OCR Biology A - 3.1.3 - Transport in Plants Part 3

H2O can rise up a narrow tube against the force of gravity

Water molecules sticking together

Attraction between water molecules and the walls of the xylem

Loss of H20 through leaves must be replaced by H2O in xylem

H2O moves up xylem as a result of tension, created by loss of water in leaves

As H2O moves out of xylem, the whole column gets drawn up due to cohesion

Enters through the xylem, passes through mesophyll (osmosis) and diffuses through air space in spongy mesophyll

As H2O vapour collects WP rises, when higher in the leaf --> diffuses out of stomata

Plants adapted to a habitat with adequate water

Plants adapted to a salty habitat

Plants adapted to dry habitats

Rolled leaves - reduce SA

Reduced no. and size of stomata - reduces diffusion

Sunken stomata - creates pocket of water vapour

Thick waxy cuticle - impermeable

Hairy leaves - traps water vapour

Dense spongy mesophyll - smaller surface area for evaporation

Thick stem - stores water

Plants adapted to live in freshwater

Aerenchyma - parenchyma with many air spaces (buoyancy and flotation): allows O2 to diffuse to roots for aerobic respiration

Reduced root system - water can diffuse directly into leaves, feathery roots hold up plant

Large thin leaves

Stomata on the upper surface only

End walls removed to form long tubes

No cytoplasm/cell organelles - little resistance of flow of water

Lignified (waterproofing and strengthening)

Bordered pits - allow movement of water between vessels

Form long tubes

End walls are retained

End walls contain many sieve pores (sieve plates)

Thin layer of cytoplasm

Very few organelles; no nucleus

Closely associated with sieve tube elements

Connected to sieve tube elements by many plasmodesmata

Dense cytoplasm with many mitochondria

Large nucleus

Evaporation at top of the xylem creates tension in the xylem

| Water molecules are cohesive and form a column which is then pulled up by tension

Movement of water up xylem vessels from roots to leaves (area of high hydrostatic pressure to area of low hydrostatic pressure)

Mass flow

Assimilates such as sucrose and amino acids

Assumption that water uptake by plants is the same as water loss

BUT water is used photosynthesis and is produced in respiration

Vapour around stomata is blown away

Reduces water vapour around stomata

Creates steeper wpg

H+ ions pumped out of cc using active transport

Uses conc. gradient to move back into cc w. sucrose through a cotransporter protein

Sucrose builds up and diffuses through plasmodesmata into sieve tubes

Reduces wp

Stomata opens for gas exchange for photosynthesis

Photosynthesis is necessary to make sugars

Water lost through the cuticle

Soluble so can easily travel in solution

| Metabolically inactive so not used during transport

Ice forms and pierces membranes

| Denaturing of proteins

Some poisons affect mitochondria and prevent production of ATP, when cyanide is applied to root cells, root pressure disappears

Root pressure increases w a rise in temp and decreases w/ a fall in temp --> chem reactions

If O2 levels fall or respiratory substrates so does root pressure

Guttation

Changes in diameter of trees - when transpiration is at its highest as is the tension, diameter shrinks

When a xylem vessel is broken air is drawn in rather than water leaking out

Plant can no longer move water up the stem as continuous stream is broken

Microscopy allows us to see he adaptations of cc for active transport

If mitochondria of cc are poisoned, translocation stops

Flow of sugars n phloem is 10,000x faster than diffusion ---> active process

Aphids

Ensures that water and dissolved mineral ions (especially nitrates) have to pass into the cell through the plasma membrane so the water and ions are in the cytoplasm

Prevents water from cortex going back to medulla