Ch 4 · Translocation · O Level Biology 5090
Topic 4 of 4 in Transport in Plants — O Level Biology 5090 — 38 lessons.
Introduction
Photosynthesis happens in leaves, yet roots, flowers, fruits and buds all need the sugar it produces. Delivering that sugar, with amino acids, is the job of the phloem, and the process is called translocation. Every candidate must be able to compare transport in xylem and phloem.
Key ideas — Translocation: sources and sinks
the definition. Translocation is the movement of sucrose and amino acids in the phloem, from sources to sinks.
🧑🏫 Teacher's explanation
Think of a plant like a massive city with a complex delivery network. The leaves are the 'factories' producing food (sucrose), and the roots or fruits are the 'customers' consuming it. Translocation is the delivery service that moves these goods through the phloem.
📖 Definitions
🔍 Worked example — Easy
Identify whether the following are sources or sinks: (a) A mature leaf during summer, (b) A growing root tip, (c) A potato tuber in winter.
🔍 Worked example — Standard
Explain why sucrose is transported in the phloem instead of glucose.
🔍 Worked example — Exam-style
A plant is growing in a greenhouse. In spring, the leaves are the main source of sucrose. In autumn, the plant moves sucrose from its roots into its developing fruit. Explain why the root acts as a source in autumn but a sink in summer.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Why a part can be both source and sink
storage organs change role. Whether a structure is a source or a sink depends on the stage in the plant's life, not on what the structure is. Consider a potato tuber:
🧑🏫 Teacher's explanation
Think of a plant like a complex economy. In a city, a factory might be a 'source' of goods, sending products out to shops. However, if that factory also needs to store its own surplus in a warehouse, it acts as a 'sink' for its own production. The role of the factory depends entirely on whether it is currently producing more than it consumes.
📖 Definitions
🔍 Worked example — Easy
A potato tuber is growing in a plant during the peak of summer. Identify whether the tuber is acting as a source or a sink and provide a reason.
🔍 Worked example — Standard
During spring, a potato tuber begins to sprout to grow a new plant, but the plant has no leaves yet. Explain the role of the tuber in this stage.
🔍 Worked example — Exam-style
A young, rapidly expanding leaf is being monitored. Initially, it is a sink. Describe the physiological change required for it to become a source.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Define the term 'sink' in the context of translocation. [1] (b) Explain why the carrot root acts as a source during germination. [2] [3 marks]
Key ideas — Comparing transport in xylem and phloem
This comparison is examined every year. Learn it as paired statements.
🧑🏫 Teacher's explanation
Think of a plant like a large city. The xylem is like a massive, one-way water main system: it consists of empty, reinforced pipes that move water upwards from the ground to the skyscrapers (leaves) using only the pressure created by evaporation at the top. It doesn't need electricity; it just reacts to the 'suction' from above.
📖 Definitions
🔍 Worked example — Easy
State the primary substances transported in the xylem and the phloem.
🔍 Worked example — Standard
Explain why phloem transport is an active process, unlike xylem transport.
🔍 Worked example — Exam-style
A plant is moved from a warm environment to a very cold environment. Predict and explain the effect on the rate of translocation in the phloem.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Worked example — Model answer
Question. A gardener plants a daffodil bulb in autumn. In spring the bulb produces leaves and a flower; in early summer the leaves stay green after the flower dies and the bulb swells. (a) Define translocation. [2] (b) Identify the source and the sink in spring and in early summer, giving a reason for each. [4] (c) Give two differences between transport in xylem and phloem. [2]
🔍 Worked example — Easy
Identify whether the following are sources or sinks: (i) A growing root tip, (ii) A mature leaf in bright sunlight.
🔍 Worked example — Standard
A plant's leaf produces 10.0 mg of sucrose per hour. 7.5 mg is sent to the roots, and 2.5 mg is used for local respiration. Calculate the percentage of sucrose translocated to the sink.
🔍 Worked example — Exam-style
Compare the transport in xylem and phloem regarding directionality and substances transported.
⚠️ Common errors
Common mistakes
Practice
Answers