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TRANSPIRATION






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TRANSPIRATION. What is Transpiration?. An evaporation of water in the form of water vapour from the surface of the plant to the atmosphere Where does transpiration take place?. Transpiration. Transpiration mainly takes place through openings on leaves – STOMATA
TRANSPIRATION

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Slide 2

TRANSPIRATION

Slide 3

What is Transpiration?

  • An evaporation of water in the form of water vapour from the surface of the plant to the atmosphere

  • Where does transpiration take place?

Slide 4

Transpiration

  • Transpiration mainly takes place through openings on leaves – STOMATA

  • If the stomata is opened, then water vapour will be lost into the atmosphere

  • In some cases, transpiration also takes place through the lenticels and cuticles

Slide 5

Transpiration

  • A thin film of water covers each mesophyll cell

Slide 6

1

Slide 7

Transpiration

  • A thin film of water covers each mesophyll cell

  • There are numerous air spaces between the mesophyll cells

Slide 8

2

Slide 9

Transpiration

  • A thin film of water covers each mesophyll cell

  • There are numerous air spaces between the mesophyll cells

  • Water evaporates from the film of water surrounding the mesophyll cells into the air spaces (and eventually diffuses out of the stomata into the atmosphere)

Slide 10

3

Slide 11

Transpiration

4) As water diffuses out of the cell, the water potential within the cell will decrease. Due to OSMOSIS, water from adjacent cells will be drawn into the cell to replace the water loss

These adjacent cells will in turn draw water from other neighboring cells

Slide 12

4

Slide 13

Transpiration

5) Water is drawn from the XYLEM vessels (in the veins) into the neighboring mesophyll cells

There will be a water potential difference between the xylem vessels and the mesophyll cells. As water is drawn from the xylem vessels, a suction pressure will develop and this pressure will pull water up the xylem vessels from the roots to the leaves

Slide 14

5

Slide 15

Transpiration

  • The pressure that allows water to be pulled from the roots to the leaves in the xylem vessels is called TRANSPIRATION PULL

  • It allows the transportation of water and minerals in plants

Slide 16

Investigation #1:Studying the loss of water by a plant into the atmosphere

Slide 17

polythene bag

Procedure

Slide 18

Discussions

  • What is the purpose of enclosing the pot and lower part of the plants with a polythene bag?

Ans: The polythene bag prevents evaporation of

water from the soil, and also prevents

vapour released by soil microorganisms

from affecting the result of the experiment

Slide 19

Discussions

2) What do you think will happen in the two set-ups after 2 hours?

Ans: Water would be found condensed on the

bell jar with the leafy plant inside

Slide 20

Discussions

3) How can you show that it is water?

Ans: We can use anhydrous cobalt chloride

paper to test it. It will turn the paper from

blue to pink. Alternatively, we can use

anhydrous copper sulphate. Water will

turn it from white to blue

Slide 21

Discussions

4) Why is it better to use forceps instead of fingers to hold cobalt chloride paper?

Ans: It is to avoid moisture on our fingers from

being absorbed by the cobalt chloride

paper

Slide 22

Discussions

5) How would you explain the results of this experiment?

Ans: Transpiration occurs in plants through

their leaves

Slide 23

I have no trouble absorbing sunlight, but I just keep losing and losing water. Is there any way that I can prevent excessive water loss?

Slide 24

Adaptations to Prevent Water Loss

1) Waxy layer of cuticle on the leaf’s outer surface of the epidermis

Slide 26

Adaptations to Prevent Water Loss

  • A waxy layer of cuticle covers the outer surface of the epidermis

  • More stomata are present in the lower epidermis than the upper epidermis

  • Guard cells control the closing and opening of stomata

Slide 27

Stomata

  • Stomata are pores in the epidermis where gaseous exchange takes place during photosynthesis (or respiration)

Slide 28

Stoma

Guard cell

Epidermal cell

Slide 29

Opened Stoma

Closed Stoma

Slide 30

Each stoma is surrounded by two guard cells which contain chloroplasts

It is kidney-shaped

The inner wall is thicker than the outer wall

Guard cell

Stoma

Guard Cells

Slide 33

How Guard Cells Control the Size of Stoma

  • Guard cells contain chloroplasts that carry out photosynthesis in the presence of light. Carbohydrates are formed and thus lower the water potential of the cell

