Why this unit is a differentiator
Human Physiology gets prepared first because it feels more familiar and its diagrams are more memorable. Plant Physiology gets what time remains, which is often not much. The result is that a candidate who prepares it properly gains relative to the field rather than merely keeping pace.
The unit also rewards a particular study method. Photosynthesis and respiration are sequences of stages, each with a location, an input and an output. Learned that way they are compact and answerable; learned as vocabulary lists they are neither.
How NEET actually asks Plant Physiology
NTA publishes no chapter-wise weightage for NEET, so figures circulating online are coaching estimates from past papers; confirm the current paper structure in the official NTA information bulletin.
The recurring types are: identify where a stage of photosynthesis or respiration occurs, distinguish C3 from C4 pathways, match a plant hormone to its effect, interpret a water potential comparison, and recall deficiency symptoms. Location and matching questions dominate, which is why organising by process rather than by term matters.
Key concepts, compressed
- ✓Water movement is governed by water potential, which falls as solute is added and rises as pressure is applied.
- ✓The ascent of sap is driven mainly by transpiration pull acting through the cohesion and adhesion of water in the xylem.
- ✓Phloem transport is an active, bidirectional process explained by the pressure flow hypothesis.
- ✓Photosynthesis has a light-dependent stage on the thylakoids and a light-independent stage in the stroma.
- ✓The C4 and CAM pathways are adaptations that concentrate carbon dioxide around RuBisCO to suppress photorespiration.
- ✓Respiration proceeds through glycolysis in the cytoplasm, the Krebs cycle in the mitochondrial matrix, and the electron transport system on the inner membrane.
Facts worth fixing before the questions
| Process | Location | Note |
|---|---|---|
| Light reaction | thylakoid membrane | produces ATP and NADPH |
| Calvin cycle | stroma | fixes CO₂ |
| Glycolysis | cytoplasm | in all cells |
| Krebs cycle | mitochondrial matrix | |
| Electron transport system | inner mitochondrial membrane | |
| First CO₂ acceptor in C3 | RuBP, a 5-carbon sugar | first product PGA, 3-carbon |
| First CO₂ acceptor in C4 | PEP, a 3-carbon acid | first product OAA, 4-carbon |
| C4 anatomy | Kranz anatomy | bundle sheath cells |
| Photorespiration | occurs in C3, wasteful | RuBisCO binds O₂ |
| Auxin | apical dominance, cell elongation | |
| Gibberellin | stem elongation, bolting | |
| Cytokinin | cell division, delays senescence | |
| Abscisic acid | stress hormone, closes stomata | |
| Ethylene | fruit ripening, senescence | the only gaseous hormone |
The five mistakes that cost the most marks
- ✓Confusing where each stage occurs. Light reactions are on the thylakoids and the Calvin cycle in the stroma; glycolysis is cytoplasmic and the Krebs cycle mitochondrial.
- ✓Mixing up C4 and CAM. C4 separates the carboxylations in space, CAM in time - both to avoid photorespiration.
- ✓Reversing water potential reasoning. Water moves from higher to lower potential, and pure water is the maximum at zero.
- ✓Treating transpiration as purely wasteful. The transpiration pull is what drives the ascent of sap in tall plants.
- ✓Learning hormones one at a time rather than as a comparison table, which is how matching questions present them.
Practice set 1: transport in plants
1. What is the difference between diffusion, osmosis and imbibition?
Diffusion is the movement of any substance from higher to lower concentration. Osmosis is the movement of solvent specifically, across a semi-permeable membrane, from higher to lower water potential. Imbibition is a special case of diffusion where a solid colloid absorbs water and swells - as when dry seeds take up water before germination.
2. What is water potential and what affects it?
The potential energy of water relative to pure water, measured in pascals and denoted by the Greek letter psi. Pure water has the highest value, taken as zero, so all solutions have negative water potential. Adding solute lowers it; applying pressure raises it. Water always moves from higher to lower water potential.
3. What is plasmolysis?
The shrinkage of the protoplast away from the cell wall when a plant cell is placed in a hypertonic solution and loses water. It is reversible - placing the cell in a hypotonic solution causes deplasmolysis. This is the plant-cell equivalent of the shrinkage an animal cell shows, but the rigid wall makes the separation visible.
4. What are root pressure and transpiration pull?
Root pressure is a positive pressure generated in the root xylem by active ion uptake, which pushes water upward. Transpiration pull is the negative pressure created as water evaporates from the leaves, drawing the column upward. Transpiration pull is the major mechanism; root pressure is minor and mainly explains guttation.
5. What is transpiration and how is it regulated?
The loss of water vapour from aerial parts, mostly through stomata. It is regulated by guard cells, which open the stomatal pore when turgid and close it when flaccid. Beyond driving the ascent of sap, transpiration cools the leaf and assists mineral uptake - so it is a cost with essential functions attached.
