Light intensity — supplies energy to split water (light-dependent reactions).
Carbon dioxide concentration — the raw carbon source fixed in the Calvin cycle.
Temperature — controls the speed of the enzymes (e.g. RuBisCO) doing the chemistry.
(Water is technically needed too, but a plant short of water closes its stomata and wilts, so it
is usually treated as a stress factor rather than the everyday rate-limiter.)
A factor in short supply that restricts the rate; increasing it speeds the process, increasing others does not.
Name the three main limiting factors of photosynthesis.
Light intensity, carbon dioxide concentration, and temperature.
Which stage does light intensity limit?
The light-dependent reactions (production of ATP and NADPH).
Which stage does CO₂ concentration limit?
The light-independent reactions / Calvin cycle (carbon fixation).
Why does temperature limit photosynthesis?
It controls enzyme (e.g. RuBisCO) activity via collision rate and denaturation.
On a rate graph, what does a plateau tell you?
The factor on the x-axis is no longer limiting; another factor has become the bottleneck.
Why does the temperature curve fall after its optimum, unlike light/CO₂?
Enzymes denature at high temperature, permanently losing active-site shape.
How can a grower prove CO₂ was limiting on a light plateau?
Raise CO₂ and watch the rate jump higher.
Recall Feynman: explain to a 12-year-old
Imagine baking cookies. You need flour, sugar, AND an oven that's hot enough. If you run out of
sugar, having a mountain of flour won't make more cookies — sugar is the thing slowing you down.
A plant making food needs light, CO₂ gas, and a warm-enough temperature. Whichever
one is "running low" is the boss controlling how fast the plant makes food. Give the plant more
of THAT one and it speeds up. But careful with heat — too hot and the plant's tiny "tools"
(enzymes) melt out of shape and stop working completely.
Dekho, photosynthesis ek assembly line jaisa hai — plant ko food banane ke liye light, CO2 gas,
aur sahi temperature teeno chahiye. "Limiting factor" wo cheez hai jo sabse kam available hai aur
isi wajah se rate ruk jaata hai. Agar light bahut hai par CO2 kam hai, to extra light kuch nahi
karega — CO2 hi boss hai us waqt. Jab tum kam wali cheez badhaoge, tabhi rate badhega.
Graph padhna simple hai. Jab curve upar chadh raha hai, matlab x-axis wala factor abhi limiting
hai (use badhao to rate badhega). Jab curve flat ho jaata hai (plateau), matlab ab wo factor enough
hai aur koi doosra factor limiting ban gaya. Light aur CO2 ke graph plateau dete hain. Lekin
temperature alag hai — wo pehle optimum tak chadhta hai, phir gir jaata hai, kyunki zyada garmi me
enzymes (jaise RuBisCO) denature ho jaate hain — unka active site shape kho deta hai.
Kyu important hai? Greenhouse farmers isi science ka use karte hain — wo CO2 add karte hain,
warmth dete hain aur light badhate hain taaki koi factor limit na kare, aur yield maximum ho. Exam
me trap yeh hai ki "more light always = more photosynthesis" — galat! Sirf rising part par sach
hai. Yaad rakho: LCT — Light, CO2, Temperature.
