Phloem Transport
Pressure-flow hypothesis of translocation from source to sink. (Biology › Transport in Plants and Mineral Nutrition, NEET UG syllabus.)
What is Phloem Transport?
The long-distance movement of organic solutes (food) through the phloem from source to sink.
Key points
- Describe the process of translocation in plants.
- Explain the Pressure-Flow Hypothesis in detail.
- Differentiate between a 'source' and a 'sink' in the context of phloem transport.
- Identify the roles of active transport and osmosis in phloem loading and unloading.
Common exam trap
Confusing the function of phloem (food transport) with xylem (water and mineral transport).
Definitions
- Term
Translocation
- Meaning
The long-distance movement of organic solutes (food) through the phloem from source to sink.
- Term
Pressure-Flow Hypothesis
- Meaning
The widely accepted mechanism for phloem transport, driven by a pressure gradient created by osmotic water movement into and out of sieve tubes.
- Term
Source
- Meaning
A plant part that produces or releases sugars (e.g., mature leaves, storage organs breaking down starch).
- Term
Sink
- Meaning
A plant part that consumes or stores sugars (e.g., roots, fruits, growing buds, young leaves).
- Term
Phloem Loading
- Meaning
The active transport of sucrose from mesophyll cells into sieve tube elements at the source, increasing solute concentration.
- Term
Phloem Unloading
- Meaning
The active transport of sucrose out of sieve tube elements into sink cells, decreasing solute concentration.
Learning objectives
Describe the process of translocation in plants.
Explain the Pressure-Flow Hypothesis in detail.
Differentiate between a 'source' and a 'sink' in the context of phloem transport.
Identify the roles of active transport and osmosis in phloem loading and unloading.
List the key cellular components involved in phloem transport.
Relate the pressure gradient to the mass flow of phloem sap.
Prerequisites
Basic knowledge of plant anatomy (xylem, phloem, leaves, roots).
Understanding of osmosis, diffusion, and active transport.
Concept of water potential and turgor pressure.
Basic understanding of photosynthesis and respiration.
Common mistakes
Confusing the function of phloem (food transport) with xylem (water and mineral transport).
Assuming phloem transport is always unidirectional like xylem transport.
Not understanding that both loading and unloading of sucrose are active processes requiring ATP.
Misinterpreting the role of water potential and osmosis in creating the pressure gradient.
Believing that only leaves can be sources; storage organs can also act as sources.
Keywords
Phloem
Translocation
Pressure-Flow Hypothesis
Mass Flow
Source
Sink
Sucrose
Sieve tube
Companion cell
Active transport
Osmosis
Turgor pressure
Water potential
Practice preview
Which of the following events occurs immediately after sucrose is actively loaded into the sieve tube elements at the source?…
medium
Which statement accurately describes the directionality of phloem transport?…
medium
A girdling experiment involves removing a ring of bark (including phloem) from a tree trunk. Which of the following observations would most likely occur above the girdle after several weeks?…
hard