CAIE A-Level Biology 14 Homeostasis Question Bank

Practise negative feedback, kidney function, osmoregulation, glucose and temperature control by identifying receptors, coordinators and effectors in data and mechanisms.

[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) ;

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Covered syllabus points

14. Homeostasis question 1

Question (a)

Question (i)

Question (ii)