Doubts

Stuck on a question? Ask here and your subject teacher answers, usually the same day. Arvind sir answered your C4 doubt this morning, and 1 doubt is waiting for a teacher.

Ask a doubt

Goes to your subject teacher

My doubts

Biology Photosynthesis in Higher Plants Answered

In C4 plants, why doesn't RuBisCO waste energy on photorespiration in the bundle sheath cells? It can still bind O₂, right?

Asked Sun 4 Oct, 7:40 pm 1 photo

AM Arvind MenonBiology · answered today, 8:15 am

Yes — it is the same RuBisCO and it could still bind O₂. What changes is the amount of CO₂ around it.

In mesophyll cells, PEP carboxylase fixes CO₂ first. It has no oxygenase activity, so it never mistakes O₂ for CO₂. The 4-carbon acid it makes (oxaloacetate, then malate) moves into the bundle sheath and is decarboxylated to release CO₂ right next to RuBisCO.

With CO₂ that concentrated, RuBisCO works as a carboxylase almost all the time, so photorespiration is nearly absent. The thick walls of the bundle sheath (Kranz anatomy, no intercellular spaces) stop that CO₂ from leaking away.

NCERT XI · Photosynthesis in Higher Plants — read "The C4 pathway" and "Photorespiration" together. This is why C4 plants tolerate high temperatures and give more yield.

1:42
Physics Laws of Motion Teacher asked a question

Full mock test 5 · Q27: a block on a rough incline. Why is the acceleration g(sin θ − μ cos θ) and not g(sin θ + μ cos θ)?

Asked Sat 3 Oct, 9:05 pm Full mock test 5 · Q27

AP Arun PrakashPhysics · yesterday, 6:30 pm

Both can be right. Is the block sliding down or was it pushed up the incline? Friction always opposes the motion, so its direction decides the sign. Send me a photo of your free-body diagram and I'll check it.
Chemistry Classification of Elements Waiting for a teacher

Why is the second ionisation enthalpy of sodium so much higher than that of magnesium, when the first is lower?

Asked today, 9:12 am Dr. Sreeja Nair usually answers by evening

Batch FAQ

NEET Long-term A · names hidden

Why is the RQ of fats less than 1? BiologyRespiration in Plants · asked by 6 students

$\text{RQ} = \dfrac{\text{CO}_2\ \text{released}}{\text{O}_2\ \text{used}}$. Fats have far less oxygen than carbon and hydrogen, so more O₂ is needed to oxidise them than CO₂ comes out. For tripalmitin: \ce{2 C51H98O6 + 145 O2 -> 102 CO2 + 98 H2O}, so $\text{RQ} = \dfrac{102}{145} \approx$ 0.7.

Carbohydrates: RQ = 1. Proteins: about 0.9.

Arvind Menon · Biology
Do we write 36 or 38 ATP for one glucose? BiologyRespiration in Plants · asked by 5 students

NCERT calculates a net gain of 38 ATP per glucose in aerobic respiration, and says this is a theoretical figure that assumes a smooth, sequential pathway. Use 38 in NEET unless the question asks something else.

Arvind Menon · Biology
Why doesn't a magnetic field change a charged particle's speed? PhysicsMoving Charges and Magnetism · asked by 4 students

The force F = q(v × B) is always perpendicular to the velocity, so it does no work. Kinetic energy and speed stay the same; only the direction turns. In a uniform field the particle moves in a circle of radius r = mv ⁄ qB.

Arun Prakash · Physics
Why is o-nitrophenol steam volatile but p-nitrophenol is not? ChemistryAlcohols, Phenols and Ethers · asked by 4 students

In o-nitrophenol the –OH and –NO₂ groups are side by side, so the hydrogen bond forms inside the same molecule (intramolecular). The molecules don't hold on to each other, so it boils easily. In p-nitrophenol the groups are far apart, so it hydrogen-bonds with neighbouring molecules (intermolecular) and has a higher boiling point.

Dr. Sreeja Nair · Chemistry
Why is a dihybrid test cross 1 : 1 : 1 : 1? BiologyPrinciples of Inheritance and Variation · asked by 3 students

The F1 plant (RrYy) makes four kinds of gametes — RY, Ry, rY, ry — in equal numbers, because the two genes assort independently. The recessive parent (rryy) gives only ry. So each F1 gamete shows up directly as one phenotype, in equal numbers.

Arvind Menon · Biology

Other students never see your name on a doubt.

Doubts

Ask a doubt

My doubts

Nothing here

Quick actions