6. How does the pressure flow hypothesis explain phloem transport?
Sugars are actively loaded into the phloem at the source, lowering water potential so water enters from the xylem and raises the pressure. At the sink, sugars are unloaded and water leaves, lowering the pressure. The resulting pressure gradient drives bulk flow from source to sink - which is why phloem transport is bidirectional overall while any single stream flows one way.
7. How does water rise to the top of a tall tree?
By the cohesion-tension mechanism. Evaporation at the leaves creates tension, and water molecules cohere to one another through hydrogen bonding while adhering to the xylem walls, so an unbroken column is pulled upward. The xylem's narrow, reinforced vessels are what allow the column to withstand that tension without collapsing.
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Practice set 2: mineral nutrition
8. What distinguishes macronutrients from micronutrients?
The quantity required, not the importance. Macronutrients such as nitrogen, phosphorus, potassium, calcium, magnesium and sulphur are needed in large amounts. Micronutrients such as iron, manganese, zinc, copper, boron, molybdenum and chlorine are needed in trace amounts. A deficiency of either is damaging.
9. Why do deficiency symptoms of mobile and immobile elements appear on different leaves?
Mobile elements such as nitrogen, phosphorus and potassium are withdrawn from older leaves and redirected to young growing tissue, so symptoms appear on older leaves first. Immobile elements such as calcium and sulphur cannot be relocated, so symptoms appear on young leaves. Which leaves are affected is therefore diagnostic, and NEET asks it directly.
10. What is biological nitrogen fixation?
The conversion of atmospheric nitrogen into ammonia by nitrogenase-containing microorganisms. Rhizobium in legume root nodules is the symbiotic example; Azotobacter and Clostridium are free-living. Nitrogenase is inactivated by oxygen, which is why nodules contain leghaemoglobin to keep the internal oxygen concentration low.
11. What is hydroponics and what was it used to establish?
Growing plants in a nutrient solution without soil. Historically its scientific value was in identifying which elements are essential - by omitting one element at a time and observing the effect, researchers established the list of essential nutrients that soil-based experiments could not resolve.
12. What criteria make an element essential?
It must be necessary for normal growth and reproduction, its deficiency must prevent the plant completing its life cycle, its role must be direct rather than through an indirect effect on another element's availability, and the requirement must be common to all plants. All four conditions distinguish an essential element from one that is merely beneficial.
Practice set 3: photosynthesis
13. Where does photosynthesis occur within the cell?
In the chloroplast - light reactions on the thylakoid membranes and the Calvin cycle in the stroma. The thylakoids house the pigment systems and electron carriers, while the stroma contains the enzymes for carbon fixation. This division of labour between membrane and matrix mirrors the arrangement in mitochondria.
14. What happens during the light reaction?
Light energy is absorbed by photosystems, water is split releasing oxygen, and electrons pass along a transport chain generating ATP and NADPH. The oxygen released comes entirely from water, not from carbon dioxide - a point established by isotope labelling and asked about directly.
15. What is the difference between cyclic and non-cyclic photophosphorylation?
Non-cyclic involves both photosystems, splits water, and produces ATP, NADPH and oxygen. Cyclic involves only photosystem I, does not split water, and produces ATP alone. Cyclic photophosphorylation operates when the cell needs more ATP than NADPH, which is one way the plant balances the two.
16. What happens in the Calvin cycle?
Carbon dioxide is fixed to RuBP by RuBisCO, producing two molecules of the three-carbon PGA. PGA is reduced to sugar using the ATP and NADPH from the light reaction, and RuBP is regenerated. The three phases are carboxylation, reduction and regeneration, and the cycle occurs in the stroma.
17. What are the main differences between C3 and C4 plants?
C3 plants fix carbon dioxide directly with RuBisCO, giving the three-carbon PGA as first product. C4 plants first fix it with PEP carboxylase in mesophyll cells, giving the four-carbon OAA, then release it around RuBisCO in bundle sheath cells. C4 plants show Kranz anatomy, have negligible photorespiration and perform better at high temperature.
18. What is photorespiration and why is it wasteful?
The process in which RuBisCO binds oxygen instead of carbon dioxide, producing phosphoglycolate rather than useful sugar. No ATP or NADPH is gained and carbon dioxide is released, so it reduces net productivity. It increases at high temperature, when RuBisCO's oxygenase activity rises relative to its carboxylase activity.
19. What is Kranz anatomy?
The characteristic leaf structure of C4 plants, in which large bundle sheath cells with thick walls surround the vascular bundles and are themselves surrounded by mesophyll cells. The bundle sheath cells have no intercellular spaces and are impermeable to gases, which is what allows carbon dioxide to be concentrated around RuBisCO.
