Q.Product of photosynthesis is transported from the leaves to various parts of the plants and stored in some cell before being utilised. What are the cells/ tissues that store them?
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.
The sugars and other organic substances made in the leaves during photosynthesis are carried through the phloem (as sieve-tube conduction) to other parts of the plant, and along the way, and at their destination, they are held in storage mainly by parenchyma cells — in particular the phloem parenchyma that lies alongside the conducting sieve tubes, as well as the parenchyma cells of the cortex, pith and medullary rays in stems and roots. …
Photosynthate is moved through the phloem and stored in parenchyma cells — chiefly phloem parenchyma, and also the general parenchyma of the cortex, pith and medullary rays.
Photosynthesis in the leaf mesophyll produces organic food substances that the plant needs to move to non-photosynthetic parts and hold in reserve until they are needed. Two things happen in sequence: transport, and then storage.
Transport is carried out by the phloem, whose sieve tubes conduct the dissolved organic food from the leaves (the source) to other regions of the plant (the sinks). …
Method: Tracing the Journey from Source to Sink, Then Asking Where It Stops
Rather than naming the storage tissue directly, trace the entire journey of a sugar molecule from where it is made to where it ends up, and ask specifically what kind of cell would make sense as a "parking spot" along that route.
Step 1 — where is the sugar made? In the leaf mesophyll, during photosynthesis (the "source").
Step 2 — how does it travel? It enters the phloem's sieve tubes, which conduct the dissolved sugar to other parts of the plant (the "sinks") — roots, developing fruits, storage organs, growing tissue.
Step 3 — what happens once it "arrives"? The sieve tube itself is not a storage compartment — a sieve tube element has no nucleus and minimal organelles; it is built purely as a conducting pipe, not a warehouse. So the sugar must be handed off to some OTHER cell type once it reaches its destination (or even along the way, for local use). …