxide) between the leaf and against pathogens. the atmosphere. Water Regulation: They also help control water loss through evaporation. Botany MIDTERMS (by:thea) ➔ Mechanism: Veins (Vascular Bundles) The guard cells can change shape due - The veins, or vascular bundles, are scattered to the movement of water in and out of throughout the mesophyll and consist of: them. When they take in water, they Xylem: Transports water and become turgid (inflated), causing the minerals from the roots to the stomata to open. Conversely, when leaves. water is lost, the guard cells deflate, Phloem: Distributes sugars and leading to the closing of the stomata. carbohydrates produced during photosynthesis throughout the plant. ○ Surrounding these tissues is the bundle sheath, made of thicker-walled parenchyma cells. 2. Leaf Skeleton: ○ The veins provide structural support, giving the leaf its “skeleton.” They run in various directions, especially in dicots, allowing for a network that These guard cells, which originate from the same facilitates efficient transport. parental cell, are part of the epidermis, but they, unlike most of the other cells of either epidermis, contain Monocots: chloroplasts. Typically have parallel veins and do not show differentiation into palisade and spongy Mesophyll mesophyll layers. Some monocots have large, - divided into two distinct regions: thin-walled bulliform cells that help reduce ○ Palisade Mesophyll: This upper layer transpiration by folding or rolling the leaf during consists of tightly packed, barrel-shaped dry conditions. parenchyma cells. It is where most photosynthesis occurs, containing over Dicots: 80% of the leaf’s chloroplasts. Exhibit a more complex vein structure and ○ Spongy Mesophyll: Located beneath distinct mesophyll layers, allowing for greater the palisade layer, this region has flexibility and efficiency in photosynthesis. loosely arranged parenchyma cells with numerous air spaces. This structure The evolution of broad leaves took about 40 million facilitates gas exchange and also years, likely influenced by atmospheric conditions. contains chloroplasts, though in lower Initially, high carbon dioxide levels reduced the need for densities than the palisade layer. many stomata, leading to potential overheating of leaves. As CO2 levels dropped, more stomata Chlorenchyma: evolved, allowing leaves to cool themselves and grow The term "chlorenchyma" refers to parenchyma larger, which ultimately enhanced photosynthesis. tissue containing chloroplasts, found in both the mesophyll and outer parts of herbaceous plant stems. Moisture Regulation: The surfaces of mesophyll cells in contact with air are moist. If moisture levels drop, the stomata close to minimize water loss, protecting the leaf from excessive drying. Botany MIDTERMS (by:thea) Leaf vs. Leaflet External Structure Leaf: The complete structure that includes all external features, such as the petiole, lamina, A typical angiosperm leaf consists of several key parts: and stipules. Leaflet: A smaller, individual segment of a Leaf Base (Hypopodium): compound leaf. A compound leaf is made up of This is the part of the leaf that attaches to the multiple leaflets, which can resemble separate stem. It plays a critical role in supporting the leaves but are all part of a single leaf structure. leaf. Petiole (Mesopodium): The petiole is the stalk that connects the leaf blade (lamina) to the stem. Leaves with a petiole are termed petiolate, while those without are called sessile. Petioles can vary in shape and size: ○ They may be short, long, or cylindrical. ○ In some plants, like lemon, they can be flattened (winged petiole). ○ In climbing plants, such as clematis, petioles may modify into tendrils. ○ Aquatic plants may have swollen petioles that help them float by enclosing air. ○ In certain species like the Australian acacia, the petiole can enlarge and function like a leaf, known as a phyllode. Lamina (Leaf Blade or Epipodium): The lamina is the broad, flat part of the leaf Types of Leaves where photosynthesis primarily occurs. Its thickness can vary based on the plant's habitat: 1. Simple Leaves ○ Thick in xerophytes (plants adapted to Definition: A simple leaf has a single leaf blade dry conditions). (lamina). ○ Thin in hydrophytes (aquatic plants). Characteristics: ○ Intermediate in mesophytes (plants ○ It may be stipulate (having stipules) or growing in moderate conditions). exstipulate (lacking stipules). ○ It can be petiolate (having a petiole) or Stipules: sessile (without a petiole). These are small outgrowths located at the ○ Always has an axillary bud in its axil. base of the leaf, which can protect the axillary Forms: The lamina may be: bud. ○ Undivided (entire). Leaves may have stipules (stipulate) or lack ○ Lobed (e.g., in plants like Digitalis, them (ex-stipulate). Eucalyptus, and Carica). Some stipules may have specialized functions, such as photosynthesis or water storage. 