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NDA Biology · Plant Biology

Photosynthesis

Photosynthesis is how green plants use light, water and CO₂ to make glucose and release oxygen; the light reaction splits water (giving the O₂) and the dark reaction fixes CO₂ into sugar.

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Why this matters

10 PYQs — a mix of mechanism questions (what splits, what's a product) and crop/botany recall. The mechanism core is small but reliably tested: the oxygen released comes from SPLITTING WATER, not CO₂; O₂ is a PRODUCT not a requirement; and chlorophyll reflects green (so leaves look green) while absorbing red and blue. A cluster of crop facts (golden rice, sugarcane, intercropping) and one odd botany fact (nettle sting) round it out.

Concept 1 of 5

Photosynthesis — requirements, equation and products

Intuition

Think of a leaf as a tiny solar food factory. It needs four things to run: a pigment to catch the light (chlorophyll), a carbon source (CO₂), a hydrogen source (water), and the energy itself (light). Feed it those and it makes glucose and gives off oxygen. The oxygen is OUTPUT — never list it as an input.

Definition

Photosynthesis converts light energy into chemical energy stored in glucose. The four requirements are chlorophyll, carbon dioxide, water, and light. The products are glucose and oxygen. Only about 1% of the sunlight falling on leaves is actually captured for photosynthesis.

Overall photosynthesis equation

6 CO2+6 H2O→light, chlorophyllC6H12O6+6 O26\,\text{CO}_2 + 6\,\text{H}_2\text{O} \xrightarrow{\text{light, chlorophyll}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\,\text{O}_2
  • CO2\text{CO}_2carbon dioxide — the carbon source (a requirement)
  • H2O\text{H}_2\text{O}water — the hydrogen source (a requirement)
  • C6H12O6\text{C}_6\text{H}_{12}\text{O}_6glucose — the food made (a product)
  • O2\text{O}_2oxygen — released (a PRODUCT, never a requirement)
ChloroplastLight reaction(thylakoid)Light + H₂O→ splits waterreleases O₂ ↑makes ATP +NADPHATP/NADPHDark reaction(stroma · Calvin cycle)CO₂ fixedusing ATP +NADPH→ glucose(no light needed)6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂

Worked example

From this list — chlorophyll, oxygen, carbon dioxide, water — which one is NOT needed to RUN photosynthesis?
Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 1Plant BiologyEASY
Which one of the following is NOT a requirement for photosynthesis?

[Q93 · Sep · 2023]

Oxygen is a PRODUCT, not a requirement

The 'which is NOT a requirement' question is a classic. Chlorophyll, CO₂ and water are all needed; oxygen is released, so it is the odd one out. Don't confuse the inputs with the outputs.

Only ~1% of sunlight is used

Green plants capture only about 1% of the solar radiation that hits them — distractors offer 5%, 8%, 10%. Most light is reflected or turned to heat.
Drill 1 more on photosynthesis — requirements, equation and products

Concept 2 of 5

The light reaction — splitting water releases oxygen

Intuition

Photosynthesis runs in two stages. The light reaction comes first: light energy is used to SPLIT water molecules. That splitting (photolysis) is where the released oxygen comes from — NOT from carbon dioxide. The dark reaction then uses CO₂ to build sugar, with no light needed directly.

Definition

The light reaction (light-dependent) uses light energy to split water (photolysis) into hydrogen ions, electrons and oxygen — so the oxygen evolved comes from water, not CO₂. The dark reaction (Calvin cycle) then fixes CO₂ into glucose using the products of the light reaction.

Photolysis of water (the source of O₂)

2 H2O→light4H++4e−+O22\,\text{H}_2\text{O} \xrightarrow{\text{light}} 4\text{H}^+ + 4e^- + \text{O}_2
  • H2O\text{H}_2\text{O}water — the molecule that is split
  • O2\text{O}_2oxygen released — sourced from water, not CO₂

Worked example

A student is told the oxygen released in photosynthesis is labelled with a special atom. Should they label the carbon dioxide or the water to track where the released oxygen comes from?
Practice this conceptself-check · 3 quick reps

From the bank · past-year question

Example 2Plant BiologyEASY
The oxygen evolved during photosynthesis comes from splitting of

[Q85 · Sep · 2018]

Released O₂ comes from WATER, not CO₂

A favourite trap offers 'carbon dioxide' as the source of the oxygen. The oxygen evolved comes from splitting water (photolysis). CO₂ is reduced to sugar in the dark reaction — its oxygen is not the gas released.

Water splits in the LIGHT reaction

The correct statement is 'water molecule splits into hydrogen and oxygen'. The misleading distractor 'light energy is directly used to split water' overstates it — water splitting happens within the light-dependent machinery, not by light acting alone.
Drill 1 more on the light reaction — splitting water releases oxygen

Concept 3 of 5

Chlorophyll and why leaves look green

Intuition

Chlorophyll is choosy about light. It grabs the red and blue parts of sunlight to power photosynthesis but bounces the green part back. Your eye sees that reflected green light — which is exactly why leaves look green.

Definition

Chlorophyll absorbs mainly red (~680 nm) and blue (~430 nm) light and reflects green light. Because the reflected light reaching your eye is green, leaves appear green. The absorbed red and blue wavelengths supply the energy for photosynthesis.

