Q.A living conducting element of the phloem that loses its nucleus at maturity but remains alive is the:
🔒You're viewing a preview — the full solution, concept, methods & PYQ mapping are locked.
🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Phloem Elements
Phloem Elements – The Plant's Food Highway
Think of a tree. The leaves make food (sugar) through photosynthesis. That food has to travel down to the roots, and also up to growing buds and fruits. Water moves up through xylem, but sugar moves through a completely different system: phloem.
Phloem is the living tissue that transports organic nutrients — mainly sucrose — from where they are made (source) to where they are needed (sink). Unlike xylem, which is mostly dead at maturity, phloem cells stay alive and work together like a team.
The Four Cell Types of Phloem
Phloem is not one kind of cell. It is a composite tissue made of four distinct elements, each with a specific job.
1. Sieve Tube Elements – The Conduit
These are the actual pipes for sugar transport. They are elongated cells arranged end-to-end, forming long sieve tubes. The end walls have clusters of pores, looking like a sieve — hence the name.
Sieve tube elements lose their nucleus, ribosomes, and most organelles at maturity. They are alive but dependent on companion cells for metabolic support. Without a nucleus, they cannot make proteins or manage their own life.
The cytoplasm of adjacent sieve tube elements is connected through the sieve pores, allowing a continuous stream of sap to flow.
2. Companion Cells – The Manager
Each sieve tube element has one or more companion cells right next to it, connected by numerous plasmodesmata. The companion cell has a dense cytoplasm and a prominent nucleus.
The companion cell controls the life of the sieve tube element. It supplies ATP, proteins, and signalling molecules. It also loads sugar into the sieve tube against a concentration gradient — an active process that drives the entire transport.
Without companion cells, sieve tubes would die within hours.
3. Phloem Fibres – The Support
These are sclerenchyma cells — long, thick-walled, and dead at maturity. They provide mechanical strength to the phloem tissue. In some plants, like jute and flax, phloem fibres are commercially important for making ropes and textiles.
Phloem fibres are not involved in transport. They are purely structural. In primary phloem, they are often absent or few; in secondary phloem, they can form prominent bundles.
4. Phloem Parenchyma – The Storage
These are living, thin-walled cells that store starch, fats, and other organic compounds. They also help in lateral transport of solutes within the phloem. Unlike companion cells, they are not directly connected to sieve tubes.
How They Work Together
The flow of sap in phloem is explained by the pressure flow hypothesis. Companion cells actively pump sucrose into sieve tubes at the source (e.g., leaves). This makes the sap hypertonic, drawing in water from xylem. The increased pressure pushes the sap towards sink regions (roots, fruits), where sucrose is unloaded and water returns to xylem.
Pressure Flow Model
Sourceactive loadingHigh pressurebulk flowSinkunloadingLow pressure …
The sieve tube element is a living phloem cell that loses its nucleus at maturity while continuing to c …
A sieve tube element is unusual among plant cells: it stays alive and functional at maturity, yet it loses its nucleus, retaining only a thin peripheral layer of cytoplasm — an adaptation that leaves more space for the transport of food through its perforated end walls (sieve plates). Companion cells (option a) stay alive AND keep their nucleus, which is exactly what lets them regulate the neighbouring, nucleus-less sieve tube element. P …
The phrase 'living but loses its nucleus' is the defining, unusual signature of the sieve tube elem …
- Picking the companion cell, which stays alive but does NOT lose its nucleus — the opposite pairing. …
- CBSE 2026Set ANNUAL1 markMCQQ.What is the main function of companion cells in phloem ?(a) Conduction of water(b) Providing support(c) Assisting sieve tube elements(d) Storage of food
›Reveal solutionSolution
Companion cells assist the sieve tube elements in the phloem, so the answer is (C).
Phloem transports food (mainly sucrose). Its conducting cells are the sieve tube elements, which lose their nucleus at maturity. Each sieve tube element is associated with a specialised living, nucleated companion cell connected to it through plasmodesmata (pit fields).
…
- CBSE 2025Set botany-hz21 markMCQQ.Companion cells are closely associated with:(a) Sieve elements(b) Vessels(c) Trichomes(d) Guard cells
›Reveal solutionSolution
Companion cells are found in phloem, closely associated with sieve tube elements (sieve elements), whose functions they support.
In angiosperm phloem, each sieve tube element is closely associated with a specialised parenchyma cell called a companion cell. Both arise from the same mother cell by longitudinal division and remain connected throughout life by numerous plasmodesmata (pit connections). Because mature sieve tube elements lose their nucleus and much of their organelle machinery, they depend on the companion cell to maintain their metabolic activity, and the companion cell also helps in the loading and unloading of photosynthates (sugars) into and out of the sieve tubes.
…
- CBSE 2023Set ANNUAL1 markQ.Fill in the blank: Food is transported in plants through ____.
›Reveal solutionSolution
Phloem is the vascular tissue that conducts the food (organic solutes made during photosynthesis) from source (leaves) to sink (roots, fruits, storage organs) throughout the plant.
Phloem is composed of sieve tube elements, companion cells, phloem fibres, and phloem parenchyma. Unlike xylem (which conducts water and minerals upward, mostly by transpiration pull, in dead conducting cells), phloem transport:
- Moves in living sieve tube elements (which lack a nucleus but stay alive with the help of adjoining companion cells). …
- CBSE 2023Set ANNUAL1 markMCQQ.Companion cells are found in :(a) epidermis(b) endodermis(c) xylem(d) phloem
›Reveal solutionSolution
Companion cells occur in phloem.
Companion cells are living cells that lie alongside the sieve tube elements of the phloem and help control their functioning (loading/unloading of food). They are a component of phloem, not of xylem or the ep …
- CBSE 2021Set ANNUAL1 markMCQQ.Phloem fibres are made-up of:(a) Parenchymatous cells(b) Collenchymatous cells(c) Sclerenchymatous cells(d) None of these
›Reveal solutionSolution
Phloem (bast) fibres are elongated, thick-walled, lignified sclerenchyma cells that provide mechanical strength.
Phloem is a complex tissue made of four elements: sieve tube elements, companion cells, phloem parenchyma and phloem fibres (also called bast fibres). Unlike the first three, which are living conducting/associated cells, phloem fibres are sclerenchymatous — elongated, dead at maturity, with thic …
- CBSE 2020Set ANNUAL1 markMCQQ.Companion cells are associated with :(a) Tracheid(b) Sclerenchyma(c) Sieve tube(d) Vessel
›Reveal solutionSolution
Companion cells are living cells found alongside sieve tube elements in phloem, connected by numerous plasmodesmata, and they help maintain the metabolic activity of the enucleate sieve tubes.
Phloem is composed of sieve tubes, companion cells, phloem parenchyma, and phloem fibres. Sieve tube elements lose their nucleus at maturity, so they depend on their adjoining companion cells (which retain a nucleus and dense cytoplasm) for the proteins and metabolic support needed to load/transport sugars (translocation). This intimate structur …
- CBSE 2020Set ANNUAL1 markQ.What do you mean by translocation?
›Reveal solutionSolution
[!TLDR]
Translocation is the transport of organic food (mainly sucrose) manufactured in leaves to other parts of the plant through the phloem.
Method …
🎓Unlock everything free for 14 days
- ✓Full step-by-step solutions
- ✓Concept-first explanations
- ✓Methods, shortcuts & mistakes
- ✓PYQ mapping + timed mock tests
Full access for 14 days. No credit card required.