The combination of numbered terms that completes all of the following statement is: i. Transport through
the phloem is __1___ while transport through Xylem is __2__ ii. Loading of sugar into the phloem is __3___
at the source iii. Movement of water out of the phloem at the sink is –4– a. 1-bidirectional, 2-unidirectional,
3-by active transport, 4-by osmosis b. 1-by osmosis, 2-by turgor pressure, 3- by passive transport, 4-
bidirectional c. 1-unidirectional, 2-bidirectional, 3-by diffusion, 4-unidirectional d. 1-by diffusion, 2-
unidirectional, 3-bidirectional, 4-by osmosi

🌱 Introduction

Transport systems in vascular plants are essential for distributing nutrients and water throughout the plant body. Two key tissues involved are:

• Phloem – responsible for transporting organic nutrients like sugars (especially sucrose)

• Xylem – responsible for transporting water and dissolved minerals

Understanding the mechanisms of transport, including directionality, energy requirements, and processes at source and sink, is critical for grasping how plants maintain homeostasis and growth.

🔍 The Question Breakdown

Let’s analyze the statements provided and identify the correct combination:

i. Transport through the phloem is (1), while transport through Xylem is (2)

• Phloem transport is bidirectional – it moves sugars from sources (like leaves) to sinks (like roots or fruits), and the direction depends on the plant’s needs.

• Xylem transport is unidirectional – water and minerals only move upward from roots to shoots.

ii. Loading of sugar into the phloem is (3)___ at the source

• Sugars are actively loaded into the phloem using active transport, which requires ATP.

iii. Movement of water out of the phloem at the sink is (4)___

• Water exits the phloem by osmosis after sugars are unloaded at the sink, lowering the osmotic pressure.

✅ Correct Answer:

(a)
1 – bidirectional
2 – unidirectional
3 – by active transport
4 – by osmosis

💡 Detailed Explanation:

1. Bidirectional Transport in Phloem

• The phloem uses pressure flow mechanism or mass flow hypothesis.

• Sugars can move up or down based on the source-sink relationship.

2. Unidirectional Transport in Xylem

• Driven by transpiration pull, capillary action, and root pressure.

• Movement is strictly upward – from roots to leaves.

3. Active Loading of Sugars

• Active transport loads sucrose into phloem sieve tubes at the source (e.g., leaves).

• This raises the osmotic pressure in phloem, pulling in water.

4. Osmotic Movement of Water at Sink

• At the sink (e.g., roots or developing fruits), sucrose is removed from the phloem.

• This decreases osmotic pressure, causing water to move out by osmosis into surrounding tissues.

🧠 Key Takeaways:

• Phloem transport is dynamic, serving various plant parts depending on metabolic needs.

• Xylem only transports water/minerals upward, without requiring energy.

• Phloem loading is energy-dependent, crucial for maintaining pressure flow.

• Osmosis plays a vital role in water movement at both source and sink.

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