AP Biology Transpiration and Stomata

Living things must exchange matter with the

environment to survive, Example:

Gas Exchange in Plants

photosynthesis

cellular

respiration

1. During which hours does a plant produce a

net excess of O2? Explain why:

2. During which hours does a plant consume

more O2 than it makes? Explain why:

1. Describe the path

of H20 through a

land plant.

2. Describe and

explain the

direction of gas-

exchange in the

Plant’s

a. Roots

b. Leaves

1. Where is water

entering and where

is it exiting the plant?

2. What is the direction

of water movement?

3. How is water moved

against gravity?

4. What is the direction

of sugar movement?

5. What is the

mechanism of this

movement?

6. What is the role of

light energy in both

processes?

1. What is the common ancestor to all plants?

2. When did the first terrestrial plants arise?

3. When did vascular plants arise?

Aquatic Plants

• Diffusion of O2 and

CO2 from the water

directly into and out

of cells on the plant’s

surface

• Cold water holds

more gas (more

primary productivity)

• No water limitation

Early Terrestrial Plants

• Life emerged onto

land – ex. Moss

• Tied in moist areas

• Very simple vascular

tissue to transport

water over short

distances (restricts

plant size)

• Mobile sperm (require

water)

1. Describe the structure of the root-system shown

above?

2. What is the function of the plant’s root system?

3. How do root hairs affect total surface area of the

root system?

Terrestrial Plants: Xylem• Xylem cells line up end-to-

end; ends are porous

• Water is transported through the xylem (like water through pipes)

• This allows water to move from roots to leaves over long distances (plants can become tall)

waxy cuticle

Stoma = a pore that

allows CO2 in and O2

and H2O out

Adaptations to prevent excessive water-loss:

• Thick, waxy cuticle on leaves

• Stomata

Terrestrial plants need adaptations to exchange matter with the environment:

• Roots (H2O & minerals)

• Xylem for water-transport

Stomata opening and closing:

http://www.youtube.com/watch?v=cFX4JrsPaUs

1. Name & describe two

structural features of the

leaf that are designed to

limit water-loss:

2. As it relates to water,

what is the function of a

terrestrial plant’s roots?

3. When a leaf is in the

light and its stomata are

open, in which direction

are the following moving

through the stomata:

H2O, CO2, O2?

Stoma

Trade-Offs

• Plants must open

stomata to obtain CO2

during photosynthesis

• But, when stomata are

open a plant

experiences water-loss

• Excess water-loss will

cause wilting and the

stomata will close

• Photosynthesis stops

without continued CO2Guard cell

Plant-Reactions to Water-Stress

• Water stress results in release of

ABA, which causes stomata to close

• Some plants can fold their leaves

• Ocotillo shed their leaves in

response to seasonal drought

Adaptations

Sunken stomata

further reduce

water-loss

Adaptations

• Cacti only open

their stomata at

night when it is

cooler (they

practice a different

form of

photosynthesis)

• Cacti also have

water-storage

capabilities

Driving Speed Homeostasis: Negative Feedback Loop

Plant Water

Potential (hydration

homeostasis)

Water potential

high, light,

[CO2)] ↓

Stomata open

[CO2] ↑ water potential drops

Water stress:

Hormone ABA

released

Water potential ↑ [CO2] ↓

Stomata Close

Plant Water

Potential

(hydration

homeostasis)

Water

potential

high, light,

[CO2)] ↓

Stomata open

[CO2] ↑ water potential drops

Water stress:

Hormone

ABA

released

Water

potential ↑ [CO2] ↓

Stomata

Close

The Story in the Stomata1. Describe the trade-off involving stomata:

2. Summarize and explain the relationship between

stomata density and atmospheric [CO2]:

3. Explain how could plant fossils be used as indicators

of past climates and atmospheric conditions:

1. Evidence for

communication?

Via shoot or

root-system?

Justify:

2. Why block

chemicals in the

soil?

3. Why after 1 hr.

plants 9-11

similar to 7-8?

4. Why the

control?

Species Density of stomata top

Density of stomata bottom

1 75 125

2 0 75

3 0 0

5 57 0

Match the species # with the description of its environment (justify your match):

• Submerged (aquatic) plant• Tree that grows in the open savannah grasslands• Small bush that grows in the tropical rainforest• A plant that grows in a lake, but whose leaves float on

the surface of the water

How can you calculate the density of stomata?

Diameter = 0.4 mm

# stomataπ(r)2

6 stomata = 6 stomataπ(0.2 mm)2 0.1256 mm2

= 48 stomata/mm2X40 objective

For which type of plant would you expect to find:

• Stomata on both top and bottom leaf surfaces

• No stomata

• Stomata only on the top Justify your selection:

For which type of plant would you expect to find:

• Stomata on both top and bottom leaf surfaces

Emergent plant such as the cattails because both

surfaces are exposed to air and the stomata are

necessary for gas-exchange but also help to limit

water-loss due to transpiration.

• No stomata The aquatic plant will do gas

exchange via diffusion of gases from water

through the entire leaf surface area. Water is also

not limiting because the plant is submerged.

