In C4 photosynthesis, CO2 combines with PEP (3-carbon) to form oxaloacetate (4-carbon compound) via the enzyme PEP carboxylase.

Root pressure is measured using a manometer connected to the cut end of a plant stem, showing the positive pressure generated by roots.

During stomatal opening, potassium ions (K+) actively accumulate in guard cells, increasing osmotic potential and causing water influx.

Abscisic acid (ABA) accumulates during water stress and environmental changes, promoting abscission layer formation and leaf fall.

High auxin to low cytokinin ratio promotes callus formation, while the ratios determine the type of organogenesis.

Osmotic potential (Ψs) = -iCRT, where it depends on solute concentration (C) and absolute temperature (T).

# 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.

- Companion cells: Support sieve tubes metabolically but don't transport sugars themselves

- Phloem fibers: Provide mechanical support only

- Sieve tube elements: Primary transport vessels with thin cytoplasm lining and functioning sieve pores

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.

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.