2. Compound Leaves Definition: A compound leaf consists of multiple leaf blades, referred to as leaflets or pinnae. Examples: Common examples include Senna, Tamarind, and Acacia. Botany MIDTERMS (by:thea) Classification of Compound Leaves The veins, which are vascular bundles, serve several important functions: a. Pinnate Compound Leaves Transport: They carry water and minerals from Characterized by a single rachis (the axis the roots to the leaf and distribute the food bearing the leaflets). produced during photosynthesis to other parts of Further classified into: the plant. 1. Unipinnate: One rachis with leaflets. Support: Veins provide strength and shape to Paripinnate: Even number of the leaf. leaflets (e.g., Tamarind, Gul Prominence: The central vein, known as the Mohor). midrib, is the most prominent vein in the leaf. Imparipinnate: Odd number of leaflets (e.g., Rose, Margosa). Types of Venation in Flowering Plants 2. Bipinnate: Contains a primary rachis and secondary rachis bearing the Reticulate Venation: leaflets (e.g., Acacia). Description: In this pattern, many veins and 3. Tripinnate: Features primary, veinlets are arranged in a network or reticular secondary, and tertiary rachis, with the pattern. tertiary bearing the leaflets (e.g., Characteristic: This type is typical of Moringa, Oroxylon). dicotyledonous leaves (dicots). 4. Decompound: Highly divided leaves, Example Plants: Common examples include appearing irregularly (e.g., Coriander, beans, maples, and sunflowers. Carrot, Anise). Parallel Venation: b. Palmate Compound Leaves Description: Here, the veins and veinlets run In this type, the leaflets arise from the petiole. parallel to one another along the length of the Further classified based on the number of leaf. leaflets: Characteristic: This type is mainly found in 1. Unifoliate: One leaflet (e.g., Lemon). monocotyledonous plants (monocots), with 2. Trifoliate: Three leaflets (e.g., Bael, some exceptions. Wood Apple). Variations: 3. Multifoliate: More than three leaflets ○ Unicostate Parallel Venation: (e.g., Bombax, Alstonia). Characterized by a single major vein running parallel to the leaf's edge. ○ Multicostate Parallel Venation: Involves multiple major veins running parallel to each other. Example Plants: Common examples include grasses, lilies, and palms. Venation - refers to the arrangement of veins within the lamina (leaf blade) of a leaf. Botany MIDTERMS (by:thea) Phyllotaxy - refers to the arrangement of leaves on the stem, which is crucial for maximizing sunlight exposure, essential for photosynthesis. 1. Alternate (or Spiral) Phyllotaxy: Description: In this arrangement, each node bears a single leaf, and the leaves spiral around the stem. Examples: Common in plants such as tobacco, mustard, and sunflower. 2. Opposite Phyllotaxy: Description: This occurs when two leaves arise Leaf Modification at the same node, positioned opposite each other. - leaves can undergo various modifications to Subtypes: perform secondary functions such as support, ○ Opposite Decussate: Each pair of protection, storage, and more. leaves at one node is at right angles to the pair at the next node. Examples Leaf Tendrils: include madder, sacred basil, and vinca. ○ Opposite Superposed: Leaves are Description: Leaves transform into slender, arranged directly above one another coiled structures that help plants climb and in the same plane. Examples include support themselves. Rangoon creeper and Ixora. Examples: Common in Lathyrus (peas) and Gloriosa. 3. Whorled Phyllotaxy: Leaf Spines: Description: More than two leaves emerge from a single node, arranged in a circular Description: Some leaves evolve into sharp pattern around the stem. spines for protection against herbivores. Examples: Seen in plants such as nerium and Examples: Found in plants like Aloe, Argemone, alstonia. and Acacia. 4. Leaf Mosaic: Phyllode: Description: Leaves are arranged to avoid Description: The petiole becomes flattened overlapping, ensuring all leaves receive and leaf-like to reduce transpiration. adequate light. Older leaves have longer Examples: Notably seen in Australian Acacia. petioles, while younger leaves have shorter petioles, filling the gaps left by the older ones. Scale Leaves: Visual: This arrangement resembles a mosaic, with leaves positioned without shading each Description: Modified leaves that protect other. terminal buds or store food. Examples: Found in plants like Oxalis and Examples: Present in ginger and potato (protect Acalypha. buds) and in onion and garlic (store food). Botany MIDTERMS (by:thea) Pitcher and Bladder Leaves: Insect-Trapping Leaves: Description: Specialized leaves that capture Description: Specialized leaves that trap and digest insects, typical of carnivorous insects to supplement nutrient intake, plants. particularly nitrogen. Examples: Seen in Utricularis (Bladderwort) Function: While they can photosynthesize, and Nepenthes (pitcher plant). these plants benefit from the nutrients obtained through digestion of trapped insects. Storage Leaves: Examples: Common in