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.
Watch out
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.
Important
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.
Note
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.
Phloem is made of sieve tube elements, companion cells, phloem parenchyma and phloem fibres; it translocates food (sugars) throughout the plant.
Constituents of phloem (a complex tissue with four elements):
Sieve tube elements — long tube-like conducting cells joined end to end through perforated sieve plates; they lack a nucleus at maturity.
Companion cells — living, nucleated cells that control and support the sieve tubes.
Phloem parenchyma — stores food and other substances.
Phloem fibres (bast fibres) — dead, thick-walled cells that provide mechanical support.
Functions of phloem:
It translocates (conducts) food, mainly in the form of sucrose, from the source (leaves, where food is made) to the sink (roots, fruits, growing regions). …
Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.
CBSE 2026Set ANNUAL1 markMCQ
Q.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).
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.
Q.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). …
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 …
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 …
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 …