Plant Physiology — Photosynthesis, Respiration & Growth

Why This Chapter Matters

Plant physiology quietly delivers 2-3 marks in almost every SSC CGL, CHSL, MTS and RRB NTPC General Awareness paper, and it repeats far more than its share of attention suggests. Photosynthesis factors, the xylem-phloem pair, and plant hormone names show up year after year, often as the exact same question reworded. This is one of the cheapest scoring zones in the whole Biology syllabus if you fix the right facts once.

Here is the shape of what's coming: the photosynthesis equation and what speeds it up or slows it down, how water and food actually move inside a plant, the five hormones that control growth, how a plant breathes without lungs, and how it bends toward light or gravity without a nervous system. The single biggest mistake aspirants make here is mixing up xylem and phloem, and then compounding it by mixing up which hormone does what. You will see students confidently write that phloem carries water, or that gibberellin causes leaf fall. Fix the direction of flow and the hormone-to-function pairing early, and this entire chapter becomes a formality on exam day.

1. Photosynthesis — How Plants Make Their Own Food

Photosynthesis is the process by which green plants trap sunlight and convert carbon dioxide and water into glucose, releasing oxygen as a by-product. Every food chain on Earth, including the one that put your breakfast on the table, starts here.

The balanced equation you must be able to write from memory:

6CO₂ + 6H₂O + sunlight → C₆H₁₂O₆ + 6O₂

(in the presence of chlorophyll)

Chlorophyll, the green pigment sitting inside chloroplasts, absorbs sunlight and starts the reaction. Photosynthesis happens mainly in the leaf's mesophyll tissue, which is packed with chloroplasts, and it happens in two broad stages: the light reaction (occurs in the thylakoid membranes, splits water, releases oxygen, produces ATP and NADPH) and the dark reaction or Calvin cycle (occurs in the stroma, does not need light directly, uses ATP and NADPH to fix CO₂ into glucose).

Exam trap: the dark reaction does not mean it happens only at night. It simply does not require light directly, though it depends on the products of the light reaction. SSC has tested this distinction more than once with a "which of the following is true" format.

Analogy: think of a photosynthesis like a dhaba kitchen at rush hour. The light reaction is the tandoor, converting raw energy (sunlight) into usable heat and prepared ingredients (ATP, NADPH) as fast as it can. The dark reaction is the cook at the counter, who does not need to look at the tandoor directly but cannot plate a single dish without what the tandoor already sent over. Cut the tandoor's fuel (sunlight) and the whole kitchen eventually stalls, even the part working "in the dark."

Factors Affecting the Rate of Photosynthesis

Exams love asking which factor is "limiting" under given conditions. The rule to remember: whichever factor is in shortest supply controls the rate, no matter how much of everything else is available.

Factor	Effect on photosynthesis rate
Light intensity	Rate rises with intensity, then plateaus once other factors become limiting; too much can damage chlorophyll
CO₂ concentration	Rate rises as CO₂ rises, up to a saturation point (normal atmospheric CO₂ is often the limiting factor outdoors)
Temperature	Rate rises with temperature up to an optimum (roughly 25-35°C for most plants), then falls sharply as enzymes denature
Water availability	Scarcity closes stomata to conserve water, which cuts off CO₂ intake and slows the rate
Chlorophyll content	More chlorophyll means more light-trapping capacity, up to the leaf's structural limit

Exam trap: a very common wrong-option trick is calling oxygen a "raw material" for photosynthesis. Oxygen is the output, not the input. The raw materials are only carbon dioxide and water.

2. Plant Tissues — Xylem and Phloem, the Plant's Plumbing

A plant has no heart and no blood vessels in the animal sense, yet it moves water from root to leaf-tip and food from leaf to root with total reliability. Two specialised tissues do this job, and confusing them is the single most common mistake in this chapter.

