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Summary of IB Biology Topic 9 (Plant Biology)
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Summary of IB Biology Topic 9 (Plant Biology)
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Topic 9 - Plant BiologySubtopic 9.1 - Xylem Transport
Transpiration
Key question: how do plants transport water from their roots?
Transpiration: loss of water vapour from stems/leaves of plants
• Light energy evaporates water in leaves via stomata
• New water is absorbed from roots —> high pressure below, low pressure above
• Water ows along pressure gradient through the xylem (transpiration stream)
Stomata: pores on underside of leaf (open and close)
Evaporation
Regulating transpiration rate: stomata open and close to control how much water can escape
• Stomata are anked by guard cells
• When plant becomes dehydrated, its dehydrated mesophyll cells release ABA (plant hormone)
• ABA triggers the e ux of potassium from guard cells, decreasing water pressure and making
them accid
• Flaccid guard cells = closed stomata, no water can escape
Turgid stomata (lots of water) = open
Closed stomata (less water) = closed
Other factors which a ect transpiration rates:
• Photosynthesis levels
• Humidity, temperature, light intensity, wind, etc
Transpiration Stream
Transpiration stream: ow of water via xylem from roots to leaves against gravity
Cohesion: allows transpiration to occur in a continues stream
Adhesion: allows capillary action, where the water adheres to the xylem walls to pull itself
up (generating further tension)
Xylem structure:
• Tube composed of dead, hollow plants cells (water ows through these)
• Cells contain pits (pores) which transfer water between cells
• Cell walls are thick and reinforced by lignin to provide strength (to withstand tension)
Can be composed of:
• Tracheids (cells which only exchange water via pits —> slower rate of water transfer)
• Vessels (ends are fused —> continuous tube —> faster rate of water transfer)
Lignin arrangements:
flfl fl ffl ff fl fl
, • Individual, circular rings at equal distances from each other
• Helix
Root Uptake
Roots need maximal surface area to take water and mineral ions from the soil
• Some plants have extensive root system
• Other plants have a main tap root with lateral branches
Epidermis: surface cells
Root epidermis may have root hairs (protrusion in cell), further increasing surface area
Stele: central core of stem containing the xylem
1. Water and minerals are absorbed by epidermis
2. Di use across cortex
3. Pumped across endodermis layer (active transport, allows rate of uptake to be
controlled)
Mineral uptake:
Examples of minerals needed by plants:
• Mg2+
• Nitrates
• Na+
• K+
• PO43-
Mineral uptake process:
1. Positively charged mineral ions attach to clay particles (anions)
2. Root cells expel H+ ions (via proton pumps) that displace the mineral ions
3. Mineral ions passively di use into root (via gradient)
- and -
Negatively charged mineral ions bind to H+ ions to be reabsorbed
Water uptake:
ff
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