carnivorous plants found in nutrient-poor environments. Description: These leaves have large, thin-walled parenchyma cells that store water and nutrients. Adaptation: Many succulents exhibit CAM (Crassulacean Acid Metabolism) photosynthesis. Examples: Found in onions, lilies, and other bulbous plants. Flower-Pot Leaves: Description: Some epiphytic plants develop leaves into pouches that house ant colonies. Function: Ants bring in soil and nitrogenous waste, aiding in nutrient absorption for the plant. Examples: Notably in Dischidia. Window Leaves: Description: These leaves are shaped like cones and are buried in sand, exposing only the tip. Function: The exposed part allows light to Morphology of Flowers, Seeds, Fruits penetrate to chloroplasts in the leaf while minimizing water loss. Examples: Found in certain desert plants. Morphology of Flowers Reproductive Leaves: Flower - a specialized structure for reproduction, serving as a modified shoot for seed production. Description: Some leaves produce new plantlets at their tips or along their margins. Function: This allows for vegetative reproduction even if the leaf is detached from the parent plant. Examples: Seen in walking ferns and air plants. Floral Leaves (Bracts): Description: Modified leaves that surround flowers, often brightly colored to attract pollinators. Examples: Found in poinsettia, where the colorful bracts function like petals. Botany MIDTERMS (by:thea) 4 Distinct Whorls: Additional Flower Features ➔ Calyx: Perianth: When the calyx and corolla are Description: The outermost whorl of similar in color and shape, they are the flower, usually green. collectively referred to as perianth (e.g., garlic, Components: Individual parts are called onion, asparagus). sepals. Function: Protects the developing Symmetry of Flowers flower bud. Regular/Symmetrical (Actinomorphic): Can be divided into equal halves by any vertical ➔ Corolla: plane (e.g., Ipomoea, rose, datura). Description: The second whorl, often Irregular/Asymmetrical (Zygomorphic): brightly colored (white or vibrant). Cannot be evenly divided by a single vertical Components: Individual parts are known plane. as petals. Function: Attracts pollinators. ➔ Androecium: Description: The third whorl, representing the male reproductive part of the flower. Components: Each unit is called a stamen, composed of: ○ Filament: The stalk that supports the anther. Special Terms for Floral Structures ○ Anther: Produces pollen grains. Epipetalous: When stamens arise from the ○ Connective: Connects the petals rather than the thalamus. anther to the filament. Gynostegium: The fusion of stamens with the ➔ Gynoecium: gynoecium. Description: The fourth whorl, the Cohesion: The union of stamens among female reproductive part of the flower. themselves. Components: Each unit is called a carpel or pistil, composed of: Types of Stamens and Carpels ○ Stigma: The receptive surface for pollen. Monadelphous: Filaments united to form a ○ Style: The structure that single bundle. connects the stigma to the Diadelphous: Filaments forming two bundles. ovary. Syngenesious: Anthers united to form a ○ Ovary: Contains ovules, which column while the filaments remain free. develop into seeds after Monocarpellary: Ovary with a single carpel. fertilization. Polycarpellary: Ovary with multiple carpels. Apocarpous: Free carpels in the ovary. Flower Classification Syncarpous: United carpels in the ovary. Complete Flower: Contains all four whorls Arrangement of Floral Parts on Thalamus (calyx, corolla, androecium, gynoecium). ➔ Thalamus - thickened part of the stem where Incomplete Flower: Lacks one or more of the the flower parts are attached four whorls. Hermaphrodite/Bisexual Flower: Contains (i) Hypogynous Flowers (Superior Ovary) both stamens and carpels. Description: In these flowers, the thalamus is Unisexual Flower: Contains either stamens or conical, flat, or convex. The sepals, petals, carpels, but not both. and stamens are attached at the base of the ovary, which is situated at the apex. Botany MIDTERMS (by:thea) Example Plants: 2. Axile Placentation ○ Brinjal (Eggplant) Description: Found in polycarpellary ○ China Rose (Hibiscus) syncarpous ovaries that are bilocular or ○ Mustard multilocular. The ventral sutures of each carpel Key Feature: The ovary is superior, meaning it meet at the center, with marginal placentation is above the other floral parts. in each carpel. Example Plants: Onion, China Rose, Ipomoea (ii) Perigynous Flowers (Half-Superior Ovary) Description: The thalamus is flat, with the 3. Parietal Placentation sepals, petals, and stamens growing around Description: Characteristic of polycarpellary the ovary. This creates a cup-like structure. syncarpous ovaries. The placentae develop on Example Plants: the ventral sutures, but the ovary remains ○ Rose unilocular (having one chamber). ○ Strawberry Example Plants: Papaya, Cucurbita (pumpkin, ○ Peach squash) Key Feature: The ovary is partially embedded in the thalamus, making it half-superior. 