Back to: Botany 300 Level
Hello, my brilliant friend! I hope you’re doing well today! Have you ever wondered how the food made by leaves reaches all parts of a plant, including the roots? Just as humans have blood vessels to transport nutrients, plants have a special transport system called the phloem. The phloem is like a food delivery service, ensuring that energy reaches every part of the plant.
Today, we’ll explore the structure, function, and sieve elements of the phloem, and by the end of this lesson, you’ll understand how plants distribute food efficiently!
Phloem: Structure, function, and sieve elements
Structure of Phloem
Phloem is a complex tissue made up of different specialised cells that work together to transport food. It consists of four main components:
Sieve elements – The main conducting cells that transport food.
Companion cells – Help sieve elements function properly.
Phloem fibres – Provide strength and support.
Phloem parenchyma – Stores nutrients and helps in transport.
Unlike xylem, which transports water only upwards, the phloem transports food in all directions, depending on the plant’s needs.
Function of Phloem
The phloem plays a vital role in plant survival by performing these key functions:
1. Transporting Food (Translocation)
Phloem carries sugar (mainly sucrose) and other nutrients from the leaves (where photosynthesis occurs) to all parts of the plant, including the roots, stems, and growing buds. This process is called translocation.
2. Transporting Hormones and Other Molecules
Apart from sugars, the phloem also transports hormones and amino acids to help with growth and development.
3. Supporting Growth and Storage
Phloem delivers food to growing parts like new leaves, flowers, and fruits.
It also transports excess sugar to storage organs like tubers (e.g. yam) and roots (e.g. carrot).
Sieve Elements: The Main Conducting Cells of Phloem
Sieve elements are the most important part of the phloem because they are responsible for transporting food throughout the plant. They come in two types:
Sieve tube elements
Found in flowering plants (angiosperms).
Arranged end-to-end to form long tubes for food transport.
Have sieve plates (perforated end walls) that allow food to pass through.
Lack nuclei and rely on companion cells for survival.
Sieve cells
Found in non-flowering plants (gymnosperms).
Smaller than sieve tube elements and do not have sieve plates.
The Role of Companion Cells
Since sieve tube elements lack nuclei, they need companion cells to help them function. Companion cells:
Provide energy and support for sieve elements.
Help load and unload sugars into the sieve tubes.
Act like assistants, ensuring that the food transport system runs smoothly.
A Simple Story to Understand Phloem
Imagine a food delivery service in a big city:
Sieve elements are like food delivery bikes carrying meals (sugar) to different locations.
Companion cells are the managers who organise and guide the delivery bikes.
Phloem fibres are the roads and bridges providing strength.
Phloem parenchyma is like a food storage warehouse, keeping extra food for later use.
Just like a well-organised delivery system ensures food reaches customers, phloem ensures that all parts of a plant get the energy they need!
Summary
Phloem transports food (sugars) from the leaves to other parts of the plant.
It consists of sieve elements, companion cells, phloem fibres, and phloem parenchyma.
Sieve tube elements are the main food transport cells and work with companion cells.
Phloem moves food in all directions, unlike xylem, which moves water only upward.
It also helps transport hormones and nutrients needed for plant growth.
Evaluation
- What is the main function of phloem?
- What is the difference between sieve tube elements and sieve cells?
- Why do sieve tube elements need companion cells?
- How does phloem differ from xylem?
- What is translocation in plants?
You are doing an amazing job! Look at you, learning how plants transport their food like a well-organised delivery system! Keep looking at plants around you, and you’ll start noticing how they use phloem to stay healthy. Keep learning with Afrilearn, and I’ll see you in the next exciting lesson. Stay curious and keep growing!
School Admin? Grow your school faster with Africa's most trusted school management softwareGet more class notes, videos, homework help, exam practice on Android [DOWNLOAD]
Get more class notes, videos, homework help, exam practice on iPhone [DOWNLOAD]