Mechanisms of Plant Nutrition and Synthesis

NUTRITION IN PLANTS

1. Introduction to Nutrition

• Nutrition involves activities by which an organism obtains, processes, and uses food for life functions.
• Energy is crucial for synthesizing organic substances and for life functions.
• Energy is derived from chemical compounds or light.

2. Types of Nutrition

2.1. Autotrophic Nutrition

• Organisms manufacture their own food.
• Derived from 'auto' (self) and 'trophein' (feed).
• Occurs either by using sunlight (phototropism/photosynthesis) or inorganic chemicals (chemotropism/chemosynthesis).
• Types:
  • Photosynthesis
  • Chemosynthesis
• Examples: Green plants, algae, some bacteria.

2.2. Heterotrophic Nutrition

• Organisms cannot manufacture their own food and obtain it from various sources.
• Derived from 'heteros' (different).
• Food is obtained by ingestion and digestion of organic compounds from the environment.
• Examples: Non-green plants (e.g., Fungi).

3. Chemosynthesis

• Process by which colourless, aerobic bacteria use energy from the oxidation of certain inorganic compounds to synthesize carbohydrates and other organic compounds.
• Does not require light as an energy source. Energy comes from chemical reactions.
• Chemotrophs do not use water as the hydrogen donor to reduce CO₂, so oxygen is not liberated.

3.1. Chemosynthesis Equation

• Equations vary based on the chemical energy source.
• Reactants:
  • Carbon-containing inorganic compound (e.g., CO₂, methane) – carbon source.
  • Chemical energy source (e.g., hydrogen gas, hydrogen sulphide, ferrous iron).
• Products:
  • Organic compound (e.g., sugar, amino acid).
  • Transformed version of the energy source (e.g., elemental sulfur, ferric iron).
• Example (Sulphur Bacteria):
  • 12HS + 6CO₂ → C₆H₁₂O₆ + 6H₂O + 12S
  • 2HS + CO₂ → C H O + H₂O + 2S (simplified)

3.2. Chemosynthetic Bacteria Types

• Sulphur bacteria:
  • Grow in sulphur springs and stagnant water containing hydrogen sulphide.
  • Obtain energy by oxidizing hydrogen sulphide (HS).
  • Consume hydrogen sulphide gas and produce solid elemental sulfur as a waste product.
• Iron bacteria:
  • Grow in lakes, marshes, and water containing ferrous iron.
  • Obtain energy by oxidizing ferrous hydroxide to ferric hydroxide.
  • Cause bog iron ore and iron oxide deposits, and often make water reddish.
• Nitrifying bacteria:
  • Live in soil containing ammonia (toxic inorganic nitrogen compound).
  • Convert ammonia (NH₃) into nitrites, and then to nitrates.
  • Energy liberated from ammonia oxidation is used to produce metabolic products, with CO₂ absorbed from the air.
  • Two types:
    • Nitrite bacterium (Nitrosomonas): 2NH₃ + 3O₂ → 2HNO₂ + 2H₂O (Ammonia to Nitrite)
    • Nitrate bacterium (Nitrobacter): 2HNO₂ + O₂ → 2HNO₃ (Nitrite to Nitrate)

4. Photosynthesis

• Chemical process where plants, algae, and certain bacteria convert carbon dioxide and water into glucose using sunlight and chlorophyll.
• Oxygen is liberated (from water) as a by-product.
• It is an oxidation-reduction process: water is oxidized, and carbon dioxide is reduced to carbohydrates.
• Essential for life on Earth as it provides oxygen.
• Plants obtain gases through stomata (pores of intercellular spaces).
• Plants require oxygen for respiration and carbon dioxide for photosynthesis.