Xylem carries water and dissolved minerals, and it moves them in only one direction: upward, from root to shoot and leaves. Xylem is made of dead, hollow, tube-like cells (tracheids and vessels) that offer a low-resistance pipeline. The upward pull is driven mainly by transpiration (water evaporating from leaf surfaces through stomata), which creates a suction that drags the water column up from the roots. This whole mechanism is called the transpiration pull or the cohesion-tension theory, resting on the fact that water molecules stick to each other (cohesion) strongly enough to be pulled up as a continuous thread without breaking.

Phloem carries food, mainly sugars made during photosynthesis, and it moves them in both directions depending on where the plant needs the food: from leaves (the "source") to roots, fruits, or growing shoots (the "sink"). Unlike xylem, phloem cells (sieve tubes and companion cells) are living. This movement of food is called translocation, and the leading explanation for it is the pressure-flow hypothesis: sugar loaded into the phloem at the source raises pressure there, pushing the sap toward low-pressure sink regions.

Memory hook: remember "Xylem = eXit upward only, Phloem = Passes both ways." X comes before P in the alphabet, and water's one-way upward journey happens first, before the plant even has sugar to send anywhere.

Exam trap: SSC questions often ask "which tissue is responsible for transpiration" as a trick, when transpiration is a process that happens at the leaf surface through stomata, not inside the xylem itself. Xylem only supplies the water that gets transpired; do not credit xylem with causing transpiration.

Both xylem and phloem run together inside a vascular bundle, visible as the "veins" in a leaf and the rings in a woody stem.

Transpiration — Necessary Evil or Useful Tool?

Transpiration loses a plant a large amount of water, sometimes over 90% of what its roots absorb, purely as vapour through stomata. It sounds wasteful, but it does three useful jobs: it pulls water up against gravity, it cools the leaf surface (just like sweat cools your skin), and it helps move dissolved minerals along with the water. Stomata are tiny pores, mostly on the underside of leaves, guarded by a pair of guard cells that open and close them based on water availability and light.

3. Plant Hormones — The Five Chemical Messengers

Plants have no nerves, yet they respond to light, gravity, injury, and the seasons with precision. Chemical messengers called phytohormones do this work, each produced in tiny quantities but with outsized effect.

Hormone	Main site of production	Key function	One SSC-tested fact
Auxin	Shoot tip (apical meristem)	Cell elongation, promotes apical dominance, causes phototropism/geotropism bending	First plant hormone discovered; IAA (indole acetic acid) is the natural form
Gibberellin	Young leaves, seeds, roots	Stem elongation ("bolting"), seed germination, breaks dormancy	Discovered from a fungus (Gibberella) infecting rice, causing abnormally tall, weak "foolish seedling" plants
Cytokinin	Root tips, developing seeds/fruits	Promotes cell division, delays leaf senescence (ageing)	Works opposite to auxin in controlling lateral bud growth
Abscisic acid (ABA)	Leaves, stems, roots (stress response)	Induces dormancy, closes stomata under drought stress, inhibits growth	Called the "stress hormone" of plants
Ethylene	Ripening fruits	Fruit ripening, promotes leaf and flower abscission (falling)	The only plant hormone that is a gas at normal temperature

Memory hook: "A Good Cop Always Ends it." Auxin (elongation and bending), Gibberellin (growth/germination), Cytokinin (cell division), Abscisic acid (stress and dormancy), Ethylene (ripening and ending, as in fruit drop and leaf fall). The phrase ends with "ends it," matching ethylene's role in ending a fruit's growth phase by ripening it.

Exam trap: gibberellin and auxin are both linked to "growth," and SSC frequently swaps them in options. The distinguishing fact to hold onto: auxin is about bending toward a stimulus and suppressing side branches, gibberellin is about overall stem elongation and breaking seed dormancy. If the question mentions a fungus or "foolish seedling," it is gibberellin.

Real-world grounding: the artificial ripening of fruit using calcium carbide (banned in India for food safety reasons) works by mimicking ethylene gas. Farmers have also known for generations that keeping one ripe banana next to raw ones speeds up their ripening; the ripe banana is releasing natural ethylene.