  • Water enters the guard cells from adjacent cells by osmosis and guard cells become turgid

Slide 34

How Guard Cells Control the Size of Stoma

  • The inner wall is thicker than the outer wall, so the cell stretches to the outer side and stoma is opened

  • At night, there is no photosynthesis. Guard cells become flaccid and so they return to the original shape and stoma is closed

Slide 36

Distribution of Stomata in Leaves

Slide 37

Distribution of Stomata in Leaves

1) Normal Plants

- Mainly on the lower surface of plants

2) Floating plants

- Mainly on the upper surface of plants

- Leaves may also have air sacs to keep them

afloat. These sacs can be used in gaseous

exchange

Slide 38

Distribution of Stomata in Leaves

3) Submerged Aquatic Plants

- No stomata (not required since gaseous

exchange can be carried out by diffusion

though the leave surface)

- No cuticle (the primary function of cuticle is

to prevent excess water transpiration which

is not present in aquatic plants)

Slide 39

Distribution of Stomata in Leaves

4) Plants in dry and hot conditions

- usually have much less stomata to reduce

the amount of water loss

Slide 40

Investigation #2:Investigating stomatal distribution in a leaf by using cobalt chloride paper

Slide 41

cobalt chloride paper

sellotape

Which piece of cobalt chloride paper will turn pink first?

Ans: The piece of cobalt chloride paper attached to the lower epidermis of the leaf will turn pink first.

Slide 42

Investigation #3: Comparing the abundance of stomata on the upper and lower surfaces of a leaf

Slide 43

Introduction to Investigation

  • This investigation allows us to compare the amount of stomata present on the upper and lower surfaces of a leaf by putting the leaf in hot water and observing the amount of bubbles appeared

Slide 44

forceps

hot water

leaf

Procedure

Slide 45

Discussions

1) Which surface has more air bubbles coming off?

Ans: There should be more air bubbles appearing

on the lower surface of the leaf

Slide 46

Discussions

2) Why do air bubbles appear on the leaf surfaces?

Ans: Air in the air spaces between the mesophyll

cells in leaf expands on heating and passes

out through stomata of the leaf, forming air

bubbles

Slide 47

Discussions

3) What does the result show?

Ans: The result shows that more stomata are

present on the lower epidermis of a leaf

Slide 48

Investigation #4: Comparing the abundance of stomata on the upper and lower surfaces of leaves by weighing

Slide 49

Introduction to Investigation

  • As water is lost by evaporation, the weight of a detached leaf will decrease with time. In this investigation, leaves will be treated differently (with vaseline) and the loss in weight will then be compared among the leaves.

Slide 50

Procedure

A

B

C

D

Smear with vaseline on both surfaces of the leaf

Smear with vaseline on lower surface of the leaf only

Smear with vaseline on upper surface of the leaf only

Do not apply vaseline on the leaf

Slide 51

Discussions

  • What is the function of vaseline in this experiment?

Ans: It is used to block the stomata so as to

prevent water being lost by evaporation

Slide 52

Discussions

2) Which leaf would show the greatest/least change in weight?

Ans: Leaf D > Leaf C > Leaf B > Leaf A

Slide 53

Discussions

3) What do the results of leaves B and C indicate?

Ans: The change in weight of leaf B is less than

that in leaf C, which indicates that more

water is lost in leaf C. This suggests that

there are more stomata present in the lower

surface than the upper surface of leaves

Slide 54

Xerophytes

Plants living in hot and dry environment

Slide 55

Adaptations of Xerophytes

  • They have numerous epidermal hair - trap moisture

Slide 57

Adaptations of Xerophytes

  • They have numerous epidermal hair - trap moisture

  • Some have sunken stomata - also trap moisture

Slide 59

Adaptations of Xerophytes

  • They have numerous epidermal hair - trap moisture

  • Some have sunken stomata - also trap moisture

  • Some have rolling leaves which enclose the stomata – reduce contact between stomata and the environment

Slide 61

Adaptations of Xerophytes

  • They have numerous epidermal hair - trap moisture

  • Some have sunken stomata - also trap moisture

  • Some have rolling leaves which enclose the stomata - reduce contact between stomata and the environment

