Legumes and the Nitrogen Cycle

Overview and Vocabulary of Legumes

• Definitions:     * Legume: Plants belonging to the family Fabaceae, characterized by specialized fruit and often nitrogen-fixing capabilities.     * Bean: A legume seed containing high levels of both proteins and lipids (fats/oils).     * Pea: A legume seed containing high levels of protein but no lipids (fats/oils).
• Core Concepts:     * Nitrogen Cycle: The process by which nitrogen is converted between its various chemical forms.     * Nitrogen Fixation: The conversion of atmospheric nitrogen ($N_2$) into organic compounds or forms usable by plants.     * Nitrogen-Fixing Bacteria: Bacteria that can convert nitrogen gas into ammonia; legumes specifically pair with Rhizobia.     * Root Nodules: Specialized structures on legume roots that house nitrogen-fixing bacteria.     * Cotyledon: Seed leaves that provide nutrition for the developing embryo; legumes are eudicots with two cotyledons.     * Phytoestrogens: Molecules found in legumes (especially soybeans) that mimic human estrogen, though they are much weaker than mammal estrogen.     * Forage Crops: Plants grown specifically to be grazed by livestock or harvested as hay (e.g., Alfalfa).     * Biodiesel: A renewable fuel made from vegetable oils (lipids) via chemical processes, often used in diesel engines.
• Other Key Terms:     * Glycerol: A component of lipids that bonds with fatty acid chains.     * Fatty Acid Chain: Carbon atoms bonded to hydrogen; the composition determines if a fat is saturated or unsaturated.     * Pigeon Peas, Peanuts, Soybeans: Primary legumes consumed by humans.     * Canola Oil: An oil often used for cooking or biodiesel.     * Drying, Semidrying, and Nondrying Oils: Classifications of oils based on their reaction to oxygen and tendency to harden.

Legume Evolution and Taxonomy

• Classification:     * Domain: Eukarya     * Kingdom: Plantae     * Division: Angiosperm (fruiting and flowering plants)     * Class: Eudicots/Dicots (having two cotyledons)
• Geographical Distribution: Legumes exist globally in most terrestrial environments but are notably absent from many waterways.
• Phylogenetic Relationships:     * The Lentil (Lens culinaris) is closely related to the Garden Pea (Pisum sativum).     * The Lotus (L. japonicus) is a legume but is distantly related to many common bean/pea varieties.     * Key Lineages (Estimates):         * Papilionoideae: $45-50$ Million Years Ago (MYA).         * Phaseoleae: $41-42$ MYA.         * Trifolieae: $37-38$ MYA.         * Viceae: $25-29$ MYA.

Common Characteristics of Legumes

• Germination and Structure:     * Cotyledons: These are the first structures seen above ground after germination; they are seed leaves, not true leaves.     * Hypocotyl: The stem-like structure that connects the cotyledons to the root system.     * Vining Habit: Many legumes grow as vines, utilizing long stems with specialized tendrils to wrap around structures and climb toward sunlight.
• Leaf Morphology:     * Most legumes have pinnately compound leaves, where the leaf blade is divided into multiple segments connected to a single petiole (the structure connecting the leaf blade to the stem).
• Floral Structure:     * Banner: Two fused petals that create a large, sailboat-like banner at the top of the flower.     * Reproductive Protection: Three fused petals hide the reproductive structures.     * Pollination: Because the flower is "closed up," it requires strong insects (like bees) to pry the petals apart to reach nectar, causing pollen to transfer to the insect.
• Fruit Morphology:     * Botanically, there are no vegetables; legumes produce fruit.     * Dehiscent Follicle Fruit: The fruit is dry at maturity and splits along at least two seams.     * Harvesting: Humans typically harvest and eat legume fruits (pods) before they reach full maturity.     * Seed Dispersal: At maturity, the pod is meant to split open and scatter seeds, reducing competition among new plants.

Symbiotic Nitrogen Fixation

• Relationship with Rhizobia: Legumes form a symbiotic relationship with bacteria from the genus Rhizobium (Rhizobia).
• Mechanisms of Infection:     1. The legume root hair secretes a chemical called NOD factor into the soil.     2. Rhizobia "smell" the NOD factor and migrate toward the root hair.     3. The root hair corrals the bacteria and builds a nodule around them.     4. Nodule Function: A "house" for bacteria; inside, they are visualized as pink, hot-dog-shaped structures or dark blobs in cross-sections.
• The Exchange:     * Rhizobia capture atmospheric nitrogen and convert it into a form of fertilizer for the plant.     * The legume plant provides sugars produced via photosynthesis to the bacteria.     * Bacteria are kept separate from the plant's vascular system.

