Plant Physiology Transport Mechanisms

Plant Physiology and Sucrose Transport

• Chloroplast and Internal Systems

  • The chloroplast has three membranes; the inner envelope plays an essential role in sucrose transport.
  • Sucrose transport occurs both day and night, allowing plants to store resources from photosynthesis (during the day) for use at night.

• Calvin Cycle and Starch Production

  • During the day, the Calvin cycle produces glyceraldehyde-3-phosphate, which is often stored as transient starch.
  • Transient starch accumulates during the day and gets broken down at night, distinguishing it from long-term starch reserves like those found in potatoes.

• Source and Sink Concept

  • Source tissues (e.g., mature leaves) produce sugars while sink tissues (e.g., roots, fruits) consume them.
  • Xylem transports water and nutrients up from roots, while phloem transport is more variable, depending on the plant’s developmental stage.
  • In young plants, roots may function as sinks before the leaves mature and take on that role.

• Xylem vs. Phloem Transport

  • Xylem transport starts from roots and ends in leaves, driven by tension and water loss (transpiration).
  • Phloem transport can move sugars (such as sucrose) from source to sink tissue, which can be far apart.

• Mechanisms of Phloem Transport

  • Sucrose moves from source to sink via osmotic pressure and flows through the phloem’s sieve plates.
  • Companion cells aid sieve elements by providing metabolic support, allowing the sieve cells to live long.

• Water Potential and Movement

  • Water moves via osmosis from areas of higher water potential to lower water potential, establishing pressure gradients between source and sink.
  • In the leaves, negative water potential creates tension, aiding water movement up via xylem, while high sugar concentration in phloem generates positive pressure for nutrient flow.

• Flow Loading in Phloem

  • Apoplastic Loading: Sugars accumulate in companion cells via energized transport, resulting in higher osmotic pressure in phloem.
  • Symplastic Loading: Involves diffusion and partitioning of sugars, using specialized plasmodesmata filters, preventing larger carbohydrates from backflowing, thus maintaining pressure in phloem.

• Cellular Structures and Functions

  • Sieve plates, which have pores, enable nutrient flow between adjacent sieve elements.
  • Companion cells are closely linked to sieve elements, responsible for transporting sugars while ensuring the sieve cells’ survival and function.
  • Electron microscopy is often used to study these structures, providing detailed insight into their anatomical arrangement.

• Development of Source and Sink Tissues

  • Transition from sink to source is developmental; once mature, tissues can no longer import sugars.
  • Old leaves cannot become sinks again, as they accumulate waste over time and effectively "quit" being productive, serving instead as a burden on the plant.

• Recap on Concepts

  • Understanding water potential, pressure gradients, and the evolutionary adaptations allows insights into how plants regulate their physiology and optimize nutrient transport.
  • Emphasis placed on water potential calculations and the physical flow versus diffusion of nutrients across various distances within the plant.

• Exam Preparation Focus Areas

  • Importance of the sieve plate structure and its role in phloem functionality.
  • Key differences between source and sink tissues concerning their metabolic roles.
  • Mechanisms of phloem loading and unloading, along with conditions affecting transport efficiency.
  • The interplay of development, physiologic function, and structural anatomy in plant growth and function.