O Level Biology Transport in Plants: Questions and Answers

Subject: Biology

Level: O Level

Topic: Transport in Plants

This resource provides a complete set of O Level Biology Questions and Answers on transport in plants – how water, minerals, and food are moved throughout the plant. You will explore the structure and function of xylem and phloem, the process of transpiration, and how plants adapt to different environments. Perfect for mastering these essential concepts.

---

Topic Overview

Plants have two transport systems: xylem and phloem. Xylem transports water and mineral ions from roots to leaves and provides structural support. Phloem transports sucrose and amino acids from sources (e.g., leaves) to sinks (e.g., roots, fruits) in a process called translocation. Transpiration is the loss of water vapour from leaves, which drives water movement up the xylem. Factors such as light, temperature, humidity, and wind affect transpiration rate. Adaptations like thick cuticles and sunken stomata help plants reduce water loss.

---

Exam Questions and Answers

Question 1

Question:
State the function of xylem and phloem in plants.

Answer:
Xylem transports water and dissolved mineral ions from the roots to the leaves. It also provides mechanical support. Phloem transports sucrose and amino acids (organic nutrients) from sources (e.g., leaves) to sinks (e.g., roots, fruits, storage organs).

Explanation:
Xylem consists of dead cells (vessels and tracheids) with lignified walls. Phloem consists of living sieve tube elements and companion cells. The movement in phloem is bidirectional and is called translocation.

Exam Tip:
Use the terms “xylem – water and minerals” and “phloem – food (sucrose and amino acids)”. Remember that xylem provides support.

Question 2

Question:
What is transpiration? Explain its importance in plants.

Answer:
Transpiration is the loss of water vapour from the aerial parts of a plant, mainly through stomata in leaves. It is important because it creates a transpiration pull that draws water up the xylem, cools the plant, and supplies water for photosynthesis and mineral transport.

Explanation:
Water evaporates from mesophyll cells into air spaces and diffuses out through stomata. The resulting tension pulls water upward, and minerals are carried with it. Cooling prevents overheating in direct sunlight.

Exam Tip:
Mention “transpiration pull”, “cohesion‑tension theory”, and “cooling”. Link transpiration to water and mineral uptake.

Question 3

Question:
Describe how the structure of xylem vessels is adapted to their function.

Answer:
Xylem vessels are long, hollow tubes with no end walls (continuous column) to allow efficient water flow. Their walls are thickened with lignin for strength and support, preventing collapse under tension. The lignified walls also have pits that allow water to move sideways between vessels.

Explanation:
The absence of cell contents reduces resistance to water flow. Lignin provides rigidity, helping the plant stay upright.

Exam Tip:
Use keywords: “lignin”, “hollow”, “no end walls”, “pits”. Explain how each feature aids water transport or support.

Question 4

Question:
Explain how water is transported from the roots to the leaves of a tall tree.

Answer:
Water is absorbed by root hair cells by osmosis. It moves across the root cortex and into xylem vessels. Transpiration from leaves creates a negative pressure (tension) in the xylem. Water molecules stick together by cohesion and are pulled upward as a continuous column. This is the cohesion‑tension theory.

Explanation:
Cohesion (water molecules attract each other) and adhesion (water molecules stick to xylem walls) maintain the column. Transpiration pull is the main driving force.

Exam Tip:
Include the terms “cohesion‑tension”, “transpiration pull”, and “osmosis”. Mention that root pressure may also contribute but is minor.

Question 5

Question:
What is translocation and why is it important?

Answer:
Translocation is the movement of sucrose and amino acids from where they are made (source) to where they are used or stored (sink) via phloem sieve tubes. It is important because it distributes organic nutrients to all parts of the plant, supporting growth, respiration, and storage.

Explanation:
Sources include leaves (photosynthesis) and storage organs (e.g., tubers) during regrowth. Sinks include roots, young leaves, flowers, and fruits. The process requires living cells and energy.