4. Respiration in Plants — Breathing Without Lungs

Every living plant cell respires, all day and all night, to release the energy locked in glucose. The overall equation is the reverse of photosynthesis:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (ATP)

Plants have no dedicated respiratory organs like lungs. Gas exchange happens through stomata in leaves, lenticels (small pores) in woody stems and bark, and directly through root-hair surfaces in the soil. Respiration occurs in the mitochondria, often called the "powerhouse of the cell," and it runs continuously, unlike photosynthesis, which needs light.

Exam trap: a favourite SSC question asks whether plants "photosynthesise and respire at the same time" during the day. The answer is yes; both processes run simultaneously in daylight, but photosynthesis produces far more oxygen than respiration consumes, so the net effect is oxygen release. At night, with no photosynthesis, only respiration continues, consuming oxygen and releasing CO₂. This is exactly why doctors and elders advise against keeping too many plants in a closed bedroom at night, since the room's CO₂ can rise slightly.

Respiration can be aerobic (using oxygen, releasing more energy) or anaerobic (without oxygen, as in waterlogged roots or during seed germination in low-oxygen soil, releasing less energy and often producing ethanol or lactic acid as by-products). Rice fields, which are frequently waterlogged, are a classic real example of plant roots relying partly on anaerobic respiration.

5. Tropisms — How Plants Move Without Muscles

A plant cannot walk toward sunlight, but it can grow toward it. Directional growth in response to an external stimulus is called a tropism. Growth toward the stimulus is positive, growth away from it is negative.

Tropism	Stimulus	Example
Phototropism	Light	Shoot bends toward light (positive), root bends away (negative)
Geotropism (gravitropism)	Gravity	Root grows downward (positive), shoot grows upward (negative)
Hydrotropism	Water	Roots grow toward moist soil
Thigmotropism	Touch/contact	Tendrils of climbers like grapevine or money plant coil around a support
Chemotropism	Chemicals	Pollen tube grows toward the ovule, guided by chemical signals

Tropisms are driven mainly by auxin, which accumulates on the shaded or lower side of a shoot, making cells there elongate faster than cells on the lit or upper side. That uneven elongation bends the shoot toward light and away from gravity.

Analogy: picture a crowded local train platform where passengers on one side get pushed forward faster than the other, so the queue itself curves. In a plant shoot, auxin is the crowd, piling up unevenly on one side of the stem and making that side's cells stretch faster, which physically bends the whole stem toward the light.

Exam trap: do not confuse tropism (directional growth response, permanent and growth-linked) with nastic movements (non-directional, reversible responses like the touch-me-not plant, Mimosa pudica, folding its leaves when touched, or a flower opening and closing with temperature). Tropism direction depends on where the stimulus is; nastic movement direction does not.

6. Photoperiodism and Vernalisation — Timing Growth to the Calendar

Plants also track the length of day and night to decide when to flower, a response called photoperiodism. Based on this, plants are classified as short-day plants (flower when night length exceeds a critical value, e.g. rice, chrysanthemum), long-day plants (flower when day length exceeds a critical value, e.g. wheat, spinach), and day-neutral plants (flowering unaffected by day length, e.g. tomato, cotton).

Vernalisation is a related but separate concept: some plants need a period of low temperature (a cold spell) before they will flower, which is why certain wheat varieties are sown in winter. Do not merge vernalisation (temperature-triggered) with photoperiodism (day-length-triggered); SSC has tested the distinction directly.

