plant organisation

is the leaf an organ?

yes

transpiration

the loss of water by evaporation from leaves through the stomata

transpiration process

• water vapour from leaf cells evaporates through stomata (on underside of leaf) into surrounding air

• water potential inside cells of leaf lowered (due to loss of water)

• water moves from xylem to leaf cells down water potential gradient

• water pulled up xylem due to (strong) cohesive forces between molecules, forming continuous column of water

• water moves into xylem from root cells, lowering water potential inside root cells so water moves into them by osmosis

transpiration stream

a continuous column of water and dissolved minerals through a plant, from the roots up to the leaves, and out into the atmosphere as water vapour

factors affecting rate of transpiration (3)

• temperature

• humidity

• wind speed

how does temperature affect the rate of transpiration?

as temperature increases rate increases (KE of particles has increased) but in extreme temperatures stomata will close so rate stops (conserves water but prevents photosynthesis)

what is humidity?

how saturated the air is

how does humidity affect the rate of transpiration?

as humidity increases rate decreases due to osmosis (high humidity = shallow gradient)

how does wind speed affect the rate of transpiration?

as wind speed increases rate increases as wind moves water vapour in boundary layer (allowing more water vapour to take its place)

translocation

the movement of food molecules through the phloem tissue

role of xylem

transports water and dissolved mineral ions around plant

direction of movement in xylem

always UP plant:

roots —> stem —> leaves

role of phloem

transports dissolved sucrose (from leaves for immediate use or storage) and amino acids around plant

direction of movement in phloem

either upwards OR downwards

xylem and phloem arrangement

arranged together in vascular bundles in roots, stem and leaves

leaf structure

see image

what is the palisade mesophyll adapted for?

adapted to absorb light efficiently (for photosynthesis)

how is the palisade mesophyll adapted?

• many chloroplasts

• column shaped and arranged closely together

• towards upper surface of leaf

what is the spongy mesophyll adapted for?

adapted for efficient gas exchange

how is the spongy mesophyll adapted?

• tissue loosely packed

• cells covered by thin layer of water (gases dissolve in this so diffuse easily)

what is the waxy cuticle adapted for?

reduces water loss by evaporation

how is the waxy cuticle adapted?

is a waterproof waxy layer on top leaf

what is the upper epidermis adapted for?

lets light straight through to palisade cells

how is the upper epidermis adapted?

• single layer of cells covering too of leaf

• are clear

lower epidermis

single layer of cells covering bottom of leaf

what are stomata?

tiny holes in lower epidermis

role of stomata

control water loss and gas exchange

how are the stomata controlled?

guard cells surround stomata which allow stomata to open and close

what does a potometer measure?

the rate of water uptake by a plant

how does a potometer measure this?

by estimating the rate of transpiration (as most water absorbed by a plant is lost through transpiration)