Biology Lecture: Translocation in Plants

Introduction to Translocation

• Organic substances such as sugars are moved around plants by translocation.
• Key focus: Movement of glucose produced during photosynthesis in leaves.

Photosynthesis Overview

• Leaves are responsible for photosynthesis.
• Equation: Carbon dioxide + water -> glucose + oxygen.
• Glucose is an organic substance needed for respiration in plant cells.

Role of Phloem in Transport

• Phloem tissue: Responsible for transporting organic substances.
• Components of Phloem:
  • Sieve Tube Elements: Living cells, no nucleus, few organelles.
  • Companion Cells: Provide ATP for active transport.

Mass Flow Hypothesis

• Concept of 'Source to Sink':
  • Source: Leaves where sucrose is created.
  • Sink: Cells where sucrose is used.
• Model Explanation:
  • Source cells (leaf) have lower water potential due to soluble sugars.
  • Water moves into source cells by osmosis, increasing hydrostatic pressure.
  • Sink cells (respiring cells) have higher water potential as sugars are used.
  • Water moves out of sink cells, decreasing hydrostatic pressure.

Detailed Translocation Process

Step 1: Sucrose Movement

• From Source to Companion Cells:
  • Sucrose transported by facilitated diffusion.
  • Hydrogen ions actively transported creating a gradient.
• Co-Transport of Sucrose:
  • Sucrose moved into sieve tubes alongside hydrogen ions.

Step 2: Sieve Tube Element Transport

• Dissolved sucrose lowers water potential.
• Water moves into sieve tubes from xylem by osmosis.
• Increased volume and pressure push solution towards sink.

Step 3: Movement to Sink

• Respiration/degradation of sucrose at sink lowers concentration.
• Sucrose actively transported into sink cells.
• Water follows by osmosis, maintaining pressure gradient.

Investigating Phloem Transport

Tracer Experiments

• Use radioactively-labeled carbon dioxide.
• Sugars become labeled and can be tracked.
• X-ray film shows where sugars are transported.

Ringing Experiments

• Removal of bark and phloem ring.
• Swelling occurs above the removed ring.
• Proves sugar transport is in the phloem.

Summary

• Translocation involves movement from source to sink due to pressure changes.
• Tracer and ringing experiments demonstrate phloem's role.

Additional Resources

• Reference video on disaccharides for sucrose overview.
• Year 13 Biology channel for more lessons.

Note: These notes provide a summary for revision purposes and are based on the explanation of translocation, including its mechanisms and investigation methods.