  • Some have small, needle-like or spiny leaves, which have a small surface area

Slide 63

Adaptations of Xerophytes

  • They have numerous epidermal hair

  • Some have sunken stomata

  • Some have rolling leaves which enclose the stomata

  • Some have small, needle-like or spiny leaves, which have a small surface area

  • Some have fleshy stems or leaves to help store water

Slide 65

Some trees also shed their leaves during dry seasons (e.g. autumn) in order to reduce the rate of transpiration

Adaptations of Xerophytes

Slide 66

Investigation #5: Comparing the rates of transpiration of a leafy shoot under different environmental conditions using a bubble potometer

Slide 67

Introduction to Investigation

  • Bubble potometer can be used for estimating the rate of water uptake by a plant. That is, it can be used as an indirect method in measuring the rate of transpiration. In this investigation, the transpiration rates of a plant under different conditions will be compared

Slide 68

graduated capillary tube

leafy shoot

air/water meniscus

tap

reservoir

Procedure

Slide 69

Results Table

Slide 70

Discussions

1) Why is it important to cut the leafy shoot under water?

Ans: This prevents air from entering the xylem vessels of the stem and blocking the water uptake

Slide 71

Discussions

2) What is the relationship between transpiration rate and the distance traveled by the bubble?

Ans: The rate of movement of the bubble is proportional to the transpiration rate of the plant. Under normal conditions, the rate of water absorption of a plant is equal to the rate of transpiration

Slide 72

Discussions

3) Compare the transpiration rate of the leafy shoot under the different conditions.

Slide 73

Under Sunlight

  • The plant has faster transpiration rate when it is placed under sunlight. Stomata open wider when the plant is placed under sunlight. Therefore the diffusion rate of water vapour to the atmosphere, and thus the transpiration rate, increases. High light intensity also increases the atmospheric temperature which in turn increases the rate of transpiration

Slide 74

Near a Fan

  • The plant has faster transpiration rate when it is placed near a fan. Greater air movement carries water vapour away from the surface of the leaf at a faster rate. This keeps the concentration of water vapour around the stomata at a low level. Hence, water vapour diffuses out of the leaf faster and the rate of transpiration is increased

Slide 75

Covered with Plastic Bag

  • The plant has a slower transpiration rate when it is covered with a plastic bag. High humidity in the surrounding air decreases the concentration gradient of water vapour between the leaf inside and that of the atmosphere. Hence, water vapour diffuses out of the leaf more slowly and the rate of transpiration is decreased

Slide 76

Limitations

  • The potometer can only measure the rate of water uptake by the leafy shoot but cannot directly measure the rate of transpiration. If a dehydrated plant is used, the rate of water absorption is higher than the transpiration rate. Under dry condition, the transpiration rate of the plant may exceed its rate of water absorption.

  • Therefore, when using a bubble potometer, assume that the rate of water absorption by the plant is equivalent to the rate of transpiration

Slide 77

Investigation #6: Comparing the rates of transpiration of a leafy shoot under different environmental conditions using a weight potometer

Slide 78

Introduction to Investigation

  • Another way to measure the a plant’s rate of transpiration is to measure its loss in weight using an electrical balance. This is called a weight potometer. In this investigation, the transpiration rates of a plant under different conditions will be compared

Slide 79

Procedure

Slide 80

Discussions

1) What is the function adding a layer of oil in the set-up?

Ans: The layer of oil can prevent evaporation of water in the flask, which can affect the result

Slide 81

Discussions

2) What is the relation between the transpiration rate and the change in weight of the plant?

Ans: Since the water loss from the set-up is due to transpiration only, the change in weight of the set-up is directly proportional to the transpiration rate of the plant

Slide 82

Discussions

3) What limitations may lead to inaccurate results in the experiment?

Ans: The initial weight of the set-up may not be accurate because water may be present on the wall of the apparatus and also on the leafy shoot

Slide 83

Discussions

4) Compare the weight potometer and the bubble potometer. Which one is easier to use? Which one is more accurate?