4. Free Central Placentation Description: Found in polycarpellary syncarpous (iii) Epigynous Flowers (Inferior Ovary) ovaries that are unilocular. The ovules are Description: In these flowers, the thalamus is attached to a central axis and are not fused with the ovary wall, placing the calyx, connected to the walls of the ovary. corolla, and stamens above the gynaecium (the Example Plants: Dianthus (carnation), female reproductive part). Saponaria, Portulaca Example Plants: ○ Sunflower 5. Basal Placentation ○ Cucumber Description: Characteristic of polycarpellary and ○ Apple unilocular ovaries. Only one ovule is present, Key Feature: The ovary is inferior, meaning it and it arises from the base of the ovary. is located below the other floral parts. Example Plants: Sunflower Pollination - the process by which pollen grains are Placentation transferred from the anther of a flower to the - the arrangement of placentae (the tissues that stigma of the same flower or another flower of nurture the developing seeds) within the ovary of the same or related species. a flower. Self-Pollination (Autogamy) 1. Marginal Placentation Description: This type is characteristic of a Description: Pollen from the anther of a flower monocarpellary ovary, where the placenta fertilizes its own stigma. This can happen in forms along the ventral suture (the line of two ways: fusion). ○ Homogamy: The anthers and stigmas Example Plants: Bean, Pea mature at the same time, allowing for immediate self-fertilization. Botany MIDTERMS (by:thea) ○ Cleistogamy: Occurs in flowers that (A) Racemose or Indefinite Inflorescence do not open (e.g., some species of - peduncle continues to grow and produce flowers Commelina), or in underground flowers, in an acropetal manner (older flowers at the facilitating self-pollination without bottom and younger ones at the top). external agents. 1. Raceme: Cross-Pollination (Allogamy) ○ Long peduncle with stalked flowers arranged acropetally. Description: Pollen is transferred from the ○ Examples: Mustard, radish. anther of one flower to the stigma of another 2. Spike: flower, which can be from the same or a different ○ Similar to a raceme but with sessile plant species. This process can be facilitated by flowers (no stalks). various agents: ○ Examples: Rangoon creeper. ○ Insects: Known as entomophily, this is 3. Spadix: a common mode of pollination where ○ Short peduncle with numerous small flowers attract insects through color, unisexual flowers covered by a nectar, and scent. boat-shaped bract called spathe. ○ Wind: Many plants rely on wind ○ Examples: Banana, arum. (anemophily) to disperse their pollen. 4. Catkin: ○ Water: Some aquatic plants can achieve ○ A spike with unisexual sessile flowers pollination through water (hydrophily). on a long peduncle. ○ Animals: Pollination can also be aided ○ Examples: Mulberry, oak. by animals such as bats, birds, and 5. Umbel: even humans. ○ Shortened axis with flowers having equal stalks arranged centripetally. Pollination Mechanism: Pollen is carried to the stigma ○ Examples: Coriander, cumin. by insects, wind, or water. 6. Spikelet: ○ Characteristic of the Gramineae family; Importance of Insect Pollination: Entomophilous small, branched spikes with bracts. plants often exhibit adaptations such as vibrant colors, ○ Contains glumes and a bracteole called enticing scents, and nectar to attract pollinators. palea. 7. Corymb: Genetic Diversity: Cross-pollination generally promotes ○ Short peduncle with bracteate greater genetic diversity, which can enhance the flowers, oldest flowers at the bottom resilience of plant populations. and youngest at the top, all at the same level. Morphology of Inflorescence 8. Capitulum or Head: ○ Flattened receptacle with small sessile flowers (florets); can have both ray Inflorescence - refers to the arrangement and pattern florets and disc florets. of flowers on a plant. ○ Examples: Sunflower, zinnia. 9. Capitate: The stalk of the inflorescence is called the peduncle, ○ Similar to umbel but with sessile and individual flower stalks are known as pedicels. flowers. ○ Example: Acacia. Botany MIDTERMS (by:thea) (B) Cymose Inflorescence - main axis terminates in a flower, leading to a definite sequence of flowering. 1. Solitary Cyme: ○ Ends in a single flower. ○ Examples: Datura, China rose. 2. Uniparous or Monochasial Cyme: ○ Main axis ends in a flower, with one lateral branch producing another 1. Seed Coat flower. Definition: The outermost protective layer of ○ Subdivided into: the seed. Hellicoid Uniparous: Structure: Branching on one side. ○ Dicotyledonous Seeds: Typically have Scorpioid Uniparous: a hard seed coat with two layers: Alternating side branching. Testa: The thick outer layer. 3. Biparous or Dichasial Cyme: Tegmen: The thin inner layer. ○ Ends in a flower with two lateral ○ Monocotyledonous Seeds: Generally branches each also ending in flowers. have a thinner seed coat, which may ○ Examples: Ixora, jasmine. fuse with the fruit wall. 4. Multiparous or Polychasial: ○ Main axis ends in a flower with several 2. Embryo laterally produced flowers. Definition: The main part of the seed that ○ Examples: Nerium, calotropis. develops into a new plant. 