Photosynthesis & Plant Physiology – NDA Biology Notes

Every plant is a food factory. It takes simple raw materials, carbon dioxide from the air and water from the soil, and converts them into glucose using sunlight. This process is photosynthesis. This is the foundation of almost all life on Earth. Plants make food; animals eat plants or eat animals that ate plants. The energy that powers every food chain begins with photosynthesis.

For the NDA, plant physiology is a chapter with consistent, wide-ranging questions: mushrooms as heterotrophs (tested twice), modified stems, living vs dead xylem components, intercalary meristems in grasses, plant hormones, guttation vs dew, nitrogen fixation, and xerophyte adaptations. Every section below marks exactly where exam questions originate.

1. What is Photosynthesis?

Photosynthesis is the process by which green plants produce glucose from carbon dioxide and water using light energy. Oxygen is released as a by-product. The overall equation is:

6CO₂  +  6H₂O  +  Light energy  →  C₆H₁₂O₆  +  6O₂

Carbon dioxide and water are the raw materials. Light energy is the energy source. Glucose is the product. Oxygen is the by-product released into the atmosphere.

2. Where Photosynthesis Happens: The Chloroplast

Photosynthesis occurs in the chloroplast, the organelle in plant cells that contains chlorophyll.

Not in the nucleus (which contains DNA and controls cell functions). Not in the ribosome (which synthesises proteins). Not in the chromosome. Only the chloroplast hosts chlorophyll and therefore only the chloroplast can carry out photosynthesis.

3. What Light Does: Reduction of CO₂

Chlorophyll absorbs light energy. This absorbed energy is used to reduce carbon dioxide: adding hydrogen atoms to CO₂ to form glucose.

CO₂ is reduced, not oxidised. Reduction means gaining hydrogen (or electrons). Oxidation is the opposite. The reaction that produces glucose is specifically a reduction of CO₂ by hydrogen derived from water.

★ IMPORTANT The one correct statement about what chlorophyll does with light energy: Chlorophyll absorbs light energy, which causes the reduction of CO₂.   Wrong options that appear in the exam: → Light energy is converted to kinetic energy: WRONG → Chemical energy reduces water: WRONG (water is split, not reduced) → Chlorophyll absorbs chemical energy: WRONG (it absorbs light energy) → CO₂ is oxidised: WRONG (CO₂ is reduced to produce glucose)

4. Which Light is Absorbed?

Chlorophyll absorbs primarily red and blue wavelengths of visible light. Green wavelengths are reflected, which is why leaves appear green. Yellow wavelengths are minimally absorbed.

This absorption spectrum has a direct practical consequence: green light is least useful for photosynthesis, and red or blue light is most effective for growing plants under artificial lighting.

5. Carotenoids: Accessory Pigments

Carotenoids are accessory pigments in the chloroplast. They serve three roles:

Role	Detail
Autumn leaf colour	Responsible for the yellow-orange colours of autumn leaves: when chlorophyll breaks down, carotenoids are exposed
Photoprotection of chlorophyll	Protect chlorophyll from damage by excess light (photo-oxidation)
Eye protection	Photoprotective role in the human eye (lutein and zeaxanthin are carotenoids)
NOT the reaction centre	The reaction centre of the photosystem belongs exclusively to special chlorophyll-a molecules (P680 and P700): NOT carotenoids

6. Magnesium: The Central Atom of Chlorophyll

Magnesium (Mg) is the essential element required for chlorophyll formation. Every chlorophyll molecule has one magnesium atom at its centre.

Without magnesium, chlorophyll cannot be made. Without chlorophyll, photosynthesis cannot occur. This is why magnesium is a plant macronutrient, needed in relatively large amounts.

Not calcium. Not iron. Not potassium. Only magnesium is at the centre of the chlorophyll ring. This is analogous to iron being at the centre of haemoglobin in blood.

7. Xylem : The Water Transport System

Xylem is the vascular tissue that carries water and dissolved minerals upward from roots to all parts of the plant.

Xylem transports water AND dissolved minerals, not food (food is transported by phloem), not gases.