4.1. Photosynthesis Equation

6CO₂ (Carbon Dioxide) + 6H₂O (Water) + Light Energy → C₆H₁₂O₆ (Glucose) + 6O₂ (Oxygen)

4.2. Photosynthesis Stages

• Takes place in the chloroplast (of the leaf), which contains the green pigment chlorophyll.
• Light energy is trapped by chlorophyll.
• Maximum photosynthesis occurs in red and blue light due to high absorption.
• Two stages:
  1. Light reaction
  2. Dark reaction

4.2.1. Light Reactions

• First stage, occurs in the presence of light in the grana (singular: granum, stack of thylakoids) of the chloroplast.
• Photolysis of water: Water supplies hydrogen ions for the reduction of oxidized NADP into NADPH. This is the source of oxygen released.
• Chlorophyll absorbs light energy, which is converted into chemical energy and stored as:
  • Adenosine triphosphate (ATP)
  • Nicotinamide Adenine dinucleotide phosphate (NADPH)

4.2.2. Dark Reactions

• Second stage, not dependent on light and chlorophyll.
• Occurs in the stroma of the chloroplast.
• Chemical energy stored (ATP and NADPH) from the light reaction is utilized.
• CO₂ is converted to carbohydrates (carbon fixation).
• Also known as the Calvin cycle.

4.3. Differences between Light and Dark Reactions

5. Limiting Factors of Photosynthesis

• Effect of light intensity:
  • Rate of photosynthesis increases with light intensity.
  • Light saturation: point where further light addition doesn't benefit, and other factors become limiting (usually 10,000 lux).
• Carbon dioxide concentration:
  • Rate increases with CO₂ concentration up to a point.
  • Beyond this point, further increase in CO₂ does not increase the rate.
• Temperature:
  • Both light and dark reactions are enzyme-controlled, making them temperature-sensitive.
  • Rate increases with temperature up to an optimum, then decreases as enzymes denature.
• Other factors:
  • Chlorophyll concentration
  • Water
  • Pollution

6. Heterotrophic Nutrition in Plants

• Heterotrophic plants cannot manufacture their own food; they get carbohydrates from various sources.
• Parasites: Depend on other living plants or animals for food.
  • Total parasites (e.g., dodder, broomrape): Never green, cannot make their own food, draw all nourishment from host.
  • Partial parasites (e.g., mistletoe): Green, not entirely dependent on host.
  • Parasitic fungi: Produce mycelia into host tissue to absorb food.
  • Parasitic bacteria: Infect living organisms to absorb food.
• Saprophytes: Depend on organic material in soil or dead bodies of plants and animals.
  • Saprophytic plants (e.g., Indian pipe, some orchids), saprophytic fungi, and bacteria grow on decaying matter and absorb organic food.

7. Holozoic Nutrition in Plants (Carnivorous/Insectivorous Plants)

• A mode of heterotrophic nutrition involving the intake of solid food pieces.
• Carnivorous/insectivorous plants capture lower animals (especially insects), digest them, and absorb nitrogenous products.
• Examples: Venus fly-trap, pitcher plant, bladderwort.

8. Mineral Requirements in Plants

• Plants obtain 16 essential elements from soil, water, and air.
• Macronutrients: Six elements required in relatively large amounts.

  Nutrient	Symbol	Source	Form Used by Plant
  Oxygen	O	Air or Water	H₂O
  Hydrogen	H	Air or Water	H₂O
  Carbon	C	Air or Water	CO₂
  Nitrogen	N	Soil	NO₃⁻, NH₄⁺
  Phosphorus	P	Soil	H₂PO₄⁻, HPO₄²⁻
  Sulphur	S	Soil	SO₄²⁻
  Potassium	K	Soil	K⁺
  Calcium	Ca	Soil	Ca²⁺
  Magnesium	Mg	Soil	Mg²⁺

• Micronutrients: Seven elements required in very small amounts (minor or trace elements).

  Nutrient	Symbol	Source	Form Used by Plant
  Iron	Fe	Soil	Fe²⁺
  Manganese	Mn	Soil	Mn²⁺
  Boron	B	Soil	H₂BO₃⁻
  Molybdenum	Mo	Soil	MoO₄²⁻
  Copper	Cu	Soil	Cu²⁺
  Zinc	Zn	Soil	Zn²⁺
  Chlorine	Cl	Soil	Cl⁻

RESPIRATION IN PLANTS

1. Introduction to Respiration

• Process by which plants use oxygen to break down sugars (e.g., glucose) produced during photosynthesis.
• Releases energy in usable form (Adenosine Triphosphate, ATP), carbon dioxide, and water.
• Occurs in all living plant cells (roots, leaves, stems), day and night.
• Plants do not have specialized organs for gaseous exchange but use:
  • Stomata (in leaves)
  • Lenticels (in stems)
  • Root hairs (in roots)

2. Respiration Equation

C₆H₁₂O₆ (Glucose) + 6O₂ (Oxygen) → 6CO₂ (Carbon Dioxide) + 6H₂O (Water) + Energy (ATP)

• Oxygen is utilized during respiration.
• Carbon dioxide, water, and energy are products of respiration.

