9.1 Nutrition in Plants | Diversity In Plant Functions | Biology 11

Nutrition refers to the process by which organisms obtain and utilize food for energy, growth, and maintenance. Plants, unlike animals, are capable of manufacturing their own food and also exhibit specialized modes of nutrition when environmental conditions are limiting.

1. Autotrophic Nutrition (Photosynthesis)

Plants are photoautotrophs — they synthesize organic compounds from simple inorganic raw materials using light energy.

Raw Materials:

Carbon dioxide ( $C O_{2}$ ) — absorbed from the air through stomata
Water ( $H_{2} O$ ) — absorbed from the soil through roots
Light energy — absorbed by photosynthetic pigments (mainly chlorophyll)

Overall Equation: $6 C O_{2} + 12 H_{2} O light + chlorophyll C_{6} H_{12} O_{6} + 6 O_{2} + 6 H_{2} O$

Note: 12 water molecules are used (not 6) because water is split during photolysis in the light reactions.

2. Mineral Nutrition

Beyond $C O_{2}$ and $H_{2} O$ , plants require mineral elements absorbed from the soil as ions. These are classified into:

Macronutrients

Required in large quantities for structural and major metabolic functions:

Element	Symbol	Role
Nitrogen	N	Proteins, nucleic acids, chlorophyll
Phosphorus	P	ATP, nucleic acids, phospholipids
Potassium	K	Stomatal regulation, enzyme activation
Calcium	Ca	Cell wall (middle lamella), cell signalling
Magnesium	Mg	Central atom of chlorophyll molecule
Sulfur	S	Amino acids (cysteine, methionine)

Micronutrients (Trace Elements)

Required in minute quantities, often as enzyme co-factors:

Element	Symbol	Role
Iron	Fe	Cytochromes, electron transport
Manganese	Mn	Oxygen Evolving Complex (OEC) — photolysis of water
Boron	B	Cell wall synthesis, pollen germination
Zinc	Zn	Enzyme co-factor (e.g., carbonic anhydrase)
Copper	Cu	Plastocyanin in electron transport
Molybdenum	Mo	Nitrogen fixation (nitrogenase enzyme)

Deficiency Symptoms

Nitrogen ( $N$ ) deficiency:

Chlorosis (yellowing) of older/lower leaves first — because N is mobile and remobilized from old leaves to young growing tissues
Stunted growth — N is essential for protein and nucleic acid synthesis

Magnesium ( $M g$ ) deficiency:

Interveinal chlorosis — yellowing between leaf veins while veins remain green
Occurs because Mg is the central atom of chlorophyll; without it, chlorophyll cannot be synthesized

Phosphorus ( $P$ ) deficiency:

Purple/reddish coloration of leaves (anthocyanin accumulation)
Poor root development

3. Heterotrophic Nutrition in Plants

Some plants cannot photosynthesize (or do so insufficiently) and obtain nutrition from other organisms or organic matter.

A. Saprophytic Nutrition

Definition: Obtaining nutrients from dead and decaying organic matter by secreting digestive enzymes externally and absorbing the products.

Example: Neottia (Bird's nest orchid) — lacks chlorophyll; depends on mycorrhizal fungi associated with decaying wood.

B. Parasitic Nutrition

Definition: Obtaining nutrients from a living host plant, causing harm to the host.

Types:

Holoparasites — completely dependent on host, no chlorophyll
- Example: Cuscuta (Dodder) — twines around host and penetrates with haustoria
Hemiparasites — partially parasitic, have some chlorophyll
- Example: Viscum album (Mistletoe) — photosynthesizes but obtains water and minerals from host

C. Symbiotic Nutrition

Definition: A mutualistic relationship where both partners benefit.

Examples:

Rhizobium–Legume symbiosis: Rhizobium bacteria in root nodules fix atmospheric $N_{2}$ into $N H_{3}$ for the plant; the plant provides carbohydrates and shelter.
Mycorrhizae: Fungi associated with plant roots increase surface area for water and mineral absorption; plant provides photosynthates.

D. Insectivorous (Carnivorous) Nutrition

Why? These plants grow in nitrogen-deficient, acidic, waterlogged soils (bogs and marshes). They supplement their nitrogen intake by trapping and digesting insects.

