Maarg Manthan · Topic 4.4

Plant Nutrition and Classification by Energy/Carbon Source

Science & Technology › Plant Biology and Nutrition · Topic 4.4

Plant nutrition is the way plants obtain the energy and materials they need to live. Organisms are classified by their source of energy and carbon. Autotrophs make their own organic food from inorganic carbon dioxide: photoautotrophs use light, as green plants and cyanobacteria do, and chemoautotrophs use energy from chemical reactions, as nitrifying bacteria do. Heterotrophs get carbon from organic compounds: photoheterotrophs use light as energy, as some purple bacteria do, and chemoheterotrophs, such as animals, fungi and most bacteria, use chemical energy. Most plants are photoautotrophs and also take up mineral nutrients from the soil. A few plants are parasites, saprophytes or insectivorous. Essential elements are grouped as macronutrients and micronutrients.

5 min read · UPSC Prelims · Science & Technology · Plant Biology and Nutrition

At a Glance

Photoautotroph Light energy; carbon dioxide
Chemoautotroph Chemical energy; carbon dioxide
Macronutrients C, H, O, N, P, K, Ca, Mg, S
Insectivorous Drosera, Nepenthes, Utricularia
Soilless culture Hydroponics
Exam link UPSC Prelims, Science & Technology: Plant Biology and Nutrition

Where Does It Fit?

This is the fourth topic of the macro on plant biology and nutrition. It places photosynthesis in a wider classification of nutrition and adds the mineral needs of plants. Previous topic: Reproduction in Plants: Sexual and Asexual Methods. Next topic: Animal Nutrition.

How Are Organisms Classified by Energy and Carbon Source?

Type	Energy and carbon source, with examples
Photoautotroph	Light for energy and carbon dioxide for carbon. Green plants, algae and cyanobacteria
Chemoautotroph	Chemical reactions for energy and carbon dioxide for carbon. Nitrifying bacteria such as Nitrosomonas and Nitrobacter, and sulphur bacteria
Photoheterotroph	Light for energy and organic compounds for carbon. Some purple non-sulphur bacteria
Chemoheterotroph	Chemical energy and organic carbon. Animals, fungi and most bacteria
Mixotroph	Can switch between modes, such as Euglena and some insectivorous plants

How Do Special Plants Obtain Food?

Parasitic plants: Cuscuta, or dodder, is a stem parasite with no chlorophyll, and mistletoe is a partial parasite on trees. Rafflesia, a root parasite, has the largest flower. Striga, or witchweed, damages cereals.
Saprophytic plants: Monotropa and many fungi feed on dead organic matter.
Insectivorous plants: grow in nitrogen-poor soil and catch insects: Drosera or sundew with sticky tentacles, Nepenthes or pitcher plant, Utricularia or bladderwort with a trap door, and Dionaea or Venus flytrap. India has Nepenthes khasiana in Meghalaya.
Symbiotic nutrition: Rhizobium in root nodules of legumes fixes nitrogen, lichens combine a fungus and an alga, and mycorrhizae help roots to take up phosphorus.
Epiphytes: orchids that grow on other plants for support but make their own food.

What Are the Essential Elements?

Group	Elements and role or deficiency symptom
Macronutrients	Needed in large amounts: carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium and sulphur
Nitrogen	Part of proteins and chlorophyll. Lack causes yellowing of older leaves, called chlorosis, and stunted growth
Phosphorus	Part of ATP and DNA. Lack causes dark or purple leaves and poor roots
Potassium	Controls stomata and enzymes. Lack causes brown leaf edges
Magnesium	Centre of chlorophyll. Lack causes yellow patches between veins
Micronutrients	Needed in small amounts: iron, manganese, zinc, copper, boron, molybdenum, chlorine and nickel. Iron lack causes yellow young leaves, and boron lack causes brittle tissue. Molybdenum lack causes whiptail in cauliflower

What Else Should You Remember and Why Does It Matter?

