Rucete ✏ Campbell Biology In a Nutshell
Unit 6 PLANT FORM AND FUNCTION — Concept 37.2 Plant Roots Absorb Many Types of Essential Elements from the Soil
Plants require a wide range of essential elements from the soil to complete their life cycles. Although most of their mass comes from water and CO₂, inorganic minerals play critical roles in structure, enzyme function, and energy transfer.
Plant Composition and Essential Elements
- 80–90% of a plant's fresh mass is water.
- 96% of dry mass is carbohydrates (C, H, O from CO₂ and H₂O).
- Only ~4% of dry mass is inorganic minerals, but these are vital.
- An element is essential if it’s required for life cycle completion and reproduction.
Macro- and Micronutrients
Macronutrients (needed in large amounts)
- C, H, O, N, P, S: Core components of organic molecules.
- K, Ca, Mg: Structural and signaling roles.
- Nitrogen: The most limiting nutrient; crucial for proteins, nucleic acids, and chlorophyll.
Micronutrients (needed in trace amounts)
- Fe, Mn, Zn, Cu, B, Cl, Mo, Ni
- Often serve as enzyme cofactors.
- Example: Iron is essential for electron transport in mitochondria and chloroplasts.
- Sodium (Na⁺) is required by C₄ and CAM plants.
Diagnosing Mineral Deficiencies
- Hydroponic culture reveals essential nutrients by growing plants in defined solutions.
- Removing one nutrient shows symptoms of deficiency:
- Nitrogen deficiency: yellowing of older leaves
- Phosphorus deficiency: stunted growth, purple veins
- Potassium deficiency: weak stems, dry edges on leaves
- Deficiency symptoms vary by:
- Element function
- Its mobility in the plant:
- Mobile nutrients (e.g., N, Mg): older leaves affected first
- Immobile nutrients (e.g., Fe): symptoms appear in young leaves first
Human Connection: Nutritional Quality of Plants
- Humans rely on plants for micronutrients like Fe, Zn, and I.
- Micronutrient malnutrition affects people even where calories are sufficient.
- Modern crops may have lower nutrient content due to:
- Elevated CO₂: more carbohydrates, but diluted minerals
- Selective breeding for yield over nutrition
Application: Soil and Fertilization
- Soil pH affects mineral availability:
- Slightly acidic soils help cation exchange
- Highly acidic soils release toxic Al³⁺, damaging roots
- Some plants (e.g., tobacco, papaya) can be genetically modified to secrete citric acid, which binds Al³⁺.
- Moderation is key: Excess nutrients can be harmful.
In a Nutshell
Plants absorb 17 essential elements—nine macronutrients and eight micronutrients—besides water and CO₂. These nutrients are indispensable for photosynthesis, DNA synthesis, and cellular function. Understanding their roles helps improve crop health, human nutrition, and sustainable agriculture.