• Stomata only on the top The water lily only has its

upper surface exposed to air. Here it will want to

control water-loss via stomata regulation. The

bottom surface is exposed to water, plant does

not have to control for water loss here.

Why do many terrestrial plants have

stomata only on the bottom of their

leaves (or more on the bottom than top)?

Why do many terrestrial plants have stomata

only on the bottom of their leaves (or more

on the bottom than top)? The top of the

leaves are exposed to more sunlight, which

would speed evaporation from stomata.

Concentrating stomata on the leaf bottoms

helps to slow water loss via transpiration.

There are trees over

330 ft. (100 m) tall.

They don’t have

hearts to pump fluids,

yet they move water

this great distance

against the force of

gravity.

Which acronym can

help you remember

the mechanism?

Water is a Polar Molecule

• Oxygen is more

electronegative than

Hydrogen - it exerts

a stronger pull on

the electrons

partial+ partial+

partial-

• Water is a polar molecule.

• Oxygen has a partial negative (δ-) charge.

• Hydrogen has a partial (δ+) charge.

δ-

δ +

δ+δ+

δ +δ-

δ-

• Hydrogen bonds are electrostatic attraction between δ + and δ–

• Hydrogen bonds are weaker than covalent bonds

• In liquid water they constantly break and reform

• Hydrogen bonds are responsible for water’s special properties such as cohesion and adhesion

Hydrogen bond

Emergent Properties of Water Molecules

1. Describe the role of

adhesion: H2O H-

bonds with the

xylem walls;

opposes downward

pull of gravity

2. Describe the role of

cohesion: H2O H-

bonds with each

other, exerting a

pull on the water-

column as each

H2O molecule exits

a stomata

1. What is the pattern in

Ψ from the soil to the

roots?

2. Why does water move

from the soil into the

roots?

3. What is the pattern in

Ψ from the roots to the

shoots?

4. What is the pattern in

Ψ from the leaves to

the outside air?

5. Why does water exit

the stomata into the

air?

1. What is the pattern in Ψ

from the soil to the roots?

Ψ decreases

2. Why does water move

from the soil into the

roots? Ψ decreases

3. What is the pattern in Ψ

from the roots to the

shoots? Ψ decreases

4. What is the pattern in Ψ

from the leaves to the

outside air? Ψ

decreases

5. Why does water exit the

stomata into the air? H2O

from ↑Ψ → ↓Ψ

Transpiration

Adhesion

Cohesion

Tension

Water diffuses through plant

tissues, entering through the

roots and exiting via leaf-pores

called stomata

Water molecules adhere to xylem

vessels due to H-bonding

Water molecules “stick” to

each other due to H-bonding

As H2O molecules evaporate from

the stomata, they create a pull on the

molecules below, creating a tension

that pulls water up against gravity

An acre of actively growing corn can

transpire 3,000 – 4,000 gallons

(11,000 – 15,000 liters) of water a day

Describe & explain the relationship between

the variables from 0 – 60% open stomata.

Do the same for >60% open stomata.

As % open stomata ↑ from 0 to 60, so does the

rate of transpiration because with more stomata

open there is ↑ diffusion between the leaf and the

environment. Above 60%, there is no change in

the rate of transpiration because another factor

becomes limiting (i.e. rate of water movement,

humidity, etc.).

1. Calculate the average rate of transpiration for

species A.

2. Calculate the average rate of transpiration for

species B.

3. Which species had the higher rate of transpiration?

Justify:

4. List and discuss three structural or physiological

adaptations that could account for the differences:

Because the slope of line A is steeper than B and 0.24 mL

H2O/100g/min is higher than 0.14 H2O/100g/min

b.

1. Show a graph of the relationship

between temperature and the rate

of transpiration (make sure you

label your axes)

2. Discuss the mechanism by which

temperature effects the rate of

transpiration:

As temp. ↑, kinetic

energy of molecules ↑,

thus the speed of

diffusion (evaporation of

water from stomata

openings) will increase.

At a certain temp. the

rate will level off

because other factors

now limit the rate of

transpiration.

1. Show a graph of the relationship

between relative humidity and the

rate of transpiration (make sure

you label your axes and note that

relative humidity runs on a scale

from 0 to 100%)

2. Discuss the mechanism by which

humidity effects the rate of

transpiration:

As relative humidity ↑, transpiration rate ↓,

because as humidity ↑, the gradient in Ψ

between the leaf and the air ↓, thus water loss ↓.

Boundary layer of high humidity,

difference in Ψ inside and

outside the leaf is ↓, so

transpiration rate ↓

Wind blows away

boundary layer

Difference in Ψ (or osmotic

potential) inside and outside the leaf ↑, so transpiration rate ↑

The effect of light on the rate of transpiration

https://www.youtube.com/watch?v=ce-4Q2NxiNE

See video on potometer set-up

Which condition would result in a higher

rate of transpiration? Explain:

1. Full sunlight or shady environment

2. Humid or dry environment

3. Still or breezy environment

Go and inspect your

lab station:

Discuss methods

Discuss how to

measure mL using

the pipette