Examples and Trapping Mechanisms:

Plant	Common Name	Trapping Mechanism
Dionaea muscipula	Venus Flytrap	Snap-trap (modified leaves with trigger hairs)
Drosera spp.	Sundew	Sticky tentacles (adhesive mucilage)
Sarracenia purpurea	Pitcher Plant	Pitcher trap (modified leaf with digestive fluid)
Utricularia	Bladderwort	Suction trap (aquatic)

1. Autotrophic Nutrition (Photosynthesis)

Plants are photoautotrophs — they synthesize organic compounds from simple inorganic raw materials using light energy.

Raw Materials:

Carbon dioxide ( $C O_{2}$ ) — absorbed from the air through stomata
Water ( $H_{2} O$ ) — absorbed from the soil through roots
Light energy — absorbed by photosynthetic pigments (mainly chlorophyll)

Overall Equation: $6 C O_{2} + 12 H_{2} O light + chlorophyll C_{6} H_{12} O_{6} + 6 O_{2} + 6 H_{2} O$

Note: 12 water molecules are used (not 6) because water is split during photolysis in the light reactions.

2. Mineral Nutrition

Beyond $C O_{2}$ and $H_{2} O$ , plants require mineral elements absorbed from the soil as ions. These are classified into:

Macronutrients

Required in large quantities for structural and major metabolic functions:

Element	Symbol	Role
Nitrogen	N	Proteins, nucleic acids, chlorophyll
Phosphorus	P	ATP, nucleic acids, phospholipids
Potassium	K	Stomatal regulation, enzyme activation
Calcium	Ca	Cell wall (middle lamella), cell signalling
Magnesium	Mg	Central atom of chlorophyll molecule
Sulfur	S	Amino acids (cysteine, methionine)

Micronutrients (Trace Elements)

Required in minute quantities, often as enzyme co-factors:

Element	Symbol	Role
Iron	Fe	Cytochromes, electron transport
Manganese	Mn	Oxygen Evolving Complex (OEC) — photolysis of water
Boron	B	Cell wall synthesis, pollen germination
Zinc	Zn	Enzyme co-factor (e.g., carbonic anhydrase)
Copper	Cu	Plastocyanin in electron transport
Molybdenum	Mo	Nitrogen fixation (nitrogenase enzyme)

Deficiency Symptoms

Nitrogen ( $N$ ) deficiency:

Chlorosis (yellowing) of older/lower leaves first — because N is mobile and remobilized from old leaves to young growing tissues
Stunted growth — N is essential for protein and nucleic acid synthesis

Magnesium ( $M g$ ) deficiency:

Interveinal chlorosis — yellowing between leaf veins while veins remain green
Occurs because Mg is the central atom of chlorophyll; without it, chlorophyll cannot be synthesized

Phosphorus ( $P$ ) deficiency:

Purple/reddish coloration of leaves (anthocyanin accumulation)
Poor root development

Holoparasites — completely dependent on host, no chlorophyll
- Example: Cuscuta (Dodder) — twines around host and penetrates with haustoria
Hemiparasites — partially parasitic, have some chlorophyll
- Example: Viscum album (Mistletoe) — photosynthesizes but obtains water and minerals from host

C. Symbiotic Nutrition

Definition: A mutualistic relationship where both partners benefit.

Examples:

Rhizobium–Legume symbiosis: Rhizobium bacteria in root nodules fix atmospheric $N_{2}$ into $N H_{3}$ for the plant; the plant provides carbohydrates and shelter.
Mycorrhizae: Fungi associated with plant roots increase surface area for water and mineral absorption; plant provides photosynthates.

D. Insectivorous (Carnivorous) Nutrition

Why? These plants grow in nitrogen-deficient, acidic, waterlogged soils (bogs and marshes). They supplement their nitrogen intake by trapping and digesting insects.

Examples and Trapping Mechanisms:

Plant	Common Name	Trapping Mechanism
Dionaea muscipula	Venus Flytrap	Snap-trap (modified leaves with trigger hairs)
Drosera spp.	Sundew	Sticky tentacles (adhesive mucilage)
Sarracenia purpurea	Pitcher Plant	Pitcher trap (modified leaf with digestive fluid)
Utricularia	Bladderwort	Suction trap (aquatic)