Criteria of essentiality: Arnon and Stout set that an element is essential if the plant cannot complete its life without it, and no other element can replace it.
Hydroponics: growing plants in a nutrient solution without soil, first developed using solutions of Sachs and Knop. Aeroponics sprays roots with nutrient mist, and both save water.
Nitrogen cycle: fixation by bacteria and lightning, nitrification, assimilation and denitrification return nitrogen to the air, as in Prokaryotes: Bacteria and Archaea.
Fertilisers: urea has 46 per cent nitrogen, and NPK fertilisers supply nitrogen, phosphorus and potassium. Overuse causes eutrophication.
Biofertilisers: Rhizobium, Azotobacter, blue-green algae and phosphate-solubilising microbes are cheaper and safer.
Soil and pH: nutrient availability depends on soil pH, with most nutrients available at pH 6 to 7.
Photosynthesis link: food made in photosynthesis is the basis of nutrition for all, as in Photosynthesis: Light Reaction and Dark Reaction.
Policy link: India’s nutrient-based subsidy scheme and the Soil Health Card scheme guide fertiliser use.
Heterotrophs among plants: heterotrophs in the plant world include Cuscuta, Rafflesia, fungi and many bacteria, which get all or some of their food from other organisms.
Fixing of nitrogen by legumes: legume crops such as gram, pea and groundnut leave nitrogen in the soil, so farmers use crop rotation with pulses to restore fertility.
Mineral transport: minerals enter roots by active transport, moving against the concentration gradient using ATP, and then travel in the xylem.

As an analytical point, plants make food from light and air but need minerals from the soil. Balanced use of fertilisers, with more organic and biological sources, is needed to keep soils healthy.

Why Is This Topic Important?

Facts Examples and deficiency asked
Concept Energy and carbon source
Link Fertilisers and soil

What Came Next?

The next topic covers animal nutrition. Next topic: Animal Nutrition.

Key People and Terms

Autotroph Makes its own food
Heterotroph Depends on others for food
Chlorosis Yellowing from lack of chlorophyll
Macronutrient Needed in large amounts
Hydroponics Growing plants in nutrient solution
Symbiosis Partnership of two organisms

Exam Corner

Points to Remember

Cuscuta: total stem parasite.
Drosera, Nepenthes, Utricularia: insectivorous.
Lack of nitrogen: chlorosis.
Nitrifying bacteria: chemoautotrophs.
Urea: 46 per cent nitrogen.

Do Not Confuse With

Parasite and saprophyte Feeds on living host versus dead matter
Macro and micronutrient Large versus small amount
Photoautotroph and chemoautotroph Light versus chemical energy

Memory Hook

Photo uses light, chemo uses reaction; Cuscuta parasite, Drosera eats insects; nitrogen lack makes yellow leaves.

Mains Angle

Questions link plant nutrition with soil and agriculture. These points add depth.

Types Energy and carbon source
Minerals Essential elements
Farming Fertilisers and hydroponics

A Question You May Face

An original practice question, not a past paper question.

“Classify organisms according to their modes of nutrition and discuss the importance of mineral nutrients for plants.”

How to Answer

Types Auto, hetero.
Special Parasites, insectivores.
Minerals Macro, micro.
Farming Biofertilisers.

GS Relevance

Prelims: nutrition types, essential elements and deficiency symptoms, parasitic and insectivorous plants, hydroponics. GS Paper 3: agriculture, science and technology.

Frequently Asked Questions

What is the difference between autotrophs and heterotrophs?

Autotrophs make their own organic food from simple inorganic substances such as carbon dioxide, using light, as in green plants, or chemical energy, as in nitrifying bacteria. Heterotrophs cannot make their own food and obtain carbon from organic compounds made by other organisms, as animals, fungi and most bacteria do.

Why do insectivorous plants catch insects?

Insectivorous plants such as Drosera, Nepenthes, Utricularia and Venus flytrap grow in soils that have little nitrogen. They carry out photosynthesis but catch and digest insects to get nitrogen and other minerals, so they are both autotrophs and partly heterotrophs, and India has the pitcher plant Nepenthes khasiana in Meghalaya.

What are macronutrients and micronutrients in plants?

Macronutrients are the elements needed in large amounts: carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium and sulphur. Micronutrients, such as iron, manganese, zinc, copper, boron, molybdenum, chlorine and nickel, are needed in small amounts but are still essential, and a lack of either group causes deficiency symptoms.

What is hydroponics?

Hydroponics is a method of growing plants without soil, with their roots in a solution that contains all the essential mineral nutrients. It saves water, allows control of nutrition and is used for vegetables and flowers in greenhouses, and aeroponics is a related method in which roots are sprayed with nutrient mist.

Which bacteria are chemoautotrophs?

Nitrifying bacteria such as Nitrosomonas and Nitrobacter are chemoautotrophs. They get energy by oxidising ammonia to nitrite and nitrite to nitrate, and use it to make food from carbon dioxide, and in doing so they change nitrogen into a form that plants can take up from the soil.

PYQ Practice — Statement Analysis

1 Cuscuta is a parasitic plant without chlorophyll.

True

It lives on host stems.

2 Nitrogen deficiency causes yellowing of older leaves.

True

Called chlorosis.

3 Venus flytrap is a root parasite.

False

It is an insectivorous plant.

4 Nitrifying bacteria are photoautotrophs.