A concise and clear overview of the key concepts covered in chapter 9. These notes make biology easy to understand and are a good place to start your revision and boost your grades.
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Transport in plants
Chapter 9
The need for plant transport systems
• Metabolic demands: green zones can photosynthesise but underground and internal parts can’t . They need oxygen and
glucose transported to them and waste products removed.
• Hormones made in one part of the plant must be transported to where they are intended for
• Mineral ions absorbed by roots need to be transported to all cells to make proteins required for enzymes and structure of the
cell
• Size: continuously growing so get very large and every part of the plant needs water/food etc
• Surface area:volume ratio: small SA:V ratio for gas exchange so cannot rely on diffusion alone for everything they need
Transport systems of dicotyledonous plants Vascular tissues
•Seeds contain 2 cotyledons - food stores for developing •
embryo plant + form the first leaves when seeds germinate
•2 types: herbaceous and woody
• Herbaceous: die after growing season / soft tissues Stem
• Woody: hard tissues / long life cycle
•Have vascular system - series of transport vessels running
through stem, roots and leaves
•In dicot plants the vessels are known as the xylem and
phloem
•They run together to form vascular bundles in stems, leaves
and roots
Roots
Phloem on outside!
Xylem structure and function
•Transports water and nutrients
•Non living (no organelles)
•Structural support Leaf
•Main structure in xylem is xylem vessels - long hollow
structures made by several columns of cells fusing together
•Other tissues associated with xylem include - xylem
parenchyma - packs around xylem vessels, storing food and
containing tannin (to stop herbivores eating it)
•And xylem fibres which are long lignified secondary walls
that provide extra support but don’t transport water
•Lignin forms spirals, rings or relatively solid tubes with lots
of small unlignified areas called bordered pits. This is where
water leaves the xylem and moves into other parts of the
plant
, The structure and function of the phloem
•Living tissue which transports food in the form of organic solutes (sucrose)
around the plant from leaves where they are made in photosynthesis.
•Supplies cells with sugar and AAs
•Flows up and down plant
• Main transporting vessels are the sieve tube elements. They form a
hollow tube like the xylem but are not lignified.
• In the areas between the cells they are perforated forming sieve plates.
•The tonoplast, nucleus and some other organelles break down
•The phloem matures to have no nucleus
• Linked to sieve tube elements are companion cells. Linked to sieve tube
elements by plasmodesmata - channels though the cellulose cell walls
linking the cytoplasm of adjacent cells.
•Companion cells maintain their nucleus and organelles and are very
active. They have many mitochondria for active transport
•Phloem also contains supporting tissues including fibres and sclereids - cells
with extremely thick walls
•Perforations between sieve tube elements get blocked after injury or
disease to limit the spread of infection
Water
•Water is needed for:
• Turgor pressure helps provide support for the stems and leaves
• Turgor drives cell expansion - so plants can grow
• Loss of water through evaporation keeps plants cool
• Mineral ions and products of photosynthesis are transport in it
• photosynthesis
Movement of water into the root Movement of minerals
•Root hair cells have a large SA, thin into the root
cell wall and cytoplasm and vacuolar •Minerals absorbed by active
sap has sugars and mineral ions so a transport as cells have more
low water potential to bring water in minerals than soil
by osmosis. They are microscopic to
penetrate between soil
Movement of water across the root
• Once the water is in the root it reaches the xylem in two pathways: In the xylem
•Symplast pathway - 10% of water moves through the cytoplasm and •In the vascular bundle water returns to the
plasmodesmata (osmosis). Moves into next cell along which has lower apoplast pathway to enter the xylem itself and
w.p and maintains steep gradient move up the plant. The active pumping of
•Apoplast pathway - 90% of water moves through the cell walls and minerals into the xylem produces root
intercellular spaces. Water moves into xylem due to cohesive forces pressure which is independent of any effects
which creates tension meaning there is a continuous flow of water, of transpiration.
through the open cellulose walls which offer little resistance •Root pressure gives water a push of up to 1m
• Vacuolar pathway - small amounts of water pass through the vacuole
and cytoplasm, usually included with the symplast pathway
Evidence for the role of active transport
Movement of water into the xylem in root pressure
• When water reaches the endodermis (the cells surrounding the •If the mitochondria are poisoned then root
vascular tissue) it meets the casparian strip. pressure disappears
•The casparian strip is a band of waxy material called suberin that •Root pressure increases with a rise in
forms a waterproof layer. The water in the apoplast pathway joins the temperature and vice versa, indicating
symplast pathway chemical reactions are involved
•If levels of oxygen falls, so does root pressure
• Potentially toxic solutes in soil can’t enter through the selectively
permeable membrane •Xylem sap may exude from cut end of stems
• Endodermal cells move mineral ions into xylem by active transport.
This means the water potential of the xylem is lower than the
endodermal cells so water is pushed up the xylem by osmosis.
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