5. Special Types: Structure: ○ Includes unique structures like: ○ Consists of an axis with: Hypanthodium (e.g., peepal, Apical Meristem: Develops into fig). the plumule (shoot). Verticillasters (e.g., sacred Radicle: Develops into the root. basil, mentha). Cotyledons: One or two seed Cymose-Umbel (e.g., onion). leaves, determining whether the plant is a monocot (one cotyledon) or dicot (two cotyledons). 3. Endosperm Definition: Nutritive tissue that supports the embryo. Classification: ○ Endospermic (Albuminous) Seeds: Endosperm remains during germination and is partially absorbed by the embryo (e.g., wheat, rice). ○ Non Endospermic (Exalbuminous) Morphology of Seeds Seeds: The endosperm is fully absorbed by the embryo during Seeds - crucial part of plant reproduction and are development, leaving no trace (e.g., defined as fertilized ovules. sunflower, tamarind). ○ Perispermic Seeds: The nucellus Phanerogams (seed-producing plants) develops into a large storage tissue along with the embryo and endosperm (e.g., nutmeg, cardamom). Botany MIDTERMS (by:thea) Morphology of Fruits Fruits - the mature ovaries of flowering plants (Phanerogams) that develop after fertilization. The outer covering of the fruit is known as the pericarp, which consists of three layers: Descriptive Terms for Seeds ➔ Epicarp: The outermost layer, which can be thin or thick. ➔ Hilum: Attachment point of the seed to its stalk. ➔ Mesocarp: The middle layer, which may be ➔ Micropyle: Small opening through which water fleshy or spongy. enters during germination. ➔ Endocarp: The innermost layer, which can be ➔ Raphe: Longitudinal marking of the stalk in thin, thick, or woody. anatropous ovules. ➔ Funicle: Stalk that attaches the ovule to the placenta. ➔ Chalaza: The basal portion of the ovule where the stalk attaches. Special Features of Seeds - assist in their dispersal and survival Classification of Fruits Aril: A fleshy growth from the hilum that covers the seed (e.g., nutmeg). Arillode: An outgrowth from the micropyle covering the seed (e.g., cardamom). Arista (Awn): Stiff, bristle-like appendages found in some grass flowers. Caruncle: A warty outgrowth from the micropyle (e.g., castor). Hairs: Hair-like projections on the seed (e.g., cotton). 1. Simple Fruits - develops from a single carpel or a syncarpous gynoecium ➔ Types: Dry Fruits: Classified based on whether they dehisce (open) or remain indehiscent. Dehiscent Fruits: ○ Legume (Pod): Opens along both margins (e.g., pea, Functions of Seeds tamarind). 1. Reproduction: Germinate to form new plants. ○ Capsule: Opens in various 2. Species Spread: Facilitate the dispersal of the ways; can be multi-chambered species. (e.g., cotton, poppy). 3. Perennation: Ensure the survival of species ○ Follicle: Opens along one through successive seed formation. margin (e.g., calotropis). ○ Siliqua: Two-chambered, opens from the base (e.g., radish). Botany MIDTERMS (by:thea) Indehiscent Fruits: 2. Indehiscent Fruits ○ Achene: One-seeded, pericarp free of seed coat (e.g., rose). ➔ Achene: One-seeded, pericarp separate; e.g., ○ Caryopsis (Grain): clematis. One-seeded, pericarp fused ➔ Caryopsis: Seed fused with pericarp; e.g., rice. with seed coat (e.g., maize). ➔ Nut: Hard pericarp; e.g., areca nut. ○ Nut: Hard, one-seeded (e.g., ➔ Samara: Winged fruit; e.g., dioscorea. cashew). ➔ Schizocarp: Splits into mericarps; e.g., cumin. ○ Samara: Winged, one- or two-seeded (e.g., shorea). 3. Fleshy Fruits ○ Schizocarp: Splits into indehiscent pieces called ➔ Drupe (Stone Fruit): Fleshy with hard mericarps (e.g., coriander). endocarp; e.g., mango. ➔ Berry: Many-seeded, fleshy; e.g., guava. 2. Aggregate Fruits ➔ Pepo: Pulpy, many-seeded; e.g., cucumber. Definition: Formed from many carpels of a single ➔ Pome: Fleshy thalamus; e.g., apple. flower or apocarpous gynoecium. ➔ Hesperidium: Many-seeded with citrus Example: Raspberry. characteristics; e.g., orange. 3. Compound Fruits Definition: Develop from multiple flowers. Examples: Figs, pineapples. 4. False Fruits (Pseudocarps) Definition: Fruits where other floral parts contribute to the fruit structure. Examples: ○ Strawberry (thalamus) ○ Cashew nut (peduncle and thalamus) ○ Apple (thalamus) Uses of Fruits 1. Nutritional Value: Fruits are important sources of carbohydrates, vitamins, and minerals. 