Ans: A weight potometer is easier to use and is more accurate to measure the transpiration rate of plants. It is because the weight potometer can measure the rate of transpiration directly whereas the bubble potometer can only measure the rate of water uptake of plants

Slide 84

Environmental Factors Affecting the Rate of Transpiration

  • There are FIVE environmental factors which affect the rate of transpiration. They are:

(IV) Wind Speed

(I) Light Intensity

(II) Temperature

(V) Water Supply

(III) Humidity

Slide 85

Light Intensity

  • More stomata open wider in light, so plants can get enough carbon dioxide from atmosphere for carrying out photosynthesis

  • Light will increase temperature

  • Rate of diffusion/evaporation of water vapour through stomata will increase

  • Rate of transpiration increases

Slide 86

Effect of Light Intensity on Rate of Transpiration

Rate of transpiration

Maximum rate

Light intensity (lux)

Slide 87

Temperature

Temperature

Rate of evaporation of water from mesophyll cells

Ability of air to hold water vapour

Rate of diffusion of water vapour from intercellular space in leaf to outside

Rate of transpiration

Slide 88

Rate of transpiration

Effect of Temperature on Rate of Transpiration

Temperature (0 C)

Slide 89

Humidity Outside

A decrease in humidity makes the diffusion gradient of water vapour from the moist intercellular space of a leaf to the external atmosphere steeper, therefore the rate of diffusion of water vapour increases

Humidity

Rate of Transpiration

Slide 90

Rate of transpiration

Effect of Relative Humidity on Rate of Transpiration

Relative humidity (%)

Slide 91

Wind Speed

Wind blows

Under very windy conditions, stomata will be closed to reduce water loss

Water vapour around the leaf is swept away

Transpiration rate increases

Diffusion gradient between intercellular space in leaves and outside becomes steeper

Slide 92

Wind speed (km/hr)

Effect of Wind Speed on Rate of Transpiration

Rate of transpiration

Slide 93

Availability of Water

Lack of water (plants become dehydrated)

Soil dries, plant wilts and stomata close

Transpiration rate decreases

Slide 94

Wilting – the leaves and stems become flaccid due to dehydration

Slide 95

Enrichment Reading 9.1

Daily Changes in Transpiration Rate

Slide 96

Plant Tissues

Plant tissues fall into 3 fundamental categories:

  • Dermal tissues

  • Ground tissues

  • Vascular tissues

    *These tissues can be found in roots, stems

    and leaves

Slide 98

Dermal Tissues

  • Provides a protective layer around the plant

  • Exists as a single layer of cells called the epidermis

  • No chloroplasts

Slide 99

Dermal Tissues

  • In the shoot system the epidermis secretes a waxy layer called the cuticle (a protective barrier to retard water loss and to prevent infection)

  • Stomata are found on the epidermis to allow gas exchange to occur

  • Guard cells control the closing and opening of stomata

  • Guard cells are the only cells in epidermis that contain chloroplasts

Slide 101

Ground Tissues

  • Make up the bulk of the plant

  • Ground tissues are needed for storage, mechanical support and energy production

  • They can be classified into four broad categories – parenchyma, chlorenchyma, collenchyma and sclerenchyma

Slide 102

Parenchyma Cells

  • The most abundant ground tissues

  • Loosely arranged with intracellular spaces

  • Have thin walls and large vacuoles

  • Metabolically active

  • Perform a variety of functions, such as photosynthesis, repair, food storage and secretion

Slide 103

Parenchyma Cells

Slide 105

Vascular Tissues

  • Continuous throughout the plant

  • Usually embedded in ground tissues

  • Composed of two complex conducting tissues which form the vascular bundles

  • Xylem + Phloem = Vascular bundles

Slide 107

Xylem

  • Conduct water and minerals (one way only: from roots to shoot)

  • Provide support to plant

  • The cell walls of xylem cells derive most of their strength from lignin, a chemical compound produced only by plants

  • It is composed of tracheids, vessel elements, fibers, and parenchyma cells

Slide 108

Xylem

1) Tracheids – long, thin cells with closed ends and are dead at maturity. Contain numerous pits through which water moves

2) Vessel elements – similar to tracheids but contain holes at each end and are joined end-to-end forming vessels. They are thick-walled and non-living (no cytoplasm and no nuclei)

Slide 112

Phloem

  • Transport organic materials (glucose) synthesized by the plant from leaves to the rest of the plant

  • It is composed of sieve tube members, companion cells, fibers and parenchyma cells

Slide 113

Phloem

1) Sieve tube members - a sieve tube, like xylem vessels, is a series of cells (sieve elements) joined end to end. The cross walls between successive sieve elements are perforated, forming sieve plates. The cell walls are thin. Although the cells are living, they lack a nucleus. Unlike xylem vessels, the cells walls are not thickened by lignin

