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 parenchyma is the living, thin-walled component of the phloem tissue that runs alongside the conducting sieve elements. Its main function is to store food materials (and other substances such as resins or tannins) and to help move some of that food laterally, in addition to the main longitudinal conduction that is carried out by the …
Phloem parenchyma is the living packing and storage tissue of the phloem, chiefly responsible for storing food materials near the conducting sieve elements.
Phloem is a complex tissue, made of several different cell types that divide the labour of transport between them. The sieve tubes and their companion cells do the main work of conducting dissolved food along the plant, but they are accompanied by phloem parenchyma and, in many plants, phloem fibres.
Phloem parenchyma cells are living, thin-walled cells that lie among the sieve tubes.
Because they are living and unspecialised for conduction, their principal role is to store food materials (and sometimes other substances) that are being transported through the adjoining sieve tubes, effectively acting as a reserve alongside the main conducting pathway. …
Rather than stating phloem parenchyma's function in isolation, understand it by explicitly contrasting what EACH of phloem's four cell types is built to do, so the parenchyma's role becomes obvious by process of elimination of what the others already cover.
List the team and their specialities:
Sieve tube elements: built for LONG-DISTANCE CONDUCTION — elongated, end-to-end, connected by sieve pores, but with no nucleus and minimal organelles (so they cannot manage their own metabolism).
Companion cells: built for METABOLIC SUPPORT of the sieve tubes — nucleated, densely cytoplasmic, connected to their sieve tube partner by many plasmodesmata, essentially "running" the sieve tube's business decisions.
Phloem fibres: built for MECHANICAL STRENGTH — dead, lignified support cells (where present).
Now ask: what job is left unassigned? Conduction is covered by sieve tubes; metabolic management of the sieve tube is covered by companion cells; mechanical strength is covered by fibres. The one common, general-purpose job every plant tissue needs somewhere nearby — STORAGE — has not yet been assigned to anyone on this team. …