Flashcards in Transport of Water in Plants (Chapter 7) Deck (49)
Why do transport systems needed in plants?
1) to move substances from where they are absorbed to where they are needed e.g. water and mineral ions from roots to other parts of the plant
2) to move substances from where they are produced to where they are needed for metabolism e.g. glucose from leaves to all parts of the plant
3) to move substances to different parts of the plant for storage
Why are transport systems not needed to transport CO2 and O2?
Because it is relatively easy for CO2 and O2 to diffuse into and out of the bodies of plants, reaching and leaving every cell quickly enough down diffusion gradients
For transport of what substances are transport systems needed?
Distribution of water, inorganic and organic nutrients, and plant hormones
What are the two transport systems in plants?
Describe the characteristics of the xylem
- Carries mainly water and inorganic ions from roots to above ground
- The xylem sap can only move in one direction
Describe the characteristics of the phloem
- Carries substances made by photosynthesis from to leaves to other areas of the plant
- Phloem sap can be moving in different directions in different parts of the plant
What are the 3 main organs involved in plant transport?
Stems, roots and leaves
What are stem, roots and leaves composed of?
More than one tissue, each a collection of cells specialised for a particular function
What are the two types of angiosperms (flowering plants)?
Monocots and dicots
What are the leaves of monocotyledons like?
Long and narrow e.g. grass
What do the leaves of dicotyledons typically have?
Blades and stalks
What is the difference between monocots and dicots?
There are differences in the distribution of xylem and phloem in their roots, stems and leaves
What is xylem made up of?
Tubes (vessels) made from dead cells (xylem vessel elements)
Describe the walls of xylem vessel elements
Reinforced with a strong, waterproof material called lignin
What is the function of xylem?
1) long distance transport of water and mineral salts
2) provides mechanical support and strength
Describe the structure of vascular tissue in roots
Xylem is at the centre with a series of 'arms' between which phloem is found
Describe the structure of vascular tissue in stems
The xylem and phloem are found in vascular bundles around the pith (xylem on the inside, phloem on the outside
Describe the structure of vascular tissue in leaves
The xylem and phloem are around in vascular bundles linearly (xylem at the top, phloem at the bottom
What do the outside of vascular bundles have?
Caps made of sclerenchyma fibres which provide extra support for the stem
What are sclerenchyma fibres?
Long, dead, empty cells with lignified walls to provide support (do not transport water)
Why is the distribution of xylem and sclerenchyma different in roots and stems?
Because the organs are subjected to different stresses and strains:
- stems need to be supported in air
- roots are usually spreading through soil and subject to pulling strains from parts above the ground
How is wood formed in trees and shrubs?
Extra xylem is made in roots and stems
Describe the pathway of water as it is transported through a plant
1) transpiration of water vapour through open stomata into the air (mainly from the underside of the leaf)
2) evaporation of water into leaf air spaces
3) water moves from xylem into leaf cells
4) water moves up xylem
5) water enters xylem
6) water uptake near root tips
How is the movement of water passive?
Because it is driven by evaporation in the leaves
Describe how water moves from roots to leaves
1) the energy of the sun causes water to evaporate from the leaves (transpiration)
2) this reduces the water potential in the leaves and sets up a water potential gradient
3) water moves down this gradient from soil into the plant e.g. through its roots hairs
4) water then moves across the root into the xylem tissue in the centre
5) once inside the xylem vessels, the water moves upwards through the root to the stem and from there into the leaves
What is transpiration?
- The loss of water vapour from a plant to its environment, by diffusion down a water potential gradient
- It is an inevitable consequence of gas exchange in plants
Where does most transpiration take place?
Through stomata in leaves
Why is the air inside the leaf usually saturated with water vapour?
Because the walls of the mesophyll cells are wet and some of this water evaporates into the air spaces
When will water vapour diffuse out of the leaf?
When there is a higher water potential inside the leaf than outside the lead and ∴ a water potential gradient