Worked example

If chlorophyll absorbs red and blue light strongly, what colour does it reflect, and how does that explain leaf colour?
Practice this conceptself-check · 3 quick reps

From the bank · past-year question

Example 3Plant BiologyEASY
Leaves of most plants appear green because the chlorophyll present in it :

[Q120 · Apr · 2024]

Leaves look green because chlorophyll REFLECTS green

A reversed distractor says chlorophyll 'absorbs green'. It is the opposite — chlorophyll absorbs red and blue and reflects green, and we see that reflected green.

Concept 4 of 5

Meristematic tissue drives plant growth

Intuition

Plants grow because certain tissues never stop dividing. Those dividing tissues are the meristems. They are not dead wood, not the thick-walled support tissue, and not confined to the bark — they are the living dividing zones at tips and along the cambium.

Definition

Meristematic tissue consists of actively dividing cells, and plant growth occurs because of the division of these cells. It is living (not dead wood), distinct from the thick-walled flexibility tissue (collenchyma), and found at growing tips and the cambium — not only in the bark.

Worked example

Which single property of meristematic tissue makes a plant grow taller and thicker?
Practice this conceptself-check

From the bank · past-year question

Example 4Plant BiologyEASY
Which one of the following statements about meristematic tissues in plants is correct?

[Q74 · Apr · 2018]

Meristem is LIVING and dividing — not dead wood

Distractors call meristem 'dead tissue forming wood', 'flexibility tissue', or 'only in bark'. The correct statement is that growth occurs due to the division of meristematic cells — they are living, dividing cells at tips and the cambium.

Concept 5 of 5

Crop and botany facts — golden rice, sugarcane, cropping, nettle

Intuition

A handful of stand-alone recall facts cluster around crops and plant products. They don't share a mechanism — just memorise the pairs the bank repeats: what golden rice makes, what sugarcane yields, the name for two crops grown together, and why nettle stings.

Definition

High-yield stand-alone facts:

  • Golden rice — genetically engineered to make beta-carotene (provitamin A / Vitamin A) in its grain, to fight vitamin-A deficiency.
  • Sugarcane — grown to obtain sucrose (table sugar), not glucose or starch.
  • Intercropping — growing two or more crops simultaneously on the same field (in a definite row pattern); distinct from mixed cropping (no defined rows) and crop rotation (different crops in sequence).
  • Nettle (Urtica) sting — injects methanoic acid (formic acid) through hollow hairs, causing the burning sensation.
FactAnswerNote
Golden rice producesVitamin A (beta-carotene)GM crop targeting vitamin-A deficiency
Golden rice = Vitamin A, NOT omega-3 or vitamin B/C (NDA 2017).
Sugarcane is grown forSucroseThe cash-crop sugar; not glucose/fructose/starch
Two+ crops grown togetherIntercroppingSame land, same time, defined rows
Intercropping = simultaneous; crop rotation = in sequence (NDA 2020).
Nettle sting injectsMethanoic (formic) acidThrough hollow stinging hairs
Stand-alone recall pairs — no shared mechanism, just memorise them.
Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 5Plant BiologyEASY
Golden rice is a genetically-modified crop plant where the incorporated gene is meant for biosynthesis of

[Q68 · Sep · 2017]

Golden rice = Vitamin A

Golden rice is engineered for Vitamin A (beta-carotene), not omega-3 fatty acids or vitamins B/C. It targets vitamin-A deficiency.

Intercropping (simultaneous) vs crop rotation (sequential)

Intercropping = two or more crops at the SAME TIME on the same land. Crop rotation = different crops in SEQUENCE across seasons. Don't swap them.
Drill 3 more on crop and botany facts — golden rice, sugarcane, cropping, nettle

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (2)

  • Photosynthesis — requirements, equation and products

    Overall photosynthesis equation

    6 CO2+6 H2O→light, chlorophyllC6H12O6+6 O26\,\text{CO}_2 + 6\,\text{H}_2\text{O} \xrightarrow{\text{light, chlorophyll}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\,\text{O}_2
  • The light reaction — splitting water releases oxygen

    Photolysis of water (the source of O₂)

    2 H2O→light4H++4e−+O22\,\text{H}_2\text{O} \xrightarrow{\text{light}} 4\text{H}^+ + 4e^- + \text{O}_2

Reference tables (1)

Crop and botany facts — golden rice, sugarcane, cropping, nettle4 rows
FactAnswerNote
Golden rice producesVitamin A (beta-carotene)GM crop targeting vitamin-A deficiency
Golden rice = Vitamin A, NOT omega-3 or vitamin B/C (NDA 2017).
Sugarcane is grown forSucroseThe cash-crop sugar; not glucose/fructose/starch
Two+ crops grown togetherIntercroppingSame land, same time, defined rows
Intercropping = simultaneous; crop rotation = in sequence (NDA 2020).
Nettle sting injectsMethanoic (formic) acidThrough hollow stinging hairs
Stand-alone recall pairs — no shared mechanism, just memorise them.

Watch out for (8)

Drill every past-year question on this subtopic

10 questions from the bank — paginated, with cart and Word-export support.

Drill the 10 questions