2. Commercial Uses: Fleshy fruits are used for extracting pectin and oils. 3. Spices: Fruits like chilies and black pepper are widely used in cooking. Types of Fruits 1. Dehiscent Fruits ➔ Legume (Pod): Opens along both sides; e.g., senna, pea. ➔ Capsule: Multi-chambered; e.g., cardamom, digitalis. ➔ Follicle: Opens at one side; e.g., rauwolfia. ➔ Siliqua: Two-chambered; e.g., mustard. Botany MIDTERMS (by:thea) Plant Metabolism Aerobic Cellular Respiration - Plants and other organisms utilize aerobic Energy and ATP cellular respiration to convert sugars into usable energy (ATP). Energy is defined as the ability to do work, and it exists in several forms: 1. Glycolysis: This initial stage occurs in the cytoplasm, where glucose is partially broken ➔ Kinetic Energy: The energy of motion, such as down into pyruvate, generating a small amount a rolling ball or moving air. of ATP and electron carriers (NADH). ➔ Heat Energy: Energy transferred between systems without performing work, influencing 2. Krebs Cycle (Citric Acid Cycle): Occurring in molecular movement. the mitochondria, this cycle further processes ➔ Potential Energy: Stored energy based on an pyruvate to produce more electron carriers object's position. For instance, a ball at the top (NADH and FADH₂) and ATP. of a hill has potential energy. ➔ Chemical Energy: A form of potential energy 3. Electron Transport Chain (ETC): Located in stored in the bonds of molecules. When the inner mitochondrial membrane, the ETC high-energy, less stable molecules react to form uses electrons from NADH and FADH₂ to lower-energy, more stable products, energy is create a proton gradient. Oxygen serves as released. the final electron acceptor, combining with electrons to form water. The energy from this Adenosine Triphosphate (ATP) gradient is used to synthesize ATP through a - Energy currency of the cell process called oxidative phosphorylation. ➔ Adenine: A nitrogenous base. Key Points: ➔ Ribose: A five-carbon sugar. ➔ Photosynthesis produces glucose, which is ➔ Three Phosphate Groups: The bonds between then broken down in cellular respiration to these phosphate groups are unstable, and generate ATP. breaking them releases energy. ➔ ATP provides the energy needed for various cellular activities. Phosphorylation ➔ Oxygen is vital for aerobic respiration, forming - addition of a phosphate group (PO₄³⁻) to a water and releasing carbon dioxide as molecule, particularly in synthesizing ATP from byproducts. adenosine diphosphate (ADP). ➔ NADH (Nicotinamide Adenine Dinucleotide) ➔ FADH₂ (Flavin Adenine Dinucleotide) Mitochondria are the cell's energy factories, composed of two lipid bilayer membranes. The inner membrane has folds called cristae, while the space between the membranes is the intermembrane space. The central region is the matrix. ATP, the main energy currency of the cell, is produced along the inner membrane by various molecules working together. Botany MIDTERMS (by:thea) 2. Conversion: ○ Each pyruvate (3-carbon) is converted glucose + oxygen → carbon dioxide + water + energy into acetyl-CoA (2-carbon). ○ The process involves the removal of one Step 1: Glycolysis carbon, which is released as carbon - the first step in the process of cellular dioxide (CO₂). respiration, where glucose is broken down to 3. Electron Capture: extract energy. ○ The electrons released during the - provides energy without oxygen and produces conversion are captured by NAD⁺, NADH for later ATP generation in oxidative reducing it to NADH. phosphorylation. 4. Production: ○ For each glucose molecule (yielding 2 ➔ Location: Cytoplasm pyruvate), 2 acetyl-CoA molecules and ➔ Inputs: 1 molecule of glucose (C₆H₁₂O₆), 2 NAD⁺, 2 NADH molecules are produced. 2 ATP (initial investment) ➔ Outputs: 2 molecules of pyruvate, 4 ATP Step 3: The Citric Acid (Krebs) Cycle (gross), 2 NADH, 2 H₂O - crucial for energy extraction from carbohydrates, fats, and proteins, providing electron carriers Process: (NADH and FADH₂) that will be used in the next 1. Glucose Activation: step of cellular respiration. ○ Glucose is phosphorylated using 2 ATP, forming fructose-1,6-bisphosphate. ➔ Location: Mitochondrial matrix 2. Cleavage: ➔ Inputs: 2 acetyl-CoA ○ Fructose-1,6-bisphosphate splits into ➔ Outputs: 2 ATP, 6 NADH, 2 FADH₂, 4 CO₂ two glyceraldehyde-3-phosphate (G3P) molecules. Process: 3. Energy Harvesting: 1. Entry of Acetyl-CoA: ○ Each G3P is oxidized, producing 2 ○ Each acetyl-CoA from pyruvate NADH and generating 4 ATP through oxidation enters the citric acid cycle. substrate-level phosphorylation. 2. Enzyme Activity: ○ Net gain is 2 ATP after accounting for ○ The cycle involves various enzymes that the initial investment. facilitate the breakdown of acetyl-CoA, 4. Formation of Pyruvate: releasing energy. ○ The end products are 2 molecules of 3. Energy Production: pyruvate. ○ For each turn of the cycle (one acetyl-CoA): Step 2: Pyruvate Oxidation 1 ATP is produced directly. - links glycolysis to the citric acid cycle (Krebs 3 NADH and 1 FADH₂ are cycle) and is crucial for continuing aerobic generated, which serve as respiration by preparing acetyl-CoA for further high-energy electron carriers. energy extraction. 