OCR A, OCRA Biology Alevel and AS Level Biology.
Module 3.3Transport in plants.
H020 and H420.
OCR Specification.
Breadth and depth of biology.
Features in Paper 1,2,3
Summary notes and revision notes.
9.1: Transport systems in dicotyledonous plants:
The need for plant transport systems:
1. Metabolic demands – internal and underground parts of the plants do not
photosynthesise so they need oxygen and glucose to be transported to them and
waste products to be removed. Hormones need to be transported from where there
made to where there needed as well as mineral ions which need to be transported
to all cells to make proteins required for enzymes.
2. Size- Some plants are large which means plants need very effective transport
systems to move substances both up and down.
3. Surface area: Volume ratio- Leaves are adapted to have a large SA:V ratio but when
the stem roots and trunks are considered, plants have a small SA:V ratio. This means
that diffusion alone isn’t enough to supply their cells with everything they need.
Transport systems in dicotyledonous:
• Cotyledons – organs that act as food stores for the developing embryo plant and
form the first leaves when a seed germinates.
• Dicotyledonous plants (dicots) – make seeds that contain two cotyledons.
• In the stem, the vascular bundles are around the edge to give strength and support
• In the roots, the vascular bundles are in the middle to withstand the tugging strains
of the wind
The structure and functions of the xylem:
• Xylem – largely non-living tissue that has two main functions: transport of water and
mineral ions and mechanical support
• Xylem vessels – long, hollow structures made by several columns of cells fusing
together end to end.
• There are 2 other tissues associated with xylem in herbaceous dicots
1. Thick walled parenchyma packs around the xylem vessels, storing food, and
containing tannin deposits. tannins are bitter chemicals that protect plants
from attacks by herbivores
2. Xylem fibres are long cells with lignified secondary walls that provide
mechanical strength but do not transport water
Observing xylem vessels in living plant stems:
, 1. Place the stem in water containing a strongly coloured dye for 24 hours, then
remove the plant from the dye and rinse it.
2. In one specimen make clean transverse cuts across the stem with a sharp blade on a
white tile
3. Observe and draw the position of the xylem vessels which should show up as
coloured spots
4. In another specimen make a careful longitudinal cut through a region you expect the
xylem vessels to be.
5. Repeat step 3
The structure and functions of the Phloem:
• Phloem – living tissue that transports food in the form of organic solutes. The
sucrose is dissolved in water to form sap.
• The main transporting vessels of the phloem are the sieve tube elements
• Elongated sieve tube elements are joined end to end to form sieve tubes. They
contain no nucleus and very little cytoplasm, leaving space for the mass flow of sap.
• At the end of the sieve tube elements are perforated cross walls called sieve plates
which allow the movement of sap from one element to the other.
• Sieve tubes have very thin walls and when seen in transverse section are usually 5 or
6 sided
• At first sight the sieve plates appear to have no real function and actually obstruct
the free flow of sap but they may act to support the tube by keeping the lumen
open. They serve as a mechanism to block the sieve tube after injury or infection.
The pores in the sieve plate can very rapidly become blocked by deposition of callose
which prevents loss of sap and inhibits the transport of pathogens.
• Companion cells are small cells with a large nucleus and dense cytoplasm. They have
numerous mitochondria to produce the ATP needed for active processes. They carry
out the metabolic processes needed to load assimilates actively.
• Plasmodesmata- microscopic channels through the cellulose cell walls linking the
cytoplasm of adjacent cells.
9.2 Water transport in multicellular plants:
Water transport in plants:
• Turgor pressure as a result of osmosis in a plant cell provides a hydrostatic skeleton
to support the stems and leaves.
• Turgor also drives cell expansion – the force that enables plant roots to force their
way through tarmac and concrete
• The loss of water through evaporation helps to keep it cool
• Mineral ions and the products of photosynthesis are transported in aqueous
solutions
• Water is a raw material for photosynthesis
Movement of water into root:
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