Transport in Plants Flashcards
State how transport occurs in plants and identify the substances transported.
Internal transport in plants occurs by mass flow with
water and mineral salts travelling in the xylem tissue and manufactured foods in the form of sugar and amino acids carried in phloem tissues.
Describe the structure of xylem tissue and explain its role in plant transport.
Long hollow tubes with dissolved end walls and the absence of organelles allow for the continuous flow of water. The lignified wall makes walls impermeable to water for the transport of water and dissolved mineral salts. Lignified walls provide strength to the xylem vessel and offer structural support so that the xylem vessel will not collapse on itself.
Define translocation
Translocation is defined as the process by which molecules synthesized (sucrose, amino acids and hormones) are transported around in the plant through the phloem tissues of the vascular bundles.
Relate the structure of the root hair cell to its function.
Root hairs are extensions of the epidermal cells. It dramatically increases the surface area of the root hair cell for the absorption of water and dissolved ions.
Describe the uptake of water by roots.
Water moves from a region of higher water potential from the soil to a region of lower water potential in the roots through osmosis. There is also the active uptake and diffusion of soluble ions into the roots depending on the concentration in the soil and that of root cells.
State and describe the two pathways of water movement from root hair to xylem vessels.
- The apoplast pathway of water movement. Water is transported by mass flow via interconnected “free space” between the cellulose fibres in cell walls.
- The symplast pathway of water movement. Water flow is slower as the water travels through the cell membrane by osmosis and move through the cytoplasm.
State and describe the two main forces enabling the movement of water against gravity.
- Root pressure
- Active transport of ions into the root xylem vessel lowers the water potential resulting in the accumulation of water in the roots by osmosis, thus the water flows upwards up to a few meters because of root pressure.
However, this is insufficient in tall trees. - Transpiration
- As water leaves the xylem vessels in the leaf through transpiration, a tension is set up on the entire water column in the xylem tissue. Since water has extremely strong adhesive and cohesive properties, this results in the flow of water up the plant. Therefore the xylem sap moves up the plant without energy expenditure.
State and describe the two main forces enabling the movement of water against gravity.
- Root pressure
- Active transport of ions into the root xylem vessel lowers the water potential resulting in the accumulation of water in the roots by osmosis, thus the water flows upwards up to a few meters because of root pressure.
However, this is insufficient in tall trees. - Transpiration streamS
- As water leaves the xylem vessels in the leaf through transpiration, a tension is set up on the entire water column in the xylem tissue. Since water has extremely strong adhesive and cohesive properties, this results in the flow of water up the plant. Therefore the xylem sap moves up the plant without energy expenditure.
Describe how transpiration occurs in a plant.
Water continuously move out of the mesophyll cells to form a thin film of moisture over their surfaces and evaporate into the intracellular air spaces. Thus creating a higher water vapour concentration inside the leaf than the surrounding air. Water vapour diffuses through the stomata into the drier air outside the leaf down a water vapour concentration gradient, to replace the moisture layer, mesophyll cells absorb water by osmosis from cells deeper into the leaf surface.
State when the size of the stomata in daytime and nighttime.
Stomata open during the daytime to take in CO2 and release water vapour. Stomata close during the nighttime.
Explain why transpiration is a consequence of gaseous exchange.
The stomata must be open for gaseous exchange. Gas exchange involves the uptake of carbon dioxide and the release of oxygen by diffusion as carbon dioxide is needed for photosynthesis while oxygen is a by-product of photosynthesis. As a result, water diffuses out of the plant through the open stomata in transpiration.
Define and describe transpiration stream
Transpiration stream is the flow of water up the stem.
As water leaves the xylem vessels in the leaf through transpiration, a tension is set up on the entire water column in the xylem tissue. Since water has extremely strong adhesive and cohesive properties, this results in the flow of water up the plant. Therefore the xylem sap moves up the plant without energy expenditure.
State the factors affecting transpiration.
- Humidity
- Temperature
- Wind or air movement
- Light intensity
- Water supply
State the effect of transpiration on the plant.
- Evaporation of water from the leaves has a strong cooling effect.
- The transpiration stream and the tension created by the water column in the xylem vessels by transpiration enables dissolved ions to be passively carried in the xylem.
- All cells in the plant receive water by lateral movements down a water potential gradient – allows all cells to be fully hydrated
- Turgor pressure of these cells provide support to the whole leaf
Describe and explain how wilting occurs and state its effects.
Under strong sunlight, the rate of transpiration exceeds the rate of absorption of water by roots, thus plant cells lose their turgor and become flaccid and the plant wilts
The rate of transpiration is reduced during wilting by the reduction of exposure of leaf surface area to sunlight.
The closure of stomata also prevents excessive loss of water.
