Summary
Summary Module 3.3: Transport in plants
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OCR biology A level A summary of key concepts ranging from xylem and phloem to transpiration and translocation
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TRANSPORT IN PLANTS Transpiration- Transpiration stream = consequence of gas exchange
Need for transport system - Water evaporate from mesophyll, air spaces, stomata
1) Metabolic demands – photosynthesis, waste - Evapo ↓ of mesophyll so water in from adjacent
products, respiration, hormones, mineral ions cell by osmosis (symplast) + diffusion (apoplast)
2) Size – continue to grow (roots + stem) - Water moves out of xylem by osmosis
3) SA:V – needed for meeting gas exchange needs - Water H bonds with carbos in xylem vessel (adhesion)
+ other water molecules (cohesion) = capillary action
Structure - Water up stem = transpiration pull, tension, soil-roots
Dicotyledonous plants
- Herbaceous = soft, short lifecycle (or woody) Evidence for cohesion tension theory
- Sclerenchyma – support + lignified 1) Change tree diameter – at night ↓ transpiration, ↓
- Collenchyma – flexible wind resistance xylem tension, ↑ diameter
- Parenchyma – soft packing tissue, store starch in 2) Xylem vessel broken – air drawn in, water no longer
roots, air spaces in water (afloat) drawn up as cohesive forces broken
Root Stomata
Vascular bundle in middle, - Guard cells control rate
help tugging strain of transpiration
- Control water loss – waxy
Stem cuticle, wilting, stomata
Vascular bundles at edge, on bottom, close at night, lose leaves
strength + support - Asymmetric – inner wall less flexible
- Extends lengthways (cellulose hoops prevent lateral)
Leaf - Pump in solutes by AT = ↑ turgor (hormones affect)
Midrib = vascular tissue,
branch veins = structure Factors affecting transpiration
1) Light – photosynth, stomata gas exchange, ↑ evapo
Xylem 2) Humidity – reduce gradient
- Transports water + mineral ions upwards 3) Temp – ↑ kinetic, ↑ evap, warm air holds ↑ water
- Non living tissue, long hollow structure 4) Air movement – hairs trap water, ↓ gradient
- Bordered pits – water leaves 5) Soil water availability – water stress ↓ transpiration
- Long cells, lignified secondary walls (annular, helical) ,
mechanical strength, support Water pathways
- Parenchyma around xylem vessels store food + tannin - Water for photosynth, struc, transport, cooling
deposits (bitter chemical, herbivores) - Turgor / hydrostatic pressure (cell expansion)
- Toluidine blue stain for lignin – pink if present, blue - Mineral ions + photosynth products transported
/green if non lignified (xylem,schlerenchyma) - Mass flow = fluid ↓ pressure gradient
Root hair cell adaptations – small between soil particles,
large SA:V for rapid diffusions / osmosis, thin surface,
conc of solutes in cytoplasm maintains ᴪ gradient
Movement into root
- Soil water has low conc of dissolved minerals = ↑
- Cytoplasm/sap have many dissolved solvents = ↓
- Water moves in by osmosis down gradient
Phloem Movement across root
- Living tissue, move up + down 1) Symplast – water thru cytoplasm + plasmodesmata
- Solutes, sugars, amino acids, assimilates by osmosis, ↑ than next cell, maintains gradient
- Sieve plates between cells allow sap flow 2) Apoplast pathway – diffuse thru intercellular space +
- Sieve tube elements long, hollow, non-lignified cell wall, pulled by cohesive forces, continuous flow +
- No membrane bound organelles, no nucleus tension, open structure = no resistance
- Companion cells linked by plasmodesmata cytoplasm
(Large nucleus, dense cytoplasm, ↑ mitochondria)
- Mass flow hypothesis – how solutes transported
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