Slide 114

Phloem

2)Companion cells – specialized parenchyma cells that develop alongside a sieve tube member. They are elongated, thin-walled and possess a nucleus. Companion cells are linked with the sieve tubes by small canals filled with cytoplasm, which are smaller than pits. Companion cells help to regulate the metabolic activities of sieve tube elements, and help to load and unload the food for transport

Slide 117

Different Parts of a Dicot Plant

  • Let’s examine the distribution of different tissues in various parts of a dicotyledonous plant:

  • Root

  • Stem

  • Leaf

Slide 121

Layer of thin-walled cells between xylem and phloem

The cambium produces new layers of phloem on the outside and new layers of xylem on the inside, thus increasing the diameter of the stem

Cambium

Slide 123

Structure of Dicot Root

  • Outermost layer: epidermis – no cuticle

  • Cortex – thin walled parenchyma allow movement of water and minerals

  • Vascular tissues in centre of root

  • Tip: root cap

Slide 124

Root Cap

  • A protective layer at the very tip of root

  • To protect the delicate cells of root from being damaged as the root grows down through the soil

  • Growing point is behind root cap – by active cell division

Slide 125

Functions of Roots

1) Water and minerals absorption

2) Anchorage

Slide 126

Adaptations for Absorption of Water and Minerals

  • Extensive branching system – what is advantage?

  • Outermost layer consists of epidermal cells that lack cuticle – what is advantage?

  • Some epidermal cells near the root tip have root hairs – what is advantage?

Slide 127

Adsorption of Soil Water by Root Hairs

  • Soil water is a dilute solution of salts – it is more dilute than the cell sap and cytoplasm of root hair

  • Water will pass by OSMOSIS into root hair through cell wall and cell membrane

Slide 129

Absorption of Minerals by Root Hairs

  • Concentration of minerals in soil is usually lower than that inside the root epidermal cells

  • Can minerals be taken up by osmosis???

Slide 130

Water and Minerals Transportation

  • Can you design an experiment to show that water and minerals are transported along the xylem vessels only?

Slide 131

Adhesion

Cohesion

Against gravity

Capillary Action

Slide 132

Root Pressure

  • Minerals are actively transported from root to xylem

  • If transpiration rate is low, salts accumulate in xylem and thus water potential is lowered

  • Therefore, water enters xylem by osmosis

  • Pressure builds up, pushing the content in the xylem upwards

  • If stem is cut, water can be seen gushing out (“stem-bleeding”)

Slide 133

Mesophyll cells in leaves (high [ ] of carbohydrates) -> sieve tubes of phloem (low [ ] of carbohydrates) -> active growing areas (e.g. root tips) /storage areas (e.g. fruits)

Transportation of Organic Substances

Slide 134

Why do plants need a support system?

Slide 135

Why do plants need a support system?

  • Prevent leaves from shaded by other plants

  • Allow leaves to receive maximum amount of sunlight for photosynthesis

  • Display flowers as to facilitate dispersal of pollen or seeds

Slide 136

Support in young plants or non-woody parts of plants is contributed mainly by turgidity of the thin-walled cells (parenchyma) in the cortex and pith

Support in Young Dicot Plants

Slide 137

Support in Young Dicot Plants

  • Thin-walled cells in cortex and pith with large central vacuole

  • Water enters the cell vacuole by osmosis

  • Pressure exerted on the tough epidermis

  • Stem becomes hard and upright

Slide 138

Water absorbed by osmosis

Water lost by osmosis

turgid

flaccid

Support in Young Dicot Plants

Wilting

Slide 139

Support in Young Dicot Plants

  • Xylem vessels contain cells walls with lignin – thick and rigid

  • Vascular bundles arranged in a ring near the epidermis – prevent bending

  • In roots – vascular bundles in centre to give roots more penetrating power and more resistance to stretching (prevent uprooting)

Slide 140

Support in Old Dicot Plants

  • Majority of xylem is thick-walled cells (e.g. xylem vessels) – make stem rigid

  • Plant no longer rely on turgidity for support

  • Support is now contributed mainly be rigidity of the lignified cells in xylem

Slide 141

Support in Old Dicot Plants

  • Thin-walled cells, cambium, found between xylem and phloem

  • As plant matures, cambium cells divide to form new cells -- inner side -> new xylem

    -- outer side -> new phloem

  • Accumulation of xylem tissues -> stem becomes woody and increases in diameter

Slide 142

Support in Old Dicot Plants


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