General Science - MCQ Practice Questions

In hypertonic solutions, water moves out of the cell due to osmosis, causing protoplasm to shrink away from the cell wall - this is plasmolysis. This is a fundamental osmotic principle tested in SSC/Railway exams.

Root pressure is generated by active transport of mineral ions into the xylem sap, creating a concentration gradient that draws water in osmotically. This is distinct from transpiration pull and is a key concept in plant physiology.

# Light Absorption Spectra in Photosynthesis

Phytochrome is a specialized photoreceptor protein that uniquely absorbs light in the red (660 nm) and far-red (730 nm) wavelength regions, making it distinct from other photosynthetic pigments.

## Step 1: Understanding Photoreceptor Types

Different pigments in plants absorb different wavelengths of light based on their molecular structure. Photosynthetic pigments like chlorophyll and carotenoids primarily absorb blue and red light for energy conversion, while phytochrome specifically evolved to detect red and far-red light for signaling purposes rather than energy production.

## Step 2: Comparing Other Options

Chlorophyll a absorbs primarily in blue (430 nm) and red (662 nm) regions for photosynthesis; carotenoids and xanthophyll absorb in blue-green regions (400-550 nm) and serve as accessory pigments. Only phytochrome has its primary functional absorption in the far-red region (730 nm), which is critical for seed germination, flowering, and shade avoidance responses in plants.

The correct answer is (B) Phytochrome, as it is the only pigment primarily responsible for absorbing light specifically in both the red and far-red regions of the electromagnetic spectrum.

In C4 plants, PEP carboxylase fixes CO2 into oxaloacetate (4-carbon compound), unlike C3 plants where RuBisCO produces 3-PGA. This adaptation reduces photorespiration and is efficient in hot climates.

Photomorphogenesis refers to light-dependent development processes including light-mediated seed germination. Phytochrome is the photoreceptor involved. This is distinct from photoperiodism (flowering response to day length).

The correct radial arrangement in dicot stems is Epidermis (outermost) → Cortex → Vascular bundles (Phloem outer, Xylem inner) → Pith (center). This NCERT-based concept is frequently asked in SSC exams.

Ethylene is the senescence hormone that promotes leaf yellowing, abscission, and fruit ripening. Cytokinins antagonize ethylene effects. This hormone physiology is important for understanding plant development.

The reduction phase of Calvin cycle requires 3 ATP per CO2 fixed. NADPH requirement is 2 molecules per CO2. These are stoichiometric ratios essential for understanding photosynthetic efficiency.

2,4-D is a synthetic auxin widely used as a selective herbicide. Natural auxin is IAA (Indole-3-acetic acid). This is application-based knowledge relevant for agricultural science in UPSC.

The Casparian strip is a band of suberin (waterproof lipid polymer) that prevents passive transport across the endodermis, forcing minerals to undergo active transport. This ensures selective ion uptake - a key NCERT concept.

Stomata generally close at night (except CAM plants) to reduce transpiration. Guard cells are rich in mitochondria and chloroplasts to provide ATP and regulate osmotic potential. This circadian regulation is fundamental to plant physiology.

CO2 saturation point is where further increase in CO2 doesn't increase photosynthesis rate because other factors (like enzyme activity) become limiting. Compensation point is where photosynthesis equals respiration.

Monocots typically have fibrous root systems with adventitious roots. They lack secondary growth (no cambium) and have scattered vascular bundles. Dicots have tap roots and show secondary growth.

ABA is a stress hormone that accumulates during drought, regulating stomatal closure and promoting seed dormancy. It antagonizes gibberellins and cytokinins. This is crucial for understanding plant responses to environmental stress.

Chlorophyll absorbs blue and red light but reflects and transmits green light, making plants appear green. This is why plants cannot utilize green light efficiently for photosynthesis - a fundamental physics-biology concept.

Wood is composed of secondary xylem with thick-walled tracheids and vessel elements that provide mechanical support and water transport. Annual rings visible in wood represent secondary xylem produced by vascular cambium.

Rhizobium bacteria form symbiotic associations with legume roots, fixing atmospheric N2 into ammonia. The bacteria live in root nodules. This symbiotic relationship is agriculturally significant and frequently tested in competitive exams.

C4 plants have higher photosynthetic efficiency (~60%) compared to C3 plants (~30%) because they concentrate CO2 around RuBisCO, reducing photorespiration. This adaptation makes them suited for hot climates - important agricultural knowledge.

Protons accumulate in the thylakoid lumen (pH 4-5) due to water photolysis and electron transport, creating a chemiosmotic gradient for ATP synthesis. The stroma maintains pH ~8. This proton gradient is essential for understanding light reactions.

Photosystem I is an integral membrane protein complex with multiple transmembrane domains, making it hydrophobic. Rubisco is soluble, while ferredoxin and plastocyanin are soluble electron carriers. This biochemical distinction is important for understanding photosynthetic mechanisms.