Plant Transport and Physiology

Transpiration occurs primarily through stomata, but also to a lesser extent through the cuticle (cuticular transpiration) and lenticels (lenticular transpiration).

The plasma membrane of a plant cell is selectively permeable, meaning it allows certain molecules or ions to pass through while blocking others.

The pressure flow hypothesis (also called mass flow hypothesis) is the accepted mechanism for phloem translocation. It explains how osmotic pressure gradients drive sugar movement from source to sink.

While phloem sap consists primarily of water and sucrose, it also transports amino acids, hormones, signaling molecules, and other organic compounds. D is correct as phloem carries all these substances.

Xylem transport is driven by transpiration pull, which moves water and minerals from the roots upward to the leaves, making it a unidirectional process.

Phloem transport is bidirectional because the source and sink relationship can change depending on the plant's developmental stage or seasonal needs.

In early spring, stored sugars in roots or stems are mobilized to act as a source for developing buds, which function as sinks.

Phloem transports sucrose from photosynthetic sources (mature leaves) to various sinks including growing tissues, storage organs, and developing fruits. This is the fundamental translocation pathway.

While xylem is primarily for water and minerals, it can transport some organic nitrogen and hormones; similarly, phloem can transport some inorganic ions.

Plants recycle essential minerals by moving them from senescing (dying) leaves to developing tissues, which is a key survival strategy for nutrient conservation.

The endodermis contains the Casparian strip (suberin) that blocks apoplastic flow, forcing all solutes through selectively permeable transport proteins. This creates a one-way valve allowing controlled entry into the stele toward xylem.

The endodermis acts as a checkpoint for water and solutes entering the vascular cylinder (xylem) from the root cortex, ensuring selective uptake.

C4 plants lose less water because they concentrate CO2 in bundle sheath cells, reducing the need to open stomata widely. The CO2-concentrating mechanism allows higher water-use efficiency - approximately half the water loss per CO2 fixed compared to C3 plants.

Transpiration is an evaporative process that consumes heat energy, which effectively cools the leaf surface by approximately 10 to 15 degrees Celsius.

The cohesion-tension theory relies on cohesion (water-water attraction), adhesion (water-wall attraction), and surface tension to maintain the continuous water column during transpiration.