Quick Revision — One-Line Facts

• Photosynthesis equation: 6CO₂ + 6H₂O + sunlight → C₆H₁₂O₆ + 6O₂, in the presence of chlorophyll.
• Photosynthesis has two stages: the light reaction (thylakoid) and the dark reaction/Calvin cycle (stroma).
• Chlorophyll is located inside the chloroplast, mainly in the leaf's mesophyll layer.
• Oxygen released in photosynthesis comes from the splitting of water, not from carbon dioxide.
• CO₂ concentration, light intensity, temperature, and water availability are the four classic limiting factors of photosynthesis rate.
• Xylem carries water and minerals upward only, from root to leaf.
• Phloem carries food (sugars) in both directions, from source to sink; this movement is called translocation.
• Xylem cells are dead; phloem cells (sieve tubes, companion cells) are living.
• The upward movement of water through xylem is explained by the cohesion-tension theory, driven by transpiration pull.
• Stomata are leaf pores controlled by guard cells; they are the main site of both transpiration and gas exchange.
• Transpiration cools the plant and helps pull water and minerals upward, despite the water loss involved.
• Auxin (IAA) causes cell elongation and controls apical dominance and tropic bending.
• Gibberellin causes stem elongation and breaks seed dormancy; it was first discovered in a rice-infecting fungus.
• Cytokinin promotes cell division and delays leaf ageing (senescence).
• Abscisic acid (ABA) is the plant's stress hormone, inducing dormancy and closing stomata during drought.
• Ethylene is the only plant hormone that exists as a gas; it triggers fruit ripening and leaf fall.
• Plant respiration equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy.
• Plants respire continuously, day and night; they photosynthesise only in light.
• Plants exchange gases through stomata (leaves), lenticels (woody stems), and root-hair surfaces.
• Anaerobic respiration in plants can occur in waterlogged roots and during low-oxygen seed germination.
• Tropism is directional growth in response to a stimulus; positive tropism grows toward it, negative grows away.
• Phototropism: shoots show positive response to light, roots show negative response.
• Geotropism: roots show positive response to gravity, shoots show negative response.
• Thigmotropism explains tendril coiling in climbers like grapevine and money plant.
• Auxin's uneven distribution on the shaded/lower side of a stem causes tropic bending.
• Nastic movements (e.g. touch-me-not folding) are non-directional and reversible, unlike tropisms.
• Short-day plants flower when nights are long (e.g. rice); long-day plants flower when days are long (e.g. wheat).
• Vernalisation is flowering triggered by prior cold exposure, distinct from day-length-based photoperiodism.
• The mitochondrion is the site of respiration in plant cells, just as in animal cells.
• Photosynthesis and respiration run simultaneously in daylight, with photosynthesis's oxygen output outweighing respiration's oxygen use.
• A vascular bundle is the combined xylem-phloem structure visible as leaf veins and stem rings.

Memory Tables

Table A — Plant Hormones at a Glance

Hormone	One-line role	Physical state/origin fact
Auxin	Elongation, bending, apical dominance	First hormone discovered; natural form is IAA
Gibberellin	Stem elongation, breaks seed dormancy	Discovered via a fungus causing "foolish seedling" disease
Cytokinin	Cell division, delays leaf ageing	Works opposite to auxin on lateral bud growth
Abscisic acid	Stress response, dormancy, closes stomata	Known as the plant stress hormone
Ethylene	Fruit ripening, leaf/flower fall	Only plant hormone that is a gas

Table B — Xylem vs Phloem vs Tropisms Quick Contrast

Feature	Xylem	Phloem
Carries	Water and minerals	Food (sugars)
Direction	One-way, upward	Both directions (source to sink)
Cell state	Dead	Living
Driving mechanism	Transpiration pull (cohesion-tension)	Pressure-flow hypothesis

Tropism	Stimulus	Positive response example
Phototropism	Light	Shoot bends toward light
Geotropism	Gravity	Root grows downward
Hydrotropism	Water	Root grows toward moisture
Thigmotropism	Touch	Tendril coils around support

Practice MCQs

Q1. What is released as a by-product of photosynthesis? (a) Carbon dioxide (b) Oxygen (c) Nitrogen (d) Water vapour only

Q2. Which pigment is essential for trapping light energy during photosynthesis? (a) Carotene (b) Xanthophyll (c) Chlorophyll (d) Anthocyanin

Q3. Which plant tissue transports water and minerals from root to leaves? (a) Phloem (b) Xylem (c) Cambium (d) Epidermis

Q4. The plant hormone responsible for fruit ripening is: (a) Auxin (b) Cytokinin (c) Gibberellin (d) Ethylene

Q5. Which of the following is the correct equation for photosynthesis? (a) C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (b) 6CO₂ + 6H₂O + sunlight → C₆H₁₂O₆ + 6O₂ (c) 6O₂ + 6H₂O → C₆H₁₂O₆ + 6CO₂ (d) C₆H₁₂O₆ + 6CO₂ → 6H₂O + 6O₂