4. Carbon Dioxide Release: ○ During the cycle, 2 carbon atoms are ➔ Location: Mitochondrial matrix released as CO₂ for each acetyl-CoA ➔ Inputs: 2 pyruvate, 2 NAD⁺ that enters (totaling 4 CO₂ for 2 ➔ Outputs: 2 acetyl-CoA, 2 CO₂, 2 NADH acetyl-CoA). Process: 1. Transport: ○ Pyruvate molecules produced in glycolysis are transported into the mitochondria. Botany MIDTERMS (by:thea) Step 4: Oxidative Phosphorylation ATP Synthase: Protons flow back into the - the final stage of cellular respiration and mitochondrial matrix through ATP synthase, a produces the majority of ATP, powering cellular turbine-like enzyme. The flow causes the processes. enzyme to rotate, converting the stored energy - Oxygen serves as the terminal electron acceptor into kinetic energy. in the ETC, combining with electrons and ATP Production: This kinetic energy is then protons to form water. used to convert ADP and inorganic phosphate (Pi) into ATP, completing the energy production ➔ Location: Inner mitochondrial membrane. process in cellular respiration. Process: Photosynthesis Overview 1. Electron Transport Chain (ETC) - process by which photoautotrophs (like plants, Process: NADH and FADH₂ donate their algae, and certain bacteria) convert sunlight into electrons to protein complexes in the inner chemical energy stored in carbohydrates. membrane, activating these complexes. Proton Pumping: As electrons are transferred ➔ Photoautotrophs: Organisms like plants, algae, through the complexes, H⁺ ions are pumped and certain bacteria (e.g., cyanobacteria, green from the mitochondrial matrix into the and purple sulfur bacteria) that use sunlight to intermembrane space, creating a proton produce their own food. gradient. ➔ Heterotrophs: Organisms such as animals, Oxygen's Role: At the end of the chain, oxygen fungi, and most bacteria that rely on sugars acts as the terminal electron acceptor, produced by photosynthetic organisms for combining with electrons and protons to form energy. water (H₂O). This is essential; without oxygen, ➔ Chemoautotrophs: A unique group of bacteria the chain cannot function. that synthesize sugars by extracting energy from inorganic compounds, rather than using sunlight. Photosynthesis Equation Substrates: ➔ Sunlight: Provides the energy needed for the reaction. ➔ Carbon Dioxide (CO₂): A low-energy molecule that serves as a carbon source. ➔ Water (H₂O): Acts as a reactant and is also produced during the process. Products: ➔ Glyceraldehyde-3-phosphate (GA3P): A simple carbohydrate that can be further converted into glucose, sucrose, and other sugars necessary for life. 2. Chemiosmosis ➔ Oxygen (O₂): Released as a waste product. Proton Flow: The proton gradient created during the ETC generates potential energy, similar to a battery. Botany MIDTERMS (by:thea) Structures of Photosynthesis Steps of Photosynthesis ➔ Photosynthesis - primarily in the mesophyll layer of leaves, although other plant parts, like Light-Dependent Reactions: stems, can also engage in this process. Location: Thylakoid membranes of chloroplasts. 1. Mesophyll Process: Chlorophyll absorbs sunlight, The middle layer of leaf cells where converting it into chemical energy. photosynthesis predominantly takes place. Inputs: Requires water (H₂O). Outputs: Produces oxygen (O₂) and energy in 2. Stomata the form of ATP and NADPH. Tiny openings (stoma) located mainly on the underside of leaves. Light-Independent Reactions (Calvin Cycle): Facilitate gas exchange, allowing carbon dioxide (CO₂) to enter and oxygen (O₂) to exit. Location: Stroma of chloroplasts. Flanked by guard cells that regulate their Process: Utilizes ATP and NADPH generated opening and closing to minimize water loss. from the light-dependent reactions to convert carbon dioxide (CO₂) into sugar molecules. 3. Chloroplasts Functionality: While these reactions don’t Organelles where photosynthesis occurs in directly use light, they rely on the products of autotrophic eukaryotes. the light-dependent reactions. Certain enzymes Double Membrane Envelope: Comprises an involved are activated by light, outer and an inner membrane. Thylakoids: Disc-shaped structures stacked in ➔ NADPH (nicotinamide adenine dinucleotide the chloroplast. phosphate) ○ Chlorophyll: A pigment embedded in the thylakoid membrane that absorbs light energy. ○ Thylakoid Lumen: The internal space enclosed by the thylakoid membrane. ○ Grana: Stacks of thylakoids. 4. Stroma The liquid-filled space surrounding the grana within the chloroplast. Contains enzymes and molecules necessary for the light-independent reactions of photosynthesis. Light-Dependent - The primary goal of light-dependent reactions is to convert solar energy into chemical energy in the form of ATP and NADPH, which are then used in the light-independent reactions to produce sugar molecules. Botany MIDTERMS (by:thea) What is Light Energy? 