Question: 100% NCERT covered from Particular Keyword.Explanations are 100% from NCERT | [Difficult level: Easy] From NCERT NEET Derived Question.
Function of companion cells is
(a) providing energy to sieve elements for active transport
(b) providing water to phloem
(c) loading of sucrose into sieve elements by passive transport
(d) loading of sucrose into sieve elements
Ans: (d) loading of sucrose into sieve elements
Companion cells
👉Phloem transports food materials, usually from leaves to
other parts of the plant. Phloem in angiosperms is composed
of sieve tube elements, companion cells, phloem parenchyma
and phloem fibres. Gymnosperms have albuminous cells and sieve cells.
They lack sieve tubes and companion cells. Sieve tube elements are
also long, tube-like structures, arranged longitudinally and are
associated with the companion cells. Their end walls are perforated in a
sieve-like manner to form the sieve plates. A mature sieve element
possesses a peripheral cytoplasm and a large vacuole but lacks a nucleus.
The functions of sieve tubes are controlled by the nucleus of companion
cells. The companion cells are specialised parenchymatous cells, which
are closely associated with sieve tube elements. The sieve tube elements
and companion cells are connected by pit fields present between their
common longitudinal walls. The companion cells help in maintaining the
pressure gradient in the sieve tubes. Phloem parenchyma is made up
of elongated, tapering cylindrical cells which have dense cytoplasm and
nucleus. The cell wall is composed of cellulose and has pits through which
plasmodesmatal connections exist between the cells. The phloem
parenchyma stores food material and other substances like resins, latex
and mucilage. Phloem parenchyma is absent in most of the
monocotyledons. Phloem fibres (bast fibres) are made up of
sclerenchymatous cells. These are generally absent in the primary phloem
but are found in the secondary phloem. These are much elongated,
unbranched and have pointed, needle like apices. The cell wall of phloem
fibres is quite thick. At maturity, these fibres lose their protoplasm and
become dead. Phloem fibres of jute, flax and hemp are used commercially.
The first formed primary phloem consists of narrow sieve tubes and is
referred to as protophloem and the later formed phloem has bigger sieve
tubes and is referred to as metaphloem.
👉The Pressure Flow or Mass Flow Hypothesis:
The accepted mechanism used for the translocation of sugars from source
to sink is called the pressure flow hypothesis. (see Figure 11.10). 
As
As
glucose is prepared at the source (by photosynthesis) it is converted to
sucrose (a dissacharide). The sugar is then moved in the form of sucrose
into the companion cells and then into the living phloem sieve tube cells
by active transport. This process of loading at the source produces a
hypertonic condition in the phloem. Water in the adjacent xylem moves
into the phloem by osmosis. As osmotic pressure builds up the phloem
sap will move to areas of lower pressure. At the sink osmotic pressure
must be reduced. Again active transport is necessary to move the sucrose
out of the phloem sap and into the cells which will use the sugar –
converting it into energy, starch, or cellulose. As sugars are removed, the
osmotic pressure decreases and water moves out of the phloem.
To summarise, the movement of sugars in the phloem begins at the
source, where sugars are loaded (actively transported) into a sieve tube.
Loading of the phloem sets up a water potential gradient that facilitates
the mass movement in the phloem.