Q6. Downward growth of roots in response to gravity is called: (a) Phototropism (b) Hydrotropism (c) Geotropism (d) Thigmotropism

Q7. Which structures on the leaf surface regulate transpiration? (a) Lenticels (b) Guard cells and stomata (c) Trichomes (d) Root hairs

Q8. Which plant hormone is known as the "stress hormone" because it helps plants cope with drought? (a) Ethylene (b) Cytokinin (c) Abscisic acid (d) Gibberellin

Q9. The transport of food from leaves to other parts of the plant is called: (a) Transpiration (b) Translocation (c) Absorption (d) Guttation

Q10. Gibberellin was first discovered in connection with which organism? (a) A bacterium causing root rot (b) A fungus causing "foolish seedling" disease in rice (c) A virus infecting tobacco (d) An alga in stagnant water

Q11. During which stage of photosynthesis does the splitting of water molecules occur? (a) Dark reaction (b) Calvin cycle (c) Light reaction (d) Glycolysis

Q12. Coiling of a tendril around a support, as seen in grapevine, is an example of: (a) Geotropism (b) Chemotropism (c) Thigmotropism (d) Hydrotropism

Q13. Which of the following statements about anaerobic respiration in plants is correct? (a) It never occurs in any plant tissue (b) It occurs in waterlogged roots and low-oxygen seed germination, releasing less energy than aerobic respiration (c) It releases more energy than aerobic respiration (d) It occurs only in chloroplasts

Q14. A plant that flowers only when night length exceeds a critical duration, such as rice, is classified as a: (a) Long-day plant (b) Day-neutral plant (c) Short-day plant (d) Vernalised plant

Q15. Which of the following correctly distinguishes xylem cells from phloem cells? (a) Xylem cells are living, phloem cells are dead (b) Xylem cells are dead, phloem cells are living (c) Both are living (d) Both are dead

Answer Key

Q	Answer	One-line reason
Q1	(b)	Oxygen comes from the splitting of water during the light reaction; CO₂ and water vapour are not products, they are involved as inputs or lost separately through transpiration.
Q2	(c)	Chlorophyll, housed in chloroplasts, is the pigment that absorbs light energy to drive photosynthesis; carotene and xanthophyll are accessory pigments.
Q3	(b)	Xylem is the one-way, root-to-leaf pipeline for water and dissolved minerals, driven by transpiration pull.
Q4	(d)	Ethylene, the only gaseous plant hormone, triggers fruit ripening and is why a ripe banana speeds up ripening of fruit kept beside it.
Q5	(b)	This is the balanced photosynthesis equation; option (a) is actually the respiration equation, a common trap when options are swapped.
Q6	(c)	Geotropism is growth in response to gravity; roots show a positive response by growing downward.
Q7	(b)	Guard cells control the opening and closing of stomata, the main sites of transpiration and gas exchange on the leaf.
Q8	(c)	Abscisic acid induces dormancy and closes stomata under drought stress, earning it the "stress hormone" label.
Q9	(b)	Translocation specifically refers to phloem's movement of sugars from source (leaves) to sink (roots, fruits, growing points).
Q10	(b)	Gibberellin was isolated from Gibberella, a fungus that made rice seedlings grow abnormally tall and thin, hence "foolish seedling" disease.
Q11	(c)	The light reaction, occurring in the thylakoid membrane, splits water molecules and releases oxygen while generating ATP and NADPH.
Q12	(c)	Thigmotropism is growth in response to touch or contact, exactly what makes a tendril coil around whatever it brushes against.
Q13	(b)	Anaerobic respiration happens under low-oxygen conditions like waterlogged soil, and it yields far less ATP than aerobic respiration.
Q14	(c)	Short-day plants like rice flower once the uninterrupted night period crosses a critical length, the reverse logic of long-day plants like wheat.
Q15	(b)	Xylem is built from dead, hollow cells that act as a passive pipeline, while phloem's sieve tubes and companion cells remain alive to actively load and move sugars.