PLANT EXCRETION.ppt

TOPIC 11

PLANT EXCRETION

WATER MOVEMENT THROUGH THE PLANT

Water movement through a plant begins with evaporation from the leaf surface

98% of the water taken up by a plant is lost to the atmosphere by transpiration

PLANTPlants do not really produce any solid,

organic wastesThe main waste product of photosynthesic

plants in the light is oxygen gas, which escapes mainly through leaf stomata (tiny pores in the leaf surface)

PLANTWater is not really a waste product, it just evaporates from leaves as a consequence of keeping stomata open to admit carbon dioxide gas required for photosynthesis

EVAPORATIONEvaporation of water into the air means that

there is less water in the leaf cells and xylem vessels.

Water moving up the stem replaces this lost water.

The water is pulled up the plant using the force from evaporation of the water

EVAPORATION FROM LEAVESWater evaporates from the parts of a plant

that are exposed to the atmosphereThe greatest loss of water takes place

through the stomata (singular stoma), minute pores on the leaf surface

There are more stomata on the lower surface of leaves than on the upper surface

EVAPORATION FROM LEAVESThe lower surface is less exposed to the

warming effects of the Sun’s radiation , which would speed up the evaporation rate

Water cannot diffuse into the leaf through the stomata because the air spaces inside the leaf are completely saturated

This flow of water through the plant to replace the losses by evaporation from the leaf is called the transpiration stream

THE LEAF AND WATER LOSSWater is lost by evaporation and diffusion

from the leaf surfaceThis water loss happens because the stomata

need to open so the leaf can take in carbon dioxide as a raw material for photosynthesis

Plants can open and close the stomata, which helps to minimise water loss whilst allowing photosynthesis to continue

ADAPTATIONS OF PLANTSPlants may be adapted to the availability of

water 1) Upper epidermis – a complete covering

which is usually one cell thick. It is transparent to allow the free passage of light and has the major function of preventing the entry of disease-causing organisms such as bacteria and fungi

ADAPTATIONS OF PLANTS2) Palisade mesophyll – tall thin cells arranged

in columns and separated by very narrow air spaces. Cell contains many chloroplasts and the dense packing of these cells allows the absorption of the maximum amount of light energy

3) Vein – the transport system in and out of the leaf. The xylem vessels deliver water and minerals salts and the phleom sieve tubes carry away the organic products of photosynthesis

ADAPTATIONS OF PLANTS4) Spongy mesophyll

these cells are rather loosely packed and are covered with a thin layer of water. The air spaces between them aid the diffusion of gases through the leaf. The air spaces are saturated with water vapour so water diffuses out of the leaf

ADAPTATION OF PLANTS5) Stomata

These minute pores are mainly present in the lower epidermis. This surface is less exposed to the sun’s radiation so that evaporation of water is kept to a minimum. The stomata can be closed when no carbon dioxide intake is needed (in the dark, for example)When a plant is short of water, the guard cells become flaccid, closing the stomataWhen a plant has plenty of water, the guard cells become turgid. The cell wall on the inner surface is very thick, so it cannot stretch as much as the outer surface. So as the guard cells swell up, they curve away from each other, opening the stoma

TRANSPIRATIONWater uptake occurs by osmosis from the soil

solution (high water potential) into the root cells (lower water potential)

Water moves up the stem in the xylem due to the tension caused by water loss from the leaves

Water moves from xylem to enter leaf tissues down water potential gradient

Transpiration – water evaporates from leaves and lowers the water potential in the leaf tissues

TRANSPIRATIONWater diffuses through stomata down

water potential gradientWater evaporates into internal air spaces

from surfaces of leaf cellsLeaf cells have a layer of moisture to aid

diffusion of carbon dioxide into the cellsStomata allow the uptake of CO2 for

photosynthesis. When stomata are open, water evaporated from the spongy mesophyll can diffuse out of the leaf down the water potential gradient

LEAF STRUCTURELeaf structure may reduce transpiration:1. Thick, waxy cuticle reduces evaporation

from epidermis2. Stomata may be sunk into pits which

trap a pocket of humid air3. Leaves may be rolled with the stomata

on the inner surface close to a trapped layer of humid air

4. Leaves may be needle shaped to reduce their surface area

EXTERNAL FACTORS External factors may affect the rate of transpiration :

1) Wind moves humid air away from the leaf surface and increases transpiration

2) High temperatures increase the water-holding capacity of the air and increase transpiration

EXTERNAL FACTORS3) Low humidity increases the water potential gradient between leaf and atmosphere and increases transpiration4) High light intensity causes stomata to open ( to allow photosynthesis) which allows transpiration to occur

EXTERNAL FACTORS4) High light intensity causes stomata to open ( to allow photosynthesis) which allows transpiration to occur

5) Soil water – If water becomes increasingly unavailable then less is taken in and transpiration is reduced

INTERNAL FACTORSTranspiration increases as the surface area increases or if the surface –area –to volume ratio increases. The thinner the cuticle the greater the rate of cuticular transpiration. Transpiration rate also increases the more stomata there are perunit area

INTERNAL FACTORSMineral elements area absorbed as ions from the soil solution via root hairs. They enter passively by diffusion or are selectively absorbed by active transport. After their initial delivery, the ions may be recirculated to other parts of the plant by the phloem

TRANSPIRATIONTranspiration means the loss of water vapour from the surface of the plant. It may occur via stomata in leaves, lenticels in stems and through the cuticle

LEAVES ARE CRYING : IS IT NORMAL?

•Guttation means the loss of liquid water from the surface of a plant•It is most apparent on the larger leaves• The plant must have healthy root system and ample water•Prolific gutation is a sign of good health•Many plants will guttate pure water under appropriate conditions : wet soil and high humidity

SUMMARYWater is absorbed into the root hairs by

osmosisWater travels through the plant in xylem

vesselsThe flow of water through the plant is called

the transpiration streamWater evaporates from the leaves causing

more water to be pulled up the stemMinerals salts are absorbed by diffusion or

active transport in both animals and plants