CIE iGCSE Biology-8.4 Translocation- Study Notes- New Syllabus
CIE iGCSE Biology-8.4 Translocation- Study Notes – New syllabus
CIE iGCSE Biology-8.4 Translocation- Study Notes -CIE iGCSE Biology – per latest Syllabus.
Key Concepts:
Supplement
- Describe translocation as the movement of sucrose and amino acids in phloem from sources to sinks
- Describe:
- (a) sources as the parts of plants that release sucrose or amino acids
(b) sinks as the parts of plants that use or store sucrose or amino acids - Explain why some parts of a plant may act as a source and a sink at different times
What is Translocation in Plants?
📌 Definition:
Translocation is the movement of sucrose and amino acids through the phloem from sources (where they are produced) to sinks (where they are used or stored).
🔄 What is Moved in Translocation?
- Sucrose – a soluble sugar made in photosynthesis
- Amino acids – used to make proteins for growth and repair
🌞 What Are Sources and Sinks?
| Term | Meaning | Example |
|---|---|---|
| Source | Where substances are produced (mainly photosynthesis) | Leaves (especially in daytime) |
| Sink | Where substances are used or stored | Roots, growing shoots, fruits, seeds |
Substances move from high concentration (source) to low concentration (sink), and this can be upward or downward depending on where the sink is located.
🚚 How Does Translocation Work?
- Sucrose and amino acids are actively loaded into the phloem at the source (e.g. leaf cells).
- Water enters from xylem by osmosis, creating high pressure in the phloem.
- This pressure pushes the contents toward sinks (mass flow).
- At the sink, nutrients are unloaded and used or stored.
- Water exits the phloem, reducing pressure and allowing flow to continue.
🧠 Key Characteristics of Phloem Translocation:
- It is an active process – requires energy (ATP).
- Bidirectional – substances move both upward and downward.
- Involves companion cells with special transport proteins.
📊 Summary Table:
| Feature | Phloem (Translocation) |
|---|---|
| What it carries | Sucrose and amino acids |
| Direction of transport | Upward and downward (bidirectional) |
| Cells involved | Sieve tube elements and companion cells |
| Requires energy? | Yes – active transport is involved |
| Main goal | Distribute nutrients to growing or storage parts |
Understanding Sources and Sinks in Plants
In the process of translocation, plants move important nutrients like sucrose and amino acids through the phloem from areas where they are produced or released (sources) to areas where they are used or stored (sinks).
(a) What Are Sources?
Sources are the parts of a plant that produce or release sucrose and amino acids into the phloem.
- Actively supply nutrients into the transport system
- Usually photosynthesizing organs (e.g., mature leaves)
- May include storage organs during certain seasons (e.g., tubers in spring)
🌞 Examples of Sources:
- Leaves during the day – produce sucrose by photosynthesis
- Storage organs like tubers – release sugars during early growth
(b) What Are Sinks?
Sinks are the parts of a plant that use, or store sucrose and amino acids delivered by the phloem.
- Consume or store nutrients for growth, repair, or energy
- Can change depending on the plant’s stage of growth
- Often have low sugar concentrations, allowing nutrients to flow in
🌱 Examples of Sinks:
- Roots – store sugars as starch
- Growing shoots and leaves – need amino acids to build new cells
- Flowers, fruits, and seeds – require energy and materials to develop
🔄 Dynamic Roles:
A plant part can act as a source at one time and a sink at another.
Example: A potato tuber is a sink in summer (stores sugars) and a source in spring (releases sugars for new growth).
🧠 Final Summary:
| Term | Definition | Common Examples |
|---|---|---|
| Source | A plant part that releases sucrose or amino acids | Mature leaves, germinating seeds, tubers |
| Sink | A plant part that uses or stores sucrose or amino acids | Roots, flowers, fruits, young leaves |
Why Can Some Plant Parts Act as Both Sources and Sinks at Different Times?
Plants are highly adaptable organisms. The same part of a plant like a leaf or a tuber can act as a source during one part of its life cycle and as a sink during another. This depends on the plant’s growth stage, energy demands, and seasonal changes.
🔁 Dynamic Role of Plant Organs
What Is a Source?
A source is any part of the plant that produces or releases sucrose and amino acids into the phloem for transport.
What Is a Sink?
A sink is any part of the plant that uses or stores these nutrients for growth, repair, or storage.
🌱 Examples of Parts That Change Role:
1. Leaves
- Young leaves: Act as sinks – they can’t photosynthesize enough yet and rely on sugars from older leaves.
- Mature leaves: Become sources – they produce excess sucrose via photosynthesis and export it to sinks.
2. Storage Organs (e.g., tubers, roots, bulbs)
- Summer (growth season): Act as sinks – store sugars from photosynthesizing leaves as starch.
- Spring or germination: Act as sources – break down starch into sugars to support new shoot growth.
3. Fruits and Seeds
- Developing fruits: Act as strong sinks – require high amounts of sucrose and amino acids for growth.
- After maturity (in some plants): May become sources – mobilize sugars to developing seeds.
📌 Why This Happens
Switching roles between source and sink helps the plant:
- Adapt to different stages of growth
- Balance the nutrient supply and demand
- Survive seasonal or environmental changes
- Optimize energy use for reproduction, repair, or survival
🔄 Summary Table
| Plant Part | When It’s a Source | When It’s a Sink |
|---|---|---|
| Mature Leaf | During active photosynthesis | Rarely acts as a sink |
| Young Leaf | Rarely a source | While still developing |
| Tuber/Bulb/Root | During sprouting or regrowth | During active storage (summer) |
| Fruit/Seed | Sometimes after maturity (to supply seeds) | During growth and development |