BIOL10004 Lecture Notes - Lecture 10: Active Galactic Nucleus, Tracheid, Plant Evolution
14 Jun 2018
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BIOL – Lecture 10
Water and transport
Movement of water and solutes
• Plant evolution is about adaptation to a terrestrial environment
o Water balance
o Gas exchange
o Structural support
o Reproduction
• Land plant ancestors were aquatic
o Uptake and loss of water weet a issue, ad taspot wast euied
o Buoyancy provided support
• Primitive land plants have no conducting tissues or roots
o Small and grow close to substrate
o More common in moist environments
Why plants need a conducting system
• Small molecules diffuse c. 50 micrometres in 1 second
• Molecule that diffuses across cell in 1 second requires 8 years to diffuse 1m
o Cytoplasmic streaming increases this to 5cm/hour – ok for large algal cells
Vascular plants have two transport systems
Xylem transports water from the roots up to the leaves
• Xylem conducting cells are tracheids and vessels
o Dead when functionally mature
o Thick cell walls reinforced with lignin
o Can withstand hydrostatic pressure
o Tracheids – long, narrow, entire cells that overlap one another
o Vessels – wide cells with degenerated end-walls and are more efficient at
water transport than tracheids
• Xylem elements that mature in elongating stems have spiral thickening to allow
stretching
Phloem transports sugars produced in photosynthesis from the leaves (where they are
produced) to sites where they are used or stored
• Sieve cells – conducting cells that are accompanied by companion cells
o Pores in sieve are open, allowing movement between connected cells
o Sieve and companion cells form from same mother cell
▪ Sieve lost most organelles except mitochondria, companion retain
active nucleus and all organelles – ribosome rich
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