Components of Xylem : Living vs Dead

Xylem Component	Living or Dead?	Function
Tracheids	Dead at functional maturity	Water conduction through bordered pits
Vessels	Dead at functional maturity	Main water conduction: wider than tracheids
Xylem parenchyma	Living [NDA 2019-II]	Lateral transport and storage
Xylem fibres	Dead at functional maturity	Mechanical support for the plant

Among the four xylem components, only xylem parenchyma consists of living cells. Tracheids, vessels, and xylem fibres are all dead at functional maturity.  [NDA 2019-II]

The endodermis is the innermost layer of the root cortex. It surrounds the vascular bundle from outside: it is NOT part of the vascular bundle itself. The vascular bundle consists of xylem, phloem, and cambium.

8. How Water Moves Upward : The Complete Chain

Water must travel from roots to the tops of tall trees, often tens of metres against gravity. Three forces work together:

Force	Mechanism	Role
Transpiration pull	Water evaporates from leaf surfaces through stomata → creates tension (negative pressure) at the top of the xylem column	Pulls water upward: the driving force
Cohesion	Water molecules stick to each other → keeps the water column continuous (one unbroken thread from root to leaf)	Maintains the integrity of the water column
Adhesion	Water molecules stick to the walls of xylem vessels → prevents the column from breaking away from the walls	Prevents column breakage at vessel walls

Together: Transpiration pull + Cohesion + Adhesion = upward water movement.

The ultimate driving force is transpiration. Without water evaporating from leaves, there would be no pull and no upward movement

9. Transpiration and Stomata

Transpiration, the loss of water vapour from a plant, primarily through tiny pores called stomata on the leaf surface.

Stomata are the structures responsible for transpiration, not xylem, not roots, not bark. The stomata control how much water vapour leaves the plant.

In dry environments, plants evolve smaller leaves to reduce the surface area available for transpiration. Less surface area = less water loss. This is a water-conservation adaptation.

Cooling Under Trees: Evaporative Cooling

On a hot day, the air under a tree feels cooler. Trees lose large amounts of water through transpiration. When water evaporates from leaf surfaces, it absorbs latent heat from the surroundings, removing heat from the air near the leaves.

This is evaporative cooling, the same physical process that makes sweating cool the human body.

10. Guttation: Liquid Drops on Leaf Tips

In the early mornings, water droplets appear on the tips and margins of grass leaves. These are NOT dew. They are produced by a different process called guttation.

Process	What is Released	Route	Driving Force
Transpiration	Water vapour	Through stomata on leaf surface	Evaporation (transpiration pull)
Guttation	Liquid water droplets	Through hydathodes at leaf tips and margins	Root pressure (when soil water potential is high)
Dew	Liquid water from atmosphere	Condenses on cool leaf surfaces	Atmospheric moisture: NOT from inside the plant

Morning water drops on grass leaf tips = guttation through hydathodes, not dew, not vapour from stomata. The drops come from inside the plant, not from the air.

11. Meristems: Where Plants Grow

Plants grow only in specific regions called meristems, zones of active cell division. Three types are distinguished by location and function.

Meristem Type	Location	Function	Key Exam Facts
Apical meristem	Root tips and shoot tips	Primary growth: increase in LENGTH of root and shoot [NDA 2025-I]	Produces all primary tissues from each tip
Intercalary meristem	Base of leaves or internodes: especially in grasses	Regrowth after grazing or mowing	Why grasses regrow from base after cutting: growth zone is at base, not tip
Lateral meristem (Cambium)	Sides of stem and root [NDA 2025-I]	Secondary growth: increase in GIRTH	Produces new xylem (wood) inward and new phloem outward each year

Cambium is a lateral meristem. It runs along the sides of stems and roots, producing new xylem inward and new phloem outward, making the stem thicker each year.  [NDA 2025-I]

Intercalary meristem in grasses is specifically located at the base of leaves, not at root tips, not at shoot tips, not on the sides. This is why grasses regrow quickly from the base after grazing or mowing. The growth zone is intact even after the top is removed.

