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OCR Biology AS Level (9)
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Chapter 9: Transport in PlantsChapter9.1: Transport systems in dicotyledonous plants
The need for plant transport systems
There are 3 main systems because multicellular plants need transport systems:
o Metabolic demands
The cells of the green parts of the plant make their own glucose and oxygen by
photosynthesis – but many internal and underground parts of the plant do not
photosynthesise.
They need oxygen and glucose transported to them and the waste products of cell
metabolism removed.
Hormones made in one part of a plant need transporting to the areas where they have an
effect.
Mineral ions absorbed by the roots need to be transported to all cells to make the proteins
required for enzymes and the structure of the cell
o Size
Some plants are very small but because plants continue to grow throughout their lives,
many perennial plants are large and some of them are enormous.
This means plants need very effective transport systems to move substances both up and
down from the tip of the roots to the topmost leaves and stems.
o Surface area: volume ratio
Surface area: volume ratios are not simple in plants. Leaves are adapted to have a relatively
large SA:V ratio for the exchange of gases with the air.
However, the size and complexity of multicellular plants means that when the stems, trunks,
and roots are considered they still have a relatively small SA:V ratio.
This means they cannot rely on diffusion alone to supply their cells with everything they
need.
Transport systems in dicotyledonous plants
Dicotyledonous plants make seeds that contain two cotyledons, organs that act as food stores for the
developing embryo plant and form the first leaves when the seed germinates.
Dicotyledonous plants have a series of transport vessels running through the stem, roots, and leaves.
o This is known as the vascular system.
There are herbaceous dicots, with soft tissues and a relatively short life cycle and woody dicots, which have
hard, lignified tissues and a long-life cycle.
In herbaceous dicots this is made up of two main types of transport vessels, the xylem, and the phloem.
These transport tissues are arranged together in vascular bundles in the leaves, stems, and roots of
herbaceous dicots
The structure and functions of the xylem
The xylem is a largely non- living tissue that has two main functions in a plant – the transport of water and
mineral ions, and support.
The flow of materials in the xylem is up from the roots to the shoots and leaves.
Xylem is made up of several types of cells, most of which are dead when they are functioning in the plant.
The xylem vessels are the main structures.
They are long, hollow structures made by several columns of cells fusing together end to end.
There are two other tissues associated with xylem in herbaceous dicots.
Thick-walled xylem parenchyma packs around the xylem vessels, storing food, and containing tannin
deposits.
Tannin is a bitter, astringent-tasting chemical that protects plant tissues from attack by herbivores.
Xylem fibres are long cells with lignified secondary walls that provide extra mechanical strength but do not
transport water.
Lignin can be laid down in the walls of the xylem vessels in several different ways.
It can form rings, spirals, or relatively solid tubes with lots of small unlignified areas called bordered pits.
This is where water leaves the xylem and moves into other cells of the plant.
The structure and functions of the phloem
, Phloem is a living tissue that transports food in the form of organic solutes around the plant from the leaves
where they are made by photosynthesis.
The phloem supplies the cells with the sugars and amino acids needed for cellular respiration and for the
synthesis of all other useful molecules.
The flow of materials in the phloem can go both up and down the plant.
The main transporting vessels of the phloem are the sieve tube elements.
Like xylem, the phloem sieve tubes are made up of many cells joined end to end to form a long, hollow
structure.
Unlike xylem tissue, the phloem tubes are not lignified. In the areas between the cells, the walls become
perforated to form sieve plates, which look like sieves and let the phloem contents flow through.
As the large pores appear in these cell walls, the tonoplast, the nucleus and some of the other organelles
break down.
The phloem becomes a tube filled with phloem sap and the mature phloem cells have no nucleus.
Closely linked to the sieve tube elements are companion cells, which form with them.
These cells are linked to the sieve tube elements by many plasmodesmata - microscopic channels through
the cellulose cell walls linking the cytoplasm of adjacent cells.
They maintain their nucleus and all their organelles.
The companion cells are very active cells, and it is thought that they function as a 'life support system' for
the sieve tube cells, which have lost most of their normal cell functions
Phloem tissue also contains supporting tissues including fibres and sclereids, cells with extremely thick cell
walls.
Chapter 9.2: Water transport in multicellular plants
Water transport in plants
Water is key in both the structure and in the metabolism of plants:
o Turgor pressure because of osmosis in plant cells provides a hydrostatic skeleton to support the
stems and leaves.
o Turgor also drives cell expansion – it is the force that enables plant roots to force their way through
tarmac and concrete.
o The loss of water by evaporation helps to keep plants cool.
o Mineral ions and the products of photosynthesis are transported in aqueous solutions. Water is a
raw material for photosynthesis.
To understand the role of water in plants we need to look at how water moves into a plant from the soil and
how it moves around the plant.
Movement of water into the root
Root hair cells are the exchange surface in plants where water is taken into the body of the plant from the
soil. A root hair is a long, thin extension from a root hair cell, a specialised epidermal cell found near the
growing root tip.
Root hairs are well adapted as exchange surfaces:
o Their microscopic size means they can penetrate easily between soil particles.
o Each microscopic hair has a large SA: V ratio and there are thousands on each growing root tip.
o Each hair has a thin surface layer through which diffusion and osmosis can take place quickly.
o The concentration of solutes in the cytoplasm of root hair cells maintains a water potential gradient
between the soil water and the cell.
Soil water has a very low concentration of dissolved minerals, so it has a very high-water potential.
The cytoplasm and vacuolar sap of the root hair cell contain many different solvents including sugars,
mineral ions, and amino acids so the water potential in the cell is lower.
As a result, water moves into the root hair cells by osmosis.
Movement of water across the root
Once the water has moved into the root hair cell it continues to move across the root to the xylem in one of
two different pathways:
The symplast pathway
Water moves through the symplast - the continuous cytoplasm of the living plant cells that is connected
through the plasmodesmata - by osmosis.
The root hair cell has a higher water potential than the next cell along.
This the result of water diffusing in from the soil, which has made the cytoplasm more dilute.
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