General Science - MCQ Practice Questions

# Phloem Tissue and Sugar Translocation

Sieve tube elements are specialized living cells in the phloem that form a continuous network for transporting dissolved sugars and organic compounds throughout the plant.

## Step 1: Understanding Phloem Composition

The phloem tissue consists of multiple cell types, each with different functions in nutrient transport and support.

## Step 2: Identifying the Transport Cells

Sieve tube elements are elongated, living cells connected end-to-end through sieve plates (perforated cell walls) that allow rapid movement of sugars and other organic solutes through their cytoplasm via mass flow.

The correct answer is (A) Sieve tube elements, as they are the only cells with the structural and functional capacity to translocate sugars bidirectionally throughout the plant.

Transpiration is the loss of water vapour from aerial parts of plants, primarily through stomata during daytime.

Water photolysis (2H2O → O2 + 4H+ + 4e-) releases electrons that are transferred through electron transport chains.

RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) catalyzes CO2 fixation with RuBP in the Calvin cycle.

CAM plants like pineapple, cactus, and agave fix CO2 at night into organic acids and release during day for photosynthesis.

Cohesion-tension theory explains that water is pulled upward due to transpirational pull creating negative pressure in xylem.

Rhizobium bacteria in root nodules of legumes fix atmospheric nitrogen into ammonia through nitrogenase enzyme.

Physiological wilting occurs when transpiration rate exceeds water absorption rate, though soil moisture is sufficient.

Double fertilization: one sperm fuses with egg nucleus (embryo), other fuses with polar nuclei (endosperm - 3n).

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.