The Nitrogen Cycle and Human Environmental Impact

• Atmospheric Composition: Earth's atmosphere consists of approximately $78\%$ Nitrogen, $20\%$ Oxygen, and $2\%$ other gases (Water, $CO_2$, Methane).
• The Natural Cycle:     * Bacteria convert Nitrogen into a plant-usable form.     * Plants store Nitrogen in cells.     * Animals eat plants to obtain Nitrogen.     * Decomposers (fungi/bacteria) break down dead matter, releasing Nitrogen back into the atmosphere.     * Lightning: A rare event that can fix Nitrogen into the soil.
• Human Alteration of the Cycle:     * Haber Process: Humans create synthetic fertilizer by burning Natural Gas (fossil fuel) to convert nitrogen gas into a usable form.     * Historical Note (Fritz Haber): A Jewish scientist who pioneered fertilizer production to aid German explosives for WWI; he also developed Zyklon B gas (used in the Holocaust) and was eventually exiled from Germany.
• Consequences of Fertilizer Use:     * Runoff: Excess fertilizer is washed into waterways by rain.     * Algae Blooms: Nitrogen causes massive algae growth in water.     * Dead Zones: When algae die, decomposers consume all available oxygen to break them down. This creates a place in a body of water with little to no oxygen where nothing can live.     * Mississippi River Example: Runoff creates a barrier layer in the Gulf of Mexico, cutting off saltier water below from air oxygen, leading to mass fish kills.

Biochemistry of Lipids: Fats and Oils

• Composition: Lipids consist of a Glycerol group and a Fatty Acid Chain (Carbon atoms bonded to Hydrogen atoms).
• Saturated Lipids:     * Every Carbon atom is bonded to as many Hydrogen atoms as possible.     * The chain is linear and straight, allowing molecules to pack tightly.     * Stable and solid at room temperature (e.g., butter, coconut oil).     * Health Warning: Doctors recommend limiting saturated fat; coconut oil is saturated and not necessarily healthier than butter.
• Unsaturated Lipids:     * Not every Carbon atom is full of Hydrogen; a double bond between Carbon atoms exists.     * The chain is kinked at the site of the double bond.     * Molecules move fluidly; stable as liquids at room temperature (e.g., Corn, Soybean, Peanut, Canola oils).
• Hydrogenation: The chemical process of forcing a double bond in an unsaturated lipid to break and bond with Hydrogen, making the chain straight. This turns liquid oil into a solid at room temperature (e.g., margarine).

Notable Legume Varieties

• Peanut (Arachis hypogaea):     * Origin: South America.     * Nutrition: Approximately $25-30\%$ protein and $45-50\%$ lipids per seed.     * Geocarpic Growth: Flowers are above ground; once pollinated, the structure (pedicel/peg) dives into the soil where the fruit develops underground.     * Structure: The two halves of a peanut are the two cotyledons.     * Allergy: Occurs when the immune system treats peanut protein as poison. GM (Genetically Modified) peanuts without the problem protein are in development.     * Peanut Oil: A non-drying oil; can be used directly as biodiesel in diesel engines.
• Soybean (Glycine max):     * Origin: Asia.     * Nutrition: Approximately $30-50\%$ protein and $13-25\%$ lipids per seed.     * Genetics: Unique for having a genome composed of $60\%$ jumping genes.     * Pollination: Requires bees; declining bee populations force farmers to hire beekeepers.     * Usage: Used for phytoestrogens, Soy Lecithin (emulsifier), and biofuel.
• Alfalfa (Medicago sativa):     * Origin: Asia.     * Usage: Forage crop (hay) for animal feed. Used as Green Manure (grown to fix nitrogen, then tilled back into the soil to reduce chemical fertilizer use).     * Distinction: "Marsh Hay" is actually Reed Canary Grass (invasive) and is nutritionally poor for cows.
• Lead Tree (Leucaena leucocephala):     * Origin: Central America.     * Usage: Rapid growth makes it a replacement for pine in paper products.     * Features: Puff-ball flowers made of many individual fused flowers; deeply pinnately compound leaves.

Other Legumes of Interest

• Carob: Chocolate substitute.
• Copaifera: Resin for paints and lacquers.
• Fenugreek: Spice.
• Indigo: Dye.
• Licorice: Herb/extract.
• Mesquite: Charcoal.
• Rosary Pea: Jewelry (seeds).
• Rosewood: Timber.
• Senna Pods: Laxative.
• Tamarind: Seasoning.
• Tuba-root: Insecticide.

Lab Application and Procedures

• Lipid Testing: Iodine is added to oils. Iodine attaches to the double bonds of unsaturated fatty acid chains, causing the iodine color to disappear.
• Oil Classification: Oils are classified as Drying, Semi-drying, or Non-drying based on their reaction to oxygen.
• Saponification (Soap Making):     * Recipe: Soybean oil, olive oil, and coconut oil.     * Process: Fat + Lye ($NaOH$) + Heat + Water $\rightarrow$ Glycerol + Soap.     * Trace: The ridge of soap left on the surface when a lift out spoon drips, indicating the soap is ready to be poured.