IGCSE BIO 8. transport in plants

xylem vessel

role :

transport water & mineral ions

has lignin on cell wall, in spiral pattern

hollow vessel with no organelle & cytoplasm & no end walls --> to give low resistance to large volume of water flow

waterproof --> prevents water leaking out

bordered pits in the lignified walls --> allows lateral movement of water between vessels

role of lignin in xylem

- gives structure support

- makes xylem waterproof

- prevents inward collapse of plant

- allows flexibility & bending

- allows adhesion of water --> continuous flow of water as transpiration stream

phloem tissue

carries sucrose & amino acids, from source --> sink

contains different types of cell within it

vascular tissues in a dicot root

xylem --> star shape

phloem --> circular, surrounding xylem inside

endodermis / cortex / root hair...

vascular bundles in a stem

xylem inside, phloem outside

vascular bundles arranged in a ring near the outer edge

pith (inside the ring) / cortex (outside) / epidermis...

vascular bundles in a leaf

xylem above, phloem below

root hair cell

long cytoplasmic extension + vacuole extension along it --> large surface area --> increases the uptake of water & mineral ions

functions :

absorb water by osmosis & mineral by active transport

anchors the plant into the soil

water uptake process

enters root hair from soil (by osmosis) --> enters cortex cell by osmosis --> moves between adjacent cortex cells by osmosis & creating root cortical pressure --> the pressure pushes water into xylem through phloem --> rises through xylem due to capillary action & transpiration pull --> in leaves, move from xylem --> mesophyll cells by osmosis

water can also move through the permeable cortex cell walls only, without entering the cells

transpiration

loss of water from plant leaves by evaporation from the surface of the mesophyll cells into the air spaces, followed by loss of water vapour through the stomata

translocation

movement of sucrose and amino acids in the phloem, from sources to sinks

source

regions of production / site of photosynthesis / storage, if energy supply is low

sink

regions of storage (if excess present) & growth & utilisation for respiration & metabolism

what affects water loss by transpiration?

large internal surface area (interconnecting air spaces between spongy mesophyll cells)

size & number of stomata

capillary action

adhesion + cohesion

factors that help raise the water column

transpiration pull / capillary action / root (cortical) pressure

water movement in stems due to transpiration

loss of water from leaf by transpiration --> decrease in water potential in leaf --> water move into leaf from xylem --> water potential gradient is created with higher water potential in the xylem of roots --> creates transpiration pull --> water moves up the xylem by mass flow, due to cohesion (of water molecules) & adhesion (between lignin <-> water)

factors affecting transpiration

directly proportional :

- temperature : evaporates faster

- wind speed : sweeps away moist air and replace it with dry air (maintains the gradient)

- sunlight : stomata opens up

inversely proportional :

- humidity : diffusion gradient decreases

- water deprivation : stomata closes up

stomata

thick cell wall inside / thin outside

when short of water : guard cell becomes flaccid --> stomata remains closed

when plenty of water : guard cell becomes turgid (swells) --> as inner surface is very thick, it cannot stretch as much as outer surface --> guard cell bends outside --> opens up the stomata

in germination, source? sink?

source : seed / cotyledon (starch stored)

sink : emerging shoot & root (as seedling cannot photosynthesize yet)

in spring / summer (when light intensity is high), source? sink?

source : leaves (photosynthesize --> makes sucrose & amino acid)

sink : all other plant organs and storage organs

at the start of growing season (early spring), source? sink?

source : vegetative storage organs (roots, tubers...) (stores starch)

sinks : all growing parts of shoot (as it cannot photosynthesize yet till leaves develop)