Exam Tip:
Use “source to sink”. Emphasize that translocation occurs in phloem and is bidirectional. Contrast with xylem transport which is unidirectional.

Question 6

Question:
List four environmental factors that affect transpiration rate and explain how each influences it.

Answer:
1. Light intensity: increases stomatal opening, increasing transpiration.
2. Temperature: increases kinetic energy and evaporation, increasing transpiration.
3. Humidity: high humidity reduces the water vapour gradient, decreasing transpiration.
4. Wind: removes water vapour near leaf surface, increasing gradient and transpiration.

Explanation:
Stomata open in light to allow CO₂ in, so transpiration is higher. Warmer temperatures increase evaporation rate. High humidity means air is nearly saturated, slowing evaporation. Wind carries away water vapour, maintaining a steep gradient.

Exam Tip:
Learn to explain each factor with reference to the water vapour gradient or stomatal behaviour. A table may help in revision.

Question 7

Question:
Describe two adaptations of xerophytic plants (plants in dry conditions) that reduce water loss.

Answer:
1. Thick cuticle: a waxy layer on leaves reduces water evaporation.
2. Sunken stomata: stomata are located in pits, creating a still air layer that traps moisture, reducing transpiration.

Explanation:
Other adaptations include rolled leaves, reduced leaf surface area (spines), and hairy leaves to trap humidity. These minimize water loss in arid environments.

Exam Tip:
Be specific: name the adaptation and explain how it reduces water loss. Give examples like cacti or marram grass.

Question 8

Question:
Describe how you would investigate the effect of wind speed on transpiration using a potometer.

Answer:
Set up a potometer with a leafy shoot. Place it in front of a fan at different speeds (or at different distances). Measure the distance a bubble moves in the capillary tube per minute (rate of water uptake) as an estimate of transpiration rate. Repeat at each wind speed and control other factors (temperature, light, humidity).

Explanation:
The potometer measures water uptake, which is approximately equal to water loss by transpiration. Higher wind speed increases transpiration because it removes water vapour from the leaf surface, steepening the gradient.

Exam Tip:
Include controls: keep the same plant, same light intensity, same temperature. Mention that you should take multiple readings and calculate average.

---

Key Concepts Summary

• Xylem: transports water and minerals (unidirectional, dead cells, lignified walls).
• Phloem: transports sucrose and amino acids (bidirectional, living cells, sieve tubes).
• Transpiration: water loss from leaves; driven by stomata; pulls water up xylem (cohesion‑tension).
• Factors affecting transpiration: light, temperature, humidity, wind.
• Translocation: movement of organic nutrients from source to sink via phloem.
• Xerophytes have adaptations (thick cuticle, sunken stomata) to reduce water loss.

---

Frequently Asked Questions

What is the difference between transpiration and evaporation?

Transpiration is the loss of water vapour specifically from plant surfaces (mainly leaves) through stomata. Evaporation is a physical process that occurs from any wet surface, not necessarily involving living tissues.

Why do plants need both xylem and phloem?

Xylem transports water and minerals needed for photosynthesis and other processes. Phloem distributes the products of photosynthesis (sugars) and other organic nutrients to non‑photosynthetic parts. Both are essential for plant survival.

How does a potometer measure transpiration?

A potometer measures the rate of water uptake by a cut shoot. Water uptake is assumed to equal water loss by transpiration (assuming no water is used for growth or storage during the experiment). The movement of an air bubble along a capillary tube indicates volume of water taken up per unit time.

---

Related Topics

• O Level Biology Cell Structure Questions and Answers Live
• O Level Biology Movement In and Out of Cells Questions and Answers Live 
• O Level Biology Plant Nutrition Questions and Answers

---

Conclusion

Mastering O Level Biology Questions and Answers on transport in plants gives you a solid understanding of how water, minerals, and food move within a plant. Learn the roles of xylem and phloem, the mechanism of transpiration and translocation, and how plants adapt to their environment. Practice these questions, review the key concepts, and use the exam tips to succeed in your exams.