20. What are the limiting factors of photosynthesis?
Light intensity, carbon dioxide concentration, temperature and water availability. Blackman's law of limiting factors states that when a process is affected by several factors, the rate is determined by the one nearest its minimum value. So increasing light will not help if carbon dioxide is the limiting factor.
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Practice set 4: respiration in plants
21. Where does glycolysis occur and what does it produce?
In the cytoplasm, in all living cells, and it requires no oxygen. One glucose molecule yields two pyruvate, a net two ATP and two NADH. Because it is cytoplasmic and anaerobic, it is common to both aerobic and anaerobic respiration - which is why it is described as the universal pathway.
22. Where does the Krebs cycle occur and what does it produce?
In the mitochondrial matrix. Each turn oxidises acetyl CoA completely, producing three NADH, one FADH2, one GTP or ATP, and two carbon dioxide. Since each glucose gives two pyruvate, the cycle turns twice per glucose. It requires oxygen indirectly, because the reduced coenzymes must be reoxidised by the electron transport system.
23. Where does the electron transport system operate?
On the inner mitochondrial membrane. NADH and FADH2 donate electrons that pass along carrier complexes, pumping protons into the intermembrane space. The resulting gradient drives ATP synthase, and oxygen serves as the final electron acceptor, forming water. This is oxidative phosphorylation.
24. What is the theoretical ATP yield from one glucose molecule?
38 ATP by the figure NCERT uses - two from glycolysis, two from the Krebs cycle, and the remainder from oxidative phosphorylation. It is a theoretical maximum that assumes ideal conditions and no losses; actual yields are lower because of the cost of transporting cytoplasmic NADH into the mitochondrion. Quote the NCERT figure and note that it is theoretical.
25. What is the respiratory quotient and what does it indicate?
The ratio of carbon dioxide released to oxygen consumed. It is 1 for carbohydrates, less than 1 for fats at around 0.7, slightly below 1 for proteins, and greater than 1 for organic acids. So the value identifies which substrate is being respired, which is exactly how NEET frames the question.
Practice set 5: growth and plant hormones
26. What are the phases of plant growth?
The lag phase of slow initial growth, the log or exponential phase of rapid growth, and the stationary phase where growth slows and stops. Plotted against time this gives a sigmoid growth curve. Note the parallel with the logistic population curve in Ecology - the same S-shape arising from the same kind of resource limitation.
27. What are the main effects of auxins?
Promoting cell elongation, maintaining apical dominance by suppressing lateral buds, initiating root formation in cuttings, and preventing premature fruit and leaf drop. Removing the apical bud releases lateral buds from that suppression, which is why pruning makes a plant bushier - a standard applied question.
28. What do gibberellins and cytokinins do?
Gibberellins promote stem elongation, cause bolting in rosette plants, break seed dormancy and stimulate the production of amylase during germination. Cytokinins promote cell division, help break bud dormancy, and delay senescence - the last effect being the basis of the Richmond-Lang effect.
29. What are the roles of abscisic acid and ethylene?
Abscisic acid is the stress hormone - it closes stomata during water shortage, induces dormancy, and generally inhibits growth, acting as an antagonist to gibberellin. Ethylene is a gas that ripens fruit, promotes senescence and abscission, and induces flowering in pineapple. It is the only gaseous plant hormone.
30. What are photoperiodism and vernalisation?
Photoperiodism is the response of flowering to relative day and night length, classifying plants as short-day, long-day or day-neutral. Crucially, it is the length of the uninterrupted dark period that matters rather than the day length. Vernalisation is the promotion of flowering by a period of low temperature, which is why some crops must be sown before winter.
How to study this chapter efficiently
- ✓Learn photosynthesis and respiration as sequences with location, input and output for each stage. Most questions ask exactly that.
- ✓Build one hormone table - name, main effects, one application. Matching questions become immediate.
- ✓Keep C4 and CAM straight by their separation strategy: C4 in space, CAM in time.
- ✓For water movement, always state the direction as higher to lower water potential rather than reasoning about concentration.
- ✓Note which deficiency symptoms appear on old versus young leaves - it is diagnostic and directly asked.
- ✓Do not deprioritise this unit. It is where relative advantage is available precisely because most candidates leave it late.
Turn this into active practice
The location questions in this unit are unforgiving - a stage is either in the stroma or on the thylakoid, and there is no partial credit for knowing what it does. That kind of precision comes from retrieval rather than from rereading a diagram.
The NEET Plant Physiology quiz on QUFF generates fresh questions across transport, nutrition, photosynthesis, respiration and growth regulators, marks them instantly and explains each answer. Do mixed sets and note whether your errors are about location, sequence or function - each needs a different kind of revision.