Home / iGCSE Biology (0610) 2.2 Size of specimens iGCSE Style Questions Paper 4

iGCSE Biology (0610) 2.2 Size of specimens iGCSE Style Questions Paper 4

IGCSE Biology (0610) Practice Questions Paper 4 - All Chapters

Question

Xerophytes are plants that show adaptive features for dry habitats.

(a) Complete the sentence to define the term adaptive feature.

An adaptive feature is an …………………… feature that helps an organism to …………………. and ………………… in its environment.

(b) Marram grass is a xerophytic plant.
Fig. 4.1 is a photomicrograph of a cross-section of a marram grass leaf.

Using Fig. 4.1, describe the visible features that identify this as a xerophyte leaf.

(c) (i) The mean cell wall thickness of an epidermal cell from one species of xerophyte is \(1.276 \, \mu\text{m}\).
Convert \(1.276 \, \mu\text{m}\) to mm.
(ii) State the name of the carbohydrate molecule used to build cell walls in xerophytes.
(iii) Suggest one way the stem of a xerophyte is adapted for dry habitats.

(d) Fig. 4.2 is a diagram showing the root systems of two different species of xerophyte.

Describe the two root systems shown in Fig. 4.2 and suggest how each one is an adaptation for living in a dry environment.

▶️ Answer/Explanation
Detailed solution

(a)
An adaptive feature is an inherited feature that helps an organism to survive and reproduce (or breed/produce offspring) in its environment.

(b)
Based on the cross-section of the marram grass leaf (Fig. 4.1), the visible xerophytic features include:

  • Rolled or curled leaf: The leaf is rolled inwards to reduce the surface area exposed to the drying effects of the wind and to trap moist air inside.
  • Hairs: Epidermal hairs are visible lining the inside of the roll. These trap water vapour, creating a humid microclimate that reduces the water potential gradient and slows down transpiration.
  • Sunken stomata: The stomata are located in pits or grooves on the inner surface (often protected by the hairs), further shielding them from air currents.
  • Thick cuticle: A thick waxy layer on the outer surface prevents water loss through the epidermis.

(c) (i)
To convert micrometers (\(\mu\text{m}\)) to millimeters (mm), you divide by 1000 (since \(1 \text{ mm} = 1000 \, \mu\text{m}\)). $$ 1.276 \div 1000 = 0.001276 \text{ mm} $$ Alternatively expressed as \(1.276 \times 10^{-3} \text{ mm}\).

(c) (ii)
Cellulose. (This is the standard structural carbohydrate found in the cell walls of all plants, including xerophytes).

(c) (iii)
Adaptations of a xerophyte stem may include:

  • Swollen / Succulent: The stem acts as a water storage organ (e.g., in cacti).
  • Photosynthetic (Green): The stem contains chloroplasts to carry out photosynthesis, allowing the plant to reduce leaf size (or modify leaves into spines) to minimize water loss.
  • Thick waxy cuticle: To reduce evaporation from the stem surface.

(d)
The two species show different root adaptations for water absorption in dry environments:

  • Species 1 (Shallow, spreading roots): The roots are shallow and spread out over a wide area close to the soil surface.
    Adaptation: This allows the plant to absorb water quickly and immediately after light rainfall before it evaporates from the surface soil.
  • Species 2 (Deep, vertical roots): The roots grow very deep into the soil (tap roots).
    Adaptation: This allows the plant to access deep water sources or the water table, which provides a supply of water even when the surface soil is completely dry.
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