2. These electrons are transferred through a series of proteins (pheophytin, Electromagnetic Radiation: The sun emits plastoquinone, etc.) and ultimately help various forms of electromagnetic radiation, of in ATP synthesis via which visible light is only a small part. Light photophosphorylation. travels in waves, and each type has a specific 3. The electrons lost from PSII are wavelength, which determines its energy level. replenished by oxidizing water, releasing oxygen as a byproduct. Wavelength and Energy: Shorter wavelengths 4. Photosystem I (PSI) absorbs more (like violet light) carry more energy than longer light, and its excited electrons are used wavelengths (like red light). The visible spectrum to reduce NADP+ to NADPH. ranges from approximately 400 nm (violet) to 700 nm (red). Absorption of Light Pigments: Light energy is absorbed by pigments in the chloroplasts, which have specific ranges of wavelengths they can absorb. Photosynthetically Active Radiation (PAR): The wavelengths between 400 nm and 700 nm Photosystems and Light Energy are essential for photosynthesis. Conversion Chlorophylls and Carotenoids: ○ Chlorophyll a and b are the primary pigments in plants, absorbing blue and Photosynthesis involves two types of photosystems red light but reflecting green, making found in the thylakoid membrane: Photosystem II (PSII) leaves appear green. and Photosystem I (PSI). ○ Carotenoids (like β-carotene) absorb ➔ Structure: blue and green light, reflecting yellow, ◆ Antenna Complex: Contains 300–400 orange, and red. They help dissipate chlorophyll a and b molecules and other excess energy and protect the plant pigments like carotenoids, which absorb from damage. light energy. ◆ Reaction Center: Contains a pair of ➔ Spectrophotometer - an essential tool for special chlorophyll a molecules that can studying pigments, as it can determine which release energized electrons. wavelengths of light a substance absorbs. Understanding Absorption Spectra Each pigment has a unique absorption spectrum, which shows the wavelengths it can absorb. This allows plants to harness a broader range of light energy, essential for photosynthesis, especially in environments with limited light. The Process of Light-Dependent Reactions Location: These reactions occur in the thylakoid membranes of chloroplasts. Z-Scheme: The pathway of electron flow: 1. Photosystem II (PSII) absorbs light energy, exciting electrons. Botany MIDTERMS (by:thea) Light Absorption and Energy Transfer Energy Buildup: This buildup of protons creates potential energy due to their mutual repulsion, as they all 1. Photon Absorption: When a photon (light carry a positive charge. particle) is absorbed by any chlorophyll, it excites the chlorophyll molecule. Proton Flow: Protons flow back into the stroma through 2. Energy Transfer: The excited energy is a specialized protein channel called ATP synthase. This transferred from chlorophyll to chlorophyll within movement is akin to water rushing through a hole in a the antenna complex, eventually reaching the dam. reaction center after about a millionth of a second. ATP Synthesis: The energy released from this proton 3. Electron Excitation: In the reaction center, the flow allows ATP synthase to attach a third phosphate energy excites the chlorophyll a molecules, group to ADP, forming ATP. allowing them to oxidize and release high-energy electrons. Chemiosmosis: The process of protons moving from a region of high concentration (thylakoid lumen) to low Electron Transport Chain concentration (stroma) through a semipermeable membrane is known as chemiosmosis. The excited electrons from PSII (specifically from the reaction center known as P680) are transferred to the primary electron acceptor. From there, the electrons move through an electron transport chain: 1. Plastoquinone (Pq): Accepts electrons from P680. 2. Cytochrome b6f Complex: Transfers electrons from Pq to plastocyanin (Pc), using energy from the electron flow to pump protons into the thylakoid lumen. Water Photolysis: 3. Plastocyanin (Pc): Carries the electrons to PSI. In photosystem II (PSII), the lost electron from Proton Pumping chlorophyll (P680) is replaced by splitting water (H₂O). As electrons move through the cytochrome b6f This process releases: complex, protons are pumped from the stroma ○ Two electrons (to replace P680’s into the thylakoid lumen, creating a proton electron) gradient that will be used to generate ATP later ○ Two protons (H⁺), which accumulate in in the photosynthesis process. the thylakoid lumen, aiding ATP synthesis. ○ One atom of oxygen, which pairs to Photophosphorylation form diatomic oxygen (O₂). About 10% of this oxygen is used for respiration in - the process of synthesizing ATP during the the leaf; the rest is released into the light-dependent reactions of photosynthesis. atmosphere. Proton Gradient Formation: As electrons move Reduction of NADP+: through the electron transport chain in the thylakoid Electrons arriving at photosystem I (PSI) need membrane, protons (H⁺ ions) are pumped into the re-energizing, which occurs when PSI absorbs thylakoid lumen, creating a high concentration of protons another photon. inside the lumen compared to the stroma. The energized electrons from PSI (P700) are transferred to NADP⁺, reducing it to NADPH. Botany MIDTERMS (by:thea) ➔ PSII generates a proton gradient for ATP production, while PSI produces NADPH. Photorespiration and Photosynthetic ➔ Both photosystems