Transport in Plants Flashcards
Define vascular bundle.
- Vascular system in herbaceous dicotyledonous plants.
- Consists of two transport vessels, the xylem and the phloem.
Describe the structure and function of the vascular system in the roots of dicotyledons.
Xylem arranged in an X shape to provide resistance against force. Phloem found as patches between the arms. Surrounded by endodermis, aiding water passage.
Describe the structure and function of the vascular system in the stem of dicotyledons.
Vascular bundles organised around a central pith. Xylem on the inside of the bundle to provide support and flexibility, phloem on the outside. Cambium is found between the two.
What structure in plants is adapted for the uptake of water and minerals?
Root hair cells.
How is water taken up from the soil?
- Root hair cells absorb minerals by active transport reducing the water potential of the root.
- Water potential of root hair cells is lower than that of the soil.
- Water moves into the root by osmosis.
Outline how plant roots are adapted for the absorption of water and minerals.
Plant roots are composed of millions of root hair cells which have:
- Long hairs that extend from the cell body, increasing the surface area for absorption.
- Many mitochondria which produce energy for the active transport of mineral ions.
State the three pathways by which water moves through the root.
- Apoplast
- Symplast
- Vacuolar.
Describe the apoplast pathway.
Water moves through intercellular spaces between cellulose molecules in the cell wall. It diffuses down its water potential gradient by osmosis.
Describe the symplast pathway.
Water enters the cytoplasm through the plasma membrane and moves between adjacent cells via plasmodesmata. Water diffuses down its water potential gradient by osmosis.
Describe the vacuolar pathway.
Water enters the cytoplasm through the plasma membrane and moves between vacuoles of adjacent cells. Water diffuses down its water potential gradient by osmosis.
Describe the structure and function of the the endodermis.
- Innermost layer of the cortex of a dicot root.
- Impregnated with suberin which forms the Casparian strip.
- Endodermal cells actively transport mineral ions into the xylem.
What is the function of the Casparian strip?
- Blocks the apoplast pathway, forcing water through the symplast route.
- Enable control of the movement of water and minerals across the root into the xylem.
What molecule makes the Casparian strip waterproof?
Suberin.
Relate the structure of the xylem to its function.
- Long, continuous columns made of dead tissue, allowing the transportation of water.
- Contain bordered pits, allowing the sideways movement of water between vessels.
- Wall impregnated with lignin, providing structural support.
Define transpiration.
- The loss of water vapour from the parts of a plant exposed to the air due to evaporation and diffusion.
- Consequence of gaseous exchange; occurs when the plant opens the stomata to exchange oxygen and carbon dioxide.
What is the transpiration stream?
The flow of water from the roots to the leaves in plants, where it is lost by evaporation to the environment.
How does water move up the stem?
- Root pressure.
- Cohesion tension theory.
- Capillarity.
What is root pressure?
The force that drives water into and up the xylem by osmosis due to the active transport of minerals into the xylem by endodermal cells.
Explain the cohesion-tension theory.
- Water molecules form hydrogen bonds with each other, causing them to ‘stick’ together.
- Surface tension of the water also creates this sticking effect.
- Therefore as water is lost through transpiration, more is drawn up the stem from the roots.
Define capillarity.
The tendency of water to move up the xylem, against gravity, due to adhesive forces that prevent the water column dropping back.
State the factors that affect the rate of transpiration.
- Light
- Temperature
- Humidity
- Air movement
How does temperature affect the rate of transpiration?
A higher temperature increases random motion and rate of evaporation, therefore increases the rate of transpiration.
How does light affect the rate of transpiration?
A higher light intensity increases the rate of photosynthesis, causing more stomata to open for gas exchange, therefore increases the rate of transpiration.
How does humidity affect the rate of transpiration?
High humidity means the water content of the air next the the leaf is high. This reduces the concentration gradient, therefore decreases the rate of transpiration.
How does air movement affect the rate of transpiration?
Large amounts of air movement blow moist air away from the leaves, creating a steep concentration gradient, therefore increases the rate of transpiration.
What is a hydrophyte?
A plant that is adapted to live and reproduce in very wet habitats, e.g water lilies.
Give adaptations of hydrophytes that allow them to live in wet conditions.
- Thin or absent waxy cuticle.
- Stomata often open.
- Wide, flat leaves.
- Air spaces for buoyancy.
What is a xerophyte?
A plant that is adapted to live and reproduce in dry habitats where water availability it low, e.g. cacti.
Give adaptations of xerophytes that allow them to live in dry conditions.
- Small/rolled leaves.
- Densely packed mesophyll.
- Thick waxy cuticle.
- Stomata often closed.
- Hairs to trap moist air.
What are mesophytes?
- Terrestrial plants adapted to live in environments with average conditions and an adequate water supply.
- They have features that enable their survival at unfavourable times of the year.
Relate the structure of the phloem to its function.
- Sieve tube elements transport sugars around the plant.
- Companion cells designed for active transport of sugars into tubes.
- Plasmodesmata allow communication and the exchange of substances between sieve tubes and companion cells.
What are cytoplasmic strands?
Small extensions of the cytoplasm between adjacent sieve tube elements and companion cells.
Describe the function of cytoplasmic strands.
- Allow communication and the exchange of materials between sieve tubes elements and companion cells.
- Hold the nucleus in place.
Define translocation.
The movement of organic compounds in the phloem, from sources to sinks.
Summaries the mass-flow hypothesis of translocation.
- Sugar loaded into sieve tubes via active transport.
- Lowers water potential, causing water to move in from the xylem.
- Hydrostatic pressure causes sugars to move towards the sink.
Give evidence for the mass-flow hypothesis.
- Sap is released when stem is cut so must be pressure in phloem.
- Sap exuding from the stylet of an aphid inserted into sieve tubes provides evidence that sugars are carried in the phloem.
- There is a higher sucrose concentration in the leaves than the roots.
- Autoradiographs produced using carbon dioxide labelled with radioactive carbon provide evidence for translocation in the phloem.
What is autoradiography?
A technique to record the distribution of radioactive material within a specimen.
What is a potometer?
An apparatus used to measure water uptake from a cut shoot.
