7. TRANSPORT IN PLANTS Flashcards
Why is light required for photosynthesis
it is a source of energy
How have leaves evolved for photosynthesis
have large surface area:volume ratio
- for efficient capture of carbon dioxide and light
How do plants obtain mineral salts and water for nutrition
through extensive root systems below ground
Why do plants need magnesium ions
ions part of the chlorophyll molecule which is required for photosynthesis
Why are transport systems needed
- to move substances from where they are absorbed to where they are needed
- to move substances from where they are produced to where they are needed for metabolism
- to move substances to a different part of the plant for storage
Functions of the xylem
- long distance transport of water and mineral salts
- mechanical support and strength
What are the caps outside vascular bundles made of
- sclerenchyma fibres
- provide extra support for the stem
What are sclerenchyma fibres
- long, dead, empty cells with lignified cell walls
- only provide extra support for the stem
Features of xylem
- dead cells: no protoplasm to get in the way of transport
- no end walls
- lignified cell walls: prevent xylem collapsing due to pressure differences
narrow diameter: helps to prevent air locks
pits in wall: allow water to bypass air locks. allow water to move out of xylem to living cells
Support of stems and roots
STEMS
need to be supported in air
ROOTS
subject to pulling strain from above ground
What is wood
in the stems and roots of trees and shrubs, extra xylem is made, forming wood
What are the strengthening tissues
xylem and sclerenchyma
What is hydrostatic pressure
- pressure exerted by a liquid
- pressure at top becomes lesser than pressure at bottom of xylem vessel as water is removed from the leaves due to transpiration
- pressure difference causes water to move up the xylem vessels in continuous columns
What is mass flow
- movement of water in xylem
- all the water molecules and any dissolved solutes move together as a body of liquid
Cohesion and adhesion
COHESION: water molecules are attracted to each other by hydrogen bonds
ADHESION: water molecules are attracted to the cellulose and lignin in the cell walls of xylem
help to move water by mass flow in a continuous column in xylem
Position of vascular bundles in stems
- arranged in rods around the centre
- xylem inside and phloem outside separated by cambium
- sclerenchyma caps at the ends for support
- strong rods help to support the stem
Process of movement of water through the xylem
largely passive from high water potential to lower water potential
What is root pressure
- cells surrounding xylem vessel elements actively pump solutes such as mineral ions into water in xylem
- decreasing water potential of the solution in the xylem
- drawing water from the surrounding root cells
- this influx of water increases the hydrostatic pressure at the base of the xylem vessel
What are assimilates
substances which the plant itself has made for example sugars, which are made by photosynthesis in leaves
What are the functions of Casparian strips
- allow control of substances entering xylem
- help with the generation of root pressure