12. Plant Tissues: Sclerenchyma and Parenchyma

Tissue	Cell State	Wall Characteristics	Function
Sclerenchyma	Dead at functional maturity	Thick, lignified walls	Provides rigidity and mechanical support: NOT flexibility [NDA 2020-I]
Collenchyma	Living	Unevenly thickened walls	Provides flexibility in young plant parts
Parenchyma	Living	Thin walls with intercellular spaces	Most basic plant tissue : photosynthesis, storage, basic metabolism

13. Plant Hormones: The Complete Portfolio

Plant hormones (phytohormones) are small organic molecules that regulate plant growth and development. They belong to chemical classes including indole compounds (auxins), adenine derivatives (cytokinins), and terpenes (gibberellins, abscisic acid). They are NOT carbohydrates, NOT fats, NOT proteins, NOT vitamins.

Thyroxine is NOT a plant hormone. Thyroxine is an animal hormone produced by the thyroid gland. When it appears in a list of plant hormones, it is always the wrong option.

Hormone	Primary Function	Key Exam Fact
Auxin	Cell elongation; phototropism (bending toward light); gravitropism [NDA 2013-I]	Migrates to shaded side → cells on shaded side elongate more → shoot bends toward light
Gibberellin	Stem elongation; seed germination; fruit development	Promotes elongation (not division): different from cytokinin
Cytokinin	Cell division; shoot growth	Specific answer for “which hormone promotes cell division”: NOT gibberellin, NOT auxin
Abscisic acid (ABA)	Inhibits growth; seed dormancy; stomatal closure under drought	The ONLY major inhibitory plant hormone: the “stress hormone”
Ethylene	Fruit ripening; leaf abscission	Promotes ripening: why fruits placed together ripen faster

Cytokinin promotes cell division. It is the specific and correct answer to any question asking which plant hormone stimulates cell division. Abscisic acid inhibits. Gibberellin promotes elongation (not division). Auxin promotes elongation (not division). Only cytokinin directly stimulates cell division.  

Abscisic acid (ABA) is the only major plant hormone that inhibits growth. It closes stomata to prevent water loss under drought, induces seed dormancy, and is the “stress hormone” of plants.  

14. Nutrition Modes: Who Can and Cannot Make Food

★ IMPORTANT Mushrooms (Agaricus) are NOT autotrophs. They cannot make their own food. Mushrooms are fungi: not plants. They obtain nutrition by saprotrophic nutrition: secreting digestive enzymes onto dead organic matter and absorbing the soluble products. This is the most consistently tested nutrition-mode error in plant science. When a question asks “which organism cannot make its own food through photosynthesis?” The mushroom is always the answer if it appears.

Nutrition Mode	Organisms	How They Feed
Autotrophic (make own food)	All green plants, Ulothrix, Riccia, Cladophora, carrot, cabbage	Photosynthesis: carbon dioxide + water + light → glucose
Saprophytic (dead matter)	Mushroom (Agaricus), Rhizopus (bread mould)	Secrete enzymes onto dead organic matter; absorb soluble products
Holoparasitic	Cuscuta (dodder): no chlorophyll at all	Entirely dependent on host plant; cannot photosynthesise
Partial root parasite	Sandalwood (Santalum album)	Can photosynthesise AND draws nutrients from host roots
Carnivorous	Nepenthes (pitcher plant)	Traps and digests insects to obtain nitrogen: photosynthesises for carbon

15. Modified Stems: Potato, Onion, Garlic

Three plants that grow underground are commonly mistaken for modified roots. They are actually modified stems.

★ IMPORTANT Potato, Onion, and Garlic are modified STEMS: NOT roots.   The diagnostic proof: Roots NEVER bear buds. Only stems can produce buds. The “eyes” on a potato are NODES bearing BUDS. Buds grow into new shoots. This is definitive proof that a potato is a stem, not a root.   Carrot and Turnip are modified TAP ROOTS: NOT stems. They do not have buds.   Common mistake: grouping carrot with potato/onion/garlic and asking which is NOT a modified stem. Carrot is the exclusion.

