CAIE A-Level Biology A2 14.2 Homeostasis in Plants Questions

Practise plant-homeostasis questions by explaining stomatal rhythms, guard-cell ion movement and ABA signalling during water stress and changing conditions.

Exam points

link stomatal opening or closure to light, CO2, humidity, temperature and water stress
sequence guard-cell H+, K+, Cl- and water movements that change turgor and pore size
explain ABA signalling with receptors, Ca2+ second messengers and potassium efflux

Covered syllabus points

14.2.1—Stomata response to environmental changes 14.2.2—Daily rhythms of stomatal opening and closing 14.2.3—Guard cells 14.2.4—Abscisic acid and stomata

[Maximum number: 6]

There are more than 600 plant species in the genus Ipomoea. Many species are grown for their attractive flowers, and some species are used as crop plants.

(a)

Scientists investigated the response of stomata to changing carbon dioxide ( $\mathrm{CO}_{2}$ ) concentrations in the beach morning glory, I. pes-caprae.

The scientists placed I. pes-caprae plants in chambers. They measured the width of open stomata (stomatal apertures) after the plants had been exposed to different $\mathrm{CO}_{2}$ concentrations for 40 minutes. Light intensity and temperature were kept constant.

The relationship between $\mathrm{CO}_{2}$ concentration and the mean width of stomatal apertures is shown in Fig. 3.2.

Fig. 3.2

[ 6 ]

(i)

In 2016, a study measured the atmospheric $\mathrm{CO}_{2}$ concentration as $400 \mu \mathrm{~mol} \mathrm{~mol}^{-1}$ .

In the future, climate change may reduce water availability and increase atmospheric $\mathrm{CO}_{2}$ concentrations in some habitats.

Suggest how the stomatal response shown in Fig. 3.2 would allow I. pes-caprae to survive the effects of climate change.

[ 2 ]

any two from:
1 (as climate changes) increase in $\mathrm{CO}_{2}$ concentration causes, stomatal aperture to decrease / stomata to be less open ;
2 less water, lost / evaporated / transpired (though stomata) ;
3 compensates for / allows survival in, low water availability / dry conditions ;

(ii)

Under certain conditions, the closure of stomata is controlled by abscisic acid.

Describe how abscisic acid causes the closure of stomata.

[ 4 ]

any four from:
1 (abscisic acid / ABA) binds to receptor on, (guard) cell surface membrane ;
$2 \quad \mathrm{H}^{+}$ stops moving, out of cell/into cell wall ;
$3 \mathrm{Ca}^{2+}$ ions enter, cytoplasm / cell
or
$\mathrm{Ca}^{2+}$ ions act as second messenger ;
$4 \mathrm{~K}^{+}$ ions leave cell ;
5 water leaves cell, by osmosis / down water potential gradient ;
6 less turgid / flaccid, guard cell(s) close stoma(ta) ;

[Maximum number: 5]

Fig. 4.1 shows a diagram of a stoma, its guard cells and adjacent epidermal cells.

Fig. 4.1

(a)

Guard cells have chloroplasts while epidermal cells do not have chloroplasts.

State one other difference, visible in Fig. 4.1, between guard cells and epidermal cells.

[ 1 ]

(b)

During stomatal closure:

[ 4 ]

(i)

state precisely where abscisic acid (ABA) binds

[ 1 ]

(ii)

identify the ion that diffuses from the guard cells to epidermal cells

[ 1 ]

$\mathrm{K}^{+} /$ potassium ;

(iii)

compare the relative water potential of the guard cells with that of epidermal cells

[ 1 ]

(iv)

describe the change in volume of the guard cells.

[ 1 ]

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Covered syllabus points

Question 1

Question (a)

Question (i)

Question (ii)

Question 2

Question (a)

Question (b)

Question (i)

Question (ii)

Question (iii)

Question (iv)