PREPARED BY :EN. MUHD FAZLI B. DOLLAH

BIOLOGY FORM 5 CHAPTER 1 : TRANSPORT

LEARNING OUTCOMES• To state what translocation is.• To explain the importance of

translocation in plants.• To describe the process of transpiration.• To explain the importance of

transpiration.• To describe the pathway of water from

the soil to the leaves. • To state external conditions affecting

transpiration

TRANSLOCATION• The transport of dissolved organic

food substances by the phloem.

IMPORTANCE OF TRANSLOCATION• Ensures a plant’s survival• Organic food molecules can be transported :

– To the growing regions in the plant for growth & development

– To the plant cells for metabolism– Downwards from the leaves to the storage organs

such as the roots

TRANSPIRATION• The loss of water as water vapour

from a living plant to the atmosphere due to evaporation.

• Most of the water is lost through the stomata of leaves.

• A small amount is lost through the cuticle or through the lenticels in woody stem

• Three main sites of transpiration are :

– STOMATA – 90% of transpiration takes place through minute pores on leaves called stomata

– CUTICLE – 10% of transpiration takes place through the cuticle. The amount of water loss varies with the thickness of the waxy cuticle

– LENTICELS – small pores in the stem which allow gaseous exchange. Only small amounts of water are lost through the lenticels.

• Transpiration occurs mainly during the day when the stomata are opened.

• Heat from the sun causes the water to evaporate from the walls of the spongy mesophyll cells.

• The water vapour accumulates in the substomatal air chambers before diffusing from the plant through the open stomata

•Water move into the mesophyll cells by osmosis from the adjacent cell

•Water flows from the xylem vessels along a chain of cells to the outermost mesophyll cell.

•Water is pulled through the xylem vessels from the root to the leaves.

The Importance of Transpiration

• Creates a transpirational pull that pulls water & dissolved mineral ions from the root to the leaves.

• Water supplied to the plant is used for photosynthesis & cell metabolism. Mineral ions is used for cell activities & growth & development of the plant.

• Water keeps the cell turgid give support to leaves & young stems (herbaceous plants).

• To keep the plant cool, an important effect particularly in hot condition.

• Water evaporates from the plants to the atmosphere faster than it can be absorbed from the soil, it is withdrawn from the plant cells flaccid – leading to wilting

PATHWAY OF WATER FROM THE SOIL TO THE

LEAVES• Water & mineral ions are absorbed by young roots with thin epidermis. (osmotic forces + active transport)

• The pressure of root hairs increases the surface area for absorption.

• The cell sap in the root hair cell contains sugars, amino acids, & minerals more concentrated than the surrounding dilute soil solution.

• Water diffuses into the root cells by osmosis.

• The entry of water into the root cells dilutes its cell sap.

• The cell sap of the adjacent cortex cell is now more concentrated water molecules diffuse into the adjacent root cell.

• The process continues & water is transported from one root cell to another by osmosis.

• In the cells of the endodermis, mineral ions are actively pumped from these cells into the solution in the xylem vessels in the root.

• Solution in xylem more concentrated water enters xylem by osmosis.

• This process creates an upward force called ROOT PRESSURE.

• Helps to push water in the xylem up to a certain height in the plant.

• Root pressure alone is insufficient to force the water to the top part of a tall tree.

• Need CAPILLARITY ACTION & TRANSPIRATIONAL PULL.

GUTTATION• In small plants, root pressure can

push water all the way up the stem & out of special pores called hydathodes at the edges of leaves Guttation

• Occurs on cool humid morning when the air is too saturated for the water drops to evaporate from the leaves.

THE EXTERNAL CONDITIONS AFFECTING THE RATE OF

TRANSPIRATION

• The external factors that affect the rate of evaporation.– Air movement– Temperature– Light intensity– Relative humidity

AIR MOVEMENT• Movement of air carries away water

molecules that have diffused from the leaves.

• The faster the air movement (wind speed) the greater the rate of transpiration.

• When there is little air movement/ in still air, water vapour diffusing out of the leaf accumulates around the stomata.

• Decreases the concentration gradient of water, the transpiration rate reduces.

TEMPERATURE• A rise in temperature increases the

kinetic energy of water molecules & increase the rate of evaporation (transpiration).

• Lowers the relative humidity of the air increase the rate of transpiration.

• Temperature low, kinetic energy of water molecules is decreased.

• Relative humidity of the air is higher decrease the rate of transpiration

LIGHT INTENSITY• The light intensity increased, the

rate of evaporation increases.

• Light not effects evaporation but causes the stomata to open increasing water loss from the plant.

• At night @ dark, stomata close rate of transpiration decreases

• The rate of evaporation depends on the

difference in concentration of water molecules in the sub-stomatal spaces & in the surrounding atmosphere.

• The lower the relative humidity of the surrounding atmosphere, the faster will the water vapour escape from the stomata.

• High relative humidity reduces water loss.

ROOT PRESSURE• If a stem is cut just above ground

level, a considerable amount of sap will exude from the cut stump, showing that there is a force pushing water up the stem from the roots.

• Its ROOT PRESSURE The result of an active process in which energy from ATP is used.

ROOT PRESSURE• Root pressure alone is not strong

enough to drive water to the tops of tall trees at the speeds achieved during a normal growing season.

COHESION & ADHESION OF WATER

• COHESION = the force of attraction between the same molecules

• The cohesive forces between water molecules hold the continuous columns of water together & when water transpires from the leaf, the whole of the water column moves up the xylem vessels.

• ADHESION = the force of attraction between different molecules

• Water molecules tend to adhere to the walls of the xylem vessels.

• The adhesive forces can support a considerable mass of water.

• Cohesion + adhesion = capillarity (the rise of water in a capillary tube).

OPENING & CLOSING OF STOMATA• Dicotyledonous leaves = on the lower

surface

• Monocotyledonous leaves = both the upper & lower surfaces.

• Each stoma consists of a pair of kidney-shaped guard cells surrounding a stomatal pore.

• A guard cell has a thinner elastic outer cell wall & a thicker, less elastic inner cell wall. It has a nucleus, cytoplasm & chloroplasts.

• The opening & closing of the stomata in the leaf controls the diffusion of CO2 & O2 into & out of a leaf as well as the rate of transpiration.

STOMATAL OPENING• DAY : photosynthesis occurs & CO2 level is

low in the leaf.

• Potassium ions (K+) diffuse from surrounding epidermal subsidiary cells into the guard cells.

• In many plant species, starch stored in the guard cell is converted into malate.

• The accumulation of K+ & malate ions increase the osmotic pressure in the guard cell.

• Water enters by osmosis from surrounding epidermal cells into the guard cells

• Two guard cells become turgid & curve outward & the stoma opens.

STOMATAL CLOSING• NIGHT : no photosynthesis & CO2 level

is high. Potassium ions move out of the guard cells into the epidermal cells.

• Malate ions are converted to starch. The osmotic pressure in the guard cells decreases.

• Water diffuses out to the surrounding epidermal cells by osmosis.

• Guard cells become flaccid & stoma closes.

• The stomata also close, when there is insufficient water supply & the transpiration rate is high.

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