Plant	Underground Structure	Modified Structure Type	Key Identifier
Potato	Stem tuber	Modified STEM	Eyes = nodes bearing buds : proof of stem nature
Onion	Bulb	Modified STEM (with fleshy scale leaves)	Layered structure, central growing point
Garlic	Bulb	Modified STEM	Individual cloves = modified leaves on stem disc
Carrot	Taproot	Modified ROOT	No buds : taproot stores food [NOT a stem]
Turnip	Taproot	Modified ROOT	No buds : swollen tap root for food storage

16. Seed Dispersal

Seeds are dispersed by three agents:

• Wind: winged or feathery seeds (dandelion, maple)
• Water: buoyant seeds (coconut)
• Animals: through fruit consumption, adhesion to fur, or internal transport

Fungi do NOT disperse seeds. Fungi produce and disperse their own spores, not seeds. Fungi are never agents of seed dispersal for flowering plants.

17. Nitrogen Fixation and Soil Fertility

Rhizobium: Symbiotic in Legumes

Rhizobium bacteria live in the root nodules of pulse plants (legumes): peas, beans, lentils, chickpeas. They form a symbiotic relationship: bacteria fix nitrogen for the plant; the plant provides shelter and nutrients for the bacteria. The fixing agent is bacterial, not fungal, not protozoan, not viral.  Azotobacter: Free-Living Biofertilizer

Azotobacter is a free-living soil bacterium. It fixes atmospheric nitrogen independently in the soil, without associating with any plant roots.

Azotobacter is used as a biofertilizer, applied to soil or seeds to enhance nitrogen availability without synthetic chemicals. It can be used alongside organic fertilisers.

Anabaena and Nostoc are nitrogen-fixing cyanobacteria. Azolla is a water fern that contains nitrogen-fixing cyanobacteria (Anabaena azollae): Azolla itself is a plant, not a bacterium.

Sunnhemp: Green Manuring

Green manuring in India is done by growing sunnhemp (Crotalaria juncea), a leguminous crop, and ploughing it back into the soil. This adds organic matter and fixes nitrogen.

Wheat, cotton, and rice are NOT green manure crops.

Excess Fertiliser: Plasmolysis

If excess fertiliser is applied to soil without adequate water, the soil solution becomes hypertonic, more concentrated than plant cell contents. Water moves out of root cells by osmosis. Cell contents shrink away from the cell wall, a process called plasmolysis. The plant dies.

More fertiliser without water = plasmolysis = plant death. Not better growth.

18. Plant Ecology: Adaptations

Xerophytes: Dry Conditions

Xerophytes are plants adapted to very dry environments. Their features:

Xerophyte Feature	Purpose
Smaller leaves	Reduce transpiration surface area
Waxy cuticle	Reduce water loss through the epidermis
Stomata in pits	Reduce air movement across stomata, lower evaporation rate
Fewer stomata	Less water loss overall

A large number of stomata is NOT a xerophyte feature. More stomata = more water loss = the opposite of what a desert plant needs.

Halophytes: Saline Conditions

Halophytes grow in saline (salt-rich) soils, not acidic, not sandy. Mangroves are the classic example.

Eucalyptus and Neem

Eucalyptus is preferred in plantation forestry because it grows very fast and its wood is easily converted into pulp for the paper industry. Eucalyptus does NOT make soil more fertile. It actually depletes soil moisture. The “soil more fertile” option is always wrong.

Neem trees are planted along roadsides in India to absorb vehicular pollutants. Neem has demonstrated capacity to absorb and detoxify atmospheric pollutants.

19. Special Plant Facts

Banyan tree (Ficus benghalensis). Its branches send down prop roots that touch the ground and become new trunks. This allows the tree to spread over a vast area and survive for centuries.

Jute, the plant fibre known as the “golden fibre” of India. Named for its golden-brown colour and economic importance as India’s principal bast fibre crop.

Plant sexual reproduction sequence: Pollination → Fertilisation → Division of zygote → Formation of embryo → Seedling.  Any sequence placing embryo or seedling formation before fertilisation is wrong.

Nucellus, the diploid (2n) nutritive tissue in the ovule of angiosperms. Synergids, the egg cell, and antipodal cells are all haploid (n). The nucellus is the correct answer to “Which is NOT a haploid cell in the ovule?”

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