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Questions and Answers
What is the main function of xylem vessels?
What is the main function of phloem sieve tube elements?
What is the mechanism by which water moves into root hair cells from the soil?
What is the pathway by which water moves through the xylem vessels?
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What is the main function of companion cells in phloem?
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What is the purpose of the Casparian strip in the root hair cell?
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What is the process by which water vapour is lost from the aerial parts of the plant?
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What is the main function of the pit in xylem vessels?
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What is the main adaptation of phloem sieve plates?
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What is the main reason for the arrangement of xylem and phloem in the root?
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Study Notes
Structure of Transport Tissues
- Plan diagram of leaf shows the arrangement of xylem and phloem in the middle, with xylem above phloem
- Plan diagram of root shows the arrangement of xylem and phloem in the centre, enclosed by the endodermis
- Plan diagram of stem shows the arrangement of xylem and phloem on the periphery, with xylem inwards and phloem outwards
Xylem and Phloem
Xylem
- Consists of dead cells
- Form a long, narrow, and hollow tube to increase capillarity
- Transport water unidirectionally from roots to leaves
- Features:
- Hollow and no cell contents
- No end walls
- Wide lumen
- Lignified cell walls
- Cellulose cell walls
- Pits for lateral movement of water
Phloem
- Contains two types of cells: sieve tube elements and companion cells
- Features:
- Sieve tube elements: no organelles, no vacuole and nucleus
- Companion cells: metabolically active, high numbers of mitochondria and ribosomes
- Sieve pores for easy flow
- Sieve plates to prevent bursting
- Little cell contents
- Plasmodesmata for flow to/from companion cells
- Thin walls for rapid entry of water
Transport Mechanisms
- Water and minerals can be transported within plants dissolved in water
- Water reaches the root hair from the soil through osmosis
- Root hairs increase the surface area for water and ion uptake
- Ions may be taken up with water by osmosis or separately through facilitated diffusion or active transport
- Two main pathways for water to reach the xylem from the root hair:
- Apoplast pathway: movement through cell walls and intercellular spaces
- Symplast pathway: movement through the cytoplasm and living material
Transport in the Xylem
- Transpiration: the loss of water in the form of water vapour from the aerial parts of the plant
- Water vapour in air spaces diffuses out of the leaf through stomata down the water potential gradient
- Water evaporates from the cell walls of spongy mesophyll cells into the intercellular air spaces
- Water moves from the cell's cytoplasm into the cell wall, and then into the xylem vessels through pits
- A cohesion-tension and transpiration stream is formed
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Description
This quiz covers the structure and function of transport tissues in plants, including xylem and phloem, and their arrangement in leaves, roots, and stems.