3. Gas Exchange in Plants

• In leaves:
  • Gaseous exchange occurs through diffusion via minute pores (stomata).
  • Opening and closing of stomatal pores are controlled by guard cells.
• In stems:
  • Gaseous exchange occurs through stomata in small herbaceous plants.
  • In woody plants, gaseous exchange occurs through lenticels (specialized groups of loosely packed cells).
• In roots:
  • Gaseous exchange occurs through diffusion via root hairs (extensions of epidermal cells).
  • Root hairs absorb oxygen from air spaces in soil particles and transport it to other root cells.
  • Root hairs expel carbon dioxide (by-product).
  • In waterlogged environments, plants develop pneumatophores (breathing roots) that grow upward to access atmospheric oxygen for aerobic respiration.
  • Excess water in soil can deprive plants of oxygen, leading to harmful anaerobic respiration.

4. Photosynthesis vs. Respiration

• Photosynthesis utilizes CO₂, water, and energy to produce glucose and oxygen.
• Respiration utilizes glucose and oxygen to produce CO₂, water, and energy.
• Yes, photosynthesis can be considered the opposite of respiration in terms of overall reactants and products, though they are distinct processes.

5. Different Types of Plant Respiration

• Aerobic respiration:
  • Primary source of energy for plants.
  • Occurs in the presence of oxygen in both cytoplasm and mitochondria.
  • Involves complete oxidation of glucose into carbon dioxide and water, releasing high energy (e.g., 36 ATP).
  • Involves the Krebs cycle, citric cycle, and electron transport chain.
• Anaerobic respiration:
  • Occurs in the absence of oxygen in the cell cytoplasm.
  • Involves partial oxidation of glucose to form ethyl alcohol and carbon dioxide as end products.
  • Involves glycolysis.

6. Factors Affecting Respiration in Plants

• Temperature: Rate increases with increasing temperatures up to an optimum, then decreases as enzymes are destroyed.
• Oxygen concentration: Rate increases with increasing oxygen concentration. Inadequate oxygen leads to alternative fermentation.
• Carbon dioxide concentration: Higher environmental CO₂ concentration leads to lower respiration rate.
• Water content: Adequate water in the soil increases respiration rate.
• Light: Rate increases with increasing light intensity because the surrounding temperature rises with increased light intensity.
• Presence of Salt: Rate increases in the presence of salt.
• Age of the cell: Aging plant tissue negatively affects respiration rate. Younger tissues exhibit higher respiration rates.

EXCRETION IN PLANTS

1. Introduction to Excretion

• Process of eliminating waste materials and excess water.
• Ensures the plant's internal environment remains balanced and healthy.
• Plants do not have a specific excretory system like animals.
• Plants have lower metabolic activity, thus producing lesser waste products compared to animals.

2. Methods of Excretion in Plants

• Removal of gaseous waste:
  • Carbon dioxide, oxygen, and water vapor are diffused through stomata (pores on leaves) and lenticels (pores on stems).
• Transpiration:
  • Process through which excess water evaporates from the plant's surface.
• Guttation:
  • Certain shrubs and grasses may excrete excess water as liquid droplets (especially when stomata are closed).
  • Occurs from leaf tips or edges through modified pores called hydathodes.
• Storage and shedding:
  • Plants store waste products like tannins, resins, and gums in leaves and bark.
  • These parts are then shed by the plants.
  • Some waste products accumulate in older xylem, contributing to heartwood formation.
• Excretion into the soil:
  • Organic acids, amino acids, and sugars are excreted into the soil through plant roots.
• Salt glands:
  • Some plants have specialized salt glands that excrete excess salts from their tissues, often in the form of crystals.