The Vascular Tissue System: Phloem
A complex tissue responsible for the translocation of food material, primarily from leaves to other parts of the plant, composed of sieve tube elements, companion cells, phloem parenchyma, and phloem fibres.
What is The Vascular Tissue System: Phloem?
A complex vascular tissue in plants that transports soluble organic compounds (food) synthesized during photosynthesis from the leaves to other parts of the plant.
Key points
- Describe the structure and components of the phloem tissue.
- Explain the function of each phloem component.
- Differentiate between protophloem and metaphloem.
- Understand the process of translocation of food in plants.
Common exam trap
Confusing phloem with xylem (food vs. water transport).
Definitions
- Term
Phloem
- Meaning
A complex vascular tissue in plants that transports soluble organic compounds (food) synthesized during photosynthesis from the leaves to other parts of the plant.
- Term
Sieve Tube Elements
- Meaning
Elongated, living cells of the phloem that lack a nucleus at maturity and are responsible for the bulk of food transport.
- Term
Sieve Plates
- Meaning
Perforated end walls of sieve tube elements that allow the passage of materials.
- Term
Companion Cells
- Meaning
Specialized parenchymatous cells associated with sieve tube elements, regulating their activity and aiding in loading and unloading of sugars.
- Term
Phloem Parenchyma
- Meaning
Parenchymatous cells in the phloem involved in storage of food materials and other substances.
- Term
Phloem Fibres (Bast Fibres)
- Meaning
Sclerenchymatous cells in the phloem that provide mechanical support to the plant.
- Term
Translocation
- Meaning
The process of transport of organic food materials (sugars) from the site of synthesis (source) to the site of utilization or storage (sink) in plants.
- Term
Pressure Flow Hypothesis (Mass Flow Hypothesis)
- Meaning
The most accepted mechanism explaining the transport of sugars through the phloem, driven by a pressure gradient created by osmotic movement of water.
Learning objectives
Describe the structure and components of the phloem tissue.
Explain the function of each phloem component.
Differentiate between protophloem and metaphloem.
Understand the process of translocation of food in plants.
Relate the structure of phloem components to their functions.
Prerequisites
Basic understanding of plant cell structure.
Knowledge of plant tissues (meristematic vs. permanent, simple vs. complex).
Understanding of photosynthesis and respiration.
Familiarity with the overall plant body plan.
Common mistakes
Confusing phloem with xylem (food vs. water transport).
Believing phloem transport is only unidirectional (it's bidirectional, but net flow is source to sink).
Forgetting that sieve tube elements are living but lack a nucleus at maturity.
Incorrectly identifying the function of companion cells.
Assuming phloem parenchyma is present in all plants (absent in most monocots).
Keywords
Phloem
Sieve Tube Elements
Companion Cells
Phloem Parenchyma
Phloem Fibres
Translocation
Pressure Flow Hypothesis
Bast
Source
Sink
Vascular Tissue System
Complex Tissue
Practice preview
Phloem fibres, also known as bast fibres, are primarily composed of:…
medium
The primary function of phloem in vascular plants is:…
easy
Companion cells are closely associated with sieve tube elements and are responsible for:…
medium